Tricritical point and critical exponents of La 0.7Ca 0.3− xSr xMnO 3 ( x = 0, 0.05, 0.1, 0.2, 0.25) single crystals

Abstract

The nature of ferromagnetic phase transition and its critical properties in La 0.7Ca 0.3− x Sr x MnO 3 ( x = 0, 0.05, 0.1, 0.2 and 0.25) single crystals have been studied systematically. Based on magnetic measurements and critical behavior analysis using Banerjee criterion and Kouvel–Fisher method, we demonstrate the existence of a tri-critical point, with critical exponents ( β = 0.26 ± 0.01, γ = 1.06 ± 0.02) at x ∼ 0.1, that separates “discontinuous” first-order magnetic transition (FOMT) for x < 0.1 compositions from “continuous” second-order magnetic transition (SOMT) for x > 0.1 compositions. Above the tricritical point, the system (e.g. x = 0.2) shows a SOMT with the critical exponents ( β = 0.36 ± 0.01, γ = 1.22 ± 0.01) belonging to the Heisenberg universality class ( β = 0.365 ± 0.003, γ = 1.336 ± 0.004). This suggests that the magnetic interaction in these manganites is of short-range type. Our systematic studies show that chemical (internal) pressure induced by substituting larger Sr ions for smaller Ca ions, cooperative Jahn–Teller distortions, antiferromagnetic coupling, and formation of ferromagnetic clusters all have significant impact on the nature of the ferromagnetic phase transition, the conduction mechanism and colossal magnetoresistance (CMR) in the doped manganites.