Weakly coupled antiferromagnetic planes in single-crystal LiCoPO
4
D. Vaknin,
1
J. L. Zarestky,
1
L. L. Miller,
1
J.-P. Rivera,
2
and H. Schmid
2
1
Ames Laboratory and Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011
2
Department of Inorganic, Analytical and Applied Chemistry, University of Geneva,
Sciences II, 30 quai E. Ansermet, CH-1211-Geneva 4, Switzerland
Received 21 December 2001; published 30 May 2002
Neutron-scattering and magnetic susceptibility studies of single-crystal LiCoPO
4
are reported. The neutron-
diffraction results indicate that in the antiferromagnetic phase the moments are not strictly aligned along the b
axis, as previously reported R. P. Santoro et al., J. Phys. Chem. 27, 1192 1996, but are uniformly rotated
from this axis by a small angle ( 4.6°). This rotation breaks the mirror symmetry along the orthorhombic b
axis. Symmetry considerations based on this rotation, on the magnetoelectric effect, and on a recently observed
weak spontaneous magnetization along the spin direction, implying a so-far-unknown ferrimagneticlike kind of
weak ferromagnetism, allow one to postulate the monoclinic magnetic point group 2 ' . The diffraction data are
analyzed in terms of weakly coupled two-dimensional Ising antiferromagnets. The large anisotropy in the
susceptibility is explained in terms of the single-ion anisotropy and anisotropic exchange interactions. We
argue that the alignment of the magnetic moments in the antiferromagnetic phase is determined by the single-
ion anisotropy even though the exchange along this direction is the weakest.
DOI: 10.1103/PhysRevB.65.224414 PACS numbers: 75.25.+z, 75.50.Ee, 78.20.Ls
I. INTRODUCTION
LiCoPO
4
belongs to a class of materials that exhibit prop-
erties intermediate to two- and three-dimensional 2D and
3D systems. It consists of buckled CoO layers that are
stacked along the crystallographic a axis. Nearest neighbors
in the plane are coupled magnetically by a relatively strong
superexchange interaction through an M -O -M oxygen
bond
1–4
which with the influence of crystal field, renders an
Ising-like character to the exchange interaction. There is no
direct or indirect exchange coupling between the Co
2 +
mo-
ments ( S =3/2) in different planes, and only higher-order
exchange interaction involving the phosphate group is pos-
sible via Co-O-P-O-Co, as suggested by Mays.
1
The
M -O-P-O- M ( M =transition metal ion is the only super-
exchange in some related 3D frameworks, such as
Li
3
Fe
2
(PO
4
)
3
, where the 3D antiferromagnetic AF order-
ing occurs at relatively large temperatures.
5
This suggests
that this type of magnetic coupling, although of a higher
order, is not negligible. Crystal-field effects, in these sys-
tems, play an important role in reducing the spin symmetry
of the magnetic moments. In particular, the orthorhombic
symmetry introduces crystal-field terms that give rise to dou-
blet ground state.
LiCoPO
4
is an insulator that is isostructural with the oli-
vine family of lithium orthophosphates LiM PO
4
( M =Mn,
Fe, Co, and Ni,
6
space group Pnma with lattice constants
a =10.093 Å, b =5.890 Å, and c =4.705 Å at room tem-
perature. It consists of two types of polyhedra: CoO
6
octa-
hedra that are corner shared and cross-linked with the PO
4
tetrahedra, forming a three dimensional network, with tun-
nels that are occupied by Li ions along the 010 and 001
directions. In this network, nearly close-packed oxygens in
hexagons can be found. Interest in inorganic Li phosphates
for potential rechargeable batteries has risen recently.
7
Ac-
cording to Ref. 3, LiCoPO
4
undergoes an antiferromagnetic
phase transition with a colinear arrangement of the Co
2 +
spins, as shown in Fig. 1. The in-plane spin configuration
with a propagation vector along the 010 is similar in all
LiM PO
4
members, and differs only in spin orientation from
one member to another. In LiCoPO
4
the spin direction was
found to be along the b axis.
2
However, recent magnetoelec-
tric effect ME and magnetic susceptibility studies of
LiCoPO
4
by Rivera
8,9
indicated strong anisotropic proper-
ties, and suggested that the spin configuration might be of a
lower symmetry than the one shown in Fig. 1. We have un-
dertaken this study to determine the detailed magnetic ar-
rangement of LiCoPO
4
in a single crystal and to characterize
the behavior of the magnetic system at low temperatures.
II. EXPERIMENTAL DETAILS
Neutron-scattering measurements were carried out on the
HB1A triple axis spectrometer at the High Flux Isotope Re-
actor at Oak Ridge National Laboratory. A monochromatic
neutron beam of wavelength =2.358 Å 14.712 meV and
k
o
=2 / =2.653 Å
-1
) was selected by a double mono-
FIG. 1. Projection of LiCoPO
4
on the b -c plane, showing two
layers of Co and Li atoms and the magnetic model as determined by
Santoro et al. One layer of Co
2+
is represented by open circles, and
the adjacent layers in the a direction by filled circles.
PHYSICAL REVIEW B, VOLUME 65, 224414
0163-1829/2002/6522/22441410/$20.00 ©2002 The American Physical Society 65 224414-1