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NiO bilayers
Journal of Magnetism and Magnetic Materials 198}199 (1999) 500}502
Asymmetry of the remagnetization processes in exchange-biased NiFe/NiO bilayers V.I. Nikitenko *, V.S. Gornakov , L.M. Dedukh , A.F. Khapikov , A.J. Shapiro, R.D. Shull, A. Chaiken Institute of Solid State Physics, Russian Academy of Sciences, Chernogolovka, Moscow District 142432, Russian Federation, Russia National Institute of Standards and Technology, Gaithersburg, MD 20899, USA Hewlett-Packard Laboratory, Palo Alto, CA 94304, USA
Abstract The magnetization reversal of NiFe/NiO exchange-coupled "lms with direct (NiFe/NiO) and inverse (NiO/NiFe) deposition ordering of the ferro- and antiferromagnetic layers onto the MgO substrate were investigated by means of the magneto-optical indicator "lm technique and vibrating sample magnetometry. Processes of the domain formation and growth as well as the magnitude of the macroscopic coercive force were found to depend drastically on the growth order. oreover all of our bilayers were characterized by an asymmetry in the activity of crystal lattice defects as domain wall nucleation centers. The results are discussed in terms of variations in the real crystal structure of the antiferromagnetic layer. 1999 Elsevier Science B.V. All rights reserved. Keywords: Ferro/antiferromagnetic bilayer; Unidirectional anisotropy; Remagnetization; Domains
The magnetic properties of exchange-coupled ferro/antiferromagnetic (FM/AFM) bilayers have recently attracted a great deal of attention because of their potential usage in spin valve magnetoresistance sensors and read heads in future computer generations. The unidirectional anistropy [1,7] possessed by these bilayers may be used for stabilization of the ferromagnetic layer magnetization. Controlling the relationship between the exchange "eld H and the coercivity H of the FM/AFM bilayers is critical for improving the reliability of such magnetoresistive sensors. It is well known that the coercivity of FM/AFM bilayers is large compared to that observed in &free' FM layers. Under the assumption of coherent rotation of the FM layer spins [2,3], di!erent mechanisms of unidirectional anisotropy cannot explain the enhanced coercivity. In recent work [4] we demonstrated that the easy axis magnetization reversal of epi-
* Corresponding author. Tel: #7-095 524 5063; fax: #7096 576 4111. E-mail address: [email protected] (V.I. Nikitenko)
taxial NiFe/NiO bilayers grown on a MgO substrate proceeds by a domain wall nucleation and motion. Moreover, an unexpected phenomenon of asymmetry in the activity of domain nucleation centers with respect to the applied "eld sign was revealed. This asymmetry has been attributed to a spatial variation in the anisotropy constant of the antiferromagnet. Here we report further evidence of the physical origins for the coercivity enhancement of the FM/AFM bilayers. In this study the magnetization behavior of NiFe/NiO bilayers with direct (NiFe/NiO) and inverse (NiO/NiFe) layer deposition order onto the MgO substrate was examined. Recently, there was shown to be a dependence of the macroscopic characteristics of the magnetization reversal processes on the growth order of the FM and AFM layers (see for example [5]). However, the precise nature of this phenomenon remains obscure and demands further investigations. We have studied the elementary events of the magnetization processes in NiFe/NiO bilayers which were grown by ion beam sputtering [6] onto MgO (0 0 1) substrates. A unidirectional inplane anisotropy was
0304-8853/99/$ } see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 9 8 ) 0 1 1 9 7 - 4
V.I. Nikitenko et al. / Journal of Magnetism and Magnetic Materials 198}199 (1999) 500}502
501
Fig. 1. Reduced magnetization (M/M ) vs. applied "eld (H) along the [1 0 0] direction for biased bilayers with direct NiFe/NiO/MgO (A) and inverse NiO/NiFe/MgO (B) orders of the growth.
established with permanent magnets producing a #30 mT uniform bias "eld along the [1 0 0] direction during deposition. Dislocations were also introduced into the substrate during cleaving prior to the thin "lm deposition. These dislocations were localized near the specimen edges. Screw dislocation slip planes were observed on the bilayer surface due to steps parallel to [1 0 0] directions. Edge dislocation (1 1 0) slip planes were revealed in transmitted polarizing light due to stress birefringence [4]. A magneto-optical indicator "lm (MOIF) technique was used to study magnetic domain structures during the magnetization reversal of the bilayers. The stray "elds around domain walls and bilayer edges were analyzed through the intensity change of the Faraday e!ect in an indicator "lm with in-plane anisotropy lying on the top of a sample. Fig. 1 shows unidirectional axis hysteresis bilayer loops with direct NiFe(100 As )/NiO(500 As )/MgO (A) and inverse (B) ordered deposition of the magnetic layers. Both loops are shifted along the "eld axis by approximately the same value (k H "2 and 1.7 mT, respectively). The coercive force of the "lm with the direct ordering of magnetic layers exceeds the coercivity of its counterpart roughly by a factor of three (k H "2.6 and 0.9 mT, respectively). Note that the coercive force of both bilayers is larger than the coercivity of the &free' NiFe/MgO FM "lm (k H "0.2 mT) [4]. The set of MOIF images in Fig. 2 demonstrates features of the domain behavior of the NiFe/NiO/MgO specimen during remagnetization. It is seen that edge dislocations only acted as domain nucleation centers during remagnetization of the bilayers to the ground state (Fig. 2a}c). Inside the specimen away from the dislocations, the domain walls had no preferential orientations and they formed on random distributed imperfections at smaller external magnetic "elds (Fig. 2d). When the external magnetic "eld was opposite to the exchange "eld direction, domain walls did not appear on the dislocations, but formed instead on the other imperfections or near crystal edges due to magnetostatic stray "eld. However in this latter case a strong pinning of domain walls on the dislocation slip planes is observed (Fig. 2e and f).
Fig. 2. MOIF images of the sample during the [1 0 0] unidirectional-axis remagnetization in the NiFe/NiO/MgO sample. Figs. a}f correspond to the conditions indicated by the circles labeled by the same letters on the hysteresis loop in Fig. 1. `da is a nondeformed area.
Fig. 3. MOIF images of the sample during the unidirectionalaxis remagnetization in the NiO/NiFe/MgO sample. Figs. a}f correspond to the conditions indicated by the circles labeled by the same letters on the hysteresis loop in Fig. 1.
Fig. 3 displays the domain nucleation and propagation in the NiO(500 As )/NiFe(100 As )/MgO bilayer with the inverse ordering of magnetic layers. In this case the domains nucleate also at the "lm edge (Fig. 3a and b) when the magnetic "eld is opposite to the unidirectional
502
V.I. Nikitenko et al. / Journal of Magnetism and Magnetic Materials 198}199 (1999) 500}502
anisotropy. When the magnetic "eld orientation coincides with the unidirectional anistropy, the reversal process starts somewhere in the bilayer interior and ends when the domain walls reach the "lm edge (Fig. 3c and d). However, we "nd di!erences between the magnetization reversal of bilayers with di!erent layer ordering. In the sample with the inverse ordering we did not observe the e!ect of dislocations on the nucleation and motion of the domain walls. In this case dislocations did not penetrate into the bilayer "lm during deposition because of its polycrystalline structure (as revealed by XRD data). Thus the asymmetry in the activity of domain nucleation centers is the intrinsic feature of the magnetization reversal processes in exchanged-biased bilayers, and this asymmetry does not depend on the growth order of the layers or their crystal structure. This behavior can be understood in terms of a local variation of the AFM anisotropy [4]. Magnetization reversal from the ground state of "lms with unidirectional anisotropy proceeds by domain wall formation in those places with the strong magnetostatic "elds or decreased crystallographic anistropy of the AFM "lm caused by imperfections. When the external magnetic "eld coincides with the direction of the exchange bias, regions of the bilayer with increased AFM anistropy play a role as places for FM domain nucleation. The local increase in energy of the Mauri-like [2] planar domain wall consisting of twisted spins in the NiO favors its untwisting in these locations. The di!erence in the coercive force and in the domain behavior of the bilayers with direct and inverse layer ordering could be attributed to di!erences in the defect structures, which occur in the "lms. These defect structures vary with the layer ordering because the di!erent interfaces (NiO/MgO, NiO/NiFe, NiFe/MgO) will possess di!erent mis"t stresses. For bilayers, the domain wall
in the thin FM layer is accompanied by a magnetic inhomogeneity in the thicker AFM "lm. Therefore, a defect structure in the antiferromagnet is the principal factor determining the coercive force of the bilayer. Our experiments showed that in the bilayer with an inverse layer order, the NiO "lm is polycrystalline and the MgO dislocations do not propagate into the NiO. The observation of domains along edge dislocation slip planes in the bilayers with the FM deposited on the AFM indicates that dislocations propagate into the bilayer from the substrate, and that "lms grew epitaxially on the MgO. In both cases of layer ordering, variatons in the e!ective anistropy of the antiferromagnet due to di!erent kinds of imperfections and mis"t stresses determine variations in the potential energy surface for domain wall motion. In the theory of conventional ferromagnets, the coercive force increases with increasing potential energy barriers. A similar correlaton is also observed in our bilayers. This work was partially supported by Grant 97-0216879 from the Russian Foundation for Basic Research and a NIST Visiting Researcher's Program.
References [1] [2] [3] [4] [5] [6] [7]
W.H. Meiklejohn, C.P. Bean, Phys. Rev. 102 (1956) 1413. D. Mauri et al., J. Appl. Phys. 62 (1987) 3047. A.P. Malozemo!, Phys. Rev. B 35 (1987) 3679. V.I. Nikitenko et al., Phys. Rev. B 57 (1998) 8111. T. Ambrose et al., J. Appl. Phys. 81 (1997) 5007. R.P. Michel et al., IEEE Trans. Magn. 32 (1996) 4651. W.H. Meiklejohn, C.P. Bean, Phys. Rev. 105 (1957) 904.
Asymmetry of the remagnetization processes in exchange-biased NiFe/NiO bilayers V.I. Nikitenko *, V.S. Gornakov , L.M. Dedukh , A.F. Khapikov , A.J. Shapiro, R.D. Shull, A. Chaiken Institute of Solid State Physics, Russian Academy of Sciences, Chernogolovka, Moscow District 142432, Russian Federation, Russia National Institute of Standards and Technology, Gaithersburg, MD 20899, USA Hewlett-Packard Laboratory, Palo Alto, CA 94304, USA
Abstract The magnetization reversal of NiFe/NiO exchange-coupled "lms with direct (NiFe/NiO) and inverse (NiO/NiFe) deposition ordering of the ferro- and antiferromagnetic layers onto the MgO substrate were investigated by means of the magneto-optical indicator "lm technique and vibrating sample magnetometry. Processes of the domain formation and growth as well as the magnitude of the macroscopic coercive force were found to depend drastically on the growth order. oreover all of our bilayers were characterized by an asymmetry in the activity of crystal lattice defects as domain wall nucleation centers. The results are discussed in terms of variations in the real crystal structure of the antiferromagnetic layer. 1999 Elsevier Science B.V. All rights reserved. Keywords: Ferro/antiferromagnetic bilayer; Unidirectional anisotropy; Remagnetization; Domains
The magnetic properties of exchange-coupled ferro/antiferromagnetic (FM/AFM) bilayers have recently attracted a great deal of attention because of their potential usage in spin valve magnetoresistance sensors and read heads in future computer generations. The unidirectional anistropy [1,7] possessed by these bilayers may be used for stabilization of the ferromagnetic layer magnetization. Controlling the relationship between the exchange "eld H and the coercivity H of the FM/AFM bilayers is critical for improving the reliability of such magnetoresistive sensors. It is well known that the coercivity of FM/AFM bilayers is large compared to that observed in &free' FM layers. Under the assumption of coherent rotation of the FM layer spins [2,3], di!erent mechanisms of unidirectional anisotropy cannot explain the enhanced coercivity. In recent work [4] we demonstrated that the easy axis magnetization reversal of epi-
* Corresponding author. Tel: #7-095 524 5063; fax: #7096 576 4111. E-mail address: [email protected] (V.I. Nikitenko)
taxial NiFe/NiO bilayers grown on a MgO substrate proceeds by a domain wall nucleation and motion. Moreover, an unexpected phenomenon of asymmetry in the activity of domain nucleation centers with respect to the applied "eld sign was revealed. This asymmetry has been attributed to a spatial variation in the anisotropy constant of the antiferromagnet. Here we report further evidence of the physical origins for the coercivity enhancement of the FM/AFM bilayers. In this study the magnetization behavior of NiFe/NiO bilayers with direct (NiFe/NiO) and inverse (NiO/NiFe) layer deposition order onto the MgO substrate was examined. Recently, there was shown to be a dependence of the macroscopic characteristics of the magnetization reversal processes on the growth order of the FM and AFM layers (see for example [5]). However, the precise nature of this phenomenon remains obscure and demands further investigations. We have studied the elementary events of the magnetization processes in NiFe/NiO bilayers which were grown by ion beam sputtering [6] onto MgO (0 0 1) substrates. A unidirectional inplane anisotropy was
0304-8853/99/$ } see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 9 8 ) 0 1 1 9 7 - 4
V.I. Nikitenko et al. / Journal of Magnetism and Magnetic Materials 198}199 (1999) 500}502
501
Fig. 1. Reduced magnetization (M/M ) vs. applied "eld (H) along the [1 0 0] direction for biased bilayers with direct NiFe/NiO/MgO (A) and inverse NiO/NiFe/MgO (B) orders of the growth.
established with permanent magnets producing a #30 mT uniform bias "eld along the [1 0 0] direction during deposition. Dislocations were also introduced into the substrate during cleaving prior to the thin "lm deposition. These dislocations were localized near the specimen edges. Screw dislocation slip planes were observed on the bilayer surface due to steps parallel to [1 0 0] directions. Edge dislocation (1 1 0) slip planes were revealed in transmitted polarizing light due to stress birefringence [4]. A magneto-optical indicator "lm (MOIF) technique was used to study magnetic domain structures during the magnetization reversal of the bilayers. The stray "elds around domain walls and bilayer edges were analyzed through the intensity change of the Faraday e!ect in an indicator "lm with in-plane anisotropy lying on the top of a sample. Fig. 1 shows unidirectional axis hysteresis bilayer loops with direct NiFe(100 As )/NiO(500 As )/MgO (A) and inverse (B) ordered deposition of the magnetic layers. Both loops are shifted along the "eld axis by approximately the same value (k H "2 and 1.7 mT, respectively). The coercive force of the "lm with the direct ordering of magnetic layers exceeds the coercivity of its counterpart roughly by a factor of three (k H "2.6 and 0.9 mT, respectively). Note that the coercive force of both bilayers is larger than the coercivity of the &free' NiFe/MgO FM "lm (k H "0.2 mT) [4]. The set of MOIF images in Fig. 2 demonstrates features of the domain behavior of the NiFe/NiO/MgO specimen during remagnetization. It is seen that edge dislocations only acted as domain nucleation centers during remagnetization of the bilayers to the ground state (Fig. 2a}c). Inside the specimen away from the dislocations, the domain walls had no preferential orientations and they formed on random distributed imperfections at smaller external magnetic "elds (Fig. 2d). When the external magnetic "eld was opposite to the exchange "eld direction, domain walls did not appear on the dislocations, but formed instead on the other imperfections or near crystal edges due to magnetostatic stray "eld. However in this latter case a strong pinning of domain walls on the dislocation slip planes is observed (Fig. 2e and f).
Fig. 2. MOIF images of the sample during the [1 0 0] unidirectional-axis remagnetization in the NiFe/NiO/MgO sample. Figs. a}f correspond to the conditions indicated by the circles labeled by the same letters on the hysteresis loop in Fig. 1. `da is a nondeformed area.
Fig. 3. MOIF images of the sample during the unidirectionalaxis remagnetization in the NiO/NiFe/MgO sample. Figs. a}f correspond to the conditions indicated by the circles labeled by the same letters on the hysteresis loop in Fig. 1.
Fig. 3 displays the domain nucleation and propagation in the NiO(500 As )/NiFe(100 As )/MgO bilayer with the inverse ordering of magnetic layers. In this case the domains nucleate also at the "lm edge (Fig. 3a and b) when the magnetic "eld is opposite to the unidirectional
502
V.I. Nikitenko et al. / Journal of Magnetism and Magnetic Materials 198}199 (1999) 500}502
anisotropy. When the magnetic "eld orientation coincides with the unidirectional anistropy, the reversal process starts somewhere in the bilayer interior and ends when the domain walls reach the "lm edge (Fig. 3c and d). However, we "nd di!erences between the magnetization reversal of bilayers with di!erent layer ordering. In the sample with the inverse ordering we did not observe the e!ect of dislocations on the nucleation and motion of the domain walls. In this case dislocations did not penetrate into the bilayer "lm during deposition because of its polycrystalline structure (as revealed by XRD data). Thus the asymmetry in the activity of domain nucleation centers is the intrinsic feature of the magnetization reversal processes in exchanged-biased bilayers, and this asymmetry does not depend on the growth order of the layers or their crystal structure. This behavior can be understood in terms of a local variation of the AFM anisotropy [4]. Magnetization reversal from the ground state of "lms with unidirectional anisotropy proceeds by domain wall formation in those places with the strong magnetostatic "elds or decreased crystallographic anistropy of the AFM "lm caused by imperfections. When the external magnetic "eld coincides with the direction of the exchange bias, regions of the bilayer with increased AFM anistropy play a role as places for FM domain nucleation. The local increase in energy of the Mauri-like [2] planar domain wall consisting of twisted spins in the NiO favors its untwisting in these locations. The di!erence in the coercive force and in the domain behavior of the bilayers with direct and inverse layer ordering could be attributed to di!erences in the defect structures, which occur in the "lms. These defect structures vary with the layer ordering because the di!erent interfaces (NiO/MgO, NiO/NiFe, NiFe/MgO) will possess di!erent mis"t stresses. For bilayers, the domain wall
in the thin FM layer is accompanied by a magnetic inhomogeneity in the thicker AFM "lm. Therefore, a defect structure in the antiferromagnet is the principal factor determining the coercive force of the bilayer. Our experiments showed that in the bilayer with an inverse layer order, the NiO "lm is polycrystalline and the MgO dislocations do not propagate into the NiO. The observation of domains along edge dislocation slip planes in the bilayers with the FM deposited on the AFM indicates that dislocations propagate into the bilayer from the substrate, and that "lms grew epitaxially on the MgO. In both cases of layer ordering, variatons in the e!ective anistropy of the antiferromagnet due to di!erent kinds of imperfections and mis"t stresses determine variations in the potential energy surface for domain wall motion. In the theory of conventional ferromagnets, the coercive force increases with increasing potential energy barriers. A similar correlaton is also observed in our bilayers. This work was partially supported by Grant 97-0216879 from the Russian Foundation for Basic Research and a NIST Visiting Researcher's Program.
References [1] [2] [3] [4] [5] [6] [7]
W.H. Meiklejohn, C.P. Bean, Phys. Rev. 102 (1956) 1413. D. Mauri et al., J. Appl. Phys. 62 (1987) 3047. A.P. Malozemo!, Phys. Rev. B 35 (1987) 3679. V.I. Nikitenko et al., Phys. Rev. B 57 (1998) 8111. T. Ambrose et al., J. Appl. Phys. 81 (1997) 5007. R.P. Michel et al., IEEE Trans. Magn. 32 (1996) 4651. W.H. Meiklejohn, C.P. Bean, Phys. Rev. 105 (1957) 904.