Sizable magnetic entropy change in bismuth-substituted La_0.75Bi_0.1Na_0.15MnO₃ manganite

The bismuth (Bi)-substituted La_0.85-BiNa_0.15MnO₃ (x = 0 and x = 0.1) manganites have been investigated for achieving room temperature magnetic entropy change at low magnetizing intensities. A 10% Bi ion substitution at the La site has led to the relatively higher symmetry R3̄c rhombohedral structure showing improvement in Mn-O-Mn bond angles. The chemical additive Bi₂O₃ has assisted in grain growth to form densified specimens even at low sintering temperatures. The sample has a soft ferromagnetic nature with a single magnetic transition occurring at 275 K. Phenomenal improvement in magnetic sensitivity at low magnetic fields was achieved with T_C close to room temperature. Nearly 100-500 percent increases in magnetic entropy change was realised over existing parent compounds and the entropy changes are greater than the prototype, Gd of active magnetic refrigeration. The magnetic interactions are governed by the Tri-Critical Mean Field model with emphasis on the coexistence of long-range and short-range interactions. The characteristics, viz. soft ferromagnet of second-order phase transition, abrupt phase change at T_C, the resultant large magnetic entropy change of 4.4 J kg^-1 K^-1 and temperature averaged entropy change of 3.914 J kg^-1 K^-1 (10 K) at the low magnetic field of 1.5 T are expected to facilitate application prospects as a magnetic refrigerants for room temperature working range.