Correlation between magnetism and magnetocaloric effect in the
intermetallic compound DyNiAl
Niraj K. Singh, K. G. Suresh, R. Nirmala, A. K. Nigam, and S. K. Malik
Citation: J. Appl. Phys. 99, 08K904 (2006); doi: 10.1063/1.2166589
View online: http://dx.doi.org/10.1063/1.2166589
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JOURNAL OF APPLIED PHYSICS 99, 08K904 共2006兲
Correlation between magnetism and magnetocaloric effect
in the intermetallic compound DyNiAl
Niraj K. Singha兲 and K. G. Sureshb兲
Department of Physics, Indian Institute of Technology Bombay, Mumbai 400076, India
R. Nirmala, A. K. Nigam, and S. K. Malik
Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai 400005, India
共Presented on 2 November 2005; published online 19 April 2006兲
Magnetization studies carried out in polycrystalline sample of DyNiAl show the presence of two
magnetic transitions, one at 15 K and the other at 30 K. The low-temperature transition is attributed
to the onset of antiferromagnetic ordering, while the other one corresponds to the ferro-para
transition. Thermomagnetic irreversibility found in the temperature dependence of magnetization
data is attributed to the domain-wall pinning effect and also to the magnetic frustration.
Magnetocaloric effect is found to be negative in the antiferromagnetic phase and positive above the
Néel temperature. © 2006 American Institute of Physics. 关DOI: 10.1063/1.2166589兴
I. INTRODUCTION
The magnetocaloric effect 共MCE兲 is intrinsic to all magnetic materials and is induced via the coupling of magnetic
sublattice with that of the applied magnetic field. The occurrence of large MCE in Gd5共Si, Ge兲4 has made the rare-earth共R兲 based intermetallic compounds a natural choice in the
search of giant magnetocaloric materials.1–3 Magnetic materials with large MCE over a considerable span of temperature find application as working substances in magnetic refrigerators. In general, MCE is found to be maximum near
magnetic transition temperatures and therefore, magnetic
materials with multiple magnetic transitions are of importance in the search for “tablelike” MCE.4 Intermetallic compounds of the general formula RNiAl are known to possess a
complex magnetic structure and undergo multiple magnetic
transitions5,6 and therefore are potential candidate to probe
the MCE behavior. The neutron-diffraction measurements on
RNiAl compounds with R = Ho, Dy, and Tb have shown that,
apart from a magnetic transition at their respective ordering
temperature, they undergo an additional magnetic transition
at roughly half of their ordering temperature.5 Though the
magnetic properties of these compounds have been studied
in detail, their magnetocaloric properties require a detailed
experimental investigation. In this paper we report our results on the magnetic and magnetocaloric properties of the
polycrystalline compound DyNiAl. The MCE behavior has
been determined both in terms of isothermal entropy change
as well as adiabatic temperature change. Based on our results, we have tried to establish the correlation between magnetism and MCE in this compound.
II. EXPERIMENTAL DETAILS
The polycrystalline sample of DyNiAl was prepared by
arc melting of the constituent elements 共of at least 99.9%
a兲
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b兲
0021-8979/2006/99共8兲/08K904/3/$23.00
purity兲 in high-purity argon atmosphere. The ingot was
melted five to six times to ensure homogeneity. Structural
analysis was performed by collecting the powder x-raydiffraction 共XRD兲 pattern at room temperature. Magnetization measurements, in the temperature range of 2–100 K and
up to a maximum field of 120 kOe, were performed using a
vibrating-sample magnetometer 共VSM兲 共Oxford Instruments兲 and a superconducting quantum interference device
共SQUID兲 magnetometer 共Quantum Design兲. The temperature
dependence of magnetization data was obtained, both under
“zero-field-cooled” 共ZFC兲 and “field-cooled” 共FC兲 conditions, in maximum applied field of 20 kOe. The heatcapacity measurements in the range of 2–200 K were performed using physical property measurement system
共PPMS兲 共Quantum Design兲.
III. RESULTS AND DISCUSSION
The room-temperature powder x-ray diffractogram reveals that DyNiAl has formed in a single phase, with the
ZrNiAl-type hexagonal structure 共space group P6̄2m, No.
189兲. The lattice parameters are found to be a
= 6.998± 0.001 Å and c = 3.849± 0.001 Å. Figure 1 shows the
temperature dependence of magnetization 共M兲 data obtained
in applied fields 共H兲 up to 20 kOe, both under ZFC and FC
conditions. It can be seen from the figure that the ZFC magnetization data collected in H = 200 Oe show magnetic transitions at about 30 and 15 K. M-T plots obtained in higher
fields reveal that the 30 K transition shifts towards higher
temperatures whereas the 15 K transition moves towards
lower temperatures, with increasing fields. These observations indicate the presence of a ferromagnetic transition with
TC = 30 K along with an antiferromagnetic transition at TN
= 15 K. It has indeed been reported6 that DyNiAl possesses a
complex magnetic structure and undergoes a ferromagnetic
transition along the c axis at 31 K and antiferromagnetic
transition within the basal plane at 15 K. It may also be noted
from the figure that, apart form multiple magnetic transitions, the M-T data colleted in FC and ZFC modes show
99, 08K904-1
© 2006 American Institute of Physics
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08K904-2
Singh et al.
J. Appl. Phys. 99, 08K904 共2006兲
FIG. 2. Field 共H兲 dependence of magnetization 共M兲 isotherms collected at
various temperatures.
FIG. 1. Temperature 共T兲 dependence of magnetization 共M兲 of DyNiAl under
zero-field-cooled 共ZFC兲 and field-cooled 共FC兲 conditions in various applied
magnetic fields. The arrows indicate the antiferromagnetic transition.
thermomagnetic irreversibility even in fields as high as 20
kOe. Generally, the FC-ZFC difference is observed in a narrow domain-wall system and/or magnetically frustrated
systems.7 Due to the low ordering temperature and large
anisotropy,8 DyNiAl may be classified as a narrow domainwall system and therefore the thermomagnetic irreversibility
may be partially attributed to the domain-wall pinning effect.
However, there are reports9 in literature which indicate that
the pinning effect is dominant only in low fields 共⬃ less than
5 kOe兲 and therefore the existence of thermomagnetic irreversibility at fields as high as 20 kOe indicates the presence
of magnetic frustration. It has been reported that in DyNiAl,
the Dy3+ ions form a triangular lattice in the basal plane and
therefore the presence of antiferromagnetism leads to the
magnetic frustration.5
The M-H isotherm collected at 2 K indicates that the
magnetization does not saturate even in a field of 120 kOe
and attains a value of about 9 ␮B / f.u. at the highest field 共the
gJ value of Dy3+ is 10␮B兲. The remanence ratio 共M r / M s兲 at 2
K is found to be ⬃13%, which decreases to ⬃4% near TN
and to ⬃2% near TC. This indicates that the magnetic hardness in DyNiAl is present only at low temperatures. The
high-temperature susceptibility obeys the Curie-Weiss law
with a paramagnetic moment of 10.5␮B which is close to the
free-ion value for Dy3+ ion 共10.6␮B兲.
The M-H isotherms obtained at various temperatures between 18 and 56 K are shown in Fig. 2. It may be seen from
the figure that the isotherms obtained below 30 K show a
curvature at low fields, which is characteristic of an ordered
state. However, it may be further noticed that the magnetization isotherms obtained at temperatures well above TC also
show curvatures at low fields. The presence of low-field curvature above TC seems to arise from magnetic polaroniclike
effect arising from the polarization of 3d band of Ni. The
neutron-diffraction measurements performed on HoNiAl
compound also suggest the presence of small Ni moment
共about 0.1␮B兲 in this compounds.10 Such a behavior has indeed been reported in other Ni-based intermetallic compounds as well.11
Figure 3 shows the temperature dependence of heat capacity 共C兲 data of DyNiAl. It can be seen from the figure that
the C-T data also show two transitions, one at about 30 K
and the other at about 15 K, which is consistent with the
magnetization data. The temperature dependence of zerofield entropy of the present compound is shown as an inset of
the figure.
The magnetocaloric effect of DyNiAl, in terms of isothermal magnetic entropy change 共⌬SM 兲, has been determined from the M-H isotherms in the temperature range of
2–100 K, using the Maxwell’s relation. Figure 4 shows the
temperature variation of ⌬SM for field changes 共⌬H兲 of 20
and 50 kOe. It is of interest to note that below a certain
temperature 共⬃12 K兲, DyNiAl shows a negative MCE 共positive ⌬S M 兲. In general negative MCE is observed in antiferromagnets with considerable anisotropy and therefore the
presence of negative MCE in the low-temperature region in
the present case lends additional support to the existence of
the antiferromagnetic phase at low temperatures.12 It has indeed been reported that DyNiAl possesses large uniaxial
magnetic anisotropy.8 It can also be seen from the figure that
⌬S M vs T plots for DyNiAl show a maximum near TC, with
FIG. 3. Temperature variation of heat capacity 共C兲 data of DyNiAl. The
inset shows the temperature dependence of entropy.The arrows indicate the
anomalies seen in the C-T data.
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08K904-3
J. Appl. Phys. 99, 08K904 共2006兲
Singh et al.
FIG. 4. Temperature 共T兲 variation of isothermal entropy change 共⌬SM 兲 of
DyNiAl for field changes of 20 and 50 kOe.
values of 10 and 19 J kg−1 K−1 for field changes of 20 and 50
kOe, respectively. The maximum values of ⌬S M 共⌬Smax
M 兲 of
promising magnetic refrigerants such as 共Er,Gd兲NiAl vary in
the range of 10–22 J kg−1 K−1 for ⌬H = 50 kOe.4 The ⌬Smax
M
values of RNi2 compounds,13 for the same field change, vary
in the range of 22–28 J kg−1 K−1.
The temperature dependence of MCE in terms of adiabatic temperature change 共⌬Tad兲, calculated from the entropy
change and the temperature variation of zero-field heatcapacity data, is shown in Fig. 5. The maximum values of
⌬Tad are found to be 3.5 and 7 K, for field changes of 20 and
50 kOe, respectively. In the case of 共Er,Gd兲NiAl compounds
the maximum values of ⌬Tad vary in the range of 4–7 K, for
⌬H = 50 kOe.4
FIG. 5. Temperature 共T兲 dependence of adiabatic temperature change 共⌬Tad兲
of DyNiAl for field changes of 20 and 50 kOe.
It can be seen from Figs. 4 and 5 that the MCE values of
DyNiAl remain considerable even at temperatures well
above the ordering temperature, which is a criterion for good
magnetic refrigerants. The large MCE seen at high temperatures may be attributed to the presence of spin fluctuations
arising from the polarization of 3d band of Ni. It has indeed
been reported that the presence of spin fluctuations smear out
the near-TC MCE peak and drag it to higher temperatures.12
However, since the MCE associated with the lowtemperature transition is negligibly small, the presence of
multiple transitions in this compound does not contribute to
the enhancement of MCE below TC.
IV. CONCLUSIONS
In conclusion, the present study shows that DyNiAl has
a predominantly antiferromagnetic structure at low temperatures. The magnetic frustration along with large anisotropy
leads to thermomagnetic irreversibility even at fields as high
as 20 kOe. The shape of the MCE peak suggests the existence of the magnetic polaronic behavior of the 3d band.
Temperature variations of both the entropy change and the
temperature change suggest that DyNiAl is a promising material for magnetic refrigeration applications in the temperature range of 20–60 K.
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