Magnetic and magnetothermal properties of La1−xNdxFe11.5Al1.5
compounds
Pramod Kumar, Niraj K. Singh, K. G. Suresh, and A. K. Nigam
Citation: J. Appl. Phys. 103, 07B338 (2008); doi: 10.1063/1.2836711
View online: http://dx.doi.org/10.1063/1.2836711
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Published by the American Institute of Physics.
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JOURNAL OF APPLIED PHYSICS 103, 07B338 共2008兲
Magnetic and magnetothermal properties of La1−xNdxFe11.5Al1.5 compounds
Pramod Kumar,1 Niraj K. Singh,1,a兲 K. G. Suresh,1,b兲 and A. K. Nigam2,c兲
1
Department of Physics, I.I.T. Bombay, Mumbai 400076, India
Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai 400005, India
2
共Presented on 6 November 2007; received 11 September 2007; accepted 5 November 2007;
published online 7 April 2008兲
In this paper, we report the structural, magnetic, and magnetocaloric effect 共MCE兲 of
La1−xNdxFe11.5Al1.5 共x = 0.1, 0.2兲 compounds. Temperature dependence of magnetization data shows
that with Nd substitution, the nature of magnetic transition changes from second order transition to
multiple first order transition. This observation is confirmed by the thermodynamic analysis using
the Landau theory of phase transitions. The MCE has been calculated in terms of the isothermal
magnetic entropy change 共⌬S M 兲 using the magnetization isotherms obtained at temperatures close to
the transition temperature. The maximum values of ⌬SM are found to be 5.4 and 4.6 J kg−1 K−1 of
x = 0.1 and x = 0.2, respectively, for a field change of 50 kOe, whereas the value for the undoped
compound is about 3 J kg−1 K−1. The refrigerant capacity has been calculated to be 544 J / kg K for
x = 0.1 and 470 J / kg K for x = 0.2. © 2008 American Institute of Physics. 关DOI: 10.1063/1.2836711兴
INTRODUCTION
Recently, research activities concerning magnetocaloric
effect close to room temperature has considerably
enhanced.1–7 It is reported that the compounds based on
La1−xCaxMnO3,10
LaFe11.4Si1.6,11
and
Gd5Si2Ge2,8,9
MnFeP1−xAsx 共Refs. 12 and 13兲 exhibit considerable magnetocaloric effect 共MCE兲 due to a first order ferromagnetic to
paramagnetic transition near their transition temperature. The
cubic NaZn13-type phase does not exist at the La–Fe binary
phase diagram. However, it can be stabilized by partial substitution of Si or Al for Fe, with the help of prolonged
annealing.14,15 Recently, it has been shown that the annealing
time can be reduced by using the melt-spinning technique.16
Among the members of this class, LaFe13−xSix and
LaFe13−xAlx quasiternary compounds have been extensively
studied because of their unusual magnetic properties. The
LaFe13−xSix compounds are ferromagnets but exhibit the phenomenon of itinerant electron metamagnetism in a narrow
temperature range above the ordering temperature.17,18 For
the LaFe13−xAlx system, the antiferromagnetic ground state is
found in the concentration range of 1.04艋 x ⬍ 1.82, whereas
ferromagnetism is realized for 1.82艋 x ⬍ 4.94.19 Wang et al.
have studied the magnetic properties of LaFe11.5Al1.5 and
found that it is antiferromagnetic with a Neel temperature of
about 192 K.20 In this paper, we investigate the relationship
between the magnetic and magnetocaloric properties of Ndsubstituted LaFe11.5Al1.5 compounds.
EXPERIMENTAL DETAILS
Polycrystalline samples of La1−xNdxFe11.5Al1.5 共x
= 0.1, 0.2兲 were synthesized by arc melting and subsequent
a兲
Present address: Ames Laboratory, Iowa Sate University, Iowa, Ames,
Iowa 50011-3020, USA.
b兲
Author to whom correspondence should be addressed. Electronic mail:
[email protected].
c兲
Electronic mail: [email protected].
0021-8979/2008/103共7兲/07B338/3/$23.00
annealing at 1273 K for 20 days. The samples were characterized by power x-ray diffraction 共XRD兲 using Cu K␣ radiation. The magnetization 共M兲 was measured using a vibrating
sample magnetometer 共physical property measurement system, Quantum Design兲 in the temperature 共T兲 range of
10– 330 K, up to a field 共H兲 of 50 kOe under “zero field
cooled” 共ZFC兲 mode.
RESULTS AND DISCUSSION
Rietveld analysis of the powder x-ray diffraction data of
the polycrystalline samples shows that the compounds are
nearly single phase. Both the compounds are found to crystallize in the cubic NaFe13 structure belonging to the space
group F − 3mc, as in the case of LaFe11.5Al1.5.21 The refinement has shown a small amount 共⬃4 wt % 兲 of ␣-Fe phase
as impurity. The lattice parameter is found to be
11.589⫾ 0.001 Å and 11.582⫾ 0.001 Å for x = 0.1 and 0.2,
respectively. The lattice parameter of LaFe11.5Al1.5 is reported to be 11.580 Å.20
Figures 1共a兲 and 1共b兲 show the ZFC M versus T plots of
La0.9Nd0.1Fe11.5Al1.5 and La0.8Nd0.2Fe11.5Al1.5 compounds. It
can be seen from Fig. 1 that the onset of magnetic ordering is
accompanied by a multistep change in the magnetization.
The transition temperatures for both the compounds have
been determined from the 共dM / dT兲 plots obtained at
1000 Oe. The inset of Fig. 1 shows the temperature variation
of dM / dT, which shows the multiple peaks with peak width
less than 1 K. This behavior is indicative of multiple first
order magnetic transitions. Another feature seen from these
plots is that while the compound with x = 0.1 is antiferromagnetic 共AFM兲, the compound with x = 0.2 is ferromagnetic
共FM兲. Therefore, it is clear that the antiferromagnetic nature
gets gradually suppressed with Nd substitution. Liu et al.22
have reported the magnetization and heat capacity measurements on La0.9Nd0.1Fe11.5Al1.5 and found that the compound
is antiferromagnetic below 199 K in the zero applied field
limit. These authors have also observed that there is a com-
103, 07B338-1
© 2008 American Institute of Physics
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07B338-2
Kumar et al.
FIG. 1. Temperature dependence of magnetization of La1−xNdxFe11.5Al1.5
共x = 0.1, 0.2兲 compounds in an applied field of 1000 Oe. Inset shows the
temperature variation of the dM / dT for La0.8Nd0.2Fe11.5Al1.5.
petition between the AFM and FM interactions in this compound and at low fields, the former dominates the magnetization behavior. The multiple step behavior seen in the
present case is also in agreement with the observations of
Liu et al.
Figure 2 shows the isothermal field dependence of magnetization for La0.9Nd0.1Fe11.5Al1.5 and La0.8Nd0.2Fe11.5Al1.5
at 100 K. The measurements were carried out after cooling
the sample from 300 to 100 K in the ZFC mode and subsequently varying the field with a sweep rate of 100 Oe/ s.
Metamagnetic transitions are clearly seen in the positive field
variation cycles.
Figure 3 shows the Arrott plots for La0.9Nd0.1Fe11.5Al1.5.
It can be seen that the Arrott plots are S shaped. A similar
observation has been made for the compound with x = 0.2. It
is well known that the S-shaped Arrott plots are seen in compounds which possess negative contribution of higher order
terms in the Landau free energy expansion.23 Since such
compounds are expected to show first order transitions, the
S-shaped plots in the present case are indicative of first order
magnetic transition.
FIG. 2. 共Color online兲 M-H isotherms of La1−xNdxFe11.5Al1.5 共x = 0.1, 0.2兲
compounds at 100 K.
J. Appl. Phys. 103, 07B338 共2008兲
FIG. 3. 共Color online兲 Arrott plots of La0.9Nd0.1Fe11.5Al1.5.
In order to understand the effect of Nd substitution on
the magnetic properties, we have further studied the nature of
magnetic transition occurring in these compounds. This has
been done by calculating the temperature variation of the
Landau coefficients. It is well known that the magnetic free
energy F共M , T兲 in general can be expressed as Landau expansion in the magnetization and the temperature and magnetic field dependencies of F共M , T兲 determine the nature of
magnetic transition.23 The Landau coefficients can be calculated using the equation of state given by ␮0H = AM + BM 3
+ CM 5. It may be noted from this equation that the magnetization isotherms obtained at various temperatures allow one
to determine the temperature variation of the Landau coefficients. It is well known that the temperature dependence of
the Landau coefficients may be utilized to distinguish between the first and second order transitions of magnetic materials. Generally, the compounds with first order transition
共FOT兲 possess positive values for A共TC兲 and B共TC兲 and negative value for C共TC兲, where TC is the magnetic transition
temperature. Furthermore, it has been reported that the magnitude of B at temperatures well below TC determines the
magnitude of MCE in giant magnetocaloric materials.3,23
The Landau coefficients of both the compounds have
been determined from the M-H isotherms obtained at various
temperatures. The temperature variation of A of all the compounds exhibits a minimum near the transition temperature.
Figure 4 shows the temperature variation of the coefficient B
of La1−xNdxFe11.5Al1.5 共x = 0.1, 0.2兲 compounds. It may be
noted from the figure that for both compounds, the sign of B
is negative at low temperatures and that its magnitude decreases with an increase in temperature. Therefore, the temperature variation of B of both the compounds indicates the
presence of FOT. With an increase in the temperature, the
sign of B of all the compounds changes from negative to
positive, thereby ruling out the possibility of FOT after a
limited temperature range above the transition temperature.
From the figure, it can also be seen that the magnitude of B
is much larger in the compound with x = 0.2, as compared to
that in x = 0.1, which implies that the strength of FOT increases with Nd concentration.
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07B338-3
J. Appl. Phys. 103, 07B338 共2008兲
Kumar et al.
istence is more in the compound with x = 0.1 may be the
reason for the larger RC in that compound. The RC values
observed in the present case compare well with those of
many potential materials such as LaFe11.4Si1.6, Gd5Si2Ge2,
MnAs, etc.,3 whose magnetic transitions are nearly in the
same range as in the present case.
CONCLUSIONS
FIG. 4. Temperature dependence of the Landau coefficient B for
La1−xNdxFe11.5Al1.5 共x = 0.1, 0.2兲 compounds.
The magnetocaloric effect in these compounds has been
measured in terms of isothermal magnetic entropy change
共⌬S M 兲 for various temperatures and applied magnetic fields.
We have calculated the ⌬SM using Maxwell’s equation.3 Figure 5 shows the temperature variation of isothermal magnetic
entropy change of La0.9Nd0.1Fe11.5Al1.5 in various fields.
These plots show a maximum with the value of 5.2 J / kg K
for x = 0.1, at 50 kOe. The corresponding value for the compound with x = 0.2 is 4.8 J / kg K. The entropy change observed in the case of LaFe11.5Al1.5 for a field of 50 kOe is
⬃3 J / kg K3. Another interesting point is that the magnetocaloric effect in the Nd-substituted compounds is significant
over a wide temperature range, 共i.e., “table-type MCE”兲 due
to the multiple first order transitions. We have also calculated
the refrigerant capacity 共RC兲 using the methods reported
elsewhere.1–3 RC values for La0.9Nd0.1Fe11.5Al1.5 and
La0.8Nd0.2Fe11.5Al1.5 compounds are found to be 544 and
470 J / kg K, respectively. The fact that the AFM-FM coex-
FIG. 5. 共Color online兲 Temperature variation of isothermal entropy change
of La0.9Nd0.1Fe11.5Al1.5 in various fields.
In conclusion, we have found that the partial substitution
of Nd for La causes a reduction in the strength of antiferromagnetic interactions. Nd substitution is also found to result
in multiple first order transitions. The nature of the magnetic
transition has been analyzed using the Landau model of magnetic phase transitions as well as Arrott plots. The magnetocaloric effect is found to increase with Nd substitution. The
refrigerant capacity is also quite large in the Nd-substituted
compounds, which has been attributed to the tablelike MCE
arising from the multiple transitions.
ACKNOWLEDGMENT
One of the authors 共K.G.S兲 thanks I.S.R.O., Government
of India for proving financial support for this work.
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