Magnetocaloric effect and electrical properties of (0.95)La0.45Nd0.25Sr0.3MnO3/(0.05)CuO composites

In this work, the structural, magnetic and magnetocaloric properties of 0.95La0.45Nd0.25Sr0.3MnO3/0.05CuO composites materials were investigated. The samples have been synthesized by solid-state reaction route. The XRD patterns of composites powders show the presence of both perovskite La0.45Nd0.25Sr0.3MnO3 and monoclinic Tenorite CuO materials. The microstructural characterization performed using Scanning Electron Microscope shows that copper oxide nanostructure is located in the grains boundaries after pressing. According to the isothermal magnetization measurements around the Curie temperature, the maximum isothermal magnetic entropy change is calculated to be 4.128 J/(Kg.K) at 5 T for the pellet with an interesting enhancement compared to the powder sample 2.7 J/(Kg.K). The relative cooling power is about 212.8 J Kg−1. Resistivity measurements under different magnetic fields were performed in order to investigate the magnetoresistance properties. The obtained magnetocaloric properties show that 0.95La0.45Nd0.25Sr0.3MnO3/0.05CuO composite was an attractive candidate material for magnetic refrigeration application. About magnetoresistance properties the (% MR) is found to be 32.78% around 320 K under a magnetic field of 5 T.


Introduction
The magnetic refrigeration (MR) is a promising alternative approach for the cooling product, compared to conventional refrigeration thanks to its important advantages such as environmentally friendly and energyefficient technology [1][2][3]. This technology is based essentially on the magnetocaloric effect (MCE), which is an intrinsic property of the magnetic materials that are heating up when they are posed in an extern magnetic field and cooled when they are took-out [4,5]. Recently magnetocaloric effect has attracted interest in several domains of applications such as domestic refrigeration [6], industrial refrigeration markets [7], catalysis [8] and medicine [9]. The first application of magnetic refrigeration in low temperature was reported by (Giuaque 1927) using the gadolinium sulfate as magnetocaloric materials [10]. The discovery of giant magnetocaloric effect by pycharsky (1997) is the main scientific event that leads in researches to the application of magnetic refrigeration at room temperature [1]. The two major parameters for the application in magnetic refrigeration are the isothermal entropy change ΔS which is proportional to the cooling power of refrigerators, and the adiabatic temperature change ΔT ad that contributes largely to the enhancement of the efficiency of an active magnetic regenerator cycle [7]. Several important published works were contributed to fulfill and meet the increasing demands and needs by developing new magnetocaloric materials with a various category such as Gd, GdGeSi series [11][12][13][14], LaFeSi family [15][16][17], husler intermetallic [5,18,19], and the oxide ferrite and perovskite types [20][21][22][23][24][25]. The low cost and the good chemical stability represent the two essential reasons for our choice of the perovskite type materials with the general formula ABO 3 [26]. Depending on the type and the overcrowding of the A cations in the vacancy site: the system migrates from the cubic to orthorhombic or rhombohedra structure [27]. In addition to the magnetocaloric effect, perovskite materials are applied in several technological applications such as thermoelectric in the case of titanates perovskite [28]. Cerates and zirconates have high proton conductivity, which give them an important place in hydrogen fuel cell and sensors applications [29,30]. As for manganite perovskite, very good magnetic properties caused the reservation of this family of perovskite to magneto-resistance and magnetocaloric effect [31][32][33]. The perovskite type ABO 3 materials could be synthesized by all usual ceramic reaction methods like solid state reaction [34], solution technologies including sol-gel and co-precipitation and others [35], hydrothermal, hydrothermal-electrochemical by using an electric field, Microwave synthesis and Physical vapor deposition (PVD) methods-laser ablation or Molecular beam epitaxy (MBE) for thin films [36]. Recently, we have reported the synthesis and magnetocaloric properties of (La 0.45 Nd 0.25 )Sr 0.3 MnO 3 (LNSMO)-based composites [37,38]. We have shown that the 0.95LNSMO/0.05CuO samples with micro particles of copper oxide, for a magnetic field changing from 0 to 1.5 T, the corresponding isothermal entropy change was found to be 2.55 J kg −1 K −1 . In the course of our studies on magnetocaloric properties of LNSMO composites and following our objective for their enhancement, we focused our efforts on the synthesis of polycrystalline 0.95LNSMO/0.05CuO composites using nanoparticles of copper oxide instead of the micro particles. We have then study the effect of the size of the secondary phase (CuO) on the magnetic and magnetocaloric properties of the composite. In addition, we report the electrical properties of our composite such as resistivity and heat capacity. Our results show that the magnetic entropy change calculated with the isothermal data increases from 2.69 J/(Kg.K) for the powder to 4.12 J/(Kg.K) in the case of pellet.
The preparation process was been achieved by annealing the sample at 1473 K for 12 h under air. The polycrystalline composites LNSMO/0.05CuO were been obtained by mixing the copper oxide (CuO) with the calcined LNSMO in a 5% weight ratio. The resulting product has been subjected to a heat treatment at 1473 K for 2 h under air. In the case of pellet, the resulting product was thoroughly mixed with PEG in water for 2 h, then the powder was dried, pressed and calcined at 1473 K for 2 h. The structural characterization was made by x-ray powder diffraction using Model D8 Discover Bruker AXS) with Cu Kα radiation (λ Cu =1.5407 Å). Scanning Electron Microscope (FEI Quanta FEG 450) equipped with Energy Dispersive x-ray (EDX) detector. The magnetic and magnetocaloric properties were performed using Magnetic Properties Measurement System (MPMS-7XL), with a Quantum Design XL-SQUID magnetometer. The electrical properties and the specific heat properties were created using Quantum Design PPMS-9. Figure 1 shows the x-ray diffraction patterns of LNSMO/CuO powder and pellet. According to the XRD data, the co-existence of two different phases showed, which confirms the formation of the LNSMO/CuO composite materials. The rhombohedra phase of LNSMO perovskite with the space group R-3c (167) is detected which can be indexed according to PDF NO 01-082-1152. While two clear peaks at (2ϴ=35.43 and 38.47) are attributed to CuO phase. The low intensity of the CuO peak is due to the low content (5%) of the CuO in the composite materials. No other phase is detected implies that no chemical reaction between the perovskite and copper oxide. In order to study the microstructure and the morphology of our samples, the Scanning Electron Microscopy (SEM) has been used to observe the surface of the samples. Figure 2(a) shows the SEM images of the studied powders, we can see clearly that the particles have two different sizes. Also, we can separate two different chemical compositions; one corresponds to the particles of the perovskite material with the average size of 1 μm and the second with medium-size particles of 300 nm corresponding to the particles of copper oxide which are distributed on the surface of the grains and in the grains boundaries. Figure 2(b) presents the micrograph of the pellet. The chemical composition and EDS analyses indicate that the copper oxide occupies majorly the grains boundaries due to the pressure effect [38]. Figure 3(a) illustrates the magnetization as a function of the temperature from 200 K to 400 K of the powder and the pellet samples under a magnetic field of 500 Oe. Both materials exhibit a magnetic transition from ferromagnetic to paramagnetic order. The magnetic moment at a low temperature of the pellet is found to be 41.8 emu g −1 , which is the double of the obtained value for the powder perovskite (20.47 emu g −1 ). Figure 3(b) shows the temperature depending on the derivative of magnetization (dM/dT) in the temperature range of 150 K and 400 K. T C values determined from the flexion points in M(T) curves are about 336.16 and 321.01 K for powder and pellet respectively. The Tc of the pellet decreases with 15 K, which is in good agreement with the study of the effect of pressure on the Tc reported by Masahiro shikano [39], also the copper oxide, that occupies largely the grains boundaries in the pellet, is characterized by an antiferromagnetic order with Néel transition at  230 K [40]. The isothermal magnetization as a function of a magnetic field up to 5 tesla and at different temperatures from 200 K to 400 K for the powder and pellet are shown in figures 4(a) and (b), respectively. We can clearly distinct two regions: one of the ferromagnetic phase characterized by non-linear comportment, with a tendency of saturation at higher fields, this region is connected by the transition temperature with the second region, which exhibits a linear behavior when the temperature is above of Tc. This result confirms the paramagnetic phase in this region. The Banerjee criterion is frequently recommended to determine the nature of the magnetic transition. It is based on H/M depending on M 2 ; when the slope is negative the magnetic transition is of first order, on the other hand when the slope is positive the magnetic transition becomes of second order [41]. Figures 5(a) and (b) show the Arrott plots (M 2 as function H/M) for the LNSMO/CuO powder and pellet samples, respectively. From figure 5, the slope of the Arrott plots is positive which corresponds to the second order transition.

Magnetocaloric characterization
The magnetic entropy change ΔS M is the most recommended parameter to evaluate the magnetocaloric effect of magnetic materials. It can be calculated by integrating Maxwell's relation using the isothermal magnetization depending on the magnetic field as data [42]: 6(a) and (b) represent the magnetic entropy change as a function of temperature for the powder and the pellet under the different fields, respectively. From figure 6, the magnetic entropy has a maximum around  the transition temperature and decreases when moving away from the transition temperature to right and left. In the case of the pellet, the operating temperature decreases. The value of the maximum entropy change is found to be 2.69 J/(Kg.K). and 4.12 J/(Kg.K). For the powder and the pellet, this increase between the powder and the pellet may be due to the external pressure. These values are in the same range compared with other manganite or manganite composites [43][44][45][46][47], some intermetallic materials [48,49], and they are very large compared with other oxides [50,51].
The relative cooling power (RCP) is an important parameter to evaluate the efficiency of the magnetocaloric material for magnetic refrigeration. The RCP is giving by the following formula: Where ΔS max is the maximum magnetic entropy change and δT FWHM is the full width at half maximum of the magnetic entropy change. Figure 7(a) regroups the values of ΔS max under various magnetic field from 1 to 5 tesla for both powder and pellet samples, ΔS max increases linearly with increasing the magnetic field. We observed that the obtained values for pellet are superior compared to the powder sample. Figure 7(b) shows the relative cooling power RCP as a function of the magnetic field from 1 to 5 T. The obtained values show a linear dependence on the magnetic field with a noticeable decrease from 296.92 J Kg −1 for the powder to 208.44 J Kg −1 for the pellet under 5 tesla. That is due essentially to the operating temperature caused by the application of external pressure in the pellet. Figure 8 presents the electrical resistivity of the pellet as a function of the temperature from 10 to 350 K without and under different magnetic fields from 1 to 5 T. it is found that our pellet sample exhibits a metal-insulator transition in the range of 300 K and 315 K. Moreover, we observed that the resistivity is inversely proportional to  the applied magnetic field, which can be due to the influence of the magnetic field on the delocalization of the charge carriers, causing the increase of the electrical conductivity, which is related to the diminution of the electrical resistivity [52,53].

Electrical properties
From the electrical resistivity data, we calculated the magnetoresistance using the following formula: Where R 0 and R H represent the resistivity without and under the applied magnetic field respectively. The temperature dependence of the magnetoresistance (MR) is shown in figure 9(a). The peak of the MR was found up to 32.78% around 320 K for a magnetic field of 5 T. The observed value of the MR is in the same range of La 0.71 Sr 0.29 MnO 3 at low field (MR=30%) [54]. And higher than that calculated in other simple perovskite manganite reported in the literature such as Pr 0.67 Sr 0.33 MnO 3 (MR=30%) [55] La 0.7 Sr 0.2 Na 0.1 MnO 3 (MR=25%) [56], and lower than that of Nd0.6Sr0.4MnO3 and La 1-x Ca x MnO3 prepared by sol gel method (MR=99.84%) [45,57] and La 0.67−x RE x Ca 0.33 MnO 3 (RE=Nd, Sm, and Gd, x=0.0, 0.1) [58]. In addition, as it is shown in figure 9(b), the maximum of the MR value of LNSMO/CuO around the transition temperature increases from 9.82% for a magnetic field of 1 T to 32.77% for 5 T, which reveals the magnetoresistance sensitivity of LNSMO/CuO composites to the external applied magnetic field.   Figure 10 presents the heat capacity of (LNSMO/CuO) in the absence and under 1 tesla of the external magnetic field. It is found that the heat capacity Cp under and without magnetic field are superposed bellow and after magnetic transition. While, in the range of the transition temperature, we observed that the peak of the heat capacity in the absence of an external magnetic field, which is related to the magnetic transition shown previously in the magnetization measurement, is higher and narrow that when applied a magnetic field of 1 T. This comportment accords well with the typical characteristic of ferromagnetic materials as reported in the literature [59].

Conclusions
In summary, 0.95La 0.45 Nd 0.25 Sr 0.3 MnO 3 /0.05CuO composites were synthesized using solid-state reaction process. The existence of two phases (rhombohedra perovskite and monoclinic of copper oxide) are confirmed by x-ray diffraction. Copper oxide occupied the grains boundaries for the powder and pellet samples. The magnetic entropy change calculated with the isothermal data increases from 2.69 J/(Kg.K) for the powder to 4.12 J/(Kg.K) in the case of the pellet. This result indicates that the perovskite/CuO composites with a pellet design are more favorably than the powder magnetic refrigeration applications.