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 wit h the cubic perovskite structure have been studied by a Monte Carlo analysi s using both the uniaxial Ising model and the isotropic Heisenberg model. T he importance of a weak superexchange involving next nearest neighbours (nn n), in addition to the much stronger nearest neighbour (nn) superexchange, has been assessed. In the Ising case, below the critical temperature, antif erromagnetic domains are formed in which the domain walls are `pinned' by t he non-magnetic atoms such that increasing dilution causes substantial inte rpenetration of the domains, which is accentuated by nnn superexchange. Usi ng the Heisenberg model, the vector nature of the spins prevents pinning, b ut a nnn superexchange of only 3-5% can cause spin rotation in localised cl usters, and ultimately destroys the coherence of long-range order even thou gh individual spins are still strongly coupled. The observed Mossbauer rela xation 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 behavio ur 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.