Proceedings of JCACS 2007, Nov. 14-17, Toyama,Japan
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JCACS
2007
Magnetic properties of GdNi1-xCux and Gd(Co1-xNix)2
N.Hatakeyama, K.Nishimura*, S.Kogura, D.Tamei, K.Mori
Graduate School of Science and Engineering for Research,University of Toyama, Toyama 930-8555, Japan
Abstract
Magnetization and specific heat measurements were carried out using pseudo-binary GdNi1-xCux and Gd(Co1-xNix)2 compounds. The
estimated adiabatic temperature change for GdNi1-xCux was found to bring about a peak with the sample of x = 0.3 at about 70 K. The
magnetic transition temperatures of Gd(Co1-xNix)2 vary from 73 K with GdNi2 to 277 K with Gd(Co0.8Ni0.2)2, which are in an almost linear
relation with x. The experimental result of the adiabatic temperature change of Gd(Co1-xNix)2 appeared to be rather independent of x.
Keywords: magnetocaloric effect, GdNi, GdCu, GdCo2, GdNi2
1. Introduction
2. Experimental
There have been intensive studies of magnetic
refrigeration applications using rare earth intermetallic
compounds in recent years. The suitable materials for the
application provide a large isothermal magnetic-entropy
change S or adiabatic temperature change T known
as magnetocaloric effect (MCE). Compounds showing
the first order magnetic transition often produce a large
MCE. GdNi is a ferromagnetic compound of the Curie
temperature TC of 70 K [1]. The crystal structure is the
orthorhombic CrB-type (space group Cmcm) [2]. GdCu
has complex magnetic structures associated with the
change in the crystal structure [3]. This compound
undergoes a martensitic structural transformation from the
cubic CsCl-type to the orthorhombic FeB-type at about
250 K as the temperature decreases.
Two
antiferromagnetic order temperatures TN were observed at
about 150 K and 45 K. The crystal and magnetic
structures of GdNi1-xCux have been also investigated in
detail [4]. The aim of this work is to examine if the
instability originated in the magnetic and crystal
structures brings about a large MCE in GdNi1-xCux system
at a certain concentration of x. Another interesting system
is Gd(Co1-xNix)2, with which we can expect a large MCE
in a wide temperature range since TC varies from 73 K
with GdNi2 to 403 K with GdCo2. This paper presents
the experimental results of T of GdNi1-xCux and Gd(Co1xNix)2 from magnetization and specific heat measurements.
———
The polycrystalline samples of GdNi1-xCux and
Gd(Co1-xNix)2 were prepared by arc melting the
constituent elements (of at least 99.9% purity) in an argon
atmosphere. The samples of Gd(Co1-xNix)2 were annealed
in vacuum for a week at 1273 K. X-ray diffraction data
were recorded at room temperature with CuKα radiation.
Magnetization was measured in the temperature range
from 2 to 300 K with external fields B up to 7 T using a
SQUID (MPMS). The specific heat data were obtained
with PPMS from 2 K.
* Corresponding author. Tel.: +76-445-6844; fax:+76-445-6703.
E-mail address: nishi@eng.u-toyama.ac.jp.
3. Results and discussion
Magnetization data (M) of GdNi1-xCux were
accumulated at various temperatures around TC or TN.
Figure 1 shows the M vs. B curves observed at 10 K. The
M values at 7 T are not in a simple linear relationship with
the Cu concentration x. In the M vs. B curves for x = 0.5
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Proceedings of JCACS 2007, Nov. 14-17, Toyama,Japan
Fig 1. Field dependence of the magnetization obtained at 10K for
GdNi1-xCux compounds
Fig 3. Adiabatic temperature change for GdNi1-xCux compounds for
magnetic field variation from 0 to 4 T.
and 0.6, metamagnetic behavior is noticeable. These
results are consistent with those reported previously, and
have been explained by the changes of the electron
density and the crystal structure due to the substitution of
Ni by Cu [4]. (The helical magnetic structure has been
suggested for x = 0.6.) The measured M data enabled us
to evaluate the S values by using the Maxwell relation
[5]. The maximum S values, which were derived from
an integration of the M data between B = 0 and 4 T, were
2.1, 2.2, 2.4, 2.7, 2.2, 1.7, and 1.4 J/mol K for x = 0.0, 0.1,
0.3, 0.35, 0.4, 0.5, and 0.6, respectively.
Similar measurements of the magnetization and
specific heat were carried out with Gd(Co1-xNix)2
compounds. Following the same procedure for the data
analysis described above, the T values were calculated
using the M data (0 - 4 T) as shown in Fig. 4. It was
found that the T values are rather independent of the Ni
concentration x, and TC decreases as x increases. From
the specific heat data, TC were deduced to be 277, 238,
199, 165, 107, and 73 K, for x = 0.2, 0.3, 0.5, 0.7, 0.9 and
1.0, respectively.
The present work implies that GdNi1-xCux can be
utilized as a refrigeration-material in the temperature
region of liquid nitrogen, and Gd(Co1-xNix)2 is possibly
useful from room temperature to liquid nitrogen
temperature.
This work is partially supported by the Takahashi
Industrial and Economic Research Foundation.
Experimental results of the specific heats (C)
measurements exhibit typical peaks for the magnetic
phase transitions as shown in Fig. 2. (A solid line in Fig.
2 indicates the C values of non-magnetic LaNi, which
gives the approximate magnitude of the electron and
phonon contributions to C.) Combining these C data with
the evaluated S values, the T values are estimated using
the relation [6]: T = TS/C. In Fig. 3, all the T peaks
appear fairly symmetric, which is a character of the
second order magnetic transition. It is worth noting that
the peak positions of T are almost independent of x
between 0.0 and 0.6. The maximum T value was
observed for x = 0.3, which is the boundary concentration
between the ferromagnetic ordering (x below 0.3) and
aniferromagnetic ordering (x above 0.4).
Fig 4. Adiabatic temperature change for Gd(Co1-xNix)2 compounds
for magnetic field variation from 0 to 4T .
References
Fig 2. Temperature dependence of the specific heat of GdNi1-xCux
compounds.
[1] R.E. Walline et al., J. Chem. Phys. 41 (1964) 1587.
[2] A.E. Dwight et al., Acta Crystallogr. 18 (1965) 837..
[3] J.A. Blanco et al., Phys. Rev. B59 (1999) 5121.
[4] D. Paccard et al., Solid State Science 7 (2005) 776
[5] N.H.Duc, D.T.K.Anh, J. Magn. Magn. Mater. 242-245 (2002) 873875.
[6] N.V.Tristan et al., J. Magn. Magn. Mater 258-259 (2003) 583-585
Proceedings of JCACS 2007, Nov. 14-17, Toyama,Japan
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