Tricritical point and critical exponents of La0.7Ca0.3−xSrxMnO3 (x = 0, 0.05, 0.1, 0.2, 0.25) single crystals

The nature of ferromagnetic phase transition and its critical properties in La0.7Ca0.3−xSrxMnO3 (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.

Research highlights▶ The present study provides a clear understanding of the nature of magnetic transitions, the transition from the “colossal” to “canonical” magnetoresistance behavior, and the mechanism of a metal-like conductivity in the paramagnetic region in La0.7Ca0.3−xSrxMnO3 (x = 0, 0.05, 0.1, 0.2 and 0.25) single crystals, prepared by the floating-zone method. ▶ Based on magnetic measurements and critical behavior analysis using Banerjee criterion and Kouvel–Fisher method, we demonstrate the existence of a tri-critical point at x ∼ 0.1, which 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 shows a SOMT with the critical exponents close to those of the Heisenberg universality class, suggesting that the magnetic interaction in these manganites is of short-range type. ▶ Our systematic studies show that chemical (internal) pressure induced by substituting larger Sr2+ ions for smaller Ca2+ 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 in the doped manganites. This study thus provides a first case for study of the influence of internal pressure on the critical properties of doped manganites.