RAPID COMMUNICATIONS
PHYSICAL REVIEW B 89, 020402(R) (2014)
Hidden magnetic order in Sr
2
VO
4
clarified with μ
+
SR
Jun Sugiyama,
1 , *
Hiroshi Nozaki,
1
Izumi Umegaki,
1
Wataru Higemoto,
2
Eduardo J. Ansaldo,
3
Jess H. Brewer,
3, 4
Hiroya Sakurai,
5
Ting-Hui Kao,
5, 6
Hung-Duen Yang,
6
and Martin M˚ ansson
7, 8
1
Toyota Central Research and Development Laboratories Incorporated, Nagakute, Aichi 480-1192, Japan
2
Advanced Science Research Center, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japan
3
TRIUMF, 4004 Wesbrook Mall, Vancouver, British Columbia, Canada V6T 2A3
4
Department of Physics and Astronomy, University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z1
5
National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan
6
Department of Physics, National Sun Yat-Sen University, Kaohsiung 804, Taiwan
7
Laboratory for Quantum Magnetism (LQM),
´
Ecole Polytechnique F´ ed´ erale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland
8
Laboratory for Neutron Scattering, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland
(Received 17 September 2013; revised manuscript received 20 December 2013; published 6 January 2014)
In order to elucidate the magnetic ground state of Sr
2
VO
4
, we have measured muon spin rotation and relaxation
(μ
+
SR) spectra of a powder sample in the temperature range between 1.8 and 140 K. As a result, we have clarified
that the transition at 105 K is not magnetic but structural and/or electric in origin and found the appearance of
static antiferromagnetic (AF) order below 8 K. Moreover, the distribution of the internal AF field was found to
be very broad, even at the lowest temperature measured. These results are consistent with the formation of an
orbital-stripe order with collinear AF order for the magnetic ground state of Sr
2
VO
4
.
DOI: 10.1103/PhysRevB.89.020402 PACS number(s): 75.25.Dk, 75.50.Ee, 75.70.Tj, 76.75.+i
Tetragonal Sr
2
VO
4
with a K
2
NiF
4
-type structure has been
considered as an analog of a parent compound of the first
superconducting cuprate, i.e., La
2
CuO
4
, since the electron
configuration of the V
4+
ion is t
1
2g
with S = 1/2 in a
tetragonal crystal field of a VO
6
octahedron [1–7]. Although
susceptibility (χ
m
) measurements clearly show a magnetic
anomaly at 105 K (=T
c
)[6], a past neutron diffraction study
reported the absence of magnetic peaks even at 5 K [2]. Since
x-ray diffraction studies revealed a sudden enhancement of the
c/a ratio below T
c
[6], it was proposed that an orbital-ordering
transition occurs at T
c
. In fact, first principles calculations
predicted an antiferromagnetic (AF) and orbital-ordered state
with a nontrivial and large unit cell structure in the ground state,
because of the coexistence and competition of ferromagnetic
and AF exchange interactions [8,9].
Recently, a more exciting possibility was proposed, namely,
that a hidden magnetic order is induced by spin-orbit coupling
for a t
1
2g
electron system [10]. Such work also predicted an
unconventional magnetic octupolar ordering at low T for
Sr
2
VO
4
. Furthermore, alternating spin-orbital order in the ab
plane was proposed by considering the effects of spin-orbit
coupling, crystal field, and superexchange on the energy levels
of the V
4+
ions [11].
Despite the above attractive predictions, there are few
reports on Sr
2
VO
4
utilizing microscopic magnetic techniques
[12]. Macroscopic magnetic measurements such as χ
m
and
neutron scattering usually give us significant insight into the
ground state of magnetically ordered solids. However, such
techniques are sometimes not suitable, particularly for the
materials exhibiting order with a broad field distribution,
i.e., when short-range order, random, or nearly random order
appears in a material, due to the absence of periodic structure
and/or the presence of rapid fluctuations. In contrast, the muon
spin rotation and relaxation (μ
+
SR) technique is very sensitive
*
e0589@mosk.tytlabs.co.jp
to local magnetic environments with a different time window
compared to neutron scattering; thus μ
+
SR has provided
crucial information on the magnetic ground state of materials
[13,14]. We have therefore performed a μ
+
SR experiment on
Sr
2
VO
4
and found the appearance of magnetic order not below
T
c
but below 8 K.
A powder sample of Sr
2
VO
4
was prepared from a precur-
sor orthorhombic β -Sr
2
VO
4
[15], which was made from a
stoichiometric mixture of SrO, V
2
O
3
, and V
2
O
5
by a solid
state reaction at 1400
◦
C in an Ar gas flow for 12 h. SrO was
obtained by thermal decomposition of SrCO
3
at 1100
◦
C for
12 h under vacuum, and V
2
O
3
was prepared from V
2
O
5
by
reduction in an H
2
gas flow at 600
◦
C for 4 h, and then at
800
◦
C for 2 h. The obtained precursor was sealed in an Au
capsule and then heated at 1300
◦
C under 4 GPa.
Powder x-ray diffraction (XRD) analyses revealed that
the obtained sample was almost single phase of a K
2
NiF
4
-
type structure with I 4/mmm space group [2]. Sr
3
V
2
O
8
was
present in the sample at a level below 2%. This suggests
that the average valence of the V ion in the whole sample
is above 4+, implying the absence of oxygen deficiencies
in Sr
2
VO
4
. χ
m
(=M/H , where M is magnetization) was
measured below 400 K under a H 10 kOe field with
a superconducting quantum interference device (SQUID)
magnetometer. (See Fig. 1.) A Curie-Weiss fit in the T range
between 110 and 320 K provided the T -independent term
[χ
0
= 0.000 07(2) emu/mol], Weiss T [
CW
=−28(4) K],
and effective magnetic moment [μ
eff
= 1.36(2)μ
B
]. These
values are consistent with the literature [1–5]. The μ
+
SR
time spectra were measured at TRIUMF in Canada in the
T range between 1.8 and 140 K. The experimental techniques
are described in more detail elsewhere [13].
Figures 2(a) and 2(b) show the zero field (ZF) and
longitudinal field (LF) μ
+
SR spectra at 1.8 K. Here LF
means the field is parallel to the initial muon spin polarization
(
S
μ
). The ZF spectrum exhibits rapid damping with a first
minimum at t ∼ 0.25 μs, indicating the presence of a broad
1098-0121/2014/89(2)/020402(4) 020402-1 ©2014 American Physical Society