ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 310 (2007) e610–e612
Structural transition of spinel compound NiCr2O4 at ferrimagnetic
transition temperature
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H. Ishibashi , T. Yasumi
Department of Physical Science, Osaka Prefecture University, Sakai, Osaka 599-8531, Japan
Available online 30 November 2006
Abstract
Magnetic properties and crystal structure of spinel compound NiCr2O4 have been investigated by magnetization and high-resolution
X-ray powder diffraction measurements. The structural transition from tetragonal to orthorhombic symmetry was observed at
ferrimagnetic transition temperature. This crystal distortion is related to the magnetic ordering of ferrimagnetic component.
r 2006 Elsevier B.V. All rights reserved.
PACS: 61.10.ꢀi; 75.50.Gg; 61.50.Ks; 75.30.Kz
Keywords: Spinel compound; Ferrimagnetic ordering; Structural transition; X-ray diffraction
Spinel compound NiCr2O4 exhibits a ferrimagnetic
ordering below TC ¼ 74 K. Recently, another magnetic
transition at TS ¼ 31 K was found by Tomiyasu et al. [1],
i.e., only the ferrimagnetic component shows an ordering
at TC and the ordering of the antiferromagnetic compo-
nent is followed below TS. A unique magnetic structure
model below TS was proposed from neutron diffraction
and magnetization measurements [1], which is different
from a spiral ordering usually observed in spinel chromites
such as CoCr2O4 and MnCr2O4. On the other hand,
NiCr2O4 with a normal spinel structure shows a structural
transition from cubic to tetragonal symmetry at ꢁ310 K
due to a Jahn–Teller effect on Ni2þ ions at a tetrahedral
site. Further, another structural transition was found at
65 K [2], which is slightly different from the magnetic
transition temperature. However, the correlation between
magnetic and structural properties has not been investi-
gated so far. In the present study, the details of the
magnetic properties and the crystal structure of NiCr2O4
were investigated by magnetization and high-resolution X-
ray powder diffraction measurements.
dence of magnetization was measured by SQUID magnet-
ometer. High-resolution X-ray powder diffraction
experiments using synchrotron radiation were performed
at a beam line of BL-3A at the Photon Factory, KEK in
Tsukuba, Japan. A wavelength of incident beam used was
˚
1:600 A. A flat Si(1 1 1) crystal analyzer was used in order
to improve the angular resolution. The temperature
dependence of the diffraction profiles of specific reflections
were measured at temperatures between 12 and 100 K.
The temperature dependence of magnetization of
NiCr2O4 under a magnetic field of 100 Oe is shown in
Fig. 1(a). There are two magnetic transition at TC ꢂ 75 K
and TS ꢂ 30 K, which are fairly good agreement with
reported one [1]. Fig. 1(b) shows the inverse of magnetic
susceptibility wꢀ1. Above TC, the T ꢀ wꢀ1 curve is nearly
represented by a hyperbola, therefore it is found that
NiCr2O4 indicates a ferrimagnetic behavior below TC.
Fig. 2 shows the temperature dependence of powder
diffraction profiles of 004t and 400t reflections, where the
indices of Bragg reflections are based on a tetragonal cell
˚
with the lattice constants of aꢁ8:18 and cꢁ8:55 A. Above
NiCr2O4 sample was prepared by heating the mixture of
high purity NiO and Cr2O3 in air. Temperature depen-
ꢁ70 K, 400 reflection in a cubic phase splits into two peaks
of 400t and 004t reflections due to a tetragonal distortion.
However, the peak profile of 400t reflection becomes
broader with decreasing temperature below ꢁ60 K and
clearly splits into two peaks at the lower temperatures. This
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0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved.