Rounding of a first-order magnetic phase transition in Ga-doped La_0.67Ca_0.33MnO₃

Abstract

The effect of disorder on the critical properties of the ferromagnetic phase transition in colossal magnetoresistive manganite La_0.67Ca_0.33MnO₃ has been studied by substituting Ga for Mn. It is found that, upon 10% Ga substitution, the peak in the specific heat at the Curie point T_C changes drastically and appears as a small anomaly. Static magnetization data analyzed in the asymptotic critical region using modified Arrott plots and the Kouvel-Fisher method give values for the critical exponents β = 0.387(6), γ = 1.362(2), and δ = 4.60(3). The results show that the first-order transition in La0.67Ca0.33MnO3 becomes continuous by Ga substitution. The critical properties of the rounded transition in La_0.67Ca_0.33MnO₃ suggest that the magnetic subsystem in this mixed-valent perovskite is close to that of a conventional isotropic ferromagnet belonging to the Heisenberg universality class with short-range interactions. It is concluded that the first-order magnetic transition in pure La_0.67Ca_0.33MnO₃ is induced by fluctuations from a competing mode, which couples to the magnetic subsystem.