Plasma Focus Rawat 12

Copyright © 2012 American Scientific PublishersAll rights reservedPrinted in the United States of America

Nanoscience andNanotechnology Letters

Vol. 4, 1–24, 2012

High Energy Density Pulsed Plasmas in PlasmaFocus: Novel Plasma Processing Tool for

Nanophase Hard Magnetic Material Synthesis

R. S. RawatNatural Sciences and Science Education, National Institute of Education,

Nanyang Technology University, Singapore 637616

This paper presents the review of the applications of pulsed high energy density pinch plasmas (withenergy densities in the range of 1−10×1010 J/m3�, accompanied by self generated energetic highflux instability accelerated ion beams, from a dense plasma focus device in nanophase material syn-thesis, in particular the magnetically hard nanoparticle thin films with possible applications in highdensity magnetic data storage. The plasma focus device, a non-cylindrical z-pinch device, being amultiple radiation source of ions, electron, soft and hard X-rays, and neutrons, has routinely beenused for several applications such as lithography, radiography, imaging, activation analysis, radioiso-topes production and more recently for material processing and thin films depositions. This reviewpaper highlights and critically discusses the key features and traits (such as the plasma dynamics,plasma characteristics and energetic ions and electron emission characteristics) of plasma focusdevice to understand the novelties, opportunities and mechanisms of processing and synthesis ofnanophase hard magnetic materials using this device. The results of recent key experimental inves-tigations performed on the modification of various physical properties of PLD grown thin films ofFePt by energetic ion exposure in plasma focus device and the deposition of nanostructured CoPtthin films in plasma focus device are reviewed. The FePt and CoPt thin films are believed to be themost prominent candidates for ultra-high density magnetic data storage. The prime requirements forultra-high density magnetic data storage are: (i) small size of nanoparticles in the range of ∼3–4 nmwith narrow size distribution (ii) nanoparticles should be in fct phase, which is achieved by postsynthesis annealing at about 600 �C, with high magnetocrystalline anisotropy (Ku) to overcome thesuperparamagnetism, and (iii) reduced exchange coupling effects with well-separated distributionof nanoparticles. The reduction of phase transition temperature for transformation from low Ku fcc-structured magnetically soft A1 phase to high Ku fct-structured magnetically hard L10 phase, controlof the fct-(001) orientation of thin films and minimizing grain growth are three key challenges forpractical application of FePt and CoPt nanoparticle thin films in data storage These technologicallychallenging issues were successfully resolved, to some extent, using novel high energy densityplasmas of plasma focus device.

Keywords:

CONTENTS

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22. Dense Plasma Focus: Device Details, Principle of

Operation and Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32.1. Types of Dense Plasma Focus Devices . . . . . . . . . . . . . . . 32.2. Physical Layout, Operational Principle and Key

Characteristics of DPF Device . . . . . . . . . . . . . . . . . . . . . . 32.3. Characteristics of Pinch Plasma and Energetic Charged

Particles in DPF device . . . . . . . . . . . . . . . . . . . . . . . . . . . 52.4. DPF Devices for Material Processing and Synthesis:

Mechanism and Applications . . . . . . . . . . . . . . . . . . . . . . . 5

3. Nanophase Magnetic Materials . . . . . . . . . . . . . . . . . . . . . . . . . 73.1. Nanophase Magnetic Materials in Data Storage . . . . . . . . . 73.2. Magnetic Recording Media . . . . . . . . . . . . . . . . . . . . . . . . 8

4. Nanophase Hard Magnetic Materials Using DPF Device . . . . . . 84.1. Lower Phase Transition Temperature in PLD Grown

FePt Thins Films Using DPF Device as EnergeticIon Irradiation Facility . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

4.2. Hard Magnetic Nanophase CoPt Thins Film SynthesisUsing DPF Device . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

5. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21References and Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Nanosci. Nanotechnol. Lett. 2012, Vol. 4, No. 3 1941-4900/2012/4/001/024 doi:10.1166/nnl.2012.1318 1

High Energy Density Pulsed Plasmas in Plasma Focus Rawat

1. INTRODUCTION

The high energy density plasmas refers to the ionizedmatter at extremely high density and temperature whichare heated and compressed to a point that the storedenergy in the matter reaches approximately to 1011 J/m3

(the energy density of a hydrogen molecule).1 This cor-responds to a pressure of approximately 1 million atmo-spheres or 1 Mbar. It has, however, been widely acceptedthat the plasmas with energy densities in the range of�1−10�×1010 J/m3 are also classified as high energy den-sity plasmas. The types of experimental facilities avail-able for high energy density plasma research include (i)high-energy relatively long-pulse (0.2–25 ns) lasers suchas the one being developed at National Ignition Facility,US or OMEGA at University of Rochester, US,2�3 (ii) highpower very short pulse (tens to hundreds of fs) TW orPW lasers,4 (iii) pulsed-power devices (which include thefast high current z-pinch devices),5 (iv) high-current par-ticle beam accelerators,6 and (v) combinations of these.The plasma focus device, focus of this review paper, is anon-cylindrical z-pinch device which belongs to the cate-gory of pulsed-power devices where high current electri-cal discharge efficiently heat and compress the plasmas ina pinched plasma column. The energy density parameter(ratio of the electrical energy stored in the device to thevolume of cylindrical pinched plasma column) of variousdense plasma focus devices are reported to be in the rangeof �1�2−9�5�×1010 J/m37 making it a high energy densityplasma facility.Dense Plasma Focus (DPF) device is a hydromagnetic

coaxial plasma accelerator, developed independently invariant forms during 1960s by Mather in US8 and by Filip-pov et al. in erstwhile USSR,9 which involves the storageof magnetic energy behind a moving current sheath and the

R. S. Rawat, did his B.Sc.(Hons) Physics, M.Sc. Physics and Ph.D. from University ofDelhi, Delhi in 1985, 1987 and 1994. He lectured at Department of Physics and Electronics,SGTB Khalsa College, University of Delhi, from 1992 to 2000. He joined National Insti-tute of Education, Nanyang Technological University, Singapore in Dec 2000 as AssistantProfessor and then later became Associate Professor in Oct 2005. Rajdeep’s main researchinterest involves performing fundamental studies on pulsed plasma devices such as DensePlasma focus (DPF) and Pulsed Laser Deposition (PLD) and their applications to soft X-raylithography, radioisotopes synthesis, soft and hard X-ray imaging, material modification,and nano-structured material synthesis. He has extensive experience in various basic plasmadiagnostic techniques like laser shadowgraphy, X-ray spectrometry, X-ray imaging, opticalemission spectroscopy, ion beam analysis, optical streak photography, neutrons and charged

particles measurement etc, and various material characterization techniques such XRD, SEM, EDX, XPS, PL, VSM,MOKE, UV-VIS, FTIR etc. He pioneered the application of plasma focus device for processing of thin films by energeticions and later used it extensively for various nanophase thin film synthesis. He is leading the research efforts at PlasmaRadiation Source Lab at NSSE/NIE and have developed several plasma focus devices there which include two 3-kJ,three 200-J and one 20 kJ PF systems. He has published 116 journal papers and over 60 conference papers and has anH-index of 16. He is currently the Secretary of Asian African Association of Plasma Training (an ICTP OEA supportedgroup with 41 member institutes in 21 countries), Council Member of Institute of Physics, Singapore and Guest Editorfor IEEE Transactions on Plasma Science.

pumping of this energy into pinched plasma column duringthe rapid radial collapse phase, producing short duration(∼10 to 50 ns), high temperature (∼1 keV) and high den-sity plasma (∼1019 cm−3). This hot dense plasma is a richsource of energetic and multiple radiations like relativisticelectrons, soft/hard X-rays, fast ions and neutrons.10

The DPF attracted much attention from the scientificcommunity during the 1960s as it was considered as anefficient fusion device, producing an intense burst of neu-trons when operated with Deuterium/ Deuterium-Tritiumas filling gas. This led to intensive research on DPF in var-ious laboratories, during 1970s, all over the world to attaincontrolled thermonuclear fusion using this alternative mag-netic confinement scheme.11�12 Since then, the significantprogress in the associated pulsed power technology hasenabled the successful operation of large as well as tabletop miniature plasma focus devices13–16 leading to the bet-ter understanding of plasma dynamics and neutron produc-tion mechanisms in this device.The essential problem to be resolved in DPF research

has always been to discover the physics, which domi-nates the neutron yield limitation/saturation at higher bankenergies.17�18 It was realized later that the neutrons arenot of purely thermonuclear origin rather they are mainlydue to beam target mechanism.19�20 The better physi-cal understanding and viability of other fusion technolo-gies such as tokamaks and high-power laser driven fusionexperiments downgraded the importance of plasma focusdevices as controlled thermonuclear fusion device. Butnevertheless, this device was soon recognized as a power-ful and compact source of energetic radiations producingfast neutrons21�22 intense X-rays,23–25 energetic ions26�27

and electrons.28–30 These intrinsic, energetic and abundantmultiple radiations from the plasma focus device set it

2 Nanosci. Nanotechnol. Lett. 4, 1–24, 2012

Rawat High Energy Density Pulsed Plasmas in Plasma Focus

apart from other devices as a prime candidate for variousapplications,31 such as(i) a neutron source for pulsed activation analysis,32

(ii) a spectroscopic source for production of highly ion-ized species,33

(iii) a pump source for lasers,34

(iv) an X-ray source for lithography, radiography andimaging35–39

(v) an electron beam source microlithography40 and(vi) an energetic ion source for short lived radioisotopeproduction.41

The most important and extensive use of DPF device,in the recent time however, is the increasing applicationsof instability generated energetic pulses of ions and elec-trons from this device for the modification/processing ofbulk and thin film materials and the deposition/synthesisof various thin films, which are discussed in detailin section 2.4 later. This paper reviews the applica-tion of plasma focus device, as one of the novel highenergy density plasma processing tools offering extremelynon-equilibrium conditions, in emerging field of plasmananoscience and nanotechnology for the synthesis ofnanostructured materials such as nanoparticles, nanoclus-ters or nanoparticle agglomerates, specifically of magneticmaterials.

2. DENSE PLASMA FOCUS: DEVICEDETAILS, PRINCIPLE OF OPERATIONAND APPLICATIONS

The plasma focus is a simple device with complex physicalphenomena and multiple dynamic processes. This sectionprovides the details of:(i) types of DPF configurations,(ii) brief description of the physical layout, operationalprinciple, and key characteristics of DPF device,(iii) characteristics of hot dense pinch plasma and ener-getic charged particle of interest in DPF device and(iv) applications of this device for bulk/thin film process-ing and/or synthesis.

These details are necessary to explain not only the speci-fication and operational details the device but also its keyplasma characteristics for a non-specialist to understandand appreciate the wide spectrum of unique features orplasma environment that this relatively new novel plasmaprocessing tool can provide for the fast emerging field ofplasma nanoscience and nanotechnology.42

2.1. Types of Dense Plasma Focus Devices

Traditionally dense plasma focus devices are categorisedas either ‘Mather’ or ‘Filippov’ types, called so after theirinventors.8�9 They can be differentiated into one of thesetwo categories according to their anode aspect ratio A,with A = z0/2a in which z0 is the effective anode length

and a is the anode radius. The ‘Mather’ configuration isdefined by A > 1 (typically 5–10) whereas the ‘Filippov’configuration has A < 1 (typically ∼0.2). Both configura-tions however exhibit similar(1) dynamics of the current sheath,(2) scaling laws for neutron emission and(3) characteristic emission of energetic ion and electronbeams, X-rays, microwaves etc.

Even though both types of DPF devices behave similarly;the ‘Mather’ type is preferred due to its simpler design,convenient access to various diagnostics, distinguishablephases of current sheath dynamics and high neutron yieldfor same driver energy. Two DPF devices used and dis-cussed in this review paper are essentially of ‘Mather’ typewith A > 1.

2.2. Physical Layout, Operational Principle and KeyCharacteristics of DPF Device

The schematic of the high performance 3.2 kJ (28.8 �f,450 kA @ 15 kV) repetitive NX2 DPF at Plasma Radia-tion Source Lab (PRSL) of Nanyang Technological Uni-versity, Singapore, depicting the physical layout and thekey subcomponents, is shown in Figure 1. It is composedof two coaxial cylindrical electrodes (central anode sur-rounded by multiple cathode rods), closed and electricallyinsulated at one end and open at the other end, whichare placed inside a vacuum chamber. The insulator sleeve,used at the closed end of the electrode assembly, providesa voltage standoff between the electrodes at the breachand also the site for the initiation of the gas breakdownthat later results in plasma sheath formation. The electrodeassembly is contained in a vacuum chamber filled withthe desired gas (e.g., hydrogen, deuterium, neon, argon,nitrogen etc.), at a pressure typically ranging from 0.5 to10 mbar depending on the gas used. The central electrode(i.e., anode) of this device is connected to the high voltageterminal of an energy storage capacitor or capacitor bankthrough a low inductance fast high current switch and theouter electrode (i.e., cathode) is grounded. The NX2 isa 4-module system with 4 capacitor banks (each having12 capacitors of 0.6 �F) with each capacitor bank con-nected to the DPF, as a load, through 4 pseudo spark gap(PSG) switches which are synchronously activated througha master trigger system as shown in Figure 1. The capaci-tor bank is charged using high coulomb transfer rate highvoltage power supply. The use of large number of capaci-tors (48 altogether) in 4 parallel modules, 4 PSG switchesin parallel and parallel transmission line assembly (ratherthan cables) reduces the system inductance, and hence thesystem impedance, by a great amount resulting in muchhigher discharge current through this system. The otherplasma focus system used in the present review paper fornanophase magnetic material synthesis is 3.0 kJ UNU-ICTP (United Nations University – International Centre

Nanosci. Nanotechnol. Lett. 4, 1–24, 2012 3


Fig. 1. The schematic of NX2 dense plasma focus device with key physical components: (i) the coaxial electrode assembly (consisting of anode andcathode rods, refer inset) with insulator sleeve at closed end, (ii) vacuum chamber housing the electrode assembly and the adjustable substrate holder,(iii) pseudo spark gap switches (one for each capacitor bank module) and the triggering module to transfer the electrical energy from capacitor banksto the electrode assembly, (iv) the 4-module capacitor bank and (v) high voltage power supply to charge the capacitor bank.

for Theoretical Physics) plasma focus facility.10 The keyphysical characteristics of these two plasma focus devices(NX2 and UNU-ICTP) used in this review and a FastMiniature Plasma Focus (FMPF-1) which has also beenrecently used for nanophase material synthesis are sum-marized in Table I for their cross comparison.The operational principle and plasma dynamics in DPF

device is as follows: when the electrical energy storedin the capacitor bank is rapidly transferred to the elec-trodes by means of a fast switch; an electrical over-volt

Table I. Comparative electrical characteristics of three DPF devices thathave been used in Plasma Radiation Source Lab at NIE/NTU, Singaporefor processing and synthesis of materials.

Main NX2 UNU-ICTP FMPF-1characteristics Device22�24 Device10 Device15

Energy bank capacitance (Co) 28.8 �F 30.0 �F 2.4 �FMaximum charging voltage (Vo) 15 kV 15 kV 14 kVMaximum stored energy (Eo) 3.2 kJ 3.3 kJ 235 JMaximum current 450 kA 170 kA 87 kA(under short circuit) (Isc) @15 kV @14 kV @14 kV

Operating voltage range 8–14 kV 12–14 kV 12–14 kVEquation circuit inductance (Leq) 26±2 nH 110±5 nH 27±2 nHEquation circuit resistance (Req) 12±1 m� 60±3 m� 66±3 m�

Quarter time of discharge (T /4) ∼1.35 �s ∼3.0 �s ∼400 nsMaximum discharge repetition rate(DRR)

10.0 Hz Once per minute 0.5 Hz

filamentary discharge is initiated across the insulator sleeveat the closed end of the electrode assembly which rapidlydevelops into a sheath of plasma, which evolves axiallyalong the electrode assembly under the effect of �J × �Bforce. Due to this Lorentz force action, the conductingplasma sheath accelerates towards the open end of elec-trode assembly i.e., from position 1 through position 2shown in Figure 2. In the final phase (i.e., from position 3to position 4), the sheath collapses on axis with a zipperingaction forming the pinch plasma column. This zipperingaction compresses the pinched plasma to a high densityand temperature. The pinch has duration of a few to sev-eral tens of nanoseconds, and coincides temporally witha sudden, sharp drop in the total current signal, causedby a decrease in plasma conductivity due to strong con-finement. The focus then disrupts due to the growth ofthe m = 0 mode instabilities in the pinch, or from radia-tive collapse. The m = 0 mode instabilities enhances theinduced electric field locally, which, coupled with the mag-netic field, breaks the focused plasma column by accel-erating the ions of the filling gas species, to very highenergies,43�44 towards the top of the chamber and electronsto relativistic energies (100 keV and above)29�45 towardsthe positively charged anode.



Fig. 2. Current sheath dynamics under various phases of plasma focusoperation.

2.3. Characteristics of Pinch Plasma and EnergeticCharged Particles in DPF device

Irrespective of configuration, the typical parameters ofpinch plasma, current sheath dynamics and energeticcharged particle characteristics for a wide energy range ofplasma focus devices are:(i) current sheath speed in axial phase:46�47 0�2× 105–1�0×105 ms−1, the enormously high speed in axial phaseitself shock-heat the plasmas to very high temperatureswith electron and ion temperatures reaching about 100 and300 eV, respectively,48

(ii) current sheath speed in radial compression phase:46

typically 2 to 2.5 times that of the axial speed,(iii) pinch plasma electron/ion densities:13�49 5× 1024–1026 m−3,(iv) pinch plasma electron temperatures:50�51 200 eV–2 keV,(v) ion temperatures of pinch plasmas:13 300 eV–1.5 keV,(vi) energies of instability accelerated electrons28�45 – tensof keV to few hundreds of keV and(vii) energies of instability accelerated ions43�44–tens ofkeV to few MeV.

It may also be noted that the DPF is a pulsed plasmadevice with all phenomena of interest being transient innature. The durations of pinch plasma and energetic par-ticles in DPF device are of the order of tens of ns toabout hundred or several hundred ns.27�30�52�53 The extremefeatures of the high energy density plasmas (that includevery high densities and temperatures of pinch plasmasand very high energies and flux of instability acceleratedcharged particles combined with their transient nature) inDPF devices offers a kind of plasma that is very differ-ent other plasmas conventionally used (typically low tem-perature plasmas) in plasma-nanoscience for plasma-aidednanofabrications.42

2.4. DPF Devices for Material Processing andSynthesis: Mechanism and Applications

The first ever application of DPF device for(i) processing of bulk material was reported by Feugeaset al.;54 a wear reduction by a factor of about 20 inAISI 304 stainless steel was observed after nitrogen ionimplantation,(ii) processing of thin film was reported by Rawat et al.;55

crystallization in amorphous lead zirconate titanate thinfilm was achieved by single shot energetic argon ion expo-sure, and(iii) deposition of thin film, of carbon, was reported byKant et al.56

It may thus be noticed that the materials related work fromDPF device can be classified into two broad categories:(i) processing of bulk or thin film target materials, placeddown stream the anode axis, by the complex mix of insta-bility accelerated energetic ions from the pinch plasmaand the energetic decaying plasma and ionization waveand shock front propagating in the forward direction if noaperture assembly is used (however if the aperture assem-bly is used then the processing is mostly done by energeticion beams only) and(ii) deposition of thin films of metals or alloys (which canbe fixed to the anode top) and their nitrides, carbides andoxides using inert gases, hydrogen, nitrogen, methane oroxygen as operating gases.

2.4.1. Processing of Bulk and Thin Films UsingDPF Device

The formation of a hot, dense pinch plasma column, at theend of radial collapse phase, is followed by the onset ofinstabilities14�57 which enhance the induced electric fieldlocally leading to the acceleration of ions of filling gasspecies with very high energies (few keV to about fewMeV) in the forward direction towards the top of theplasma focus chamber. The instability accelerated ener-getic ions are one of main components that process thebulk or thin film samples that are placed down the anodestream.The pulse duration for the nitrogen ion beam emitted

from a 2.3 kJ DPF device (32 �F, 12 kV, 190 kA) wasestimated to be ∼140 ns by Hassan et al.,58 who usedenergetic nitrogen ions for processing the Ti substrate totransform its top layer into TiN. Jiaji et al.59 used a Fara-day cup to investigate the ions emitted from the hydrogenoperated plasma focus device and deduced ion energies ofH+ ions to be in the range of about 35 keV to 1.5 MeV.Following the method reported by Sanchez and Feugeas,60

they reported that the total number of ions passing througha 0.5 mm diameter pinhole placed 10 cm above the anode,with in 35 keV–1.5 MeV energy range, is about 1�29×1011

with a mean energy of 124 keV per ion. The ion flux at5 cm was estimated to be 2�63×1014 ions cm−2.



The pulse duration of energetic ion beam depends onpinch phase duration of plasma focus device which inturn depends on the characteristic time (∼√

L0C0; whereL0 and C0 are system inductance and capacitance respec-tively) of the device. Most plasma focus devices operatein typical voltage range of 10–20 kV and hence the storedenergy of DPF devices is largely dependent on the capaci-tance of the system. The high energy plasma focus devicesuse bigger capacitor bank (1 MJ PF1000 uses 1320 �Fbank)13 while the lower energy plasma focus devices usesmaller capacitor banks (3 kJ UNU-ICTP10 and 0.2 kJFMPF-115 have 30 and 2.4 �F banks respectively). In lowand mid energy plasma focus the energetic ion beam dura-tion is of the order of several tens to about hundreds ofns while in bigger plasma focus device it might be sev-eral hundred ns. The transient nature of energetic ions aswell as that of hot decaying plasma in DPF device causesthe transient processing of the exposed/irradiated sample.The ion energy range is not affected, significantly, by thechange in stored energy of the device and found to remainin several tens of keV to MeV range. The ion energy flux,on the sample being exposed, however changes with thestorage energy of the DPF device even if the anode topto sample distance is kept same as the number of ions inpinch plasma column changes due to the change in thesize (both radius and length) of the pinched plasma col-umn with the change in the storage energy of the system.The increase/decrease in capacitance (for DPFs of differ-ent storage energy) increases/decreases the quarter timeperiod of the discharge current; and then the anode dimen-sion has to be suitably increased/decreased to achieve themaximum compression efficiency in the pinch phase bysynchronizing the occurrence of pinch phase (refer Fig. 2)with current maximum at about the quarter time periodof the discharge pulse. The change in anode dimensionschanges the size of the pinched plasma column, and hencethe number of ions in the pinched plasma column, asit is well know through X-ray pin hole imaging resultsthat the radius and length of the pinched plasma col-umn are approximately one tenth and equal to radius ofthe anode, respectively.61 This basically implies the inten-sity/degree of processing of the ion irradiated/exposedsamples depends on the storage energy of the DPF dueto changes in the ion pulse duration and ion energy flux.Sanchez and Feugeas,60 considering(i) the geometry and physical characteristics of the ionbeams of a typical mid energy range DPF device,(ii) the single-ion-solid interaction process and(iii) the thermal properties of the materials; estimated thegeneration of transient heating slopes and heating speedsas high as ∼3600 K �m−1 and ∼40 K ns−1 respectively.

They showed that the maximum temperatures reached thematerial evaporation point at the surface layers and thecooling down process turned out to be fast enough to pro-duce the complete thermal relaxation of the target in only afew microseconds after the end of the ion beam incidence.

It may be important to highlight over here that the sam-ple exposed in DPF devices are bombarded not only bytransient pulse of instability accelerated ions but also by(i) strong shock wave that moves in front of the part of thecurrent sheath that moves axially while radial compressionis going on,(ii) fast moving ionization wave front that is observedshadowgraphically (refer Fig. 6(c) of a paper by Leeet al.62 showing “the bubble” formation which is an indi-cation of fast moving ionization wavefront), and(iii) hot and still relatively dense decaying plasma afterthe pinched plasma column is broken by the m = 0instability.

The DPF device thus offers a very complex mix of highenergy density plasmas, intense shock wave and instabil-ity accelerated energetic ion beam that transiently processthe material under irradiation in a very complex way andhence the exact material processing mechanism will be dif-ficult to predict. However, broadly speaking, these intensetransient phenomena with high energy density can causestrong thermal effect on the exposed material surface dueto extremely high temperature rise rate followed by rapidquenching which process the exposed material to bring outthe changes in their several physical properties and com-positional characteristics.A significant amount of work has been done to inves-

tigate the effects of the pulsed ion irradiation in DPFdevice on various bulk and thin film materials by vari-ous research groups across the globe. The effects of argonions irradiation on the amorphous lead zirconate titanate,Sb2Te3 and oriented CdI2 thin films have been reportedby Rawat et al.55�64�64 The amorphization of the crystallineCdS thin film and bulk Si (c-Si) using the DPF ion irra-diation were reported by Sagar and Srivastava65 and Sadiqet al.,66 respectively. The DPF device has also been usedfor processing of various bulk and thin films such as: fordiode formation on polyanilene thin film,67 to induce phasechange from non-magnetic �-Fe2O3 to magnetic Fe3O4

phase,68 to enhance the Tc of superconducting BPSCCOthick films,69 to irradiate American diamond (high purityzirconia),70 to implant/irradiate energetic nitrogen ionsfrom DPF device on various materials such as zirconium,aluminum, stainless steel, graphite, ZnO, Ti, nickle ferriteetc.71–79

2.4.2. Deposition of Thin Films Using DPF Device

The DPF device, in addition to its application in processingof bulk and thin films, has also been used extensively forthe deposition of thin films. During the pinch phase of aplasma focus shot (operation), two key events from thethin film deposition point of view occur:(i) the instability generated high energy ions of fill-ing/operating gas species accelerate towards the top of thechamber and bombard the substrate surface and



(ii) at the same time instability accelerated relativisticelectrons (which move towards the DPF anode) and thehigh energy density pinch plasma interact with the anodetip material.

A hollow anode (central electrode) is, therefore, conven-tionally used in DPF device to minimize the ablation ofmetal impurities into the pinch plasma to minimize thecooling of the pinched plasma by radiation losses to max-imize the thermonuclear neutron yield from deuteriumoperated plasma focus device. For the deposition of thinfilms using DPF device, this efficient ablation of anodetop material is rather used as an advantage wherein a solidanode (or hollow anode fitted with solid anode tip) ofthe desired material is used to maximize the ablation forhigher deposition rates.80 The plasma of the ablated anodematerial, which may or may not interact with the plasmaof the filling gas species depending on whether the fillinggas species is reactive or inert, moves towards the sub-strate and is deposited on to it in the form of a thin film.A suitable (inert or reactive) operating gas and anode tipmaterial combination is used for the deposition of varioustypes of thin films.Typically, multiple plasma focus shots are required for

the deposition of thin film of a suitable thickness. Theinstability accelerated energetic ions during any givenplasma focus shot, transiently process the thin film (in away that has been explained in the previous section) thathas already been deposited before that particular shot. Inother words, the plasma focus device provides an ener-getic ion assisted deposition that leads to high packingdensity in the deposited films and simultaneous process-ing (by transient heating) of thin films during deposi-tion leading to direct synthesis of crystalline thin filmswithout any need for post deposition annealing. Due tothese novel features of dense plasma focus device, manygroups have used it for synthesis of various thin films.Kant et al. reported the application of it for carbon56 andfullerene81 thin film deposition. Rawat et al. have suc-cessfully deposited thin films of titanium carbide,82 tita-nium nitride,83 titanium dioxide84 and Fe85 thin films.Many researchers have deposited diamond like carbon thinfilms using plasma focus device.81�86–89 Other thin filmsthat have been successfully deposited using plasma focusdevice are that of FeCo,90 tungsten nitride,91 aluminiumnitride,92 silicon carbide,93 hafnium oxide94 etc.The focus of this review paper is the synthesis of

nanophase magnetic materials using DPF devices whichhas been achieved by both the above mentioned mecha-nisms i.e., by the direct deposition of nanophase magneticmaterial in the plasma focus facility and by processingthe PLD grown thin films of magnetic materials. A briefreview of nanophase magnetic materials is given in thenext section.

3. NANOPHASE MAGNETIC MATERIALS

The nanophase magnetic materials can broadly be dividedinto three different types:(i) nano-particle type: used as recording media, magneticfluid, magnetic medicine and wave absorption materials,(ii) nano-microcrystal type: such as magnetically hard andsoft microcrystal materials, and(iii) nano-structured type: which include magnetic thinfilm, multilayer films and tunnel junctions.

Nanophase magnetic materials have different propertiesfrom the normal magnetic materials because the physi-cal lengths relating to magnetic characteristics (such assingle magnetic domain, spin diffusion length, domainwall thickness, exchange coupling length etc.) are about1–100 nm. Interests in the study of these fascinatingproperties have been driven by various promising appli-cations of nanophase magnetic materials; ranging fromultra-high density magnetic data storage, biosensors, gassensor, toner material for high quality colour copier andprinter, new generation electric motor and generator, envi-ronment friendly refrigerants, biomedicine etc.95 In addi-tion, due to their comparable size to biological entities(such as 10–100 �m cells and 20–250 nm viruses), possi-bilities of surface bio-functionalization and motion controlthrough external magnetic field, nanophase magnetic mate-rials will find applications in bio-medicine and biotech-nology such as therapeutic drugs, gene and radionuclidedelivery, radiofrequency-induced destruction of cells andtumors via hyperthermia, and contrast-enhancement agentsfor magnetic resonance imaging etc.96–101

3.1. Nanophase Magnetic Materials in Data Storage

In recent decades, the recording areal density in data stor-age industry has been increasing at a rate of about a factorof 10 every 10 years. The introduction (by IBM) of diskread heads based on the magnetoresistive effect102 and thegiant magnetoresistive effect103 allowed the areal data den-sity growth for magnetic disk drives to accelerate to about60% compound growth rate during 1990s and towardsthe end of the 20th century to an even greater rate of100% (doubling every year). Starting in 2002, as areal den-sity increase toward 100 Gbit/in2 with longitudinal mag-netic recording104 accompanying with the appearance ofthe thermal stability limitation, maintaining this incrediblerate was no longer possible. Today, the technology is mov-ing towards 1 Tbit/in2 areal storage density105 which is the-oretically available at perpendicular magnetic recording106

at a more sustainable pace of about 30% and 40% per year.One of the main reasons for such rapid advances in arealdensity is the significant achievement in the understand-ing and control of nanoscaled magnetic films. In orderto continue to achieve this exponential growth in arealdensity, some major obstacles in both recording mediaand recording head must be overcome. The most severe



areal density limitations result from the magnetic relax-ation (superparamagnetic limit), the separation of head tomedium, the saturation of write head, the sensitivity ofread head and servo tracking bandwidthThe data areal density limitation on the recording media

resulted from super-paramagnetic limit which is wellestablished when the particle size reaches the critical vol-ume. There is a certain amount of energy required to “flip”a magnetic domain on a disk surface from one orienta-tion to the other (representing the flip of data bit from “0”to “1”). When this amount of energy becomes compara-ble to the thermal energy (=kT; where k is the Boltzmannconstant and T the temperature) that a domain has at roomtemperature, the domains will spontaneously flip orienta-tion at random. This so called “superparamagnetic limit”puts a ceiling on the areal density that could be achieved.The transition noise of the media is, and will be, the

dominant noise source in the recording systems. This noiseoriginates mainly from the roughness of the recorded bitboundaries. This zigzag shape is due to the magneticexchange coupling and magnetostatic interactions betweencrystalline grains. To reduce this noise the following twonecessary conditions must be satisfied:(i) the grain size of the media must be reduced accord-ingly with the decreasing recorded bit size, and(ii) the grains must be magnetically isolated.

The later condition requires that the magnetic and crys-tal grain sizes be similar. However, the thermal stabil-ity of the recorded information depends on the magneticgrain size and the uniaxial anisotropy. A rapid decreaseof the magnetic grain size required an increase in mag-netic anisotropy in order to maintain the thermal stabilityof the recorded information. Also, the non-uniform grainsize will drastically reduce the thermal stability of therecorded bit due to the thermal decay. Therefore, in orderto push the superparamagnetic limit, uniform grain size isextremely important.

3.2. Magnetic Recording Media

Magnetic recording is the central technology of the infor-mation storage, which uses the different patterns of mag-netization on a magnetic recording media (magneticallycoated surface) to store information.107 Bits can be writtenwith the magnetization orientation parallel or perpen-dicular to media plane which are referred as longitudi-nal magnetic recording (LMR) or perpendicular magneticrecording (PMR) respectively. One of the most commonlyfollowed practices, these days, to overcome the super-paramagnetic limit, to increase thermal stability, is to usefilms of materials, such as CoSm and CoCr, CoPt,108–110

FePt109–112 and CoCrPtM (M= Ta, Nb, B, C, etc.) alloys,which have higher values of magnetic anisotropy Ku forthe magnetic recording media. The FePt and CoPt filmswith atomic percentage of 50:50 are believed to be the

most prominent candidates for ultra-high density magneticdata storage, which requires:(i) small particle size (critical value is around 3 or 4 nm)with tight size distribution(ii) fct phase with high magnetocrystalline anisotropyand preferred crystal orientation to overcome the super-paramagnetic limit, and(iii) reduced exchange coupling effects with well-separated distribution of nanoparticles.

Therefore, reduction of ordering temperature, control ofthe fct-(001) orientation of thin films and decrease of grainsize are three key challenges for practical application ofL10 phase FePt and CoPt thin films nowadays.

4. NANOPHASE HARD MAGNETICMATERIALS USING DPF DEVICE

In this section the synthesis of nanophase hard magneticmaterials of FePt/CoPt using DPF devices with probablepotential applications in ultra-high density magnetic datastorage will be reviewed. As mentioned before the thermaleffects in any system, due to preferably room temperatureoperation requirement of devices for economic feasibil-ity, are unavoidable. There exists a thermal limit, knownas super-paramagnetic limit, where the magnetic energystored in the grains becomes comparable to their thermalenergy, making them impossible to store magnetic states.The relaxation of the magnetization orientation of eachdata storage bit is determined by = 0e

KuV /2kT , in which is the relaxation time at one orientation, Ku is the stor-age media’s anisotropy constant, V is the volume of thestorage bit, k is the Boltzmann’s constant, and T is tem-perature. As the size of data storage bit decreases therecomes a point where the magnetic energy KuV becomescomparable to the thermal energy kT and then the mag-netization of the data bit starts to fluctuate from one ori-entation to another (magnetization reversal) due to shortrelaxation time making it impossible to store the magneticstate. In general, the ratio of the stored magnetic energyKuV to that of the thermal energy kT needs to exceed 40for the data stability greater than 10 years. Therefore, tohave smaller size of storage bit for higher storage density,the storage media should be made of materials with higheranisotropy constant.The FePt and CoPt binary alloy nanoparticles, contain-

ing a near-equal atomic percentage of Fe/Co and Pt, arean important class of magnetic nanomaterials as they arechemical stabile and exhibit the possibility of existing ina phase with high magnetic anisotropy constant Ku. Asshown in Figure 3, the FePt is known to have two solidphases; CoPt also has exact similar solid structures with Fesimply being replaced by Co. These two phases basicallyare in two types of crystal structures:113

(i) chemically disordered face-centered-cubic (fcc) struc-tured A1 phase and



(a) (b)

Fig. 3. Schematic diagram of the unit cell of (a) chemically disorderedfcc and (b) chemically ordered fct FePt. For CoPt the Fe is simplyreplaced by Co.

(ii) chemically ordered face-centered-tetragonal (fct)structured L10 phase.

The FePt and CoPt exhibit a fcc structure with a ran-dom distribution of Fe/Co and Pt atoms in corner posi-tions and face centred positions, refer to Figure 3(a). Inthis A1 phase, FePt and CoPt have lower anisotropy con-stant Ku with smaller coercivity value and exhibit magnet-ically soft character. The fully ordered fct phase of FePtor CoPt can be viewed as alternating atomic layers ofFe/Co and Pt stacked along the (001) direction (c-axis), asshown in Figure 3(b), which has high anisotropy constantKu with a high coercivity value and thus exhibits mag-netically hard character. The FePt and CoPt in ordered fctL10 phase are found to have high uniaxial magnetocrys-talline anisotropy constants of �6�6−10�× 107 erg/cm3

and 4�9×107 erg/cm3, respectively.114 Due to their excep-tionally high Ku value these materials can exhibit ferro-magnetic character up to the nanoparticle size of about3–4 nm.It may, however, be noted that the as-deposited, when

deposited at room temperature, FePt and CoPt show adisordered fcc structure which is magnetically soft (lowcoercivity) and need to undergo a phase transition fromthe disordered fcc structured A1 phase to the orderedfct structured L10 phase after post-deposition annealingat an elevated substrate temperature of about 600 to700 �C to exhibit hard magnetic character required fordata storage.115 However, the high annealing temperatureneeded for the phase transition, which is reported to begreater than 500 �C,116�117 may cause nanoparticle agglom-erations or grain growth and hence the particles may nolonger stay isolated which lead to the deterioration ofmagnetic properties and also the increase in particles sizeis undesirable for ultra-high data storage density. Hence,achieving low temperature phase transition in FePt/CoPtthin films from magnetically soft low Ku fcc-structuredA1 phase to magnetically hard high Ku fct-structured L10phase is one of the key challenges in ultra-high densitymagnetic data storage.

4.1. Lower Phase Transition Temperature in PLDGrown FePt Thins Films Using DPF Device asEnergetic Ion Irradiation Facility

Several methods have been adopted to lower the order-ing temperature of FePt. The addition of a third element(Cu, Ag, and Au) into these alloys was reported to beeffective for reducing the ordering temperature.118�119 Linet al.120 reported the lowering of the phase transition tem-perature to 300 �C in FePt:Al2O3 nanocomposite thin filmssynthesized by magnetic trapping assisted pulsed laserdeposition. Another approach for lowering down the phasetransition temperature is using ion irradiation.121�122 Ionirradiation has been considered as an effective techniqueto lower down phase transition temperature and simul-taneously tune the structural and magnetic properties offilms. The mechanism involved in continuous ion beamirradiation has been discussed by Devolder et al.123 He+

irradiation was reported to control the degree of chemicalordering in FePt/FePd films.124�125 High energy (350 keV)He+ irradiation was used by Wiedwald et al. to lower theordering temperature of FePt film by more than 100 �C.126

It was also reported that the ordered L10 FePt phase wasdirectly achieved by using continuous 2 MeV He+ irradi-ation for about 1 hour.127 However, all of these ion irradi-ation results are obtained by continuous ion sources with,typically, hours of irradiation duration. In the followingsub-section, the lowering of fcc to fct phase transitiontemperature of the PLD grown FePT thin films using thepulsed energetic ion beam from DPF device is reviewed.

4.1.1. FePt Thin Films by PLD

The FePt with atomic percentage of 50:50 is found tohave highest anisotropy constant. The inherent advantageof stoichiometry preservation in pulsed laser deposition(PLD)128 made it an appropriate choice for us for thesynthesis of FePt thin films and FePt based nanocom-posite thin films, as compared to other deposition meth-ods. In PLD, the atomic mobility of ablated species isdependent on their kinetic energy and thermal energy.129

These ablated species are generally generated with alarge amount of internal energy and the main paths oftheir stabilization are radiative, evaporative and collisionalcooling.130 The first two mechanisms are evidently dom-inant in plume expansion under vacuum and the last oneis present during the diffusive expansion of the plumeinto the ambient gas.131 Hence, in vacuum or very lowambient gas pressure, the kinetic or thermal energy ofablated species is high since there is no collision or veryless collisions with ambient gas, thus their surface mobil-ity is high132 leading to smooth and uniform thin filmformation.133 It was demonstrated through our work134

that the morphological features of deposited coatings canin the form of smooth thin films, nanoparticle agglom-erates and/or floccules-like nanoparticle networks which



are essentially governed by the kinetic energy of ablatedspecies and, in turn, their surface mobility on the substratesurface, which can be tailored through the deposition con-ditions, such as ambient gas pressure, laser energy andtarget-substrate distance and geometry.The FePt in smooth thin form and in the forms of

nanoparticle, nanoparticle agglomerates and floccules-likenanoparticle networks were successfully synthesized byboth conventional PLD and backward plume deposition(BPD).59�134�135 FePt nanoparticles were also synthesizedby thermal annealing to induce nanostructuring of pulsedlaser deposited smooth thin films. The thermal anneal-ing induced FePt nanoparticles are well-separated anduniformly-distributed but their coercivity value is low dueto the short annealing duration.134 FePt:Al2O3 nanocom-posite thin films, which embed FePt nanoparticles insidenon-magnetic matrix material, were successfully synthe-sized by conventional PLD using two lasers and specially-shaped target to reduce their exchange coupling effectsand to minimize typical annealing effects of grain growthand agglomeration.136 However, a large number of laserdroplets of aluminium oxide, were observed on depositednanocomposite thin films. To reduce the number of laserdroplets, special target-substrate geometry and strong mag-netic field were introduced during the PLD depositionof FePt:Al2O3 nanocomposite thin films using PLD. Thismagnetic trapping assisted PLD not only significantlyreduced the number of laser droplets but also lowereddown the fcc to fct phase transition temperature from600 �C to significantly lower value of 300 �C.120

4.1.2. Processing of PLD grown FePt Thin Films inPlasma Focus Device

The FePt thin films with two different thicknesses of about67 and 100 nm were grown on Si(001) substrates at roomtemperature by pulsed laser deposition (PLD)59�137 in vac-uum (better than 3×10−5 mbar). The Continuum Nd:YAGlaser (532 nm, 10 Hz, 10 ns and 80 mJ) was focusedon FePt (50:50 at%; Kurt Lesker, 99.99%) target discwith energy density of about 1× 103 J/cm2, and sam-ples were deposited at the target-substrate separation of3 cm. The FePt thin films then were irradiated by highlyenergetic H+ ions using UNU/ICPT (United Nation Uni-versity/International Center for Theoretical Physics) DPFdevice10 whose details are given in Table I. The sam-ples were placed axially along the anode axis at variousdistances from the anode top and were also exposed todifferent numbers of plasma focus irradiation shots. Theworking gas used is hydrogen, which was kept at a fillinggas pressure of 5 mbar. Full details of the experimentalsetup and irradiation procedures can be found in our papersby Lin et al.59 and Pan et al.137�138

After exposure, the FePt thin films were heated invacuum furnace to various temperatures, at the rate of

60 �C/min, and maintained for 1 hour before beingcooled down naturally. The exposed samples were ana-lyzed before and after annealing for their morphological,structural and magnetic properties using various character-ization tools listed in our papers.137�138

4.1.3. H+ Ion Irradiation on 67 nm FePt Thin FilmUsing DPF Device

The PLD grown FePt samples were irradiated by hydro-gen ions from UNU-ICTP DPF device at a distance ofthe 5 cm from the anode top.59 The morphology of as-deposited sample and samples after ion irradiation isshown in Figure 4. The as-deposited FePt thin films, referFigure 4(a), were smooth with laser droplets dispersing onthe surface. The morphology of FePt thin films changedfrom that of the smooth uniform film to film with uni-form and isolated nanoparticles, refer Figure 4(b), after asingle DPF shot exposure. The average particle size werefound to be 9�1±2�3 nm. As the number of ion irradiationDPF shots were increased to two, the morphology of irra-diated samples changed to nanoparticle agglomerates ofbigger size of about 51�3±7�4 nm, as seen in Figure 4(c).The change in morphology with the increase in number ofexposure shots is because of the increased energy beingimparted by energetic ions from next shot to the nanopar-ticles created in the first exposure which causes them tomigrate and form bigger agglomerates to reduce their sur-face energy.The X-ray diffraction (XRD) spectra of as deposited,

single shot irradiated and annealed samples are shown inFigure 5. The as-deposited thin films exhibited fcc phasewith a broad peak of (111) at about 41� which after singleshot DPF exposure almost disappeared which might bebecause of:(i) continuous accumulation of defects which destabilizethe crystal structure at some critical levels, and(ii) rapid quenching of irradiation induced liquid thermalspike regions.139

The thermal spike is referred to a damaged and amor-phous structure which is formed due to the local melting ofthe implantation region and consequently a rapid quench-ing of the liquid phase. The amorphization of the crys-talline material by energetic ion exposure in DPF devicehas also been reported before.65�66 The XRD spectrum ofsingle shot irradiated sample and later being annealed at300 �C showed (111) and (002) diffraction peaks of fccphase with slight splitting indicating that some of the FePtnanoparticles might have been converted to ordered fctphase. By increasing the annealing temperature to 400 �C,more peaks appeared on the XRD pattern. The appear-ance of (001) peak at around 24� and the splitting of thefundamental (111) and (002) peaks indicated the phasetransition to the long-range L10 ordered fct phase. Theenhancement in (001) and (002) peaks indicates that most



Fig. 4. SEI micrograph of (a) as-deposited sample and samples after (b)1 shot and (c) 2 shots ion irradiation by plasma focus device. Reprintedwith permission from [59], J. J. Lin et al., J. Phys. D: Appl. Phys. 41,135213 (2008). © 2008, IOP Publishing Ltd.

of the FePt nanoparticles have their (001) planes parallelto substrate surface, which would give rise to high mag-netocrystalline anisotropy and high coercivity in in-planehysteresis behavior. However, for the samples without ionirradiation, they remained in fcc phase after annealing at400 �C, which would not convert to fct phase unless theannealing temperature was raised up to 600 �C. The lowphase transition temperature of ion irradiated samples can

Fig. 5. XRD patterns of as-deposited sample and samples after 1 shotion irradiation and annealing at different temperatures. Reprinted withpermission from [59], J. J. Lin et al., J. Phys. D: Appl. Phys. 41, 135213(2008). © 2008, IOP Publishing Ltd.

be explained by the impact of energetic ions and neutralspecies which provoke significant adatom mobility and adecrease of the activation energy for atomic ordering byincreasing point defects such as vacancies and interstitials.The in-plane hysteresis loops, depicted in Figure 6,

show that(i) as-deposited FePt thin film were weakly ferromagneticwith coercivity of about 75 Oe,(ii) sample irradiated with one shot ion irradiation, referFigure 6(b), was almost similar with marginal increase incoercivity to about 96 Oe,(iii) by annealing the ion irradiated sample at 300 �C, thecoercivity increased to about 447 Oe which may be due tothe partial transition of FePt nanoparticles to L10-orderedfct phase as indicated by the slight splitting of (111) and(002) fundamental peaks, and(iv) a drastic increase in coercivity to about 1563 Oe (witha 16 fold increase), refer Figure 6(d), was observed for thesample irradiated by one DPF shot and then annealed at400 �C.

The many fold increase in the coercivity of the last sam-ple was explained by the phase transition from low Ku

fcc-FePt to high Ku fct-FePt nanoparticles. The rema-nence ratio S (=Mr/Ms; where Mr is the remanencemagnetization and Ms is the saturation magnetization) isequal to 0.5 for the materials with randomly orientednanoparticles undergoing coherent rotations without inter-action. The remanence ratio decreases below 0.5 whenthe nanoparticles exhibit magnetostatic interaction, while itincreases when the nanoparticles exhibit exchange-coupledinteraction (which should be minimized to increase thesignal-to-noise ratio and to reduce the media noise).The remanence ratio for the single shot irradiated sam-ple annealed at 400 �C was estimated to be about 0.4,which is smaller than 0.5 and hence the main intergran-ular interaction was desirable magnetostatic rather than



Fig. 6. Hysteresis loops of (a) as-deposited sample and samples after (b) 1 shot ion irradiation and annealing at (c) 300 �C and (d) 400 �C. Reprintedwith permission from [59], J. J. Lin et al., J. Phys. D: Appl. Phys. 41, 135213 (2008). © 2008, IOP Publishing Ltd.

undesirable exchange coupling. More detailed results andtheir discussions can be found in our paper.59

4.1.4. H+ Ion Irradiation on 100 nm FePt Thin FilmUsing DPF Device

The PLD grown slightly thicker samples of FePt thin films,with thickness of ∼100 nm, were irradiated by hydrogenions from UNU-ICTP DPF device(i) at a distance of the 4 cm from the anode top usingdifferent number of focus shots137 and(ii) at different distances of 5, 6 and 7 cm using singlefocus shots.138

The changes in surface morphology of ∼100 nm FePt thinfilm samples after exposure to different number of focusshots was similar the one reported for ∼67 nm FePt thinfilm samples as they all changed from smooth thin films tonanoparticles. Single shot ion-irradiation led to the forma-tion of very small and uniform nanoparticles with averageparticle size of about 8±2�5 nm, along with some biggersized (∼30–40 nm) nanoparticles agglomerates. The twoshot irradiation lead to slight increase in the size of par-ticle agglomerates but most of the particles forming thebackground were still very uniform and small; howeverwith the three shot irradiation uniformity in the particle

size distribution was destroyed as the particles with thesizes ranging from about 20 to 100 nm were observed.137

The XRD results, shown in Figure 7 for 100 nm thickFePt thin film samples, shows that the intensity of diffrac-tion peaks for sample exposed to single DPF irradiationshot, without annealing,(i) increased as compared to that of as-deposited sampleand(ii) is almost similar to that of un-irradiated sampleannealed at 400 �C implying thereby that single shot ofpulsed plasma focus ion irradiation provides almost equalamount of energy that is offered by conventional thermalannealing for 1 hour at 400 �C.137

Similarly, the samples exposed at the distances of 5, 6and 7 cm showed enhanced crystallinity, refer Figure 8,after single DPF shot exposure.138 This was in contrast toresults obtained for ∼67 nm film exposed to one DPF shotat 5 cm where the irradiated sample became almost amor-phous indicating thereby the change in the thickness of thefilm can significantly affect the degree of transient thermalprocessing caused by the energetic ions.59 The increasein number of plasma focus irradiation shots to two andthree, however, provided more than enough energy andmay result in more defects and lattice distortion whichunfortunately can destroy the crystallinity of FePt samples,



Fig. 7. XRD patterns of as-deposited and samples (of ∼100 nm FePtthin films) irradiated using different numbers of DPF shots at 4 cm fromanode top, before and after annealing at 400 �C. Reprinted with permis-sion from [137], Z. Y. Pan et al., Appl. Phys. A 96, 1027 (2009). © 2009,Springer.

Fig. 8. XRD patterns of as-deposited and samples (of ∼100 nm FePtthin films) irradiated at different distances from anode top, before andafter annealing at 400 �C. Reprinted with permission from [138], Z. Y.Pan et al., Thin Solid Films 517, 2753 (2009). © 2009, Elsevier.

leading to their disordering as noticed by the decrease indiffraction peak intensity in Figure 7. The annealing ofall ion irradiated sample, whether irradiated by differentnumber of DPF shots at 4 cm or irradiated at differentdistances, at 400 �C lead to the phase transition from dis-ordered fcc structured A1 phase to chemically ordered fctstructured L10 phase indicated by the appearance of thesuperlattice tetragonal (001), (110) and (002) peaks (referFigs. 7 and 8).The TEM bright-field images, refer Figure 9, for sin-

gle shot ion irradiated sample at 4 cm from anode topafter annealing confirmed the formation of well separatednarrow size distributed FePt nanoparticles with the aver-age particle size of 11�6±3�4 nm. The selected area elec-tron diffraction (SAED) pattern, shown in the inset of

Fig. 9. TEM images, at different magnifications, of FePt nanoparti-cles induced by single shot ion irradiation at 4 cm from anode top andannealed at 400 �C. Inset in (a) is the corresponding SAED pattern.



Figure 9 and analyzed by JEMS@JEOL, confirmed theFePt nanoparticles to be in polycrystalline fct phase. Thephase transition at lower temperature, for single focus shotirradiation, had restricted nanoparticles grain size growthand agglomeration resulting in relatively small (about11�6± 3�4 nm) and well separated magnetically hard fctphase FePt nanoparticles which is important for highermagnetic data storage density.The as-deposited FePt thin film showed weak ferro-

magnetism, with coercivity of about 78 Oe, which afterannealing at 400 �C increases slightly as the coercivityincreased to 177 Oe; as shown in Figure 10(a). The sam-ples irradiated by different numbers of focus shots beforeannealing, refer Figures 10(b)–(e), exhibited soft mag-netic property with coercivity similar to that of the as-deposited sample. After annealing at 400 �C, as shownin Figures 10(b) and (f), the samples irradiated to single

Fig. 10. In-plane hysteresis loops of 100 nm FePt thin films samples (a) as-deposited and annealed at 400 �C, (b–d) before and after 400 �C annealingthe samples irradiated to 1, 2 and 3 focus shots at the distance of 4 cm from anode top sample and finally the samples irradiated at 5, 6 and 7 cm(e) before annealing, and (f) after annealing at 400 �C. (a)–(d): Reprinted with permission from Ref. [137], Z. Y. Pan et al., Appl. Phys. A 96, 1027(2009). © 2009, Springer; (e and f): Reprinted with permission from Ref. [138], Z. Y. Pan et al., Thin Solid Films 517, 2753 (2009). © 2009, Elsevier.

DPF shot at different distances from anode top showed anenormous increase in coercivity to values >5000 Oe. Thesharp increase in coercivity was due to(i) the transformation from magnetically soft disorderedfcc to magnetically hard ordered L10 fct phase, asobserved in XRD results as well, and(ii) the formation of separated nanoparticles on irradiatedsample surface resulting in lower inter-particle exchangecoupling.

The increase in the number of ion irradiation shots, referFigures 10(c) and (d), resulted in the sharp decrease ofcoercivity due to lower ordering degree of (001) diffrac-tion peak, which according to Lim et al.140 coincides withmagnetic easy axis of the ordered L10 fct phase. The order-ing degree of (001) diffraction peak was estimated usingtexture coefficient141 of this peak for all irradiated andannealed samples and was found to be maximum for single



focus shot ion irradiated and annealed sample which grad-ually decreased by 4.4% and 24.7% for two and three shotsion irradiated and annealed samples.

4.1.5. Plausible Mechanisms of Nanostructurizationand Lowering of L10 Phase TransitionTemperature in FePt Thin Films UsingDPF Facility

In above experiments which are taken from our previ-ous reported work,59�137�138 the instability accelerated tran-sient energetic ions, with typical pulse duration of about>100 ns and energies in the range of 35 keV to 1.5 MeV,are one of main components that processed the FePt thinfilm samples placed down the anode stream as most ofthe decaying hot plasma, ionization wavefront and shockwaves were effectively stopped by an aperture assemblyplaced in front of the anode to avoid deposition of copperdebris ablated from the anode rim by electron beam. Theinteraction of energetic ion beams with matter has beenextensively studied and it has been reported that as theenergetic ions penetrate into the sample material, they havean almost constant energy loss in the beginning and a veryhigh energy loss at the end where they stop, the so calledBragg peak. The mean energy of the H+ ions in our sys-tem has been estimated to be 124 keV59 with the projectedrange of about 521 nm in FePt and can create about 0.4vacancies per ion, as estimated from SRIM®.142 As thethickness of FePt thin films was only about 67 or 100 nm,most of the H+ ions stop and deposit bulk of their energyin silicon substrate heating it to very high temperature ina very short span of time. The thermal energy is thenconducted to the FePt thin films and causes the diffusionof metal atoms either through the lattice or along grainboundaries. The diffusion releases the thermal expansionmismatch stresses between the silicon oxide layer of thesubstrate surface and the PLD coated FePt thin film, lead-ing to the formation of nanoparticles at the surface layer ofFePt thin films. Another possible reason for the formationof FePt nanoparticles is that the weak interaction betweenFePt and the oxide layer of silicon substrates, resulting ina low activation energy barrier to FePt migration whichcan result in nanoparticle formation or weakly bound indi-vidual atoms upon ion irradiation. According to Wiedwaldet al.,126 one of the ways to lower down the annealing tem-perature at which the phase transition to L10 ordered fctphase occurs is to reduce the activation energy, ED, fordiffusion say by increasing the number of point defects,such as vacancy and interstitial, in the crystal structures.The phase transition can be related to the characteristicdiffusion length during post annealing, which is givenby = √

DtA with D = D0 exp�−ED/kBTA�, wherein, Dis the diffusion coefficient, kB is Boltzmann’s constant, tAis the annealing time and TA is the annealing temperature.Since the energetic ion irradiation of FePt thin film sam-ples is expected to create higher defect concentration in

these samples and hence the activation energy for diffusionwill be lower in irradiated samples leading to phase transi-tion from low Ku fcc to high Ku fct phase at lower anneal-ing temperature of 400 �C.

4.2. Hard Magnetic Nanophase CoPt Thins FilmSynthesis Using DPF Device

In this section, the deposition of nanostructured magneticCoPt thin films at room temperature on Si substrates usingNX2 DPF device operated at sub-kJ storage energy ofabout 880 J, which is reported by us in detail in Ref. [143]is reviewed.

4.2.1. Experimental Conditions of CoPt Thin FilmsSynthesis Using NX2 DPF Facility

The conventional central hollow copper anode of NX2device, shown in Figure 1, was replaced by a high purity(50:50 at%; Kurt Lesker, 99.99%) solid CoPt tip fitted cop-per anode. The hydrogen was used as the filling gas asaccording to Zhang et al.28 the NX2 device produces highenergy electrons with higher yields for this gas resultingin most efficient ablation of the anode target material. Twosets of experiments were done:143

(i) the gas pressure was changed from 2 to 8 mbar keepingthe other parameters fixed, and(ii) the number of plasma focus deposition shots waschanged from 25 to 200 shots for fixed filling gas pressureof 6 mbar hydrogen, the optimum gas pressure investi-gated in first set, to study the effects of the various numberof focus deposition shots.

The distance between the substrate holder and the anodetop was fixed at a relatively bigger distance of 25 cm andthe NX2 DPF was operated at the lower charging voltageof 8 kV, with storage energy of 880 J, for both sets ofdepositions.

4.2.2. Nanophase CoPt Thin Films

The morphological features of the nanostructured CoPtthin films were found to depend strongly on the filling gaspressure and the number of DPF deposition shots used.The SEM images in Figures 11(a)–(d), show the mor-phology of CoPt thin films samples deposited at differentfilling gas pressures using 25 DPF deposition shots. TheSEM image of the sample synthesized at the filling gaspressure of 2 mbar, Figure 11(a), showed a two layeredstructure with the top layer sintering together to form bigisland-like structures and a sub layer of some agglomer-ates between the nanoislands. For 4 mbar deposition, referFigure 11(b), only the particle agglomerates with the aver-age agglomeration size of 35±6�0 nm were observed andthe island like structures disappeared. With the increase infilling gas pressure to 6 and 8 mbar average size of these



(a) (b)

(c) (d)

Fig. 11. SEM images of nanostructured CoPt thin films grown using 25 focus shots at hydrogen filling gas pressures of: (a–d) 2, 4, 6, and 8 mbar,respectively. Reprinted with permission from Ref. [143], Z. Y. Pan et al., J. Phys. D: Appl. Phys. 42, 175001 (2009). © 2009, IOP Publishing Ltd.

nanoparticles/particle agglomerate was found to decrease,refer Figures 11(c) and (d), to 15± 3�0 and 10± 2�0 nmrespectively. The cross-sectional SEM images that thick-nesses of the samples deposited at 2, 4, 6, and 8 mbar areestimated to be about 94�0± 4�0, 58�0± 3�0, 44�0± 2�0and 22�0± 1�0 nm, respectively. The film thickness andhence the deposition rate is found to decrease with theincreasing filling gas pressure. The reduction of particleagglomerate size and the decrease in thin film depositionrate (film thickness) with the increasing filling gas pressurewas because:(i) the focusing efficiency decreases with the increase infilling gas pressure which results in lesser number ofablated ions from the anode target with lower kinetic ener-gies, and(ii) the collision frequency between the ablation ions andthe ambient gas will increase with increasing operatingpressure which causes the ablated ions to lose more energyduring its movement from the anode to the substrate.

The SEM images of CoPt thin films samples deposited at6 mbar using different numbers of DPF deposition shots(25, 50, 100, 150 and 200 shots respectively) showedthat with the increase in the number of deposition shots,the morphology changes from well separated small sizednanoparticles (for 25 shot deposition with particle size ofabout 15 nm) to nanoparticle agglomerates of increasingsize e.g., the average size of agglomerate is estimated to

be 32�8±7�0 nm for the sample deposited with 50 shots.A typical cross-sectional SEM image of 200 shot depo-sition sample is shown in Figure 12(a). The thicknessesof as-deposited samples, estimated from cross-sectionalSEM images, are found to be about 44�0±2�0, 72�5±4�0,208�1±8�0, 294�5±14�0 and 331�7±12�0 nm for 25, 50,100, 150 and 200 shots deposition samples respectively.The trend of thickness with number of deposition shotsis shown in Figure 12(b). The slope of linear fitted curverevealed that the average deposition rate of CoPt nanopar-ticle thin film was about 1.78 nm/shot for NX2 plasmafocus device operating at 880 J at 6 mbar hydrogen ambi-ence at the substrate distance of 25 cm from the anodetop. This deposition rate is more than 30 times higher ascompared to that of conventional PLD which is found tobe about 0.50 Å/shot.144 The cross-sectional image of the200 shot deposition sample, refer Figure 11(b), shows rel-atively uniform thickness and ultra-dense crack free film.The different morphological features and thickness

observed on the samples deposited by different numbersof plasma focus shots can be explained on the basisof following understanding of the processes of deposi-tion in plasma focus devices. The ablated species ions(ions of Co and Pt) accelerate upward along the anodeaxis and reach the Si substrate surface with some kineticenergy. On the substrate surface, the impinging ablatedions/molecules undergo a diffusion process, due to the



Fig. 12. (a) Typical SEM cross-section image of CoPt deposited using200 focus shots at 6 mbar hydrogen at anode top-substrate distance of25 cm and (b) the deposition layer thickness as a function of the numberof plasma focus deposition shots. Adapted and Reprinted with permissionfrom Ref. [143], Z. Y. Pan et al., J. Phys. D: Appl. Phys. 42, 175001(2009). © 2009, IOP Publishing Ltd.

surface mobility they have owing to their kinetic energy,to cluster together to form nanoparticles. The formation ofclusters/nanoparticles is also possible during the gas phasedynamics. The ablated material moves from anode top tothe substrate through few mbar of ambient gas and thus itis slowed down due to its interaction with the ambient gastaking more time to reach the substrate surface (comparedto what it would have taken if it expands in vacuum). Theablated material thus interacts more among itself leading topossible cluster/nanoparticle formation during the expan-sion in gas phase. Most plasma focus based depositionsare multiple shot depositions. When the second deposi-tion shot is fired, a complex mix of high energy ions ofthe filling gas species, sufficiently hot and dense decay-ing plasma from the post pinch phase, fast moving ion-ization wavefront and a strong shockwave will reach thesubstrate surface (as in this case no aperture assembly isused) and transfer their energy to the nanoparticles/clusters(deposited during the first shot) and also to the substrate

surface. This provides a transient annealing of the sam-ple. This transient heating of the sample promotes the dif-fusion of clusters/nanoparticles on the surface and theymay coalesce together leading to the formation of biggersize nanoparticles. The ions/clusters/nanoparticles of theablated anode material reach afterward which owing totheir own kinetic energies will also promote coalescence.As more and more deposition shots are fired the clus-ters/nanoparticle size will grow and even the nanoparticlewill start to bind together to form particle-agglomerates.The average thickness of the deposition will increase withthe increasing number of shots due to the fact that moreand more material is transferred to the sample surface.The structural analysis of CoPt thin films deposited

using 25 DPF shots at different hydrogen ambient gaspressures from 2–8 mbar showed that all of as depositedsamples were in fcc phase with a broad and weak peak of(111) plane at about 41�, refer Figure 13. The crystallinityin general improved with one hour annealing at 500 �C,but the sample remained in fcc phase. The transition todesirable L10 fct phase was seen to occur only after theannealing temperature was raised to 650 �C. The average

Fig. 13. XRD patterns of CoPt nanostructures synthesized at differentgas pressures by 25 plasma focus shots annealed at different tempera-tures. From (a) to (d): 2, 4, 6 and 8 mbar. Reprinted with permissionfrom Ref. [143], Z. Y. Pan et al., J. Phys. D: Appl. Phys. 42, 175001(2009). © 2009, IOP Publishing Ltd.



Fig. 14. Hysteresis loops for CoPt nanostructures synthesized at different gas pressures and annealed 650 �C. From (a) to (d): 2, 4, 6 and 8 mbar.Reprinted with permission from Ref. [143], Z. Y. Pan et al., J. Phys. D: Appl. Phys. 42, 175001 (2009). © 2009, IOP Publishing Ltd.

crystallite size of samples deposited at 2, 4, 6 and 8 mbar,after being annealed at 650 �C, were estimated to be about15.6, 12.0, 12.3 and 11.6 nm using the most preferred(111) CoPt peak respectively. The average feature (nanois-lands/agglomerate) sizes for 2 and 4 mbar depositionsfrom SEM images were much bigger than the averagecrystallite size estimated by Scherer’s formula indicatingthat these features are actually the agglomerates of smallersized nanoparticles. The ordering degree of annealed sam-ples, calculated from the ratio of peak intensities of I�001�and I�002� i.e., I�001�/I�002� in XRD patterns, were found tobe about 0.90, 0.71, 0.65 and 0.60 for the 650 �C annealedsamples deposited at 2, 4, 6 and 8 mbar respectively. Theinvestigation of magnetic properties of as-deposited sam-ples without annealing showed very weak coercivity, referFigure 14, which were in confirmation with the fcc phaseobserved for these samples in structural analysis. The coer-civity of the samples after annealing at 650 �C were sig-nificantly enhanced to about 1700, 3014, 618 and 696 Oefor samples deposited at 2, 4, 6 and 8 mbar respectively.The enhanced coercivity after annealing was due to thephase transition from low magnetic anisotropy disorderedfcc phase to ordered fct phase which exhibits a high mag-netic anisotropy. It was also noticed that the coercivityafter annealing was decreased with the increasing gas pres-sure, which is due to the reducing ordering degree trendmentioned earlier.The XRD analysis of CoPt thin films deposited at

6 mbar of hydrogen as ambient gas using different numberof focus shots (25, 50, 100, 150 and 200 focus deposi-tion shots) showed that all of as deposited samples werein fcc phase with a broad peak of (111) plane at about 41�,

refer Figure 15. The intensities of fundamental (111) and(200) peaks increased continuously, for all samples, as theannealing temperature was raised from 400 to 700 �C indi-cating that the crystallinity of the samples was increased.When the annealing temperature was increased to 600 �C,weak fct superlattice peaks of (001), (110), and (201)appeared clearly as a result of the phase transition to fctphase. The intensity of superlattice peaks increased sig-nificantly when the annealing temperature is increased to700 �C, and a clear splitting of fundamental (200) peakinto fundamental (200) and superlattice (002) peaks wasalso observed and all fct peaks became narrow and intenseindicating that the samples were converted into a highlyordered L10 phase.The average crystallite sizes, estimated from diffraction

results and shown in Table II, for all as-deposited sam-ples were found to be in a very narrow range of 6�1±0�5 nm and did not increase with the increase in num-ber of shots. This was because all these depositions weredone at a much bigger distance of 25 cm and simple esti-mates predict that the temperature rise at this distance willbe insignificant to increase the crystallite size. The aver-age crystallite size increases for the annealed samples at afaster rate for the sample deposited using bigger numberof plasma focus shots as shown in Table II. It may also benoted that the average crystallite size at the given anneal-ing temperature, say 600 �C, increases with the increas-ing number of plasma focus deposition shots. This maybe due to the fact that even though no significant tran-sient annealing was done due to the bigger distance ofdeposition (which reduces the ion energy density on thesample surface), the defects were created by energetic ion



(e) (f)

Fig. 15. XRD patterns of CoPt nanostructures synthesized by (a) 25 (b) 50 (c) 100 (d) 150 and (e) 200 plasma focus shots annealed at differenttemperature (400, 500, 600 and 700 �C), and (f) the average crystallite size variation as a function of the number of plasma focus deposition shots.Reprinted with permission from Ref. [143], Z. Y. Pan et al., J. Phys. D: Appl. Phys. 42, 175001 (2009). © 2009, IOP Publishing Ltd.



Table II. The average crystallite sizes of as-deposited CoPt sample andsamples after annealing at different temperatures and synthesized by dif-ferent numbers of plasma focus deposition shots.

Annealing Temp.

Room 500 �C 600 �C 700 �CNumber of shots (As-deposited) (nm) (nm) (nm) (nm)

25 6�1 8�8 11�7 14�750 5�9 9�1 14�2 15�2100 5�9 10�2 16�4 17�7150 5�9 10�5 18�2 18�9200 6�6 10�8 19�9 21�4

bombardment and more the number of shots more will bethe defects on the deposited sample. The increased defectconcentration will lead to greater reduction in the acti-vation energy for diffusion and hence the annealing pro-motes more crystallinity in the samples with bigger defectconcentration.The hysteresis loops of as-deposited nanostructured

CoPt thin film samples synthesized using 25, 50, 100, 150and 200 shots, refer Figure 16(a) and/or Table III, showweak ferromagnetic signal as they were in low Ku softmagnetic fcc phase. The total saturation moments of the

Fig. 16. Hysteresis loops of CoPt nanostructures synthesized by different numbers of plasma focus shots (a) before annealing, and after annealing at(b) 500, (c) 600, and (d) 700 �C. Reprinted with permission from Ref. [143], Z. Y. Pan et al., J. Phys. D: Appl. Phys. 42, 175001 (2009). © 2009, IOPPublishing Ltd.

Table III. The coercivity of as-deposited CoPt sample and samplesafter annealing at different temperatures and synthesized using differentnumbers of plasma focus deposition shots.

Annealing Temp.

Room 500 �C 600 �C 700 �CNumber of shots (As-deposited) (Oe) (Oe) (Oe) (Oe)

25 20 260 726 60750 73 450 3060 1338100 50 402 4962 6850150 66 592 5664 7529200 64 742 5477 8973

as-deposited samples increased with the increasing numberof deposition shots due to the increase in the thicknessof the deposited sample. The coercivity of the samplesannealed at 500 �C annealed samples though remained lowbut it increased with the increase in number of DPF shotsbecause the crystallinity of fcc phase was enhanced due toannealing even though it was not transformed to fct phase.

After being annealed at 600 �C, a very significantincrease in the coercivity was observed for the most ofthe samples, except for sample deposited with 25 shots,which can be attributed to the start of phase transition from



magnetically soft fcc phase to magnetically hard fct phaseat this temperature which was also noticed in the XRDresults shown in Figure 15. A further increase in annealingtemperature to 700 �C resulted in extremely high coerciv-ity, in excess of 6800 Oe for samples deposited using 100and 150 shots and reaching to about 9000 Oe for 200 shotdeposition sample, due to achievement of highly orderedfct structure L10 phase in these samples.

5. CONCLUSIONS

The paper reviewed the successful application of denseplasma focus device, a novel pulsed high energy den-sity plasma source, in plasma nanoscience as an ener-getic ion irradiation facility for processing of magneticthin films grown by PLD as well as a high depositionrate source for magnetic thin film nanostructured magneticmaterial synthesis. The DPF device offers a complex mix-ture of high energy ions of the filling gas species withenergies ranging from few tens of keV to several MeV,immensely hot and dense decaying plasma from the postcompression phase with plasma temperature of few keVand plasma density of the order of 1023 m−3, fast movingionization wavefront with speed in excess 20 cm/�s and astrong shockwave that provides a unique plasma and phys-ical/chemical environment that is completely unheard ofin any other conventional plasma based deposition or pro-cessing facility. The results of two key experimental inves-tigations on the modification of various physical propertiesof PLD grown thin films of FePt and the deposition ofnanostructured CoPt thin films were reviewed.The DPF device, due to use of an aperture assembly,

was essentially used as an intense pulsed ion source toinvestigate the effects of ion irradiation on the PLD grownFePt thin films of two different thickness of about 67 and100 nm. By adjusting the operation parameters of ion irra-diation using UNU/ICTP DPF device, the specific findingswere:(1) Nanostructuring of PLD synthesized FePt thin films,particularly in nanoparticles form, can be successfullyachieved by energetic plasma focus H+ ion irradiation.(2) The annealing temperature for phase transition of FePtthin films from low Ku fcc phase to high Ku fct phasewas successfully reduced to 400 �C by plasma focus ionirradiation from conventionally required 600 �C for suchphase transition. It was postulated that the ion irradiationlower down the activation energy for diffusion by inducingthe vacancy or interstitial point defects and hence lowerdown the annealing temperature for phase transition.(3) The hard magnetic properties were significantlyenhanced by single shot plasma focus ion irradiation atlower post deposition annealing temperature of 400 �Cresulting in lesser grain growth or agglomeration ofnanopartilces with reduced exchange coupling effect; ahighly desirable trait for application of these nanoparticlesin ultra high density data storage.

The advantage of energetic ion irradiation using the DPFdevice is that it can reduce the phase transition temperatureto hard magnetic phase in a single shot ion exposure withion pulse duration of the order of about hundred to fewhundred nanoseconds. The use of transient pulsed ener-getic ions from DPF device, as opposed to that of longexposure time irradiation from continuous ion sources,provides a fast and effective way to lower the phase tran-sition temperature with significantly enhanced hard mag-netic properties with lower particle size.The NX2 plasma focus device, operating in sub-kJ

range, was successfully used to synthesize CoPt nanopar-ticles thin film of as small particle size as possible withnarrow size distribution using:(i) different hydrogen filling gas pressures keeping thesubstrate-anode top distance and the number of plasmafocus deposition shots fixed at 25 cm and 25 shots respec-tively and(ii) using different numbers of plasma focus depositionshots at fixed substrate-anode distance of 25 cm at fillinghydrogen gas pressure of 6 mbar. The specific findingswere:

(1) The morphological features, such nanostructure for-mation and their shapes and sizes, showed a strong depen-dence on the filling gas pressure and the number of plasmafocus deposition shots. A recipe to synthesize well sepa-rated and narrow-size distributed CoPt nanoparticles wasobtained by tuning the filling gas pressure and the numberof plasma focus deposition shots.(2) The deposition rate at a deposition distance of 25 cmand at an optimized gas pressure of 6 mbar was estimatedto be about 1.78 nm/shot which is more than 30 timeshigher as compared to that of conventional PLD whichis reported to be about 0.50 Å/shot. This proves that theDPF device provides very high deposition rate facility.This deposition rate can be further enhanced by a factorof 4 by reducing the distance to half of its current value.The uniform deposition can be obtained using a rotatingtarget holder. Hence, in principle, the energetic electronbeam and hot dense plasma based physical evaporation ofthe anode target material in DPF device can provide depo-sition rates two orders of magnitude higher compared tothat of PLD. The DPF deposition has an added advantagesimultaneously processing of the deposited material by acomplex plasma/ion beam/shockwave mixture providingan environment unavailable in any other device.(3) The phase transition, from low Ku fcc structured A1phase to high Ku fct structured L10 phase, took placewhen the annealing temperature was about 600 �C result-ing in significantly enhanced hard magnetic properties.For example, very high coercivities of 6850, 7529 and8973 Oe are achieved on the samples deposited using 100,150 and 200 focus deposition shots after 700 �C annealing.These samples also had higher ordering degree indicatingthat most of fcc phase was transformed to fct phase.



To conclude, the dense plasma focus is indeed a novelhigh energy density pulsed plasma device with immensepotential for synthesis of nanophase hard magnetic mate-rials which should further be explored to other potentialhard magnetic materials and possibly for direct room tem-perature synthesis of high Ku fct-structured L10 phase hardmagnetic phase.

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Received: 13 September 2011. Accepted: 13 December 2011.


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