Pressure induced magnetic and magnetocaloric properties in NiCoMnSb
Heusler alloy
Ajaya K. Nayak, K. G. Suresh, A. K. Nigam, A. A. Coelho, and S. Gama
Citation: J. Appl. Phys. 106, 053901 (2009); doi: 10.1063/1.3208064
View online: http://dx.doi.org/10.1063/1.3208064
View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v106/i5
Published by the American Institute of Physics.
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JOURNAL OF APPLIED PHYSICS 106, 053901 共2009兲
Pressure induced magnetic and magnetocaloric properties in NiCoMnSb
Heusler alloy
Ajaya K. Nayak,1 K. G. Suresh,1,a兲 A. K. Nigam,2 A. A. Coelho,3 and S. Gama3
1
Department of Physics, Magnetic Materials Laboratory, Indian Institute of Technology Bombay,
Mumbai 400076, India
2
Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai 400005, India
3
Instituto de Física ‘Gleb Wataghin’, Universidade Estadual de Campinas-UNICAMP, CP 6165,
Campinas 13 083 970, Sao Paulo, Brazil
共Received 13 April 2009; accepted 22 July 2009; published online 1 September 2009兲
The effect of pressure on the magnetic and the magnetocaloric 共MC兲 properties around the
martensitic transformation temperature in NiCoMnSb Heusler alloy has been studied. The
martensitic transition temperature has significantly shifted to higher temperatures with pressure,
whereas the trend is opposite with the application of applied magnetic field. The maximum magnetic
entropy change around the martensitic transition temperature for Ni45Co5Mn38Sb12 is 41.4 J/kg K at
the ambient pressure, whereas it is 33 J/kg K at 8.5 kbar. We find that by adjusting the Co
concentration and applying suitable pressure, NiCoMnSb system can be tuned to achieve giant MC
effect spread over a large temperature span around the room temperature, thereby making it a
potential magnetic refrigerant material for applications. © 2009 American Institute of Physics.
关doi:10.1063/1.3208064兴
I. INTRODUCTION
Recently, Ni–Mn based ferromagnetic shape memory alloys have drawn much attention due to their multifunctional
properties in various fields. Among these alloys, Ni–Mn–Ga
has been studied extensively since it shows large magnetic
field induced strain.1,2 The most interesting property of these
alloys is that they undergo a first order structural transition
from high magnetic austenite phase to a considerably low
magnetic martensitic phase, which gives rise to many important properties like shape memory effect, magnetocaloric effect 共MCE兲 and magnetoresistance.3–8 As many of these alloys show martensitic transitions and consequently large
MCE around room temperature, they can be potential candidates as room temperature magnetic refrigerants. Discovery
of this new series with giant MCE has enhanced the prospect
of “near room temperature magnetic refrigeration,” which
has emerged as an energy-efficient and ecofriendly alternative to the commonly used gas-based refrigeration technology. Therefore, these materials are being considered, along
with Gd and Gd5共Si1−xGex兲4, for room temperature magnetic
cooling.9
Recently, several studies have been reported on the magnetostructural and magnetothermal properties of Ni–Mn–Sb
Heusler system.10–13 In general, the off-stoichiometric alloys
show first order structural transition around room temperature, giving rise to large MCE. This alloy generally shows
large inverse 共negative兲 MCE, i.e., the entropy change is
positive on applying the magnetic field. The magnetic moment of the compound mainly arises from the Mn atoms. The
Mn–Mn exchange interaction is strongly dependent on the
Mn–Mn distance, which can be changed by chemical substia兲
Author to whom correspondence should be addressed. Electronic mail:
[email protected].
0021-8979/2009/106共5兲/053901/4/$25.00
tutions or by application of hydrostatic pressure. These
changes are expected to have a profound influence on the
magnetic properties. Recently, the effect of pressure on some
of the Ni–Mn based systems has been reported.14–17 Apart
from these, there are only a very few pressure studies on the
Heusler systems, especially on their MCE behavior. Recently, we have reported a large enhancement of MCE with
Co substitution in NiMnSb.18 In this paper, we report the
effect of pressure on the magnetic and MC properties of
Ni50−xCoxMn38Sb12. It was found that the substitution of Co
for Ni reduces the martensitic transition temperature. Since
we have focused in the temperature region near room temperature, we have studied the pressure effect in the compounds with x = 4 and 5, which have the martensitic transition close to room temperature.
II. EXPERIMENTAL DETAILS
The methods of preparation of polycrystalline samples
of Ni50−xCoxMn38Sb12 and their structural characterization
are reported in Ref. 18. The magnetization measurements
under various applied pressures 共P兲 have been performed
using a superconducting quantum interference device magnetometer 共quantum design兲 attached with a Cu–Be clamp
type pressure cell with a maximum pressure of 12 kbar.
III. RESULTS AND DISCUSSION
The powder x-ray diffraction patterns of the samples
with x = 4 and 5 taken at room temperature show that they
possess the austenite 共cubic兲 phase. However, the compound
with x = 4 showed traces of martensitic phase, as this phase
starts just below the room temperature. No such peaks corresponding to the martensitic phase have been seen in the
compound with x = 5. The variation of the lattice parameters
with Co concentration was negligible. The temperature de-
106, 053901-1
© 2009 American Institute of Physics
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J. Appl. Phys. 106, 053901 共2009兲
Nayak et al.
MS
35
p=0 kbar
p=1.4 kbar
p=3 kbar
p=7 kbar
p=9 kbar
M (emu/g)
30
25
20
15
10
0
270
285
(a)
MS
50
M (emu/g)
40
30
300
T (K)
315
330
0
Ni45Co5Mn38Sb12
1 kOe
20 kOe
50 kOe
80 kOe
45
30
p=0 kbar
p=1.1 kbar
p=2.7 kbar
p=5.7 kbar
p= 8.5 kbar
MF
240
260
280
T (K)
300
0
100
150
200 250
T (K)
300
350
FIG. 2. 共Color online兲 Temperature dependence of magnetization with various fields for Ni45Co5Mn38Sb12 at ambient pressure.
Ni45Co5Mn38Sb12
20
10
60
15
MF
5
(b)
Ni46Co4Mn38Sb12
M (emu/g)
053901-2
320
FIG. 1. 共Color online兲 Temperature dependence of magnetization with various pressures for 共a兲 Ni46Co4Mn38Sb12 and 共b兲 Ni45Co5Mn38Sb12.
pendence of magnetization with various pressures is shown
in Figs. 1共a兲 and 1共b兲. For each pressure, the measurements
were done in the field cooled 共FC兲 and the field heated 共FH兲
modes in a field of 1 kOe. For better clarity, the magnetization curves are shown in the martensitic transition region
only. With decrease in temperature, the materials undergo the
martensitic transition, followed by a sharp decrease in the
magnetization. The temperatures corresponding to the onset
and the end of this transition, on cooling, are called the martensitic start 共M S兲 and the martensitic finish 共M F兲 temperatures, as shown in Figs. 1共a兲 and 1共b兲.
It is clear from Fig. 1 that the martensitic transition temperature 共M S or M F兲 increases with pressure in both the compounds, which indicates that the martensitic phase becomes
more stable with pressure. This may be due to the lower
volume of the martensitic phase as compared to that of the
austenite phase. It is important to note that the change in
electronic structure due to pressure decides the transition
temperature, as martensitic transition is related to the formation of Ni d and Sb p hybrid state.19 The increase in pressure
would enhance the hybridization between Ni and Sb, thereby
strengthening the Ni-Sb bond. This necessitates more thermal energy for the martensite-austenite transition to occur,
leading to an increase in the martensitic transition temperature. The magnetization at both M S and M F is found to decrease with pressure. However, there is a significant decrease
in the magnetization value of the austenite phase at M S,
which leads to a reduction in the magnetization step across
the martensitic transition. The fact that the magnetization at
M S is found to decrease considerably with pressure indicates
that there is no significant enhancement of the Curie temperature of the austenite phase 共TCA兲 with pressure. This is in
contrast to a recent report which shows that in NiMnIn,14 the
magnetization in the austenite phase remains constant with
pressure. The low magnetization in the martensitic phase indicates the presence of some antiferromagnetic component,
even at ambient pressure. From the dM / dT versus T plots, it
is found that the Curie temperature of the martensitic phase
共TCM 兲 increases marginally with pressure. It is also observed
that the magnetization in the martensitic phase around M F
decreases nominally with pressure. The net result is that, in
both the alloys, the sharpness of the martensitic transition
decreases with pressure, which is expected to reflect in the
MCE. However, the thermal hysteresis observed between FC
and FH at all pressures indicates that the first order nature of
the structural transition from austenite to martensitic phase is
retained even at the highest pressure.
As mentioned earlier, the magnetic ordering in these
compounds are mainly determined by the Mn–Mn indirect
exchange interaction, which depends on the Mn–Mn distance. Because of the off-stoichiometry, a part of the Mn
atoms may occupy the Sb sites.20 At ambient pressure, the
exchange interaction is known to be ferromagnetic within the
Mn sublattices while the interaction between the Mn atoms
occupying the regular Mn sites and Sb sites is antiferromagnetic in both the austenite and martensitic phases. The net
result is the competition between the two types of interactions. Depending on these interactions, the austenite to martensitic transition temperature and the magnetization in both
austenite and martensitic phases show different behaviors
with substitution of Co 共for Ni兲, application of pressure and
field, etc.
In order to get an understanding on the effect of magnetic field and pressure on the martensitic transition, the thermomagnetic variation with different fields, at ambient pressure, is shown in Fig. 2. It can be seen that the martensitic
transition shifts to lower temperatures with increase in field.
For Ni45Co5Mn38Sb12, M S is 270 K in a field of 1 kOe, which
decreases to 252 K at 80 kOe. Therefore, by comparing Figs.
1 and 2, it is clear that the effect of applied field is opposite
to that of pressure. This fact is also highlighted in Fig. 3,
which shows the variation of M S with pressure, whereas the
inset shows the shift of M S with field. The difference between the application of field and pressure is also reflected in
the magnetization jump 共⌬M兲 between M S and M F. While
the ⌬M increases with field, it decreases with pressure in
both the compounds. These observations indicate that the
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053901-3
J. Appl. Phys. 106, 053901 共2009兲
Nayak et al.
FIG. 5. 共Color online兲 Magnetic entropy change 共⌬SM 兲 at various pressures
as a function of temperature for 共a兲 Ni46Co4Mn38Sb12 and 共b兲
Ni45Co5Mn38Sb12
FIG. 3. 共Color online兲 Variation of M S with pressure for Ni50−xCoxMn38Sb12
共x = 4 , 5兲. The inset shows the change in M S with magnetic field at ambient
pressure for x = 5.
The MCE 共⌬SM 兲 was calculated from the isothermal
magnetization curves using the Maxwell equation
冕冉
H
pressure stabilizes the martensitic state, whereas the magnetic field prefers the austenite state. The increase in the
martensitic transition temperature 共⌬T兲 with pressure in the
case of x = 4 is about 25 K, while it is 28 K for x = 5, for the
highest pressure. On the other hand, the decrease is only
about 15 K even at the highest field of 80 kOe at ambient
pressure in x = 5. It is also found that the shift in M S with
pressure in the present case is much larger than the one reported in other Ni–Mn based Heusler alloys.
Figure 4 shows the magnetization isotherms at the ambient
共P = 0兲
and
the
highest
pressures
for
Ni50−xCoxMn38Sb12 with x = 4 and 5. For x = 4, at P = 0 关Fig.
4共a兲兴, the magnetization isotherms at 292–294 K shows a
metamagnetic transition, which is absent at P = 9 kbar. In the
case of x = 5, the metamagnetic transition seen at ambient
pressure is present even at higher pressures, as is evident
from Figs. 4共c兲 and 4共d兲. The metamagnetic character in the
M-H isotherms is associated with a field-induced reverse
martensitic transformation from a low magnetization martensitic state to a higher magnetization austenite state. In general, it can be seen that the critical fields for the metamagnetic transition increases with pressure, which implies that
the strength of the antiferromagnetic component in the martensitic phase increases with pressure. Decrease in the
Mn–Mn bond length must be the reason for this increase.
Another feature seen from Fig. 4 is that the magnetization
value at the highest field as well as the difference in the
values between two consecutive isotherms around the martensitic transition temperature decrease with pressure. This
reduction would determine the MCE of these compounds
with pressure.
⌬S M 共T,H兲 =
0
⳵ M共T,H兲
⳵T
冊
dH.
共1兲
H
Figures 5共a兲 and 5共b兲 show the ⌬SM as a function of temperature for applied fields 共⌬H兲 of 20 and 50 kOe for
Ni46Co4Mn38Sb12 and Ni45Co5Mn38Sb12, respectively. It can
be seen that the entropy change is positive and that the
共⌬S M 兲max decreases with pressure. For Ni46Co4Mn38Sb12,
there is a continuous decrease in 共⌬S M 兲max from 32.3 J/kg K
for P = 0 to 16.5 J/kg K for P = 9 kbar. For Ni45Co5Mn38Sb12
at P = 0, the 共⌬S M 兲max is 41.4 J/kg K which increases to 46
J/kg K at p = 1.1 kbar and finally decreases to 32.5 J/kg K at
P = 8.5 kbar. The decrease in the MCE can be attributed to
the weakening of the first order transition, which results in
smaller difference in the magnetization between the austenite
and the martensitic phases. This causes a reduction in the
⳵ M / ⳵T value. Another important observation from Fig. 5 is
the tunability of the martensitic transition temperature with
pressure, even while retaining significant entropy change.
This result in large MCE over a wide temperature spans
around the room temperature and makes this system quite
interesting from the point of view of applications
It can also be seen from Fig. 5 that, at any pressure, the
entropy change is larger in x = 5, as compared to that in x
= 4. This may be due to the fact that Co has a larger magnetic
moment as compared to Ni and hence substitution of Co for
Ni would increase the net magnetic moment. As mentioned
earlier, there exist both ferromagnetic 共FM兲 and antiferromagnetic 共AFM兲 interactions between the Mn atoms at different sublattices. Substitution of Co for Ni would increase
the ferromagnetic coupling. It has been suggested that the
enhancement of ferromagnetic coupling may originate from
the altered electronic structure due to the presence of the Co
3d electrons.7
The suitability of a magnetic refrigerant is judged on the
basis of its refrigeration capacity 共RC兲, which can be calculated using the equation
RC =
冕
T2
关⌬S M 共T兲兴⌬HdT,
共2兲
T1
FIG. 4. 共Color online兲 Magnetization isotherms of Ni50−xCoxMn38Sb12 for
x = 4 and 5 at the ambient 共P = 0兲 and the highest pressures.
where T1 and T2 are the temperatures of the cold and the hot
sinks. In the present case, the RC is calculated by integrating
⌬S M 共T兲 curve over the temperature span corresponding to
the full with at half maximum points. It is observed that for
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053901-4
both x = 4 and 5, RC decreases marginally with pressure. The
maximum RC values obtained at ambient pressure for x = 4
and 5 are 95 and 143 J/kg respectively, which decreases to 78
and 128 J/kg, respectively, at highest pressure. We have also
calculated the hysteresis loss at ambient pressure for
Ni46Co4Mn38Sb12. The average hysteresis loss is estimated to
be 21 J/kg for the same temperature interval used for calculating the RC. The large hysteresis loss arises due to the
metamagnetic transition in the MH isotherms 共Fig. 4兲. The
effective RC value for this compound turns out to be 84 J/kg
at ambient pressure. It is found that the RC values observed
in Ni50−xCoxMn38Sb12 system is much larger than the values
reported for the parent system namely NiMnSb.10 However,
the RC value in present case cannot be compared with that of
Gd5共Si1−xGex兲4,9 as it is calculated in very narrow temperature interval.
IV. CONCLUSIONS
We have studied the effect of pressure on the magnetic
and MC properties of NiCoMnSb Heusler alloys. Pressure
enhances the stability of the martensitic phase, leading to an
upward shift in the martensitic transition temperature. The
isothermal magnetic entropy change is found to decrease
with pressure, which is a result of the reduction in the magnetization step at the martensitic transition. A detailed neutron diffraction study would be essential to unravel the magnetic moment variation brought about by Co substitution and
pressure.
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