The role of rotatable anisotropy in the asymmetric magnetization reversal of exchange biased NiO/Ni bilayers

Abstract

The angular dependence of asymmetric magnetization reversal in exchange biased NiO/Ni bilayers was investigated using a vectorial vibrating sample magnetometer. Different angular dependences of the exchange field, coercivity, and asymmetry were obtained by tuning the NiO layer thickness (tNiO). Comparing the experimental data with the theoretical calculations, we found that the asymmetric magnetization reversal of bilayers with larger tNiO was dominated by competing uniaxial and unidirectional anisotropies, and an additional rotatable anisotropy was required in the simulation to correctly describe the magnetization reversal of bilayers with smaller tNiO. The irreversible magnetization switches of a large amount of antiferromagnetic grains in thin NiO layers are responsible for the rotatable anisotropy, which may also significantly affect the shape of the hysteresis loops and the range of angles with symmetric magnetization reversal.

The angular dependence of the exchange bias under the planar domain wall model

“…7 Among these studies, the jump phenomenon was regarded as an attractive feature of the ADEB. This peculiar phenomenon has been confirmed by both experimental observations [8][9][10][11] and theoretical calculations. 12,13 It is reported that H EB reaches its maximal value and H C disappears suddenly at the jumping points.…”

“…This feature is also similar to the one of ADEB described by RAF model. [8][9][10][11][12][13] For J = 0.45, it can be observed from Figs.5(d), 5(e) and 5(f) that both the scopes of H C (θ H ) and H EB (θ H ) curves exhibit a nonmonotonic variation with increasing the values of γ. The scopes of H C (θ H ) and H EB (θ H ) curves will increase gradually when γ increases from γ < 1 to γ = 1, while it will decrease gradually for further increasing the values of γ in the case of γ > 1.…”

“…This feature is similar to that of the jump phenomenon studied by RAF model. [8][9][10][11][12][13] The angular regions where H C = 0 are gradually increased with increasing J. These angular regions are within the range of θ F6 − 180 o ≤ θ H ≤ θ F2 and θ F6 ≤ θ H ≤ θ F2 + 180 o .…”

“…12,13 It is reported that H EB reaches its maximal value and H C disappears suddenly at the jumping points. [8][9][10][11] Therefore, an intensive understanding of the jump phenomenon is essential to obtain the maximal H EB and the minimal H C simultaneously, which is very useful for designing the involved magnetoresistance devices. [14][15][16] In our previous works, 12,13 the jump phenomena of the exchange bias have been explained by the rigid antiferromagnetic moments model (RAF model), which is a modified M-B model 17,18 assuming that the uniaxial anisotropy of the AFM layer is infinite, i.e., the AFM magnetization keeps rigid during the magnetization reversal of the FM layer.…”

Determination of the Fe magnetic anisotropies and the CoO frozen spins in epitaxial CoO/Fe/Ag(001)

“…This scenario picture corresponds to the "spin-flop" mechanism in an antiferromagnet and has been proposed theoretically for the uniaxial magnetic anisotropy due to the AFM/FM interfacial interaction [7]. In experiment [48,49,50], the strength of the uniaxial anisotropy due to the AFM/FM coupling could vary from several tens Oe in NiMn/Co [50] and NiO/Ni [51] to several hundreds Oe in Fe/MnPd [48], NiFe/NiO [49], and Ni 81 Fe 19 /Cr 2 O 3 [52]. The strength of the uniaxial anisotropy in our CoO/Fe/Ag(001) system after field cooling is above 3000 Oe, which is much greater than the previous experimental data.…”