Magnetic, electrical, and magnetothermal properties in Ni–Co–Mn–Sb
Heusler alloys
Ajaya K. Nayak, K. G. Suresh, and A. K. Nigam
Citation: J. Appl. Phys. 107, 09A927 (2010); doi: 10.1063/1.3368109
View online: http://dx.doi.org/10.1063/1.3368109
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JOURNAL OF APPLIED PHYSICS 107, 09A927 共2010兲
Magnetic, electrical, and magnetothermal properties in Ni–Co–Mn–Sb
Heusler alloys
Ajaya K. Nayak,1,a兲 K. G. Suresh,1 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Presented 19 January 2010; received 18 October 2009; accepted 18 December 2009;
published online 21 April 2010兲
We have studied the magnetic, magnetoresistance, and thermal properties of Ni50−xCoxMn38Sb12 for
x = 0 – 7. The martensitic transition temperature decreases with increase in x and there is no
martensitic transition observed for x = 8. The martensitic transition is accompanied by a large change
in magnetization as well as in electrical resistance. Due to the large change in magnetization, a large
magnetic entropy change in 68 and 43 J kg−1 K−1 is observed for x = 5 and 4, respectively, around
the room temperature. A large magnetoresistance of 34% is observed for x = 7. Both the
magnetocaloric effect and the magnetoresistance are associated with the martensitic transition,
which can be tuned significantly by varying the Ni/Co composition. The results obtained in this
system suggest that it may act as a potential magnetic refrigerant as well as a magneto resistive
material. © 2010 American Institute of Physics. 关doi:10.1063/1.3368109兴
I. INTRODUCTION
Materials exhibiting magnetic first order transition
共MFOT兲 have received a great attention because many of
them show multifunctional behavior. Owing to MFOT, materials such as Gd5共Si1−xGex兲4,1 Mn–Fe–As,2 La共Fe, Si兲13,3
and MnFePAs 共Ref. 4兲 exhibit a large magnetocaloric effect
共MCE兲, which can be exploited for magnetic refrigeration.
Recently, the ferromagnetic 共FM兲 shape memory 共FSM兲 alloys became a new member of the MFOT family, as it shows
first order magnetostructural transition. Among the FSM alloys, Ni–Mn–X 共X = Ga, In, Sn, and Sb兲 Heusler alloys attract a great deal of attention due to their multifunctional
properties such as shape memory effect, giant MCE, giant
magnetoresistance 共MR兲, and exchange bias behavior.5–10 All
these properties are critically dependent on the austenite to
martensitic structural transition, which as well modifies the
magnetic state as a result of a strong magnetostructural coupling. The martensitic transition temperature of these alloys
depends on the chemical composition. So, the martensitic
temperature can be tuned by adjusting the chemical composition of Ni and Mn or by substituting Co, Cu, or Fe in place
of Ni or Mn.12,13 In this paper, we have studied the effect of
Co on the magnetic, electrical, and magnetothermal properties of Ni50−xCoxMn38Sb12 共x = 0 – 7兲.
der x-ray diffractograms 共XRDs兲. The magnetization measurements were carried out using a vibrating sample magnetometer attached to a physical property measurement system
共PPMS兲 共Quantum Design, PPMS-6500兲/superconducting
quantum interference device magnetometer and the resistivity measurements were done by the four probe method using
the PPMS. The heat capacity measurement was also performed using PPMS. The differential scanning calorimetry
共DSC兲 measurements were performed using the TA Q100
setup with a cooling/heating rate of 20 K min−1.
III. RESULTS AND DISCUSSION
The XRD patterns taken at room temperature show all
the compounds are single phase. The samples with x ⱖ 4 possess L21 austenite 共cubic兲 phase, whereas for x ⱕ 3 show 10
M modulated orthorhombic martensitic structure at room
temperature. The Rietveld refinement has shown that for the
alloys having the same crystal structure at room temperature,
the variation in lattice parameter with Co is negligible. Figure 1 shows the DSC studies for alloys with x = 3, 4, and 5.
The temperature versus heat flow measurements were per-
II. EXPERIMENTAL DETAILS
Polycrystalline ingots of Ni50−xCoxMn38Sb12 共x = 0 – 7兲
were prepared by arc-melting the appropriate amounts of Ni,
Co, Mn, and Sb with purity more than 99.99% in pure argon
atmosphere. For better homogeneity, the ingots were annealed in evacuated quartz tubes at 850 ° C for 24 h. The
structural characterization of the materials was done by powa兲
Electronic mail: [email protected].
0021-8979/2010/107共9兲/09A927/3/$30.00
FIG. 1. 共Color online兲 Heat flow as a function of temperature for x = 3, 4,
and 5, both in cooling and heating modes.
107, 09A927-1
© 2010 American Institute of Physics
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09A927-2
Nayak, Suresh, and Nigam
FIG. 2. 共Color online兲 Temperature dependence of heat capacity in zero
field for x = 4 and 5.
formed in both heating and cooling modes. In the heat flow
curve the vertical displacements 共peaks兲 are proportional to
the heat capacity of the sample and is expected to change
with composition, which is clearly seen from Fig. 1. The
start and finish transition temperatures are taken at the temperature where the heat flux curve start changing the slope.
In the cooling curve, the start and finish points of change in
slope indicate the martensitic start 共M S兲 and martensitic finish temperature 共M F兲. Similarly in heating curve, the start
and finish points of change in slope indicate the austenite
start 共AS兲 and austenite finish 共AF兲 temperature. The martensitic transition temperature is defined as T M = 共M S + M F兲 / 2
and the of austenite transition as TA = 共AS + AF兲 / 2. For x = 4,
the various transition obtained from DSC are M S = 294 K,
M F = 281 K, AS = 290 K, and AF = 306 K and for x = 5, M S
= 264 K, M F = 252 K, AS = 258 K, and AF = 278 K. From
this, for x = 4, T M = 288 K and TA = 298 K and that for x = 5,
T M = 258 K and TA = 268 K have been calculated. The transition temperatures obtained from the DSC are in very good
agreement with those obtained from the magnetization data.
Figure 2 shows the variation in heat capacity 共C兲 with
temperature for x = 4 and 5, measured in heating mode in
zero field. For x = 5, the TA occurs at 266 K and that for x
= 4 is at 295 K, which indicate the structural transition from
martensitic to austenite phase, which is nearly same as the TA
observed in DSC curve. However, we find that the DSC values are more than two times the heat capacity values when
expressed in the same unit. The peak height obtained from
HC is 2.9⫻ 103 J / mole K for x = 4 and 2.6⫻ 103 J / mole K
for x = 5. The same obtained from DSC is 8.5⫻ 103 and 6.8
⫻ 103 J / mole K for x = 4 and 5, respectively. This may be
due to the fact that the coupling between the sample and the
sample puck decreases to about 70% around the first order
transition region, which affects the heat capacity value.
Figure 3 shows the temperature dependence of magnetization measured in zero field cooled 共ZFC兲, field cooled
cooling 共FCC兲, and fieled cooled warming 共FCW兲 mode. All
measurements are performed in presence of 1 kOe field. The
FM to paramagnetic transition temperature of the austenite
phase 共TCA兲 for all the compounds occurs above 330 K. As
temperature decreases below TCA, the magnetization increases to a maximum value at M S and then attains the minimum at M F. For x = 2 – 5, below M F, the magnetization
shows an increase, corresponding to the magnetic ordering
temperature of the martensitic phase 共TCM 兲, whereas there is
J. Appl. Phys. 107, 09A927 共2010兲
FIG. 3. 共Color online兲 Temperature dependence of magnetization measured
in 1 kOe field. The half open symbols represent ZFC, the close symbols
represent FCC, and the open symbols represents FCW magnetization curve
for Ni50−xCoxMn38Sb12 共x = 2 – 7兲.
no sign of TCM for x = 6 and 7. It is interesting to note that as
x increases beyond 7, there is no martensitic transformation
observed in the system. The T M decreases monotonically
with increasing Co concentration, e.g., T M for x = 2 is 310 K
and for x = 7, it is around 100 K. The sharp decrease in magnetization around T M indicates the presence of some non-FM
entities around the M S-M F regime. It is expected that in the
entire martensitic region, there is a coexistence of both FM
and antiferromagnetic 共AFM兲 components. This is because in
the X2YZ共50: 25: 25兲 compound having Fm-3m space group,
the X = Ni occupies the 共1/4,1/4,1/4兲 site, Y = Mn occupies
the 共0,0,0兲 site, and Z = Sb occupies the 共1/2,1/2,1/2兲 site. On
the other hand, in the nonstoichiometric compound 共as in the
present case兲 the extra Mn atoms occupy the Sb site. The
coupling between the regular Mn sites is FM; the one between Mn atoms occupying the regular Mn and the Sb sites
is AFM.14 It is also observed that there is an increase in the
magnetization of the austenite phase with Co. This may be
due to the fact that Co has a larger moment than Ni and
substitution of Co may increase the FM ordering. At low
temperatures for all the samples the drop in the ZFC magnetization indicates the presence of AFM in the martensitic
phase.
Figure 4 shows the temperature dependence of electrical
resistivity 共␳兲, measured both in cooling and heating mode,
in zero and 50 kOe applied fields. At low temperatures the
resistances remain constant for all samples except for x = 5,
where there is a downward trend both in zero field and 50
kOe curves. Another important characteristic observed in the
FIG. 4. 共Color online兲 Temperature dependence of electrical resistance for
Ni50−xCoxMn38Sb12 共x = 2 – 7兲 in zero field 共close symbol兲 and at 50 kOe
共open symbol兲.
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09A927-3
J. Appl. Phys. 107, 09A927 共2010兲
Nayak, Suresh, and Nigam
FIG. 5. 共Color online兲 ⌬S M as a function of temperature for x = 4 and 5 in
various fields.
FIG. 6. 共Color online兲 Temperature dependence of MR for x = 0 – 7. The MR
is calculated from ␳ ⬃ T curve taken at zero and 50 kOe field.
low temperature region is that the resistance in the zero field
and 50 kOe curves remains same except for x = 7, in which
the resistance decreases with field. The martensitic to austenite transition is followed by a sharp decrease in electrical
resistance and the hysteresis between the cooling and heating
data confirms the first order nature of transition. Above the
martensitic transition the resistance increases slowly with
temperature. The lower magnetization of the martensitic
phase is reflected in the ␳ ⬃ T curves with higher resistance.
The MCE 共⌬S M 兲 was calculated from the isothermal
magnetization curves obtained at intervals of 1 K, using the
Maxwell equation.7 Fig. 5 shows the magnetic entropy
change 共⌬SM 兲 as a function of temperature in various applied
fields 共⌬H兲. Since in x = 4 and 5, the martensitic transition
occurs near the room temperature, the ⌬SM is calculated at
various applied fields for these compounds. The 共⌬S M 兲max of
68 and 43 J kg−1 K−1 have been observed for x = 5 and 4,
respectively, whereas 共⌬SM 兲max in parent Ni50Mn38Sb12 is
only about 7 J kg−1 K−1 and monotonically increase in
共⌬S M 兲max with Co. The large ⌬S M is due to the sharp change
in magnetization 共giving large ⳵ M / ⳵T兲 and large difference
in the magnetization between the austenite and martensitic
phases. The increased magnetostructural coupling induced
by Co also gives rise to large ⌬S M .
However, there is a large discrepancy observed in ⌬S M
calculation using Maxwell relation as observed in MnAs
compound, which also undergoes first order transition.15 To
overcome this we have calculated the refrigeration capacity
共RC兲 and the effective RC for x = 4 compound after subtracting the hysteresis loss. The RC is calculated by integrating
the ⌬S M 共T兲 curve over the full width at half maximum and
given by 95 J/kg for x = 4. We have calculated the average
hysteresis loss of 21 J/kg for x = 4 for the same temperature
interval used for calculating the RC. The effective RC value
for this compound turns out to be 74 J/kg and is much larger
as observed in parent system.11
The variation in MR with temperature for
Ni50−xCoxMn38Sb12 共x = 0 – 7兲 is shown in Fig. 6. The MR
around the martensitic transition was determined by using
the relation MR= 关兵␳共H , T兲 − ␳共0 , T兲其 / ␳共0 , T兲兴ⴱ100. A maximum MR of 34% is obtained for x = 7; however there is no
systematic variation in MR with Co. In all the compounds,
the maximum MR is obtained at the martensitic transition
temperature, as there is a large decrease in resistance with
applied field around this temperature.
IV. CONCLUSIONS
In conclusion, we have studied the magnetic, MR, and
magnetothermal properties in Ni–Co–Mn–Sb Heusler alloys.
The observation of a sharp peak in the DSC and heat capacity data confirms the structural transition from austenite to
martensitic phase. This first order nature of structural transition is confirmed by the hysteresis between the cooling and
heating curves, both in the case of magnetization and resistivity. A large MCE as well as a large MR is observed around
the martensitic transition temperature. Therefore, the present
system appears to be a potential material for multifunctional
applications.
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