Journal of Magnetism and Magnetic Materials 258­259 (2003) 19­24 Magneto-optical indicator film studyof the hybrid exchange spring formation and evolution processes V.I. Nikitenkoa,b,c,*, V.S. Gornakova,b, Yu.P. Kabanova, A.J. Shapirob, R.D. Shullb, C.L. Chienc, J.S. Jiangd, S.D. Baderd a Institute of Solid State Physics, Russian Academy of Sciences, Chernogolovka, Moscow District 142432, Russia b National Institute of Standards and Technology, Gaithersburg, MD 20899, USA c The Johns Hopkins University, Baltimore, MD 21218, USA d Argonne National Laboratory, Argonne, IL 60439, USA Abstract The elementaryevents of the remagnetization processes in nanocomposite magnetic bilayers were investigated using iron-garnet indicator films with in-plane anisotropy. We have observed hybrid domain walls consisting of both ferromagnetic and antiferromagnetic sections perpendicular to the interface. The external magnetic field shifts onlythe ferromagnetic part of the domain walls. This leads to the formation of a hybrid exchange spin spring parallel to the interface. The processes of spring nucleation and untwisting occur at different locations. With the field oriented antiparallel to the macroscopic unidirectional anisotropy, remagnetization of the soft ferromagnet layer in the hard/soft nanocomposite starts bythe formation of an exchange spring consisting of micrometer-scale sub-domains with opposite direction spin twisting. A rotating magnetic field (smaller than some critical value) creates firstlya single-chiral spin spiral; this spiral then loses stability, incoherently untwists and gradually inverts its chirality with increasing field rotation. Untwisting of the hybrid exchange spring at higher fields leads to the creation of unusual hybrid non-1801 domain walls. The initial (ground) state of the bilayer with such noncollinear magnetized domains is not restored after stopping the field rotation and returning it to zero. The revealed phenomena are attributed to the influence of the dispersion in the unidirectional anisotropy induced by magnetization frustration in the interface and bilayer crystal lattice defects. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Magnetic bilayers: domain walls; Exchange springs; Magnetic imaging The science and technologyof heterophase thin film inequivalent. The magnetization reversal of such a magnetic materials are currentlyattracting worldwide nanocomposite ferromagnetic bilayer is determined by attention because of the unique physical properties and the nucleation and evolution of hybrid spin spirals important practical applications of these materials [1,2]. (exchange springs) that differ dramaticallyfrom topo- Spin coupling across the interface between thin layers logicallystable domain walls (DWs) in bulk conven- possessing different magnetic order leads to a broken tional ferromagnets. magnetic symmetry. Ferromagnets of this type are For bulk materials, the sample size is greater than the characterized byhaving a unidirectional anisotropy: domain dimensions and much greater than the domain wherein opposite spin directions are energetically wall width. Large regions of uniform spin orientation in conventional bulk ferromagnets are separated by narrow topologicallystable spin spirals­domain bound- *Corresponding author. Institute of Solid State Physics, Russian Academyof Sciences, Chernogolovka, Moscow Dis- aries [3]. In conventional single-phase ferromagnets, the trict 142432, Russia. remagnetization process proceeds primarilybythe E-mail address: nikiten@issp.ac.ru (V.I. Nikitenko). motion of DWs as quasi-particles of classical nature. 0304-8853/03/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 2 ) 0 0 9 9 8 - 8 20 V.I. Nikitenko et al. / Journal of Magnetism and Magnetic Materials 258­259 (2003) 19­24 Theywere nucleated and formed in a verysmall part of we visualized for the first time the domain wall in the AF the sample. In contrast, the thickness of each ferromag- layer coupled to an FM DW, and studied the evolution netic layer in a nanocomposite bilayer is usually the of such a hybrid domain wall (HDW) in a magnetic same order of magnitude (or less) as the domain wall field. width. The remagnetization of the nanocomposite Fig. 2 shows the domain structure for the AC- ferromagnet in a heterophase bilayer proceeds primarily demagnetized AF/FM bilayer as the external field is through the formation process of hybrid spin spirals. cycled. One can clearly see the 1801 domains in the Experimental observation of the nucleation and initial ground state (Fig. 2a). Theyare revealed due to development process of DW-type spin spirals is an the well-defined black­white contrast on their bound- extremelyimportant but difficult problem. Even in aries as well as on the sample edges. The component of uniform bulk ferromagnets, until now, there has not the magnetization, M; perpendicular to the wall (as well been a direct experimental studyof the formation as to the sample surfaces), gives rise to magneto-static process. In this article we present the results of an poles, the type of which depends on the relative investigation of the formation and evolution of hybrid orientation of the magnetic moments in the domains. spin spirals in two types of heterophase nanocomposite In the described case, a head-to-head M position leads bilayers: (1) in exchange coupled ferromagnet/antiferro- to the black tint in the MOIF image of the wall. The magnet (FM/AF) thin films and (2) in magneticallysoft light tint appears in case of a tail-to-tail orientation of and hard ferromagnets (sF/hF). It was possible to do the M in the region adjacent to the DW. this using the advanced magneto-optical indicator film The FM layer magnetization changes with the H (MOIF) technique [4­6]. This technique is based on the application not onlybecause of displacement of the use of the Faradayrotation of linearlypolarized light in initial ground state DWs, but also due to nucleation of an indicator film with in-plane anisotropy, placed on the new walls (and their subsequent motion) in domains sample. Strayfields emanating from its edges, DWs, and originallymagnetized in the direction opposite to the other magnetic inhomogeneities deflect an indicator film field. The domain structure corresponding to the half- magnetization out of its plane. As a result, domain loop to the right of the origin in Fig. 1a is presented in structure of the sample is imaged via Faradayeffect in Fig. 2b­e. The corresponding pictures for the half-loop the indicator film. on the left side of Fig. 1a are shown in Fig. 2f­j. Note We studied a 160 (A thick Ni81Fe19 permalloyfilm that that the approach to saturation under positive fields was grown on an epitaxial Fe50Mn50(300 (A)/Cu(300 (A)/ (Fig. 2b and c) and remagnetization back into the Si structure [7]. It was cooled in zero magnetic field after demagnetized state (Fig. 2c­e) occur in a domain FM film demagnetization in a decreasing AC magnetic depicted in the central part of the picture, while under field at T ¼ 400 K, a temperature which is higher than negative fields (Fig. 2f­j) the remagnetization process the MnFe blocking temperature and lower than the proceeded onlyin the adjacent domains. Fig. 2 clearly Curie temperature of the permalloy. The magnetization evidences the asymmetry of the nucleation centers reversal of the FM in the magnetic field m0H ¼ 20 mT activity [8,9] during field cycling of this sample. was characterized bytwo separate hysteresis loops, each Note also in Fig. 2b that onlyone of the two adjacent shifted bythe same amount but in opposite direction walls separating domains in the FM in the demagnetized along the H-axis (Fig. 1a). This means that the FM film state moves upon application of positive fields. The DW contains regions with oppositelyoriented axes of configuration is drasticallydifferent during application unidirectional anisotropy. Using the MOIF technique, of negative fields (Fig. 2g). The most unexpected result Fig. 1. Minor hysteresis loops of Py(160 (A)/FeMn(300 (A) at 300 K (a) after demagnetizing at 400 K, and zero field cooling and (b) after field cooling in a 20 mT field. V.I. Nikitenko et al. / Journal of Magnetism and Magnetic Materials 258­259 (2003) 19­24 21 Fig. 1b), such faint DWs were not observed at satura- tion. Thus, the presented results clearlyshow that the MOIF technique permits one to investigate the domain structure in an AF thin film exchange coupled with an FM layer. AF domains arise under the influence of 1801 domains in the FM layer during cooling of the bilayer at temperatures below the blocking temperature point. The FM layer also plays the role of a sensitive sensor in which strayfield along the intersection of the interface and the AF DW is created. The ferrimagnet indicator film can visualize magneto-opticallythis ultra-weak strayfield and reveal the evolution of the HDW under the application of external magnetic fields. Stationary DWs in an AF stabilize the domain structure of an FM layer near H ¼ 0: Theybecome visible due to specific black and white contrast on graybackground when the movable section of the HDW in the FM moves away from them. A parallel to the interface exchange spin spring forms in this place connecting the shifted FM and the stationaryAF sections of the HDW. The bilayer domain structure and its evolution in the magnetic field are shown schematicallyin Fig. 3. In the ground state, the crystal contains a threading HDW (those penetrating the layers and lying normal to the magneto-coherent phase interface) that consist of FM and AF sections (Fig. 3a). Theyare formed near the blocking temperature (during the cooling process of the bilayer) when the magnetization vector directions in FM Fig. 2. MOIF images of domain structure taken during unidirectional-axis remagnetization of the zero field cooling Py/FeMn bilayer. (a)­(e) correspond to the right loop and (a), (f)­(j) to the left hysteresis loop at Fig. 1a. (a) m0H ¼ 0; (b) +1.8, (c) +6.0, (d) +0.6, (e) +0.6 after 20 s, (f) 1.15, (g) 1.2, (h) 6.0, (I) 0.4, (j) 0.35 mT, The top band perpendicular to the unidirectional axis is the edge of the bilayer. was the appearance of DW contrast even when the FM was magnetized to saturation (Fig. 2c and h), and this contrast was located in the same place where the DW had been located in the ground state (Fig. 2a). It is obvious that this contrast is associated with the stationaryHDW section, which is situated in the AF layer. In this sample cooled in a constant saturating field Fig. 3. Schematic diagram of the spin structures in the FM/ (characterized byone unidirectional anisotropy , AFM bilayer of Fig. 1 during remagnetization. 22 V.I. Nikitenko et al. / Journal of Magnetism and Magnetic Materials 258­259 (2003) 19­24 domains have determined the local orientation of the antiferromagnetism vectors. The external magnetic field exerts pressure onlyon the FM part of the HDW. Consequently, only the FM section moves while the AF section of the HDW remains stationaryas the field is applied. However, the rotating spins in the ferromagnetic section of the HDW twist the AF spins near the intersection of the interface and the HDW. This latter spin twisting leads to the formation and growth of the novel HDW segment that is located in Fig. 4. Hysteresis loop of SmCo/Fe bilayer. the AF/FM interface perpendicular to the stationaryAF and shifting FM sections of the HDW. This remagne- tized interface area is a specific exchange spin spring, consisting of FM and AF spins (Fig. 3b and c). Such a ``partial'' DW was considered in Ref. [10]. The intersec- tion of this partial DW with the DW in the ferromagnet is a high-energyboundaryline which possesses a unique spin configuration determined also bythe ``coherent'' spin configuration between the unchanged AF/FM interface on the other side of this line. This boundary line (node) plays a key role in determining the resistance to motion of the FM domain wall. In order for the FM DW to move, a hybrid exchange spring must be grown or eliminated on one side of this node depending on the direction of the wall motion. During growth of the exchange spring (i.e. as H is increased) the ground state is firstlyovercome in regions with low values of the AF ffiffiffiffiffiffiffiffiffiffi p ffi anisotropy Ka and exchange Aa energies: EE AaKa: Fig. 5. MO images of the SmCo/Fe bilayer region near the When the magnetic field decreases, its pressure on the 300 mM hole at different reverse fields. The field is applied at FM spins weakens so that at some critical field the angle a ¼ 01 (a­c) and 101 (d­f). amount of energystored in the spin spring becomes enough to drive its untwisting. This process starts in the regions where Ka and Aa are high (but not as low as it was during the spring nucleation). The heterogeneous byDC magnetron sputtering onto a Cr(200A)-buffered FM/AF exchange spring subsequentlybegins leaving the single-crystalline MgO(1 1 0) substrate (see Refs. [11,14] AF and graduallytransforming into a pseudo-mono- for details). In this material, the epitaxial growth of phase FM spin spring (a quasi-1801 DW in the FM). It SmCo gives rise to an in-plane uniaxial magnetic leads to the nucleation and growth of domains in the anisotropy and a high coercivity. The hysteresis loop FM layer and the sample's remagnetization back into of the sample (Fig. 4) is typical for exchange spring the ground state (Fig. 3d). magnets. In a soft ferromagnet/hard ferromagnet bilayer, the In order to analyze the magnetization processes of the spins in the soft film start gradual rotation at reverse exchange spring sample, a 300 mm hole was bored external magnetic fields larger than some critical value. through the magnetic layers and leakage fields around The angle of rotation increases with increasing distance the hole were monitored. The location and intensityof from the interface, resulting in a spin spiral similar to the bright and dark areas around the hole (Fig. 5) are that in a conventional Bloch DW. At the beginning of determined bythe direction and magnitude of the the remagnetization process, the exchange spring is leakage field and provide a means for estimating the localized mainlyin the soft ferromagnet of sF/hF orientation and value of the average magnetization M bilayer. But unlike in the AF/FM bilayer, its motion [13]. A set of MOIF images in Fig. 5 shows the can lead to domain formation in the second constituent remagnetization process of the Fe layer when the reverse of the composite. We have continued the MOIF field H is either aligned with the unidirectional investigation [11­13] of sF/hF nanocomposite remagne- anisotropyaxis (a­c) or deviates from it 101 in the tization to studyfeatures of the hybrid spin spiral clockwise (d­f) direction. If the applied field is perfectly formation and motion in exchange spring magnets. In aligned with the easyaxis (Fig. 5a­c) the average order to examine such a system, we applied the MOIF magnetization direction remains collinear with the H; technique to an Fe(500A)/Sm2Co7 (350 A) film prepared but there is a dramatic change in MO contrast during V.I. Nikitenko et al. / Journal of Magnetism and Magnetic Materials 258­259 (2003) 19­24 23 the reversal. It decreases, practicallydisappears and re- appears with a new sign as the reversed H increases in magnitude. Unlike theoretical predictions, in this case, as in the case of (1 0 0)-oriented MgO [12,13], the magnetization reversal process of a uniaxial sF/hF bilayer starts and proceeds bythe formation and evolution of a two- dimensional ES. This is a natural consequence of incoherent spin rotation with opposite chiralities in adjacent local areas upon increasing the field. The small- angle dispersion in the sF anisotropydirection caused by weak random deviations of the easyaxis in the grains of the bilayer system from the mean direction has a crucial effect on the thin structure of the exchange spring. A single-chiral spin spiral can be formed when the deviation of the remagnetizing field from the direction of the macroscopical unidirectional anisotropyexceeds the maximal easyaxis dispersion (Fig. 5d­f) or in a rotating field, as was revealed [13] in Fe/SmCo bilayer with an in-plane four-fold magnetic anisotropygrown on MgO (1 0 0) substrate. MOIF studies of the exchange-spring behavior in SmCo/Fe films with uniaxial in-plane anisotropyin a rotating magnetic field uncovered a new unusual phenomenon. Fig. 6 shows the response of the Fe layer when the applied field is fixed in magnitude (m0H ¼ 0:06 T) but is rotated byangle a in the film plane. From the easymagnetization direction (Fig. 6a), when a increases, the average magnetization smoothly rotates with the field with some phase delayin the same wayas in the sample with four-fold symmetry[13]. This is easilyrevealed bythe rotation of the symmetryof the hole magneto-optical contrast (Fig. 6b and c). After the field reaches a critical value of the angle a; the local magnetization in some places begin to rotate in the opposite direction while the field direction continues to Fig. 6. MO images of the SmCo/Fe bylayer region near the change in the original direction. This leads to nucleation hole during an in-plane field rotation for m0H ¼ 60 mT. The and growth of macroscopical domains with noncollinear white and black arrows indicate the directions of H and M; respectively. The graph shows the rotation angle j of M vs. the magnetizations that are bounded bynon-1801 walls magnetic field rotation angle (a) for different field amplitudes (Figs. 6b­e). The subsequent rotation of the total measured from the MOIF images. magnetization, after completion of this stage of DW motion and annihilation, is again in synchronization with the field. In this final stage of magnetization reversal, M takes the phase lead over H up to the point bythe motion of the line singularityconnected to the where theyboth coincide with the unidirectional intersection of the DW and the exchange spring. anisotropyaxis and the MO contrast in the sample However, the ground state does not restore after again becomes homogeneous (Fig. 6f). The single interrupting the field rotation and decreasing it to zero domain ground state is fullyrestored at this point. That if noncollinear DWs have arisen in the specimen (Fig. 7). there is hysteresis in the magnetization rotation is shown Upon decreasing the field there is no visible domain wall bythe data in Fig. 6g. motion, onlya weakening of its MO contrast is Fig. 6 shows DWs, which are formed because of observed. At small values of the magnetic field a clear untwisting of the exchange spring in different locations contrast of micro-scale domains elongated along the and subsequent adjacent changes in chirality. In a uniaxial anisotropydirection arises (Fig. 7b­d). This rotating field these specific domain walls shift, annihilate finding provides an evidence that the exchange spring and leave the sample, leading it again in the single- has penetrated into the hard ferromagnet and led to the domain state. In this case, the remagnetization is limited formation of micro-domains. These micro-domains 24 V.I. 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