Direct observation of magnetization switching in focused-ion-beam-fabricatedmagnetic nanotubesSakhrat Khizroev, Mark H. Kryder, Dmitri Litvinov, and David A. Thompson Citation: Applied Physics Letters 81, 2256 (2002); doi: 10.1063/1.1508164 View online: http://dx.doi.org/10.1063/1.1508164 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/81/12?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Lock in of magnetic stripe domains to pinning lattices produced by focused ion-beam patterning J. Appl. Phys. 98, 054306 (2005); 10.1063/1.2030412 A study of magnetic interactions of Ni 80 Fe 20 arrays using ultrasensitive microcantilever torque magnetometry J. Appl. Phys. 95, 7010 (2004); 10.1063/1.1682912 Magnetic force microscopy and x-ray scattering study of 70×550 nm 2 pseudo-spin-valve nanomagnets J. Appl. Phys. 93, 7927 (2003); 10.1063/1.1558076 Fabrication of spin-current-induced domain-wall-nucleation device in planar configuration J. Vac. Sci. Technol. B 20, 2814 (2002); 10.1116/1.1518014 Lorentz transmission electron microscopy of focused ion beam patterned magnetic antidot arrays Appl. Phys. Lett. 77, 3063 (2000); 10.1063/1.1323737

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Direct observation of magnetization switching in focused-ion-beam-fabricated magnetic nanotubes

Sakhrat Khizroev,a) Mark H. Kryder, and Dmitri LitvinovSeagate Research, Pittsburgh, Pennsylvania 15222

David A. ThompsonIBM Almaden Research Center, San Jose, California 95120

~Received 26 June 2002; accepted for publication 30 July 2002!

In this letter, a direct measurement of ‘‘easy’’ magnetization switching indicatingzero-magnetization remanence in a magnetic probe with a cross section as narrow as 60360 nm2, and as tall as 750 nm, is presented. Magnetic force microscopy was utilized to testfocused-ion-beam-fabricated nanomagnetic probes. The data directly indicate that unlike a regularsolid probe, a probe with a tubelike ending~nanotube! provides substantially ‘‘easier’’switching. © 2002 American Institute of Physics.@DOI: 10.1063/1.1508164#

One of the most fundamental questions which needs tobe answered before stepping into the era of nanomagnetictechnologies is the ability of a magnetic probe with dimen-sions in the nanoscale range to controllably conduct mag-netic flux.1 The process of the magnetization switching inmagnetic devices with dimensions in the nanoscale range isof critical importance for a number of high technologies,including magnetic random access memory, magnetic nano-electromechanical systems, magnetic recording at densitiesbeyond 100 Gbit/in.2, and others.1–4 For these technologies,the core process is determined by the magnetic characteris-tics of a magnetic device~probe! in the nanoscale range.5,6

For example, at a 100 Gbit/in.2 density, one square bit has across section of 80380 nm2. In this case, considering thatthe magnetic domain wall width in a typical ‘‘soft’’ magneticmaterial is of the order of 500 nm, the Maxwell’s equations~macroscopical by their nature! cannot describe the processof the magnetization switching.7 Therefore, a reliable experi-ment is vital. In this letter, a direct measurement of ‘‘easy’’magnetization switching indicating zero-magnetization re-manence in a magnetic probe with a cross section as narrowas 60360 nm2, and as tall as 750 nm, is presented. Magneticforce microscopy~MFM! is utilized to test focused-ion-beam~FIB! fabricated nanomagnetic probes.

To study magnetics of such narrow probes, a recordinghead of the type utilized in perpendicular recording~thecutting-edge magnetic recording technology! isconsidered.8,9 At an areal density beyond 100 Gbit/in.2, sucha head has a pole tip in the shape of a probe with a crosssection, W3W, smaller than 80380 nm2. Two basic typesof magnetization distribution in such a narrow probe areshown in Figs. 1~a! and 1~b!, respectively.10 A nonzero-remanent magnetization state in the probe, as shown in Fig.1~a!, results in a nonzero-recording field even in the absenceof the electrical current in the flux drive coil.11 The remanentrecording field can be sufficiently large to cause uncontrol-lable erasure of previously recorded information.9 In this

case, the nonzero-remanent magnetization state causes a sys-tem failure. No remanent magnetic field is generated in thestate without magnetization remanence, as shown in Fig.1~b!, and therefore, the recording field is entirely controlledby the electric current in the flux drive coil. The narrower theprobe is, the more it is inclined to have nonzero magnetiza-tion remanence because in this case quantum exchange stiff-ness and shape anisotropy dominate over magnetic anisot-ropy. The probe assumes a single-domain state that issaturated in the long direction,L. The head design problemis: ‘‘What are the smallest probe dimensions which avoidmagnetization remanence and preserve easy magnetizationswitching?’’ In recognition of earlier theoretical conclusions,a rectangular probe with a magnetic void~tubelike structureof the probe tip! at the air bearing surface~ABS! in the formof a physical cavity, as shown in Fig. 1~c!, was explored.6

The intention was to reduce the usable probe cross-sectiondimensions as much as possible, while maintaining a largeprobe length. Further, in this letter, the authors refer to thisprobe structure as to the magnetic nanotube. It is expectedthat the magnetic nanotube favors the no-remanence state byeliminating the high-energy Bloch line at that location in thedomain configuration of Fig. 1~b!.12

Magnetic probes with a rectangular cross-section of 60360 nm2 and with a probe length~throat height! which wasvaried in the range of 100 to 1000 nm were fabricated usingFIB trimming of regular recording heads.13 One set of headswas chosen for fabricating magnetic nanotubes. Each mag-

a!Author to whom all correspondence should be addressed; electronic mail:sakhrat@prodigy.net

FIG. 1. Two basic magnetization configurations in a soft magnetic probe:~a!with and ~b! without remanence.~c! A diagram of a probe with a magneticsingularity ~void! at the ABS~magnetic nanotube!.

APPLIED PHYSICS LETTERS VOLUME 81, NUMBER 12 16 SEPTEMBER 2002

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netic nanotube had a 40 nm deep cavity with a 40 nm diam-eter created at the ABS of the probe tip via FIB. Both mag-netic nanotubes and regular probes were made of a Ni/Fe~45/55! alloy with saturation magnetization of 1.6 T and withanisotropy field of approximately 20 Oe. FIB images of aregular head trimmed into the nanotube shape are shown inFig. 2.

MFM was utilized to image the magnetic field generatedby a probe at its ABS.14 Consecutively taken MFM imagesof the field generated by a probe with a 60360 nm2 crosssection and a 500 nm throat height at three values of thedrive current,2500 ~initial saturated state!, 0 ~remanentstate!, and 1500 mA turn ~second saturated state!, of theregular and nanotube types, are shown in Fig. 3~a!. It shouldbe noted that the current of 500 mA turn was sufficient tosaturate the magnetization. The substantial reduction of thesignal at the remanent state for the nanotube clearly indicatesnegligible magnetization remanence of this probe. On thecontrary, for the regular probe, the signal does not drop tozero in the remanent state, thus directly indicating substantialnonzero-remanence for the investigated sizes.

Values of the field at the center of a probe with a 60

360 nm2 cross section versus the drive current at differentvalues of the probe length for the two probe types are shownin Fig. 3~b!. It can be noted that the crossovers between zero-and nonzero-remanent states occur at approximately 1000nm with the cavity, and only 300 nm without the cavity at theABS. The fact that the pole length crossover occurs at amuch larger length value for the case with the cavity at theABS indicates that the presence of the cavity indeed pro-motes the state with no magnetization remanence. Anotherobservation is the fact that a drive current of less than 500mA turn is sufficient to saturate this head. For comparison,the saturation current for a regular size recording head is ofthe same order of magnitude.13 This indicates the preserva-tion of sufficient magnetic ‘‘softness’’ even at such smalldimensions.

Another experiment performed was MFM imaging oftwo adjacent 65 nm wide tracks recorded into a CoCr-basedperpendicular recording medium with a soft underlayer usingthe aforementioned magnetic nanotube. Well-defined 65 nmwide tracks with clear transitions, as shown in Fig. 4, alsoindicate relatively ‘‘easy’’ magnetization switching in such asmall magnetic probe.

In conclusion, this letter presents a study on a directMFM observation of the magnetization switching in mag-netic nanotubes, with a cross section as small as 60360 nm2 and a length in the range of 100 to 1000 nm. Suchnanomagnetic probes were fabricated via FIB etching ofregular thin-film magnetic heads. The MFM measurementsindicate easy magnetization switching with negligibly smallmagnetization remanence in a magnetic probe with a crosssection as narrow as 60360 nm2, and as tall as 750 nm.

This work was supported by an IBM doctoral fellowshipfor one of the authors~S.K.! and an NSF grant under No.ECD-8907068.

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FIG. 2. FIB images of a typically FIB trimmed head.

FIG. 3. ~a! MFM images of the magnetic field generated at the ABS by aprobe with a 60360 nm2 cross section at three values of the drive current,2500, 0, and1500 mA turn, for the nanotube and regular probe types.~b!Digitized values of the MFM signal at the center of the ABS of a probe witha 60360 nm2 cross section for a set of three length values for the two probetypes.

FIG. 4. An MFM image of two adjacent 65 nm wide tracks recorded with amagnetic nanotube.

2257Appl. Phys. Lett., Vol. 81, No. 12, 16 September 2002 Khizroev et al.

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