Large Magnetocaloric Effect in Cu-Doped La_0.7Ca_0.3MnO₃ Compounds

Abstract

The structural characterization, and the electronic, magnetic and magnetocaloric properties of polycrystalline samples of La_0.7Ca_0.3Mn_1−xCu_xO₃ (x = 0, 0.04, 0.06, 0.08) have been investigated. X-ray powder diffraction analysis indicates all samples having an orthorhombic structure, belonging to the Pbnm space group. X-ray absorption fine structure spectra reveal that Mn is in the mixed state of Mn³⁺ and Mn⁴⁺ while Cu has divalent state (Cu²⁺). With the substitution of Cu²⁺ for Mn, the Curie temperature, T_C, decreases monotonically from 248 K for x = 0 to 156 K for x = 0.08, which is due to weakened exchange interactions. The downturn in the temperature dependencies of the inverse magnetic susceptibility, χ⁻¹(T), curves observed above T_C for x = 0 and 0.08 is characteristic of the Griffiths-like phase. The analysis of isothermal magnetization data M(T, H) based on the Banerjee’s criteria has indicated x = 0, 0.04, and 0.06 samples undergoing a first-order magnetic phase transition. However, the x = 0.08 sample, the coexistences of second-order magnetic phase transition at low magnetic fields below 8 kOe and first-order magnetic phase transition at high magnetic fields were observed. The maximum magnetic entropy change measured at a magnetic field span of 50 kOe occurring near the T_C decreases from 10.3 to 4.8 J/kg.K with increasing x from 0 to 0.08. However, the relative cooling power (RCP) tends to increase, in which a maximum RCP of 360 J/kg for x = 0.08 that is about 1.3 times greater than that observed for the parent sample (x = 0).

1. Introduction

Perovskite-type manganites with the formula R_1−xA_xMnO₃ (R = rare-earth element, such as La, Pr, Nd, …; and A = alkaline-earth element, such as Ca, Ba, Sr, …) owing unique magnetic and electrical properties are promising candidates for technology applications of magnetic refrigerators (MRs), and magnetic switches.¹⁾ MRs are based on the principle of the magnetocaloric (MC) effect; an effect which refers to the intrinsic property of all magnetic materials. MRs has been the subject of intense research due to its efficient cooling performance than a conventional vapor compression-based refrigerating system in addition to its environment friendly aspect.²⁾

Concerning La_1−xA_xMnO₃, the parent compound of LaMnO₃ is an anti-ferromagnetic insulator, in which the valence state of Mn ions is 3+ (Mn³⁺).³⁾ The La-site substitution with a divalent alkaline-earth ion (A, such as Ca²⁺, Ba²⁺ or Sr²⁺) would lead to the mixed valence of Mn³⁺/Mn⁴⁺ occupying the Mn site. The double exchange (DE) interaction is associated with the ferromagnetic coupling of Mn³⁺ and Mn⁴⁺, and other attributes (such as Mn³⁺/Mn⁴⁺ ratio, mismatch of the ionic size, tolerance factor t, Mn–O bond length, and doping content). These components are usually used to explain the colossal MC and magnetoresistance effects of doped manganites La_1−xA_xMnO₃. The tolerance factor t is specified as $t = (\langle R_{A}\rangle + R_{O})/\sqrt{2} (\langle R_{B}\rangle + R_{O})$, with ⟨R_A⟩ and ⟨R_B⟩ as the cations’ average radii situated at A site and B site, respectively, in ABO₃ – type perovskite. A rise in t extends the lengths of Mn^3+,4+–O²⁻ resulting in the T_C increase. Numerous researchers have based on these features to tune the magnetic properties, the magnetic entropy change (ΔS_m) - an important parameter characteristic of the MC effect, the insulator-metal transition temperature, and the T_C by managing the relative ratio of Mn³⁺/Mn⁴⁺, and doping concentration with the substitution in La site and/or Mn site.^3–6) For La_1−xCa_xMnO₃, Ramirez et al.³⁾ mentioned that the FM insulator ground state is at 0 < x < 0.2, while the FM conducting ground state exist only for 0.2 < x < 0.5, and the ground state is anti-ferromagnetic and charge ordered for x > 0.5. Additionally, a large change in resistivity only occurred near x ≈ 0.3. Bally et al.⁷⁾ have turned T_C toward the room temperature and have changed the maximum ΔS_m from 1.66 to 1.25 J/kg.K by adjusting the Ca content in La_0.55Ca_xSr_0.45−xMnO₃ (x = 0–0.25).

A direct substitution of Mn sites will modify not only the crucial Mn³⁺–O²⁻–Mn⁴⁺ network, but also will lead to more intricate interactions within Mn ions and dopants. The effect of doping and underlying mechanisms are different from type and concentration of dopants at the Mn sites. Up to now, there are serval reports on Cu-doped manganites. Wang et al.⁸⁾ have studied the structural, magnetic and transport interactions in La_0.7Ca_0.3Mn_1−xCu_xO₃ (0 < x < 0.2) and found that for x = 0.15, the maximum magnetoresistance, MR, is about 10⁴ times maximum MR for sample x < 0.1. In addition, for x < 0.05, the Cu remains in the Cu³⁺ ionic form, meanwhile Cu²⁺ ions begin to recur and then escalate with further increasing of the Cu content. Bouzaiene et al.⁹⁾ reported the rhombohedral structure for Bi and Ca doped La_0.7Sr_0.3Mn_0.9Cu_0.1O₃ and ΔS_m values under an applied field of 50 kOe increases when Bi and Ca doping were added into the compound. They are 3.06 J/kg.K (without doping), 3.39 J/kg.K (Bi of x = 0.1), and 3.65 J/kg.K (Bi of x = 0.1 and Ca of x = 0.05), respectively. Kim et al.¹⁰⁾ reported the effect of La_0.7Sr_0.3Mn_1−xCu_xO₃ (0 < x < 0.2) on its structural, magnetic and transport behaviors. They observed that all samples show the rhombohedral structure and the Cu³⁺ state presents in a few of the Cu ions. The resistivity measurement shows that for x ≥ 0.15, metal is transitioning to insulator state. Moreover, for sample x = 0.15, the MR value is at its highest due to the Cu³⁺–Cu²⁺ co-existence and the Cu-doping dilution effect on the DE interaction. The earlier reports are believed to be rather dispersed, with less clarity on the effects of Cu doping. For more insightful details on the effects of Cu dopant on the structural, local atomic, magnetic and magnetocaloric traits of La_0.7Ca_0.3Mn_1−xCu_xO₃, we present here a systematic study of the mentioned properties for La_0.7Ca_0.3Mn_1−xCu_xO₃ (x = 0–0.08). Arrott plots were applied to analyze the sample’s magnetic order. The valence state of Cu and Mn ions and the influence of Cu-doping on the MC properties were also investigated.

2. Experimental Details

Polycrystalline compounds of La_0.7Ca_0.3Mn_1−xCu_xO₃ (x = 0, 0.04, 0.06, and 0.08) were prepared using a conventional solid-state reaction method by mixing stoichiometric quantities of high-purity (99.9%) powders of La₂O₃, CaCO₃, CuO, and MnO in the ambient atmosphere. The powders were then combined, carefully ground, and heated at 900°C for 24 h in air. After a few intermediate grindings were added and continued with heating at 1000°C for 24 h, the powders were shaped into pellets and finally sintered at 1200°C for 12 h in air. Following the annealing and room temperature cooling stage, the crystalline structure and purity phase of the obtained samples were analyzed using X-ray powder diffraction tooling with a Cu-K_α radiation source (λ = 1.5406 Å) at room temperature. In order to reduce errors generated during the calibration process of position of the X-ray incident beam, a 5 mass% of standard Si powder was included into the powder samples. The oxidation number of Mn and Cu was determined using an X-ray absorption fine structure (XAFS) spectrometer at room temperature. The XAFS spectra were measured for the Mn, Cu at K-edge of the samples with MnO, MnO₂, Mn₂O₃, Cu₂O, and CuO as reference standards. A superconducting quantum interference device (SQUID) was utilized to study the magnetic and MC properties at temperatures and magnetic fields of T = 5–300 K and H = 0–50 kOe, respectively. The dimension of the samples is 2.5 mm × 2.5 mm × 2 mm with tetragonal shape. The sensitivity of the magnetization measurements is of the order of 1 × 10⁻⁶ emu.

3. Results and Discussion

Figure 1 shows the XRD spectra at room temperature of all samples. The XRD data were recorded with 2theta angle from 15° to 70° with a speed of 3° min⁻¹ and a step size of 0.02°. These results indicated that the patterns of Cu-doped samples have a similarity with that of the La_0.7Ca_0.3MnO₃ parent compound. The diffraction peaks reflect the formation of orthorhombic structure with the space group of Pbnm for all Cu concentrations. No secondary or impurity phases were observed within the experimental limits of the XRD equipment. However, as the Cu content (x) increases, the diffraction peak shifts slightly to a higher angle as compared to that of the parent compound, not shown here. Table 1 presents the lattice parameters increase with the increase in x, for all samples. This result was attributed to the larger ionic radius owned by Cu²⁺ (0.73 Å), as compared to those of smaller radii of Mn³⁺(0.645 Å) and Mn⁴⁺(0.53 Å). Ionic radii are taken from the Shannon’s work with six-coordinate radius.¹¹⁾ A similar result was found in our previous work¹²⁾ for La_0.7Ca_0.3Mn_1−xZn_xO₃. The doping of Zinc leads to an increment of the lattice constants and the volume of the unit cell because of the larger Zn²⁺ ionic radius (0.74 Å), when compared to that of Mn³⁺(0.645 Å). However, our findings in this work are different from Zhang’s report for the same compounds.¹³⁾ They concluded that the contracted lattice triggered by the Cu ions substitution is due to the presence of trivalent Cu³⁺ ions (0.54 Å) which was confirmed by the samples’ X-ray absorption spectra of Cu 2p. Furthermore, Table 1 shows that the Cu²⁺ substitution for Mn³⁺ subtly lessens the t value from 0.916(2) for x = 0 to 0.912(8) for x = 0.08. The decrease in t backwards lower values (corresponding to t → 1) denotes a depletion of the bond-angle ⟨Mn–O–Mn⟩. These findings are on par with those of the Zn doped La_0.7Ca_0.3Mn_1−xZn_xO₃,¹²⁾ and the Sc doped La_0.7Ca_0.3Mn_1−xSc_xO₃.¹⁴⁾

Fig. 1

Miller-indexed XRD spectra of La_0.7Ca_0.3Mn_1−xCu_xO₃ measured at room temperature.

Table 1 Lattice parameters (a)–(c), unit cell volume (V), tolerance factor (t), Curie temperature (T_C), Curie-Weiss temperature (θ), experimental effective PM moment ($\mu_{\textit{eff}}^{\textit{exp}}$) and calculated effective PM moment ($\mu_{\textit{eff}}^{\textit{cal}}$) of the samples. The number in the parentheses denotes the error bars of the associated parameters.

To further inspect the valence state of Mn and Cu in all samples, the Mn K-edge and Cu K-edge XAFS spectra were recorded and presented in Fig. 2. The spectra of Mn³⁺, Mn⁴⁺, Cu¹⁺, and Cu²⁺ are referred to as Mn₂O₃, MnO₂, Cu₂O, and CuO, respectively; all of which are also included in Fig. 2 for comparison. In Fig. 2(a), the absorption–edge of all studied samples is near to the edge of Mn₂O₃, MnO₂, indicating that both Mn³⁺ and Mn⁴⁺ states coexist in these samples. Additionally, Fig. 2(a) shows a little change in shape and position of Mn K-edge with increasing x. Particularly, the threshold energy was established as the first inflection point (maximum of the first derivative) of the absorption spectrum, equals to 6553.5 eV for x = 0 and 6554.4 for x = 0.04, and 0.06, and 6554.8 eV for x = 0.08. This signifies that the slight alteration in the average manganese valence and Mn⁴⁺ ions were initiated at greater x values. The results are consistent with another work on La_0.7Ca_0.3Mn_1−xZn_xO₃¹²⁾ where the absorption-edge is shifted upon higher energies of Mn⁴⁺ with increasing x. Maurin et al.¹⁵⁾ reported that in the La_1−xCa_xMnO₃ series and self-doped LaMnO_3+δ system, the absorption edge is shifted to high energies when Ca²⁺ concentration increases, and the Mn oxidation state increases from Mn³⁺ to Mn⁴⁺, respectively. In contrast, Ulyanov et al.¹⁴⁾ observed no significant change in the shape and the position of Mn K-edge with increasing Sc-doping content in La_0.7Ca_0.3Mn_1−xSc_xO₃ indicating no modification in the average manganese valence. Moreover, Zhang et al.¹³⁾ concluded that the normalized XAFS spectra at Mn K-edge of La_0.7Ca_0.3Mn_1−xCu_xO₃ (x = 0–0.1) are alike to one another, suggesting the identical-like local structure of the samples, even with increase in the Cu ions doping concentration.

Fig. 2

(a) Mn K-edge XAFS spectra and (b) Cu K-edge XAFS spectra for La_0.7Ca_0.3Mn_1−xCu_xO₃. Inset of (a) shows FT magnitude of Mn K-edge for all studied samples.

The inset of Fig. 2(a) displays the Fourier transformed (FT) results into radial coordinates (R-space). These FT data are all weighted by k³. As we know, the strongest peak (first peak) corresponds to the Mn–O bond (the first coordination) in which the Mn atom is surrounded by six oxygen atoms. Clearly, it is shown that for the first peak (near 1.4 Å), its intensity along with its form shows minimal changes with the doping of Cu ions. The Mn–O bonds distance R-value narrowly enhances from 1.41 Å at x = 0 to 1.43 Å at x = 0.08. This implies that the Cu ion substitution at the Mn site has a minor effect on the Mn–O bonds distribution.

In Fig. 2(b), the spectrum of Cu₂O is different from those of all studied sample while the absorption edge of CuO is close to the edge of all studied samples. This result demonstrates that the absence of the Cu¹⁺ state and the existence of Cu²⁺ state in these samples. This result is different from report of Zhang et al.¹³⁾ in La_0.7Ca_0.3Mn_1−xCu_xO₃ system. They observed that the Cu valence state exemplifies trivalent state for x ≤ 0.06 and the divalent Cu²⁺ ions start to appear with further doping. Kim et al.¹⁶⁾ on La_0.7Sr_0.3Mn_1−xCu_xO₃ has also demonstrated the coexistence of Cu²⁺ and Cu³⁺ with Cu²⁺ as the dominant state.

Following the structure, and electronic analysis, we investigated the significance of Cu doping on the La_0.7Ca_0.3Mn_1−xCu_xO₃ magnetic properties. Figure 3 shows the temperature dependences of the zero-field-cooled (ZFC) and field-cooled (FC) magnetization, M_ZFC/FC(T), which was measured at magnetic field, H, of 100 Oe for all studied samples. All the measurements displayed are warming curves. For x = 0, x = 0.04, and x = 0.06 samples, in M_ZFC(T) curves, as shown in Fig. 3(a)–(c), magnetization M escalate steadily with increasing temperature T starting from 5 K. The maximum is reached at a specific temperature near the ferromagnetic (FM) - paramagnetic (PM) phase transition. With further increasing T, the value of M is quickly reduced. This is due to the FM-PM phase transition, in which the thermal activation energy causing the weakening of Mn³⁺–Mn⁴⁺ FM coupling. This FM–PM transition is in a tight temperature range for these samples. Contrastingly, for the x = 0.08 sample, the broadening in the PM-FM transition was exposed. Moreover, in M_ZFC(T) curve of this sample, there was the drop of the M_ZFC below 40 K. It is well known that for the system that shows a valid long-range FM order, M_ZFC for T < T_C is nearly independent of T.¹⁷⁾ In opposite, M_ZFC of x = 0.08 sample decreases on cooling below T_C signifying the absence of a proper long-range FM order in this sample. This behavior could be due to the existences of a cluster spin glass state and/or FM cluster at temperature below 40 K. On a similar note, this feature of the M_ZFC is also discovered in La_0.7Ca_0.3Mn_1−xZn_xO₃,¹²⁾ La_0.7Ca_0.3Mn_1−xSc_xO₃,¹⁸⁾ La_0.67Ca_0.13Ba_0.2Mn_1−xCo_xO₃,¹⁹⁾ La_0.7Ca_0.3Mn_1−xCo_xO₃.¹⁷⁾

Fig. 3

M_ZFC(T) and M_FC(T) curves for La_0.7Ca_0.3Mn_1−xCu_xO₃ measured at H = 100 Oe. The insets of the figures show the first derivative of M_ZFC(T)/dT.

Specifically, the M_ZFC and M_FC branches bifurcated at T = 240, 228, 200, and 150 K for x = 0, 0.04, 0.06, and 0.08, respectively. The splitting is explained by the irreversible magnetization contribution, which can be seen by calculating the difference, ΔM = M_FC − M_ZFC.²⁰⁾ The irreversible magnetization behavior is regarded as the result of the intrinsic inhomogeneity.²¹⁾ Figure 4(a) shows the ΔM versus temperature T for all samples at H = 100 Oe. It is shown that the ΔM increases as T decreases. This behavior was due to the strong magnetic anisotropy at low T values. Ferromagnets are considered good when the ΔM is small and temperature independent. The value of ΔM is large when the magnetic anisotropy is greater than the applied field H, meaning the magnetic moment of the Mn ions is confined in random directions. Therefore, ΔM starts to increase rapidly at the temperature where the magnetic anisotropy is far higher than the H over cooling. When the temperature is adequately high, the system can be unblocked, hence the decreasing ΔM. Finally, at a sufficiently high temperature (close to T_C), ZFC path merges with FC one leading to ΔM was zero. Additionally, the previous report stated that the greater the magnetic field is, the less ΔM becomes.²¹⁾ The results in Fig. 4(a) suggest that the sample with x = 0 is eminently anisotropic and that the magnetic moments of Mn ions cannot be aligned because of the weak 100 Oe of H, leading to a larger ΔM at a low T. For Cu-doped samples, ΔM decreases as x increases inferring that the system becomes less anisotropic with increasing doping content.

Fig. 4

(a) M_FC − M_ZFC with respect to T, (b) χ⁻¹(T) plots (open symbol) in the PM probed at H = 100 Oe. The CW law (solid lines) is used to fit the plots for La_0.7Ca_0.3Mn_1−xCu_xO₃. The inset of the figure (b) shows first derivative of χ⁻¹(T)/dT for La_0.7Ca_0.3Mn_1−xCu_xO₃ with x = 0 and 0.08.

The T_C values, described as the minima of dM_FC/dT vs. T curves, which inset in each frame of Fig. 4 and are listed in Table 1, are determined to be 248, 230, 204, 156 for x = 0, 0.04, 0.06, and 0.08, respectively. This result shows a monotonic decrease in T_C as x increases and the observed T_C decrease can be explained with two reasons, namely the dissimilar Mn³⁺ and Cu²⁺ electron configurations and the disparity of Mn³⁺ (0.645 Å) and Cu²⁺ (0.73 Å) ions radii.¹⁴⁾ The substitution of Cu²⁺ at the Mn site reduces the amount of Mn³⁺ and converts Mn³⁺ to Mn⁴⁺ for charge neutrality. This substitution has a direct effect on the DE interaction as well as weakens the FM coupling between Mn³⁺ and Mn⁴⁺ ions, leading to the lower ordering of FM transition temperature. Notably, the T_C of the un-doped sample presented here is lower than those formerly published^8,22,23) and in agreement with those documented by Ulyanov et al.¹⁴⁾ This difference could be due to the varied preparation conditions of the samples. Ulyanov et al.²³⁾ suggested that the high sintering temperature during synthesis triggers the oxygen deficiency in manganites, creating a decrease in the T_C.

The temperature dependencies of the inverse magnetic susceptibility χ⁻¹(T) of all samples are shown in Fig. 4(b). The fitting for paramagnetic straight line of each curve was computed using the Curie-Weiss law which is expressed as χ(T) = C/(T − θ) with C and θ are the Curie constant and the ordering temperature, respectively. The values of θ for all samples are specified in Table 1. As x increases, the θ value decreases, indicating the FM–DE interaction is weakened, whereas the increasing contribution of AFM interaction is attributable to the super-exchange pairs of Mn³⁺–Mn³⁺ and Mn⁴⁺–Mn⁴⁺.

It is well known that the experimental effective PM moment per formula unit, $\mu_{\textit{eff}}^{\textit{exp}}$, can be calculated basing on the relation between C and $\mu_{\textit{eff}}^{\textit{exp}}$ which is expressed as: $C = \frac{N_{\textit{AV}}}{3k_{B}}\mu_{\textit{eff}}^{2\textit{exp}}$, where k_B is Boltzmann’s constant (1.38016 × 10⁻²³ J/K) and N_AV is the Avogadro’s number (6.023 × 10²³ mol⁻¹). According, $\mu_{\textit{eff}}^{\textit{exp}}$ for all samples were obtained and listed in Table 1. Furthermore, the calculated effective PM moment, $\mu_{\textit{eff}}^{\textit{cal}}$ is defined as $\mu_{\textit{eff}}^{\textit{cal}} = \sqrt{ {(0.7 - 2x)\mu_{\textit{eff}}^{2}(\textit{Mn}^{3 + }) + (0.3 + x)\mu_{\textit{eff}}^{2}(\textit{Mn}^{4 + }) + x\mu_{\textit{eff}}^{2}(\textit{Cu}^{2 + })}\mathstrut} $ with μ_eff(Mn³⁺) = 4.9 μ_B, μ_eff(Mn⁴⁺) = 3.87 μ_B, and μ_eff(Cu²⁺) = $\sqrt{3} $ μ_B. The calculated values of $\mu_{\textit{eff}}^{\textit{cal}}$ are presented in Table 1. The experimental effective PM moment has values higher compared to those of the theoretical ones. These higher values are ascribed to the existence of short-range FM interactions above T_C. Prior studies revealed a behavior similar to the one inspected here.^1,2,24) Particularly, for x = 0 and 0.08, the CW law downturn deviation was observed ranging from T_C to the Griffiths temperature T_G, which was acquired from the maximum of the first order deviation with respect to temperature of χ⁻¹(T) curve (displayed in the inset of Fig. 4(b)). This H/M downturn versus T is known as a distinctive feature of Griffiths phase.¹³⁾ The temperature interval T_C < T < T_G is prescribed as the temperature region of Griffiths phase. The curve of this temperature regime is described by Griffiths model $\chi^{ - 1} \propto (T - T_{C}^{\textit{rand}})^{1 - \lambda }$, where $T_{C}^{\textit{rand}}$ is the random transition temperature and λ is a non-universal exponent with 0 < λ < 1. The value of T_G and temperature interval of Griffiths phase ΔT = T_G − T_C are 260 K and 12 K, respectively, for x = 0, and 230 K and 74 K for x = 0.08. Clearly, compared to those of x = 0, the T_G and ΔT of x = 0.08 are lesser and large, respectively. Similar results were found in the report of Pramanik et al.²⁵⁾ They deduced that the decreasing size of the clusters along with the increasing number of the finite-size spin clusters were the reasons behind the obtained results. Fitting the χ⁻¹(T) data above T_C to Griffiths model, the obtained value of $T_{C}^{\textit{rand}}$ and λ are 254 and 0.37, respectively, for x = 0, and 185 K and 0.43, respectively, for x = 0.08. The fact that λ increases with the increasing Cu²⁺ concentration demonstrates the enhancement of Griffiths phase. In the temperature region $T_{C}^{\textit{rand}} < T < T_{\text{G}}$, the dominant interaction is between the FM and PM phases, indicating the system is not a perfect FM ordering. This means that it has a complete PM behavior or it has a random distribution of FM clusters within the PM matrix.²⁶⁾ Concerning to the origin of Griffiths phase, as to the doped manganites, it has been proposed to originate from a quenched disorder emerging from radius mismatch, redistribution of magnetic lattice Mn–O–Mn network, and the appearance of short-range FM clusters in PM region.¹³⁾ In actuality, the Griffiths phase is predominantly observed in the doped perovskite-type manganite. Saleh et al.²⁷⁾ showed that Al-doping affected the structural, magnetic and electric properties of La_0.8Ba_0.2Mn_1−xAl_xO₃ (0 < x < 0.25) where they found that the Griffiths phase was evident in all samples except for x = 0, 0.1. Further, the decrease in λ with increasing Al³⁺ content suggests an additional decrease in the Griffith phase behavior and a decrease in size of the magnetic cluster. Zhang et al.¹³⁾ have also examined the Cu-doped La_0.7Ca_0.3MnO₃ with Griffiths phase observed at x = 0, 0.1, which attributes to the existence of anti-FM, FM and PM phase above T_C. They compared the enhancement of Griffiths phase at x = 0.1 and x = 0 and concluded that the enhancement originated from the larger Debye-Waller factor for x = 0.1 in comparison to that of x = 0. Moreover, T. L. Phan et al.²⁶⁾ have analyzed the system of Ba doping at La/Ca site and noticed that the Griffiths phase is reduced with the increase of Ba doping concentration.

To get insight into the nature of the phase-transition type and MC effect of the samples La_0.7Ca_0.3Mn_1−xCu_xO₃, a series of isothermal magnetization curves, M(H), were recorded at different temperatures throughout the FM-PM transition at narrow intervals of 3 K. Typically, the results can be displayed in common M(H) curves as depicted in Fig. 5(a), (b) and (d), (e) for x = 0, and 0.06, with narrow intervals of 3 K. For x = 0.08 sample, these M(H) curves was displayed with temperature interval of 6 K in order to making visible, as presented in Fig. 5(c) and (f). Figure 5(a)–(c) show that as the T increase, the nonlinear dependences of M(H) curves in the FM region convert into linear curves, which is a typical feature of PM state. For all the studied sample, at given field, the magnetization increases with the decreasing temperature. Especially, for the x = 0, 0.04 (not shown here), and 0.06, in the vicinity of T_C, the M(H) curves are of S-like shape. The curves have distinguished feature as follows. Initially, at low field, the magnetization begins to increase steadily and then starts to show a rapid pace at the intermediate field until it is slowly approaching the saturation. For x = 0.08, its M(H) curves at temperature range of 119–155 K show an abrupt change in the slope at magnetic field range between 5 and 20 kOe. This feature was explained by magnetic inhomogeneity in sample. Similar results were observed in La_0.7−xPr_xCa_0.3MnO₃ at magnetic field below 20 kOe,²⁸⁾ and in La_5/8−yPr_yCa_3/8MnO₃ at magnetic field below 26 kOe.²⁹⁾ The Arrott plots–illustrated by the H/M vs. M² curves–depict the dynamic of the FM-PM phase transition,³⁰⁾ as seen in Fig. 5(d)–(f) for x = 0, 0.06, and 0.08 samples. Figure 5(d)–(f) shows that, around the T_C, at high magnetic field, the plots are not a series of parallel straight lines, as specified by the mean field theory adapted for the long-range FM order; with no straight line at T = T_C that passes over the coordinate of origin. These attributes denote a presence of short-range FM order in the samples. The Cu substitution at Mn-site annihilates the DE interaction of Mn–O–Mn and the DE interaction between Mn³⁺ and Mn⁴⁺ ions is weakened because the Cu ions replace the partial Mn³⁺ ions. The competitive FM and AFM interactions of Mn³⁺–Mn³⁺ and Mn⁴⁺–Mn⁴⁺ ions are causing the short-range FM order. Furthermore, in the Banerjee criterion,³¹⁾ the negative/positive sign of the slope in the M² vs. H/M curve determines the characteristic of the FM-PM transition. A positive or negative slope represents a second-order magnetic transition (SOMT) or a first-order magnetic transition (FOMT), respectively. The results in Fig. 5(d)–(e) for x = 0, 0.06 and also for x = 0.04, not shown here, demonstrate that several Arrott-plot curves near T_C have negative slope, confirming the FOMT type for these samples. However, for the x = 0.08 sample, the H/M vs. M² curves (see Fig. 5(f)), at low magnetic fields (H < 8 kOe) show positive slopes, corresponding to SOMT, but at higher magnetic fields (H > 8 kOe), it becomes negative values, corresponding to FOMT. This coexistence of FOMT and SOMT in this sample could be due to magnetic inhomogeneity.

Fig. 5

M(H) data and Arrott plots (M² versus H/M) for La_0.7Ca_0.3Mn_1−xCu_xO₃ compounds with (a), (d) x = 0, (b), (e) x = 0.06, and (c), (f) x = 0.08, around their FM-PM phase transition.

A study on magnetic entropy change, ΔS_m, discloses the effect of MC in La_0.7Ca_0.3Mn_1−xCu_xO₃, which is conducted through the classical thermodynamic Maxwell relation:³²⁾   

\begin{equation} \Delta S_{m}(T,H) = \int\limits_{0}^{H} \left( \frac{\partial M}{\partial T} \right)_{H}dH \end{equation} (1)

As mentioned above, in our present study, the magnetization was measured under small, discrete field and temperature intervals. The eq. (1) can be written as a summation of   

\begin{equation} |\Delta S_{m}| = \sum\quad \frac{M_{i} - M_{i + 1}}{T_{i + 1} - T_{i}}H_{i} \end{equation} (2)

where M_i and M_i+1 are the M values measured at T_i and T_i+1 in a magnetic-field sweep of H_i, respectively. As stated in eq. (1), |ΔS_m| is achieved maximum value near T_C where the magnetization decays most rapidly. Figure 6(a)–(d) show the −ΔS_m(T) curves for various applied field ΔH values extending from 5 kOe to 50 kOe with 5 kOe intervals for x = 0, 0.04, 0.06, and 0.08, respectively. It can be seen that, for all samples, |ΔS_m| was optimized (expressed as |ΔS_max|) near the T_C and increases with increasing ΔH. In addition, the location of ΔS_max is shifted onto a higher temperature with increasing ΔH which is characteristic of materials undergoing FOMT. Furthermore, the −ΔS_m(T) curves have an asymmetric shape for all samples. The |ΔS_max| value determined at ΔH = 50 kOe is the largest for the x = 0 and declines with increasing x. Especially, the field dependencies of the |ΔS_max| around T_C could be determined by a power law of the field |ΔS_max|α Hⁿ where H varies from 0 to 50 kOe, with n as the magnetic-ordering exponent.³³⁾ To fit the |ΔS_max(H)| data to that power law function, the n values introduced are about 0.22, 0.23, 0.25, and 0.27 for x = 0, 0.04, 0.06, and 0.08, respectively, as shown in Fig. 6(e). It is well known that SOMT of a magnetic material appears at T = T_C, n = 2/3 below the mean-field approximation. The n values for all the studied samples are different from 2/3, suggesting that the mean-field behavior is not exhibited and the coexistence of short-range order in these samples.

Fig. 6

−ΔS_m(T) at various ΔH values for La_0.7Ca_0.3Mn_1−xCu_xO₃ compounds with (a) x = 0, (b) x = 0.04, (c) x = 0.06, and (d) x = 0.08. Arrows line indicate the direction of increasing H value, (e) Field dependences of |ΔS_max|. The data are fitted to the power law |ΔS_m| ∞ Hⁿ and the fittings are denoted by the solid lines, (f) Field dependences of RCP for all samples.

Aside from the assessment based on ΔS_m, the relative cooling power, RCP, is an additional variable to evaluate MCE. This variable can be computed based on the magnitude of ΔS_m and its full width at half maximum, δT, as express as RCP = |ΔS_max| × δT.³⁴⁾ Quite often, materials exemplifying a sharp −ΔS_m(T) curve possess a lower RCP than materials with a broad transition. The results for the RCP for all the samples are shown in Fig. 6(f). Clearly, for each sample, the δT (not shown here) and RCP increased with H, following the same trend of ΔS_max. The δT and RCP achieved its largest value at x = 0.08 under ΔH = 50 kOe even though the its ΔS_max value is the smallest. This is due to the spread of the transition subdues the deteriorating effects of the drop in |ΔS_max|. In Table 2, some main properties of the studied samples are compared with those of other manganites compounds exhibiting a large MCE. The |ΔS_max| and RCP values for undoped sample attained in this work are higher than those of the same compound stated by Rebello et al.³⁵⁾ The RCP value for x = 0.08 in this study is comparable to its value of La_0.7Ca_0.3Mn_0.94Zn_0.06O₃.¹²⁾ Additionally, the |ΔS_max| and RCP values measured at ΔH = 50 kOe for x = 0.04, 0.06, and 0.08 are larger than those of La_0.6Bi_0.1Sr_0.3−xCa_xMn_0.9Cu_0.1O₃ with x = 0.1 and 0.15,²⁾ La_0.67Sr_0.33Mn_0.8Fe_0.2O₃,³⁶⁾ La_0.7Ca_0.3Mn_0.93Fe_0.07O₃,³⁷⁾ and La_0.7Ca_0.25Ba_0.05MnO₃.³⁹⁾ Despite having smaller RCP than the value of Gd³⁸⁾ found in our samples, manganite with its economic value and its simplicity in turning the working temperature range, controllable T_C, is still regarded as a potential candidate for magnetic cooling devices.

Table 2 Magnetic phase transition temperatures T_C and ΔS_m, RCP values for La_0.7Ca_0.3Mn_1−xCu_xO₃ compounds compared with those for Gd and other perovskite manganites.

4. Conclusions

The structural characterization, oxidation state of Cu and Mn, the magnetic and magnetocaloric properties of La_0.7Ca_0.3Mn_1−xCu_xO₃ (x = 0, 0.04, 0.06, and 0.08) have been explored. XRD spectra display single phases for all samples without any obvious secondary phase. An increase in the lattice is due to the larger ionic radius of the Cu²⁺ (0.73 Å) than that of Mn³⁺(0.645 Å) and Mn⁴⁺(0.53 Å). Detailed XAFS analyses of the electronic structures exposed that with the increase of Cu-content in La_0.7Ca_0.3Mn_1−xCu_xO₃, more Mn⁴⁺ ions were introduced. Additionally, XAFS results indicated that the valence state of Mn ions is in mixed states of Mn³⁺ and Mn⁴⁺ while the valence state of Cu ions is in Cu²⁺. The T_C values are decreased with increasing Cu content due to suppression of the DE interaction between Mn³⁺–O²⁻–Mn⁴⁺. The Griffiths phase inspected for x = 0 and 0.08 and could be due to the presence of short-range FM clusters in the PM region. Furthermore, the characters of the Arrott plots demonstrate that the FM-PM phase transition in x = 0, 0.04, and 0.06 samples belong to the FOMT type. However, the x = 0.08 sample shows SOMT at low magnetic fields (H < 8 kOe) but FOMT at high magnetic fields (H > 8 kOe). Even though as x increases the |ΔS_max| magnitude decreases, the ΔS_m(T) peak has an exceptional broadening, which leads to the RCP consecutive increase from 290 J/kg for x = 0 to 360 J/kg for x = 0.08. Having large RCP values for La_0.7Ca_0.3Mn_1−xCu_xO₃ suggests that these types of compounds are considerably promising for magnetic-refrigerating systems at sub-room temperature.

Acknowledgement

This research is supported by Vietnam National Foundation for Science and Technology Development (NAFOSTED) under grant number 103.02-2019.335.

REFERENCES

• 1)   J.  Li,  H.  Wang,  Z.  Liang,  Y.  Li,  Q.  Chen,  H.  Zhang and  Y.  Li: Ceram. Int. 48 (2022) 36888–36899. doi:10.1016/j.ceramint.2022.08.255
• 2)   E.  Bouzaiene,  A.  Dhahri,  J.  Dhahri and  E.K.  Hlil: Inorg. Chem. Commun. 132 (2021) 108824. doi:10.1016/j.inoche.2021.108824
• 3)   A.P.  Ramirez,  S.-W.  Cheong and  P.  Schiffer: J. Appl. Phys. 81 (1997) 5337–5342. doi:10.1063/1.364538
• 4)   J.A.  Saleh,  I.A.  Sarsari,  P.  Kameli and  H.  Salamati: J. Magn. Magn. Mater. 465 (2018) 339–347. doi:10.1016/j.jmmm.2018.06.014
• 5)   M.A.  Dhahri,  K.  Jemmali,  E.  Taibi and  E.K.H.  Dhahri: J. Alloy. Compd. 618 (2015) 488–496. doi:10.1016/j.jallcom.2014.08.117
• 6)   P.  Bisht,  V.  Nagpal,  G.  Singh and  R.N.  Mahato: J. Appl. Phys. 132 (2022) 023901. doi:10.1063/5.0095083
• 7)   M.A.A.  Bally,  M.A.  Islam,  M.Z.  Ahasan and  F.A.  Khan: J. Magn. Magn. Mater. 557 (2022) 169462. doi:10.1016/j.jmmm.2022.169462
• 8)   K.Y.  Wang,  W.H.  Song,  J.M.  Dai,  S.L.  Ye,  S.G.  Wang and  J.  Fang: J. Appl. Phys. 90 (2001) 6263–6267. doi:10.1063/1.1415757
• 9)   E.  Bouzaiene,  A.H.  Dhahri,  J.  Dhahria,  E.K.  Hlil and  A.  Bajahzar: J. Magn. Magn. Mater. 491 (2019) 165540. doi:10.1016/j.jmmm.2019.165540
• 10)   M.S.  Kim,  J.B.  Yang,  P.E.  Parris,  Q.  Cai,  X.D.  Zhou and  W.J.  James: J. Appl. Phys. 97 (2005) 10H714. doi:10.1063/1.1860992
• 11)   R.D.  Shannon: Acta Crystallogr. Sec. A 32 (1976) 751–767. doi:10.1107/S0567739476001551
• 12)   T.A.  Ho,  S.H.  Lim,  P.T.  Tho,  T.L.  Phan and  S.C.  Yu: J. Magn. Magn. Mater. 426 (2017) 18–24. doi:10.1016/j.jmmm.2016.11.050
• 13)   H.  Zhang,  J.  Shi,  Y.  Li,  L.  Zhao,  Z.  Lu and  Q.  Li: J. Low Temp. Phys. 169 (2012) 77–89. doi:10.1007/s10909-012-0644-1
• 14)   A.N.  Ulyanov,  D.-S.  Yang,  H.-J.  Shin,  K.-j.  Kim,  A.V.  Vasiliev and  S.V.  Savilov: J. Alloy. Compd. 794 (2019) 223–226. doi:10.1016/j.jallcom.2019.04.290
• 15)   I.  Maurin,  P.  Barboux,  Y.  Lassailly and  J.-P.  Boilot: J. Solid State Chem. 160 (2001) 123–133. doi:10.1006/jssc.2001.9204
• 16)   M.S.  Kim,  J.B.  Yang,  J.  Medvedeva,  W.B.  Yelon,  P.E.  Parris and  W.J.  James: J. Phys. Condens. Matter 20 (2008) 255228. doi:10.1088/0953-8984/20/25/255228
• 17)   N.  Gayathri,  A.K.  Raychaudhuri,  R.  Gundakaram,  A.  Arulraj and  C.N.R.  Rao: Phys. Rev. B 56 (1997) 1345–1353. doi:10.1103/PhysRevB.56.1345
• 18)   Y.-H.  Huang,  C.-S.  Liao,  Z.-M.  Wang,  X.-H.  Li and  C.-H.  Yan: Phys. Rev. B 65 (2002) 184423. doi:10.1103/PhysRevB.65.184423
• 19)   G.  Xiao,  W.  He,  T.  Yang,  G.  Huang,  T.  Wang and  J.  Huang: Curr. Appl. Phys. 19 (2019) 424–435. doi:10.1016/j.cap.2019.01.011
• 20)   T.A.  Ho,  S.H.  Lim,  C.M.  Kim,  M.H.  Jung,  T.O.  Ho,  P.T.  Tho and  S.C.  Yu: J. Magn. Magn. Mater. 438 (2017) 52–59. doi:10.1016/j.jmmm.2017.04.038
• 21)   T.  Gao,  S.  Cao,  Y.  Liu,  T.  Zhou,  Z.  Feng and  J.  Zhang: J. Appl. Phys. 117 (2015) 163902. doi:10.1063/1.4918912
• 22)   A.  Marzouki-Ajmi,  M.  Mansouri,  W.  Cheikhrouhou-Koubaa,  M.  Koubaa and  A.  Cheikhrouhou: J. Magn. Magn. Mater. 433 (2017) 209–215. doi:10.1016/j.jmmm.2017.01.097
• 23)   A.N.  Ulyanov,  A.V.  Vasiliev,  E.A.  Eremina,  O.A.  Shlyakhtin,  S.V.  Savilov and  E.A.  Goodilin: Ceram. Int. 44 (2018) 22297–22300. doi:10.1016/j.ceramint.2018.08.354
• 24)   T.A.  Ho,  P.D.  Thang,  T.D.  Thanh,  W.B.K.  Putri,  T.O.  Ho and  S.C.  Yu: J. Superconduc. Nov. Magn. 35 (2022) 865–874. doi:10.1007/s10948-021-06125-0
• 25)   A.K.  Pramanik and  A.  Banerjee: Phys. Rev. B 81 (2010) 024431. doi:10.1103/PhysRevB.81.024431
• 26)   T.-L.  Phan,  N.T.  Dang,  T.A.  Ho,  A.D.  Phan and  S.C.  Yu: J. Alloy. Compd. 657 (2016) 818–834. doi:10.1016/j.jallcom.2015.10.162
• 27)   J.A.  Saleh,  I.A.  Sarsari,  P.  Kameli and  H.  Salamati: J. Magn. Magn. Mater. 465 (2018) 339–347. doi:10.1016/j.jmmm.2018.06.014
• 28)   T.A.  Ho,  S.H.  Lim,  T.L.  Phan and  S.C.  Yu: J. Alloy. Compd. 692 (2017) 687–692. doi:10.1016/j.jallcom.2016.09.097
• 29)   M.H.  Phan,  M.B.  Morales,  N.S.  Bingham,  H.  Srikanth,  C.L.  Zhang and  S.W.  Cheong: Phys. Rev. B 81 (2010) 094413. doi:10.1103/PhysRevB.81.094413
• 30)   A.  Arrott: Phys. Rev. 108 (1957) 1394–1396. doi:10.1103/PhysRev.108.1394
• 31)   S.K.  Banerjee: Phys. Lett. 12 (1964) 16–17. doi:10.1016/0031-9163(64)91158-8
• 32)   V.  Chaudhary,  X.  Chen and  R.V.  Ramanujan: Prog. Mater. Sci. 100 (2019) 64–98. doi:10.1016/j.pmatsci.2018.09.005
• 33)   J.Y.  Law,  V.  Franco,  L.M.  Moreno-Ramírez,  A.  Conde and  O.  Gutfleisch: Nat. Commun. 9 (2018) 2680. doi:10.1038/s41467-018-05111-w
• 34)   V.  Franco and  A.  Conde: Int. J. Refrig. 33 (2010) 465–473. doi:10.1016/j.ijrefrig.2009.12.019
• 35)   A.  Rebello,  V.B.  Naik and  R.  Mahendiran: J. Appl. Phys. 110 (2011) 013906. doi:10.1063/1.3603014
• 36)   G.F.  Wang,  Z.R.  Zhao,  H.L.  Li and  X.F.  Zhang: Ceram. Int. 42 (2016) 18196–18203. doi:10.1016/j.ceramint.2016.08.138
• 37)   P.J.  Lampen,  Y.D.  Zhang,  T.L.  Phan,  P.  Zhang,  S.C.  Yu and  H.  Srikanth: J. Appl. Phys. 112 (2012) 113901. doi:10.1063/1.4768175
• 38)   K.A.  Gschneidner, Jr. and  V.K.  Pecharsky: Annu. Rev. Mater. Sci. 30 (2000) 387–429. doi:10.1146/annurev.matsci.30.1.387
• 39)   T.D.  Thanh,  D.C.  Linh,  H.T.  Van,  T.A.  Ho,  T.  Van Manh,  L.V.  Bau,  T.-L.  Phan and  S.-C.  Yu: Mater. Trans. 56 (2015) 1316–1319. doi:10.2320/matertrans.MA201546