Phase coexistence induced by cooling across the first order transition in
Ni–Co–Mn–Sb shape memory alloy
Ajaya K. Nayak, K. G. Suresh, and A. K. Nigam
Citation: J. Appl. Phys. 108, 063915 (2010); doi: 10.1063/1.3483951
View online: http://dx.doi.org/10.1063/1.3483951
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Published by the American Institute of Physics.
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JOURNAL OF APPLIED PHYSICS 108, 063915 共2010兲
Phase coexistence induced by cooling across the first order transition
in Ni–Co–Mn–Sb shape memory alloy
Ajaya K. Nayak,1 K. G. Suresh,1,a兲 and A. K. Nigam2
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
共Received 8 May 2010; accepted 28 July 2010; published online 23 September 2010兲
The first order austenite to martensitic transition in the off-stoichiometric Heusler alloy
Ni–Co–Mn–Sb has been studied using magnetization, electrical resistivity, and heat capacity
measurements with different field cooling protocols and thermal cyclings. The coexistence of high
temperature, high magnetic austenite phase along with the low temperature and low magnetic
martensitic phase after field cooling has been confirmed by all these measurements. The time
dependence of these data quite vividly illustrates the metastability of the supercooled/superheated
phase in the disorder-broadened first order transition. © 2010 American Institute of Physics.
关doi:10.1063/1.3483951兴
I. INTRODUCTION
Over the past few years a large number of research
groups have been concentrating on the study of Heusler
alloys/shape memory alloys systems due to their multifunctional properties. Among these alloys Ni–Mn based alloys
are the most studied systems due to their anomalous properties by virtue of the first order nature of the martensitic transition in them. These alloys undergo a first order structural
transition from austenite to martensitic phase on cooling,
giving rise to different magnetic states in these two crystallographic phases due to the strong magnetostructural
coupling.1–3 This coupling between the structural and magnetic states leads to many interesting properties such as fieldinduced shape memory effect, giant magnetocaloric effect,
large magnetoresistance, and exchange bias behavior in these
alloys.4–8 The magnetostructural transition can be shifted by
the application of field as well as by chemical or hydrostatic
pressure.9 As the first order transition is accompanied by a
large thermal hysteresis between the cooling and heating
magnetization data, it is very important to study various
physical phenomena as a function of simultaneous variations
in magnetic and thermal energies.
In general, in these alloys the austenite phase is more
ferromagnetic 共FM兲 than the martensitic phase. The magnetic
state in the martensitic phase is complex due to the presence
of FM and antiferromagnetic 共AFM兲 components. The
Mn–Mn interactions in these off-stoichiometric alloys are
found to be critically dependent on the bond lengths, which
causes both FM and AFM coupling between different
Mn–Mn neighbors. Due to the first order nature of the martensitic transition, it is observed that the high temperature
共austenite兲 phase gets supercooled to low temperatures during cooling across the transition and gets released on heating.
The extent of supercooling depends critically on the cooling
protocol. The supercooling enhances the coexistence of competing magnetic phases 共FM and AFM兲 in addition to the
a兲
Electronic mail: [email protected].
0021-8979/2010/108共6兲/063915/7/$30.00
structural phases. The metastability of the supercooled state
and its kinetic arrest give rise to many interesting features in
these systems. Some of these have been studied in Ni–
Mn–Sn and Ni–Mn–In Heusler alloys and in doped CeFe2
using certain experimental probes.10–12
It has also been observed recently that the features associated with the first order transition as seen in the Heusler
alloys also manifest in certain other systems such as doped
CeFe2. It was found that the supercooling is best achieved by
cooling in presence of a magnetic field, across the first order
transition in doped CeFe2 alloys.13,14 Furthermore, the influence of the martensitic scenario on the magnetization and
related phenomena observed in doped CeFe2 has been illustrated recently.15 The anomalous magnetic properties of giant
magnetocaloric Gd5Ge4 family as well as certain colossal
magnetoresistive manganites were also explained by invoking the martensitic scenario.16–18 In the light of large magnetocaloric effect and exchange bias that we observed in Co
substituted off-stoichiometric Ni50−xCoxMn38Sb12 Heusler
alloys,9,19,20 we have now focused on the martensitic transition region in one of the concentrations namely
Ni43Co7Mn38Sb12. The main reason for selecting this particular composition is to study the metastability arising from the
super cooling of the high temperature phase by field cooling
across the martensitic transition. For the Co concentration 共x兲
less than 6, there exists a secondary transition corresponding
to the Curie temperature of the martensitic phase 共TCM兲 共Ref.
19兲, which vanishes for the present composition. Due to this
the effect of super cooling is not prominent over a considerable temperature regime for lower x. Therefore,
Ni43Co7Mn38Sb12 is the best composition to carry out a detailed study on various features induced by the martensitic
transition as revealed by magnetization, electrical resistivity
and heat capacity data. The effect of metastable and supercooled austenite phase induced by the first order transition
has been studied by using various novel experimental techniques and protocols, the results of which are presented in
this paper.
108, 063915-1
© 2010 American Institute of Physics
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063915-2
Nayak, Suresh, and Nigam
J. Appl. Phys. 108, 063915 共2010兲
FIG. 1. 共Color online兲 The x-ray diffraction patterns of Ni43Co7Mn38Sb12
along with the Rietveld refinement. The Rietveld refinement clearly shows
the single phase of the sample.
II. EXPERIMENTAL DETAILS
Polycrystalline ingot of Ni43Co7Mn38Sb12 was prepared
by arc-melting the appropriate amounts of Ni, Co, Mn, Sb of
atleast 99.99% purity in high pure argon atmosphere. About
3% extra manganese was added to compensate the weight
loss. For better homogeneity the sample was melted four
times and consequently annealed in evacuated quartz tube at
850 ° C for 24 h. The structural characterization was done by
powder x-ray diffractograms 共XRD兲 using Cu K␣ radiation.
The magnetization 共M兲 measurements up to a maximum
field 共H兲 of 90 kOe were carried out using superconducting
quantum interference device magnetometer 共Quantum Design兲 or a vibrating sample magnetometer attached to a
physical property measurement system 共Quantum Design,
PPMS-6500兲. The thermomagnetic measurements have been
performed in the temperature 共T兲 range of 5–200 K in different modes. In the zero field cooled 共ZFC兲 mode, the sample
was initially cooled to 5 K without applying any field and
then the data was taken as the temperature was increased
from 5 K by applying a field, while in the field cooled warming 共FCW兲 mode, the data was collected while heating, after
field cooling to 5 K. The magnetization data was also taken
in the field cooled cooling 共FCC兲 mode in which the data
was collected during cooling. The electrical resistivity 共␳兲
measurements were carried out by the linear four probe
method attached to the PPMS. The heat capacity 共C P兲 measurements were also performed using PPMS, using the relaxation method. The temperature dependence of resistivity and
heat capacity were performed in the temperature range of
2–150 K. We denote the cooling field as Hcool and measuring
field as Hmeasure throughout this paper.
III. EXPERIMENTAL RESULTS
The x-ray diffraction pattern at room temperature, along
with the Rietveld refinement is shown in Fig. 1. The refinement has been performed with L21 structure 共cubic兲 having
space group Fm3̄m. In X2YZ 共full heusler alloy兲 type structure the “X” occupies the 共1/4, 1/4, 1/4兲 site, Y occupies the
共0, 0, 0兲 site, and Z occupies 共1/2, 1/2, 1/2兲 site.21 In the
present case the Ni and Co occupy X site, Mn occupies Y
site, and the extra Mn 共38–25兲 and the Sb occupy the Z site.
FIG. 2. 共Color online兲 ZFC 共closed circle兲 and FCW 共open circle兲 magnetization in Ni43Co7Mn38Sb12 as a function of temperature in various fields. In
the FCW case, the cooling and the measuring fields were the same.
The Rietveld refinement confirms that the sample is single
phase 共austenite, cubic兲 having lattice parameter a = b = c
= 5.96 Å.
Figure 2 shows the temperature dependence of magnetization measured in the temperature range of 5–180 K. In all
the FCW plots, the cooling field and the measuring field
were kept the same. In general there exist two types of transitions, FM to paramagnetic 共around 340 K, not shown in the
figure兲 and the austenite to martensitic transition, around 120
K. The latter is accompanied by a large change in the magnetization. The region of the transition is believed to consist
of both martensitic and austenite components. The sharp decrease in the magnetization across the transition indicates the
presence of some non-FM components.5 In the low field
plots, one can see an additional transition below the martensitic transition, where the separation between ZFC and FCW
curve is observed. This transition corresponds to the blocking temperature 共Tb兲 observed in exchange bias systems, as
discussed in Ref. 18. This may be due to the coexistence of
FM and AFM components in the martensitic phase. It may be
noted that such a transition is absent in the FCW mode for all
cooling fields and in ZFC mode for higher fields. This clearly
indicates that this transition is connected with the AFM component, which vanishes with field. At low temperatures 共below 100 K兲 there is a large separation between ZFC and
FCW curves for fields of 0.1 and 1 kOe 关Figs. 2共a兲 and 2共b兲兴.
This indicates the presence of some magnetic frustration in
this temperature regime, possibly arising from the presence
of an AFM component trapped in the FM matrix, which cannot align with the field direction at low fields. As the measuring 共and the cooling兲 field increases further, the separation
between the ZFC and FCW curves vanishes at a field of 10
kOe. However, for fields greater than 10 kOe the FCW curve
starts to deviate from ZFC curve and the separation again
increases with increase in field. This behavior indicates the
presence of higher magnetic phases at low temperature as a
result of cooling the sample with higher field. As the sample
is cooled in higher field through the martensitic transition,
larger fraction of the austenite phase gets supercooled to the
lower temperatures, giving rise to a higher FCW magnetiza-
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063915-3
Nayak, Suresh, and Nigam
FIG. 3. 共Color online兲 FCW curve measured in 20 kOe after field cooling in
the same field and after waiting for different durations before starting the
measurement. Inset shows the dependence of FCC magnetization on the
cooling rate in Ni43Co7Mn38Sb12. Data are taken with cooling rates of 0.4, 1,
and 4 K/min in a field of 20 kOe.
tion. Using field dependent x-ray diffraction studies in the
Ni–Mn–Co–In system 共with the martensitic transition temperature around 250 K, it was shown that the high temperature phase could be brought down to temperatures as low as
8 K with field cooling through the martensitic transition.22
Therefore, it is quite clear that supercooling exists in the
present system as well. We probe this feature in further detail
with the help of various novel measurement procedures as
detailed below.
Figure 3 shows the FCW thermomagnetic curves obtained after field cooling to 50 K in 20 kOe. After reaching
50 K, the cooling field was kept on, but the magnetization
was recorded after waiting for time durations 共tw兲 of 0, 2000,
and 6000 s. It is observed that the magnetization is the lowest after the longest waiting duration. In the data obtained
with zero waiting time, the measured magnetization corresponds to the magnetization of the mixed initial martensitic
state and the supercooled austenite phase. The latter phase,
which is metastable below the transition temperature, relaxes
with time. Therefore, the FCW curve taken after 2000 s has
less magnetization, as it is contributed mainly by the equilibrium martensitic phase. This behavior reinforces our earlier proposition about the role of metastable, supercooled
phase on the net magnetization. To further investigate the
effect of supercooling induced by the field cooling, we have
measured the FCC thermomagnetic and thermoresistivity
curves with different cooling rates, which is shown in inset
of Fig. 3. Across the transition the magnetization curves
taken at 0.4, 1, and 4 K/min follow different paths. With
decrease in cooling rate, the transition is found to shift to
higher temperatures as the magnetization for a particular
temperature decreases with decrease in the cooling rate. The
magnetization below the transition temperature also decreases with decrease in the cooling rate. This indicates that
with decrease in the cooling rate, the supercooled austenite
phase gets sufficient time to relax, giving rise to lower magnetization.
The temperature dependence of resistivity measured under various protocols is shown in Fig. 4. The martensitic
transition is seen to be associated with a large thermal hysteresis between the cooling and the heating curves in both
J. Appl. Phys. 108, 063915 共2010兲
FIG. 4. 共Color online兲 Temperature dependence of electrical resistivity in
Ni43Co7Mn38Sb12 measured in different fields. The open circles represent the
data taken in zero field 共curve-1兲 and the star symbol represents the data
taken in 80 kOe 共curve-2兲, both after zero field cooling to 5 K. The triangles
represent the data taken in zero field 共curve-3兲, after field cooling in 80 kOe
and squares represent the data taken in 80 kOe 共curve-4兲, after field cooling
in 80 kOe The inset shows the expanded view of the separation between
curve-1 共circle兲 and curve-3 共triangle兲.
Hmeasure = 0 and 80 kOe. The higher magnetization of the austenite phase and the lower magnetization of the martensitic
phase are reflected in the lower resistivity of the austenite
phase and the higher resistivity of the martensitic phase. In
the low temperature region, below the martensitic transition,
a large separation is observed between the warming data of
curve-1 共circles兲 and curve-3 共triangles兲, as shown in the
inset. The large decrease in the latter case is due to the low
resistivity of the supercooled austenite phase as a result of
the field cooling. However, there is no such difference observed at low temperatures between curve-1 共circles兲 and
curve-2 共stars兲. These observations clearly highlight the role
of cooling field as compared to that of the measuring field.
Another interesting behavior observed is that at low temperatures the curve-2 共stars兲 nearly follows the path of the
curve-1 共circles兲 and eventually merges with the curve-4
around the transition region. On the other hand, the curve-3
共triangles兲 starts near the curve-4 共squares兲 and merge with
curve-1. This reveals that the supercooling of the austenite
phase does not change the martensitic transition temperature
when measuring field is same for different cooling fields.
Therefore, it is clear that the observations made from Fig. 4
clearly show the presence of field-induced supercooling as
well as the fact that the measuring field has an influence only
above the transition region.
In Fig. 5共a兲 the ZFC magnetization curve taken in 1 kOe
after various thermal cycles 共closed circles兲 is highlighted
against the normal ZFC data. After cooling the sample to 5 K
in zero field, the sample was subjected to thermal cycling at
T = 30, 40, 50, 60, and 70 K. After recording the magnetization at a particular temperature 共say 30 K兲, the sample was
cooled down to 5 K and then heated back to the next 共higher兲
temperature, in presence of the field. The magnetization measured during cooling follows the same path as that during
heating. However, at each temperature the magnetization
value increases from that of the previous one. This observation clearly suggests that, at low temperatures, even at a
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063915-4
Nayak, Suresh, and Nigam
FIG. 5. 共Color online兲 共a兲 Thermomagnetic measurements performed in
Ni43Co7Mn38Sb12 in 1 kOe field in the low temperature region 共where the
difference between ZFC and FCW magnetization plots is considerable兲. The
filled triangles show the normal ZFC data and the filled circles represent the
ZFC data measured after thermal cycling. The open circles show the FCW
data. 共b兲 Temperature dependence of resistivity measured in zero field. The
open circles represent the data taken in zero field, after zero field cooling.
The closed circles represent the curve taken in zero field on thermal cycling
after field cooling in 80 kOe. The triangles represent the normal FCW data
with Hcool = 80 kOe and Hmeasure = 0, without thermal cycling.
fixed 共low兲 field of 1 kOe, the thermal cycling is able to
convert some fraction of the AFM component to FM gradually. When the temperature increases above 80 K 共where the
ZFC and FCW merge兲, instead of getting a higher magnetization, the magnetization curve follows the FCW curve. Another striking behavior is that the ZFC curve taken without
thermal cycling and the one taken with the thermal cycling
follow the same path 共the triangles and filled circles兲. Below
80 K the thermal cycling gives rise to the gradual nucleation
and growth of the FM state out of the mixed FM and AFM
states. Therefore the temperature region of 5 K ⱕ T ⱕ 80 K
represents a highly metastable regime consisting of predominantly AFM ordering. A similar thermomagnetic curve has
been reported by Roy et al.23 for Gd5Ge4, which has a low
temperature magnetic glassy state.
To study further the metastable nature of the mixed
phase we have performed the similar sequence of measurements to record the resistivity data and the results are shown
in Fig. 5共b兲. The sample was cooled to 5 K with 80 kOe field
and then the field was reduced to zero, then the sample was
subjected to thermal cycling at T = 10, 15, 20, and 30 K. After
reaching a particular temperature the sample was cooled
down to 5 K and then heated back to the next temperature.
Figure 5共b兲 shows this data, along with the ZFC and FCW
data. Thermal cycling gradually suppresses the supercooled
FM phase produced by field cooling, giving rise to the successive increase in the resistivity. Eventually, the thermally
cycled data starts coinciding with the FCW data, at high
temperatures. It can be seen that the phenomenon observed
in Fig. 5共b兲 is a clear reflection of the magnetization behavior
seen in Fig. 5共a兲. In Fig. 5共a兲 the metastability arises in the
ZFC path due to the existence of mixed FM and AFM components and in Fig. 5共b兲 the metastability arises in the field
cooled path due to the existence of mixed austenite and martensitic phases. Therefore, the two figures give same properties when subjected to temperature cycling.
J. Appl. Phys. 108, 063915 共2010兲
FIG. 6. 共Color online兲 Magnetization isotherms and field dependence of
resistivity in Ni43Co7Mn38Sb12. 共a兲 and 共b兲 the sample was ZFC to 5 K and
then heated back to 105 K. 共c兲 and 共d兲 after directly cooling to 105K 共in zero
field兲.
In order to probe the magnetic state in the coexistence
regime, we have collected the magnetization isotherms and
the field dependent resistivity data in different sequences.
Figure 6 shows the magnetization isotherms 关M共H兲兴 and
field dependence of resistivity 关␳共H兲兴 at 105 K. The M共H兲
and ␳共H兲 obtained at 105 K after cooling to 5 K and heating
back to 105 K 关Figs. 6共a兲 and 6共b兲兴, show the virgin curve
共loop 1兲 lying outside the envelope curve 共loop 5兲. Such a
behavior has been obtained in doped CeFe2,13,24,25 MnSb2,26
some manganites,27–29 and a few Heusler alloys.10,11 However, the M共H兲 and ␳共H兲 obtained after directly cooling to
105 K 关Figs. 6共c兲 and 6共d兲兴 do not show such an anomaly.
This difference can also be explained as a consequence of
supercooling/super heating across the first order transition
and the metastable nature of the transition region. When
sample is cooled to 5 K 共in zero field兲 it remains completely
in the martensitic phase and heating back to 105 K, the state
remain in the metastable phase consisting both austenite and
martensitic phases. In Figs. 6共a兲 and 6共b兲, with application of
field the martensitic phase transforms to the austenite one
due to the field-induced transition. When the field is reduced
to zero the whole austenite phase cannot return to its initial
state, giving rise to more magnetization and less resistivity
than the previous zero field state. This gives a large irreversibility in the M共H兲 and r共H兲 isotherms. However when the
sample is directly cooled to 105 K, the state remains nearer
to the austenite phase as there is a large hysteresis between
heating and cooling across the first order transition. So the
resistivity starts with a lower value than the previous case.
Another interesting feature in this case is the separation between the end points of the fifth loop and the virgin curve.
This is due to the fact that there exists some amount of supercooled austenite phase at 105 K, though the cooling field
was zero. On applying a negative filed, the metastable austenite phase gets converted to the martensitic phase, which
gets kinetically arrested. This gives rise to a higher resistivity
along the fifth loop, resulting in the separation between the
end points. Only way to reduce this separation is to increase
the temperature. The slight upturn 共positive magnetoresis-
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063915-5
Nayak, Suresh, and Nigam
J. Appl. Phys. 108, 063915 共2010兲
FIG. 8. 共Color online兲 C P / T as a function of temperature, in
Ni43Co7Mn38Sb12 measured in zero and in 50 kOe. The open circles represent the data taken in zero field and the star symbol represents the data taken
in 50 kOe, both after zero field cooling to 2 K. The triangles represent the
data taken in zero field and square represents the data taken in 50 kOe, both
after field cooling in 50 kOe to 2 K. The inset highlights the separation of
ZFC curves and field cooled curves. All the data are measured in the heating
cycle.
FIG. 7. 共Color online兲 共a兲 Magnetization isotherms in Ni43Co7Mn38Sb12
measured at 5 K after field cooling the sample in different fields H. The
measuring field was applied in the sequence +H → 0 → −H → 0 → +H. The
inset in figure 共a兲 represents the shift in the magnetization loops to negative
direction with increase in cooling field. 共b兲 Field variation in resistivity at 5
K measured in the same sequence.
tance兲 seen in the virgin curve at low fields is due to the
relaxation of the supercooled austenite phase, again indicating its nonequilibrium state at that temperature.
As mentioned earlier, it is observed in Fig. 6共d兲 that the
end points of the virgin curve and that of the 5th loop do not
coincide when the sample is directly cooled to 150 K, because of the effect of the negative field. To throw more light
on this anomaly and to get more insight into the effect of
field cooling on the magnetic and the transport properties, we
have measured the magnetization isotherms and the field dependence of resistivity at 5 K after cooling in different fields.
The sample was field cooled from above the martensitic transition temperature with different cooling fields H and the
magnetization data was collected as the measuring field was
varied in the sequence +H → 0 → −H → 0 → +H. The magnetization isotherms thus obtained are shown in Fig. 7共a兲. The
magnetization steadily increases with the successive increase
in the cooling field, though it does not saturate even at a
measuring field of 80 kOe. On the other hand, the resistivity
shows a systematic decrease, as can be seen in Fig. 7共b兲.
However, the separation between two successive resistivity
curves decreases with cooling field. The increase in magnetization and the decrease in resistivity with higher cooling
field can be attributed to the gradual increase in the austenite
components having larger magnetization, with increase in the
cooling field. The same behavior has been observed in Cr
doped Nd0.5Ca0.5MnO3, where the FM phase increases with
higher cooling field in a mixed phase system.30 The result
obtained in this case clearly supports the result obtained in
Fig. 2, where the separation between ZFC and FCW curves
increases with increase in cooling field. It is worth noting
that for the highest cooling field, the magnetization 共resistivity兲 at the end of the field cycle is significantly lower 共higher兲
than the starting value. This may be explained as follows.
Cooling the sample with some field gives a mixture of magnetic phases, because of the supercooling. However, it may
be noted at this point that at 5 K, the martensitic phase is the
stable phase and the austenite phase is metastable, unlike at
105 K. The magnetization at the starting point 共in the first
loop兲 is predominantly due to the field-induced FM 共austenite兲 phase. As the measuring field decreases, the austenite
phase starts losing its stability. By completely reversing the
field to negative direction the martensitic phase grows at the
expense of the austenite phase, as mentioned earlier. When
the field increases further toward positive H value, the magnetization recovers only partially, as the complete restoration
of the original austenite fraction is possible only with the
help of heating to a higher temperature. This is a characteristic of a field-induced, disorder-broadened first order transition. This observation reconfirms the fact that the austenite
component loses its stability by reversing the field. This effect was also evidenced in the field dependence of resistivity
data. In fact this role of the negative field is the reason for the
separation between the end points of the envelope and the
virgin curves in Fig. 6共d兲. The inset of Fig. 7共a兲 represents
the shift in the magnetization loop to the negative field axis
with increase in cooling field. This represents the typical
exchange bias behavior in the compound. The complete
study on the exchange bias behavior of this compound has
been reported elsewhere.19
To study the effect of field cooling on the heat capacity
behavior we have measured the temperature dependence of
heat capacity in zero and in 50 kOe with different measurement protocols. All the measurements were performed in
heating mode. Figure 8 shows the temperature variation in
C P / T obtained under various protocols. The martensitic transition corresponds to the hump in the heat capacity. The tran-
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063915-6
J. Appl. Phys. 108, 063915 共2010兲
Nayak, Suresh, and Nigam
FIG. 9. 共Color online兲 Time dependence of heat capacity measured in
Ni43Co7Mn38Sb12 at 10 K. Prior to the measurement the sample was field
cooled to 10 K in 60 kOe.
sition temperature shifts to lower temperatures when the
measurement is performed in presence of a field, which implies that the martensitic phase gets destabilized with field.
The same trend is also observed in magnetization and resistivity measurements. The most striking feature is that the
heat capacity is more under field cooling, irrespective of the
value of the measuring field 共highlighted in the inset兲. However, the peak corresponding to the transition occurs at the
same temperature for the same measuring field, irrespective
of the cooling field. The behavior obtained in this case is
completely in agreement with the resistivity data shown in
Fig. 4. In field cooling, the resulting mixed phase regime
being a nonequilibrium metastable state, gives rise to larger
heat capacity than that in the ZFC case.
In view of the metastability seen in various data mentioned above, it was decided to study the time dependence of
the nonequilibrium supercooled phase using the heat capacity and magnetization probes. These results are presented in
the following sections. It may be recalled here that the heat
capacity value measured after field cooling is higher than
that measured after zero field cooling, which was attributed
to the metastability. To verify this prediction, we have cooled
the sample to 10 K in a field of 60 kOe and immediately the
field was reduced to zero. The time dependence of the heat
capacity was then measured for a time period 共t兲 of about
350 min, which is shown in Fig. 9. The heat capacity value
starts from 21.3 J mol−1 K−1 at t = 0 and attains a value of
FIG. 10. 共Color online兲 Magnetization relaxation in Ni43Co7Mn38Sb12 measured at 50 K after cooling from 200 K in different fields and then reducing
the field to zero.
18 J mol−1 K−1 at t = 350 min. The decrease in heat capacity
with time confirms the result obtained in all the previous
measurements. Cooling in 60 kOe transforms some of the
high temperature phase to low temperatures and gives a
larger heat capacity than the parent low temperature phase as
observed in Fig. 8. Since the austenite phase is metastable at
low temperature, it relaxes with time and hence the martensitic phase grows. This leads to the decrease in the heat capacity with time and tends to attain the value of the initial
martensitic phase, with time.
Figure 10 represents the relaxation measurement done
after cooling the sample to the martensitic region in various
fields. After cooling the sample to 50 K the field was immediately reduced to zero. It is observed that magnetization
decreases with time. Initially the decrease is rapid and then it
slows down with time, which is consistent with the result
obtained from the heat capacity relaxation measurement. As
the sample has a remanent magnetization at 50 K, even after
reducing the field to zero, the magnetization starts from a
nonzero value and changes slightly with different cooling
fields. The decrease in magnetization again confirms the relaxation of the austenite component with time.
IV. DISCUSSION
The results presented in the above section quite vividly
show the role of phase coexistence and metastability associated with the disorder-broadened first order transition in the
anomalous behavior of Ni43Co7Mn38Sb12. The consistency
seen in the three measurements namely magnetization, electrical resistivity and heat capacity are worth noting. The
blocking temperature 共Fig. 2兲 and the exchange bias 关Fig.
7共a兲兴 clearly suggest that there is a coexistence of competing
FM and AFM components in this alloy. Though the offstoichiometry of the compounds is a reason for the FM-AFM
coexistence in the martensitic phase, supercooling of the FM
austenite phase enhances this feature. All the measurements
carried out in the present case show that supercooling is
present over a wide temperature regime, under field cooling.
In the case of zero field cooling, the supercooling is present
only over a narrow temperature interval below the transition.
This implies that the magnetic state at low temperatures is
quite complex and metastable. The metastability has been
seen in the time and cooling field/rate dependencies of the
magnetization data. The relaxation of the heat capacity data
also points toward the same inference. Magnetization, resistivity, and heat capacity data vividly show that the supercooled FM component is present even at the lowest temperature. The kinetic arrest, as revealed by the irreversibilities in
the magnetization, resistivity, and heat capacity data, must be
attributed to the substitutional disorder, which broadens the
first order martensitic transition. Since the XRD shows a
clear single phase compound, it is quite unlikely that the
arrest is due to any impurity phases.
It was earlier found that Co substitution in
Ni50−xCoxMn38Sb12 system gives rise to a large exchange
bias for various values of x. In fact the exchange bias was
found to increase with x up to x = 5 and then it decreases for
higher x values. The striking point that we note is that the
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063915-7
J. Appl. Phys. 108, 063915 共2010兲
Nayak, Suresh, and Nigam
supercooled FM phase is only around the martensitic region
in the case of x ⬍ 7, whereas it is present down to the lowest
temperature in the case of x = 7. By comparing these observations, it is found that the supercooling become more effective if the entire martensitic temperature range is free from a
paramagnetic phase, i.e., there is no TCM. Increase in both the
moment and the magnetostructural coupling at x = 7 may be
the reason for the absence of TCM, because of the overlapping
of the martensitic and the magnetic transitions. Another interesting aspect brought about by the present study is the role
of the negative field on the magnetization and electrical resistivity behavior. Negative field is found to cause the conversion of the metastable austenite phase to the martensitic
phase and this gets kinetically arrested. This is clearly evident in the decrease in the magnetization and the increase in
the resistivity after the negative field excursion and points
toward the role of strong magnetostructural coupling in the
material. From this study, we also find that the heat capacity
in the mixed phase is more than that in the pure austenite or
the martensitic phase.
The phase coexistence and the metastability in this alloy
are associated with the disorder influenced first order phase
transition.31 The degree of super cooling and hence the metastability depend on the strength of the cooling field or the
quenching field. Therefore, for any system having first order
phase transition, the metastability can be observed due to the
competition between two states in the presence of a
quenched disorder.32 From the time dependence of heat capacity and magnetization study it is clearly observed that the
nucleation and growth of the martensitic phase at the expense of the metastable austenite phase, gives rise to a percolation type of phenomenon. In this regard Moreo et al.33
have theoretically observed that in doped manganites, clusters are induced by disorder in the exchange and the hopping
amplitudes near a first order transition. In this respect this
alloy shows striking similarities with such manganites,
doped CeFe2 and giant magnetocaloric Gd5Ge4 family.25 The
common feature in all these different classes of materials is
the strong magnetostructural coupling. Finally, we highlight
the fact that Ni43Co7Mn38Sb12 alloy is anomalous in many
respects as compared to the other members of
Ni50−xCoxMn38Sb12 series, though there are many common
features between them.
V. CONCLUSIONS
In conclusion, we have studied the effects of field cooling and zero field cooling across the first order phase transition. The arrest of high temperature phase to low temperature
has been revealed by various measurements like magnetization, resistivity, and heat capacity. These measurements
clearly bring out the various anomalous behaviors not yet
reported in this system. The metastable nature of the austenite phase in martensitic phase has been confirmed by various
FCC isotherms of magnetization and resistivity, along with
the relaxation data. The anomalous magnetic and related
properties observed in this alloy have been attributed to the
strong magnetostructural coupling brought about by Co. The
results obtained in the present case have been correlated with
the similar results observed in some intermetallic and oxide
materials which show first order transition. It is expected that
the present results may stimulate similar studies on other
systems with a strong magnetostructural coupling.
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