Thermal fluctuation aftereffect model for some systems with ferromagnetic-antiferromagnetic coupling

Abstract

A theoretical model is developed for, and applied to, some coupled-film systems consisting of an underlying ferromagnetic thin film and a surface layer of antiferromagnetic material viewed as an assembly of uniaxial small particles. The magnetization of the film biases, and is in turn biased by, the particles through an interfacial exchange coupling. Above a blocking temperature, dependent on size, particles are able to reverse rapidly due to thermal fluctuation, thus exhibiting superparamagnetic response. By assuming a physically reasonable distribution of particle sizes, good agreement is obtained between computed curves for temperature and frequency dependence of hysteresis loop displacement and coercivity based on this model and corresponding experimental results for oxidized Permalloy films, reported in a companion paper. This thermal fluctuation model is also applied successfully to the case of oxidized cobalt films as studied by Schlenker. In this case it is necessary to include in the analysis the temperature variations of the antiferromagnetic anisotropy energy and of the interfacial exchange coupling.

Rotatable anisotropy and coercivity in exchange-bias bilayers

“…Phenomenological approaches that explain both the isotropic negative FMR shift and the increased coercivity in exchange-coupled bilayers with polycrystalline AF layer were proposed in the Fulcomer and Charap model 8 ͑FC model͒ and in the Stiles and McMichael model 13 ͑SM model͒. In both models the basic assumption is that there must be two parts in the AF layer, one with ''rotatable'' 31 anisotropy and another with ''nonrotatable'' anisotropy.…”

“…No doubt, to a great extent, this is due to the important technological application the exchange bias has found in information storage technology. 6,7 Other reasons include the following: ͑i͒ despite the fact that most of the existent models [8][9][10][11][12][13][14][15][16][17][18][19] quantitatively explain the best known macroscopic magnetic property of the exchange anisotropy, the hysteresis loop shift field H eb , the enhancement of the coercivity, H c , when compared to that of an uncoupled film, is less well understood ͑such an enhancement has also been observed in mechanically alloyed AF/FM powders 20 ͒; ͑ii͒ the observation of a negative isotropic shift in the ferromagnetic resonance ͑FMR͒ field in exchange-bias films; [21][22][23] and ͑iii͒ the fact that different experimental techniques may yield different values for the exchange-bias field. 21,24 -30 These differences, at least in some cases, could be assigned either to the fact that the measurements are performed on different sets of samples, 28 or because the model used to interpret the experimental data from reversible measurement techniques may not be very plausible.…”

Out-of-plane exchange coupling between epitaxial Ni (50 Å) and NiO (600 Å) bilayers

“…According to the models proposed by Fulcomer and Charap 15 and its extension, 16 enhanced coercivity is due to the ''dragging'' of the AF moments in small AF grains at the interface which rotate when the magnetization of the FM layer is reversed, while the AF moments in large AF grains that do not rotate are responsible for the shift of the loop. In our sample, the square shape of the hysteresis loops implies that magnetization reversal for the out-of-plane direction in the FM is due to domain nucleation and domain wall propagation instead of rotation, as discussed previously.…”

Large time-dependent coercivity and resistivity modification under sustained voltage application in a Pt/Co/AlOx/Pt junction

“…18 Suppose a site i at the Co/AlOx interface has a chance P i of being occupied by a charged defect, with an energy barrier E b separating it from a site that is 2E hop lower in energy due to the applied electric field, then…”