Metastable Random Field Ising model with exchange enhancement: a simple model for Exchange Bias

We present a simple model that allows hysteresis loops with exchange bias to be reproduced. The model is a modification of the T=0 random field Ising model driven by an external field and with synchronous local relaxation dynamics. The main novelty of the model is that a certain fraction f of the exchange constants between neighbouring spins is enhanced to a very large value J_E. The model allows the dependence of the exchange bias and other properties of the hysteresis loops to be analyzed as a function of the parameters of the model: the fraction f of enhanced bonds, the amount of the enhancement J_E and the amount of disorder which is controlled by the width sigma of the Gaussian distribution of the random fields.Comment: 8 pages, 11 figure

Magnetic field in a cavity

The classical Lorentz field in a spherical cavity of 4π M s/3 does not occur in a ferromagnetic material which has reached technical saturation. This effect is due to the formation of closure domains in the material near the surface of the cavity. Measurements of a material of low saturation magnetization shows that the field in the cavity approaches 4π Ms/3 only at fields that are five times the maximum demagnetizing field in the material. These results show that cavities or non-magnetic inclusions will greatly effect the approach to saturation in ferromagnetic materials, as pointed out by Néel.La valeur du champ classique de Lorentz dans une cavité sphérique (4π Ms/3) n'est pas atteinte dans un matériau ferromagnétique à son niveau de saturation technique. Ceci est dû à la formation de domaines de fermeture dans la substance à la surface de la cavité. Les mesures effectuées sur un matériau ayant une saturation magnétique faible montrent que le champ dans la cavité n'approche de la valeur 4π Ms/3 que pour des champs égaux à cinq fois le champ démagnétisant maximum de la substance. Comme l'a montré Néel ces résultats montrent que les cavités ou les inclusions non-magnétiques perturbent considérablement l'approche à la saturation dans les matériaux ferromagnétiques

Analytical solutions for exchange bias and coercivity in ferromagnetic/antiferromagnetic bilayers

Analytical expressions have been derived for the exchange bias field, coercivity, and effective anisotropy field in ferromagnetic/antiferromagnetic bilayers in the framework of a model assuming the formation of a planar domain wall at the antiferromagnetic side of the interface with the reversal of the ferromagnetic orientation. It is shown that there are five different sets of analytical expressions for the hysteresis loop displacement and coercivity, which depend on the interfacial exchange coupling strength and ferromagnetic anisotropy, and only one expression for the effective anisotropy field. These expressions are compared with the previously reported theoretical results, and the validity of the latter is discussed. It is shown that in the framework of the present model, the hysteresis loop, ac susceptibility, and ferromagnetic resonance measurements of exchange anisotropy should give the same values for the exchange bias field. The difference between the exchange bias field values, estimated experimentally by ac susceptibility and through hysteresis loop measurements for Co/CoO bilayers, is explained as well