Magnetic properties and magnetocaloric effect in ndxla1-xfe11.5al1.5 compounds

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Vol 17 No 8, August 2008

Chinese Physics B

c 2008 Chin. Phys. Soc.
and IOP Publishing Ltd

Magnetic properties and magnetocaloric effect in
NdxLa1−xFe11.5Al1.5 compounds∗
Wang Fang(王 芳)† , Wang Guang-Jun(王光军),
Sun Ji-Rong(孙继荣), and Shen Bao-Gen(沈保根)
State Key Laboratory of Magnetism, Institute of Physics and Centre for Condensed Matter Physics,
ChineseAcademy of Sciences, Beijing 100190, China
(Received 6 November 2007; revised manuscript received 24 March 2008)
Effects of Nd-doping on the magnetic properties and magnetocaloric effects (MCEs) of Ndx La1−x Fe11.5 Al1.5 have
been investigated. Substitution of Nd leads to a weakening of the antiferromagnetic (AFM) coupling and an enhancement
of the ferromagnetic (FM) coupling. This in turnresults in a complex magnetic behaviour for Nd0.2 La0.8 Fe11.5 Al1.5
characterized by the occurrence of two phase transitions at ∼188 K (PM–AFM) and ∼159 K (AFM–FM). As a result,
a table-like MCE (9 J/kg·K) is found in a wide temperature range (160–185 K) for a field change of 0–5 T around the
transition temperature, as evidenced by both the magnetic and calorimetric measurements. Based on theanalysis of
low-temperature heat capacity, it is found that the AFM–FM phase transition modifies the electron density significantly,
and the major contribution to the entropy change comes from the electronic entropy change.

Keywords: magnetic properties, NaZn13 -type compounds, magnetic entropy change
PACC: 7530S, 6540, 7550E, 0570F

NaZn13 -type LaFe-based compounds La(Fe,Si)13
and La(Fe,Al)13have recently attracted much attention due to their potential applications in magnetic
refrigeration.[1−3] La(Fe,Al)13 compounds with different Al content show diverse magnetic properties,
and their properties are easily modified by magnetic
field, pressure, temperature and composition.[4−11]
The ground magnetic state of La(Fe,Al)13 compounds
with high Fe concentration is antiferromagnetic.Substitution of small amounts of Co for Fe or introducing carbon atoms can make the antiferromagnetic coupling collapse and result in a ferromagnetic
state.[12−19] Large magnetic entropy change is observed around the phase transition temperature without any hysteresis, which indicates that they may be
an appropriate candidate for the magnetic refrigerant.
To date, limited reports are availableabout the effects
of other rare earth elements in La(Fe,Al)13 compounds
with high Fe concentration on the structure, phase
transition and magnetic properties.
In this paper, we use light rare earth element
Nd to substitute for La and investigate the change
of magnetic properties in detail. In addition, a table∗ Project

like magnetocaloric effect (MCE) (9 J/kg·K) is found
in a wide temperaturerange (160–185 K) for a field
change of 0–5 T around the transition temperature,
as evidenced by both the magnetic and calorimetric
measurements. We also carry out a study on the origin of this large entropy change by combined magnetic
and calorimetric measurements.
Arc-melted samples Ndx La1−x Fe11.5 Al1.5 (x=0–
0.5) were annealed at 1173K for 2 weeks in vacuum, then quenched in liquidnitrogen. X-ray diffraction was used to examine the phase purity of the
compounds. The dc magnetization, ac susceptibility
at different frequencies and heat capacity were measured using a physical property measurement system
(PPMS) in the temperature range 2–295 K under applied fields up to 8 T.
X-ray diffraction patterns of compounds
Ndx La1−x Fe11.5 Al1.5 (x=0, 0.1, 0.2 and 0.3) at room
temperatureare shown in Fig.1. One can see that
the compounds Ndx La1−x Fe11.5 Al1.5 (x=0.1 and 0.2)
obtained are single-phase with a cubic NaZn13 -type
structure. When x ≥ 0.3, the alloys are mixtures containing mainly Nd1−x Lax Fe11.5 Al1.5 and α-Fe, which

supported by the National Natural Science Foundation of China (Grant Nos 50571112 and 50731007), the National Basic
Research Program of China...
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