Solid State Communications 152 (2012) 603–605

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Solid State Communications

journal homepage: www.elsevier.com/locate/ssc

Ferromagnetic resonance and magnetic force microscopy evidence for aboveroom temperature ferromagnetism in Mn doped Si made by a solid statesintering processRajen Patel a, Frank J. Owens b,∗

a Armament Research, Development and Engineering Ctr., Picatinny, NJ 07806, United Statesb Department of Physics, Hunter College, and Graduate Center City University of New York, 696 Park Ave., NY, 10065, United States

a r t i c l e i n f o

Article history:Received 19 July 2011Received in revised form18 November 2011Accepted 10 January 2012by T.T.M. PalstraAvailable online 17 January 2012

Keywords:A. Mn doped siliconB. Electric force microscopyB. Magnetic force microscopyD. Ferromagnetic resonance

a b s t r a c t

Silicon doped with manganese made by a solid state sintering process starting with nanoparticles of Siand MnO is shown to be ferromagnetic above room temperature. The evidence for ferromagnetism isobtained from ferromagnetic resonance and magnetic force microscopy measurements. The magneticforce microscopy measurements show that the ferromagnetic resonance occurs in nanosized regionsin the sample as has been suggested by Dietl. Raman and electric force microscopy measurements alsosuggest that then material is doped inhomogenously.

© 2012 Elsevier Ltd. All rights reserved.

1. Introduction

An important objective of research on semiconductors is todevelop magnetic semiconductors. Such semiconductors wouldenable switching and information storage to exist in one material,which could contribute to increasing computer speed. Theapproach has been to dope semiconductors such as ZnO, GaP andGaAsN with Cu2+ and Mn2+ paramagnetic ions. Ferromagnetismhas been observed in these alloys of the semiconductors havinga wide range of Curie temperatures [1–4]. For example Cu2+

doped GaP had an estimated Curie temperature above 700 K.Theoretical models have been developed to explain the origin offerromagnetism in these materials, referred to as dilute magneticsemiconductors (DMS). The models propose that the exchangeinteraction between the paramagnetic dopants is mediated bythe conduction electrons or holes [5]. It has also been suggestedthat the ferromagnetism in these DMS materials is occurring innanosized regions in the material [6,7].

Silicon is the workhorse of computer switches being thematerial used in complimentary metal oxide semiconductors(CMOS). Because of this it would be desirable to develop a siliconDMS material. In fact theoretical density functional calculations

∗ Corresponding author.E-mail addresses: fowens@hunter.cuny.edu, owensfj@gmail.com (F.J. Owens).

0038-1098/$ – see front matter© 2012 Elsevier Ltd. All rights reserved.doi:10.1016/j.ssc.2012.01.021

have predicted that Mn doped Si could be ferromagnetic [8]. Therealso have been some reports of ferromagnetism in Si doped withMn2+ fabricated by an ion implantation method and films madeby molecular beam epitaxy [9,10].

The objective of this paper is to show that a simple solidsintering process, starting with nanoparticles of silicon and MnO,can be used to dope Si with Mn2+ producing a ferromagneticsemiconductor of silicon. Such a simple synthesis has thepotential to lead to less expensive large scale productionprocesses in contrast to an ion implantation and molecularbeam epitaxy method. Evidence for ferromagnetism is obtainedfrom ferromagnetic resonance measurements and magnetic forcemicroscopy measurements. One of the motivations of usingmagnetic force microscope measurements is to investigate thepossibility that the magnetism is occurring in nanosized regionsin the sample as has been suggested [6,7].

2. Experimental

The paramagnetic and ferromagnetic resonance measurementswere made using a Resonance Instruments 8400 spectrometeroperating at 9.2 GHz with 100 kHz modulation employing a TE102microwave cavity. The temperature of the sample was controlledby flowing heated or cold nitrogen gas through a doubled-walledquartz tube, which is inserted through the center of themicrowavecavity. The temperature of the flowing gas is controlled by a

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150

100

50

0

Inte

nsi

ty (

cou

nts

/s)

400 450 500 550Wavenumber (cm-1)

Fig. 1. Raman spectra of Mn doped (smaller line) and undoped silicon.

Resonance InstrumentsMN82VT temperature controller.Magneticforce microscope images were obtained using a Veeco NanoScopeIV equippedwith amagnetic tip. Ramanmeasurementsweremadeusing a J.Y. Horiba confocal micro-Raman system employing a25 mWHe–Ne laser focused to a spot having a 10 µm radius and awavelength of 632.8 nm.

In a typical synthesis, 0.10 g of 50 nm silicon nanoparticleswere mixed with 0.016 g of 60 mesh MnO particles. The MnOparticles having 99.99% purity were obtained from the Aldrichchemical company and the silicon nanoparticles were obtainedfrom American Element Inc. Electron paramagnetic resonance(EPR) measurements on the Si nanoparticles prior to processingshowed no evidence of paramagnetic signals that could arise frommagnetic impurities such as Fe, Co or Ni indicating the absenceof magnetic impurities such as Fe Ni or Co in the Si material.Similarly the EPR of MnO showed only the Mn resonance andno evidence of the presence of other magnetic materials. EPRis one of the more sensitive spectroscopies available capable ofdetecting amounts smaller than one part in 1010. The Si and MnOwere thoroughly mixed and ground in a mortar and pestle for 2 hand then pressed into a pellet. The pellet was then placed on aceramic slab inside a quartz tube located in a tubular oven. Thematerial was heated to 740 °C in flowing nitrogen or helium for 4 h.After 4 h the sample was rapidly quenched to room temperature.X-ray diffraction (XRD) measurements of the Mn doped Si yieldedthe characteristic silicon lines at 2θ values of 28.338°, 47.183°,55.996° and 69.000°. These valueswere slightly shifted down fromthe lines in the undoped silicon starting material which occur at28.356, 47.216, 56.043, and 69.054 indicating a slight expansionof the lattice in the Mn doped material, which would be expectedbecause of the larger radii of Mn compared to Si. The line widthsin the doped sample were narrower than in the starting siliconnanoparticles.

3. Results

Fig. 1 shows theRaman spectra of the longitudinal optical latticemode in doped and undoped silicon (larger signal). The frequencyof the mode in the doped material has shifted down from 520 to485 cm−1 indicating an expansion of the lattice resulting from thelarger radius of Mn2+ compared to silicon consistent with the XRDresults. These results indicate that the Mn2+ is incorporated intothe lattice either substitutionally for Si or interstitially. No otherRaman lines were observed.

It is interesting to note that there is evidence of an unshiftedline in the Raman spectra of the doped material. This indicatesthat within the 10 µm spot of the focused laser there are someregions which are not doped indicating that the doping is nothomogeneous in the sample. The absence of Raman lines otherthan that due to the LO mode of silicon precludes the presence of

Fig. 2. Ferromagnetic resonance spectra of Si:Mn showing low field non resonantsignal (a) and resonant signal (b).

other phases in the sample and confirms the doped sample has thetetrahedral silicon structure.

Fig. 2(a), (b) show the ferromagnetic resonance (FMR) spectraof the sample at room temperature. The spectra consist of twocharacteristic components, a low field non resonant absorptioncentered around zero field shown in Fig. 2(a) and a resonantabsorption in the vicinity of 3200 G shown in Fig. 2(b). Thepresence of the two lines indicates the sample is ferromagnetic. Ina single crystal, the magnetic field position of the resonant FMRsignal depends on the orientation of the dc magnetic field withrespect to important symmetry direction in the crystal. The spectrashown here are from a collection of randomly oriented grainsand are powder patterns representing the sum of spectra from allorientations of the dc magnetic field. The presence of the low fieldnon resonant absorption signal is a well established indication offerromagnetism inmaterials [11,12]. The signal occurs because thepermeability in the ferromagnetic state depends on the appliedmagnetic field increasing at low fields to a maximum and thendecreasing. Since the surface resistance depends on the squareroot of the permeability, the microwave absorption depends non-linearly on the strength of the dc magnetic field resulting in a non-resonant derivative signal centered at zero field. This signal is notpresent in the paramagnetic state and emerges as the temperatureis lowered to below the Curie temperature. A distinguishingcharacteristic of a resonant FMR signal from an electron spinresonance signal is a strong temperature dependence of the linewidth and field position of the FMR signal [13]. Fig. 3 shows aplot ofthe temperature dependence of the linewidth of the resonant FMRsignal above room temperature. This data indicates that the Curietemperature is above 400 K, which is in agreement with estimatesfrom SQUID magnetization measurements on Si implanted withtheMn2+ [10]. The FMRdata discussed aboveprovides unequivocalevidence of the existence of ferromagnetism in the material.

The electric force microscope (EFM) can be used tomeasure thecharge distribution on the surface of a sample [14]. Fig. 4 showsthe image obtained by EFM on the surface of the Si;Mn pellet. The

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TEMPERATURE [K]

300 320 340 360 380 400

LIN

E W

IDT

H [

G]

215

220

225

230

235

240

245

250

255

Fig. 3. Temperature dependence of the linewidth of the resonant FMR signal aboveroom temperature.

Fig. 4. Electric force microscope image of the surface of the Si:Mn pellet.

peaks correspond to regionswhere there is charge likely due to thepresence of Mn2+. These results suggest that Mn is incorporatedinto the lattice as Mn2+ not as MnO. Also it shows that the dopingis not homogeneous as suggested by the Raman data. Fig. 5 showsthe magnetic force image (MFM) of the surface of the pellet. Thespikes are regions of magnetism and are aligned in the direction ofthe scan. The alignment may be due to domain alignment from themagnetic field of the tip.

Sample roughness is not affecting the MFM results as it shouldbe random and not yield aligned regions. A check for roughnesswas made by scanning in EFM mode with zero voltage betweenthe sample and the tip and the roughness deflections wereconsiderably less than the magnitude of magnetic deflections.The MFM image confirms the presence of ferromagnetism in thesample and shows the ferromagnetism is occurring in nanosizedregions in the sample as has been proposed [6,7]. The EFM and

Fig. 5. Magnetic force microscope image of the surface of the Si:Mn pellet.

MFM images were taken over different regions of the sample so nocorrelation between them should be expected.

4. Conclusion

FMR and MFM measurements on Si doped with Mn by asolid state sintering process show the existence of above roomtemperature ferromagnetism. The observed magnetism is not aresult of impurities in the startingmaterials, as EPRmeasurementson the starting, Si and MnO materials yield no signals fromimpurities. EPR measurements are sensitive to one part in1010. It has previously been shown that MnO particles are notferromagnetic at room temperature [15]. Dietl proposed that inDMS materials the ferromagnetic spin order is not uniformlydistributed in the sample but is often in nanosized separatedregions [6,7]. Our MFM images of the Mn doped Si confirm thissuggestion showing the magnetism in localized nanosized regionsin the sample. Raman and EFM measurements also indicate thedoping of the sample is not uniform and likely occurs in smallregions.

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