Magnetic ordering in the three-dimensional site-disordered Heisenberg model

Monte Carlo simulations have been carried out on a simple cubic ferromagnet with nearest-neighbor interactions. In order to model the effects of site frustration, a fraction f of the sites are occupied at random by moments that couple antiferromagnetically (AF) to their neighbors. When the concentration of AF sites is less than similar to 1/6, the system has one magnetic transition from paramagnet to ferromagnet at a critical temperature T-c. For f> 1/6 the system exhibits a second distinct ordering event at a lower temperature T-xy, where the transverse spin components freeze out leading to an increase in total spin length. Below T-xy the system is in a mixed state, in that the z components of the spins are ferromagnetically ordered while the transverse components exhibit AF correlations. The approximate magnetic phase diagram for our model is consistent with experimental results on site-disordered systems such as Eu1-xGdxS and Fe3-xMnxSi.