铁酸铋 磁性

Appl Phys A(2014)114:853–859DOI10.1007/s00339-013-7712-5Structural,optical,and multiferroic properties of single phased BiFeO3M.Muneeswaran·P.Jegatheesan·M.Gopiraman·Ick-Soo Kim·N.V.GiridharanReceived:26December2012/Accepted:13April2013/Published online:27April2013©Springer-Verlag Berlin Heidelberg2013Abstract A soft chemical coprecipitation method has been proposed for synthesis of nano-sized multiferroic BiFeO3 (BFO)powders.The X-ray diffraction pattern confirms the perovskite structure of BFO and Rietveld refinement re-veals the existence of rhombohedral R3c symmetry.Crys-tallite size and strain value are studied from Williamson–Hall(W–H)analysis.The transmission electron microscope (TEM)image shows that the particle size of BFO powders lies between50–100nm.4A1and7E Raman modes have been observed in the range100–650cm−1and a prominent band centered around1150–1450cm−1have also been ob-served corresponding to the two-phonon scattering.Differ-ential Thermal Analysis(DTA)shows the existence of two prominent peaks at330◦C and837◦C corresponding to the magnetic and ferroelectric ordering,respectively.From the temperature dependent dielectric studies,an anomaly in the dielectric constant is observed at the vicinity of Neel tem-perature(T N)indicating a magnetic ordering.Also,BFO shows antiferromagnetic behavior measured from the mag-netic studies.1IntroductionRecently,the interest in multiferroics is stimulated by fun-damental physics leading to multiferroism arising from cou-M.Muneeswaran·P.Jegatheesan·N.V.Giridharan( ) Department of Physics,National Institute of Technology, Tiruchirappalli620015,Indiae-mail:giri@Fax:+91-431-2500133M.Gopiraman·I.-S.KimNano Fusion Technology Research Group,Faculty of Textile Science and Technology,Shinshu University,Ueda,Nagano386-0015,Japan pling between magnetic and ferroelectric orderings,andhave been extensively studied for their possible technicalapplications,including spintronics,microelectronics,mag-netic memory,and sensors[1].The term“multiferroic”means coexistence of ferroelectric and magnetic ordering inone single phase or multiphase materials.However,thesetwo ordering parameters are mutually exclusive in principlebecause ferroelectricity requires empty d shells,while mag-netism requires partiallyfilled d shells[2].Several compos-ite materials,consisting of separate ferroelectric and mag-netic phases,have been reported to show magnetoelectriccoupling at room temperature[3].However,the availabil-ity of room-temperature single phase multiferroics is verylimited[4].Among the few room temperature single-phasemultiferroics reported so far[5],BiFeO3(BFO)is an im-portant multiferroics,which has rhombohedrally distortedperovskite crystal structure with a space group of R3c atroom temperature[6].It exhibits ferroelectric ordering be-low T C∼1083–1103K,and antiferromagnetic ordering be-low T N∼625–643K[7].BFO shows G-type antiferromag-netic structural behavior having modulated spiral spin struc-ture with long periodicity of62nm in the unit cell[8].In-terests shown by the researchers to work on these materi-als in a nanoregime is due to their size dependent proper-ties compared to the bulk.Nanosized BFO powders havebeen reported to exhibit weak ferromagnetism at room tem-perature,which is different from the magnetic property ofbulk samples[9].One important challenge in the success-ful synthesis of pure BFO is avoiding the secondary phasessuch as Bi2Fe4O9and Bi25FeO39[10].Several techniqueshave been developed to prepare pure BFO powders.Thesolid state reaction route generally involves a higher pro-cessing and requires HNO3as a leaching agent to elimi-nate the secondary phases leading to the coarse nature ofthe powders.Nanosized BFO ceramics have been prepared854M.Muneeswaran et al.by low-temperature chemical methods such as sol–gel[11], hydrothermal[12],auto combustion[13]and coprecipita-tion[14].But these processes also involve complex solu-tions and acid reagents.Hence,it is still worth in developing a soft chemical approach to obtain single-phase BFO with a homogeneous chemical composition crystallized at rela-tively low temperature.Here,we propose a novel soft chem-ical synthesis of single phase BFO powders relatively at low temperature,without using complex precursor solutions and acid-reagents.The rhombohedral structure of the synthe-sized BFO powders has been confirmed by Rietveld analy-sis.4A1and7E phonon modes in the lower frequencies and two phonon scattering in the higher frequencies have been observed from Raman-scattering studies.A distinct dielec-tric anomaly observed in the temperature dependent dielec-tric measurements and their antiferromagnetic behavior at room temperature is confirmed by the Arrott–Belov–Kouvel (ABK)plot.2Experimental detailsThe synthesis procedure is as follows.Bi(NO3)3·5H2O and Fe(NO3)3·9H2O was dissolved in200ml of double distilled water and stirred for about20minutes to form a clear solu-tion.A constant pH level of10.8was maintained by syn-chronized dropping of a mixture of ammonia solution and distilled water solution to get the reaction product.This pre-cipitate was kept at room temperature for about24hours and was washed several times with double distilled water to re-move unreacted products and thenfiltered.Thefinal product was dried in a hot air oven at100◦C for about5hours and final sintering was carried out at600◦C for2hours.The phase identification was examined on a Rigaku(D/Max ul-tima III)X-ray diffractometer using Cu Kαradiation.XRD data was collected at a slow scan rate of0.05◦/min and the simulation of the crystal structure was done based on the measured XRD data and Rietveld crystal structure refine-ment software General Structure Analysis System(GSAS). The morphology of the prepared powders was observed us-ing a Field Emission scanning electron microscope(FE-SEM,S-5000,HITACHI,Japan).Transmission electron mi-croscopy(TEM)images of the samples were taken through a JEM-2010electron microscope with an accelerating voltage of200kV.Differential Thermal Analysis(DTA)and Differ-ential scanning colorimetric(DSC)were performed using the SII Nanotechnology Inc.,Japan,and EXSTAR6200,re-spectively.The Raman spectra were recorded with a Raman spectrometer(Hololab5000,Kaiser Optical Systems Inc., (USA)with argon laser(532nm)and a Kaiser holographic edgefilter.Temperature dependant dielectric studies were performed with LCR meter(HIOKI3532-50,Japan)and the magnetic measurement were done with a vibrating sample magnetometer(Lakeshore,USA7404).3Result and discussionFigure1(a)shows the XRD patterns(as prepared and sin-tered)of BFO.As prepared powders show amorphous be-havior,while the sintered BFO was found to be well crystal-lized and formed in the rhombohedral structure(R3c)with a clear splitting of(104)and(110)peaks.A small amount of an impurity phase has also been observed due to the kinetics of formation[15].Further,the experimental XRD pattern is simulated to know the structure and the lattice parameters. Figure1(b)shows the results of the Rietveld refinement of the XRD patterns of BFO.The refinement is performed us-ing the rhombohedral crystal symmetry.The crystal struc-ture parameters derived from the simulation are listed in Ta-ble1.The Rp and wRp values are found to be higher com-pared to other literatures and may be due to the larger parti-cle size of BFO.The R andχ2values suggest that the sim-ulated XRD patterns agree well with the experimental XRD pattern.XRD data can also be utilized to evaluate the peak broad-ening in terms of the crystallite size and the lattice strain due to dislocation.Since the breadth of the Bragg peak is the combination of both instrumental and sample dependent effects,it is necessary to collect a diffraction pattern from the line broadening of a standard material such as siliconTable1Relevant parameters obtained from Rietveld refinement XRD pattern of BFO powdersLattice parameters(Å)Atom coordinates Bond length(Å)Bond angle V olume(Å3)X Y Z Rp wRpχ2a=5.5947±0.01597Bi6a00012.020.4 1.79Bi–O Fe–O–Fe 2.210161.4◦b=13.9058±0.036508Fe6a000.201Fe–O O–Bi–O376.960 1.88471.2◦O18b0.4610.0330.951Fe–O 2.140Structural,optical,and multiferroic properties of single phased BiFeO3855Fig.1(a)X-ray diffraction pattern of BFO powders (rhombohedral with R3c space group).(b)Rietveld refinement of X-ray diffraction data for BFO.The insetsfigure shows that the cells with blue,brown and red spheres correspond to Bi,Fe,and O respectively. (c)W–H plot for BFOpowdersto determine the instrumental broadening[16,17].The in-strumental corrected broadeningβhkl corresponding to the diffraction peak of BFO are estimated by using the rela-tion:βhkl=(βhkl)2measured−β2instrumental1/2(1)and strain induced broadening is given byε=βhkl/tanθ. Williamson and Hall(W–H)proposed a method of decon-voluting size and strain from the mathematical expression given byβhkl cosθ=kλD+4εsinθ(2)where“k”is the shape factor,“λ”is the X-ray wavelength,“θ”is the Bragg angle,“D”is the effective crystallite size,εis the strain,andβhkl is the full width at half maximum of the corresponding hkl plane.A plot is drawn between 4sinθalong the x-axis andβhkl cosθalong the y-axis as shown in Fig.1(c).From the linearfit to the data,the value of the strain is calculated from the slope of the line which is 0.00127±0.0003and the calculated crystallite size is42nm derived from the intersection of linear line with the vertical axis.Figure2shows typical TEM images of the BFO sample. The image indicates[Fig.2(a)]the particle sizes are in be-tween50–100nm,which is in accordance with the particle size calculated from the XRD.Figure2(b)shows images ob-tained from a portion of an individual BFO particle confirm-ing the good crystalline nature of BFO.Further,by using image analyzer software IMAGE-J on the lattice resolved TEM image,the distance between two parallel planes are found to be∼2.35Å.Figure3(a)shows the Differential Thermal Analysis (DTA)curve of BFO for the heating cycle at a rate of 10◦C/min.Two distinct peaks have been observed.A broad peak around330◦C corresponds to the magnetic order-ing[18].Though a small energy change is associated with the magnetic transition[19],but still it is reflected in the DTA curve.The same has been confirmed from the DSC measurements shown in insetfigure.The sharp peak at 837◦C corresponds to the ferroelectric to paraelectric tran-sition temperature of BFO[20].Figure4shows the polarized Raman spectrum of BFO in the frequency range100–1500cm−1.On decomposing thefitted curves into individual Gaussian components,the peak position of each component,i.e.,the natural frequency856M.Muneeswaran et al.Fig.2Transmission electron microscope images of (a )BFO powders.(b )Individual BFOparticleFig.3DTA curve of BFO powder and inset figure shows DSC mea-surement(cm −1)of each Raman active mode is as shown in Fig.4(a)and (b).At room temperature,BFO belongs to rhombohe-dral structure with the R3c space group with two formulas in one primitive cell.According to group theory,rhombohe-dral BFO has 18optical phonon modes [21]:Γopt .,R3c =4A 1+5A 2+9EThe A 1(TO)and E (LO)modes are Raman and in-frared active,while the 5A 2modes are Raman inactive modes [22]ΓRaman ,R3c =4A 1+9Ewhere,A 1and E are polar optical modes,which are Ra-man and IR active,and they can split into two modes:Longitudinal Optical (LO)and Transverse Optical modes (TO).Here,A 1-symmetry phonons are therefore longitu-dinal optical A 1(LO)while the E-symmetry phonons aretransverse optical E (TO).It has been reported that Bi–O bonds contribute mostly to A 1modes,first-and second-order E (TO)modes,and Fe–O bonds to third-and fourth-order E (TO)modes [23].In polarized Raman scattering,the A 1modes can be observed by parallel polarization,while the E modes can be observed by both parallel and crossed polarizations.Thus,the E mode is associated with the atomic motion in the “a ”and “b ”plane whereas the A 1mode is associated with the atomic motion along the “c ”axis.In our present study [shown in Fig.4(a)],we observe seven E (TO)and four A 1Raman modes,which are mentioned in Table 2along with other literature re-ports [24,25].The Raman scattering data clearly shows three intense peaks of A 1-1,A 1-2,and A 1-3modes ap-pearing at 138,170,and 214cm −1and a quite weak scat-tering intensity at 470cm −1corresponding to A 1-4mode;the modes having medium scattering intensities at 254,276,342,418,523,557,and 603cm −1assigned to E (TO)phonons.According to Yuan et al.[25],the stereo chemi-cal activity of Bi lone electron pair plays the main role in the change of both Bi–O covalent bonds,which is reflec-tive in five (E 1,A 1,A 2,A 3,and E 2)characteristic modes.These modes are responsible for the ferroelectric nature of the BFO.Most of the Raman studies on BFO are focused in the low frequency range,since all the A1and E modes fall within this low frequency region.Very few reports are avail-able at higher frequencies.Generally,the origin of the high-frequency modes in the Raman spectra is attributed to elec-tronic Raman scattering or the high-order phonon scattering [26,27].We measured the same and is as shown in Fig.4(c).Three Raman modes,namely,2A 4(LO),2E 8(TO),and 2E 9(TO)are observed at 960cm −1,1099cm −1,and 1261cm −1It has been reported that these high-frequency modes of BFO are overtones of the first-order A 4,E 8,and E 9phonon modes corresponding to 2A 4,2E 8,and 2E 9modes,respec-tively.The modes at 557cm −1(2E 8)is due to the Fe-O1Structural,optical,and multiferroic properties of single phased BiFeO3857Fig.4(a)Polarized Raman spectra of BFO.(b)A magnified view of the spectra range between100–650cm−1with their Gaussianfitted curve showing seven E modes and four A1modes.(c)Two phonon scattering observed between 850–1450cm−1Table2Observed and reportedRaman modes for BFO samples Raman modes(cm−1)Yang et al.[24]Yuan et al.[25]Present studyA1-1139152.6138A1-2172177.5170A1-3217224.2214A1-4470–470E262270254E275298.8276E307––E345354.9342E396––E429473.3418E521–523E–554.3557E615618603bonds and at603cm−1(2E9)assign to Fe–O2bonding, where O1are axial ions and O2are equatorial ions[22]. These two-phonon peaks are associated to the magnetic characters of BFO.The strong contribution of the two-phonon band to the total Raman spectrum has been at-tributed to a resonant enhancement with the intrinsic absorp-tion edge in BFO.This is similar to the two-phonon bands reported in hematite,-Fe2O3,the simplest case of iron ox-ides containing only FeO6octahedra[28].The temperature dependence of the dielectric constant of BFO measured at different frequencies is shown in Fig.5. The high values of the dielectric constant at low frequen-cies and low values at higher frequencies indicate large dis-persion due to a Maxwell–Wagner type of interfacial polar-858M.Muneeswaran et al.ization,in agreement with Koop’s phenomenological the-ory [29].A dielectric anomaly has also been observed in the temperature dependent dielectric studies for the all the fre-quencies around 315◦C at the vicinity of the Neel temper-ature (T N )of BFO.This dielectric anomaly may signify the coupling between the polarization and magnetization prop-erty of a multiferroic material.Below T N ,the material is ex-pected to be simultaneously ferroelectric and antiferromag-netic.The vanishing magnetic order on the electric order at the vicinity of T N leads to dielectric anomaly in magneto-electrically ordered systems as explained by the Landau–Devonshire theory of phase transitions [30,31].Similar di-electric anomaly in the vicinity of the Neel temperature for the both bulk BFO and thin films have been reported by sev-eral others [32,33].Fig.5Dielectric constant versus temperature plot for the BFO ceram-ics measured at various frequenciesTo study the magnetic properties of the BFO,we have measured the magnetization (M )as a function of applying magnetic field (H )at room temperature shown in Fig.6(a).The magnetic hysteresis loop of the BFO shows enhanced antiferromagnetic properties with saturated magnetization (M s ),remanent magnetization (M r ),and coercive field (H c )values of 0.11emu/gm,∼0.01emu/gm and ∼146.47Oe.To confirm the antiferromagnetic behavior of BFO,Arrott–Belov–Kouvel (ABK)plots shown in Fig.6(b)are drawn by using M –H data.The ABK plot exhibits a concave nature without any spontaneous magnetization at H =0,indicating a AFM-feature [34,35].4ConclusionsIn summary,a soft chemical coprecipitation method had been proposed for the synthesis of nanosized multiferroic BFO powders.The structural refinement of BFO reveals R3c crystal symmetry.From TEM analysis,the particle size of the BFO samples found to be between 50–100nm.The Dif-ferential Thermal Analysis (DTA)showed existence of mag-netic and ferroelectric ordering around 330◦C and 837◦C,respectively.Raman spectra of BFO over the frequency range of 100–1500cm −1showed 4A 1and 7E modes with the appearance of 2A 4,2E 8,and 2E 9modes corresponding to the two-phonon scattering.From the temperature depen-dent dielectric studies,an anomaly in the dielectric constant was observed at the vicinity of the Neel temperature (T N )indicating a magnetic ordering and coupling between polar-ization and magnetization in BFO.The magnetic studies on the BFO confirmed the antiferromagnetic behavior at roomtemperature.Fig.6(a )M–H hysteresis loop of BFO sample measured at room temperature.The inset figure shows partly enlarged M–H loop.(b )Arrott—Belov–Kouvel (ABK)plots for BFO powderStructural,optical,and multiferroic properties of single phased BiFeO3859Acknowledgements The authors would like to thank Dr.R.Na-galakhsmi for Rietveld refinement analysis and Dr.R.Justin Joseyphus for providing the thermal analysis facility.References1.R.Ramesh,N.A.Spaldin,Multiferroics:progress and prospects inthinfilms.Nat.Mater.6,21(2007)2.S.W.Cheong,M.Mostovoy,Multiferroics:a magnetic twist forferroelectricity.Nature6,13–20(2007)3.S.Shastry,G.Srinivasan,M.I.Bichurin,V.M.Petrov, A.S.Tatarenko,Microwave magnetoelectric crystal bilayers of yttrium iron garnet and lead effects in single magnesium niobate-lead ti-tanate.Phys.Rev.B70,064416(2004)4.N.A.Hill,Why are there so few magnetic ferroelectrics?J.Phys.Chem.104,6694–6709(2000)5.T.Kimura,wes,A.P.Ramirez,Electric polarization rotationin a hexaferrite with long wavelength magnetic structural.Phys.Rev.Lett.94,137201(2005)6. C.W.Huang,L.Chen,J.Wang,Q.He,S.Y.Yang,Y.H.Chu,R.Ramesh,Phenomenological analysis of domain width in rhom-bohedral BiFeO3films.Phys.Rev.B80,140101(2009)7.J.Wang,J.B.Neaton,H.Zheng,V.Nagarajan,S.B.Ogale,B.Liu,D.Viehland,V.Vaithyanathan,D.G.Schlom,U.V.Wagh-mare,N.A.Spaldin,K.M.Rabe,M.Wuttig,R.Ramesh,Epitaxial BiFeO3multiferroic thinfilm heterostructures.Science299,1719 (2003)8.Q.Xu,X.Zheng,Z.Wen,Y.Yang,D.Wu,M.Xu,Enhancedroom temperature ferromagnetism in porous BiFeO3prepared us-ing cotton templates.Solid State Commun.151,624–627(2011) 9.T.J.Park,G.C.Papaefthymiou, A.J.Viescas, A.R.Mooden-baugh,S.S.Wong,Size-dependent magnetic properties of single-crystalline multiferroic BiFeO3nanoparticles.Nano Lett.7,766–772(2007)10.S.M.Selbach,M.A.Einarsrud,T.Grande,Structure and propertiesof multiferroic oxygen hyper stoichiometric BiFe1−x Mn x O3+d.Chem.Mater.21,169(2009)11. B.Ramachandran,M.S.Ramachandra Rao,Low temperaturemagnetocaloric effect in polycrystalline BiFeO3ceramics.Appl.Phys.Lett.95,142505(2009)12.Y.Wang,G.Xu,L.Yang,Z.Ren,X.Wei,W.Weng,P.Du,G.Shen,G.Hanw,Alkali metal ions-assisted controllable synthe-sis of bismuth ferrites by a hydrothermal method.J.Am.Ceram.Soc.90,3673–3675(2007)13.J.Yang,X.Li,J.Zhou,Y.Tang,Y.Zhang,Y.Li,Factors con-trolling pure-phase magnetic BiFeO3powders synthesized by so-lution combustion synthesis.J.Alloys Compd.509,9271–9277 (2011)14.M.Y.Shami,M.S.Awan,M.A.Rehman,Phase pure synthesis ofBiFeO3nanopowders using diverse precursor via co-precipitation method.J.Alloys Compd.509,10139–10144(2011)15. C.T.Munoz,J.P.Rivera,A.Monnier,H.Schmid,Measurement ofthe quadratic magnetoelectric effect on single crystalline BiFeO3.J.Appl.Phys.Suppl.24,1051–1053(1985)16.J.S.Lee,R.J.De Angelis,X-ray diffraction patterns from anocrys-talline binary alloys.Nanostruct.Mater.7,805(1996)17.Z.Chen,C.Huang,Y.Qi,P.Yang,L.You,C.Hu,T.Wu,J.Wang,C.Gao,T.Sritharan,L.Chen,Low-symmetry mono-clinic phases and polarization rotation path mediated by epitaxialstrain in multiferroic BiFeO3thinfilms.Adv.Funct.Mater.21, 133–138(2011)18.W.Kaczmarek,Z.Pajak,Differential thermal analysis of phasetransitions in(Bi1−x La x)FeO3solid solution.Solid State Com-mun.17,807–810(1975)19.J.P.Gonjal,D.Avila,E.V.Castrejon,F.Garcia,L.Fuentes,R.W.Gomez,J.L.Mazariego,V.Marquinae,E.Moran,Structural,mi-crostructural and Mössbauer study of BiFeO3synthesized at low temperature by a microwave-hydrothermal method.Solid State Sci.13,2030–2036(2011)20.S.M.Selbach,M.A.Einarsrud,T.Tybell,T.Grande,Synthesis ofBiFeO3by wet chemical methods.J.Am.Ceram.Soc.90,3430 (2007)21.J.B.Neaton,C.Ederer,U.V.Waghmare,N.A.Spaldin,K.M.Rabe,First-principles study of spontaneous polarization in multiferroic BiFeO3.Phys.Rev.B71,014113(2005)22.R.Haumont,J.Kreisel,P.Bouvier,F.Hippert,Phonon anomaliesand the ferroelectric phase transition in multiferroic BiFeO3.Phys.Rev.B73,132101(2006)23.M.K.Singh,S.Ryu,H.M.Jang,Polarized Raman scattering ofmultiferroic BiFeO3thinfilms with pseudo-tetragonal symmetry.Phys.Rev.B72,132101(2005)24.Y.Yang,J.Y.Sun,K.Zhu,Y.L.Liu,J.Chen,X.R.Xing,Ramanstudy of BiFeO3with different excitation wavelengths.Physica B 404,171–174(2009)25.L.Yuan,S.Wing,H.Chan,Raman scattering spectra and ferro-electric properties of Bi1−x Nd x FeO3(x=0–0.2)multiferroic ce-ramics.J.Appl.Phys.101,064101(2007)26. A.Sacuto,J.Cayssol,P.Monod,D.Colson,Electronic Ramanscattering on the underdoped HgBa2Ca2Cu3O+δ8high-T c super-conductor:the symmetry of the order parameter.Phys.Rev.B61, 7122(2000)27.Y.J.Jiang,L.Z.Zeng,R.P.Wang,Y.Zhu,Y.L.Liu,Fundamentaland second-order Raman spectra of BaTiO3.J.Raman Spectrosc.27,31(1996)28.M.J.Massey,U.Baier,R.Merlin,W.H.Weber,Effects of pres-sure and isotopic substitution on the Raman spectrum ofα-Fe2O3: identification of two-magnon scattering.Phys.Rev.B41,7822 (1990)29. C.G.Koops,On the dispersion of resistivity and dielectric constantof some semiconductors at audio frequencies.Phys.Rev.83,121 (1951)30.R.Mazumder,P.Sujatha Devi,D.Bhattacharya,P.Choudhury,A.Sen,M.Raja,Ferromagnetism in nanoscale BiFeO3.Appl.Phys.Lett.91,062510(2007)31.P.Uniyal,K.L.Yadav,Observation of the room temperature mag-netoelectric effect in Dy doped BiFeO3.J.Phys.Condens.Matter 21,012205(2009)32.G.L.Yuan,S.W.Or,J.M.Liu,Z.G.Liu,Structural trans-formation and ferroelectromagnetic behavior in single-phase Bi1−x Nd x FeO3multiferroic ceramics.Appl.Phys.Lett.89, 052905(2006)33.V.R.Palkar,J.Jhon,R.Pinto,Observation of saturated polariza-tion and dielectric anomaly in magnetoelectric BiFeO3thinfilms.Appl.Phys.Lett.80,1628(2002)34.S.K.Mandaly,T.K.Nath,D.Karmakarz,Magnetic and opticalproperties of Zn1−x Fe x O(x=0.05and0.10)diluted magnetic semiconducting nanoparticles.Philos.Mag.88,265–275(2008) 35.T.K.Nath,N.Sudhakar,E.J.McNiff,A.K.Majumdar,Magneti-zation study ofγ-Fe80−x Ni x Cr20(14≤x≤30)alloys to20T.Phys.Rev.B55,12389(1997)。