Rotatable anisotropy and coercivity in exchange-bias bilayers

Abstract

A phenomenological approach for polycrystalline exchange-bias bilayers is proposed which explains the coercivity enhancement as well as its temperature and coupling strength dependences. In the model, it is assumed that uncompensated interfacial antiferromagnetic grains can switch their magnetizations irreversibly, producing a rotatable anisotropy. A preferential distribution of the antiferromagnetic easy axes is also considered. An inhomogeneous ferromagnetic magnetization reversal is allowed, assuming that the ferromagnet is divided into domains, each coupled to a stable antiferromagnetic grain only. The antiferromagnetic anisotropy distribution affects the angular dependence of the coercivity, reducing its value in the vicinity of the exchangebias direction, also smoothing the loop shift variations, more notably for small ferromagnetic uniaxial anisotropy. The inclusion of the rotatable anisotropy changes the shape of the magnetization curves and their characteristics. The larger the relative contribution of the rotatable anisotropy to the effective uniaxial anisotropy, the closer the loop shift angular variation gets to a pure cosine behavior, and no significant effect on the coercivity for strong coupling is detected. The frequently observed peak in the temperature variation of the coercivity is also explained considering the variation of the rotatable anisotropy, which is directly connected to the temperature dependence of the unstable antiferromagnetic grains' magnetization.

Discrimination between coupling and anisotropy fields in exchange-biased bilayers

“…3. The rounded shape of the magnetization curves indicates that the FM/AF interface is partly disordered which, in the model simulations, is taken into account by considering certain distributions 19,20,23 MLs. One might argue that this is the point where the transition from paramagnetic to AF state occurs for FeMn.…”

“…Among them is the fact that different measurement techniques may yield distinct values [8][9][10][11][12][13][14][15][16][17][18][19][20] for the FM/AF exchange coupling constant J E , being these differences of up to one order of magnitude. This has lead some authors to classify the techniques in two categories: reversible and irreversible.…”

“…In the framework of this model, by finding the equilibrium directions of M FM and M AF , one can numerically simulate magnetization curves, 14,16,[18][19][20]23 transverse-biased ac susceptibility 20 t , FMR field 16,18,19 H R , and anisotropic magnetoresistance ͑AMR͒ ͑Ref. 24͒ for any in-plane dc field direction given by H .…”

Domain structures during magnetization reversal in exchange-biased layers

“…7 Measurements of the exchange bias and coercive field as a function of the applied field angle 8,9 are very informative and provide a good test for the validity of existing theoretical models. 10 This article presents a study of the domain patterns during the magnetization reversal in exchange-biased Co/NiO bilayers, for different angles between the applied magnetic field and the exchange biasing axis. In some way, our measurements are similar to what has been done for the free ferromagnetic layer in spin-valve structures, 4 with a corresponding theoretical model.…”

Rotatable magnetic anisotropy of CoO/Fe/Ag(001) in ultrathin regime of the CoO layer

“…But in the ultrathin regime of the AFM layer, where the exchange bias vanishes, it is not clear whether the induced anisotropy has the same origin or not. Theoretical models have been proposed to attribute the coercivity enhancement into two different origins: one from the induced uniaxial anisotropy and the other from the so-called rotatable anisotropy [7,8]. It was found in experiment that while the uniaxial anisotropy dominates the system in thicker AFM regime as expected, the rotatable anisotropy dominates the system in thinner AFM regime [9,10].…”