A Monte Carlo analysis of spin dynamics and Mossbauer relaxation in 3-D magnetically diluted iron oxides

The spin dynamics of three-dimensional magnetically diluted iron oxides with the cubic perovskite structure have been studied by a Monte Carlo analysis using both the uniaxial Ising model and the isotropic Heisenberg model. The importance of a weak superexchange involving next nearest neighbours (nnn), in addition to the much stronger nearest neighbour (nn) superexchange, has been assessed. In the Ising case, below the critical temperature, antiferromagnetic domains are formed in which the domain walls are `pinned' by the non-magnetic atoms such that increasing dilution causes substantial interpenetration of the domains, which is accentuated by nnn superexchange. Using the Heisenberg model, the vector nature of the spins prevents pinning, but a nnn superexchange of only 3-5% can cause spin rotation in localised clusters, and ultimately destroys the coherence of long-range order even though individual spins are still strongly coupled. The observed Mossbauer relaxation can be simulated for both models, and different relaxation rates are found for spins that have different numbers of nn and nnn spins. However, the observed spectra are apparently more consistent with Ising-like behaviour in the critical region, and it is suggested that local anisotropy may be an important factor in these diluted systems, which would traditionally be expected to behave as Heisenberg magnets.