Magnetic and magnetocaloric properties of nano-sized La0.8Ca0.2Mn1−xFexO3 manganites prepared by sol–gel method

X-ray diffraction

Powder X-ray diffraction patterns (Fig.  ¹ ) show that the samples show single phase and indexed in the orthorhombic structure with Pnma group space (Fig.  ² ). Refined cell parameters such as unit cell parameters, unit cell volume, R factor, and the goodness-of-fit indicator ( χ ² ) are listed in Table  ¹ . We can deduce that the substitution of Mn ³⁺ by Fe ³⁺ ions induces an increase of the unit cell volume. The linear increase is unexpected because we substitute Mn ³⁺ having 0.0645 nm as ionic radius by Fe ³⁺ with the same ionic radius (0.0645 nm). Consequently, no change induced by this substitution is expected. Therefore, the increase could be attributed to the lattice disorder arising from the random occupancy of Fe and Mn ions on the B-site. Indeed, in the pure perovskite La _0.8 Ca _0.2 MnO ₃ system (LCMO), Mn shows a mixed valence with Mn ³⁺ /Mn ⁴⁺ ratio close to 4 ([Mn ⁴⁺ ] = 0.2 and [Mn ³⁺ ] = 0.8) with a valence of +3 for La. The partial substitution of the Mn ions by transition metal ions (Fe) in La _0.8 Ca _0.2 Mn _{1− x} Fe _x O ₃ manganites gives rise to changes in the Mn ³⁺ /Mn ⁴⁺ rate, and some Mn ³⁺ –O ²⁻ –Mn ⁴⁺ networks are substituted by Fe ³⁺ –O ²⁻ –Mn ⁴⁺ . This causes a disorder of the charge transfer mechanism. Such disorder causes a change in the Mn–O distances and Mn–O–Mn angles. Consequently, the distortion of the ideal perovskite structure in which the Mn–O–Mn angle is equal to 180° undergoes a modification. These results are similar to those obtained by Othmani et al. [ ¹⁸ ].

Fig. 1

Fig. 2

Magnetic properties

To study the effect of substitution of iron in manganese sites on the magnetic properties, we have analyzed the magnetization variation versus temperature of La _0.8 Ca _0.2 Mn _{1− x} Fe _x O ₃ ( x  = 0, 0.01, 0.15, 0.2) samples under an applied magnetic field of H  = 0.05 T ( Fig.  ³ ) . The M (T) curves reveal that all samples exhibit a ferromagnetic (FM)–paramagnetic (PM) transition at Curie temperature, T _C  = 223, 205, 114, and 70 K, for x  = 0, 0.01, 0.15, 0.2, respectively. The Curie temperature T _C , defined as the peak of d M /d T in the M (T) curves, is reported for all compositions in Table  ³ . This table gives evidence that T _C and the magnetization are sensitive to Fe content. Indeed, the increase in Fe content causes an increase in T _C accompanied by a reduction of the magnetization. Probably, both changes are attributed to the competition between the superexchange (Mn ⁴⁺ –O–Mn ⁴⁺ ) and double-exchange (Mn ³⁺ –O–Mn ⁴⁺ ) interactions. The Fe takes place at the Mn site as Fe ³⁺ (replacement of some Mn ³⁺ –O–Mn ⁴⁺ bonds by Mn ⁴⁺ –O–Fe ³⁺ bonds), giving rise to an antiferromagnetic coupling between Mn and Fe ions that favors the superexchange mechanism. The evolution of magnetization ( M ) versus the applied magnetic field ( µ ₀ H ) for x  = 0, 0.01, 0.15, and 0.2 samples, obtained at different temperatures and measured under applied magnetic field ranging from 1 to 5 T, is shown in Fig.  ⁴ . These curves show that, below the Curie temperature, the magnetization greatly increases with the magnetic field and the saturated M is reached at H  = 1 T. For T  >  T _C , the variation of M (T, µ ₀ H ) does not reach the saturation and a linear behavior appears. This result confirms that all samples present a typical ferromagnetic behavior.

Fig. 3

Fig. 4

Figure  ⁵ presents the magnetization measurements performed at 4 K under applied magnetic fields of up to 6 T, for La _0.8 Ca _0.2 Mn _{1− x} Fe _x O ₃ ( x  = 0, 0.01, 0.15, 0.2) samples. Table  ² lists the experimental and the calculated magnetic moments per Mn ion, denoted by MSatExp and MSatTheo , respectively. The values of MSatTheo have been calculated by considering that the spins of all Mn and Fe ions are aligned. The magnetic moment of La0.83+Ca0.22+Mn1-xFex0.83+Mn0.24+O3 is expressed as

Fig. 5

The magnetic moments of Mn ³⁺ , Mn ⁴⁺ , and Fe ³⁺ ions are µ _Mn ³⁺  = 4 µ _B , µ _Mn ⁴⁺  = 3 µ _B , and µ _Fe ³⁺  = 5 µ _B , respectively. The x is the iron concentration and µ _B is the Bohr magneton. We note that the magnetization saturation values MSat decrease with increasing Fe content, especially for x  = 0.2. It is worth noting that similar results are reported in [ ¹⁹ ]. The difference between the measured and the calculated values should be explained by the presence of a magnetic inhomogeneity or by spin-canted state at low temperature.

Arrott curve

To determine the nature of magnetic transition type (first or second order), we have considered the experimental criterion given by Banerjee [ ²⁰ ]. It consists in inspecting the slope of isotherm plots of µ ₀ H / M versus M ² . According to this criterion, magnetic transition is of second order if all the curves have positive slopes, while, if some of these curves show a negative slope, the transition is first order. Figure  ⁶ shows the isotherm M ² versus µ ₀ H / M above and below T _C for La _0.8 Ca _0.2 Mn _{1− x} Fe _x O ₃ ( x  = 0, 0.01, 0.15) samples. Based on the of Banerjee’s criterion, the LCM and LCMF _0.01 systems exhibit a first-order ferromagnetic-to-paramagnetic phase transition, whereas a second-order transition is confirmed for LCMF _0.15 and LCMF _0.2 .

Fig. 6

Magnetocaloric study

The MCE is defined as the heating or cooling of a magnetic material due to the application or suppression of a magnetic field, respectively. To estimate the magnetocaloric effect, the change of magnetic entropy ( ΔSM ) was calculated numerically using the equation [ ²¹ ]:

The M _i and M _{i +1} are the experimental values of magnetization measured at temperatures T _i and T _{i +1} , respectively. The H _i is the applied magnetic field. The magnetic entropy change ( ΔSM ) determined numerically using Eq. ( ² ) and the M (T, µ ₀ H ) curves are shown in Fig.  ⁷ . The ( ΔSM ) value increases with temperature increase to reach a maximum near T _C and lowers above T _c . To compare our results with previously published data for other perovskite manganites, we listed in Table  ³ the data of several magnetic materials that could be used as magnetic refrigerants. Also, the maximum magnetic entropy change of Fe-doped manganites increases gradually with increasing applied magnetic field for such materials. We noted that the maximum entropy change ΔSMmax corresponding to a magnetic field variation of 5 T for La _0.8 Ca _0.2 MnO ₃ and La _0.8 Ca _0.2 Mn _0.99 Fe _0.01 O ₃ is about 4.42 and 4.32 J/(kg K), respectively.

Fig. 7

In Table  ³ , we compared our performances of MCE with those of Gd [ ² ]-based materials as well as rare earth manganites. The highest value of the magnetic entropy change for La _0.8 Ca _0.2 MnO ₃ and La _0.8 Ca _0.2 Mn _0.99 Fe _0.01 O ₃ samples is observed with x  = 0 content and is equal to 1.96 and 4.42 J/(kg K) under magnetic fields of 1 and 5 T, respectively. In addition, similar results were observed by Shaobo Xi et al. for La _0.8 Ca _0.2 MnO ₃ [ ²⁵ ] and by S. Ghodhbane et al. for Pr _0.8 Ba _0.2 MnO ₃ compounds under applied magnetic fields of 3 and 1 T, respectively. These values are lower than that of pure Gd [2.8 J/(kg K)] in a magnetic field change of 1 T [ ² ] ) and Gd ₅ (SixGe _{1− x} ) ₄ system [ ⁹ ] which have been considered as good magnetic refrigerants. For LCMFe _0.15 and LCMFe _0.2 , the maximum value of magnetic entropy change, ΔSMmax, is 1.6 and 0.54 K/(kg K) under a magnetic field of 5 T, respectively. Similar results have been reported for La _0.7 Ca _0.15 Sr _0.15 Mn _0.9 Fe _0.1 O ₃ [ ¹⁸ ] and La _0.63 Ca _0.33 Mn _0.8 Fe _0.2 O ₃ [ ³⁰ ].

The temperature dependence of the ΔSM upon the magnetic applied field changes of 5 T is shown in Fig.  ⁸ . These curves reveal that the La _0.8 Ca _0.2 Mn _{1− x} Fe _x O ₃ ( x  = 0, 0.01) samples present large magnetic entropy change and that ΔSM decreases when increasing the Fe content ( x ). This behavior is understood as the reduction of the double-exchange mechanism between Mn ³⁺ and Mn ⁴⁺ ions for La _0.8 Ca _0.2 Mn _{1− x} Fe _x O ₃ samples when x increases.

Fig. 8

Relative cooling power (RCP)

Another useful parameter which examines the efficiency of a magnetocaloric material is the RCP or the refrigerant capacity. It expresses the heat transfer between the hot and the cold reservoirs during an ideal refrigeration cycle. This is defined as the product of peak value of change in the magnetic entropy and the full width at half maximum (FWHM) of Δ S _M versus T curve (Fig.  ⁸ , inset) [ ³¹ ].

We have represented in Fig.  ⁹ the variation of the RCP factor as a function of the applied magnetic field. The RCP values exhibit a linear rise with increasing field for LCMO and LCMO _0.01 samples. Under the influence of an applied field of 5 T, the RCP values are found to be 146 and 116 J/kg for the samples x  = 0 and 0.01, respectively. Similar RCP values at 5 T (RCP = 140 J/kg) have been reported for La _0.8 Ca _0.2 MnO ₃ [ ²⁶ ] and La _0.7 Ca _0.15 Sr _0.15 Mn _0.9 Fe _0.1 O ₃ samples [ ²⁴ ]. Another interesting feature in the MCE plot is its asymmetric shape, especially under high field. Similar behavior is observed in Fe-substituted lanthanum calcium manganite [ ³⁰ ]. For comparison, the maximum magnetic entropy change, the Curie temperature, and the relative magnetic cooling efficiency of several manganese perovskites considered useful for room-temperature magnetic refrigerators are summarized in Table  ³ . Thus, due to the high Δ S _M and RCP values, our compounds with x  = 0 and 0.01 could be considered as active magnetic refrigerants for near-room-temperature magnetic refrigeration.

Fig. 9