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

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 npz 1 <<1<<n where p is concentration and z 1 is the number of nearest neighbours (NN) on the lattice, the distribution is a universal function of Delta /np and independent of the crystal lattice. For npz 1 >>1, Delta is dominated by nearest-neighbour contributions and is normally distributed about a mean value Delta n =(2npz 1 ) 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 .