Sequence, symmetry, and magnetic fluctuations of the magnetization reversal in exchange-biased multilayers

Paul, Amitesh; Kentzinger, Emmanuel; Rücker, Ulrich; Bürgler, D. E.; Grünberg, P.

doi:10.1103/physrevb.70.224410

Cited by 28 publications

(35 citation statements)

References 24 publications

(28 reference statements)

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“…5͒ using the procedures described in Ref. 17: We only consider deviations from the purely collinear single-domain configurations and take into account the nonideal polarization efficiencies. The Co layers ͑i =1,¼,10 or 40͒ are described by their mean magnetization amplitude ͗M i ͘ and their angular deviation from the easy axis, which we describe by a single parameter .…”

Section: Resultsmentioning

confidence: 99%

Field-dependent magnetic domain structure in antiferromagnetically coupled multilayers by polarized neutron scattering

et al. 2006

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We study the magnetic structure of antiferromagnetically ͑AF͒ coupled Co/ Cu multilayers ͑MLs͒ with 10 or 40 bilayers under the influence of an external field by polarized neutron scattering in specular and off-specular geometry. We observe in the spin-flip channels off-specular intensities around the 1 2 -order Bragg position. Based on simulations of the measured data within the distorted-wave Born approximation we find vertically correlated domains, and their domain size evolves for a sufficiently large number of bilayers along the ML stack. Small domains most likely at the top and large domains presumably at the bottom coexist within a single ML. The small domains gradually get aligned with the applied field direction around 0.5 kOe, whereas the bigger domains remain AF coupled up to 3.0 kOe, which is well above the apparent saturation field measured by conventional magnetometry methods. Moreover, we observe a double-peak structure at the 1 2 -order position for the MLs with ten as well as 40 bilayers.

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Section: Resultsmentioning

confidence: 99%

Field-dependent magnetic domain structure in antiferromagnetically coupled multilayers by polarized neutron scattering

et al. 2006

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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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“…Theoretically the interpretation of the magnetization reversal was discussed in Ref. 7 where it was shown that depending on , the angle between H FC and the AF anisotropy axis, the reversal mode is either by coherent rotation for both loop branches or asymmetric with a nonuniform reversal for the decreasing branch.Very recently, Paul et al 8 have shown symmetric and sequential reversal for polycrystalline ͓Ir 20 Mn 80 ͑6.0 nm͒ /Co 80 Fe 20 ͑3.0 nm͔͒ 10 multilayers. Here sequential refers to a process, where different layers reverse their magnetization at different field strengths one after another.…”

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“…Very recently, Paul et al 8 have shown symmetric and sequential reversal for polycrystalline ͓Ir 20 Mn 80 ͑6.0 nm͒ /Co 80 Fe 20 ͑3.0 nm͔͒ 10 multilayers. Here sequential refers to a process, where different layers reverse their magnetization at different field strengths one after another.…”

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

et al. 2006

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We have studied the magnetization reversal process in continuous: ͓Co/ CoO͔ 20 and separated: ͓Co/ CoO / Au͔ 20 exchange-biased polycrystalline multilayers ͑MLs͒. For continuous ML, reversal proceeds sequentially starting with the bottom ͑top͒ Co layer for increasing ͑decreasing͒ field. Each Co layer remagnetizes symmetrically for both field branches in a nonuniform mode similarly as we have observed earlier for ͓IrMn/ CoFe͔ 3/10 MLs ͓Phys. Rev. B. 70, 224410 ͑2004͔͒. By polarized neutron reflectivity, we observe increasing exchange bias field strengths down the stack. However, usual asymmetric reversal is observed for the separated ML. We explain the different magnetization behavior within a simple and general model. The increased anisotropy energy for continuous ML is responsible for the nonuniform symmetric reversal as the angular dependencies for reversal are guided by the relative strengths of exchange, anisotropy, and Zeeman energies. Asymmetric hysteresis loops due to a different magnetization reversal process in different branches of the hysteresis loop are common 2-6 in exchange biased systems. Neutron scattering under grazing incidence with polarization analysis has been proven decisive for identification of the reversal mechanism. Two mechanisms can be distinguished: uniform magnetization reversal by magnetization rotation 3-6 and nonuniform magnetization reversal by domain nucleation and growth. Magnetization rotation is identified by a significant increase of the specular reflectivities in the spin-flip ͑SF͒ channels ͑R +− and R −+ ͒, which correspond to in-plane magnetization components perpendicular to the guiding field H a applied collinear to H FC . Reversal by domain nucleation and propagation ͑nonuniform magnetization reversal͒ does not provide enhanced SF intensities because the magnetization is always collinear to H a . Reversal mechanisms are observed for the Co/ CoO bilayer systems, 5,6 where the domain wall motion occurs for the decreasing ͑positive to negative͒ and magnetization rotation for the increasing ͑negative to positive͒ field sweeping direction of H a for the hysteresis loop with respect to negative direction of H FC . This behavior is just opposite to that reported in Ref. 4. Theoretically the interpretation of the magnetization reversal was discussed in Ref. 7 where it was shown that depending on , the angle between H FC and the AF anisotropy axis, the reversal mode is either by coherent rotation for both loop branches or asymmetric with a nonuniform reversal for the decreasing branch.Very recently, Paul et al. 8 have shown symmetric and sequential reversal for polycrystalline ͓Ir 20 Mn 80 ͑6.0 nm͒ /Co 80 Fe 20 ͑3.0 nm͔͒ 10 multilayers. Here sequential refers to a process, where different layers reverse their magnetization at different field strengths one after another. This reversal mode-symmetric, and nonuniformcorresponds to the situation = 0, considered unlikely to occur in experiments.7 Interestingly, however, the samples were multilayers ͑MLs͒ unlike the bilayer specimen...

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Sequence, symmetry, and magnetic fluctuations of the magnetization reversal in exchange-biased multilayers

Cited by 28 publications

References 24 publications

Field-dependent magnetic domain structure in antiferromagnetically coupled multilayers by polarized neutron scattering

Field-dependent magnetic domain structure in antiferromagnetically coupled multilayers by polarized neutron scattering

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

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

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