Drop of magnetocaloric effect related to the change from first- to second-order magnetic phase transition in La2/3(Ca1−xSrx)1/3MnO3

TLDR

The transition from orthorhombic to rhombohedral structure at tolerance factor t = 0.92 changes the magnetic phase transition from first to second order, and anomalous thermal expansion at the Curie temperature due to lattice effects is thought to underlie the jump in ΔSM with composition. Magnetic entropy change ΔSM was measured from initial magnetization isotherms of La₂/₃(Ca₁₋ₓSrₓ)₁/₃MnO₃ (x = 0–1) near their Curie temperatures. ΔSM decreases from 3.7 to 1.5 J kg⁻¹ K⁻¹ as Sr content increases, with a steep drop between x = 0.05 and 0.15, which we attribute to the suppression of cooperative Jahn–Teller distortions in the rhombohedral phase.

Abstract

From data of initial magnetization isotherms, the magnetic entropy change ΔSM of the series of ferromagnetic perovskites La2/3(Ca1−xSrx)1/3MnO3, with x=0, 0.05, 0.15, 0.25, 0.50, 0.75 and 1, has been measured near their Curie temperatures. The results go from 3.7 J kg−1 K−1 for La2/3Ca1/3MnO3 to 1.5 J kg−1 K−1 for La2/3Sr1/3MnO3. Nevertheless, the evolution of ΔSM with x is not monotonic, it shows a steep decrease between x=0.05 and x=0.15. This point corresponds to a tolerance factor t=0.92, at which the system changes from orthorhombic (Pbnm) to rhombohedral (R3̄c) structure, and the magnetic phase transition from first to second order. Provided that rhombohedral symmetry forbids static long-range cooperative Jahn–Teller distortions, present in orthorhombic samples, we interpret our results as the strong increase of lattice effects in them. The anomalous thermal expansion at the Curie temperature, due to these lattice effects, is thought to underlie the jump in ΔSM with x.