Chien, CL

Gornakov, VS

Liu, K

Nikitenko, VI

Shapiro, AJ

Shull, RD

Zhou, SM

Macroscopic domain structures have been realized in wedged-permalloy (Py)/uniform-FeMn bilayers during magnetization reversal. When the exchange anisotropy is established perpendicular or parallel to the wedge direction, two macroscopic domains are observed. Separating these domains are a 180° wall in the perpendicular geometry and an intermediate band containing large density of stripe-type microdomains in the parallel geometry. While the exchange field remains practically the same in both geometries, the coercivity and squareness of the loop are much less in the parallel geometry

Liu, Kai

eScholarship - University of California

UC DavisUC Davis Previously Published WorksTitleAnisotropy-dependent macroscopic domain structure in wedged-permalloy/uniform-FeMn bilayersPermalinkhttps://escholarship.org/uc/item/0084v4m2JournalJournal of Applied Physics, 87(9)ISSN0021-8979AuthorsLiu, KaiZhou, SMChien, CLet al.Publication Date2000-05-01DOI10.1063/1.373245 Peer reviewedeScholarship.org Powered by the California Digital LibraryUniversity of CaliforniaJOURNAL OF APPLIED PHYSICS VOLUME 87, NUMBER 9 1 MAY 2000Anisotropy-dependent macroscopic domain structurein wedged-permalloy Õuniform-FeMn bilayersKai Liu,a) S. M. Zhou, and C. L. Chienb)Department of Physics and Astronomy, The Johns Hopkins University, Baltimore, Maryland 21218V. I. Nikitenko,c) V. S. Gornakov,c) A. J. Shapiro, and R. D. ShullMetallurgy Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899Macroscopic domain structures have been realized in wedged-permalloy~Py!/uniform-FeMnbilayers during magnetization reversal. When the exchange anisotropy is established perpendicularor parallel to the wedge direction, two macroscopic domains are observed. Separating these domainsare a 180° wall in the perpendicular geometry and an intermediate band containing large density ofstripe-type microdomains in the parallel geometry. While the exchange field remains practically thesame in both geometries, the coercivity and squareness of the loop are much less in the parallelgeometry. ©2000 American Institute of Physics.@S0021-8979~00!59208-1#,e-thgeThourenic--nodoe.roeyoonomszaye-avrode-ot-ase.ndintocalryllelea-y di-eascut, theldngge-lesinFig.ofingremefer-anderse inallel31ceSince its discovery in 1956 by Meiklejohn and Bean1exchange coupling across a ferromagnetic~FM!/antiferromagnetic~AF! interface has been an intriguing phnomenon. Despite extensive research efforts onsubject2–14 and the technological applications of exchancoupling in spin-valve field-sensing devices,15 the under-standing of exchange coupling remains unsatisfactory.early model assuming rigid AF spin structures has seridiscrepancies with experimental results, particularly withgard to the values of the exchange fieldHE and the coerciv-ity HC .1 Several recent models have featured both FM aAF domains and provided more realistic predtions.3,4,8,11,13,16However, direct observation of the AF domains in such systems has been intrinsically difficult andyet realized to date, whereas the detail study of the FMmain wall ~DW! formation and motion is hampered by thpresence of a large number of domains during switching17Recently, we have demonstrated that simple macscopic domain structures in wedged-Py (Ni81Fe19)/uniform-FeMn (Fe50Mn50) bilayers can be created due to thinverse dependence of the exchange field on the FM lathickness.12 We have shown that when the exchange anisropy is introduced perpendicular to the wedge directimagnetization switching involves only two macroscopic dmains. Such a system can be used as a model systeprobe the domain dynamics in exchange-coupled systemIn this work, we compare the features of the magnetition reversal processes in wedged-Py/uniform-FeMn bilasystem with the exchange anisotropy induced eitherperpen-dicular or parallel to the wedge direction. Using an advanced magneto-optical indicator film~MOIF! technique,9,11we have confirmed the simple domain structure and hinvestigated the transition region between the two macscopic domains.Specimens of Py~20–300 Å wedge!/FeMn~300 Å!/Cu~300 Å!/Si have been fabricated using the processa!Present address: Dept. of Physics, UCSD, La Jolla, California 92093-0b!Electronic mail: clc@pha.jhu.educ!Also at: Institute of Solid State Physics, Russian Academy of ScienChernogolovka, Russia 142432.5050021-8979/2000/87(9)/5052/3/$17.00Downloaded 28 Dec 2000  to 164.67.164.248.  Redistribution subject tees-dt--ert-,-to.-re--scribed earlier.12 The Cu buffer layer promotes the facecentered-cubic~fcc! ~111! growth of the AF FeMn. The di-mensions of the samples are about 5 cm32 cm, where thelong dimension is the wedge direction. Unidirectional anisropy, perpendicular or parallel to the wedge direction, hbeen introduced using a suitable field cooling procedur12We will refer to these two geometries as perpendicular aparallel geometry. The wedge specimen was then cuttwo halves along the wedge direction for magneto-optiKerr effect ~MOKE! and vibrating sample magnetomet~VSM! measurements, using samples along two parawedge directions separated by about 1 cm. The MOKE msurements were made on an uncut wedged specimen brecting the laser beam along the wedge direction, wherthe VSM measurements were made on small samplesfrom the wedge specimen. During these measurementsapplied magnetic field was always along the cooling fiedirection, along which the unidirectional exchange coupliwas set. Details of the experiments in the perpendicularometry can be found in our previous publication.12 Directexperimental study of the domain structure in the samphas been performed by using the MOIF technique.Representative hysteresis loops measured by VSMboth perpendicular and parallel geometries are shown in1. In each geometry, the amount of loop shift to the leftthe origin and the width of the loop increase with decreasPy thicknesstPy, i.e., both the exchange fieldHE and thecoercivity HC increase with decreasingtPy. Furthermore,with decreasingtPy, the loop becomes increasingly moslanted, the degree of which can be characterized byDH, thewidth of switching of magnetization between6M . Betweenthe perpendicular and the parallel geometries, for the saPy thickness, the values ofHE are similar, but the value oHC in the parallel geometry is smaller, and the value ofDHis much larger, than the corresponding quantities in the ppendicular geometry. One also notes that the decreasing-the increasing-field branches of the loop for thinner Py layare not symmetrical. The asymmetry is barely appreciablthe perpendicular geometry but more apparent in the pargeometry.9.s,2 © 2000 American Institute of Physicso AIP copyright, see http://ojps.aip.org/japo/japcpyrts.html.uliv-loiqgeenlldbsrynethes inithitheeningofn isandrec--ownnddgeofe-tab-o-be-teofontryionaticardsple,intoon-theormanlyinro-pen-atic.5053J. Appl. Phys., Vol. 87, No. 9, 1 May 2000 Liu et al.Previously, we have demonstrated in the perpendicgeometry that the hysteresis loops measured by MOKE gthe same values ofHE and HC as those of the VSM measurements taken at the same corresponding locations athe wedge direction.12 The fact that the results of the MOKEand VSM measurements are the same indicates the unmacroscopic domain structure in exchange-coupled wedPy/uniform-FeMn bilayer. Interestingly, the same agreembetween MOKE and VSM has been observed in the parageometry, as shown in Fig. 2, where the measured valuesHEand HC determined by MOKE~solid symbols! and VSM~open symbols! are shown. For comparison, the corresponing values in the perpendicular geometry measuredMOKE are shown as crosses. It is clear that the value ofHEat the sametPy is essentially the same in both geometriewhereasHC is always larger in the perpendicular geometThe 1/tPy dependence of the values ofHE and HC in bothFIG. 1. Representative magnetic hysteresis loops of wedged-Py/unifFeMn bilayers at various Py thicknesses measured by VSM with exchanisotropy perpendicular~left! and parallel~right! to the wedge direction.FIG. 2. Exchange fieldHE and coercivityHC of wedged-Py/uniform FeMnbilayers at various Py thicknessestPy . In the parallel geometry, the MOKEand VSM results are represented by solid and open symbols, respectivethe perpendicular geometry, results are represented by crosses. Theshows the inverse dependence ofHE andHC on tPy .Downloaded 28 Dec 2000  to 164.67.164.248.  Redistribution subject tarengued-tel-y,.geometries are shown in the inset of Fig. 2, in which onotes that the exchange fields in both geometries sharesame slope, whereas the coercivities have different slopethe two geometries.The states with1M and 2M are two single-domainstates with magnetization pointing in the1H and the2Hdirections, respectively. In an exchange-coupled bilayer wa uniform FM layer, during the switching from1M and2M , the entire FM layer breaks up into many domains wcomplicated patterns.18 However, in the presentwedged-Py/uniform FeMn system, where we have exploited the 1/tPydependence of the exchange coupling, the transition betw6M is restricted only to a small boundary region separatthe two domains with1M and2M .In the perpendicular geometry, direct MOIF imagingthe domain structure have shown that the transition regioa 180° wall separating two macroscopic domains~Fig. 3!.Different magnetostatic charges are imaged as brightdark contrast, delineating domains and magnetization ditions. The black-white contrast in the left part in Fig. 3 images the stray fields from left edge of the sample, as shin the schematic. Magnetization reversal from the groustate begins at the two corners of the thick end of the wewhere the exchange anisotropy field (HE) is minimal and themagnetostatic field (HMS) is maximal. Reversal into theground state begins from the central part of the thin endwedge whereHE is maximal andHMS is minimal. Furtherdetails of the domain formation and DW motion in this gometry are published elsewhere.19In the specimen where the exchange anisotropy is eslished parallel to the wedge direction, two macroscopic dmains are also observed. However the transition regiontween them is not a 180° DW. It is a band of intermediaregion of considerable width containing large densitystripe-type microdomains. The evolution of magnetizatireversal revealed by MOIF imaging in the parallel geomeis shown in Fig. 4. The imaged area is in the middle portof the thick end of the wedge, as illustrated in the schemin Fig. 4. Initially, the sample is fully magnetized, insingle-domain state with a positive field applied downwa~left schematic!. Decreasing the field to244 Oe, reversaldomains begin to nucleate along the thick end of the samwhere HE is minimal andHMS is maximal. The originalsingle domain in this part of the specimen has broken upmany small and narrow stripe-type domains, which are elgated perpendicular to the wedge direction. At256 Oe,magnetization has practically switched at the thick end of-ge. InsetFIG. 3. MOIF image showing the 180° domain wall separating two macscopic domains in the specimen where unidirectional anisotropy is perdicular to the wedge direction. The imaging area is shown by the schemThe magnetization direction is indicated by the white arrow.o AIP copyright, see http://ojps.aip.org/japo/japcpyrts.html.trgl dnftlythettizolinzemlleaob-y/er-hetiontheom-try,less.cu-llelf aofgevedheu-ag-gelo-veren aintoanflipbeeti-os.tt..pl..ys.L.ndpl..l iim5054 J. Appl. Phys., Vol. 87, No. 9, 1 May 2000 Liu et al.wedge, indicated by the change of contrast from brightdark at the sample edge. However, there are still a lanumber of stripe domains encompassed by the reversamains. This band of microdomains gradually moves alothe wedge direction~middle schematic!. At a large negativefield ~266 Oe!, the band of microdomains is driven out othe thick end and moves towards the thin end. Finally asufficiently large negative field, the Py layer is completemagnetized upwards~right schematic!.The unique domain structures are also manifested inshape of the hysteresis loops. In the perpendicular geoma 180° wall sweeps across the sample during the magnetion reversal process. Correspondingly, in a hysteresis lothe width (DH) of the switching between6M represents thefield increment needed to drive the wall across the samparea. We have shown that the width of switchingDH de-pends on both the Py layer thickness and the sample si12The exchange coupling with anisotropy in the parallel geoetry also affects the switching characteristics. In the parageometry, the intermediate band is much wider~hundreds ofmicrons! than the 180° wall in the perpendicular casswitching from one single-domain state to the other is reized over much broader field range.FIG. 4. MOIF images showing the evolution of magnetization reversathe center of the thick end of the wedge. The schematics illustrate theaging area and different stages of the magnetization reversal process.Downloaded 28 Dec 2000  to 164.67.164.248.  Redistribution subject toeo-gaery,a-p,g.-el,l-In summary, two macroscopic domains have beenserved during magnetization switching in wedged-Puniform-FeMn bilayers, with the exchange anisotropy ppendicular or parallel to the wedge direction. But tmacroscopic characteristics and kinetics of the magnetizareversal in these two types samples drastically differ. Atsame Py thickness, the exchange field in the parallel geetry is the same as that in the perpendicular geomewhereas the coercivity and the squareness are muchMagnetization switching in the samples with the perpendilar geometry proceeds by a 180° DW motion. In the parageometry, remagnetization occurs due to the formation owide band of intermediate region with a large numberstripe-type microdomains and its motion along the weddirection.The different magnetization reversal patterns obserhave two likely sources. First of all, it may be due to tdifferent magnetostatic field distribution. In the perpendiclar geometry, the magnetostatic field is determined by mnetic poles localized on the left and right sides of the wed~refer to the schematic in Figs. 3 and 4!. Whereas in theparallel geometry, it is determined by magnetic chargescalized on thin and thick ends of the wedge, as well as othe whole wedge surface. Second, there may have bechange of an exchange spring penetration mechanismthe AF layer during the FM magnetization reversal. It cinclude a spin orientation phase transformation by spinor flop processes.3,4,8,16Each of these mechanisms couldresponsible for the observed characteristics during magnzation reversal processes.This work has been supported by NSF Grant NDMR96-32526 and DMR97-32763.1W. H. Meiklejohn and C. P. Bean, Phys. Rev.102, 1413~1956!; 105, 904~1957!.2C. Tang, N. Heiman, and K. Lee, J. Appl. Phys.52, 2471~1981!.3D. Mauri, H. C. Siegman, P. S. Bagus, and E. Kay, J. Appl. Phys.62,3047 ~1987!.4A. P. Malozemoff, Phys. Rev. B35, 3679~1987!; 37, 7673~1988!.5J. Nogue´s, D. Lederman, T. J. Moran, and I. K. Schuller, Phys. Rev. Le76, 4624~1996!.6B. H. Miller and E. D. Dahlberg, Appl. Phys. Lett.69, 3932~1996!.7N. J. Gökemeijer, T. Ambrose, and C. L. Chien, Phys. Rev. Lett.79, 21~1997!.8N. C. Koon, Phys. Rev. Lett.78, 4865~1997!.9V. S. Gornakov, V. I. Nikitenko, L. H. Bennett, H. J. Brown, M. JDonahue, W. F. Egelhoff, R. D. McMichael, and A. J. Shapiro, J. ApPhys.81, 5215~1997!.10T. Ambrose and C. L. Chien, J. Appl. Phys.83, 6822~1998!.11V. I. Nikitenko, V. S. Gornakov, L. M. Dedukh, Y. P. Kabanov, A. FKhapikov, A. J. Shapiro, R. D. Shull, A. Chaiken, and R. P. Michel, PhRev. B57, R8111~1998!.12S. M. Zhou, K. Liu, and C. L. Chien, Phys. Rev. B58, R14717~1998!.13M. D. Stiles and R. D. McMichael, Phys. Rev. B59, 3722~1999!.14S. F. Zhang, D. V. Dimitrov, G. C. Hadjipanayis, J. W. Cai, and C.Chien, J. Magn. Magn. Mater.198Õ199, 468 ~1999!.15B. Dieny, V. S. Speriosu, S. S. P. Parkin, B. A. Gurney, D. R. Wilhoit, aD. Mauri, Phys. Rev. B43, 1297~1991!.16T. C. Schulthess and W. H. Butler, Phys. Rev. Lett.81, 4516~1998!.17J. C. Lodder,Handbook of Magnetic Materials, edited by K. H. J. Bus-chow ~Elsevier Science, Amsterdam, 1998!, p. 324.18Z. H. Qian, M. T. Kief, P. K. George, J. Sivertsen, and J. H. Judy, J. ApPhys.85, 5525~1999!.19V. I. Nikitenko, V. S. Gornakov, A. J. Shapiro, R. D. Shull, K. Liu, S. MZhou, and C. L. Chien, Phys. Rev. Lett.84, 765 ~2000!.n-o AIP copyright, see http://ojps.aip.org/japo/japcpyrts.html.

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Anisotropy-dependent macroscopic domain structure in wedged-permalloy/uniform-FeMn bilayers

Anisotropy-dependent macroscopic domain structure in wedged-permalloy/uniform-FeMn bilayers

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