Accepted Manuscript
Magnetic and magnetocaloric properties in amorphous and crystalline Tb0.67Au0.33 alloys
G. Alouhmy, R. Moubah, A. Berrada, N. Hassanain, L. Bessais, H. Lassri
PII: S0304-8853(17)30717-5
DOI: http://dx.doi.org/10.1016/j.jmmm.2017.07.070
Reference: MAGMA 63001
To appear in: Journal of Magnetism and Magnetic Materials Received Date: 24 February 2017
Revised Date: 16 June 2017 Accepted Date: 20 July 2017
Please cite this article as: G. Alouhmy, R. Moubah, A. Berrada, N. Hassanain, L. Bessais, H. Lassri, Magnetic and magnetocaloric properties in amorphous and crystalline Tb0.67Au0.33 alloys, Journal of Magnetism and Magnetic Materials (2017), doi: http://dx.doi.org/10.1016/j.jmmm.2017.07.070
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Magnetic and magnetocaloric properties in amorphous and crystalline Tb
0.67Au
0.33alloys
G. Alouhmy1, R. Moubah1,*, A. Berrada2, N. Hassanain2, L. Bessais3, H. Lassri1
1LPMMAT, Faculté des Sciences Ain Chock, Université Hassan II de Casablanca, BP 5366, Mâarif - Casablanca, Morocco
2Laboratoire de Physique des Matériaux, Faculté des Sciences, BP 1014, Rabat, Morocco
3CMTR, ICMPE, UMR7182, CNRS Université de Paris 12, 2-8 rue Henri Dunant F-94320 Thiais, France
Abstract:
The magnetic and magnetocaloric properties of amorphous and crystalline Tb0.67Au0.33were compared. Amorphous Tb0.67Au0.33 alloys exhibit a spin glass state with a prevailing ferromagnetic behavior with a Curie temperature of 130 K, while the crystalline alloy presents an antiferromagnetic behavior with a Néel temperature of 42 K. Arrott plots show that the amorphous alloy presents a second order magnetic transition while the crystalline state exhibits a field induced a metamagnetic transition. Under magnetic field change of 20 kOe, the maximum magnetic entropy changes were found to be 1.38 and 2.03 J/kg.K, with corresponding relative cooling power of 153 and 40 J/kg for the amorphous and crystalline Tb0.67Au0.33 alloys, respectively.
Keywords:
Magneticalloys; Magnetocaloric effect; Magnetic entropy change; Magnetic transition, Relative cooling power.
*Corresponding author: [email protected]
1.
Introduction
Magnetic refrigeration is a promising technology which is actually used for liquefaction of hydrogen and helium. Magnetic refrigeration devices are based on the magnetocaloric effect which determines the tendency of a magnetic material to heat up when placed in a magnetic field and cool down once the field is removed. Measurements of magnetic entropy change can be used as a way to check the magnetic refrigeration efficiency.
Finding magnetocaloric materials having both large magnetic entropy change and adiabatic temperature change (∆TM) is important for the development of magnetic refrigerators [1,2]. One can notice that a good magnetocaloric performance was previously reported in first-order magnetic transition (FOMT) materials such as:
La(Fe,M)13(M=Co,Al,Si), or Gd5Si4-xGexe [3,4,5] . However, the irreversibility of their magnetocaloric effects limits their practical application. In addition, FOMTs occur in a narrow temperature window and the materials exhibit generally a bad mechanical stability [6]. Thus, it was required to find materials with second-order magnetic transition (SOMT) with high magnetocaloric efficiency. In this perspective, many magnetic materials studied in the past were being reinvestigated to find out the magnetocaloric effect (MCE) like GdNi and TbAl [ 7 , 8 ]. In order to use and exploit these materials a fundamental understanding of their magnetic and magnetocaloric properties is necessary. Up to now, the magnetocaloric effect in TbAu alloys remains unexplored for both the crystalline and amorphous states. In this work, we present a comparative investigation of the magnetic and magnetocaloric properties in amorphous and crystalline Tb0.67Au0.33.
2.
Experiment
Amorphous Tb0.67Au0.33 alloys were liquid-quenched using piston and anvil technique under argon atmosphere. The cooling rate was estimated to be of the order of 10°C/sec. The
obtained amorphous samples have the form of foils with surface area of 2 × 2 cm2 with a thickness of around 40 µm. The polycrystalline alloys of the same composition was synthesized by arc-melting in a high purity argon atmosphere. All samples were prepared from their constituent metals of purity 4N for Tb and 3N for Au. The obtained ingots were melted 4 times to insure a good homogeneity. Losses induced by evaporation were found to be negligible. The chemical characterization of the resulting samples were investigated using electron probe microanalysis (EPMA). The EPMA measurements have shown that all samples are uniform, and chemically homogeneous, with the expected stoichiometry with an uncertainty of around 5 %. Magnetization measurements were carried out using Faraday method using a superconducting Coil with an external applied magnetic field up to 5 T in a temperature range from 1.8 to 300 K.
3. Results and discussion
The change of magnetization as a function of temperature recorded at a magnetic field of 0.1 kOe for the amorphous and crystalline Tb0.67Au0.33 samples are displayed in Fig. 1. The magnetization M(T) for crystalline Tb0.67Au0.33 alloy presents an important magnetization decrease at low temperature which is a signature of antiferromagnetic interaction. The maximum magnetization is found to beat at 42 K which corresponds to its Néel temperature (TN). We note that Sill et al.[9] have reported a Néel temperature of 50 K in crystalline Tb2Au alloys. The difference in the magnitude of TN could probably attributed to the preparation technique or a slight composition difference. For amorphous Tb0.67Au0.33 alloys, the M (T) curve is in line with the existence of a spin glass state with a predominant ferromagnetic character. This behavior highlights a non homogeneity of the magnetic interaction, which can be understood by considering the random distribution of the interatomic distances within the amorphous matrix. Ferromagnetic interactions are favored at larger Tb–Tb interatomic distances and antiferromagnetic interactions at shorter
distances. Because of the deviations in interatomic spacing in amorphous alloys as compared with bulk crystalline materials, the exchange interaction can shift from antiferromagnetic to ferromagnetic ones [10].The Curie temperature (Tc) of the amorphous Tb0.67Au0.33 alloy was found to be around 130 K, which was deduced from the inflection point of the |
| vs. T plot. The change of the inverse susceptibilities χ-1 as a function of temperature for amorphous and crystalline Tb0.67Au0.33 alloys is shown in the top inset of Fig. 1. For both samples and at high temperatures the inverse susceptibilities follow the Curie-Weiss law which is associated with the paramagnetic state. The effective moments, deduced from Curie constants, were found to be 9.89 and 9.65 µB/at of Tb for amorphous and crystallineTb0.67Au0.33 alloys, respectively. These values are comparable to the effective moment found in free Tb3+ 9.72 µB [11].
Isothermal magnetization curves for the amorphous and crystalline Tb0.67Au0.33 alloys were measured at different temperatures (Fig. 2). For the amorphous Tb0.67Au0.33 alloy, a progressive decrease of magnetization can be observed with increasing temperature, which is due to the ferromagnetic to paramagnetic transition in the vicinity of Curie temperature. No metamagnetic transition can be seen at any temperature in this case. In the crystalline Tb0.67Au0.33 alloy case, a metamagnetic transition takes place at low temperature. At low temperature and field, the magnetization increases linearly with increasing applied magnetic field, but above a critical field, it increases abruptly. The critical field estimated by the
curve is found to be 7 kOe (see the left inset of Fig. 2(b)). One can notice that the critical field magnitude is comparable to that reported by Gamari –Seale, et al. [12] in crystalline Tb2Au alloys, which is around 6.4 kOe. Fig. 3 (a) and (b) show the Arrott plots recorded at different temperatures for the amorphous and crystalline Tb67Au33 alloys, respectively. According to the Banerjee criterion a magnetic transition is expected to be of the first-order when some of
curve slopes are negative and present a S shape, whereas it will be of second-order when all slopes are positive with almost a linear character of these curves. For all temperatures, the Arrott plots of the amorphous Tb2Au exhibits a positive slope in the surrounding of TC which highlights a second-order magnetic transition. In the crystalline Tb0.67Au0.33 alloy, it can be observed from fig. 3 that some of the Arrott plots present a S shape, highliging the existence of a field induced a first-order antiferromagnetic to ferromagnetic transition below TN. However, the Arrott plot exhibits a positive slope above TN which indicates a second-order FM–PM transition[13]. The metamagnetic transition is interpreted as the driving force of the transition from the first-order to second-order by random field and random field-induced domain structure [14].
From Maxwell relation, the magnetic entropy change (-∆SM) was determined from the isothermal magnetization curves in the vicinity of the magnetic temperature transitions. Using thermodynamic theory, the magnetic entropy change induced by a change in the applied magnetic field from 0 to µ0Hmax is given by the following formula:
T dH T M
S H T S H T S
T H
µ M
M M
MAX
∫
∂∂ =
−
=
∆
∆
0
0
) 0 , ( ) , ( ) , (
(1)
In case of discrete magnetic field changes ∆SMcan be expressed as:
∆ = ∑
∆ (2)
Mi and Mi+1 are the magnetization values at temperature Ti and Ti+1 under an applied magnetic field Hi respectively. The results of the maximum magnetic entropy change for the amorphous and crystalline Tb0.67Au0.33 alloy are displayed in Fig 4(a). In both cases, the (−∆SM) curves exhibit a positive peak at around TC, and TN. We note that the amorphous alloys present a much broader temperature distribution than the crystalline alloys. The full width at half-
maximum of the (−∆SM)(T) were found to be 110.34 and 20.06 K for the amorphous and crystalline alloys, respectively. This difference can be understood by the fact that the amorphous sample presents an interatomic distance distribution. Due to the amorphous structure, the exchange interactions should be non-homogenous, which induced a broader temperature distribution [ 15 ]. In addition, the |∆SMmax| magnitude of the crystalline Tb0.67Au0.33 alloy is higher than that of the amorphous alloy, which can be explained by the temperature dependence of magnetization, the latter varies rapidly around the temperature magnetic transition in the crystalline Tb0.67Au0.33 alloy (first magnetic order transition) than in the amorphous sample (second order magnetic transition), which is in accordance with the Arrott plots. Note that a large magnetic entropy change alone is not enough to achieve good magnetic refrigeration efficiency. Indeed, the relative cooling power (RCP) is an important parameter as the magnetic refrigeration is concerned, which is defined by of the product of
∆Smmax
and its full width at half-maximum (TFWHM) [16]:
RCP = -∆Smmax* TFWHM (2)
The RCP defines how much heat is transferred between the hot and cold ends in one ideal refrigeration cycle. The change of RCP as a function of applied magnetic field for the amorphous and crystalline Tb0.67Au0.33 alloys is displayed in Fig 4(b). For both alloys, the RCP increases with increasing magnetic field. The RCP value obtained at a field change of 2 T for the amorphous Tb0.67Au0.33 alloy is 153.04 J/kg. However for the crystalline Tb0.67Au0.33
alloy, the corresponding RCP value is 40 J/kg under the same field change. The amorphousTb0.67Au0.33 alloy exhibits a larger RCP than the crystallineTb0.67Au0.33 alloy due to its higher (TFWHM) which is associated with the non-homogenous interatomic distribution of the amorphous alloys.
Conclusion:
7
In summary, the magnetic and magnetocaloric properties of amorphous and crystalline Tb0.67Au0.33 alloys were studied. Magnetic measurements have shown the existence of a ferromagnetic order with Curie temperature 130 K for the amorphous Tb0.67Au0.33 alloy and the antiferromagnetic order with Neel temperature 42 K for the crystalline Tb0.67Au0.33 alloy.
For a magnetic field change of 20 kOe, the amorphous and crystalline Tb0.67Au0.33 alloy shave maximum magnetic entropy changes of 1.38 and 2.03 J/kg.K, with RCP values of 153 and 40 J/kg, respectively. Our comparison shows that the crystalline Tb0.67Au0.33 alloy possesses larger magnetocaloric properties than the amorphous Tb0.67Au0.33 alloys. Finally, this study will be useful to understand the magnetic and magnetocaloric properties in both amorphous and crystallineTb0.67Au0.33 alloy.
Acknowledgements:
Fig. 1: Temperature dependence of magnetization recorded at a magnetic field of 0.1 kOe of the amorphous and crystalline Tb0.67Au0.33 alloys. Bottom inset shows the zoom of the M(T) curve of the crystalline sample, and the top inset is the temperature dependence of inverse
0 50 100 150 200 250 300
0.0 0.5 1.0 1.5 2.0 2.5
Amorphous Crystalline
M(emu/g)
T (K)
0 50 100 150 200 250 300 0.00
0.04 0.08 0.12 0.16
T(K)
M(emu/g)
Crystalline
TN=42K
0 50 100 150 200 250 300 0
1 2 3 4 5 6 7
χ-1(kOe.g/emu)
T (K)
susceptibility for the amorphous and crystalline Tb0.67Au0.33 alloys (the lines represent fits of the high temperature data to the Curie–Weiss law).
0 10 20 30 40 50 60
0 20 40 60 80
0 10 20 30 40
0 2 4 6 8 10
H (kOe)
dM/dH(emu/g.kOe)
M (emu/g)
H (kOe)
4 K 9 K 13 K 18 K 23 K 28 K 32 K 37 K 42 K 46 K 51 K 56 K 60 K 65 K 70 K
0 (b) 30 60 90
120 4.2 K
18 K 32 K 46 K 60 K 74 K 88 K 100 K 116 K 130 K 144 K 158 K 172 K 186 K 200 K
M (emu/g)
(a)
Fig. 2: Isothermal magnetization curves as a function of applied field for the a) amorphous and b) crystalline Tb0.67Au0.33 alloys respectively at different temperatures. Inset shows the field dependence of the
plot at 4 K for the crystalline Tb0.67Au0.33 alloys.
Fig. 3: Arrott plots of the a) amorphous and b) crystalline Tb0.67Au0.33 alloys obtained at different temperatures.
0 3000 6000 9000 12000
15000 4,2 K
18 K 32 K 46 K 60 K 74 K 88 K 100 K 116 K 130 K 144 K 158 K 172 K 186 K 200 K
M²(emu/g)²
(a)
0.0 0.3 0.6 0.9 1.2 1.5 1.8
0 1000 2000 3000 4000 5000
6000 4 K
9 K 13 K 18 K 23 K 28 K 32 K 37 K 42 K 46 K 51 K 56 K 60 K 65 K 70 K
M²(emu/g)²
H/M(kOe.g/emu) (b)
5 10 15 20 25 30 35 40 45
0 60 120 180 240 300
360 Amorphous
Crystalline
RCP (J/kg)
H (kOe)
0 40 80 120 160 200
0.0 0.5 1.0 1.5 2.0 2.5
Amorphous Crystalline
-∆S (J/kg.K)
T (K)
Fig. 4: (a) and (b) present thetemperature dependence of magnetic entropy change -∆Sm at 20 kOe, and change of RCP as a function of the applied magnetic field for the amorphous and crystalline Tb0.67Au0.33 alloys, respectively.
We have studied the magnetic and magnetocaloric properties in amorphous and crystalline Tb0.67Au0.33 alloys.
Amorphous Tb0.67Au0.33 alloy exhibits a spin glass state with a prevailing ferromagnetic behavior, while the crystalline alloy is antiferromagnetic.
The maximum magnetic entropy change is larger in case of the amorphous alloy.
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