Bimetallic oxalates are layered molecule-based magnets with transition metals
M(II) and M'(III) coupled by the oxalate bridges ox=C2O4 in an open honeycomb
structure. Fe(II)Fe(III) bimetallic compounds with spins S = 2 and S′ = 5/2 ferrimagnetically
order below a transition temperature Tc that ranges from 30 to 48
K, depending on the cation between the layers. In small magnetic fields, several of
these compounds exhibit "giant negative magnetization" below a compensation temperature
of about 0.62 Tc. By studying the behavior of the orbital-doublet ground
state produced by a C3-symmetric crystal field, we construct a reduced Hamiltonian
that contains both exchange and spin-orbit interactions. This Hamiltonian is used to
explain all of the important behavior of the Fe(II)Fe(III) bimetallic oxalates, including
the stability of magnetic order in well-separated two-dimensional layers and the
magnetic compensation in compounds with high transition temperatures. Fitting
the parameters of this reduced Hamiltonian, we estimate that the z component of
the orbital angular momentum of the ground-state doublet in compounds exhibiting
magnetic compensation is about 0.27 and that the antiferromagnetic exchange
coupling Jc between the Fe(II) and Fe(III) spins is about 0.45 meV. In a magnetic
field perpendicular to the bimetallic layers, a spin-flop transition is predicted at a
field of about 3Jc/μB ≈ 25 T. A Holstein-Primakoff 1/S and 1/S′ expansion is used
to evaluate the spin-wave spectrum and to estimate the spin-wave gap sw ≈ 1.65
meV in compounds that exhibit magnetic compensation. This significant spin-wave
gap explains the stability of magnetic order in well-separated bimetallic layers. We
predict that the negative magnetization can be optically reversed by near-infrared
light. Breaking the C3 symmetry about each of the Fe(II) ions either through a
cation-induced distortion or uni-axial strain in the plane of the bimetallic layer is
expected to increase the magnetic compensation temperature.
