PHYSICAL REVIEW B 96, 144104 (2017)
Neutron diffraction study of the inverse spinels Co
2
TiO
4
and Co
2
SnO
4
S. Thota,
1 , *
M. Reehuis,
2 , †
A. Maljuk,
3
A. Hoser,
2
J.-U. Hoffmann,
2
B. Weise,
3
A. Waske,
3
M. Krautz,
3
D. C. Joshi,
1
S. Nayak,
1
S. Ghosh,
1
P. Suresh,
4
K. Dasari,
5
S. Wurmehl,
3
O. Prokhnenko,
2
and B. Büchner
3
1
Department of Physics, Indian Institute of Technology, Guwahati-781039, Assam, India
2
Helmholtz-Zentrum Berlin für Materialien und Energie, Hahn-Meitner-Platz 1, D-14109 Berlin, Germany
3
Leibniz Institute for Solid State and Materials Research, IFW-Dresden, D-01069 Dresden, Germany
4
Department of Physics, Indian Institute of Science, Bangalore-560012, India
5
Department of Physics, University of Puerto Rico, San Juan, Puerto Rico 00936-8377, USA
(Received 27 May 2017; revised manuscript received 2 August 2017; published 11 October 2017)
We report a detailed single-crystal and powder neutron diffraction study of Co
2
TiO
4
and Co
2
SnO
4
between the
temperature 1.6 and 80 K to probe the spin structure in the ground state. For both compounds the strongest magnetic
intensity was observed for the (111)
M
reflection due to ferrimagnetic ordering, which sets in below T
N
= 48.6 and
41 K for Co
2
TiO
4
and Co
2
SnO
4
, respectively. An additional low intensity magnetic reflection (200)
M
was noticed
in Co
2
TiO
4
due to the presence of an additional weak antiferromagnetic component. Interestingly, from both the
powder and single-crystal neutron data of Co
2
TiO
4
, we noticed a significant broadening of the magnetic (111)
M
reflection, which possibly results from the disordered character of the Ti and Co atoms on the B site. Practically,
the same peak broadening was found for the neutron powder data of Co
2
SnO
4
. On the other hand, from our
single-crystal neutron diffraction data of Co
2
TiO
4
, we found a spontaneous increase of particular nuclear Bragg
reflections below the magnetic ordering temperature. Our data analysis showed that this unusual effect can be
ascribed to the presence of anisotropic extinction, which is associated to a change of the mosaicity of the crystal.
In this case, it can be expected that competing Jahn-Teller effects acting along different crystallographic axes
can induce anisotropic local strain. In fact, for both ions Ti
3+
and Co
3+
, the 2t
g
levels split into a lower d
xy
level
yielding a higher twofold degenerate d
xz
/d
yz
level. As a consequence, one can expect a tetragonal distortion in
Co
2
TiO
4
with c/a < 1, which we could not significantly detect in the present work.
DOI: 10.1103/PhysRevB.96.144104
I. INTRODUCTION
Magnetic properties of oxides with spinel crystal struc-
ture [AB
2
O
4
= A
2+
(B
3+
)
2
O
4
for normal spinels, and
B
3+
(A
2+
B
3+
)O
4
for inverse spinels] have been the subject
of extensive research because of their unusual magnetic
behavior and wide variety of applications in high-frequency
electronic components such as transformers, tunable noise
filters, and magnetic read-write heads [1–10]. Substitution
of nonmagnetic elements (dilution) inside these oxides at
tetrahedral (A) or octahedral (B ) sites often induces disorder or
lattice distortions. Such issues may generate new pathways of
magnetic interactions leading to some complex ferrimagnetic
ordering with altered ground state [11–18] and brings about
some interesting magnetic phenomena like reentrant spin-glass
characteristics, magnetic frustration, and bipolar exchange
bias [19–29].
Among various classes of spinel compounds that exhibit
the above properties, cobalt orthotitanate (Co
2
TiO
4
), cobalt-
zinc titanates (Co
2−y
Zn
y
TiO
4
), cobalt-germanium titanates
(Co
2
Ge
x
Ti
1−x
O
4
), and cobalt orthostannate (Co
2
SnO
4
) are
some of the best known ferrimagnetic systems, which ex-
hibit glassy behavior just below T
N
[30–40]. The magnetic
ordering in these compounds has already been studied by
several authors in polycrystalline form [22–25,32–38]. In
this work, we focus on the neutron diffraction studies of
Co
2
TiO
4
[Co
2+
(Co
3+
Ti
3+
)O
4
] together with the magnetic
*
Corresponding author: subhasht@iitg.ac.in
†
Corresponding author: reehuis@helmholtzberlin.de
characterization of a single-crystalline sample and compared
the results systematically with the polycrystalline Co
2
SnO
4
.
Let us first recall that both compounds, Co
2
TiO
4
and Co
2
SnO
4
[Co
2+
(Co
2+
Sn
4+
)O
4
], exhibit ferrimagnetic behavior due to
unequal magnetic moments of Co
2+
ions at the tetrahedral
A sites [μ(A) = 3.87 μ
B
] and octahedral B sites [μ(B ) =
5.19 μ
B
and 4.91 μ
B
for Co
2
TiO
4
and Co
2
SnO
4
, respectively]
[22–25]. The nature of magnetism in polycrystalline Co
2
TiO
4
was first investigated by Sakamoto and Yamaguchi in 1962
using temperature-dependent remanence and torque measure-
ments [41]. These authors report ferrimagnetic behavior in
Co
2
TiO
4
with Néel temperature T
N
∼ 53 K together with
displaced hysteresis loops along the magnetization axis at
4.2 K [41]. Further, Ogawa and Waki report the temperature
dependence of specific heat C
P
(T ) in Co
2
TiO
4
synthesized by
cobalt ammonium sulfate and TiO
2
as precursors [42]. These
authors observed a weak anomaly across 49 K in the C
P
(T ),
which is associated with the magnetic transition, and reported
the Debye temperature
D
∼ 560 K with T
3/2
dependence for
T< 30 K [42]. Later studies by Hubsch and Gavoille reported
a semi-spin-glass transition T
SG
∼ 46 K (<T
N
) in Co
2
TiO
4
[32]. According to this report, Co
2
TiO
4
undergoes a com-
pensation temperature across 30 K where the two-sublattice
magnetizations balance with each other [32]. In 1991, Gavoille
et al. reported that the random anisotropy plays a major role
in the global magnetic behavior of Co
2
TiO
4
system [33]. Such
random anisotropy originates mainly from unsystematic lattice
distortions which screen the local charge fluctuations due to
large charge difference between Co
2+
and Ti
4+
[33].
The ac-magnetic susceptibility χ
ac
(T ) studies by Srivastava
et al. reveals few multiple transitions in Co
2
TiO
4
below
2469-9950/2017/96(14)/144104(13) 144104-1 ©2017 American Physical Society