Effect of Si substitution on the magnetic and magnetocaloric properties of
ErCo2
Niraj K. Singh, S. K. Tripathy, D. Banerjee, C. V. Tomy, K. G. Suresh et al.
Citation: J. Appl. Phys. 95, 6678 (2004); doi: 10.1063/1.1676112
View online: http://dx.doi.org/10.1063/1.1676112
View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v95/i11
Published by the American Institute of Physics.
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JOURNAL OF APPLIED PHYSICS
VOLUME 95, NUMBER 11
1 JUNE 2004
Effect of Si substitution on the magnetic and magnetocaloric
properties of ErCo2
Niraj K. Singh,a) S. K. Tripathy, D. Banerjee, C. V. Tomy, and K. G. Suresh
Department of Physics, I.I.T. Bombay, Mumbai 400076, India
A. K. Nigam
Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai 400005, India
共Presented on 6 January 2004兲
The magnetic and magnetocaloric properties of Er共Co1⫺x Six ) 2 compounds with 0⭐x⭐0.075 have
been studied to determine their suitability as magnetic refrigerant materials. The strength of itinerant
electron metamagnetism was found to decrease with Si concentration, which is responsible for the
reduction of the magnetocaloric effect. Magnetization curves at low temperatures show the
existence of a critical field for magnetization to increase, which is a consequence of domain wall
pinning. The critical field and the coercive field were found to increase with Si concentration.
© 2004 American Institute of Physics. 关DOI: 10.1063/1.1676112兴
I. INTRODUCTION
III. RESULTS AND DISCUSSION
Intermetallic compounds formed between rare earth 共R兲
and transition metal 共T兲 draw a lot of attention because of
their interest in fundamental studies as well as their use in
various applications.1–5 Recently, magnetic refrigeration has
developed into an important application of magnetic
materials.2,4,6,7 The primary requirement of a good magnetic
refrigerant material is large magnetocaloric effect 共MCE兲. It
is known that materials showing first order magnetic transitions 共FOT兲 show considerable MCE, which enables them as
potential candidates for working substances in magnetic refrigerators. Among the R-T intermetallics, RCo2 compounds
with R⫽Dy, Ho, and Er show FOT at their ordering temperatures (T C ). The refrigerant materials should possess considerable MCE values over a range of temperature. Since MCE
is large only close to T C , for applications it is desired to
have a composite material with a range of T C values. Therefore, it is of importance to study the variation of T C and
MCE as a function of various substitutions at the R and T
sites. In this paper, we report the effect of Si substitution on
the magnetic properties and MCE in ErCo2 .
II. EXPERIMENTAL DETAILS
All the compounds were prepared by arc melting the
constituent elements of at least 99.9% purity in argon atmosphere. The ingots were melted several times to ensure
homogeneity. The alloy buttons were subsequently annealed
in high purity argon atmosphere at 900 °C for a week. Lattice
parameters were determined from the x-ray diffraction
patterns taken on powder sample using Cu K ␣ radiation
at room temperature. Magnetization measurements at fields
up to 5 T were carried out using a vibrating sample
magnetometer/SQUID magnetometer in the temperature
range of 1.8 –300 K.
a兲
Author to whom correspondence should be addressed. Electronic mail:
[email protected]
0021-8979/2004/95(11)/6678/3/$22.00
6678
Powder x-ray diffraction patterns show that all the compounds have formed in single phase with MgCu2 Laves
phase structure. The lattice parameters were found to increase with Si concentration. The ordering temperature as a
function of Si concentration is given in Table I. Figure 1
shows the M -H plot for Er共Co0.95Si0.05) 2 obtained at 1.8 K.
The existence of a critical field, called propagation field
(H p ), for the magnetization to rise and then to reach the
saturation (M s ), can be seen from this figure. The M -H
plots, obtained at 1.8 K, show an increase in propagation
field and coercivity (H c ) with an increase in Si concentration. The remanence ratio (M r /M s ) at 1.8 K for the compound with x⫽0.05 is found to be about 30%. The H p and
H c values for different compounds are also given in Table I.
Figure 2 shows the M -T plots of Er共Co0.95Si0.05) 2 , under
field-cooled 共FC兲 and zero-field-cooled 共ZFC兲 conditions at
50 Oe. The first order transition at T C can be seen from these
plots. A large difference between the FC and ZFC curves,
known as thermomagnetic irreversibility, can be seen from
this figure. The observation of propagation field and thermomagnetic irreversibility is attributed to the domain wall pinning effect. In materials with low T C and high anisotropy, the
domain wall width would be comparable to that of lattice
spacing and hence, the pinning effect would be larger. The
compound ErCo2 may be classified as a narrow domain wall
system, as its T C is much lower than ErFe2 and has a higher
anisotropy due to magnetoelastic distortion in the magnetically ordered phase.5 Domain wall pinning arises due to the
intrinsic defects3 and the substitution of nonmagnetic Si.
Large remanence and coercivity are consequences of the pinning.
The Arrott plots of all the studied compounds were
found to be S shaped, which indicates the presence of metamagnetism in all the compounds. The metamagnetism arises
due to the formation of Co moments close to T C , and this is
termed as itinerant electron metamagnetism.4 Therefore, the
transition is first order in all these cases. The Si substitution
© 2004 American Institute of Physics
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Singh et al.
J. Appl. Phys., Vol. 95, No. 11, Part 2, 1 June 2004
6679
TABLE I. Ordering temperature (T C ), propagation field (H p ), and coercive
field (H c ) in Er共Co1⫺x Six ) 2 compounds.
x
T C (K)
H p (Oe)
H c (Oe)
0
0.025
0.05
0.075
36
43
58
62
650
1050
1200
1400
1150
1250
1690
2020
in these compounds increases the lattice parameter, which
leads to larger spin fluctuation4 and hence, reduces the
strength of the metamagnetism. This is reflected in MCE
values 共discussed later兲 as well.
The magnetocaloric effect in Er共Co1⫺x Six ) 2 compounds
has been measured as the isothermal magnetic entropy
change ⌬S M (T,⌬H) for various temperatures and applied
magnetic fields. The ⌬S M is calculated from magnetic isotherms M (T i ,H), obtained at a sequence of temperatures
T i , using the Maxwell’s relation
⌬S M 共 T av,i H 2 兲 ⫽
冕 冉⳵
⬇
1
T i⫹1 ⫺T i
H2
H 1 ⫽0
冊
M 共 T,H 兲
T av,i dH
⳵T
冕
H2
0
关 M 共 T i⫹1 ,H 兲
⫺M 共 T i ,H 兲兴 dH.
FIG. 2. Temperature dependence of magnetization under FC and ZFC
conditions.
behavior was also observed in ErCo2 , where ⌬S max
for
M
⌬H⫽10 kOe and ⌬T⫽1 K was found to be 39 J kg⫺1 K⫺1,
while it reduces to 31.1 J kg⫺1 K⫺1 for ⌬H⫽40 kOe and
⌬T⫽4 K. The decrease in ⌬S max
M with a larger ⌬T is due to
averaging of the ⌬S M value, which is maximum near T C . In
particular, this variation is large in compounds showing FOT.
This suggests that the calculation of MCE in such materials
is critically dependent on the choice of ⌬T.
The decrease in ⌬S max
M with increase in Si is due to the
reduction in the strength of itinerant electron metamagnetism
Here, T av,i ⫽(T i⫹1 ⫹T i )/2 is the average temperature
and ⌬T⫽T i⫹1 ⫺T i is the temperature difference between the
magnetization isotherms measured at T i⫹1 and T i , when the
magnetic field is changed from H 1 ⫽0 to H 2 . The ⌬S M values for the compounds with x⭐0.05 for field changes (⌬H
⫽H 2 ⫺H 1 ) of 10 and 40 kOe are shown in Figs. 3共a兲 and
3共b兲, respectively. It can be seen from the figures that ⌬S M
always shows a maximum (⌬S max
M ) close to T C and it decreases gradually with Si. The MCE is found to drop by
⬇30%, as x changes from 0 to 0.05, followed by an increase
in T C of about 25 K. The ⌬S max
M in Er共Co0.925Si0.075) 2 for
⌬H⫽10 kOe, calculated from M -H data taken at temperature intervals (⌬T) of 4 K, was found to be ⫺9.5 J kg⫺1 K⫺1,
while the ⌬S max
M value for the same ⌬H but with M -H data
taken at ⌬T of 1 K, was found ⫺24.5 J kg⫺1 K⫺1. Similar
FIG. 1. Magnetization as a function of applied magnetic field at T
⫽1.8 K. The inset shows the existence of the propagation field.
FIG. 3. Magnetic entropy change as a function of temperature in
Er共Co1⫺x Six ) 2 compounds for 共a兲 ⌬H⫽10 kOe and 共b兲 ⌬H⫽40 kOe.
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6680
Singh et al.
J. Appl. Phys., Vol. 95, No. 11, Part 2, 1 June 2004
due to the spin fluctuations. The spin fluctuations in Sisubstituted compounds arise due to the magnetovolume effect and also to the increased thermal contribution associated
with an increase in T C .
However, a large MCE value is not the only criterion in
determining the suitability of a material as a potential refrigerant. The magnetic hysteresis of the materials should be
very small when close to the operating temperatures, so that
the magnetization behavior is reversible in a cyclic operation
of the refrigerator. In the case of Er共Co0.95Si0.05) 2 , the
(M r /M s ) ratio close to T C is only about 8.5%, in contrast to
the value of 30% observed at 1.8 K. This implies that the
magnetic hardness is present only at very low temperatures.
Moreover, the propagation field is also negligibly small at
temperatures close to T C . All these imply that this system is
magnetically soft enough to be considered for applications as
magnetic refrigerants.6
IV. CONCLUSIONS
We find that considerable MCE is retained in
Er共Co1⫺x Six ) 2 compounds up to a Si concentration of 0.05,
along with an increase in T C of about 25 K. This is important
from the point of view of magnetic refrigeration applications,
since a practical refrigerant should possess a large MCE over
an extended temperature range. Therefore, the present study
suggests that this system is a potential candidate for refrigeration applications below 60 K. Furthermore, our results
indicate that this system is magnetically soft, which is again,
a criterion for a good refrigerant material.
ACKNOWLEDGMENT
On of the authors 共K.G.S.兲 thanks DST, Government of
India for financial support in form of a sponsored project.
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