Study of Magnetocaloric Effect and Magnetic Phase Transition of Electron-Doped La_0.9Sb_0.1MnO₃ Manganite

Abstract

An electron-doped La_0.9Sb_0.1MnO₃ manganite was prepared by a sol-gel method, and its magnetocaloric effect and magnetic phase transition (MPT) have been systematically studied. The X-ray diffraction pattern is consistent with the rhombohedral lattice structure ($R\overline 3 C$ space group). This sample shows a ferromagnetic order at T _C = 248 K. A maximum magnetic entropy change (−ΔS _M ^max) of 4.79 J kg⁻¹ K⁻¹ and a relative cooling power (RCP) of 258.15 J kg⁻¹ are observed at a magnetic field change of 5 T. It is found that the values of −ΔS _M ^max are comparable to some hole-doped manganese compounds. Nevertheless, RCP values of La_0.9Sb_0.1MnO₃ are higher than those of some hole-doped manganites under the same field change, which is very beneficial for designing the magnetic refrigeration materials in engineering. Furthermore, Arrott plots reveal a second-order MPT for La_0.9Sb_0.1MnO₃, which is also confirmed by Franco’s universal curves. In addition, the experimental observed −ΔS _M follows Lauder’s mean-field theory very well, which demonstrates that the magneto-elastic and magneto-electronic couplings have a crucial importance on the magnetic entropy change of La_0.9Sb_0.1MnO₃ manganite.

1 Introduction

Mixed-valence manganites of the type La_1−x AE _x MnO₃ (AE = Li, Na, K or Sr, Ca, Ba, etc.) with the perovskite structure have been intensively investigated for more than 60 years because of their noticeable physical properties [1–3] (colossal magnetoresistance (CMR) effect, ferromagnetic-paramagnetic transition, metal-insulator transition, magnetocaloric effect, etc.) and potential technological applications in spintronic devices [4–6]. In the recent past, many studies on the substitution of tetravalent ions (Te ⁴⁺, Zr ⁴⁺, Sn ⁴⁺, and Hf ⁴⁺) [7–10] or pentavalent ions (Sb ⁵⁺) [11] for trivalent La ³⁺ in parent compound LaMnO₃ reveal that magnetic, transport, and magnetoresistive properties similar to those of the monovalent and/or divalent ions doped (also named as hole-doped) manganese oxides are also observed in the case of these tetravalent and/or pentavalent ions substituted (named electron-doped) manganites. In order to explain the same physical properties in electron-doped manganese compounds, many researchers [7–10] have believed that the CMR effect and FM-PM transition can be explained via the double-exchange interaction between Mn ²⁺ and Mn ³⁺, instead of double-exchange interaction between Mn ³⁺ and Mn ⁴⁺ ions as in hole-doped manganites [12].

Aside from the CMR effect, more recently, a large magnetocaloric effect (MCE) was also observed in electron-doped manganites and captured tremendous interest. Yang et al. [13] reported that the magnetic entropy change (MEC) of La_0.9Te_0.1MnO₃ was 1.85 J kg⁻¹ K⁻¹ for a magnetic field change of 1.5 T at 252 K, and the corresponding relative cooling power (RCP) of this sample was 541 mJ cm ⁻³. Mahato et al. [14] studied MCE properties of nanocrystalline La_0.7Te_0.3MnO₃ and found a larger change in magnetic entropy value, namely, 12.5 J kg⁻¹ K⁻¹ under μ ₀ΔH = 5 T. Shelke et al. [15] studied the MCE of electron-doped manganites La_1−x Zr _x MnO₃(x = 0.05, 0.10, and 0.15) and found that the MEC and RCP of these compounds were higher than those of some hole-doped manganites.

Up to now, most of the pervious works have been focused on the magnetic and transport properties of electron-doped La_0.9Sb_0.1MnO₃ [16–18]. But the research on the MEC of La_0.9Sb_0.1MnO₃ compound is still lacking. The study of La_0.9Sb_0.1MnO₃ will offer a complementary understanding of the MCE in the ABO₃-type perovskite manganites and may provide a new opportunity for developing highly efficient and environmentally friendly cooling devices. In this paper, we extend our investigation to the system of La_0.9Sb_0.1MnO₃ for studying its MCE and magnetic phase transition (MPT).

2 Experimental

Polycrystalline La_0.9Sb_0.1MnO₃ was synthesized by sol-gel route with stoichiometric compositions of highly pure La₂ O ₃ (99.95 %) powders, Sb₂ O ₅ (99.9 %) powders and Mn(NO₃)₂ (50 %) solution as starting materials. The precursors were dissolved in diluted nitric acid in which an excess of citric acid was added, followed with continuous stirring for about 10 h. After all reactants were completely dissolved, the solution was heated on a water bath at 353 K leading to the formation of a brown gel. The gel was dried at 373 K in an oven and then preheated at 873 K to remove the remaining organic and decompose the nitrates of the gel. The obtained black powders were mixed, ground, and sintered at 1473 K for 12 h. This step mentioned above was repeated three times for a better chemical homogeneity. Finally, the furnace was slowly cooled down to room temperature.

The phase structure of La_0.9Sb_0.1MnO₃ was performed by x-ray diffraction (XRD) using 𝜃–2 𝜃 scans with Cu K α radiation. Temperature and magnetic field-dependent magnetization measurements were carried out by means of a quantum design superconducting quantum interference device.

3 Results and Discussion

3.1 Structure

The phase purity and homogeneity of the polycrystalline La_0.9Sb_0.1MnO₃ sample was determined with a powder X-ray diffractometer. The XRD pattern (Fig. 1) of this sample confirms its single-phase nature and rhombohedral structure with the space group of $R\overline 3 C$ (Z = 6).

Fig. 1

XRD pattern of La_0.9Sb_0.1MnO₃ manganite

3.2 Magnetic Properties

Figure 2 displays the temperature dependence of the thermal magnetization (M- T) of the La_0.9Sb_0.1MnO₃ sample in the zero-field-cooled (ZFC) mode under an external field of 0.1 T, which shows a sharp paramagnetic-ferromagnetic (PM-FM) transition at Curie temperature (T _C). Usually, T _C can be estimated from the dM/dT versus T plot and corresponds to the minimum value [19], as shown on the right hand of Fig. 2. The sample exhibits a T _Cvalue of about 248 K, in good agreement with the literature [16].

Fig. 2

Left side M(T) curve in a magnetic field of 0.1 T for La_0.9Sb_0.1MnO₃; right side plot of dM/dT versus T

3.3 Magnetocaloric Effect

In order to calculate the MCE of the La_0.9Sb_0.1MnO₃ compound, a set of isothermal magnetization curves were recorded in the temperature range of 192–288 K with a step of 3 k up to a maximum applied field of 5 T and are displayed in Fig. 3. M-H curves of the compound exhibit a rapid increase at lower fields and get saturated at higher fields for all the temperatures measured, which indicate a typical FM behavior below T _C [20]; the M−μ ₀ H data show a linear behavior as typical in the PM state at temperatures above T _C [21].

Fig. 3

Isothermal M−μ ₀ H curves for La_0.9Sb_0.1MnO₃ between 194 and 293 K in the ΔT of 3 K

Magnetic refrigeration (MR) operates on the basis of MCE and is a desirable and green cooling technology owing to the fact that it has higher energy efficiency and less impact on the environment than conventional technology based on the gas compression-expansion cycle [22]. The MCE can be characterized by an isothermal MEC or an adiabatic temperature change of a material upon the application of a magnetic field. MEC can be calculated as [23]

$$\begin{array}{@{}rcl@{}} \left| {\Delta S_{M} (T,H)} \right|&=&\left| {S_{M} (T,H)-S_{M} (T,0)} \right|\\ &=&{\int}_{0}^{H_{\max} } {(\frac{\partial S}{\partial H})}_{T} \mu_{0} dH \end{array} $$

(1)

Using Maxwell’s thermodynamic relation

$$ \left( {\frac{\partial M}{\partial T}} \right)_{H} =\left( {\frac{\partial S}{\partial H}} \right)_{T} $$

(2)

one can deduce the following expression [23]:

$$ \left| {\Delta S_{M} (T,H)} \right|={\int}_{0}^{H_{\max} } {(\frac{\partial M}{\partial T})}_{H} \mu_{0} dH $$

(3)

In practice, the MEC (−ΔS _M) can be estimated from the isothermal magnetization measured with a small interval, where MEC can be approximated as [23]

$$ {\Delta} S_{M} =\mu_{0} \sum\limits_{i} {\frac{M_{i} -M_{i+1}} {T_{i} -T_{i+1}} } {\Delta} H_{i} $$

(4)

where M _i and M _i+1 are magnetizations at T _i and T _i+1, respectively, under the external magnetic field μ ₀ΔH _i .

Figure 4 shows the temperature dependencies of the MEC for the La_0.9Sb_0.1MnO₃ compound in magnetic field intervals ranging from 1 to 5 T. The values for −ΔS _M are positive in the whole temperature range. For each magnetic field, the −ΔS _M curve grows up a maximum value (−ΔS _M ^max) close to the T _C , where the change of magnetization with temperature is strong. With μ ₀ΔH = 1, 2, 3, 4, and 5 T, the −ΔS _M ^max is about 1.65, 2.66, 3.50, 4.18, and 4.79 J kg⁻¹ K⁻¹, respectively.

Fig. 4

Temperature dependence of −ΔS _M at different applied fields for La_0.9Sb_0.1MnO₃

For the application of magnetic refrigeration, the relative cooling power (RCP) is another meaningful parameter in selecting a proper potential for magnetic refrigerant, which can be represented by the amount of heat transferred between the hot and cold sinks in the ideal refrigeration cycle. According to the definition of RCP suggested by Gschneidner et al. [24]:

$$ RCP=-{\Delta} S_{M}^{\max} (T,H)\times \delta T_{FWHM} $$

(5)

where δ T _{F W H M} is the full-width temperature span of the −ΔS _M vs. T curve at its half maximum. The RCP values of La_0.9Sb_0.1MnO₃ under magnetic fields of μ ₀ΔH = 1, 2, 3, 4, and 5 T are found to be 43.37, 84.32, 139.21, 191.67, and 258.15 J kg⁻¹, respectively. The RCP parameter is considered to be a suitable factor for comparing different MR materials. To assess the applicability of electron-doped La_0.9Sb_0.1MnO₃ manganite as a magnetic refrigerant, −ΔS _M ^max, RCP values, and other magnetic parameters for La_0.9Sb_0.1MnO₃ as well as other samples are collected in Table 1. Obviously, La_0.9Sb_0.1MnO₃ presents moderate values of −ΔS _M ^max among the listed hole- and electron-doped manganites. However, the RCP value of La_0.9Sb_0.1MnO₃ is higher than that of some hole-doped ABO₃-type manganese compounds under the same field change, which is beneficial to magnetic cooling. Though the values of −ΔS _M ^max and RCP for the present sample are smaller than those of some typical magnetic refrigerant materials such as pure Gd and Gd₅(Si_1−x Ge _x )₄, La_0.9Sb_0.1MnO₃ polycrystalline has many advantages including low production cost, negligible magnetic hysteresis, and high resistivity. Therefore, La_0.9Sb_0.1MnO₃ polycrystalline could be a promising magnetocaloric material.

Table 1 Summary of magnetocaloric properties for La_0.9Sb_0.1MnO₃ manganites. The present results of La_0.9Sb_0.1MnO₃ are compared with some of ABO3-type manganites and pure Gd

3.4 Magnetic Phase Transition

It is generally accepted that the order of MPT can be judged with Banerjee’s criterion [31]. According to this criterion, the negative slopes or inflection points in the Arrott plots (μ ₀ H/M versus M ² curves) are related to the first-order MPT, while the positive slopes and linear behavior near T _C often mean that the MPT belongs to the second order. In order to identify the nature of the MPT in La_0.9Sb_0.1MnO_3, the M- μ ₀ H curves were converted into Arrott plots at some temperatures, as shown in Fig. 5. Neither a negative slope nor inflections over the whole field intervals is found near T _C in the Arrott plots, indicating the second-order feature of the MPT.

Fig. 5

A set of typical μ ₀ H/M versus M ² curves for La_0.9Sb_0.1MnO₃ at different temperatures close to T _C

Franco and co-workers have introduced a new method to discriminate the order of MPT [32, 33]. They suggested that the order of FM-PM transition can be conveniently distinguished with the re-scaling of MEC curves, that is to say, for a second-order MPT, the rescaled ${\Delta } S^{\prime }_{M}$ curves should collapse into a single curve independent of the external magnetic field and temperature; however, for a first-order one, the rescaled ${\Delta } S^{\prime }_{M}$ curves should display a dispersive behavior. Franco’s universal curves can be constructed by (1) normalizing each ΔS _M (T) curve with its respective maximum value ΔS _M ^max (i.e., ΔS ^′ = ΔS _M (T)/ΔS _M ^max) and (2) the temperature axis is rescaled by a new variable, 𝜃, defined by the following expression [34]:

$$ \theta =\left\{\begin{array}{c} -(T-T_{C} )/(T_{1} -T_{C} ){\kern 1pt}\quad T\le T_{C} {\kern 1pt}\\ (T-T_{C} )/(T_{2} -T_{C} )\quad T>T_{C} \end{array} \right. $$

(6)

where T _r1 (below T _C ) and T _r2 (above T _C ) are two reference temperatures, which can be selected for each curve from temperature corresponding to ΔS _M (T _r1)/ΔS _M ^max = ΔS _M (T _r2)/ΔS _M ^max = 0.5.

Figure 6 shows the universal curves by plotting ΔS ^′ against 𝜃 for the La_0.9Sb_0.1MnO₃ compound. One can clearly find out from this figure that all the rescaled −ΔS _M curves under different magnetic field changes collapse onto a single universal curve, which confirms that the FM-PM transition of La_0.9Sb_0.1MnO₃ is second order.

Fig. 6

Normalized MEC (ΔS _M /${\Delta } {S_{M}}^{\max })$ as a function of the rescaled temperature 𝜃 for different applied fields for La_0.9Sb_0.1MnO₃

3.5 The Temperature Dependence of MEC

In this section, we will model the experimental values of −ΔS _M using Lauder’s theory to study the contributions in the MCE of the La_0.9Sb_0.1MnO₃ compound. Based on this theory, the magnetic energy can be included in expression of Gibb’s free energy [35]. Neglecting the higher-order parts, Gibb’s free energy can be written as in Eq. 7,

$$ G(T,M)=G_{0} +\frac{1}{2}A(T)M^{2}+\frac{1}{4}B(T)M^{4}-MH $$

(7)

where A(T) and B(T) are well known as Lauder coefficients, which are temperature-dependent parameters standing for the magneto-elastic coupling and electron condensation energy [35], and can be obtained at the equilibrium state of ∂ G/∂ M = 0

$$ \frac{H}{M}=A(T)+B(T)M^{2} $$

(8)

The values of A(T) and B(T) can be obtained from the linear region of the Arrott plots by a polynomial fitting. The values of Landau coefficients A(T) and B(T) as functions of the temperature are plotted in Fig. 7. Parameter A(T) is observed to be almost linear for La_0.9Sb_0.1MnO₃, whereas the temperature dependence of B(T) is highly nonlinear. In principle, parameter B(T) plays a key role in determining the order of MPT. Amaral et al. [35] have found that B(T _C) can be positive, zero, or negative. If B(T _C) >0, the MPT is of the second order; otherwise, it is of the first order. It is clear from this figure that B(T _C) is positive, implying that the MPT of La_0.9Sb_0.1MnO₃ belongs to the second order.

Fig. 7

The temperature dependence of Lauder’s coefficients A(T) and B(T)

The temperature dependence of MEC is theoretically determined from differentiation of the magnetic part of the free energy and can be written as follows [36]:

$$ S_{M} (T,H)= -\frac{1}{2}\frac{\partial A(T)}{\partial T}M^{2}-\frac{1}{4}\frac{\partial B(T)}{\partial T}M^{4} $$

(9)

With the values of A(T) and B(T) extracted from the data, the variation of the experimental and estimated MEC versus temperature at some fields (1–5 T in this work) for La_0.9Sb_0.1MnO₃ are shown in Fig. 8. An excellent agreement is observed between the experimental data and the one calculated by Lauder’s theory. The present analysis further proves the important role of both magneto-elastic coupling and the electron interaction in the study of the MCE of electron-doped La_0.9Sb_0.1MnO₃ manganite.

Fig. 8

Experimental and theoretical MEC for La_0.9Sb_0.1MnO₃ for μ ₀ H= 1–5 T

4 Conclusions

We prepared an electron-doped La_0.9Sb_0.1MnO₃ polycrystalline compound using the sol-gel method and then investigated its MCE and MPT. XRD pattern shows that the material crystallized in a rhombohedral structure with the $R\overline 3 C$ space group. This sample shows a FM-PM transition at T _C = 248 K. A maximum MEC of 4.79 J kg⁻¹ K⁻¹ and a RCP of 258.15 J kg⁻¹ are observed at μ ₀ H = 5 T. The values of −ΔS _M ^max are comparable to some hole-doped manganese compounds. However, the RCP value of La_0.9Sb_0.1MnO₃ is much higher than that of some hole-doped manganites under the same field change. These results suggest that the La_0.9Sb_0.1MnO₃ compound is a proper candidate material for magnetic refrigerators near sub-room temperature. Both Arrott plots and Franco’s universal curves indicate that the MPT between FM and PM states is of the second order. Moreover, the temperature dependence of the MEC can be well understood within the framework of Lauder’s theory.