Accepted Manuscript
Review Articles
Magnetocaloric effect in CoEr 2 intermetallic compound
A. Boutahar, R. Moubah, H. Lemziouka, A. Hajjaji, H. Lassri, E.K. Hlil, L.
Bessais
PII: S0304-8853(17)31331-8
DOI: http://dx.doi.org/10.1016/j.jmmm.2017.08.015
Reference: MAGMA 63049
To appear in: Journal of Magnetism and Magnetic Materials Received Date: 29 April 2017
Revised Date: 13 July 2017 Accepted Date: 2 August 2017
Please cite this article as: A. Boutahar, R. Moubah, H. Lemziouka, A. Hajjaji, H. Lassri, E.K. Hlil, L. Bessais, Magnetocaloric effect in CoEr 2 intermetallic compound, Journal of Magnetism and Magnetic Materials (2017), doi:
http://dx.doi.org/10.1016/j.jmmm.2017.08.015
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Magnetocaloric effect in CoEr 2 intermetallic compound
A. Boutahar 1,5* , R. Moubah 2 , H. Lemziouka 2,3 , A. Hajjaji 1 , H. Lassri 2 , E. K. Hlil 4 , L. Bessais 5
1 Laboratoire des Sciences de l’Ingénieur pour l’Energie. Ecole Nationale des Sciences Appliquées d’El jadida BP 1166 Université Chouaib Doukkali - El
Jadida Plateau 24002 Morocco
2 LPMMAT, Université Hassan II-Casablanca, Faculté des Sciences Ain Chock, BP 5366, Mâarif - Casablanca, Morocco.
3 Renewable Energy Laboratory and Dynamic Systems, Faculté des Sciences Ain- Chock, Université Hassan II de Casablanca, Morocco
4 Institue Néel, CNRS et Université Joseph Fourier, 25 rue des Martyrs BP 166, F- 38042 Grenoble Cedex 9, France.
5 ICMPE-CMTR, UMR CNRS 7182, 2-8, rue H. Dunant, 94320 Thiais, France
Abstract
Magnetic refrigeration based on the magnetocaloric effect has attracted much attention due to its high energy-efficiency, compactness and environmental benignness. Many research activities were devoted to explore a range of potential magnetic materials with large magnetocaloric effect. Here, we report a giant magnetocaloric effect in CoEr 2 compound.
Magnetic measurements show that the sample exhibits a second order magnetic transition with a T C of 16 K. At a magnetic field change of 5 T, large values of magnetic entropy change (-∆S max = 21 J kg −1 K −1 ) and relative cooling power (RCP= 526 J kg -1 ) below 16.5 K were observed. The CoEr 2 compound with excellent magnetocaloric effect is expected to have effective applications in low temperature range.
Keywords: CoEr 2 compound; Magnetocaloric effect; Low temperature; Magnetic transition.
*Corresponding author: (boutahar.fsac@gmail.com)
I. INTRODUCTION
Magnetic refrigeration 1 based on the magnetocaloric effect (MCE) materials 2,3,4 can be a key solution to many environmental concerns compared to conventional vapor compression refrigeration. This is due to its several advantages in terms of refrigeration efficiency, reliability, low noise and ecological friendliness 5 . Up to now, the MCE has been used in magnetic refrigerant devices mainly in the low temperature range (T < 20 K) using salt compounds 6 , which are used in technological applications such as: space science and liquefaction of hydrogen in fuel industry 7,8 . However, the actually used salt materials present many drawbacks because they are hydrated, which requires to be encapsulated in a hermetic container to prevent dehydration. In addition, the growth of salt compound is very time consuming which is not convenient for industrial processes. On the other hand, gaint magnetocaloric effects were previously reported in first-order magnetic transition materials, however, it is accompanied by some non-desirable disadvantages such as: the irreversibility of the effect, poor mechanical thermal stability or field hysteresis 9 . Therefore, many researchers have been concentrated in finding new magnetic refrigeration materials with excellent performance at low temperature range and especially those exhibiting second order magnetic transitions(SOMT) 10, 11,12,13,14,15,16,17,18,19,20,21,22,23
. SMOT materials are known to lack a very
large (−∆SM), but they do present a very high refrigeration capacity (RC), which is now
recognized as an essential parameter for obtaining high magnetic refrigeration efficiency. The
other positive features of SOMT materials are low magnetic hysteresis, higher electrical
resistivity, enhanced corrosion resistance, and tunable T C by varying the composition. The
MCE can be characterized by the magnetic entropy change (-∆S m ) and adiabatic temperature
change (-∆T ad ). Besides, the relative cooling power (RCP) has also been considered as
another important parameter to quantify the magnetocaloric properties 2,24 . Recently, R-Co
(R= rare earth elements) compounds have been have been extensively studied during the last
years due to their interesting physical phenomena counting metamagnetism, giant magnetostriction, and large magnetocaloric effects (MCE) 25,26,27,28,29,30,31,32,33 . Significant efforts were devoted to rare-earth based intermetallic compounds for low temperature magnetic refrigeration and some of them are exhibiting excellent MCE properties 34,35,36,37 . In this work, we have successfully synthesized the CoEr 2 compound by arc melting without annealing. The magnetic properties and MCE in the intermetallic compound CoEr 2 were systematically studied. A giant reversible MCE was observed. The present results show that the CoEr 2 compound is a promising candidate for magnetic refrigeration especially for hydrogen liquefaction.
II. Experimental details
Polycrystalline CoEr 2 samples were synthesized by arc melting technique using stoichiometric amounts of Co (99.9%) and Er (99.9%) under purified argon atmosphere. The sample was remelted several times (five successive melting) for having a good homogeneous composition. X-ray diffraction measurements were carried out using using D5000 Siemens diffractometer with Co-K α1 source. Magnetization measurements were carried out in the temperature range from 6 to 66 K with an external applied magnetic field up to 7 T using BS2 extraction type magnetometer, developed at Néel Institute of CNRS in Grenoble, France.
III. Results and discussion
The x-ray diffraction pattern of the CoEr 2 compound is displayed in figure 1. All the diffraction peaks can be indexed according to the monoclinic structure (space group P 2/m) of CoEr 2 suggesting the absence of spurious phases in the detection limit of the XRD experiment. The lattice parameters were determined using Rietveld method and were found to be a= 14.744 Å, b = 12.955 Å, and c= 3.509 Å with R wp =12.1 %, R p =10.8 % and χ 2 =1.79.
The average grain size was calculated using Debye-Scherrer formula: = .. λ
α . θ , where D is
the crystallite size, λ is the wavelength of x-ray (λ CoKα1 =1.788920 Å), α is the full width at half maximum (FWHM) of diffraction peaks. In order to determine the FWHM, the peaks were fitted to Gaussian distribution. We found an average grain size of around 21 nm. Figure 2 shows a SEM image of the CoEr 2 compound. We can note that the sample is fairly uniform, homogeneous, and the grain presents a layer shape. The chemical composition was checked using energy dispersive x-ray (EDX) analysis (data not shown). The EDX spectra reveals the presence of only Co and Er elements and the determined chemical composition was found to be close to the nominal one (Co: 32.73; Er : 67.27 at %).
The temperature dependence of magnetization M (T) of the CoEr 2 alloy recorded at an external magnetic field µ 0 H = 0.05 T is displayed in Fig. 3. The magnetization is found to decrease with increasing temperature. The Curie temperature is found to be Tc=16 K, which was determined from the minimum of the dM/dT curve (Fig. 3). The isothermal magnetization curves as a function of magnetic field were measured in applied magnetic fields up to 7 T and from 6 to 66 K (Fig. 4). It can be seen that even at high fields (7 T), the magnetization saturation is not fully attained which can be understood by the non-collinear magnetic structure of Er and Co sublattices. The magnetic moment calculated at 7 T is 8.8 µ B per Er atom at 6 K, which is smaller than the expected value of 9.6 for Er 3+ ions. This indicates that the Er spin structure is not completely collinear in CoEr 2 alloy. Similar behavior were previously reported in CoEr 2 amorphous alloy 28 . In addition, a sharp change in magnetization is clearly observed in Fig. 4 as the temperature nears and eventually crosses over T C from ferromagnetic to paramagnetic transitions. A noticeable feature below T C in Fig.
4 is that a large proportion of the change in magnetization occurs below 2 T. This is beneficial
for practical application in magnetic refrigeration at modest fields 38 . We have also plotted M 2
versus H/M isotherms (Arrot plots, inset of Fig 4.). The positive slope of the Arrot plot
suggests that the ferromagnetic to paramagnetic phase transition in CoEr 2 is second order in
nature 39 according to the Banerjee criterion 40 .
To confirm the magnetic phase transition nature, we have used the Inoue–Shimizu s-d model
41,42 , which has widely been used to discuss behaviors of several types of magnetocaloric materials. According to Landau theory 29,30 , the magnetic free energy F (M, H) versus magnetization and temperature can be expressed as:
MH M
T c M
T b M
T a
F 4 ( ) 6 0
6 ) 1
4 (
² 1 ) 2 (
1 + + − µ
= . (1)
The Landau coefficients are accessible through the equation of state linking M and the magnetic field:
H M
T c M T b M T
a ( ) + ( ) 3 + ( ) 5 = µ 0 . (2)
The coefficients a (T), b (T) and c (T) depend on temperature with respect to the thermal
variation of spin fluctuations amplitude and can be determined by fitting the isothermal
magnetization data using the above equation. Examination of the free energy demonstrates
that the parameter a (T) is always positive and would get a minimum value at Curie
temperature corresponding to a maximum of susceptibility. On the other hand, the sign of
b (T) determine the order of magnetic transition nature: the 1 st order transition takes place if
b (T C ) < 0, while the second order transition occurs when b (T C ) ≥ 0. Besides, c (T) is positive
at T C and, in the other temperature regions, it can be negative or positive. The magnetization
isothermal data around T C of CoEr 2 at different temperatures together with the fitted curves
are shown in Fig. 5. It can be observed that the fitted µ 0 H-M curves according to Landau
model are in good agreement with the experimental data. Fig. 6 shows the temperature
dependence of the Landau’s parameters for the CoEr 2 alloy. a (T) was found to be positive
with a minimum close to T C and b (T C ) positive indicating the occurrence of a SOMT.
Moreover, the T C magnitude deduced from classical magnetic measurements is similar to that obtained from Inoue–Shimizu s-d model.
As is known, a completely reversible MCE requires no hysteresis in the magnetization curves, which is advantageous for practical applications. From the M (H) curves, we have calculated the magnetic entropy change using Maxwell thermodynamic relation 2 .
i
i i i
i i H
M H
T T
M µ M
H T
S ∆
−
= −
∆
∆ ∑
+ +
∆
1 0 1
) ,
( , (3)
Where µ 0 is the vacuum permeability, M i and M i+1 are the magnetization values measured at temperatures T i and T i+1 at a field change ∆H i .
Fig. 7 shows the temperature dependences of magnetic entropy change of CoEr 2 alloy at different magnetic field changes. The maximum value of -∆S m was found to increase monotonically with increasing magnetic field and reaches a value of 26.9 J kg -1 K -1 at 16.5 K for a magnetic field change from 0 to 7 T. The calculated values of the magnetic entropy change (-∆S m ) in the present crystalline CoEr 2 alloy are higher than those obtained in CoEr 2
alloy in amorphous state 27 . In the intermetallic compound CoEr 2 , the major contribution of magnetization is coming from the rare earth element with large magnetic moment. The large saturation magnetization and the sharp change of magnetization as a function of T near T C
( inset of Fig. 3) result in large magnetic entropy change in the crystalline CoEr 2 alloy.
Note that a large (-∆S m ) is not enough to obtain excellent magnetic refrigeration features. The effective method for characterizing the cooling efficiency is the RCP which measures the amount of heat that can be transferred between the cold and hot sinks in one ideal refrigeration cycle and is defined as 43 :
= − × (4)
Where δT FWHM is the full width at half maximum of the (-∆S m )-(T) curve.
We have found a high relative cooling power (RCP) of our CoEr 2 alloy. The RCP increases from 60 to 526 J kg −1 when the field increases from 1 and 5 T, respectively. The summary of RCP values and other essential magnetocaloric parameters of CoEr 2 and other magnetic materials reported in literature with close transition temperatures 44,45,46,47,48 were listed in Table 1. As shown, the CoEr 2 exhibits higher (-∆S m ) values than HoCoC 2 , Dy 3 Co, Tm 2 Cu 2 Cd materials. Although that the TmZn, HoPdIn, ErMn 2 Si 2 compounds present larger (-∆S m ) than the CoEr 2 alloy , its RCP factor is found to be higher than the other compounds (526 J/kg at 16 K), which is one of the highest values reported for magnetocaloric materials at low temperature. We note that the RCP factor is an important parameter as far as the magnetic refrigeration is concerned. The high values of both (-∆S M ), and RCP factor allow us to conclude that this alloy is one suitable candidate for magnetic refrigeration in low temperature range close to 16 K.
IV. CONCLUSION
In this paper, we have studied the structural, magnetic and magnetocaloric properties of CoEr 2
compound. X-ray diffraction (XRD) indicates that the CoEr 2 compound crystallizes in
monoclinic structure. Magnetic measurements have shown the presence of a ferromagnetic
(FM) –paramagnetic (PM) transition at T C =16 K and a large reversible magnetic entropy
change with second-order magnetic transition. Our study demonstrates that the CoEr 2
compound could be considered as a potential candidate in magnetic refrigeration at low
temperature.
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Acknowledgements
This work is mainly supported by the PHC Maghreb 15MAG07.
30 35 40 45 50 55 60 65 70 75 80 85
(6 0 2 )
(4 2 2 ) (4 0 2 ) (1 3 2 )
(2 0 2 )
(4 5 1 ) (5 5 0 )
(5 5 0 )
(0 5 1 ) (5 4 0 )
(- 5 0 1 )
(2 5 0 ) (3 2 1 ) (3 1 1 )
(2 2 1 ) (2 0 1 )
CoEr 2
In te ns ity (a . u .)
2 θ (°)
Measured (Iobs) Calculated (Icalc) Bragg Position I obs - I calc
(1 1 1 )
Fig. 1. XRD pattern of the CoEr 2 compound at room temperature with corresponding Rietveld
refinement.
Fig. 2 SEM image of the CoEr 2 compound at room temperature.
0 10 20 30 40 50 60 70 80
0 3 6 9 12 15
0 10 20 30 40 50 60 70
0 30 60
-1.0 -0.8 -0.6 -0.4 -0.2 0.0
dM /d T ( em u/ g K )
T (K)
M ( em u /g )
µ 0 H= 0.05T
T (K)
M ( e m u /g )
µ0H= 4T µ0H= 3T µ0H= 2T
T C =16 K
Fig. 3. Temperature dependence of magnetization under a magnetic field of 0.05 T of
the CoEr 2 compound. The corresponding dM/dT curve was also overlaid to mark the
transition temperature. The inset shows the temperature dependence of magnetization
recorded at different applied fields of 2, 3, and 4 T.
0 1 2 3 4 5 6 7 8
0 50 100 150 200 250
0 100 200 300
0 30000 60000
µ0H/M (Oe g/emu) M2 (emu2/g2)
9 K 12 K 15 K 18 K 21 K
