Real-time magneto-optical indicator film images reveal distinct asymmetry in the motion of a single domain wall in a wedged-NiFe/uniform-FeMn bilayer due to the nucleation and behavior of an exchange spring in the antiferromagnetic layer. Magnetization reversal from the ground state begins at the thick end of the wedge where the exchange anisotropy field (HE) is minimal and the magnetostatic field (HMS) is maximal, whereas reversal into the ground state begins from the thin end where HE is maximal and HMS is minimal

Chien, CL

Gornakov, VS

Liu, K

Nikitenko, VI

Shapiro, AJ

Shull, RD

Zhou, SM

Liu, Kai

eScholarship - University of California

UC DavisUC Davis Previously Published WorksTitleAsymmetry in Elementary Events of Magnetization Reversal in a Ferromagnetic/Antiferromagnetic BilayerPermalinkhttps://escholarship.org/uc/item/2cs2k63nJournalPhysical Review Letters, 84(4)ISSN0031-9007AuthorsNikitenko, VIGornakov, VSShapiro, AJet al.Publication Date2000-01-24DOI10.1103/physrevlett.84.765 Peer reviewedeScholarship.org Powered by the California Digital LibraryUniversity of CaliforniaVOLUME 84, NUMBER 4 P H Y S I C A L R E V I E W L E T T E R S 24 JANUARY 2000Asymmetry in Elementary Events of Magnetization Reversalin a Ferromagnetic/Antiferromagnetic BilayerV. I. Nikitenko,1,2 V. S. Gornakov,1,2 A. J. Shapiro,1 R. D. Shull,1 Kai Liu,3 S. M. Zhou,3 and C. L. Chien31Metallurgy Division, National Institute of Standards and Technology, Gaithersburg, Maryland 208992Institute of Solid State Physics, Russian Academy of Sciences, Chernogolovka, Russia 1424323Department of Physics and Astronomy, The Johns Hopkins University, Baltimore, Maryland 21218(Received 19 August 1999)Real-time magneto-optical indicator film images reveal distinct asymmetry in the motion of a singledomain wall in a wedged-NiFe/uniform-FeMn bilayer due to the nucleation and behavior of an exchangespring in the antiferromagnetic layer. Magnetization reversal from the ground state begins at the thickend of the wedge where the exchange anisotropy field HE is minimal and the magnetostatic field HMSis maximal, whereas reversal into the ground state begins from the thin end where HE is maximal andHMS is minimal.PACS numbers: 75.70.Cn, 75.30.Et, 75.50.KjDomain wall (DW) nucleation and motion are amongthe most important phenomena in nonlinear physics, rang-ing from the reversal processes of magnetization in ferro-magnets, polarization in ferroelectrics, to the magnetic andelectric field response of superconductors. Domain wallsare most prominently featured in ferromagnets, for whichLandau and Lifshitz [1] proposed in 1935 an equation ofmagnetization dynamics and showed that the structure ofa 180± DW is determined by the competition between themagnetocrystalline anisotropy K and exchange A ener-gies. Within the wall width of d  ppAK , the spins aretwisted in a spiral creating a topologically stable exchangespring. In real ferromagnetic materials, the magnetizationreversal proceeds by incoherent spin rotation, which causesDW nucleation in the regions with maximal magnetostaticfield or crystal lattice defects [2].In the last few years there has been a great deal of in-terest in studying the unique properties of magnetic nano-structures, involving small magnetic entities whose sizesare comparable to the DW width d, in which there is acoexistence of ferromagnetic (FM) and antiferromagnetic(AF) exchange interactions [3–21]. One important classof nanostructures involves FM/AF bilayers [7–18], promi-nently featured in spin-valve field sensing devices withtechnological importance [19]. When a FM/AF bilayer iscooled in a magnetic field H to below the Néel tempera-ture, a unidirectional (exchange) anisotropy is created [3].This anisotropy is manifested by a shift of the hysteresisloop away from H  0 by the amount of the exchangebias field HE, and a significant increase in the coerciveforce HC of the FM [3,15–18]. To date, the investigationof the microscopic magnetization reversal mechanisms inthese systems has revealed drastic contradictions betweentheory and experiment in both the values of HE and HC.It is now well recognized that the understanding of theexchange-biased FM/AF thin film lies in the peculiaritiesof nucleation and motion of domain walls of both the con-stituent FM and AF layers. However, detailed experimen-0031-90070084(4)765(4)$15.00 ©tal study of the DW dynamics of the FM/AF bilayers hasbeen severely hampered by the complicated multidomainstructure, which occurs during switching, occurring withina narrow field range, from one single-domain state to an-other with an opposite magnetization.Recently, macroscopic domain structures in anexchange-coupled bilayer of wedged-permalloy Py Ni81Fe19uniform-FeMn Fe50Mn50 has been realizedby exploiting the inverse dependence of the HE on theFM layer thickness [16]. The magnetization switchingprocess involves only two macroscopic domains separatedby only one 180± DW, which moves along the wedgedirection. Taking advantage of this unusually simple do-main pattern, we have used the magneto-optical indicatorfilm (MOIF) technique [17] to investigate the details ofthe DW nucleation and motion.We have revealed in this work a distinct asymmetry ofthe DW motion in decreasing-field and increasing-fieldbranches of a field cycle. We have observed directexperimental evidence that DW nucleation during thedecreasing- and the increasing-field branches of thehysteresis loop proceeds at different locations of the FMlayer. Contrary to that in free FM layers, the magnetostaticfield stimulates DW formation in the exchange-coupledFM layer only in the decreasing-field branch. This distinctasymmetry in the elementary events of magnetizationreversal is due to the local penetration of the exchangespring into, and its withdrawal from, the AF layer,proceeding on different hysteresis loop branches.The sample used in this work has the structure ofPy110 Å 180 ÅFeMn300 ÅCu300 ÅSi and thedimension of 15 mm 3 6 mm, with the long directionbeing the wedge direction. The film has an in-planeunidirectional anisotropy established by field cooling withthe field applied perpendicular to the wedge direction.Magnetometry measurements and the MOIF imaging havebeen performed with the applied magnetic field H alsoperpendicular to the wedge. The MOIF imaging, which2000 The American Physical Society 765VOLUME 84, NUMBER 4 P H Y S I C A L R E V I E W L E T T E R S 24 JANUARY 2000measures the stray fields from the FM surface, has beenused to map out the domains and the DW motion.The hysteresis loop of the entire wedge sample mea-sured at room temperature in a vibrating sample magne-tometer is shown in Fig. 1(a) (the large scatter and slightlydifferent values in MMs at saturation is probably dueto the weak signal of a small sample). The correspond-ing domain patterns observed by MOIF are schematicallyshown in Fig. 1(b). Representative images taken at differ-ent stages during a hysteresis loop measurement are shownin Figs. [2–4]. The domain nucleation and DW motioncan be divided into different stages for both the decreasing-field and increasing-field branches of a hysteresis loop.In the decreasing-field branch [stages I–V in Fig. 1(b)],magnetization reversal from the fully magnetized state[stage I in Fig. 1(b)] occurs first at the two corner regionsof the thick end of the wedged-FM [stage II in Fig. 1(b)],due to the presence of magnetic poles as in the free FM.Figure 2 shows the magnetization reversal in the lowerright-hand corner of the thick end of the wedge. The MOIFtechnique detects the stray fields from a magnetic surfaceand nonuniform magnetization distribution inside thesample. Magnetic charges are revealed as dark and brightcontrast on the gray background in slightly uncrossedpolarizers, displaying the domains and the orientation ofthe magnetization within them. White arrows have beenplaced to show the magnetization direction. The samplein Fig. 2 was initially fully magnetized in a positive fieldof 14 mT, applied to the right [stage I in Fig. 1(b)]. Asshown in Fig. 2(a), upon decreasing the field to 25.5 mT,FIG. 1. Hysteresis loop (a) and schematics (b) of domain struc-ture at the different stages (I–VIII) of magnetization reversal ofa wedged NiFe/uniform FeMn bilayer.766small domains with opposite magnetization begin toappear, whereas the vast majority of the area is still mag-netized to the right. When the external field continues todecrease, the domain with a reversed magnetizationexpands and encompasses the thick FM end [Figs. 2(b)and 2(c)], consolidating the small reversed domains inFig. 2(a).The corner domain evolution represented byFigs. 2(a)–2(c) also occurs at the lower left-handcorner of the thick end of the wedge, i.e., two cornerdomains develop simultaneously at the thick end of theFM [stage II in Fig. 1(b)]. Their continued growth indecreasing magnetic field leads to the joining of the twodomains [stage III in Fig. 1(b)]. At this stage, the wedgedFM consists of only two macroscopic domains withopposite magnetizations, separated by a 180± wall [16].The resultant single DW is arc-shaped, bowing towardsthe thin end of the wedge.The arc-shaped DW is driven towards the thin end of thewedge with further decrease of the magnetic field. Nearthe end of the magnetization reversal process, not two butonly one domain is driven out of the sample in the middlesection of the thin end of the wedge [stage IV in Fig. 1(b)].This is clearly illustrated by examining the central regionof the thin end as shown in Figs. 3(a)–3(c), where onlyone domain is driven out.In the increasing-field branch of the hysteresis loop,from V to VIII [Fig. 1(b)], on a macroscopic level, themagnetization reversal seems to be just the stages of II toV in reverse. However, a closer examination shows crucialFIG. 2. MOIF images of domain structure at the lower right-hand corner of the thick end of the wedge at various points ofthe hysteresis loop corresponding to stages II (a)– (c) and VIII(d)– (f ) in Fig. 1. The white arrows indicate the direction ofmagnetization.VOLUME 84, NUMBER 4 P H Y S I C A L R E V I E W L E T T E R S 24 JANUARY 2000FIG. 3. MOIF images of domain structure in the center of thethin end of the wedge at various points of the hysteresis loopcorresponding to stages IV (a)– (c) and VI (d)– (f ) in Fig. 1.The white arrows indicate the direction of magnetization.differences between the increasing-field and decreasing-field branches due to unusual DW nucleation features andthe asymmetry in the DW motion. The magnetization re-versal process starts from a saturated magnetization point-ing to the left [Fig. 3(c) and stage V in Fig. 1(b)]. Asthe field is increased to 26.6 mT, initial nucleation ofFIG. 4. MOIF images of domain wall moving in opposite di-rections at stages III (a)– (c) and VII (d)– (f ) of the hystere-sis loop in Fig. 1. The white arrows indicate the direction ofmagnetization.the reversal domain occurs in the middle part of the thinend [Fig. 3(d)], but at a different location from the onethat the domain disappeared during the decreasing-fieldbranch of the loop [Fig. 3(b)]. As the magnetic field fur-ther increases, the newly reversed domains with magneti-zation pointing to the right extend along the thin end of thewedge to form a continuous domain. At the same time, theDW moves towards the thick end of the wedge [Figs. 3(e)and 3(f), stage VI in Fig. 1(b)]. Both the domain shapeand DW dynamics differ from those observed during thedecreasing-field branch. The DW is zigzag in shape inthe field-decreasing process, is more mobile, and has alarge propensity to creep [Fig. 3(a)]. In the field-increasingprocess, the DW is much more rounded in shape, indicat-ing a different distribution of the localized magnetostaticcharges.As the field is further increased, the DW moves to-wards the thick end of the wedge, also arc-shaped as be-fore, except that the central part of the DW is the leadingone [stage VII in Fig. 1(b)]. When this central part ofthe macroscopic DW reaches the thick end of the wedge,two corner domains remain in the sample as shown inFigs. 2(d)–2(f) [stage VIII in Fig. 1(b)]. Eventually DW’sof these two domains are driven out of the wedge corners[Figs. 2(d)–2(f)], and the sample returns to the fully mag-netized ground state shown in stage I of Fig. 1(b).In stages III and VII of Fig. 1(b) there is an arc-shapedDW. The central part of the DW is perpendicular tothe wedge direction, but with larger curvature at bothends of the DW. This is illustrated in Fig. 4, showingthe motion of the curved DW near the sample edge.Comparing the domain pattern of the same region forthe decreasing [Figs. 4(a)–4(c)] and increasing branches[Figs. 4(d)–4(f)], one immediately notices a pronouncedasymmetry in the DW-pinning site interactions. In theincreasing-field branch, the DW is strongly pinned by arectilinear crystal defect, whereas in the decreasing-fieldbranch this defect does not influence the DW motion.The key observation revealed by the MOIF images isthe acute asymmetry in the DW nucleation, its motion,and its interactions with crystal lattice defects. It hasbeen revealed in every region of the sample as shown inFigs. 2–4. This asymmetry in the DW behavior is uniqueto exchange-coupled AF/FM bilayers. In either a singleFM layer or a single AF layer, such asymmetry does notexist. As described below, the observed asymmetry inFM/AF bilayers reveals the presence of an AF domainwall, which has also been indicated by more recent mi-cromagnetic models.In the first model of exchange coupling of FM/AFbilayers, the AF layer was assumed to have a staticspin structure with an uncompensated interfacial spinstructure [3]. During switching of the exchange-coupledFM layer, the spin structure of the AF layer re-mains unchanged. Such models with a static AF spinstructure could not explain asymmetry in the DW mo-tion. The pinning centers for the DW would be the767VOLUME 84, NUMBER 4 P H Y S I C A L R E V I E W L E T T E R S 24 JANUARY 2000same for both increasing-field and decreasing-fieldbranches, such as those encountered in a free FM layer.The observed asymmetry is incompatible with modelshaving a static AF spin structure.A number of subsequent models of FM/AF exchangecoupling have concluded the existence of DWs in the AF[7,8,11,17,18]. When the magnetization is reversed by anexternal field, the AF moments at the interface will be ro-tated with the FM moments, with spiraling AF momentsbelow the interface, similar to that of the exchange springin a DW of a ferromagnet [1] or in the exchange-springmagnets [20,21]. While direct observation of such a spe-cific exchange spring in the AF layer has not been pos-sible, the asymmetry of the DW behavior in the FM layerrevealed here gives strong, albeit indirect, evidence of theformation of the proposed spin structure during bilayermagnetization reversal. However, even the recent models[7,11,14] did not take into account the inhomogeneity ofmagnetization reversal processes of the FM layer. As de-scribed below, our results indicate the formation of a newtype of hybrid AF/FM domain wall.During the magnetization reversal process, the nuclea-tion and motion of the hybrid DW involve twisting anduntwisting of the spins in both the FM and AF layers.In the decreasing-field branch of the hysteresis loop, theDW nucleation begins in the thick parts of the FM layer(Fig. 2), where the exchange field HE is minimal. In thiscase, the magnetostatic field increases the force exertedby the external field on the FM spins and thus on theferromagnetic part of the hybrid DW. At higher magneticfields, the exchange spring overcomes the pinning sites inthe FM and penetrates into the AF layer. Therefore, DWformation in the FM initiates at the locations with bothminimal HE (thick end of the wedge) and maximal HMS(the two corners at the thick end).In the increasing-field branch of the hysteresis loop,magnetization reversal begins in the thin region of thesample where HE is maximal. The large magnitude of theinterface exchange field initiates the untwisting of the ex-change spring. Since the untwisting occurs first at placeswith the largest anisotropy energy inside the AF layer, it isnot necessary to nucleate the DW in both the FM and AFlayers. Rather, the DW in the AF now moves towards andpenetrates into the FM and nucleates new domains withthe ground state magnetization. Further untwisting of theexchange spring gives rise to motion of the hybrid AF/FMDW toward the thick end of the wedge. The magneto-static field in the increasing-field branch of the hysteresisloop acts against the untwisting of the exchange spring.Therefore, DW formation in a FM initiates at sample loca-tions with both maximal HE and minimal HMS (center ofthin end).The observed asymmetry of the DW motion shown inFig. 4 is another manifestation of the antiferromagneticDW, which is sensitive to local anisotropy changes causedby crystal defects in the AF layer. If a defect is to decrease768(increase) the local crystalline anisotropy, the energy asso-ciated with the spin twisting decreases (increases). Duringthe untwisting process, the AF spins, and consequently theFM spins, will rotate less readily (more easily), leading topinned (unhindered) DW motion.In summary, we have directly observed macro-scopic domain structures in a wedged-Py/uniform-FeMnexchange-coupled bilayer with the anisotropy directionperpendicular to the wedge direction. Magnetizationreversal from a fully magnetized state starts at the thickcorners of the wedge where the exchange energy isminimal and the magnetostatic energy is maximal. Thetwo edge domains then join and form a macroscopicreversal domain, separated from the original domain by a180± macroscopic DW. The DW propagates toward andeventually vanishes at the thin end of the wedge where theexchange energy is maximal and the magnetostatic energyis minimal. The observed asymmetry in the DW motion,incompatible with a static AF spin structure, indicatesthe presence of a mobile DW (exchange spring) in theantiferromagnet.Work at JHU was supported by NSF Grant No. DMR96-32526.[1] L. D. Landau and E. M. Lifshitz, Phys. Z. Sowjetunion 8,153 (1935).[2] See, e.g., W. F. Brown, Jr., Micromagnetics (Interscience,New York, 1963).[3] W. H. Meiklejohn and C. P. Bean, Phys. Rev. 102, 1413(1956); 105, 904 (1957).[4] P. Grünberg et al., Phys. Rev. Lett. 57, 2442 (1986).[5] S. S. P. Parkin, N. More, and K. P. Roche, Phys. Rev. Lett.64, 2304 (1990).[6] J. Unguris, R. J. Celota, and D. T. Pierce, Phys. Rev. Lett.67, 140 (1991).[7] D. Mauri et al., J. Appl. Phys. 62, 3047 (1987).[8] A. P. Malozemoff, Phys. Rev. B 35, 3679 (1987); J. Appl.Phys. 63, 3874 (1988).[9] A. Berger and H. Hopster, Phys. Rev. Lett. 73, 193 (1994).[10] J. Nogues et al., Phys. Rev. Lett. 76, 4624 (1996).[11] N. C. Koon, Phys. Rev. Lett. 78, 4865 (1997).[12] N. J. Gökemeijer, T. Ambrose, and C. L. Chien, Phys. Rev.Lett. 79, 4270 (1997).[13] K. Takano et al., Phys. Rev. Lett. 79, 1130 (1997).[14] T. C. Schulthess and W. H. Butler, Phys. Rev. Lett. 81, 4516(1998).[15] D. V. Dimitrov et al., Phys. Rev. B 58, 12 090 (1998).[16] S. M. Zhou, Kai Liu, and C. L. Chien, Phys. Rev. B 58,R14 717 (1998).[17] V. I. Nikitenko et al., Phys. Rev. B 57, R8111 (1998).[18] M. D. Stiles and R. D. McMichael, Phys. Rev. B 59, 3722(1999).[19] B. Dieny et al., Phys. Rev. B 43, 1297 (1991).[20] Eckhart F. Kneller and Reinhard Hawig, IEEE Trans.Magn. 27, 3588 (1991).[21] E. Fullerton et al., Phys. Rev. B 58, 12 193 (1998).

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Asymmetry in Elementary Events of Magnetization Reversal in a Ferromagnetic/Antiferromagnetic Bilayer

https://escholarship.org/uc/item/2cs2k63n

Asymmetry in elementary events of magnetization reversal in a ferromagnetic/antiferromagnetic bilayer.

Asymmetry in elementary events of magnetization reversal in a ferromagnetic/antiferromagnetic bilayer.

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