Crystal-Field Splitting and Giant Negative Magnetization in

Abstract

Bimetallic oxalates are a class of layered organic magnets with transi- tion metals M(II) and M'(III) coupled by oxalate molecules in an open honeycomb structure. Of particular interest are the Fe(II)Fe(III) bimetal- lic compounds, which are ferrimagnetically ordered below a transition temperature that ranges from 30 to 48 K, depending on the cation that separates the layers. In small magnetic fields, several of these compounds exhibit giant negative magnetization below a compensation temperature of roughly 2/3 Tc. By studying the behavior of the Kramer's doublet ground state produced by the crystal-field splitting and constructing a reduced Hamiltonian that includes both exchange and spin-orbit and in- teractions, we can explain all of the important behavior of this class of materials: the stability of magnetic order in two dimensions and the existence of magnetic compensation in samples with high transition tem- peratures. We also provide several new predictions for the spin-wave gap and the optical flipping of the magnetic moment.