Magnetic anisotropy in the Fe(II)Fe(III) bimetallic oxalates
Randy S. Fishman and Fernando A. Reboredo
Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6065, USA
Received 7 January 2008; published 23 April 2008
Bimetallic oxalates are layered molecule-based magnets with transition metals MII and M'III coupled by
oxalate molecules ox= C
2
O
4
in an open honeycomb structure. Among the most interesting molecule-based
magnets, FeIIFeIII bimetallic compounds with spins S = 2 and S' =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 0.62T
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 FeIIFeIII
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 bime-
tallic layers, a spin-flop transition is predicted at a field of about 3J
c
/
B
24 T, where J
c
0.45 meV is the
nearest-neighbor antiferromagnetic exchange coupling. Holstein–Primakoff 1 / S and 1 / S' expansions are used
to evaluate the spin-wave spectrum and to estimate the spin-wave gap
sw
1.65 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 FeII ions through either a cation-induced
distortion or uniaxial strain in the plane of the bimetallic layer is predicted to increase the magnetic compen-
sation temperature.
DOI: 10.1103/PhysRevB.77.144421 PACS numbers: 75.50.Xx, 71.70.Ej, 75.10.Dg, 75.30.Gw
I. INTRODUCTION
Astounding progress has been achieved
1,2
during the past
two decades in the synthesis and characterization of
molecule-based magnets. Comparatively little progress has
been made in predicting their magnetic behavior. This paper
closes that gap by constructing a theoretical framework that
can be used to design the magnetic properties of an impor-
tant class of layered molecule-based magnets. Based on the
symmetry of the crystal-field potential and the energy scales
of the spin-orbit and exchange interactions, we evaluate the
magnetization, the magnetic phase diagram, and the spin-
wave SW spectrum of the FeIIFeIII bimetallic oxalates.
Due to the sensitivity of the crystal-field potential to changes
in the organic constituents, the magnetic properties of this
layered molecule-based magnet can be finely controlled.
First synthesized in 1992,
3
bimetallic oxalates are salts
with the chemical formula A MII M'IIIox
3
. Each of the
metallic layers contains two different metal atoms in the al-
ternating honeycomb structure pictured in Fig. 1, with
nearest-neighbor separations a of about 5.4 Å.
4–6
The neigh-
boring metal atoms MII with valence of +2 and M'III
with valence of +3 are connected by the oxalate molecule
ox=C
2
O
4
with valence of -2. Most commonly, MII = Mn,
Ni, Fe, Co, Cu, or Zn and M'III =V, Cr, Mn, or Fe. The
negatively charged bimetallic layers are separated by an or-
ganic cation A with a charge of +1. Depending on the metal
and organic cation species, the stacking of the bimetallic
planes can be rather complex with two to six bimetallic lay-
ers per unit cell.
4,5
For different metal atoms, a single bimetallic layer can be
either ferromagnetic MII and M'III moments aligned or
ferrimagnetic MII and M'III moments opposite. In zero
field, the magnetic moments of MII and M'III point out
of the plane. While it does not change the sign of the ex-
change coupling, the choice of organic cation A affects the
overall behavior of the system. Depending on the organic
cation, bimetallic oxalates can be optically activated,
5
metallic,
7
or disordered.
6,8
Some of the highest magnetic transition temperatures T
c
among the bimetallic oxalates are found in the ferrimagnetic
FeIIFeIII compounds,
9–13
where FeII and FeIII have
3d
6
and 3d
5
electronic configurations, respectively. By
Hund’s first rule, the spins on the FeII and FeIII sites are
S =2 and S' =5 / 2. Since the FeIII shell is half-filled, only
the FeII moments experience a magnetically anisotropic
FIG. 1. Color online A bimetallic layer with MIIpurple and
M'IIIblue in an open honeycomb structure coupled by oxalate
C
2
O
4
molecules, with oxygen atoms in red and carbon atoms in
green.
PHYSICAL REVIEW B 77, 144421 2008
1098-0121/2008/7714/14442110 ©2008 The American Physical Society 144421-1