Magnetic phase diagram of the two-dimensional antiferromagnet Ni
5
(TeO
3
)
4
Br
2
Matej Pregelj and Andrej Zorko
Institute Jožef Stefan, Jamova 39, 1000 Ljubljana, Slovenia
Oksana Zaharko
Laboratory for Neutron Scattering, ETHZ and Paul Scherrer Institute, CH-5232 Villigen, Switzerland
Rodolphe Bousier
LCMI, CNRS, 25 rue des Martyrs, 38042 Grenoble, France
Helmuth Berger
Institute of Physics of Complex Matter, EPFL, 1015 Lausanne, Switzerland
Hiroko A. Katori
Magnetic Materials Laboratory, Wako Institute, RIKEN, 351–0198 Saitama, Japan
Denis Arčon
Institute Jožef Stefan, Jamova 39, 1000 Ljubljana, Slovenia and Faculty of Mathematics and Physics, University of Ljubljana,
Jadranska 19, 1000 Ljubljana, Slovenia
Received 23 June 2008; revised manuscript received 9 December 2008; published 9 February 2009
Phase diagram of the two-dimensional antiferromagnet Ni
5
TeO
3
4
Br
2
with triangular arrangement of Ni
2+
S =1 magnetic moments within the Ni
5
O
17
Br
2
subunits has been investigated by temperature and magnetic
field dependent heat-capacity, magnetization, and magnetic-torque measurements down to 1.5 K and up to 23
T. A nonzero magnetic contribution to the heat capacity observed up to 2.3T
N
is consistent with short-range
magnetic ordering and the two-dimensional nature of the system. Below the Néel temperature T
N
=29 K
several antiferromagnetic phases were identified. The zero-field phase is characterized by a planar antiferro-
magnetic arrangement of the two in-layer neighboring Ni
5
O
17
Br
2
magnetic clusters within the magnetic unit
cell. When the magnetic field is applied along the a
crystal axis, a spin-flop-like transition to a phase with a
complex out-of-plane arrangement of Ni
2+
S =1 magnetic moments occurs at 10 T. Using a molecular-field
approach we predict that this transition will shift to higher fields with increasing temperature and that a
magnetic phase with ferromagnetic ordering of Ni
5
O
17
Br
2
magnetic clusters will occur above 24 T. We
ascribe the richness of the magnetic phases to strongly exchange-coupled clusters, being the basic building
blocks of the investigated layered system.
DOI: 10.1103/PhysRevB.79.064407 PACS numbers: 75.50.Ee, 81.30.Bx
I. INTRODUCTION
Two-dimensional 2D antiferromagnetic AFM systems
represent a fertile playground for experimental and theoreti-
cal investigations of phase transitions and critical phenom-
ena. In recent years, they were intensively studied in relation
to high-temperature superconductivity in cuprates,
1
colossal
magnetoresistance in manganites,
2
and various exotic phases
in geometrically frustrated lattices, with a kagome lattice as
an archetype.
3
Phase diagrams of these systems are very rich
and sensitively depend on a particular spin arrangement,
magnetic anisotropy, and external perturbations, such as ap-
plied magnetic field or pressure.
Lately, 2D AFM systems that are built of strongly
exchange-coupled magnetic clusters instead of individual
magnetic moments have become accessible. Their phase
diagrams, influenced by the competition between intracluster
and intercluster interactions, are expected to be even more
complex. The recently discovered compound,
4
Ni
5
TeO
3
4
Br
2
, can be regarded as a novel representative of
such systems. Here, five magnetic Ni
2+
S =1 moments con-
stitute the Ni
5
O
17
Br
2
basic building block in which they are
arranged into two Ni
2+
triangles connected with a common
central Ni site.
5
Ni
5
O
17
Br
2
entities are linked via eight cor-
ners to four nearest neighbors, which then form a layered
structure. The triangle-based structure of the Ni
5
O
17
Br
2
unit suggests a geometrical frustration if AFM interactions
between nearest-neighbor Ni
2+
moments are assumed. These
moments are situated in three different severely distorted oc-
tahedral surroundings, implying strong magnetic anisotropy.
6
The dominant magnetic interactions between the Ni
2+
are
indeed antiferromagnetic, as indicated by high-temperature
susceptibility data, implying a negative Curie temperature
-50 K. The system exhibits a long-range Néel ordered
state below T
N
=29 K.
4–8
Consequently, the empirical frus-
tration parameter, / T
N
= 1.7, is rather small. Neutron mag-
netic diffraction and unusual temperature dependence of the
antiferromagnetic resonance AFMR frequency,
6,9
however,
suggest a very complicated temperature dependence of the
Ni
2+
sublattice magnetizations in the ordered phase. This re-
flects the importance of competition between various mag-
netic terms, possibly leading to intriguing magnetic struc-
tures as a function of temperature and external magnetic
field.
PHYSICAL REVIEW B 79, 064407 2009
1098-0121/2009/796/0644077 ©2009 The American Physical Society 064407-1