ARTICLE IN PRESS
Magnetic and magnetocaloric properties of the intermetallic
compound TbNiAl
Niraj K. Singha, K.G. Suresha,, R. Nirmalab, A.K. Nigamb, S.K. Malikb
a
Department of Physics, Indian Institute of Technology Bombay, Mumbai, 400076, India
b
Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai, 400005, India
Abstract
Magnetization measurements have been carried out on the intermetallic compound TbNiAl in applied fields up to 120 kOe.
Temperature dependence of magnetization under zero-field-cooled and field-cooled conditions shows thermomagnetic irreversibility,
which is attributed to magnetic frustration. With the increase of field, the irreversibility decreases and vanishes completely at high fields.
Magnetocaloric effect has been calculated in terms of isothermal magnetic entropy change using magnetization isotherms obtained at
various temperatures. The maximum entropy change is 13.8 J kg1 K1 near the ordering temperature for a field change of 50 kOe. The
refrigerant capacity is found to be 494 J kg1 for the same field change and for a temperature difference of 52 K between the cold and the
hot sinks.
PACS: 75.30.Sg; 75.30.Kz
Keywords: Intermetallics; Magnetic frustration; Magnetocaloric effect; Refrigerant capacity
1. Introduction
The search for potential magnetic refrigerant materials is
mainly focused towards the rare earth (R)–transition metal
(TM) intermetallic compounds owing to the large magnetocaloric effect (MCE) exhibited by them near the magnetic
transitions [1,2]. The use of magnetic refrigeration as an
alternative to the conventional gas compression method
has led to an intensive research, both on experimental as
well as theoretical aspects, in the field of MCE [3–5].
Though MCE can be measured directly using calorimetric
techniques, indirect calculation methods in terms of
isothermal magnetic entropy change or adiabatic temperature change are more prevalent. Materials should possess
considerable MCE over a span of temperature in order to
be considered as practical refrigerants. In general, MCE is
found to be maximum near the magnetic ordering
temperature and, therefore, composite materials with
distributed ordering temperatures or materials with multiple magnetic transitions are preferred for refrigeration
applications [2]. In this context, RNiAl compounds, which
possess a complex magnetic structure and undergo multiple
magnetic transitions, are important and warrant a detailed
experimental study of MCE. Though the magnetic properties of these compounds have been well studied, very little
work has been reported on their magnetocaloric behavior.
In the present communication, we have tried to correlate
the magnetic and magnetocaloric properties of the intermetallic compound TbNiAl.
2. Experimental details
The polycrystalline sample of TbNiAl was prepared by
arc melting the constituent elements (of at least 99.9%
purity) in high-purity argon atmosphere. The ingot was
melted several times to ensure homogeneity and the final
weight loss was less than 1%. Lattice parameters were
determined from powder X-ray diffraction (XRD) patterns
using Cu Ka radiation. Magnetization measurements, in
ARTICLE IN PRESS
303
0.3
H = 200 Oe
TbNiAl
0.2
FC
ZFC
0.1
0.0
2.4
M (µB/f.u.)
the temperature range 2–120 K and up to a maximum field
of 120 kOe, were performed using a vibrating sample
magnetometer (VSM, Oxford Instruments). The temperature variation of magnetization, under zero-field-cooled
(ZFC) and field-cooled (FC) conditions, was measured up
to a maximum field of 20 kOe. In the ZFC mode, the
sample was cooled to the lowest temperature in the absence
of an applied magnetic field and the magnetization was
measured in a fixed magnetic field on warming. However,
in the FC mode, the sample was cooled in a constant
applied field, which was retained during warming to
measure the magnetization. ZFC and FC curves were
obtained at the same warming rate. The magnetization
isotherms in the temperature range 18–74 K were used for
the calculation of isothermal magnetic entropy change.
These isotherms were obtained in the increasing-field
mode, as the temperature was increased from 18 to 74 K.
Before measuring the magnetization isotherms, the sample
was heated to above the ordering temperature and then
cooled to the desired temperature.
H = 2 kOe
1.6
FC
ZFC
0.8
0.0
8
H = 20 kOe
6
FC
ZFC
4
2
3. Results and discussion
0
Fig. 1 shows the Rietveld-refined room temperature
powder X-ray diffractogram for TbNiAl using the Fullprof
program. The fitting revealed that the compound has
formed in single phase with the hexagonal ZrNiAl-type
structure (space group P62m, no. 189). The lattice
parameters were found to be a ¼ 6.98770.009 and
c ¼ 3.86970.005 Å. Fig. 2 shows the temperature (T)
dependence of magnetization (M), in various applied fields
(H), under ZFC and FC conditions. The M2T data show
two magnetic transitions, one at 23 and the other at 48 K.
Neutron diffraction measurements reported earlier in this
compound [6,7] have revealed the existence of antiferro-
TbNiAl
Observed
Calculated
Intensity (arb. units)
Difference
15
30
45
2θ (deg)
60
75
90
Fig. 1. Observed and fitted powder XRD patterns of TbNiAl. The
difference plot between the experimental and calculated patterns is given
at the bottom of the figure.
0
20
40
60
80
100
T (K)
Fig. 2. Temperature dependence of magnetization of TbNiAl under ZFC
and FC conditions in various applied magnetic fields. Arrows indicate the
low-temperature transition at about 23 K.
magnetic ordering with a Neel temperature (T N ) of 47 K.
Therefore, the present M2T data seem to be in agreement
with the neutron diffraction results. The low-temperature
transition appears to be weak, as can be seen from Fig. 2.
It may be noted from Fig. 2 that the magnetization data,
collected in applied fields up to 2 kOe under FC and ZFC
conditions, show thermo-magnetic irreversibility. Generally, the thermo-magnetic irreversibility is observed in
narrow-domain wall systems or geometrically frustrated
systems. It is reported that TbNiAl is a geometrically
frustrated system with antiferromagnetic ordering along
the c-axis as well as in the ab-plane of the hexagonal unit
cell [6–8]. The Tb ions form a triangular lattice in the abplane, which leads to the magnetic frustration in this
compound. The low-temperature peak (23 K), observed
in the M2T data, arises due to the ordering of these
frustrated moments, as the temperature is lowered. Though
the thermo-magnetic irreversibility and low temperature
transition exist up to an applied field of 2 kOe, they seem to
vanish for a field of 20 kOe. This may be due to the fact
that above a critical field (less than 20 kOe), the strength of
the antiferromagnetic interactions decreases, thereby reducing the effects of frustration.
Fig. 3 shows the field dependence of magnetization of
TbNiAl at 2 K. The inset shows the low-field region of the
M2H plot at the same temperature. As can be seen, the
magnetization shows an abrupt change at about 10 kOe.
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304
18 K
TbNiAl
δT ~ 4 K
8
M (µB/f.u.)
6
4
74 K
2
0
0
10
20
(a)
Fig. 3. Field dependence of magnetization obtained at 2 K. Inset shows
the low-field region of the magnetization isotherm. Arrows indicate the
data corresponding to the increasing and decreasing fields.
40
50
60
8
TbNiAl
δT ~ 4 K
18 K
6
M (µB/f.u.)
This may be due to a field-induced metamagnetic transition
from an antiferromagnetic to a predominantly ferromagnetic state. It is of interest to note that this observation is
consistent with the M2T plots shown in Fig. 2. However,
it may also be possible that this transition is a result of a
crystallographic change occurring at low temperature.
From the M2H data obtained at 2 K, the saturation
moment is calculated to be 8.3 mB/f.u., for a maximum field
of 120 kOe. Ehlers and Maletta [8] have reported that the
magnetic structure of TbNiAl can be analyzed in terms of
two sublattices with magnetic propagation vectors
ð1=2; 1=2; 1=2Þ and ð1=2; 0; 1=2Þ. Though the temperature
dependences of the magnetization of these sublattices are
different, the magnetization values are almost the same at
2 K. The average magnetic moment at 2 K is reported to be
about 8.6 mB/f.u., which is very close to the value derived
from the M2H isotherm at 2 K in the present case. It has
also been reported [9] that the nature of low-temperature
magnetization isotherms in TbNiAl and GdNiAl are not
similar to those of conventional antiferromagnets and this
has prompted these authors to assume the existence of
some ferromagnetic phase in both TbNiAl and GdNiAl.
On the other hand, these authors also report that the lowtemperature transitions in both these compounds are
pushed to lower temperatures on application of magnetic
fields, which is indicative of a low-temperature antiferromagnetic phase.
The remanence ratio (M r =M s ) estimated from the M2H
plots is found to be about 7.5% at 2 K, which decreases
to about 1% near T N . This implies that the magnetic
hardness is considerable only at low temperatures. At
temperatures above T N , the magnetic susceptibility obeys
the Curie–Weiss law with an effective paramagnetic
moment of 9.7 mB, which is equal to the free ion value
of Tb3+ ion.
30
H (kOe)
4
46 K
2
0
(b)
0
4
8
H (kOe)
12
16
Fig. 4. (a) Magnetization of TbNiAl as a function of applied magnetic
field at temperatures above and below the Neel temperature. (b) Low-field
region of the M2H plots below the Neel temperature. The temperature
difference (dT) between different isotherms is approximately 4 K.
Fig. 4a shows the field dependence of magnetization
data, collected in the temperature range 18–74 K at
intervals of 4 K, up to a maximum field of 50 kOe. The
low-field region of the M2H plots below T N is expanded
and is shown in Fig. 4b. It can be seen from Fig. 4b that,
below a critical field, the magnetization value at 46 K is
higher than that at 18 K. However, at higher fields, this
trend reverses and the low-temperature magnetization
becomes larger than the high-temperature value. This
indicates that the compound is antiferromagnetic below a
certain field. However, it can be made ferromagnetic with
the application of a suitable field. Such a transition has
indeed been observed in Fig. 3. The occurrence of a fieldinduced metamagnetic transition resulting in a ferromagnetic state has been reported in many intermetallic
compounds [10].
ARTICLE IN PRESS
305
changes of 20 and 50 kOe (with T cold ¼ 20 and
T hot ¼ 72 K) are 199 and 494 J kg1, respectively. It is of
interest to note that the q value of Gd, for a field change of
10 kOe and a temperature difference (between the cold and
hot sinks) of 60 K is about 78 J kg1 [11]. This suggests that
the refrigerant capacity in TbNiAl is comparable to that of
Gd, which is used in practical magnetic refrigerators.
14
TbNiAl
12
−∆SM (J Kg-1K-1)
10
8
6
4. Conclusion
4
In conclusion, the thermomagnetic irreversibility in
TbNiAl seems to arise due to magnetic frustration. With
the increase in magnetic field, the frustration effect
decreases, which may be attributed to a metamagnetic
transition. The low-temperature magnetization behavior
probably indicates the existence of some ferromagnetic
component along with the antiferromagnetic character. A
detailed neutron diffraction study is essential to understand
the exact magnetic state, especially at low temperatures
(2 K). The magnetocaloric properties of TbNiAl are
found to be comparable to those of many potential
magnetic refrigerants. Furthermore, the low remanence
ratio (about 1%) near T N indicates that TbNiAl is
magnetically soft, which is an additional criterion for a
good refrigerant material. The high values of entropy
change and refrigerant capacity along with magnetic
softness make the present compound a promising candidate for refrigeration applications near 50 K.
∆H = 50 kOe
2
∆H = 20 kOe
0
20
30
40
50
60
70
T (K)
Fig. 5. Temperature variation of isothermal magnetic entropy change
(DSM ), for a field change (DH) ¼ 20 and 50 kOe, in TbNiAl.
By employing Maxwell’s relation and using the magnetization isotherms, we have calculated the MCE in terms of
isothermal magnetic entropy change (DS M ). The temperature dependence of DS M for two different fields, namely 20
and 50 kOe, is shown in Fig. 5. The entropy change shows a
peak close to the Neel temperature, T N . Furthermore, a
weak secondary peak is seen at about 23 K, which may be
due to the ordering of the frustrated moments. It may also
be noted that the entropy change is negative (positive
MCE) in this case, unlike in some antiferromagnetic
compounds. The positive MCE indicates a transition from
an antiferromagnetic to a ferromagnetic phase in the
presence of suitable fields, which is consistent with the
magnetization measurements.
It can be seen from Fig. 5 that the maximum values of
DS M are 7.1 and 13.8 J kg1 K1 for field changes of 20 and
50 kOe, respectively. These values compare very well with
those obtained in potential magnetocaloric materials like
(Er,Gd)NiAl. It has been reported that the maximum
values of DS M and the adiabatic temperature change
(DT ad ) in (Er,Gd)NiAl compounds vary in the range
10–22 J kg1 K1 and 4–7 K, respectively, for DH ¼ 50 kOe
[3]. Since TbNiAl is iso-structural to (Er,Gd)NiAl compounds, it is reasonable to assume that DT ad values of
TbNiAl would also be comparable to those of (Er,Gd)NiAl
compounds. It can also be seen from Fig. 5 that DS M does
not fall off rapidly around T N .
The refrigerant capacity (q), which is a measure of heat
transfer between the cold and the hot sinks in an ideal
refrigeration cycle, is defined as
Z T hot
q¼
DSðT; P; DHÞP;DH dT,
T cold
where, T cold and T hot are the temperatures of the cold and
hot sinks, respectively. The q values for TbNiAl, for field
Acknowledgements
K.G.S. thanks DST, Government of India, for financial
support in the form of a sponsored project. The authors
would like to thank Mr. Pramod Kumar, IIT Bombay, for
help in the sample preparation.
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