Symmetry and asymmetry during magnetization reversal in exchange biased multilayers and bilayers

Paul, Amitesh; Kentzinger, Emmanuel; Rücker, Ulrich; Brückel, Thomas

doi:10.1103/physrevb.73.092410

Cited by 23 publications

(37 citation statements)

References 14 publications

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“…The non-symmetric reversal mechanism between the first and second switching points is a common feature of exchange bias systems. 52,56,[58][59][60] In the hematite system, the steplike increase towards a 90…”

Section: -8mentioning

confidence: 99%

Exchange bias in a nanocrystalline hematite/permalloy thin film investigated with polarized neutron reflectometry

et al. 2012

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We investigated a hematite α-Fe2O3/permalloy Ni80Fe20 bilayer film where the antiferromagnetic layer consisted of small hematite grains in the 2 to 16 nm range. A pronounced exchange bias effect occurred below the blocking temperature of 40 K. The magnitude of exchange bias was enhanced relative to reports for identical compounds in large grain, epitaxial films. However, the blocking temperature was dramatically reduced. As the Néel temperature of bulk α-Fe2O3 is known to be very high (860 K), we attribute the lowtemperature onset of exchange bias to the well-known finite-size effect which suppresses the Morin transition for nanostructured hematite. Polarized neutron reflectometry was used to place an upper limit on the concentration and length scale of a layer of uncompensated moments at the antiferromagnetic interface. The data were found to be consistent with an induced magnetic region at the antiferromagnetic interface of 0.5-1.0 μB per Fe atom within a depth of 1-2 nm. The field dependence of the neutron spin-flip signal and spin asymmetry was analyzed in the biased state, and the first and second magnetic reversal were found to occur by asymmetric mechanisms. For the fully trained permalloy loop, reversal occurred symmetrically at both coercive fields by an in-plane spin rotation of ferromagnetic domains. We investigated a hematite α-Fe 2 O 3 /permalloy Ni 80 Fe 20 bilayer film where the antiferromagnetic layer consisted of small hematite grains in the 2 to 16 nm range. A pronounced exchange bias effect occurred below the blocking temperature of 40 K. The magnitude of exchange bias was enhanced relative to reports for identical compounds in large grain, epitaxial films. However, the blocking temperature was dramatically reduced. As the Néel temperature of bulk α-Fe 2 O 3 is known to be very high (860 K), we attribute the low-temperature onset of exchange bias to the well-known finite-size effect which suppresses the Morin transition for nanostructured hematite. Polarized neutron reflectometry was used to place an upper limit on the concentration and length scale of a layer of uncompensated moments at the antiferromagnetic interface. The data were found to be consistent with an induced magnetic region at the antiferromagnetic interface of 0.5-1.0 μ B per Fe atom within a depth of 1-2 nm. The field dependence of the neutron spin-flip signal and spin asymmetry was analyzed in the biased state, and the first and second magnetic reversal were found to occur by asymmetric mechanisms. For the fully trained permalloy loop, reversal occurred symmetrically at both coercive fields by an in-plane spin rotation of ferromagnetic domains.

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Section: -8mentioning

confidence: 99%

Exchange bias in a nanocrystalline hematite/permalloy thin film investigated with polarized neutron reflectometry

et al. 2012

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“…8 In the case of large anisotropy ratio, one can also expect asymmetric reversal ͑nonuniform reversal for the decreasing branch͒ as shown for Ӎ 0°. 10 With further increase in H a , we approach even higher values of H X thereby increasing the anisotropy ratio again. This causes the 1,2 to approach 0°/ 180°which favors flipping of the top layers ͓Fig.…”

Section: Resultsmentioning

confidence: 80%

“…9,10 Comparing the anisotropy ratio in Fig. 4 for ഛ 15°, one can see that it is significant for the bottom layer magntization which flips with H a .…”

Section: Resultsmentioning

confidence: 92%

“…Recently we observed sequential reversal of the FM layers in ͓IrMn/ CoFe͔ 10 , 9 and also in ͓Co/ CoO͔ 20 multilayers ͑MLs͒, 10 which was directly related to the evolution of the grain size along the stack which leads to increasing exchange bias strengths along the ML stack. The nonuniform reversal of each CoFe layer was found to proceed symmetrically via domain-wall motion for both remagnetization directions as observed also by imaging techniques.…”

Section: Introductionmentioning

confidence: 87%

“…This reversal mode-symmetric, but nonuniform-corresponds to the situation for = 0°for our polycrystalline ML specimens, unlike usually observed asymmetric reversals in epitaxial bilayer specimens. [4][5][6]10 In the present paper, we investigate the same polycrystalline IrMn/ CoFe sample. We choose such a prototypical system which shows magnetization reversal that can be symmetric or asymmetric via the nonuniform 11 or uniform 13 mode which intuitively may depend upon the respective anisotropy energies involved.…”

Section: Introductionmentioning

confidence: 99%

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Magnetization reversal with variation of the ratio of the anisotropy energies in exchange bias systems

et al. 2006

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We systematically vary the ratio of the exchange and ferromagnetic anisotropies in a single multilayered system in finally unravelling the mysteries of magnetization reversal of exchange coupled systems. This is particularly possible due to increasing unidirectional exchange anisotropies in our multilayer system which we saw earlier from sequential switching of exchange coupled layers along the stack with increasing applied field strengths. Here, by introducing different directions ͑͒ of the applied field with respect to the unidirectional anisotropy direction, we have varied two different energy parameters: ͑i͒ the exchange anisotropy for each layer, ͑ii͒ the ferromagnetic anisotropy. Our polarized neutron measurements thus show a gradual crossover from layer flipping ͑domain-wall motion͒ for low torque regimes of the effective field to coherent rotation for high torque regimes. We explain these findings within a general and simple model where the angular dependencies for reversal are guided by the relative strengths of Zeeman, exchange, and anisotropy energies.

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Polarized Neutron Reflectivity and Scattering from Magnetic Nanostructures and Spintronic Materials

Zabel

Theis‐Bröhl

Toperverg

2007

Handbook of Magnetism and Advanced Magnetic Materials

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Polarized neutron reflectivity and scattering has established itself as one of the most important experimental methods for the analysis of magnetic nanostructures and spintronic materials. With this method, seminal contributions have been made to the physics of thin magnetic films, interfaces, and superlattices, including collinear and noncollinear exchange coupling. Another rich area is the exchange coupling phenomena between ferromagnetic and antiferromagnetic layers. Presently neutron scattering from laterally patterned magnetic arrays has become very active for the understanding of domain structures, domain reversals, and correlation effects. Established more than 25 years ago, polarized neutron reflectivity was focused mainly on the specularly reflected intensity. With the advent of area detectors and efficient wide‐angle spin analyzers, the off‐specular diffuse scattering could be analyzed, contributing to a more complete picture of the physics of magnetic nanostructures and patterned arrays. In this review a basic introduction into the fundamentals of polarized neutron reflectivity is provided, including specular and off‐specular scattering. For the description of the off‐specular part, the distorted wave Born approximation has rigorously been applied and features missed in the simpler Born approximation are pointed out. The methods are illustrated by a number of examples from research into magnetic thin films, spin valves, magnetic multilayers, exchange bias systems, spintronic materials, and periodic magnetic arrays of islands and stripes.

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Symmetry and asymmetry during magnetization reversal in exchange biased multilayers and bilayers

Cited by 23 publications

References 14 publications

Exchange bias in a nanocrystalline hematite/permalloy thin film investigated with polarized neutron reflectometry

Exchange bias in a nanocrystalline hematite/permalloy thin film investigated with polarized neutron reflectometry

Magnetization reversal with variation of the ratio of the anisotropy energies in exchange bias systems

Polarized Neutron Reflectivity and Scattering from Magnetic Nanostructures and Spintronic Materials

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