Journal of Advanced Ceramics

2013, 2(3): 213-217

DOI: 10.1007/s40145-013-0062-0

Research Article

ISSN 2226-4108

Magnetocaloric effect in La0.7Sr0.3MnO3/Ta2O5 composites

Mahmoud Aly HAMAD ab*

aPhysics Department, College of Science, Al Jouf University, Al Jouf, Skaka, P.O. Box 2014, Saudi Arabia bPhysics Department, Faculty of Science, Tanta University, Tanta, Egypt

Received: March 06, 2013; Accepted: April 04, 2013 ©The Author(s) 2013. This article is published with open access at Springerlink.com

Abstract: The magnetocaloric effect (MCE) achieved for La0.7Sr03MnO3/Ta2O5 composites has been investigated. The maximum value of magnetic entropy change of La07Sr03MnO3 composites is found to decrease slightly with the further increasing of Ta2O5 concentration. It is shown that La0.7Sr0.3MnO3/Ta2O5 composites exhibit much more uniform magnetic entropy change than that of gadolinium. Moreover, the results indicate that the temperature range between 100 K and 400 K can be covered using the La0.7Sr0.3MnO3/Ta2O5 composites. Therefore, MCE makes the composites promising for room-temperature magnetic cooling applications.

Keywords: magnetic materials; simulation and modeling

1 Introduction

The new refrigeration based on magnetocaloric effect (MCE) or electrocaloric effect has been demonstrated as a promising alternative technology to classical refrigeration (air conditioning, refrigeration, liquefaction of gases, etc.), and has a great potential to compete successfully with compression and relaxation of the gases for refrigeration [1-15]. The magnetic cooling technology is based on the use of MCE applied to various metallic materials and new alloys named magnetocaloric materials. The characterization and application of magnetic properties of the ferromagnetic materials become increasingly important as magnetoelectronic devices for the level reliability [16,17].

Recently, perovskite manganites, R1-xAxMnO3 (where R is a trivalent rare-earth ion, and A is a

* Corresponding author. E-mail: m_hamad76@yahoo.com

divalent ion such as Ca, Sr, Ba and Pb), have attracted the widest interest in aspects of experimental and theoretical researches due to their colossal magnetoresistance and large magnetic entropy change [18-20]. In this paper, the magnetocaloric properties of La07Sr03MnO3/Ta2O5 composites sintered at different temperatures have been investigated. It used phenomenological model to predict magnetocaloric properties of the composites, such as magnetic entropy change, heat capacity change, and relative cooling power.

2 Theoretical considerations

According to the phenomenological model in Ref. [21], the dependence of magnetization on the variation of temperature and Curie temperature TC is presented by

M = jMi ~Mf jtanh[A(TC - T)] + BT + C (1) where Mi is an initial value of magnetization at

ferromagnetic-paramagnetic transition and Mf is a final value of magnetization at ferromagnetic-paramagnetic transition as shown in Fig. 1;

2(B - SC) M - M f

; B is the magnetization sensitivity

dL at ferromagnetic state before transition; Sc is

the magnetization sensitivity at Curie

temperature T- and C = f 1- BTC.

Temperature (K)

Fig. 1 Temperature dependence of magnetization in constant applied magnetic field.

The magnetic entropy change of a magnetic system under adiabatic magnetic field variation from 0 to final value Hmax is available by

AO I /M'- M f

ASM = \ - A

sech2[A(Tc - T)] + BjH^ (2)

The result of Eq. (2) is the maximum magnetic entropy change, and ASmax (where T = Tc) can be evaluated as the following equation:

' M - Mf

ASmax = Hm

The determination of full-width at half-maximum

¿TFWHM can be carried out as follows:

= — sech A

2 A(M1 - Mf)

' A(Mi -Mf) + 2B

The magnetic cooling efficiency is estimated by considering the magnitude of magnetic entropy change ASM , and its full-width at half-maximum ^TFWHM

[22]. The product of -ASmax and STFWHM is called relative cooling power (RCP) based on magnetic

entropy change.

RCP = -ASM (T,Hmax) X^Tfwhm

= |Mi - Mf - 2B I Hmax X sech

2A(Mi -Mf) \A(M1 - Mf) + 2B

The magnetization-related change of the specific heat is given by Ref. [22]:

.¿ASM (6)

AC„h = T-

According this model [21], ACpH can be rewritten as ACp,H = -TA2(Mi -Mf)sech2[A(TC - T)]

X tanh[ A(Tc - T)]Hmax (7)

From this phenomenological model, it can simply

evaluate STF

and RCP for

La0.7Sr03MnO3/Ta2O5 composites under magnetic field variation.

3 Results and discussion

Figure 2 shows the magnetization of the precursor La0.7Sr0.3MnO3 powders mixed with Ta2O5 (weight fraction of 0%, 4%, 10% and 15%, respectively) versus temperature in 0.5 T magnetic field. The symbols represent the experimental data from Ref. [23], while the dashed curves represent the modeled data using model parameters given in Table 1. These parameters are determined from the experimental data. Figures 3 and 4 show the predicted values for changes of magnetic entropy and heat capacity as functions of temperature, respectively. ASM is expanded for different concentrations of Ta2O5. Since Fig. 3 indicates that the temperature range between 100 K and 400 K can be covered using the Lao.7Sr0.3MnO3/Ta2O5 composites, the composites is beneficial for manipulating magnetocaloric refrigeration that occurs in various temperatures. Furthermore, the ASM distribution of La07Sr03MnO3/Ta2O5 is much more uniform than that of gadolinium [24]. This feature is desirable for an Ericson-cycle magnetic refrigerator [25]. In addition, perovskite-like structured materials are easier to fabricate and possess higher chemical stability as well as higher resistivity. The high resistivity is beneficial to lowering the eddy current heating. Thus, due to these features,

La0.7Sr03MnO3/Ta2O5 composites can be considered as active magnetic refrigerant for near room-temperature magnetic refrigeration.

The values of maximum magnetic entropy change, full-width at half-maximum, and relative cooling

power at different Ta2O5 contents in 0.5 T magnetic field, are calculated by using Eqs. (3)-(5) respectively, and tabulated in Table 2. Furthermore, the maximum and minimum values of specific heat change for each sample are determined from Fig. 4.

Table 1 Model parameters for La0.7Sr03MnO3/Ta2O5 composites in 0.5 T applied magnetic field

Ta2O5 (wt%) Mi (emu/g) Mf (emu/g) Tc (K) B (emu/(g-K)) Sc (emu/(g-K))

0 72.9 3.2 309 -0.05 -0.59

4 61.9 1.9 317 -0.05 -0.59

10 53.1 1.9 323 -0.05 -0.59

15 35.1 2.4 318 -0.05 -0.40

Table 2 The predicted values of magnetocaloric properties for La07Sr03MnO3/Ta2O5 composites in 0.5 T applied magnetic field

Ta2O5 (wt%) -ASmax (J/(kg-K)) ¿TFwhm (K) RCP (J/kg) ACpMmzx) (J/(kg-K)) -ACp

H(min) (J/(kg'K))

0 0.295 122.43 36.12 1.14 -0.86

4 0.293 105.39 30.88 1.33 -1.05

10 0.295 89.94 26.51 1.56 -1.28

15 0.200 92.18 18.42 1.01 -0.83

Temperature (K)

Fig. 2 Magnetization in 0.5 T magnetic field for the La0.7Sro.3MnO3/Ta2O5 composites versus temperature. The dashed curves are modeled results and symbols represent the experimental data from Ref. [23].

Fig. 3 Magnetic entropy change as function of temperature for La0.7Sr0.3MnO3/Ta2O5 composites in 0.5 T magnetic field.

Fig. 4 Heat capacity change as function of temperature for La0.7Sr0.3MnO3/Ta2O5 composites in 0.5 T magnetic field.

In general, the magnetic entropy change in perovskite manganites has been believed to be related to the considerable variation of magnetization near TC [26]. The spin-lattice coupling in the magnetic ordering process could play a significant role in additional magnetic entropy change [27].

Magnetic refrigeration works because there are two contributions to the total entropy of the system: a magnetic entropy that is related to the order of magnetic moments, and a lattice entropy that is related to the temperature. It is convenient to start with a material with disordered magnetic moments, which is typically found with the lowest field magnitude and ambient temperature within the refrigeration cycle.

Applying a magnetic field adiabatically causes the spins in the material to align. Recalling that no heat is exchanged in an adiabatic process, the decrease in magnetic entropy must be compensated by an increase in the lattice entropy, which implies that the material must heat up. Once the moments are aligned and excess heat has been removed, the material returns to ambient temperature. The adiabatic removal of the applied field then leads to an increase in magnetic entropy, which is compensated by a decrease in the lattice entropy, and thus the temperature of the material decreases below ambient.

Due to the strong coupling between spin and lattice, the significant lattice change accompanying magnetic transition in perovskite manganites has been observed [28,29]. The lattice structural change in the Mn-O bond distance as well as Mn-O-Mn bond angle would, in turn, favor the spin ordering. Thereby, a more abrupt reduction of magnetization near TC occurs and results in a significant magnetic entropy change [30-32]. In this way, a conclusion might be drawn that a strong spin-lattice coupling in the magnetic transition process would lead to additional magnetic entropy change near TC , and consequently, favors the MCE.

4 Conclusions

The calculations show that the La07Sr03MnO3/Ta2O5 composites provide temperature span and obtain a considerable ASM against temperature variation. ASM distribution is uniform, which is desirable for Ericsson-cycle magnetic refrigerator, magnetic softness and isotropic and low priced. In addition, these samples are convenient to prepare and exhibit higher chemical stability as well as higher resistivity that is favorable for lowering eddy current heating.

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