Dipolar anisotropy and the critical behaviour of spin glasses in small AC fields

Abstract

The dipole-dipole interaction provides a mechanism for anisotropy energy affecting the rotation of magnetic clusters in a spin glass. The authors have calculated the distribution of the lowest energy barrier Delta between the two stable directions of the net magnetic moment of a cluster of n spins in the limit of large n. In the dilute or spin glass limit, i.e. for npz₁<1, Delta is dominated by nearest-neighbour contributions and is normally distributed about a mean value Delta _n=(2npz₁)^1/2 sigma with a standard deviation sigma approximately= dipolar energy between two NN spins. Thus as n to infinity , the quantity Delta /n^1/2 tends to a fixed value. Within the framework of the magnetic cluster model, the above results are used to calculate the susceptibility measured in an AC magnetic field near the glass temperature T_g.