Magnetic and magnetoelectric study of the pyroxene NaCrSi
2
O
6
Gwilherm Nénert,
1,
* Ingyu Kim,
2
Masahiko Isobe,
3
Clemens Ritter,
1
Alexander N. Vasiliev,
4
Kee Hoon Kim,
2
and
Yutaka Ueda
3
1
Institut Laue-Langevin, Boîte Postale 156, 38042 Grenoble Cedex 9, France
2
CeNSCMR, Department of Physics and Astronomy, Seoul National University, Seoul 151-747, Korea
3
Institute for Solid State Physics, University of Tokyo, 5-1-5 Kashiwa, Chiba 277-8581, Japan
4
Low Temperature Physics Department, Moscow State University, Moscow 119991, Russia
Received 29 January 2010; revised manuscript received 16 April 2010; published 7 May 2010
We investigated the magnetic, magnetoelectric, and crystal structures of the pyroxene NaCrSi
2
O
6
by super-
conducting quantum interference device SQUID magnetometry, electrical polarization measurement, and
powder neutron diffraction. Magnetic exchange couplings extracted from magnetization measurements are
found to be J
intra
/ k
b
=-0.484 K and J
inter
/ k
b
= 0.248 K. This is in perfect agreement with the antiferro-
magnetic order determined below T
N
= 2.82 K by neutron diffraction. Corroborating the determined magnetic
structure magnetic symmetry C-1', the magnetic field dependence of electrical polarization evidences a clear
magnetoelectric effect below T
N
. An induced magnetic field transition toward a ferromagnetic state magnetic
symmetry C2' / c' is observed in the SQUID data and confirmed by neutron diffraction.
DOI: 10.1103/PhysRevB.81.184408 PACS numbers: 75.85.+t, 75.10.Pq, 91.60.Pn, 75.25.-j
I. INTRODUCTION
In recent years, the coupling between magnetic and di-
electric properties in transition-metal oxides gave rise to a
significant research effort.
1–3
This effort is governed by the
emergence of new fundamental physics and potential techno-
logical applications.
2–4
Multiferroic materials exhibit simul-
taneously ferromagnetic, pyroelectric, and ferroelastic
properties. Contrary to multiferroic materials, magnetoelec-
tric materials show an induced electrical polarization by a
magnetic field. A proper understanding of the interplay be-
tween the various physical properties of these two types of
materials relies heavily on the knowledge of the detailed
crystal and magnetic structures.
Lately, three materials NaFeSi
2
O
6
, LiFeSi
2
O
6
, and
LiCrSi
2
O
6
belonging to the pyroxene family have been re-
ported as multiferroic materials. It has been shown that
LiCrSi
2
O
6
and LiFeSi
2
O
6
are, in fact, magnetoelectric mate-
rials rather than multiferroics.
5,6
Jodlauk et al.
7
suggested that several members of the py-
roxene family should have an incommensurate magnetic
structure. This suggestion was based on some theoretical as-
sumptions and linked to the geometrical magnetic frustration
present in this family.
7,8
However, for the moment it turned
out that most of the pyroxenes exhibit a simple magnetic
structure commensurate with the lattice with k =0.
5,6,9–12
Nevertheless, several of those reported magnetic structures
are compatible with a linear magnetoelectric effect.
5,6,9,11
In this contribution, we continue our investigation of the
crystal and magnetic structures of the pyroxene family by
studying NaCrSi
2
O
6
. We have investigated its magnetic
properties using superconducting quantum interference de-
vice SQUID magnetometry and powder neutron diffraction
as function of temperature and magnetic field. We show that
NaCrSi
2
O
6
contrary to NaCrGe
2
O
6
exhibits an antiferromag-
netic ground state characterized by a commensurate mag-
netic structure defined by k = 0. Contrary to Li containing
pyroxenes like LiMSi
2
O
6
M =V,Cr,FeRefs. 5, 9, and 10
the magnetic structure is described by the direct product of
two irreducible representations. The associated magnetic
symmetry is C-1' and allows a linear magnetoelectric
effect.
13
In agreement with the determined magnetic struc-
ture, the magnetic field dependence of the electrical polariza-
tion evidences a clear magnetoelectric effect. We find that the
magnetic moment of Cr
3+
is equal to 2.314
B
at 1.8 K.
Magnetic-susceptibility data are well described by a pseudo-
one-dimensional Heisenberg chain of S =3 / 2 spins giving
rise to an antiferromagnetic intrachain magnetic exchange
coupling and a ferromagnetic interchain magnetic exchange
coupling. Powder neutron diffraction at low temperature con-
firms the existence of both antiferromagnetic and ferromag-
netic exchange interactions. The magnetic structure is de-
scribed by ferromagnetic layers coupled antiferro-
magnetically while moving along the c axis. Applying
magnetic field, a transition to a ferromagnetic phase charac-
terized by the space group C2' / c' is induced. This field-
induced magnetic symmetry is identical to the one reported
for the zero-field magnetic structure of NaCrGe
2
O
6
.
12
II. EXPERIMENT
Polycrystalline samples of NaCrSi
2
O
6
were prepared by a
solid-state reaction with an appropriate molar ratio of
Na
2
CO
3
, Cr
2
O
3
, and SiO
2
. The weighted mixtures were
pressed into pellets and heated at 1273 K in air for several
days with one intermediate grinding.
Neutron-diffraction measurements were carried out on
powder of NaCrSi
2
O
6
. The precise crystal and magnetic
structures were investigated using high-resolution powder
data at room temperature, 10 and 1.8 K using the D2B dif-
fractometer at the ILL. The measurements were carried out at
a wavelength of 1.594 Å corresponding to the 335 Bragg
reflection of a germanium monochromator. The neutron de-
tection is performed with
3
He counting tubes spaced at 1.25°
intervals. A complete diffraction pattern is obtained after 25
steps of 0.05° in 2. Additional neutron-diffraction measure-
PHYSICAL REVIEW B 81, 184408 2010
1098-0121/2010/8118/1844088 ©2010 The American Physical Society 184408-1