Magnetic anisotropy in the Fe(II)Fe(III) bimetallic oxalates

Abstract

Bimetallic oxalates are layered molecule-based magnets with transition metals $M(\text{II})$ and ${M}^{\ensuremath{'}}(\text{III})$ coupled by oxalate molecules $\text{ox}={\text{C}}_{2}{\text{O}}_{4}$ in an open honeycomb structure. Among the most interesting molecule-based magnets, Fe(II)Fe(III) bimetallic compounds with spins $S=2$ and ${S}^{\ensuremath{'}}=5/2$ ferrimagnetically order at a transition temperature ${T}_{c}$ that ranges from 30 to 48 K, depending on the organic cation between the layers. In small magnetic fields, several of these compounds exhibit ``giant negative magnetization'' below a compensation temperature of about $0.62{T}_{c}$. By studying the behavior of the low-energy orbital doublet produced by a ${C}_{3}$-symmetric crystal field, we construct a reduced Hamiltonian that contains both the exchange and spin-orbit interactions. This Hamiltonian is used to explain almost all of the important behaviors of the Fe(II)Fe(III) bimetallic oxalates, including the stability of magnetic order in weakly coupled layers and the magnetic compensation in compounds with high transition temperatures. In a magnetic field perpendicular to the bimetallic layers, a spin-flop transition is predicted at a field of about $3{J}_{c}/{\ensuremath{\mu}}_{B}\ensuremath{\approx}24\text{ }\text{T}$, where ${J}_{c}\ensuremath{\approx}0.45\text{ }\text{meV}$ is the nearest-neighbor antiferromagnetic exchange coupling. Holstein--Primakoff $1/S$ and $1/{S}^{\ensuremath{'}}$ expansions are used to evaluate the spin-wave spectrum and to estimate the spin-wave gap ${\ensuremath{\Delta}}_{\text{sw}}\ensuremath{\approx}1.65\text{ }\text{meV}$ in compounds that exhibit magnetic compensation. We predict that the negative magnetization can be optically reversed by near-infrared light. Breaking the ${C}_{3}$ symmetry about each of the Fe(II) ions through either a cation-induced distortion or uniaxial strain in the plane of the bimetallic layer is predicted to increase the magnetic compensation temperature.