铁磁学

Evolution of magnetization reversal mechanism in Fe-Cr alloy films T. R. Gao, S. P. Hao, S. M. Zhou, and L. SunCitation: J. Appl. Phys. 100, 073909 (2006); doi: 10.1063/1.2357637View online: /10.1063/1.2357637View Table of Contents: /resource/1/JAPIAU/v100/i7Published by the American Institute of Physics.Additional information on J. Appl. Phys.Journal Homepage: /Journal Information: /about/about_the_journalTop downloads: /features/most_downloadedInformation for Authors: /authorsEvolution of magnetization reversal mechanism in Fe-Cr alloyfilms T.R.Gao,S.P.Hao,and S.M.Zhou a͒Surface Physics Laboratory(State Key Laboratory)and Department of Physics,Fudan University,Shanghai200433,ChinaL.SunDepartment of Mechanical Engineering,University of Houston,Houston,Texas77204͑Received8November2005;accepted29July2006;published online13October2006͒A large thinfilm sample of Fe x Cr1−x alloy with a composition gradient͑x=0.38−0.52͒has beenprepared by co-sputtering to investigate magnetic anisotropy and magnetization effects on reversalmechanisms.The single-phased Fe-Crfilms have a bcc structure with͓110͔preferred orientation.Since the magnetization decreases as the Fe content is decreased and the uniaxial anisotropy energykeeps at almost constant,the uniaxial anisotropicfield H K is increased.At the same time,the pinningfield H p͑0͒changes little since the lattice constant of Fe-Cr alloyfilm does not change much withthe composition.Consequently,the H K can be much larger than H p͑0͒for low Fe concentrations andbecomes comparable with increasing Fe concentration.As a result,the magnetization reversalmechanism evolves from a mode based on the pinning and motion of domain wall to another modebased on modified Kondorsky model as the Fe content is increased.©2006American Institute ofPhysics.͓DOI:10.1063/1.2357637͔I.INTRODUCTIONMetallic ferromagneticfilms are of crucial importance because of their potential applications in magnetoelectronics and spintronic devices.In spin valves,for example,ferro-magnetic layers with large asymmetry of spin-dependent scattering are required for high giant magnetoresistance ratio. In order to enhance the sensitivity of the device,small coer-civity and uniaxial anisotropy are desired.More importantly, the control of the magnetization reversal process in free fer-romagnetic layer can help to suppress the noise level caused by imperfect magnetization reversal.1For magnetic materials with uniaxial anisotropy,there are three mechanisms to describe magnetization reversal in magnetic singlefilms,including coherent magnetization re-versal,pinning and motion of domain wall,and the Kondor-sky model.2In the coherent rotation model,the magnetiza-tion reversal is accompanied by the coherent rotation of magnetization under an external magneticfield.In the sec-ond model,the magnetization reversal is accomplished only through the motion of the domain wall when the pinning field of the domain wall H p͑0͒along the easy axis must be much smaller than the uniaxial anisotropicfield H K͑=2K U/M S͒,where the K U is the induced uniaxial aniso-tropic energy and M S is the saturation magnetization.H p͑0͒is equal to the maximum of thefirst derivative of the areal domain wall energy with respective to the location.The switchingfield H S͑␾H͒is the magneticfield to overcome the irreversible motion of the domain wall and H S͑␾H͒=H p͑0͒sec͑␾H͒.3It is usually found in single crystal mate-rials with large intrinsic uniaxial anisotropy like CrO2, LaSrMnO3,barium ferrite,and some permanent magnetic materials.2,4–6In addition to these two mechanisms,the Kon-dorsky model is developed to describe the magnetization re-versal under the condition where the H p͑0͒and H K are com-parable,and the above two magnetization reversal mechanisms coexist.Up till now,in most studies,the mag-netization reversal mechanism has been studied case by case and few experimental studies have been performed on the evolution of the magnetization reversal mechanism in one system.On one hand,in most of the binary alloys of transition metals,magnetization can be tuned continuously through composition control,and thus these systems can provide im-portant information on the evolution of magnetization reversal.7The uniaxial anisotropy can be induced in the alloy films by deposition magneticfield and have a maximal value at a certain composition.To achieve a wide enough expan-sion of H K,single phase alloys with a wide composition range and thus a wide magnetization range are desired.In this work,we have chosen metastable single-phased Fe-Cr alloyfilms as an example to study the evolution of the mag-netization reversal mechanism with the alloy composition.It is well known that the Fe-Cr alloy exhibits antiferromag-netic,spin glass and ferromagnetic phases in the Fe concen-tration range from0.5to30at.%.8,9For the Fe concentra-tion higher than30at.%,the ferromagnetic phase can usually be observed.As the magnetization is decreased with decreasing Fe content,the magnitude of H K varies sharply with the composition.10On the other hand,since the lattice constant of the Fe-Cr alloys does not change significantly as a function of the alloy composition,the H p͑0͒of the domain wall displacement is expected to change little.Therefore,the magnetization reversal mechanism can be expected to change through modification of the ratio H p͑0͒/H K as a func-tion of the alloy composition.a͒Author to whom correspondence should be addressed.Electronic mail:shimingzhou@JOURNAL OF APPLIED PHYSICS100,073909͑2006͒0021-8979/2006/100͑7͒/073909/6/$23.00©2006American Institute of Physics100,073909-1II.EXPERIMENTSA large sample ͑1cm ϫ5cm ͒of a Fe-Cr alloy film was deposited onto a glass substrate by a multisource sputtering system.The base pressure is 5ϫ10−8Torr and the Ar pres-sure is 5mTorr during deposition.Instead of fabricating many Fe-Cr samples with different Fe-Cr compositions,which are susceptible to run-to-run variations,we have made a single specimen of Fe x Cr 1−x with a composition gradient by rf and dc sputtering of Fe and Cr targets,respectively.The composition changes from x =0.38to x =0.52across the length of 5cm,approximately as a linear function of the location.At the same time,the film thickness also changes linearly from 36to 25nm.As buffer and coverage layers,two 30nm thick Cu layers were deposited by dc sputtering.The deposition rates of the Fe-Cr and Cu layers are about 0.1nm/s.The large specimen was then cut into many small samples along the gradient direction.The composition and thickness of each small sample are essentially constant.The microstructure of Fe-Cr films was characterized by x-ray dif-fraction ͑XRD ͒.In-plane hysteresis loops of individual small samples were measured using a vibrating sample magneto-meter ͑VSM ͒of Model 7407from LakeShore Co.In-plane anisotropic properties were characterized by the ferromag-netic resonance ͑FMR ͒technique.FMR spectra were mea-sured with a fixed microwave frequency of 9.78GHz at varying azimuthal angles when the dc external field was kept in the film plane.All measurements were carried out at room temperature.III.RESULTS AND DISCUSSIONFigure 1shows the XRD spectra of Fe-Cr alloy films.There are three diffraction peaks from 2␪=35°to 55°.Thepeaks near 2␪=43°and 51°can be identified as Cu ͑111͒and Cu ͑200͒,respectively.The small peak at the right-hand side of Cu ͑111͒does not shift with the composition.In principle,it can be attributed to either body-centered-tetragonal ͑bct ͓͒110͔preferred orientation of Fe-Cr alloys or bcc ͓110͔pre-ferred orientation of Fe-Cr,or pure Fe or Cr because they are located closely to each other.11However,the small peak should be bcc ͓110͔preferred orientations of Fe-Cr alloys,as discussed below.In experiments,hysteresis loops of Fe-Cr films are squared along the easy axis as shown below.Espe-cially,there is only one resonance peak in the FMR spectra when the external magnetic field is aligned along the film plane.It is a clear indication that there is only one phase in the Fe-Cr alloy films and no granular structure exists.More-over,since the Curie temperature of bct Fe-Cr is lower than room temperature,it can also be excluded.10The present Fe-Cr films consist of bcc FeCr alloys.It is quite different from previously reported experiments,in which within the present range of Fe contents the Fe-Cr films consist of bcc and bct phases.10This might be related to either the effect of the Cu buffer layer or the co-sputtering technique.In order to get insight into the mechanism of the mag-netization reversal,it is necessary to measure in-plane hys-teresis loops of all samples at various orientations of the external magnetic field and to see the variation of the switch-ing field H S ,the coercivity H C ,and the remanent ratio.It is noted that the magnetization at H C is equal to zero while dH /dM =0at the switching field H S .Figure 2shows the typical hysteresis loops at various orientations for x =0.38.For ␾H =0,the hysteresis loop is squared and then becomes more slanted with increasing ␾H .One can find that for ␾H =0°,H C =H S ,while for large ␾H ,H C ϽH S .With increasing ␾H ,the remanent ratio is decreased.Figure 3shows the an-gular dependence of the in-plane normalized remanent ratio for Fe-Cr alloy films with different alloy compositions.It is noted that the remanent ratio is not equal to zero at ␾H =90°.For all samples,the experimental results ͑the squared symbols ͒coincide with the fitted results ͑the solid lines ͒of M r =cos ␾H at all orientations,except for near ␾H =90°.This is because without an external magnetic field,the magneti-zation should be aligned along the easy axis and the rema-nent magnetization should be equal to the project of the satu-ration magnetization along the external magnetic field.The disagreement near ␾H =90°can be attributed to a narrow dispersion of the easy axis.Figure 4shows three representative angular dependence of measured H C and H S .For all Fe-Cr films,H C is equal to H S near the easy axis,yet with increase of ␾H ,the two fields begin to deviate from each other with H C ϽH S .For a low Fe content of x =0.38,H S increases from 20to 120Oe as ␾H is increased from 0°to 90°.At the same time,H C initially increases with ␾H but exhibits a peak at around 72°.When the Fe concentration is increased to 0.456,H S increases monotonically and H C decreases monotonically with increas-ing ␾H ,respectively.For a higher Fe concentration of 0.49,H C decreases monotonically with increasing ␾H ,yet H S dis-plays a minimum at ␾H =50°.Figure 5shows a plot of 1/H S vs cos ␾H for different Fe concentrations.For x =0.38,the inverse H S is proportionaltoFIG.1.X-ray diffraction spectra of Fe x Cr 1−x films with four different Fe compositions x .The inserted number refers to the value of x .the cos ␾H for cos ␾H Ͼ0.2,i.e.,␾H Ͻ80°,in agreement with the pinning model.These observations mean that the magnetization switching is accompanied by the domain wall pinning and motion with little contribution from the magne-tization coherent rotation.Therefore,H p ͑0͒=H S ͑0͒=H C ͑0͒.For x =0.38,the value of H S ͑␾H ͒at ␾H =80°is about 3–4times larger than H S ͑0͒.For polycrystalline permalloy thin films,the H S ͑␾H ͒/H S ͑0͒is increased by a factor of 1.4.12For ␾H Ͼ80°,the variation of H S is deviated from that of the pinning model and the switching process is also accompa-nied by the magnetization coherent rotation,in addition to the pinning model.It is suggested that for ␾H Ͼ80°,the H p ͑␾H ͒is close to or larger than the switching field of the magnetization coherent rotation.For x =0.40and 0.425,the results are similar to that of x =0.38.When x is increased from 0.38to 0.425,the onset of cos ␾H for the linear depen-dence is shifted,as indicated by arrows.For x =0.40and 0.425,the inverse H S obeys the proportional relationship with cos ␾H in a narrower region of ␾H .Actually,for x Ն0.425,no linear dependence of the switching field is ob-served.As discussed above,there are three possible magnetiza-tion reversal mechanisms for a ferromagnetic film with uniaxial anisotropy:coherent rotation,domain wall pinning,and the modified Kondorsky model.2In the coherent rotation model,the H S ͑␾H ͒has the so-called S-W angular depen-dence,in which the H S first decreases to reach a minimum at ␾H =45°and then increases with increasing ␸H .The H C de-creases monotonically when ␾H is increased from 0°to 90°.In this case,the H p ͑0͒is much larger than H K .In the second model,domain pinning and domain wall motion cause mag-netization reversal and H S obeys a linear dependence on sec ␾H .This sec ␾H relation requires the strength of H p ͑0͒to be much smaller than that of the H K .The Kondorsky model has been developed to describe the magnetization reversal when H p ͑0͒approaches H K .Under this circumstance,both coherent rotation and the motion of domain wall contribute to magnetization reversal and the H S will obey neither the angular dependence of 1/cos ␾H nor that of the S-W model.Thus,the angular dependence of the H S is determined by the ratio of H p ͑0͒/H K .From the above analysis,one can know that for x =0.38and 0.40the magnetization reversal process is accompanied mainly by the motion of domain wall in a wide range of ␾H ,whereas for x Ͼ0.425the magnetization reversal process is accompanied by the modified Kondorsky model.From above analysis,one can know that the ratio H p ͑0͒/H K is far smaller than 1.0for x =0.38and 0.40and is estimated to be equal to or larger than 0.5for x =0.425,0.456,0.49,and 0.52,as shown in the inset of Fig.5.In order to study the uniaxial anisotropic field,the FMR spectra were measured at various orientations.Figure 6shows typical FMR spectra of a Fe 45Cr 55film.A smallreso-FIG.2.Representative hysteresis loops of the Fe 0.38Cr 0.62films measured with different in-plane external fieldorientations.FIG.3.Angular dependence of the normalized in-plane remnant magneti-zation for two different Fe concentrations.The lines correspond to cos ␾H .The ␾H represents the angle between the applied magnetic field and the easy axis.nance peak is observed in the low field region,which is possibly due to the switching of the magnetization upon ap-plication of the magnetic field.Only one resonance peak at high magnetic fields can be observed for all external field orientations.This confirms the homogeneity of the Fe-Cr films.The resonance field is shifted toward high magnetic fields with increasing ␾H .From the FMR spectra,one can get the angular dependence of the resonance field.Figure 7shows the results.Apparently,a uniaxial anisotropy exists.Since the resonance field is much smaller than the effective demagnetizing field,the in-plane angular dependence of the resonance field can be approximately described as follows 13:H res =␻24␲M eff ␥2−H K cos 2␾H ,͑1͒where ␻is the resonance frequency,␥is the magnetogyric ratio,and 4␲M eff is the effective demagnetizing field.The results in Fig.7can be described by Eq.͑1͒,and the uniaxial anisotropic field can be fitted.Figure 8shows the variations of H S ͑0͒and H K with the Fe content.For x Յ0.425,H K is increased sharply and is much larger than H S ͑0͒.Since the magnetization switching is accompanied by the motion of domain wall for ␾H Ͻ80°,H S ͑0͒=H p ͑0͒.Therefore,H K ӷH p ͑0͒.So the prerequisite forthe motion of the domain wall can be satisfied.For high Fe contents,H K and H E ͑0͒have comparable values.During the switching process,both the motion of domain wall and the magnetization coherent rotation coexist.In this way,H S ͑0͒is related to both H K and H p ͑0͒.Thus,it is suggested that H p ͑0͒does not change significantly with the Fe content.As is well known,the H p ͑0͒is induced by the inhomogeneity of micro-structure and stress,and is determined by the first derivative of the domain wall energy with respect to the location.7Since Fe and Cr atoms have very similar lattice parameters,the microstructure of the Fe-Cr films and thus H p ͑0͒do not change much as a function of the alloy composition.For this reason,one can understand the results in the inset of Fig.5.Figure 9͑a ͒shows the variation of the magnetization at room temperature with the alloy composition.The magneti-zation increases monotonically with the increase of the Fe content,which approximately agrees with that of Slater-Pauling curve.7According to K U =H K M S /2,one can easily calculate the uniaxial anisotropy K U as a function of the Fe concentration.Figure 9͑b ͒shows the K U dependence on the Fe content.As is well known,a uniaxial anisotropy can be induced in binary ferromagnetic alloy films.In Fe-Ni alloys,for example,the induced uniaxial anisotropy will have a maximum value when the atomic contents of Fe and Ni ele-ments are close to each other.14However,in the present Fe-Cr alloys,the uniaxial anisotropy does not changemuchFIG.4.Angular dependent in-plane switching field H S ͑squares ͒and coer-cive field H C ͑circles ͒for three different Fe contents.The inserted numbers refer to the values of x.FIG.5.Angular dependence of inverse H S for Fe x Cr 1−x thin films with x =0.38and 0.40͑a ͒,and 0.425,0.456,and 0.49͑b ͒.The solid lines refer to fitted results of the linear dependence.The arrows indicate the onset of the cos ␾H for inverse H S to deviate from the linear dependence.The inset shows the variation trend of the ratio H p ͑0͒/H K ,which is estimated from the results in Figs.4and 5.FIG.6.Typical FMR spectra of the Fe-Cr film at various orientations for x =0.45.The inset shows the value of ␾H.FIG.7.Angular dependence of the in-plane resonance field for the Fe-Cr film at various orientations for x =0.45.The solid line refers to the fitted results by Eq.͑1͒.position dependent in-plane H S ͑0͒͑a ͒and H K ͑b ͒for the single phase bcc Fe x Cr 1−x alloyfilms.FIG.9.Magnetization ͑a ͒and uniaxial anisotropy energy K U ͑b ͒of Fe x Cr 1−x alloy at the room temperature.with the Fe content in the present composition region.Thismight be related to the specific spin structure of the Fe-Cralloys.8Fortunately,the H K can be modified,changing thealloy composition since the magnetization increases with in-creasing Fe content and the K U in Fe-Cr alloyfilms changeslittle,as shown in Fig.9.IV.CONCLUSIONSIn short summary,a Fe x Cr1−x alloyfilm with a composi-tion gradient͑x=0.38−0.52͒has been prepared by co-sputtering.The Fe-Crfilm has a bcc structure with͓110͔preferred orientation along thefilm normal direction.As theFe content is decreased,the H K increases.Since the latticeconstant of the Fe-Cr alloyfilm remains almost constant,H p͑0͒changes little with respect to the alloy composition.In contrast,the uniaxial anisotropicfield shows strong compo-sition dependence since the magnetization of the alloy de-creases sharply when the Fe concentration drops in a narrowrange.So the value of H K can have significant increaseswhen Fe content is decreased.The will result in a remarkablycontrol of H p͑0͒/H K in a wide range.Therefore,the magne-tization reversal mechanism is evolved as a function of the alloy composition,from the displacement of the domain wall to modified Kondorsky model.ACKNOWLEDGMENTSThis work was supported by the National Science Foun-dation of China Grant Nos.10574026,60271013,60490292, 10021001,and10321003,and the State Key Project of Fun-damental Research Grant No.2002CB613504,Shanghai Nanotechnology Program Center͑No.0252nm004͒.1B.Dieny,V.S.Speriosu,S.S.P.Parkin,B.A.Gurney,D.R.Wilhoit,and D.Mauri,Phys.Rev.B43,1297͑1991͒.2D.V.Ratnam and W.R.Buessem,J.Appl.Phys.43,1291͑1972͒.3L.Sun,P.C.Searson,and C.L.Chien,Appl.Phys.Lett.79,4429͑2001͒. 4F.Y.Yang,C.L.Chien,E.F.Ferrari,X.W.Li,G.Xiao,and A.Gupta, Appl.Phys.Lett.77,286͑2000͒.5Z.H.Wang,G.Cristiani,and H.U.Habermeier,Appl.Phys.Lett.82, 3731͑2003͒.6K.H.J.Buschow,Rep.Prog.Phys.54,1123͑1991͒.7Physics of Ferromagnetism,edited by S.Chikazumi͑Clarendon,Oxford, 1997͒,p.173.8E.Fawcett,H.L.Alberts,V.Yu.Galkin,D.R.Noakes,and J.V.Yakhmi, Rev.Mod.Phys.66,25͑1994͒.9H.L.Alberts and J.A.J.Lourens,J.Phys.:Condens.Matter4,3835͑1992͒.10N.H.Duc,A.Fnidiki,J.Teillet,J.Ben Youssef,and H.Le Gall,J.Appl. Phys.88,4778͑2000͒.11A.A.Levin,D.C.Meyer,A.Tselev,A.Gorbunov,W.Pompe,and P. Paufler,J.Alloys Compd.334,159͑2002͒.12S.Middelhoek and D.Wild,IBM J.Res.Dev.11,93͑1967͒.13S.M.Zhou,S.J.Yuan,M.Lu,J.Du,A.Hu,and J.T.Song,Appl.Phys. Lett.83,2013͑2003͒.14Physics of Ferromagnetism,edited by S.Chikazumi͑Clarendon,Oxford, 1997͒,p.299.。