Effects of C content on the formation and magnetic properties of Nd 2 Fe 14 ( BC )/α- Fenanocomposite magnetsZ. C. Wang, H. A. Davies, and S. Z. Zhou Citation: Journal of Applied Physics 91, 3769 (2002); doi: 10.1063/1.1450037 View online: http://dx.doi.org/10.1063/1.1450037 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/91/6?ver=pdfcov Published by the AIP Publishing Articles you may be interested in High coercivity in nanocrystalline Nd 25 Fe 40 Co 20 Al 10 B 5 prepared by out-of-equilibrium techniques J. Appl. Phys. 105, 103905 (2009); 10.1063/1.3129642 Magnetic properties of ( Nd,Pr,Dy ) 2 Fe 14 B /α- Fe nanocomposite magnets crystallized in a magnetic field J. Appl. Phys. 93, 8128 (2003); 10.1063/1.1537703 Preparation and magnetic properties of melt-spun Nd 2 Fe 14 (BC) /α- Fe nanocomposite magnets J. Appl. Phys. 91, 7884 (2002); 10.1063/1.1451402 Effect of Co and Zr on magnetic properties of nanophase PrFeB alloys J. Appl. Phys. 87, 6116 (2000); 10.1063/1.372627 The effect of boron and rare earth contents on the magnetic properties of La and Cr substituted α- Fe/R 2 Fe 14B -type nanocomposites J. Appl. Phys. 83, 6271 (1998); 10.1063/1.367572

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Effects of C content on the formation and magnetic propertiesof Nd 2Fe14„BC…Õa-Fe nanocomposite magnets

Z. C. Wanga) and H. A. DaviesDepartment of Engineering Materials, University of Sheffield, Mappin Street, Sheffield S1 3JD,United Kingdom

S. Z. ZhouState Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing,Beijing 100083, People’s Republic of China

~Received 24 September 2001; accepted for publication 14 December 2001!

The phase evolution, microstructure, and magnetic properties of Nd8Fe86B62xCx ~x50, 2, 4, 5, 6!melt-spun ribbons were systematically studied as a function of C content. It was found that theaddition of C decreases the glass-forming tendency of the as-spun ribbons significantly. A uniformnanoscale exchange coupled Nd2Fe14(BC)/a-Fe microstructure with an average grain size of 20–25 nm can be developed in the directly quenched ribbons with C contents up to 4 at. %. Furtherincrease of C content tox55 leads to, in the optimally quenched ribbons, the presence of anundesirable Nd2Fe17Cx phase in addition to the 2:14:1 anda-Fe phases, whereas the alloy ribboncontaining 6 at. % C consists almost entirely of the soft magnetic Nd2Fe17Cx and a-Fe phases.Subsequent annealing induces a transformation of the 2:17:Cx phase to the 2:14:1 phase1a-Fe inthe ribbons withx55 and 6, resulting in the formation of a composite 2:14:1/a-Fe structure havingrelatively large crystallite sizes. Magnetic measurements revealed that, for the optimally processedsamples, replacement of up to 4 at. % of B by C significantly increases the coercivityiHc , with onlyslight reduction in remanenceJr ; an optimum coercivity of 542 kA/m was obtained in theNd8Fe86B2C4 ribbon compared with 430 kA/m for the Nd8Fe86B6 ribbon. Excessive substitution ofC (x.4) causes a drastic deterioration of bothiHc andJr due to the microstructural coarsening.Moreover, the Curie temperature of the 2:14:1 phase in the samples decreases progressively withincreasing C content from 312 °C forx50 to 270 °C forx56. © 2002 American Institute ofPhysics. @DOI: 10.1063/1.1450037#

I. INTRODUCTION

Because of their unusually high remanence ratioJr /Js

~whereJr is the remanence andJs the saturation polariza-tion!, high maximum energy product (BH)max, and low cost,nanocomposite permanent magnetic materials consisting ofhard and soft magnetic phases have attracted considerableattention in recent years.1–5 In order to achieve the excellentmagnetic properties, however, the optimal structures of thesematerials require a homogeneous and narrow distribution ofvery fine grains, especially soft grains with diameters com-parable with the domain wall width of the hard phase.2,6 Inaddition, the predominant microstructure should be substan-tially free from the intergranular phase which would inhibitexchange coupling between adjacent grains.7 These require-ments for the grain structure ensure that the entire volumefraction of soft magnetic phase is exchange hardened by thehard magnetic phase so that a substantially enhanced rema-nence and, up to a limit, a higher energy product8 can beobtained as compared with those of single-phase materials.Therefore, it is anticipated that the magnetic behavior ofnanocomposite magnets should be dependent on the intrinsicmagnetic properties of hard and soft phases as well as on

their microstructures. To date, many nanocomposite hardmagnetic alloy systems, including R2Fe14B/a-Fe ~R5Nd orPr!,8–14 Nd2Fe14B/Fe3B,1 and Sm2Fe17N/a-Fe4 have beenexplored by various research groups using melt spining8–14

and mechanical milling.4

It was reported that, for the Nd2Fe14B compound, partialsubstitution of C for B increases the magnetocrystalline an-isotropy field, with only nominal reductions of saturationmagnetization and Curie temperature.15,16 Complete replace-ment of B by C leads to a compound, Nd2Fe14C,17 which isisostructural with Nd2Fe14B. The magnetocrystallineanisotropies of the R2Fe14C ~R5Lu, Gd, Nd! carbides werefound to be larger than those of the respective borides.18

Coercivitiesm0Hc up to 1.25 T can be obtained for the castingots of R2Fe14C ~R5Nd, Dy!-based alloys19,20 without re-sorting to powder metallurgy or rapid solidification. How-ever, due to the sluggish formation of the tetragonal 2:14:1-type phase in the as-cast Nd–Fe–C alloys, long-termannealing of ingots is required, e.g., over 21 days at tempera-ture between 830 and 880 °C. In contrast, the R2Fe14C phaseforms rapidly on annealing melt-spun ribbons or mechani-cally alloyed powders.21–23 Coercivities of 1.16 and 1.8 Thave been achieved for annealed flakes of Nd13.5Fe79.6C6.9

and Nd13Dy2Fe77C8 , respectively. Moreover, it was foundthat Nd–Fe–~C, B! magnets with appropriate C:B ratios,produced by mechanical alloying and subsequent annealing,

a!Author to whom correspondence should be addressed; electronic mail:z.c.wang@sheffield.ac.uk

JOURNAL OF APPLIED PHYSICS VOLUME 91, NUMBER 6 15 MARCH 2002

37690021-8979/2002/91(6)/3769/6/$19.00 © 2002 American Institute of Physics

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have better magnetic properties than those of carbon freeNd2Fe14B-based magnets.23

However, in the literature, research on the influence ofpartial or full C substitution for B on the magnetic propertiesof R2Fe14B/a-Fe-type nanocomposite magnets is verylimited.24 In this article, we report a systematic study of theeffect of carbon content on the formation, structure, andmagnetic properties of melt-spun Nd2Fe14(BC)/a-Fe nano-composite magnets with nominal compositions ofNd8Fe86B62xCx ~x50, 2, 4, 5, 6!.

II. EXPERIMENTAL PROCEDURE

Ingots of Nd8Fe86B62xCx ~x50, 2, 4, 5, 6! alloy wereprepared by arc melting the constituent elements in an argonatmosphere. The alloy ingots were crushed into small piecesof mass about 3 g, which were then melt spun to ribbonsusing roll speeds ranging from 10 to 40 m/s. The as-quenched ribbons were annealed in evacuated quartz tubesfor 10 min at various temperatures in the range 500– 900 °Cand then water quenched. The magnetic phases in the ribbonsamples and their Curie temperaturesTc were identified byx-ray diffraction ~XRD! using CuKa radiation and by ther-momagnetic analysis~TMA !. The as-quenched and heattreated ribbons were magnetized with a pulsed field of about7 T, and their magnetic properties were then measured with avibrating sample magnetometer using a maximum appliedfield of 2 T. A JEOL 2010 UHR transmission electron micro-scope~TEM! equipped with an energy-dispersive x-ray de-tector was employed for microstructural and compositionalcharacterization. Wohlfarth’s remanence analysis25,26 wasemployed to determine the effect of the carbon addition onthe strength of exchange interactions between the magneti-cally hard and soft phases in the optimally processed mate-rials.

III. RESULTS AND DISCUSSION

For melt-spuna-Fe/R2Fe14B-type nanocomposite mag-nets, previous studies have shown that theJr , iHc , and(BH)max are strongly dependent upon the strength of theexchange interaction between thea-Fe and R2Fe14B phases~2:14:1 phase!. Controlling the crystallite size and avoidingthe occurrence of amorphous phase are essential to obtaininghigh Jr , iHc , and (BH)max nanocomposites.10,13 Since thecrystallite size and the phases present are determined by thequenching rate and overall composition, it should be of in-terest to establish how the C substitution affects the micro-structures and magnetic properties of the melt-spun ribbons.

Figure 1 shows XRD patterns for as-quenchedNd8Fe86B62xCx ~x50, 2, 4! ribbons melt spun at a roll speedVs of 24 m/s. It can been seen that both the C-free sampleand the sample withx52 consist of a mixture of amorphousphase and crystalline Nd2Fe14(BC) anda-Fe phases. How-ever, the proportion of crystalline phases for thex52 sampleis higher than that in the C-free sample. The XRD pattern forthe sample withx54 indicates only crystalline Nd2Fe14(BC)anda-Fe phases, with no significant amorphous phase. Thus,the volume fraction of amorphous phase in the ribbonsamples decreases with increasing C content, suggesting a

progressive decrease in the glass-forming ability. Since thepresence of amorphous phase in nanocomposite magnets isundesirable, the melt spinning parameters need to be ad-justed with carbon content to optimize the microstructureand magnetic properties.

Figure 2 shows the variation ofJr , iHc , and (BH)max

for the ribbon with both C content and roll speedVs . Foreach sample,Jr , iHc , and (BH)max first increase with in-creasingVs , reach maximum values simultaneously, andthen decrease with further increases ofVs . Initially, at lowVs , the crystallite sizes are relatively large and are thus dis-advantageous to achieving high values ofJr , iHc , and(BH)max. An optimum nanocrystalline structure, with ex-change enhanced magnetic properties, is obtained at someintermediateVs . Ultimately, at highVs , the magnetic prop-erties are dramatically reduced due to the large proportion ofthe amorphous phase. For the C-free sample, aVs of 18 m/sis found to be optimal to produce ribbon with attractive mag-netic properties. However, with increasing C content in the

FIG. 1. X-ray diffraction patterns of Nd8Fe86B62xCx ~x50, 2, 4! alloy rib-bons melt spun at the roll speed of 24 m/s.

FIG. 2. Dependence ofJr , iHc , (BH)max for the as-melt spunNd8Fe86B62xCx ~x50, 2, 4, 5, and 6! alloy ribbons on the roll speedVs .

3770 J. Appl. Phys., Vol. 91, No. 6, 15 March 2002 Wang, Davies, and Zhou

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ribbons, the optimalVs shifts significantly to higher values.This lends further support to the proposal that substitution ofC for B decreases the glass-forming ability of the alloys.

Figures 3 and 4 show XRD patterns and TMA curves,respectively, for optimally quenched Nd8Fe86B62xCx ~x50,2, 4, 5, 6! ribbons. The XRD and TMA data show that atwo-phase Nd2Fe14(B,C)1a-Fe structure was developed inthe ribbons with C contents up to 4 at. %. Further increase inC content to 5 at. % led to the formation of another ferro-magnetic phase Nd2Fe17Cx (2:17:Cx) ~rhombohedralTh2Zn17 structure,Tc5136 °C!, in addition to 2:14:1 anda-Fe phases. At room temperature, the Nd2Fe17Cx phaseshows planar anisotropy.17 Because of its presence, the vol-ume fractions of the 2:14:1 anda-Fe phases in the ribbonsdecreased accordingly. The alloy ribbon with 6 at. % C con-sisted almost entirely of the Nd2Fe17Cx anda-Fe phases with

little hard magnetic 2:14:1 phase being detected. In addition,it was evident that the Curie temperatureTc for the 2:14:1phase in the ribbon sample decreased from 312 to 270 °C asthe carbon content increased fromx50 to x56, which sug-gests that C atoms entered into Nd2Fe14B crystals and occu-pied the B sites at least partially or even completely.

The room temperature hysteresis loops of optimallyquenched Nd8Fe86B62xCx ~x50, 2, 4, 5, 6! ribbons areshown in Fig. 5. Those for thex50, 2, 4 samples showsmooth demagnetization curves which are typical of single-phase hard magnetic materials, suggesting very fine and uni-form grain sizes in the ribbons. The enhancedJr /Js ratio~0.77–0.78! indicates the existence of strong intergranularexchange interactions between the hard and soft magneticgrains in the sample. However, for the Nd8Fe86B1C5 sample,an apparent kink can be seen in the demagnetization curvenear zero field. This characteristic might be caused by thepresence of a large fraction of the Nd2Fe17Cx phase, as evi-denced by XRD~Fig. 3! and TMA analysis~Fig. 4!. In thiscase, the 2:17:Cx grains may not be uniformly distributedbetween the hard magnetic grains and thus may not be iso-lated by the grains of the 2:14:1 phase. At least some2:17:Cx grains may be in mutual contact or adjacent toa-Fegrains, hence forming extensive soft magnetic regions withdimensions larger than the exchange length. Consequently,the regions of soft magnetic phases would be only partlyexchange coupled to the adjacent hard magnetic grains. Theywould thus reverse incongruously, leading to a shoulderedhysteresis loop. The detailed study of the influence of theNd2Fe17Cx phase on the magnetic properties of the sample isin progress. In the Nd8Fe86C6 ribbon sample, which consistsmainly of 2:17:Cx anda-Fe soft magnetic phases, very lowcoercivity (m0iHc,0.1 T) is observed.

FIG. 3. X-ray diffraction patterns for optimally quenched Nd8Fe86B62xCx

~x50, 2, 4, 5, and 6! ribbons.

FIG. 4. TMA scans for optimally quenched Nd8Fe86B62xCx ~x50, 2, 4, 5,and 6! ribbons.

FIG. 5. Room temperature hysteresis loops for optimally quenchedNd8Fe86B62xCx ribbons with:~a! x50, ~b! x52, ~c! x54, ~d! x55, and~e!x56.

3771J. Appl. Phys., Vol. 91, No. 6, 15 March 2002 Wang, Davies, and Zhou

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An annealing treatment was also applied individually toall as-quenched ribbons in order to investigate if any im-provement in magnetic properties could be induced as com-pared to their optimum direct quench magnetic properties, inparticular for the 5 and 6 at. % C samples. The magneticproperties of heat treated ribbons were compared with thoseof the corresponding as-quenched ribbons. Figure 6 summa-rizes the optimum magnetic properties of the as-quenchedand the annealed Nd8Fe86B62xCx ribbons as a function of Ccontent. It was found that, for the ribbons with C contents upto 4 at. %, no further improvement in the magnetic propertieswas achieved on annealing. For the Nd8Fe86B1C5 andNd8Fe86C6 samples, however, the best magnetic propertieswere obtained by annealing the optimally quenched ribbonsat 800 °C for 10 min, at which the Nd2Fe17Cx

1Nd2Fe14(BC)1a-Fe phase mixture transformed to a com-posite structure of Nd2Fe14(BC)1a-Fe. This was confirmedby the XRD and TEM analyses, which will be discussedlater. Clearly, the Nd2Fe17Cx phase in the as-quenched statewas metastable. Figure 7 compares the hysteresis loops ofoptimally quenched Nd8Fe86B1C5 and Nd8Fe86C6 ribbonswith those for the corresponding ribbons after annealing at800 °C for 10 min. It can be seen that, for both alloys, theannealing~at 800 °C for 10 min! improves bothiHc andJr ;however, an apparent kink still persists in the demagnetiza-tion curves for both samples, even though the structure isnow the two-phase 2:14:1/a-Fe composite. This is probablya consequence of the coarse-grained structure in the sample,i.e., some of the larger soft magnetica-Fe grains are partiallydecoupled from the neighboring hard magnetic 2:14:1 grainsand reverse independently.

Table I presents, for clarity, the values of the magneticproperties for optimally processed Nd8Fe86B62xCx ~x50, 2,4, 5, 6! as a function of C content. From both Fig. 6 andTable I, it can be seen that for optimally processed ribbonsJr

decreases slightly with C content up tox54 but then dropsabruptly whenx is further increased to 6. In contrast,iHc

increases with C substitution, reaching a maximum atx;4and then decreases. Accordingly, the (BH)max remains al-most constant with increasing C content up tox54 but de-creases drastically for higher C content. It is also noted thatthe Curie temperature of the 2:14:1 phase decreases gradu-ally from 312 °C forx50 to 270 °C forx56.

Figure 8 shows bright-field TEM micrographs and elec-tron diffraction patterns for optimally processedNd8Fe86B62xCx ~x50, 4, 5, 6! ribbon samples, respectively.Both the bright- and dark-field observations indicate that auniform nanocomposite Nd2Fe14(BC)/a-Fe structure, withan average grain size of about 20–25 nm, is developed in theNd8Fe86B6 and Nd8Fe86B2C4 ribbons, as shown in Figs. 8~a!and 8~b!, respectively. The microstructures of these twosamples are very similar, with little difference in averagegrain size between them being evident. The electron diffrac-tion patterns in Figs. 8~c! and 8~d! confirm the presence ofNd2Fe14(BC) anda-Fe phases and the absence of Nd2Fe17Cx

phase in thex55 and x56 ribbons. This indicates that atransformation from the Nd2Fe17Cx1Nd2Fe14(BC)1a-Fephase mixture to a composite structure of Nd2Fe14(BC)1a-Fe occurred in both samples after the annealing treat-ment. However, grain coarsening is evident in the sampleswith x55 and 6. The average grain size for theNd8Fe86B1C5 sample is around 40 nm but the grain sizedistribution is nonuniform with somea-Fe grains being up toabout 100 nm in diameter. For the Nd8Fe86C6 sample, theaverage grain size is about 70 nm and its nonuniformity waseven more pronounced. The microstructural difference be-tween the samples with low C content~x50, 4! and thosewith high C content~x55, 6! can be attributed to the formerbeing processed by direct quenching from the melt, forwhich the high cooling rate facilitates very high nucleationfrequencies for the Nd2Fe14(BC) anda-Fe crystallites, lead-ing to a fine-grained homogeneous structure~with no influ-

FIG. 6. Optimum magnetic properties of the as-spun and the annealedNd8Fe86B62xCx ~x50, 2, 4, 5, and 6! ribbons.

FIG. 7. Room temperature hysteresis loops for optimally quenchedNd8Fe86B1C5 and Nd8Fe86C6 ribbons with and without annealing at 800 °Cfor 10 min.

3772 J. Appl. Phys., Vol. 91, No. 6, 15 March 2002 Wang, Davies, and Zhou

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ence of subsequent annealing at 600 °C on the grain size!. Inthe latter case, the fact that the composite 2:14:1/a-Fe mi-crostructure is obtained by solid state transformation fromthe metastable 2:17:Cx phase, using a high-temperature an-nealing treatment~800 °C 10 min!, is the probable reason forthe coarser-grained and inhomogeneous microstructure.

The remanence enhancement and magnetic hardening inisotropic nanocomposite magnets arise from exchange inter-actions between the magnetically hard and soft phases.2 So,without knowing the exchange coupling effect in thesamples, the microstructural observations alone are, in gen-eral, insufficient to explain in detail the variations of mag-netic properties with increasing C content. An effectivemethod of analyzing the exchange coupling is via so-calleddM @5md(H)2(122mr(H))# plots,13,14 where md is thereduced demagnetization remanence andmr the reducedmagnetization remanence.25,26 According to Wohlfarth’sanalysis, for an assembly of noninteracting single-domainparticlesdM50, whereas a nonzerodM indicates the exis-tence of interaction between the grains. For magnetostaticinteractionsdM is negative, while for exchange interaction itis positive. Figure 9 shows thedM plots as a function of theapplied field in optimally processed Nd8Fe86B62xCx ~x50,2, 4, 5, 6! ribbons. It can be seen that, despite the differencesin C concentration, the heights of the positivedM peaks forthe ribbon samples withx50, 2, 4 are nearly identical,which suggests that the exchange interactions between the

hard and soft magnetic phases are of similar magnitude inthese samples. However, the peak intensity for thex55 or 6ribbons is significantly lower than those forx50, 2, 4, indi-cating that a weaker exchange coupling exists between thehard and soft magnetic phases. From these results, the varia-tion in magnetic properties of the optimally processedsamples on increasing the C content, as shown in Table I andFig. 6, can be explained as follows.

The ribbons with C contentx<4 have the nanocompos-ite Nd2Fe14(BC)/a-Fe structure. Due to the exchange inter-actions between the magnetically hard 2:14:1 phase and thesoft a-Fe phase, the ribbons exhibit smooth hysteresis loopswith enhancedJr /Js . With increasing C content, the anisot-ropy field of the 2:14:1 phase increases while the saturationmagnetization decreases,15,16leading to higher coercivity andlower remanence. The coercivity reaches its maximum valueof 542 kA/m in Nd8Fe86B2C4 ~as compared to the 430 kA/min the C-free Nd8Fe86B6 ribbon!. Further increase of C con-tent leads to the formation of a coarser-grained inhomoge-neous microstructure, which results in the weakening of theexchange coupling and consequently the drastic deteriorationof both iHc andJr .

IV. CONCLUSIONS

The effects of C content on the formation and magneticproperties of melt-spun Nd2Fe14(BC)/a-Fe nanocomposite

FIG. 8. TEM micrographs of optimally processed Nd8Fe86B62xCx ribbons:~a! x50, ~b! x54, ~c! x55, and~d! x56.

FIG. 9. Variation ofdM with the external magnetic fields for optimallyprocessed Nd8Fe86B62xCx ~x50, 2, 4, 5, and 6! ribbons.

TABLE I. Alloy composition, processing condition, and magnetic properties of optimally processedNd8Fe86B62xCx melt-spun ribbons. TheTc is the Curie temperature of 2:14:1 phase in the ribbons.

Composition(x) Condition

Tc

(°C)J20

~T!Jr

~T!iHc

~kA/m!(BH)max

(kJ/m3)

0 18 m/s 312 14.84 1.16 430.0 148.82 20 m/s 297 14.36 1.11 491.8 148.24 24 m/s 286 13.78 1.07 542.0 150.35 26 m/s

800 °C, 10 min274 13.56 0.95 328.7 83.8

6 28 m/s800 °C, 10 min

270 13.44 0.87 260.2 59.4

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alloys with nominal compositions of Nd8Fe86B62xCx ~x50,2, 4, 5, 6! have been investigated. Increasing the C contentwas found to decrease the glass-forming tendency of as-spunribbons significantly. For the samples with C contentx<4,the best magnetic properties were achieved by directlyquenching at an optimum roll speed, by which a uniformexchange coupled nanocomposite Nd2Fe14(BC)/a-Fe struc-ture was developed. Within this composition range, C substi-tution does not significantly affect the 2:14:1 anda-Fe grainsize of the optimally quenched ribbons. However, the coer-civity was increased with only nominal reduction in rema-nence; these effects can be attributed, respectively, to an en-hancement of the magnetocrystalline anisotropy constant andto a slight reduction in the saturation magnetization of the2:14:1 phase.

For the samples with higher C content (x.4), the bestmagnetic properties were obtained by annealing the opti-mally quenched ribbons at 800 °C, at which the Nd2Fe17Cx

1Nd2Fe14(BC)1a-Fe phase mixture transformed to thetwo-phase Nd2Fe14(BC)1a-Fe composite. This phase mix-ture had a very nonuniform and coarse-grained microstruc-ture which resulted in only weak exchange interaction be-tween hard and soft magnetic phases and hence in markeddeterioration of bothiHc andJr compared with the sampleshavingx<4.

ACKNOWLEDGMENTS

Financial support for research on nanophase magneticalloys at the University of Sheffield by the U.K. Engineeringand Physical Sciences Research Council~EPSRC!, throughthe Advanced Magnetics Program, and at USTB by the Na-tional Natural Science Foundation of China is gratefully ac-knowledged.

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3774 J. Appl. Phys., Vol. 91, No. 6, 15 March 2002 Wang, Davies, and Zhou

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