Magnetic properties of the Kagomé mixed compounds „Co
x
Ni
1-x
…
3
V
2
O
8
N. Qureshi*
Institute for Materials Science, University of Technology, Petersenstrasse 23, D-64287 Darmstadt, Germany
and Institut Max von Laue-Paul Langevin, 6 rue Jules Horowitz, BP 156, 38042 Grenoble Cedex 9, France
H. Fuess and H. Ehrenberg
Institute for Materials Science, University of Technology, Petersenstrasse 23, D-64287 Darmstadt, Germany
T. C. Hansen and C. Ritter
Institut Max von Laue-Paul Langevin, 6 rue Jules Horowitz, BP 156, 38042 Grenoble Cedex 9, France
K. Prokes and A. Podlesnyak
Hahn-Meitner-Institute, Glienicker Strasse 100, D-14109 Berlin, Germany
D. Schwabe
Institute for Physics, Justus Liebig University, Heinrich-Buff-Ring 16, D-35392 Giessen, Germany
Received 23 September 2006; published 19 December 2006
The magnetic properties of the mixed compounds Co
x
Ni
1-x
3
V
2
O
8
CNVO investigated by magnetization
and neutron diffraction measurements are presented. Unlike their parent compounds Ni
3
V
2
O
8
NVO and
Co
3
V
2
O
8
CVO, only one magnetic phase transition into an antiferromagnetic phase was detected for powder
samples with x = 0.27, 0.52, and 0.76. The magnetic structures are modulated according to a propagation vector
k
= ,0,0 with being dependent on the composition parameter x. Furthermore, magnetization data of a CVO
single crystal is featured, which is qualitatively different from previous publications and exhibits a controver-
sial aspect concerning the behavior of the curve under an applied magnetic field along the b axis.
DOI: 10.1103/PhysRevB.74.212407 PACS numbers: 75.25.z, 75.30.Gw, 75.30.Kz, 75.60.Ej
I. INTRODUCTION
The orthooxovanadates of the 3d transition metals M,
which crystallize in the orthorhombic space group Cmca,
represent magnetic lattices labeled as Kagomé staircases.
The crystal structure of these M
3
V
2
O
8
compounds has been
determined by Fuess et al.
1
and again by Sauerbrei et al.
2
The structure is characterized by edge-sharing MO
6
octahe-
dra, which form buckled layers separated by nonmagnetic
V
5+
ions. Because of the structural anisotropy and two non-
equivalent magnetic sites within the staircases Fig. 1, these
compounds differ from the ideal Kagomé net, which consists
of corner-sharing equilateral triangles. The reduced symme-
try relieves the geometrical frustration and results in long-
range ordered magnetic structures and rich H-T phase
diagrams.
3
Although the parent compounds NVO and CVO
have been exhaustively investigated,
1–10
only structural data
about Co
0.5
Ni
0.5
3
V
2
O
8
has been published
11
concerning the
cation distribution on the two M sites. In the present work,
magnetization measurements of three powder samples of the
mixed compounds with x = 0.27, 0.52, and 0.76 were carried
out and compared to the pure Ni and Co compounds. The
results were confirmed by neutron powder diffraction experi-
ments revealing more information about the magnetic struc-
tures. In addition, magnetization measurements of CVO are
presented that differ from previously published data.
5,8
Pos-
sible reasons for this discrepancy will be discussed.
II. EXPERIMENTAL
Powder samples of CVO and CNVO were synthesized
with a reported technique
8
by mixing NiO and CoO in the
desired ratio. Single crystals of CVO were grown by the
Czochralski method. The crystal structures of the powder
samples were investigated by neutron diffraction experi-
ments. Therefore, measurements were carried out at the high-
resolution neutron powder diffractometers D1A CNVO and
D2B CVO at the Institut Laue-Langevin using a wave-
length of 1.911 Å at 20 K and 2.3974 Å at 14 K, respec-
tively. The single-crystal structure was checked by x-ray dif-
fraction on the single-crystal diffractometer Oxford
Diffraction XCALIBUR with Mo K
at 100 K. The mag-
netic phase transitions were examined by magnetization
measurements using a superconducting quantum interference
device SQUID magnetometer Quantum Design. The
magnetic structures of CNVO were studied at zero magnetic
field by neutron powder diffraction experiments at D1A. Ad-
ditionally, neutron single-crystal diffraction experiments on
CVO two-axis diffractometer E4 at the Hahn-Meitner-
Institut in Berlin were used to reveal the magnetic phase
transitions due to the temperature dependence of the propa-
gation vector.
FIG. 1. Color online Visualization of a Kagomé staircase with
the two crystallographic M sites M1 light gray blue, M2 dark
gray blue and the different distances between them.
PHYSICAL REVIEW B 74, 212407 2006
1098-0121/2006/7421/2124074 ©2006 The American Physical Society 212407-1