Effect of magnetic polarons on the magnetic, magnetocaloric, and
magnetoresistance properties of the intermetallic compound HoNiAl
Niraj K. Singh, K. G. Suresh, R. Nirmala, A. K. Nigam, and S. K. Malik
Citation: J. Appl. Phys. 101, 093904 (2007); doi: 10.1063/1.2724740
View online: http://dx.doi.org/10.1063/1.2724740
View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v101/i9
Published by the American Institute of Physics.
Related Articles
The effect of distributed exchange parameters on magnetocaloric refrigeration capacity in amorphous and
nanocomposite materials
J. Appl. Phys. 111, 07A334 (2012)
Particle size dependent hysteresis loss in La0.7Ce0.3Fe11.6Si1.4C0.2 firstorder systems
Appl. Phys. Lett. 100, 072403 (2012)
Magnetocaloric effect and refrigerant capacity in Sm1−xSrxMnO3 (x=0.42, 0.44, 0.46) manganites
J. Appl. Phys. 111, 07D705 (2012)
Properties of NaZn13-type LaFe13−xSix (x=1.4, 1.5) compound with the first-order phase transition
J. Appl. Phys. 111, 07E107 (2012)
Large magnetocaloric effect for magnetic refrigeration from 210 to 275 K in La0.7Ca0.3Mn1−xCoxO3
J. Appl. Phys. 111, 07D703 (2012)
Additional information on J. Appl. Phys.
Journal Homepage: http://jap.aip.org/
Journal Information: http://jap.aip.org/about/about_the_journal
Top downloads: http://jap.aip.org/features/most_downloaded
Information for Authors: http://jap.aip.org/authors
Downloaded 24 Feb 2012 to 59.162.23.76. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions
JOURNAL OF APPLIED PHYSICS 101, 093904 共2007兲
Effect of magnetic polarons on the magnetic, magnetocaloric, and
magnetoresistance properties of the intermetallic compound HoNiAl
Niraj K. Singh and K. G. Suresha兲
Department of Physics, Indian Institute of Technology Bombay, Mumbai, Maharashtra 400076, India
R. Nirmala, A. K. Nigam, and S. K. Malik
Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai, Maharashtra 400005, India
共Received 29 November 2006; accepted 6 March 2007; published online 7 May 2007兲
The magnetic, magnetocaloric, and magnetoresistive properties of the polycrystalline compound
HoNiAl have been studied. The temperature variations of magnetization and heat capacity show that
the compound undergoes two magnetic transitions, one at 14 K and the other at 5 K. The former is
due to the paramagnetic-ferromagnetic transition, while the latter is attributed to the onset of an
antiferromagnetic ordering, as the temperature is lowered. The M-H isotherm obtained at 2 K shows
a metamagnetic transition with a critical filed of about 13 kOe. The maximum values of isothermal
magnetic entropy change and adiabatic temperature change, for a field change of 50 kOe, are
estimated to be 23.6 J / kg K and 8.7 K, respectively. The relative cooling power is found to be about
500 J / kg for a field change of 50 kOe. A large magnetoresistance of about 16%, near the ordering
temperature of 14 K, is observed for a field of 50 kOe. The magnetic, magnetocaloric, and
magnetoresistance data seem to suggest the presence of magnetic polarons in this compound.
© 2007 American Institute of Physics. 关DOI: 10.1063/1.2724740兴
I. INTRODUCTION
The large magnetocaloric effect 共MCE兲 exhibited by
many rare earth 共R兲-transition metal 共TM兲 intermetallic compounds has made them potential candidates for magnetic refrigeration applications, both in the cryogenic as well as
room temperature regimes.1–3 The possibility of magnetic refrigeration emerging as an alternative to the conventional gas
compression methods has led to an intensive research, both
on experimental as well as on theoretical aspects, in the field
of MCE.4–6 Generally, the MCE is measured in terms of
isothermal magnetic entropy change or adiabatic temperature
change and the materials with significant MCE over a wide
temperature range are being considered as the active elements in a magnetic refrigerator. In general, MCE is found to
be a maximum near the ordering temperature and therefore,
composite magnetic material with distributed ordering temperatures or materials with multiple magnetic transitions are
of importance in the search of “tablelike MCE.”5 The RNiAl
compounds are known to possess a complex magnetic structure and to undergo multiple magnetic transitions.7–9 The
neutron diffraction studies on HoNiAl have shown that this
compound crystallizes in the hexagonal ZrNiAl-type structure. It is also known that it possesses nonmagnetic Ni sublattice and orders ferromagnetically at the ordering temperature 共TC兲 of 13 K. Apart from the ferromagnetic ordering
seen at TC, it also undergoes a transition to an antiferromagnetic state at ⬃5 K, leading to the presence of ferromagnetic
ordering along hexagonal c axis along with the antiferromagnetic coupling of Ho moments in the ab plane.8 Though the
magnetic properties of HoNiAl have been studied in detail,
a兲
Author to whom correspondence should be addressed; electronic mail:
[email protected]
0021-8979/2007/101共9兲/093904/5/$23.00
its magnetocaloric properties are yet to be reported. Therefore, in order to explore the application potential of HoNiAl
as a low temperature magnetic refrigerant material, the magnetocaloric properties, both in terms of the isothermal magnetic entropy change, as well as the adiabatic temperature
change, have been studied. It is well known that, like MCE,
the electrical resistivity of a magnetic material is greatly influenced by its magnetic state, and, therefore, in order to
develop a better understanding of the magnetic properties,
we have also studied the temperature and field dependences
of electrical resistivity.
II. EXPERIMENTAL DETAILS
The polycrystalline sample of HoNiAl was prepared by
arc melting the constituent elements 共of at least 99.9% purity兲 in high purity argon atmosphere. The ingot was melted
several times to ensure homogeneity. Lattice parameters
were determined from powder x-ray diffraction 共XRD兲 pattern taken using Cu K␣ radiation. Magnetization measurements, in the temperature range of 2 – 60 K and up to a maximum field of 50 kOe, were performed using a vibrating
sample magnetometer 共Oxford instruments兲/superconducting
quantum interference device 共SQUID兲 magnetometer 共Quantum Design兲. The heat capacity and the electrical resistivity
measurements, in the temperature range of 2 – 300 K and in
fields up to 50 kOe, were carried out using a physical property measurement system 共PPMS兲 共Quantum Design兲. The
heat capacity was measured using the relaxation method
whereas the electrical resistivity was measured using the linear four-probe technique.
101, 093904-1
© 2007 American Institute of Physics
Downloaded 24 Feb 2012 to 59.162.23.76. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions
093904-2
Singh et al.
FIG. 1. 共a兲 Temperature dependence of magnetization, under zero-fieldcooled 共ZFC兲 and field-cooled 共FC兲 conditions, in various applied magnetic
fields. The arrows indicate the low temperature magnetic transition. 共b兲 The
temperature variation of the inverse susceptibility fitted to the Curie-Weiss
law.
III. RESULTS AND DISCUSSION
The Rietveld refinement of the room temperature powder x-ray diffractogram reveals that the compound has
formed in single phase with the hexagonal ZrNiAl-type
structure 共space group P6̄2m, No. 189兲. The lattice parameters are found to be a = 6.991± 0.001 Å and c
= 3.825± 0.001 Å. Figure 1共a兲 shows the temperature 共T兲 dependence of magnetization 共M兲 data obtained under various
fields 共H兲, both under zero-field-cooled 共ZFC兲 and fieldcooled 共FC兲 conditions. It can be seen from this figure that
the M-T data collected in the field of 200 Oe show the presence of two magnetic transitions; one at 14 K and the other
at ⬃5 K. We denote the transitions at 14 and 5 K as Tord and
T1, respectively. The M-T data also shows that with increase
in the field, the transition at Tord shifts to higher temperatures
whereas the transition associated with T1 is pushed towards
low temperatures. These facts indicate that the magnetic
transition at Tord is ferromagnetic in character, whereas the
one seen at T1 is of antiferromagnetic nature. This observation is consistent with previous reports.7,8 Figure 1共b兲 shows
the temperature variation of the inverse dc magnetic susceptibility 共␹兲 in the high temperature regime and the CurieWeiss fit to it. The susceptibility is found to obey the CurieWeiss law with an effective moment 共␮eff兲 of 10.75␮B and a
paramagnetic Curie temperature 共␪ P兲 of 8.9 K. It is of interest to note that the observed value of ␮eff is larger than
10.6␮B, which is the theoretical gJ value of Ho3+ ion. A
J. Appl. Phys. 101, 093904 共2007兲
similar enhancement in the ␮eff of RNiAl compounds has
been reported previously also and may be attributed to the
polarization of the conduction band, which is termed as magnetic polaronic effect.10,11 The magnetic polarons mainly
arises in compounds with strong s-f exchange. Another point
of interest is that the ␪ P value obtained in this case is lower
than Tord. Such a behavior has been observed in other intermetallic compounds also and may be attributed to the presence of antiferromagnetic correlations along with the ferromagnetic ones.11
Apart from the multiple magnetic transitions, another
feature worth noting from Fig. 1共a兲 is the occurrence of thermomagnetic irreversibility between the FC and ZFC magnetization data. It may be noted from the figure that M vs T
data, collected in fields up to 2 kOe show FC-ZFC difference. Generally, the thermomagnetic irreversibility is observed in geometrically frustrated systems or narrow domain
wall systems.12,13 It is reported that in HoNiAl the Ho3+ ions
form a triangular lattice in the ab plane and undergo an antiferromagnetic transition at low temperature. Therefore, the
existence of antiferromagnetic ordering in the triangular lattice would result in the magnetic frustration effect.7 Furthermore, the low ordering temperature and the high anisotropy
associated with the hexagonal structure lead to the formation
of narrow domain walls in this compound. Owing to these
factors, the magnetization remains very small at low temperatures, in the ZFC mode. With the increase of temperature, the mobility of the domain walls increases and also the
effect of frustration decreases. Both these factors cause an
increase in the ZFC magnetization as the sample is warmed
up. However, in FC mode, the presence of magnetic field
during cooling prevents the pinning of the domain walls, and
therefore the FC magnetization will be different compared to
the ZFC magnetization. In the FC mode, the frustration is
also less dominant. The thermomagnetic irreversibility seen
in HoNiAl may be attributed to both the frustration effect as
well as the narrow domain wall nature. It is also evident
from Fig. 1共a兲 that the thermomagnetic irreversibility vanishes completely in a field of 20 kOe. This may partly be
attributed to the fact that in the presence of high magnetic
fields the domain walls, at any temperature, would have sufficient energy to overcome the barrier provided by pinning
centers and hence would give rise to a reduction in the thermomagnetic irreversibility. Furthermore, with the increase in
field, the strength of antiferromagnetic interaction decreases,
which in turn causes a reduction in the frustration effect.
Figure 2 shows the field dependence of magnetization
data of HoNiAl, obtained at 2 K. It can be seen from the
figure that for low fields 共⬍5 kOe兲 the M-H data show a
sharp increase in the magnetization, which may be attributed
to the presence of the ferromagnetic phase. With further increase in the field the magnetization increases slowly and,
associated with the antiferromagnetic phase, a metamagnetic
transition is seen at a critical field of ⬃13 kOe. A similar
metamagnetic transition has been reported previously also.9
For fields above 13 kOe, the M-H data show a linear dependence and even at the highest field of 50 kOe, the magnetization is not saturated. The nonsaturation tendency indicates
that the metamagnetic transition has not completely sup-
Downloaded 24 Feb 2012 to 59.162.23.76. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions
093904-3
Singh et al.
FIG. 2. Magnetization isotherm of HoNiAl at 2 K.
pressed the low temperature antiferromagnetic phase. The
saturation moment, as determined from the M vs 1 / H plot, is
found to be 9.2␮B / f.u. It may be mentioned that a saturation
magnetization of 8.06␮B / f.u. and 9.47␮B / f.u. has been reported by Javorsky et al. and Tuan, respectively.8 It should
be kept in mind that the gJ value for Ho3+ is 10␮B.
Figure 3 shows M-H isotherms of HoNiAl obtained at
various temperatures below and above Tord. It may be seen
from the figure that the isotherm obtained at 5 K does not
show any metamagnetic transition and therefore may be
taken as indicative of the fact that in HoNiAl the antiferromagnetic phase exists only below 5 K. Another feature worth
noting in the figure is the strong low-field curvature in the
magnetization isotherms obtained at temperatures well above
Tord. The presence of curvature indicates the existence of
short range correlations just above Tord and has been observed in many other intermetallic compounds.11 This may
also be attributed to the magnetic polaronic effect.
Figure 4 shows the temperature variation of heat capacity 共C兲 of HoNiAl in various applied magnetic fields. It can
be seen from the figure that the zero-field heat capacity data
show two peaks, one near 13 K and another at about 5 K.
With the increase in the magnetic field, the peak associated
with Tord smears out and shifts towards the high temperature
side, thereby confirming the ferromagnetic character of this
transition. The low temperature peak, on the other hand, is
found to shift towards lower temperature and therefore con-
FIG. 3. Field dependence of magnetization of HoNiAl at various temperatures close to the ordering temperature. The temperature difference 共␦T兲
between the isotherms is ⬃4 K.
J. Appl. Phys. 101, 093904 共2007兲
FIG. 4. Temperature variation of heat capacity 共C兲 of HoNiAl in various
applied magnetic fields.
firms its antiferromagnetic character. It may also be noted
from Fig. 4 that, though in a field of 20 kOe the peak associated with T1 shifts to low temperatures and remains prominent, it gets rounded off and is pushed again to high temperatures, in a field of 50 kOe. A similar behavior has been
recently reported14 in the intermetallic compound Gd7Rh3.
The magnetocaloric effect, in terms of isothermal magnetic entropy change 共−⌬S M 兲, of HoNiAl has been calculated
independently from the magnetization isotherms obtained at
temperatures close to Tord 共employing Maxwell’s relations兲,
as well as using the heat capacity data collected in various
fields. Figure 5 shows the temperature variation of ⌬S M calculated for two different fields using C-H-T data. The inset
of this figure shows the temperature dependence of ⌬S M calculated from the M-H-T data. It may be noticed from this
figure that ⌬S M values calculated from the C-H-T data compare very well with that calculated using the M-H-T data. It
can also be seen from the figure that ⌬S M vs T plots of
HoNiAl show a maximum near Tord. The maximum values of
⌬S M 共⌬Smax
M 兲 for field changes 共⌬H兲 of 20 and 50 kOe are
12.3 and 23.6 J / kg K, respectively. It is of interest to mention that the ⌬Smax
M values of 共Er, Dy兲Al2 compounds, which
are promising magnetic refrigerants in the temperature range
of 13– 60 K, lie in the range of 20– 35 J / kg K, for ⌬H
= 50 kOe.15 The ⌬Smax
M of RNi2 compounds, which have ordering temperatures less than 80 K, also lies approximately
in the same range.16 The ⌬Smax
M of Gd2PdSi3 which is pro-
FIG. 5. Temperature variation of isothermal entropy change 共−⌬S M 兲 of HoNiAl, obtained using the heat capacity data in fields of 20 and 50 kOe. The
inset shows the temperature dependence of ⌬S M obtained using the magnetization data.
Downloaded 24 Feb 2012 to 59.162.23.76. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions
093904-4
Singh et al.
FIG. 6. Temperature dependence of adiabatic temperature change 共⌬Tad兲 of
HoNiAl, obtained for a field change 共⌬H兲 of 20 and 50 kOe. The inset
shows the variation of ⌬Tad in the low temperature region.
posed as a potential magnetic refrigerant below 40 K is
⬃8 J / kg K 共for ⌬H = 40 kOe兲.17 Therefore, a comparison of
of HoNiAl with that of the potential magnetic
the ⌬Smax
M
refrigerant materials indicates that this material is suitable for
the refrigeration applications below 30 K. However, it may
be mentioned here that a comparison of mere ⌬Smax
M values is
not sufficient to examine the suitability of magnetic
refrigerants.18 Therefore, in order to get a better comparison
of the application potential of this compound, we have also
calculated the MCE in terms of adiabatic temperature change
共⌬Tad兲, which is shown in Fig. 6. It can be seen from Fig. 6
that, as expected, the temperature variation of ⌬Tad is similar
max
to that of ⌬S M . The ⌬Tad
values of HoNiAl are found to be
4 and 8.7 K, for ⌬H = 20 and 50 kOe, respectively. It has
max
values,
been reported that in the case of DyNiAl the ⌬Tad
for ⌬H = 20 and 50 kOe, are 3.5 and 6.8 K, respectively.5
max
The ⌬Tad
value, for ⌬H = 50 kOe, of the RNi2 compounds
varies in the range of 3.5– 9 K,16 whereas for 共Er, Dy兲Al2
compounds, for the same field change, this value varies bemax
tween 7 and 11 K.15 The ⌬Tad
value of GdPd, which is an
active magnetic regenerator material used in the low temperature stage of magnetic refrigerators for liquefaction of
hydrogen is 8.7 K, for a field change of 70 kOe.19,20 Theremax
value also indicates the suitability of HoNiAl as
fore, ⌬Tad
a low temperature magnetic refrigerant. Apart from the large
value of MCE, another feature worth noting from the temperature variations of ⌬S M and ⌬Tad of HoNiAl 共Figs. 5 and
6兲 is the occurrence of considerable MCE even at temperatures well above Tord. This may be attributed to the presence
of magnetic polarons above Tord, as mentioned earlier. It is of
interest to note from Figs. 5 and 6 that at very low temperatures the MCE is negative, which indicates the dominance of
the antiferromagnetic state below T1. The negative MCE can
be clearly seen from the inset of Fig. 6, which shows the
variation of the adiabatic temperature change at low temperatures.
Apart from the magnitude of MCE, another figure of
merit which is used to compare the magnetic refrigerant materials is the relative cooling power 共RCP兲. The RCP of a
magnetic material is the product of ⌬Smax
M and the full width
at half maximum of the ⌬S M vs T plot, and it is a measure of
the heat transfer between the cold and hot reservoirs in an
ideal refrigeration cycle. The RCP values of HoNiAl, for
J. Appl. Phys. 101, 093904 共2007兲
FIG. 7. Temperature dependence of zero-field electrical resistance data of
HoNiAl, normalized to its value at 300 K. The inset shows the data in the
temperature range of 2 – 40 K.
field changes of 20 and 50 kOe, are found to be 235 and
500 J / kg. It is of importance to note that RCP values, for
⌬H = 50 kOe, of the potential refrigerant materials such as
Gd and Gd5Si2Ge2 are ⬃700 and 500 J / kg, respectively
共data taken from Ref. 21兲. The RCP values of the
LaFe13-based materials have been reported to vary between
483 and 576 J / kg, for the same field change.21 Therefore, it
may be noted that RCP value of HoNiAl compares well with
that of some potential refrigerant materials, thereby rendering this material to be considered for refrigeration applications around 20 K.
In order to get further information about the magnetic
state of HoNiAl, the electrical resistivity and magnetoresistance 共MR兲 measurements have been carried out. The temperature dependence of electrical resistivity, normalized to
the value at 300 K, of HoNiAl is shown in Fig. 7. The inset
of this figure shows the normalized resistance, expanded in
the low temperature region. It can be seen from the figure
that the compound shows metallic character in the temperature range investigated. It may also be noticed from this figure 共more clearly in the inset兲 that there is a change in the
slope of the normalized resistance near Tord; however, no
anomaly corresponding to the antiferromagnetic transition
共at T1兲 could be seen. The magnetoresistance 共in percentage兲,
defined as MR= ⌬R / R = 兵关R共T , H兲 − R共T , 0兲兴 / R共T , 0兲其 ⫻ 100,
has been calculated using the field dependence of electrical
resistance data, collected at various temperatures above Tord.
Figure 8 shows the field dependence of MR at temperatures
FIG. 8. Field dependence of magnetoresistance of HoNiAl at various temperatures above the ordering temperature.
Downloaded 24 Feb 2012 to 59.162.23.76. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions
093904-5
J. Appl. Phys. 101, 093904 共2007兲
Singh et al.
above Tord. It may be seen from the figure that MR at 15 K
and for a field change of 50 kOe is negative with a magnitude of about 16%. Above Tord, the magnitude of MR decreases with increase in temperature and attains a value of
about 1% at 65 K; however, the sign of the MR still remains
negative. With further increase in temperature, the sign of the
MR becomes positive. It is well known that in a metallic
system, the application of magnetic field gives rise to a positive contribution to MR and, therefore, the existence of large
negative MR at temperatures above Tord indicates that there
is considerable magnetic contribution to the electrical resistivity, which is suppressed by the field. A similar MR behavior has been reported in other intermetallic compounds such
as R2PdSi3 共R = Tb and Dy兲, GdT2X2 共T = Ni, Cu, and Pd, and
X = Si and Ge兲, and GdNi.11,22,23 The unusual MR behavior
seen in HoNiAl could be attributed to the magnetic polarons.
The presence of magnetic polarons causes a strong confinement of the conduction electrons, thereby resulting in an increase in the resistance. However, when the magnetic field is
applied, these polarons grow in size and eventually coalesce,
causing the delocalization of the conduction electrons and
hence the reduction in the electrical resistivity. The negative
MR is a result of this reduction in the resistivity. It may be
recalled here that the signature of magnetic polarons was
evidenced in the M-H isotherms, as well as the temperature
dependence of MCE in this compound.
IV. CONCLUSIONS
In conclusion, the existence of thermomagnetic irreversibility in HoNiAl is attributed to both the magnetic frustration, as well as the domain wall pinning effect. The magnetization isotherm obtained at 2 K and the heat capacity data
collected in various applied magnetic fields suggest that the
application of a field suppresses the low temperature antiferromagnetic component in this compound. The magnetocaloric properties are found to be comparable to those of some
potential magnetic refrigerants. Magnetic, magnetocaloric,
and magnetoresistance properties are found to be influenced
by the magnetic polarons in this compound.
ACKNOWLEDGMENTS
One of the authors 共K.G.S.兲 would like to thank ISRO
Government of India for funding this work through a sponsored project. The authors acknowledge the help rendered by
Devendra Buddhikot in carrying out some of the measurements presented in this paper.
1
K. A. Gschneidner, Jr., V. K. Pecharsky, and A. O. Tsokol, Rep. Prog.
Phys. 68, 1479 共2005兲.
2
N. K. Singh, S. Agarwal, K. G. Suresh, R. Nirmala, A. K. Nigam, and S.
K. Malik, Phys. Rev. B 72, 014452 共2005兲.
3
K. A. Gschneidner, Jr. and V. K. Pecharsky, Annu. Rev. Mater. Sci. 30,
387 共2000兲.
4
V. Provenzano, A. J. Shapiro, and R. D. Shull, Nature 共London兲 429, 853
共2004兲.
5
N. K. Singh, K. G. Suresh, R. Nirmala, A. K. Nigam, and S. K. Malik, J.
Appl. Phys. 99, 08K904 共2006兲.
6
P. J. von Ranke, N. A. Oliveira, C. Mello, A. Magnus, G. Carvalho, and S.
Gama, Phys. Rev. B 71, 054410 共2005兲.
7
G. Ehlers and H. Maletta, Z. Phys. B: Condens. Matter 101, 317 共1996兲.
8
P. Javorsky, P. Burlet, E. Ressouche, V. Sechovsky, and G. Lapertot, J.
Magn. Magn. Mater. 159, 324 共1996兲.
9
P. Javorsky, H. Sugawara, D. Rafaja, F. Bourdarot, and H. Sato, J. Alloys
Compd. 323–324, 472 共2001兲.
10
G. Ehlers and H. Maletta, Z. Phys. B 99, 145 共1996兲.
11
R. Mallik, E. V. Sampathkumaran, and P. L. Paulose, Solid State Commun.
106, 169 共1998兲.
12
K. G. Suresh, S. K. Dhar, and A. K. Nigam, J. Magn. Magn. Mater. 288,
452 共2005兲.
13
N. K. Singh, K. G. Suresh, and A. K. Nigam, Solid State Commun. 127,
373 共2003兲.
14
K. Sengupta, K. K. Iyer, and E. V. Sampathkumaran, Phys. Rev. B 72,
054422 共2005兲.
15
P. J. von Ranke, V. K. Pecharsky, and K. A. Gschneidner, Jr., Phys. Rev. B
58, 12110 共1998兲.
16
P. J. von Ranke, E. P. Nobrega, I. G. de Oliveira, A. M. Gomes, and R. S.
Sarthour, Phys. Rev. B 63, 184406 共2001兲.
17
E. V. Sampathkumaran, I. Das, R. Rawat, and S. Majumdar, Appl. Phys.
Lett. 77, 418 共2000兲.
18
V. K. Pecharsky and K. A. Gschneidner, Jr., J. Appl. Phys. 90, 4614
共2001兲.
19
K. A. Gschneidner, Jr., J. Alloys Compd. 344, 356 共2002兲.
20
A. M. Tishin and Y. I. Spichkin, The Magnetocaloric Effect and its Application 共IOP, Bristol, 2003兲.
21
Proceedings of the First International Conference on Magnetic Refrigeration near Room Temperature, edited by P. W. Egolf 共Montreux, Switzerland, September 2005兲.
22
R. Mallik, E. V. Sampathkumaran, P. L. Paulose, and V. Nagarajan, Phys.
Rev. B 55, R8650 共1997兲.
23
R. Mallik and E. V. Sampathkumaran, Phys. Rev. B 58, 9178 共1999兲.
Downloaded 24 Feb 2012 to 59.162.23.76. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions