Structural, Magnetocaloric Effect and Critical Phenomena Studies of La0.8Na0.2Mn0.94Bi0.06O3 Synthesized by Sol–gel Technique

Abstract

In this actual paper, we investigated the effect of elaboration technique on the structural, magnetic, magnetocaloric effect, landau theory and critical phenomena for the compounds La_0.8Na_0.2Mn_0.94Bi_0.06O₃ synthesized by Solid-state reaction and Sol–Gel method. The structural study showed that the method of preparation does not affect the crystallization structure. Both of the compounds are indexed in the rhombohedral structure with the symmetry of R-3C. However, the unit cell volume of Sol–Gel compound is higher than Solid-state compound as well as the lattice parameters. We showed also that the length of the bandwidth of Sol–Gel compound is less than that of Solid-state compound that has a crucial role in the change of the magnetic properties. We displayed also in the structural part that the crystallite size of Sol–Gel compound is less than that of Solid-state compound due to the difference of the sintering temperature. In the magnetic study, M (T) curve displayed a ferromagnetic paramagnetic phase transition around the transitional point Tc. The several values of Tc are 310 K and 195 K for Solid-state compound and Sol–Gel compound, respectively. This difference is due to the difference between the crystallite size and in the other way, it can be explicated by the difference of the bandwidth, which creates a changed localization. The magnetic entropy change as a function of temperature displayed a broad transition from order to disorder state around the Curie temperature Tc. The several values of -ΔS_M at 5 T applied magnetic field is equal to 5.2 J/kg K and 2.5 J/kg K for SS and SG, respectively. The several values of the relative cooling power RCP are equal to 229 J kg⁻¹ and 265 J kg⁻¹ for Solid-state compound and Sol–Gel compound which are about 55% and 64% as compared with the gadolinium. The universal master curve showed that both of the compounds display a second-order phase transition. We confirmed the order of transition from the coefficient of the landau mean field theory. The comparison between the magnetic entropy change calculated through Maxwell relation and Landau theory showed a perfect coincidence between the two trends only in the high temperature part and there are some discrepancies in the low temperature section. We calculated the critical exponent β = 0.5 using the magnetic entropy change which lead to guess that the phase transition is defined in the Mean field model. The critical phenomena investigation displayed that the transition of Solid-state compound is described between the tricritical mean field model and 3D-Heseiberg model with β=0.188 and y=1.44. However, the Sol–Gel compound is completely described in the Mean field model with \(\beta =0.51\) and y=0.99. The authority of the determined exponent are confirmed with the scaling theory.