A study of superexchange interactions in the spinel Mg1+tFe2−2tTitO4 by Monte Carlo analysis

Abstract

The spin dynamics of the magnetically diluted spinel system Mg_1+tFe_2−2tTi_tO₄ have been studied by a Monte Carlo analysis using an isotropic Heisenberg model. It is shown that many of the phenomena observed by Mössbauer, neutron diffraction, and magnetization experiments can be simulated. The superexchange between A-sites is shown to be small, and an upper limit of J_AA/J_AB = 0.02 is proposed. At compositions with 0.40 ≤ t ≤ 0.60, it is found that on cooling the paramagnetic state is replaced by a dynamic collinear spin arrangement. At a much lower temperature, ‘spin-freezing’ occurs in which small clusters of spins orient in different directions. Inter-cluster interactions are small because of the diamagnetic atoms which separate them. Long-range order may disappear before the percolation limit of t = 0.68 because of frustration on the B-sites, although clusters still exist by ‘freezing’ that become smaller as t increases further.