PHYSICAL REVIEW B 68, 014418 2003 Hybrid domain walls and antiferromagnetic domains in exchange-coupled ferromagnetÕantiferromagnet bilayers C. L. Chien,1 V. S. Gornakov,2,3 V. I. Nikitenko,1,2,3 A. J. Shapiro,2 and R. D. Shull2 1Department of Physics and Astronomy, The Johns Hopkins University, Baltimore, Maryland 21218, USA 2National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA 3Institute of Solid State Physics, Russian Academy of Sciences, Chernogolovka 142432, Russia Received 12 August 2002; revised manuscript received 21 March 2003; published 15 July 2003 Magneto-optical imaging has revealed new features crucial for the understanding of the exchange bias phenomenon. We have observed hybrid domain walls consisting of ferromagnetic FM and antiferromagnetic sections and their evolution. The external magnetic field moves only the ferromagnetic section of the hybrid domain walls, leading to the formation of an exchange spring parallel to the interface. The nucleation and unwinding of the exchange spring occur at different locations and its propagation depends on the chirality of the FM domain walls. The stationary antiferromagnetic sections of the hybrid domain walls define the antifer- romagnetic domains. DOI: 10.1103/PhysRevB.68.014418 PACS number s : 75.70.Cn, 75.30.Et, 75.50.Kj The phenomenon of exchange bias between a ferromagnet hybrid FM/AF domain walls in the ground state. These ob- FM and an antiferromagnet AF has been intensely studied servations demonstrate that the underlying AF spin structure in recent years,1­4 prompted by the intriguing physics and its forms an exchange spring, whose chirality dictates the asym- prominent role in spin-valve field sensing devices.5 A unidi- metry. rectional exchange anisotropy in the FM, as revealed by a We have used a common FM/AF bilayer of shifted hysteresis loop, can be established in a FM/AF bi- Ni81Fe19(160 Å)/FeMn(300 Å) grown on a Cu seed layer of layer, most commonly by cooling the bilayer in a static ex- 300 Å deposited on a Si substrate. All layers are polycrystal- ternal magnetic field from high to low temperatures, during line with grains several hundred angstrom in sizes. When the which the AF order develops. Despite its importance, the bilayer is cooled in the usual way under a static magnetic understanding of the physics of exchange bias remains un- field from 400 to 300 K with a single-domain FM, the hys- satisfactory. However, it has been recognized that the nature teresis loop measured by a vibrating sample magnetometer of the AF spin structure holds the key to the understanding of VSM at 300 K is shifted from H 0, as shown in Fig. 1 a . exchange bias. However, as discussed below, this most commonly achieved Most theoretical models and experimental investigations ground state is not suitable for revealing the underlying AF of exchange bias have assumed a static AF spin structure for spin structure. Instead, when the same sample is first ac de- simplicity.1­4 The exchange bias is the result of interactions magnetized in a field of decreasing magnitude to zero total between the FM layer and the uncompensated interfacial AF magnetization at 400 K, and then cooled in zero magnetic spin structure, while the remaining AF spin structure is as- field to 300 K, one observes two loops, shifted to either sides sumed to be unchanged throughout the magnetization rever- of H 0, as shown in Fig. 1 b .13 This is because the ac sal process of the FM layer. Other theoretical and experimen- demagnetization creates large stripe domains with opposite tal studies of exchange bias have concluded that the AF spin magnetizations, which acquire bias fields of opposite sign, as structure could not be static.4,6­11 In particular, Mauri et al. schematically shown in the inset of Fig. 1 b . This special first showed that when the magnetization of the FM is re- ground state of stripe FM domains reveals the key features of versed, the subsequent AF layers fan out into a spiraling AF exchange bias. spin structure as an exchange spring near the FM/AF interface.6 Despite their crucial roles in exchange bias, the underlying AF spin structure and AF domains are not readily accessible to most experimental investigations. Furthermore, it has been demonstrated that the AF spin structure is drasti- cally altered after the FM layer has been deposited.12 Thus, the AF spin structure must be studied in FM/AF bilayers rather than AF layers in isolation. The understanding of the exchange bias phenomenon and the central issue of the AF spin structure can be addressed through the reversal processes of the FM layer exchange coupled to the AF layer. In this work, we report on the FIG. 1. Hysteresis loop of Ni observation of an acute asymmetry in the nucleation and 81Fe19(160 Å)/FeMn(300 Å) at 300 K after a cooled in 1 T field from 400 to 300 K, and b ac orientation of the FM domain walls DW during reversal. demagnetized at 400 K and cooled in zero field to 300 K. Points We have also revealed the AF domains that are bounded a,b,c, etc. correspond to the domain patterns in Fig. 2. The domain by stationary AF domain walls and the existence of pattern and the image area are shown in the inset. 0163-1829/2003/68 1 /014418 5 /$20.00 68 014418-1 ©2003 The American Physical Society CHIEN, GORNAKOV, NIKITENKO, SHAPIRO, AND SHULL PHYSICAL REVIEW B 68, 014418 2003 The magneto-optical indicator film MOIF technique has been used to observe on a microscopic level the reversal processes. Imaging techniques that use an electron beam as in scanning electron microscopy, or a magnetic tip as in mag- netic force microscopy, are strongly susceptible to external magnetic fields. These imaging techniques are usually ad- ministered in zero external field. In contrast, the MOIF tech- nique allows the application of an external magnetic field during imaging throughout the reversal process. As described elsewhere, the MOIF method uses an iron garnet film with in-plane magnetization and a large Faraday effect placed di- rectly above the sample.14 Any stray magnetic field in the sample due to magnetization discontinuities at the sample edge, at FM domain walls, and at atomic defects causes magneto-optical Faraday effect in the indicator film. In the domain pattern below, the magnetization in each domain is directed from the ``black'' edge towards the ``white'' edge. The MOIF microscope was focused on one area near the edge of the sample capturing three FM domains with two domain walls, as shown in Fig. 2 a . The image area is about 1 mm 1.5 mm, and thus the size of the FM domain is macroscopically large. There are thousands of grains in each sub-mm-sized magnetic domain. When an external magnetic field is applied along the exchange anisotropy axis, the evolution of the domains are shown in Fig. 2 corresponding to the specific points on the double hysteresis loop, shown in Fig. 2 b in the order of a b c d a e f g a. We first describe the domain evolution for the positive loop. Starting from the demagnetized state of a shown in the middle of Fig. 2, upon applying a positive magnetic field H of increasing magnitude, the pattern evolves from a to c via FIG. 2. MOIF image of domain patterns taken at various points b, where the central ``down'' domain reverses to ``up.'' This (a,b,c, etc. on the double hysteresis loop of Fig. 1 b with the occurs with the invading domains shown as white arrows magnetic field 0H of a 0, b 1.8, c 6, d 0.6, e 1.2, f consuming the ``up'' regions. Returning from c to a via d, by 6, g 0.35 mT. The black arrows indicate the magnetization applying a positive H of decreasing magnitude, the invading direction of the domains, whereas the white arrows indicate those of ``down'' domains reverse the central ``up'' domain. For the the invading domains. negative loop, the domain pattern changes from a to f via e. The invading ``down'' regions, marked by the white arrows, asymmetry in both domain nucleation and domain wall occur in the two outer domains until state f is reached. Upon propagation for forward and backward reversal. The acute returning from f to a via g, the two outer ``down'' domains asymmetry observed in the FM/AF bilayer vividly demon- reverse to ``up.'' strates that the underlying AF spin structure must not be In an isolated FM layer without the AF layer, at symmet- static. ric points of the hysteresis loop, the domain patterns are the MOIF studies of the reversal processes of exchange- same but with reversed magnetization. This is because the coupled FM/AF bilayers uncover yet another unusual same domain walls are pinned by the same static defects in phenomena. In Fig. 2, one observes by comparing b both forward and backward reversal. In the FM layer with d for the central domain that the invading domains exchange coupled to the AF layer, the domain patterns are not along the easy axis. The invading domains in b slant of d and e in Fig. 2 likewise by comparing b and g) to the left, while the invading domains in d slant to the right. are totally different. The domain patterns in Fig. 2 also reveal The same conclusion is reached by comparing e and g several new aspects of exchange bias. There is an asymmetry for the invading domains in the two outer domains. Thus, in the nucleation of the domains for forward and backward there is a distinct chirality, an evidence of an exchange reversal. During reversal, only one of the two DW's spring that winds and unwinds during forward and backward is shifted to accommodate the invading domains. This reversal. is clearly shown in b, in which the invading domains nucle- In FM layers, isolated or exchange coupled, all the FM ate and propagate from the right DW, whereas in d the domain walls can be swept by an external magnetic field and domains nucleate from the left DW. The same conclusion the FM becomes a single domain. It is particularly revealing is reached by comparing e and g. Thus, there is an acute to compare Fig. 2 a , in which two FM DWs separate three 014418-2 HYBRID DOMAIN WALLS AND ANTIFERROMAGNETIC . . . PHYSICAL REVIEW B 68, 014418 2003 FM domains with opposite magnetizations, with the single- domain states of 2c and 2 f of opposite magnetization. That c and f are single FM domains is unequivocal by noting the white edge in c and black edge in f. Most remarkably, even in the single-domain FM of c and f, there are still weaker but clearly visible contrasts at the original locations of the FM DW. In fact, an inspection of all the domain patterns in Fig. 2 shows that within the field range of about 2 mT to 2 mT, these contrasts at the same locations are always present regardless of the state of the FM domains or the magnetic field applied. These are the indications of the stationary AF DWs, which are not swept by the applied field, thus revealing the underlying AF domains. The weaker contrast of the AF DW is due to FM spin frustration near its intersection with the FM/AF interface. In this regard, the FM layer plays the role of a sensitive sensor through which stray fields at the AF DW can be detected. The fact that the faint contrasts at the original locations of the FM DW persist, as shown in Figs. 2 c and 2 f , after the FM layer has been aligned into a single domain state by a field of about 2 mT warrants further investigation. One might suspect that the faint contrasts might be due to certain irregu- larities in the FM layer, instead of the stationary AF DW to which we have attributed. Further imaging has been per- formed under a larger magnetic field. As shown in Fig. 3 a , these faint contrasts disappear at a larger field of 9 mT, indicating that all the FM moments, including those at the original FM DW, have now been aligned. However, at lower magnetic fields, the same faint contrasts return to the same locations, as can be barely observed at 4.2 mT Fig. 3 b and clearly observed at 1.2 mT Fig. 3 c . The same con- clusion is reached using a negative magnetic field. When all the FM moments have been aligned at 9 mT Fig. 3 d , the same contrasts return at 1.2 mT, as shown in Fig. 3 f . To further illustrate this point, the same area in Figs. 3 c and 3 f has been enlarged four times, as shown on the right. The comparison shows that the structure of the FM DW has the same details even though the overall magnetization axes are opposite in the two cases. These results demonstrate clearly that, when a large field e.g., 9 mT sweeps the FM layer FIG. 3. MOIF images of domain patterns over the same area as including those FM moments directly above the AF DW in Fig. 2, but at larger magnetic field 0H of a 9, b 4.2, c 1.2, into alignment, the external magnetic field has no effect on d 9, e 4.2, f 1.2 mT. The black arrows indicate the the underlying AF DW. When the external field is reduced magnetization direction of the domains. The same area in c and f have been enlarged four times, showing the detailed domain wall to within the lower field range of 2 to 2 mT, the faint structures. contrasts return and persist in the top FM layer, as shown in Fig. 2. The faint contrasts indeed mark the location of observed. It should be emphasized that the sizes of the FM the underlying AF DW, which cannot be otherwise directly domains and the AF domains here are several mm in length observed. and a fraction of mm in width. These dimensions are three In isolated polycrystalline AF thin films, the grain size orders of magnitude larger than the grain sizes of the FM and usually defines the AF domain, each of which has its own the AF layers. The switching behavior and the domain pat- anisotropy axis. However, in FM/AF bilayers, the FM sets tern that we have observed are, therefore, unrelated to the the anisotropy axis of the AF during field cooling. A major grain sizes of the magnetic layers. conclusion of our studies is that the AF domains have the The microscopic model that encompasses the experimen- same size as that of the FM domains. In FM/AF bilayers, tal results is schematically illustrated in Fig. 4. In the ground using the usual field cooling under a constant magnetic field state, showing in Fig. 4 a , there are striped domains sepa- e.g., Fig. 1 a , there would be only one single AF domain rated by the DWs. However, each DW is a hybrid domain spanning the entire sample. The stationary AF DWs in this wall consisting of the FM DW and the AF DW connected by case would be the edges of the sample, and thus cannot be a line singularity as schematically shown in Fig. 4 b , which 014418-3 CHIEN, GORNAKOV, NIKITENKO, SHAPIRO, AND SHULL PHYSICAL REVIEW B 68, 014418 2003 It is well known that DWs move readily in isolated FM films. In exchange-coupled FM/AF bilayers, however, the moving FM DW carries with it the mobile line singularity, which reconstructs the undisturbed AF region into an ex- change spring. This process impedes the FM DW motion during the winding of the exchange spring. This process con- tinues until the FM layer is a single domain. When the ex- ternal field decreases, its pressure on the FM spins lessens, such that at some critical fields, the stored energy in the exchange spring becomes sufficiently large for its un- winding. This process begins at regions where the anisotropy and exchange energies are the highest. At that, the heteroge- neous FM/AF exchange spring begins to retrieve, and leading to nucleation and growth of the domains in the FM layer until the ground state is reached. It should be noted that the unwinding of the exchange spring occurs at the regions where anisotropy and energies are the highest as opposed to being the lowest during exchange spring nucleation. Thus, the unwinding of the exchange spring is not winding in reverse, but is instead occurring at different locations as observed. This is the basis for the asymmetrical reversal and the asymmetrical domain growth in exchange-coupled systems. Finally, we discuss the origin of the domain wall orienta- tion revealed in this work. It is known that the DW can be one of two chiralities where the spins are twisted clockwise or counterclockwise, or a mixture of both chiralities sepa- rated by the Bloch line singularities. As shown in Fig. 4 c , the moving FM DW causes an exchange spring penetrating into the AF with a depth of , in which the spins are twisted according to the chirality of the FM DW. From this point on, the mobility and orientation of the FM DW is controlled by the spin singularity in the exchange spring. During rever- FIG. 4. Color online Schematic representations of the striped domain structure a in the ground state of the FM/AF bilayer. The sal, the magnetic field of decreasing magnitude or of boxed region in a is magnified in b showing a hybrid FM/AF opposite sign leads to different chirality in the exchange domain wall consisting of the FM domain wall and the AF domain spring, which nucleates at different locations with a different wall. c During reversal of the FM, an exchange spring is formed orientation. within the AF that connects to the moving FM domain wall at one In summary, using a FM/AF bilayer with a ground state end and the stationary AF domain wall at the other. consisting of stripe FM domains as accommodated by a spe- is a magnified version of the boxed region in Fig. 4 a . The cial field-cooling process, we have observed features that are external magnetic field exerts pressure on the FM DW, but essential for the understanding of exchange bias. We have not on the AF DW. As the FM DW moves, an exchange provided unequivocal evidences that the AF spin structure in spring parallel to the interface, is formed within the AF near exchange-coupled AF/FM bilayer is not static, contrary to the interface as shown in Fig. 4 c . The exchange spring the assumptions in some theoretical models. Our results are extends into the AF with a depth of the order of (A/K)1/2, consistent with a microscopic model for exchange bias that where A and K are the exchange stiffness and anisotropy involves a FM/AF exchange spring. At the ground state, all constant, respectively, of the AF, and the value of for FeMn DWs are hybrid FM/AF DWs. During reversal under a has been estimated to be of the order of tens of nm.6 When magnetic field, the FM DW moves, while the AF DW re- the FM DW moves, it carries with it a mobile line singularity mains stationary. The AF spins near the interface form an the hatched region in the interfacial region, while an im- exchange spring between a moving FM domain wall and a mobile line singularity the shaded region remains at the stationary AF domain wall. The shifted hysteresis loop, the location of the stationary AF. The exchange spring, bounded signature of exchange bias, involves winding and unwinding by these two line singularities, is connected to a moving FM of the exchange spring during the backward and forward DW at one side, and a stationary AF DW at the other. The reversals. mobile singularity the hatched region at the intersection of the FM DW, the exchange spring, and the undisturbed AF We thank N. J. Go¨kemeijer for the samples. Work at JHU spin structure plays key roles in the bilayer magnetization was supported by NSF Grants Nos. DMR00-80031 and processes. DMR01-01814. 014418-4 HYBRID DOMAIN WALLS AND ANTIFERROMAGNETIC . . . PHYSICAL REVIEW B 68, 014418 2003 1 W.H. Meiklejohn and C.P. 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