Magnetic entropy table-like shape and enhancement of refrigerant capacity in La1.4Ca1.6Mn2O7–La1.3Eu0.1Ca1.6Mn2O7 composite

In this work, we have investigated the structural, magnetic and magnetocaloric properties of La1.4Ca1.6Mn2O7 (A) and La1.3Eu0.1Ca1.6Mn2O7 (B) oxides. These compounds are synthesized by a solid-state reaction route and indexed with respect to Sr3Ti2O7-type perovskite with the I4/mmm space group. The substitution of La by 10% Eu enhances the value of magnetization and reduces the Curie temperature (TC). It is also shown that these compounds undergo a first-order ferromagnetic–paramagnetic phase transition around their respective TC. The investigated samples show large magnetic entropy change (ΔSM) produced by the sharp change of magnetization at their Curie temperatures. An asymmetric broadening of the maximum of ΔSM with increasing field is observed in both samples. This behaviour is due to the presence of metamagnetic transition. The ΔSM(T) is calculated for Ax/B1−x composites with 0 ≤ x ≤ 1. The optimum ΔSM(T) of the composite with x = 0.48 approaches a nearly constant value showing a table-like behaviour under 5 T. To test these calculations experimentally, the composite with nominal composition A0.48/B0.52 is prepared by mixing both individual samples A and B. Magnetic measurements show that the composite exhibits two successive magnetic transitions and possesses a large MCE characterized by two ΔSM(T) peaks. A table-like magnetocaloric effect is observed and the result is found to be in good agreement with the calculations. The obtained ΔSM(T) is ≈4.07 J kg−1 K−1 in a field change of 0–5 T in a wide temperature span over ΔTFWHM ∼ 68.17 K, resulting in a large refrigerant capacity value of ≈232.85 J kg−1. The MCE in the A0.48/B0.52 has demonstrated that the use of composite increases the efficiency of magnetic cooling with μ0H = 5 T by 23.16%. The large ΔTFWHM and RC values together with the table-like (−ΔSM)max feature suggest that the A0.48/B0.52 composite can meet the requirements of several magnetic cooling composites based on the Ericsson-cycle. In addition, we show that the magnetic field dependence of MCE enables a clear analysis of the order of phase transition. The exponent N presents a maximum of N > 2 for A, B and A0.48/B0.52 samples confirming a first-order paramagnetic–ferromagnetic transition according to the quantitative criterion. The negative slope observed in the Arrott plots of the three compounds corroborates this criterion.


Introduction
Manganese oxides exhibiting colossal magnetoresistance (CMR) and large magnetocaloric effect (MCE) are a very hot topic in materials science, not only because of the interest they have generated in basic research but also for their potential technological applications in spintronics and magnetic refrigeration (MR). 1,2 In addition, their adjustable phase transition temperatures, low-price and wear and corrosion resistance provide an additional advantage for the choice of manganese oxides as magnetic refrigerant materials for designing a "green" cooling refrigerator. [3][4][5] Magnetic refrigerators are considered an ecologically friendly technology because of several advantages they have compared to traditional refrigerators. High efficiency, small volume, and being free of harmful gas leakage are among these advantages. 6 The latter machines work on the principle of the MCE, 7,8 which describes the adiabatic temperature change of magnetic substance produced by the magnetic entropy change (DS M ) upon magnetization and demagnetization. 9 When magnetics stimulus in an adiabatic process is applied, the entropy of the spin subsystem is diminished and the transfer of energy to the lattice produces heating of the magnetic substance. Conversely, removing magnetics stimulus of the substance causes it to cool down. 10,11 The exploration of new refrigerant materials with large MCE at both ambient and cryogenic temperatures is strongly desired and is vital in accelerating the progress of magnetic cooling technology. Nevertheless, large MCE and negligible thermal and magnetic-eld hysteresis losses are required for MR. Considering the various requirements for applying magnetic cooling, La 2À2x -Ca 1+2x Mn 2 O 7 Ruddlesden-Popper phases (n ¼ 2) possess high magnetic moments and have a giant magnetocaloric effect. A giant peak value of DS M (16.8 J kg À1 K À1 ) originating from the abrupt change of magnetization observed at 5 T in La 1.4 Ca 1.6 -Mn 2 O 7 system. 12 This value is mostly close to that of systems undergoing a rst order magnetic phase transition (FOMT) such as Gd 5 GeSi 2 (18.5 J kg À1 K À1 ) and MnFeP 1Àx As x (18 J kg À1 K À1 ) alloys under the same eld change. 13 Generally, the DS M is adopted as an important index to demonstrate the refrigerant ability. Moreover, for a sample exhibiting FOMT, the value of DS M is highest near the magnetic transition temperature and falls rapidly with temperature, making its usage limited over a narrow temperature range. 10 Even though the change in magnetic entropy is large in such type of materials, they exhibit large thermal and eld hysteresis on variation of magnetization with temperature and magnetic eld, respectively. However, a considerable refrigerant capacity (RC), besides a giant peak entropy change, is also essential to obtain an excellent refrigeration efficiency. In this context, FOMT compounds do not seem to be the best choices, as their large hysteresis losses and limited temperature spans lead to signicant decreases in refrigerant capacities. From the practical application point of view, materials with a large MCE over a broad temperature range are desired. However, it is therefore interesting to search for new FOMT materials with low-level hysteresis, high performance and excellent functional stability. Among the presently known MCE materials with a rst-order magnetic transition (FOMT) the La 1.4 Ca 1.6 Mn 2 O 7 compounds fulll most of the requirements for practical applications of magnetic refrigeration. First, it has a limited thermal hysteresis at the FOMT. 14 Second, it is easy to tune the operating temperature by varying the La/Ca ratio or by substituting the Mn ion by various transition metal. Furthermore, the composition of this compound is low priced, have good chemical stability, easy to prepare, and does not contain any toxic or expensive elements such as arsenic and germanium, respectively.
It is well known that the structure of La 1.4 Ca 1.6 Mn 2 O 7 is constructed from ferromagnetic metal bilayer slices of MnO 2 sheets taken from the cubic perovskite, each slice being separated by a nonmagnetic insulating spacer layer which serves to isolate the bilayers (La, Ca) 2 O 2 stacked along the c-axis. The anisotropy and the reduced dimensionality of these compounds play a crucial role in their special properties different from those shown by the cubic perovskites. [15][16][17] Basically, the simultaneous ferromagnetic and metallic states observed in the Mnbased perovskite are explained using the double exchange mechanism (DE) caused by charge disproportionation. [18][19][20] The DE interaction in the Mn-O-Mn network in the case of bilayer manganite is expected to be much weaker along the stacking caxis direction because of the intervening rock salt layer that disrupts the interaction between the [MnO 2 ] layers. Members of this perovskite family are very responsive to small changes in composition and structure because of their layered structure. An inherent anisotropy modies the thermomagnetic properties of the layered materials.
In the context of magnetic cooling, the La 1.4 Ca 1.6 Mn 2 O 7 compound shows an abrupt change in the magnetization ((vM/ vT) H ) and illustrates his magnetic entropy (DS M (T)) with particularly peak at Curie temperature. However, in regenerative Ericsson cycle, the entropy change of the refrigerant DS M (T) should be constant (table-like MCE) over the operating temperature range of about 30 K. For this, there are number of publications [21][22][23] in which the authors have proposed different solutions to improve the cooling capacity at larger spans. Therefore, a solution is to work with a multiphase or sandwich materials. These materials extend the temperature range in which the magnetic entropy changes signicantly increase the possibility of improving performance through layering. Another simple way that increases the efficiency of magnetic cooling of bilayer manganites is the creation of the composite by a succession of magnetocaloric refrigerant samples with similar values of DS M and refrigerant capacity (RC). 24,25 In this work, an optimum molar fraction of La In addition, to extend the range of refrigeration, a composite magnetic refrigerant can be also used to increase or to optimize the refrigeration capacity (RC). This represents approximately the total thermal energy transferred from the hot to cold reservoirs over the active temperature range. Therefore, it was demonstrated that mixing of La 1.4 Ca 1.6 Mn 2 O 7 (A) and La 1.3 Eu 0.1 Ca 1.6 Mn 2 O 7 (B) provides an extra material design tool such that the optimal magnetic refrigerant material can be developed for a specic temperature range. The experimental results agree well with those calculated and discussed in the framework of an optimum regeneration Ericsson cycle. The MCE and RC of a prepared composite have been compared with those of individual bilayer manganites.

Experimental details
In this work, standard ceramic process is used to prepare two samples: La 1 with a purity of (99.9%) is prepared. These contents are mixed and grounded, then sintered for 12 h at 1200 C. Subsequently pressed into pellets, which and sintered again for 12 h at 1200 C. Aer grinding, the annealed powders are then pressed into disks and sintered at 1400 C for 12 h with intermittent grinding and slow cooling in a furnace. The obtained disk-shaped samples are well grounded again, then pelletized and sintered at 1400 C for 24 h. Finally, the sintered ceramic samples are slowly cooled to room temperature in air. As the sample have been elaborated in air, it is consequently stoichiometric in oxygen. 26,27 The composite sample is made by thoroughly mixing 48% : 52% (by weight) of polycrystalline powders of La 1.4 Ca 1.6 Mn 2 O 7 and La 1.3 Eu 0.1 -Ca 1.6 Mn 2 O 7 in an agate mortar for 30 min. The obtained compound will be referred as A 0.48 /B 0.52 . The samples are characterized using X-ray powder-diffraction measurements at room temperature in the 2q range of 20 to 80 with CuKa radiation (l ¼ 1.5406Å). The structural parameters are rened by Rietveld's prole-tting method using Fullprof soware. The temperature-dependence and the magnetic-eld-dependence of the magnetization, M(T) and M(m 0 H), are performed around the Curie temperature (T C ) using vibrating sample magnetometer developed at NEEL Institute.

Results and discussions
The XRD patterns of La 1.4 Ca 1.6 Mn 2 O 7 (A) and La 1.3 Eu 0.1 Ca 1.6 -Mn 2 O 7 (B) samples registered at 300 K and the structural renement patterns showing the observed, calculated, and difference proles for the nal t for the A and B samples, are depicted in Fig. 1(a) and (b). The phase identication and structural analysis of both samples are performed using the FullProf soware. 28,29 It is found that all diffraction peaks can be indexed with respect to Sr 3 Ti 2 O 7 -type perovskite with I4/mmm space group. As a La-bilayer-structured perovskite, these compounds are generally formed of the bilayers MnO 2 (magnetic conducting layer) separated by a monolayer rock-salttype (La, Ca) 2 O 2 (non-magnetic insulating layer) along the c axis. Meanwhile, some small secondary phases attributed to the presence of CaO impurity with space group Fm 3m and a fraction of with La 0.67 Ca 0.33 MnO 3 type orthorhombic structure with space group Pbnm are observed in both samples. Both impurities are identied with X'Pert HighScore Plus soware. For both compounds, the positions of the La 1 (B). The quality of the renement is evaluated through the goodness of the t indicator c 2 , which is 1.32% for A sample and 1.43% for B sample. This conrms that the renement is acceptable. The prole factor is found to be R p ¼ 19.3% (19.3%), weighed prole factor R wp ¼ 20.2% (20.6%) and Bragg R-factor R Bragg ¼ 7.57% (4.98%) for A sample (for B sample). The amounts of all phases present in the sample are quantied simultaneously using the Rietveld method. The phase quantication procedure involves the identication of major and minor phases. Here, quantitative phase analysis obtained by Rietveld MnO 3 and the CaO phases account for only 6.1% (7.9%) and 4.2% (4.7%), respectively. The latter phase is frequently encountered aer the nal step of the synthesis of LaCa-bilayer manganites. Given the small concentration of the impurities, we assume that the secondary phase does not have any signicant effect on the subsequent measurements of physical properties.  dissimilarity may be explained by the sensitivity of Curie temperature to the preparation conditions and the temperature of sintering 32,33 which leads to the conclusion that the preparation processes have enormous impacts on the performance of magnetic materials. Fig. 3(a) shows the magnetic hysteresis loops of both A and B samples taken at 10 K. Both loops show nearly zero coercivity, high magnetization saturation and negligible hysteresis which means that A and B bilayer manganites exhibit perfect magnetic reversibility or so ferromagnetic nature. These observed outstanding so-magnetic properties are benecial for the application as bulk magnetic refrigerants. Furthermore, it can be seen that they display scarcely any hysteresis loss, although the two compounds exhibit the nature of rst-order phase transition. This point is very attractive for magnetic refrigeration.
Isothermal magnetization M(m 0 H) curves are performed around transition temperature for each sample. Fig. 3(b) and (c) represents the recorded M(m 0 H) curves of samples A and B over a wide range of the magnetic eld ranging from 0 T to 5 T. At temperatures above 270 K for A sample (250 K for B sample) M(m 0 H) curves show a linear behaviour as expected in the paramagnetic state. Below 220 K for A sample (190 K for B sample) M(m 0 H) curves show an expected rapid increase at eld values less than 0.4 T followed by the tendency to saturation at higher elds, which indicate the existence of a ferromagnetic state in the samples.
However, it can be clearly seen that the magnetization initially increases gradually with increasing m 0 H for temperatures between 220 and 270 K for A sample (190 and 250 K for B sample). A sudden change appears above a critical magnetic eld followed by a rapid increase of magnetization thus exhibiting an 'S' shaped M(m 0 H) plot. This is a signature of a metamagnetic behaviour observed in the both samples.   kg À1 K À1 at 5 T (ref. 27)) observed in Pr 0.8 K 0.2 MnO 3 sample which showed a similar metamagnetic behavior, we investigated the MCE in both aforementioned compounds. 26 In the present work, it is interesting to evaluate the magnetocaloric effect of the A and B compounds. For this reason, we used the isothermal magnetisations measured at discrete temperatures to determine the MCE for each compound. Using Maxwell relation and magnetization curves (M-m 0 H) we obtained the value of magnetic entropy changes DS M (T, m 0 H) as a function of temperature in the magnetic eld range of 0 to 5 T for both A and B samples. Fig. 4(a) and (b) depicts the behaviour of DS M (T, m 0 H) for both compounds. The negative sign of the DS M (T, m 0 H) seen in the latter gures is referred as the normal MCE and conrms the ferromagnetic nature of these samples. [34][35][36] As one can see, the aforementioned materials illustrate signicant values of the magnetic entropy changes and show that the magnitudes of DS M increases with an increase in the applied magnetic eld. For m 0 H ¼ 5 T, the entropy change DS M exhibits a maximum value of 6.6 J kg À1 K À1 around T peak $ 245 K for A sample (6.25 J kg À1 K À1 around T peak $ 215 K for B sample) and it decreases on either side. However, the magnitude of DS M increases and the peak of DS M becomes asymmetrical with the rise of magnetic eld. While DS M diminishes abruptly with lowering temperature below the peak, it gradually falls with the rise of temperature above the peak. We can also remark that DS M curves for the both samples present higher peak values and are quite similar in the temperature range of DT ¼ T CA À T CB z 40 K. Due to the remarkable similarities in the results, the two materials provide an opportunity to manufacture a composite with high performance in the context of magnetic refrigeration.
In this context, magnetic properties for La 1.    To explore the performance of this composite, we have calculated the refrigerant capacity (RC) which is another decisive parameter for evaluating and approving cooling efficiency. 38 The RC parameter measures the amount of heat convey between the cold and hot reservoirs in the thermodynamic cycle. Thus, it has been suggested as a more suitable indicator of magnetic substances utility for solid-state refrigeration. For practical cooling systems, the RC with a broad temperature range is suitable for the active magnetic refrigeration cycle. [39][40][41] The refrigerant capacity depends not only on the maximum of ÀDS M (T), but also on the overall prole of ÀDS M (T). RC is obtained by numerical integration of the area under the ÀDS M (T) curve. The limits of the temperature integration are set by the half-maximum of the DS M (T) peak, where T Hot and T Cold correspond to the two temperatures at which the |DS M (T)| value is half of the peak value: 42 Accordingly, we report a detailed investigation of the MCE response as a function of the composite ratio; we present in Fig. 6 Fig. 6(b) for our new compound. This curve clearly shows that the investigated composite specimen exhibits two magnetic transitions because of its heterogeneous composition. It is also observed in Fig. 6(b) that the pronounced two minima in the dM/dT versus T curve conrm that the composite contains two magnetic phase transitions compared with individual A and B bilayer manganites. The later successive minima correspond exactly to T C for each of the constituent phases A and B used to prepare the A 0.48 /B 0.52 composite. It is worthwhile to mention that the magnetization magnitude of the studied composite shows a small decrease at low temperatures as compared with that of A and B bilayer manganites. The existence of two transition temperatures originating from different phases can certainly have an important effect on the MCE characteristics because the shape and behaviour of the magnetic entropy change are highly sensitive to the character of the magnetic phase transition.
It is demonstrated that the presence of two magnetic phases in the refrigerant material ensure that the material has a large MCE with a broad refrigeration temperature range and enhanced RC. In this investigation, we used the presence of two magnetic transitions to conrm our above calculation and for generating a broad range of MCE with a signicant increase in RC.
To get deeper insight into the magnetocaloric response of the prepared composite upon changing the magnetic eld from 0 to 5 T, isothermal magnetization curves of A 0.48 /B 0.52 are measured as a function of the applied eld recorded at different temperatures.
The measured M(m 0 H) plots are shown in Fig. 7(a). In Fig. 7(b) we compare the selected isothermal M-m 0 H curves plotted with applied elds between 0 and 5 T at T ¼ 10, 220, 250 and 320 K for the individual samples and the A 0.48 /B 0.52 composite. It is observed from this gure that the A 0.48 /B 0.52 sample has similar values of magnetization at 10 K and 320 K as compared to A and B samples. In addition, the M(m 0 H) curves are typical for a ferromagnetic state at 10 K and for a paramagnetic state at 320 K. On the other side, at 220 K and 250 K, the three compounds present different shapes in M(m 0 H) and the composite system shows the intermediate values of magnetization compared to that of the constituent phases A and B. In this temperature range, the slightly jump in the M(m 0 H) curves may be attributed to strong domain wall pinning in the ferromagnetic state.
The temperature dependences of magnetic entropy changes, ÀDS M (T), taken at 1, 2, 3, 4 and 5 T for the A 0.48 /B 0.52 composite is presented in Fig. 8(a). All the curves of ÀDS M (T) have a clear double-peak shape (two DS M values), resulting from the disparity in Curie temperature of both phases A and B. The latter double-peak shape is very noticeable at low m 0 H and  begins to atten gradually in favor of the table-like behaviour occurring at higher magnetic elds. This behaviour could give rise to the maximum values of DT FWHM and the RC refrigerant capacity strongly required for the ideal Ericsson cycle magnetic refrigeration over a broad temperature range. 43 Fig. 8(b) depicts experimental and theoretical entropy change curves for m 0 H ¼ 5 T of phases A and B that make up the composite with T C , A ¼ 200 K and T C,B ¼ 240 K, along with DS M (T) in the composite x ¼ 0.48. Latest gure demonstrates that the agreement between the experimental curves DS M (T) and that predicted by eqn (1) is excellent. According to this agreement, we can conclude that the numerical calculations are valid in the choice of MCE composite and can thus be used as means of designing magnetic refrigerant materials with an improved magnetocaloric response for the desired magnetic elds. The maximum value of ÀDS M (T) is found to be 4.07 J kg À1 K À1 for A 0.48 /B 0.52 in a wide temperature range. The magnitude of ÀDS M (T) is reduced in the A 0.48 /B 0.52 composite which gives a broad table-like behaviour with a wide temperature range compared to that of the pure constituent phases. Basically, in an ideal Ericsson cycle, the entropy conveyed between two heat reservoirs (T Hot and T cold ) should be as constant as possible to avoid the generation of irreversible work. 44 For this reason, the attening of ÀDS M (T, x ¼ 0.48) curve can be able to meet the latter requirements for the use of A 0.48 /B 0.52 as a composite for Ericsson-cycle-based magnetic refrigerators. 43 From eqn (2), the obtained value of RC is $205.92 J kg À1 at 5 T in B sample while it does not exceed $178.92 J kg À1 in A sample which indicate that the Eu-substitution increase the refrigerant capacity. Fig. 9(a) shows DS max M , DT FWHM and RC plots as a function of the applied magnetic eld. As displayed in Fig. 9(b), the obtained values of DS max M , DT FWHM and RC are strongly related to the magnetic eld. It is clearly observed that the A material has smaller values of DT FWHM than the B sample. Compared to gadolinium, which is considered as the typical ferromagnetic material for magnetic refrigeration, the RC values of the A and the B samples represents about $56.13% and $64.6% of the RC estimated for Gd (the value of RC is around 25% lower than that of the relative refrigerant capacity RCP for the DS M (T), 42 from ref. 45 According to the obtained result, the Eu-doped sample is still valuable for magnetic refrigeration at low temperatures. These values are much larger than that of several manganites 46,47 and are high enough for magnetic cooling. Refrigerants with wide working temperatures and high RC are in fact very benecial to magnetic cooling applications 48 and suggests that compounds can thus be used as an active magnetic refrigeration materials suggested by Barclay. 39  However, it follows that a compromise is necessary between the value of the DT FWHM and the energy losses and the efficiency of machine (due to an increase of cycles in the heat exchange medium). The investigated A 0.48 /B 0.52 composite exhibit nearly constant value of DS M (T) with width of $68.16 K. The present results conrm that the eld and the temperature range used in the numerical calculation are analogous to those explored experimentally and ensure that the large DT FWHM observed in the prepared composite have a great importance for cooling capacity.
The broadening width of DS M (T) is expected to make an increase of RC in the composite as predicted by eqn (2). In this study, it should be noted that the used T Cold and T Hot are dened as temperatures fullling DS M (T Cold ) ¼ DS M (T Hot ) ¼ DS max M /2. In order to visualize RC (T Cold ) and obtain the RC values at different values of T Cold , the integrand is evaluated from high temperature (T Hot ) to low temperature (T Cold ) as depicted in eqn (2). Fig. 9(b) shows the calculated values of RC as a function of T Cold under 5 T magnetic eld of A, B and A 0.48 / B 0.52 compounds. It is found that the RC increases as T Cold is separate from the DS M peak temperature (T peak ), at which RC is This journal is © The Royal Society of Chemistry 2019 zero due to eqn (2). In the temperature span of T Hot À  51 To make our analysis more complete, we are concerned with the nature of the magnetic phase transition in our bilayer manganites. For that reason, we have investigated the eld The temperature dependence of N is illustrated in Fig. 10(a) for A, B and A 0.48 /B 0.52 samples. The exponent N, for A and B samples has a minimum value at T C . However, for A 0.48 /B 0.52, the N(T) curves exhibits two minima whose positions are related to the critical temperatures of the existing phases (A and B) in this composite. On the one hand, it is observed in Fig. 10 that the exponent N is sensitive to the magnetic eld in the entire studied temperature range and the magnetic entropy changes, DS M . The value of N(T C ) < 0.4 at high magnetic elds indicates that our samples undergo rst order magnetic transition temperature. 54,55 In the other hand and under critical temperatures, the N(T) curves increase gradually with the temperature drop and approaches 1 for higher magnetic elds. Far above T C , the N(T) values overshoots 2 (N > 2) in the paramagnetic region (near magnetic transitions) of all three samples. This overshoot is more pronounced in sample B compared to sample A and the composite A 0.48 /B 0.52 . The observed behaviour shows that the quantitative criterion of N > 2 near the transition is valid for monophasic and biphasic materials which indicates that our samples exhibit a rst-order transition. This is in agreement with the previous observations in N(T C ) values. A similar behaviour is reported in other magnetic materials with rst-order transition. 56 However, the order of magnetic phase transition is usually revealed by the Arrott plots (M 2 vs. H/M). For more conrmation of the nature of the magnetic phase transition of A, B and A 0.48 /A 0.52 samples, the curves of M 2 vs. H/M plotted at different temperatures are exhibited in Fig. 11. The Arrott plots for the aforementioned materials just above the respective T C are displayed in the inset of the Fig. 11. According to the Banerjee criterion, the obviously negative slopes of Arrott plots verify the rst-order nature of the three samples, 57,58 which is consistent with the quantitative criterion.

Conclusion
In summary, we have rst investigated the structural, magnetic and magnetocaloric properties of La 1.4 Ca 1.6 Mn 2 O 7 and La 1.3 -Eu 0.1 Ca 1.6 Mn 2 O 7 samples prepared by the standard solid-state reaction method. The magnetic study showed that our investigated samples exhibit a PM-FM transition and present large magnetocaloric properties. Secondly, we have prepared a composite using the aforementioned samples with weight ratio of 48-52%. The latter ratio is determined numerically to obtained high magnetocaloric performances. Compared with the main polycrystalline phases, the magnetic entropy change of the prepared composite was found to be smaller. The prepared composite is characterized by important values of DT FWHM (68.16 K under 5 T) and RC (232.85 J kg À1 under 5 T). The refrigeration capacity of the composite is enhanced by 23.16% and 11.56% when compared with those of the individual La 1.4 Ca 1.6 Mn 2 O 7 and La 1.3 Eu 0.1 Ca 1.6 Mn 2 O 7 samples. Hence, the results of the A 0.48 /B 0.52 composite represent a signicant motivation to search for new suitable magnetic material with several reversible magnetic transitions originating from two different phases in order to expand working temperature with the same sign of successive magnetic entropy changes. These observations corroborate that the magnetic refrigerant compound with the more competitive characteristics may be developed in a form of a composite material that can lead to a future cheaper, more efficient and green refrigerator. In addition, our magnetocaloric investigation shows that the rst order phase transition is observed on our compounds. We showed that using the eld dependence of magnetocaloric effect, the order of the phase transition can be unambiguously determined using a quantitative criterion even for N > 2 near the transition of monophasic and biphasic magnetocaloric materials. The order of the magnetic phase transition of the three samples is corroborate by using the Banerjee criterion.

Conflicts of interest
There are no conicts of interest to declare.