Order-order transitions in spin systems with double electron exchange

Abstract

Spin systems in the presence of isotropic single and double electron exchange are investigated at nonzero temperature. We use a generalized mean field approximation that allows for a variable local axis of quantization. This leads to the introduction of an angular parameter Θ giving the angle between nearest neighbor quantization axes. It is shown that order-order transitions can occur between phases of partially aligned spins (general Θ) and pure ferro- ( Θ = 0) or antiferromagnetic ( Θ = π) spin structures for physically reasonable values of the exchange ratio J 2/ J 1 of double to single exchange when J 2 > 0. The order-order critical temperature τ 0 is determined as a function of J 2/ J 1 for the particular cases of atomic spin s = 1, 3 2 , 2, 5 2 , and the corresponding phase diagrams are presented. Biquadratic and three-atom double electron exchange effects are considered separately. Expressions for the paramagnetic transition temperature and for the spontaneous magnetization M s for general atomic spin s are given, and the discontinuous change in M s at τ 0 is also found as a function of J 2/ J 1.