Ladder approximation for uniaxial metamagnets

Abstract

The ladder approximation for metamagnets with exchange, single ion uniaxial anisotropy, and small interplane coupling is worked out. The thermal renormalization and damping of the uniform mode are given in explicit form. This result is applied to FeCl2, improving previous analysis where the self-energy of a ferromagnet was extended to metamagnetic systems. We find that the aforementioned approach is good in itself. However, we obtain non-negligible improvements by taking into account the damping effects neglected in the previous approach (for instance, a spurious divergence in the self-energy is taken off). The fitting of the energy gap and the magnon lifetime with experimental data indicates that the single ion anisotropy is pre-eminent with respect to the exchange one.