Critical behavior of randomly pinned spin-density waves

Monte Carlo simulations have been used to study a Z₆ version of the two-component random anisotropy model on a simple cubic lattice. For strong random anisotropy, there is a finite temperature second-order phase transition with critical exponents {η}=0.01±{}0.03 and {α}=-0.76±{}0.03. The specific-heat amplitude ratio is A/A'=1.00±{}0.02. The low-temperature phase is characterized by an infinite susceptibility and a {}k^-2.4 decay of two-spin correlations, but no long-range magnetic order. Although the Hamiltonian has only a twofold exact symmetry, it appears that the low-temperature phase contains two pairs of Gibbs states, which approximate a fourfold symmetry in the phase space. These results are generally in agreement with the existing experiments for the randomly pinned spin-density wave state in dilute Yr alloys, and with similar experiments on certain amorphous magnetic alloys.