Unusual Critical Behavior in La_1.2Sr_1.8Mn₂O₇Single Crystal

摘要

In this paper, we present a detailed analysis on the critical behavior of La_1.2Sr_1.8Mn₂O₇single crystal via isothermal magnetization measured at different temperatures around the paramagnetic-ferromagnetic phase transition at T_C= 85 K. Using the Landau-Lifshitz coefficients, the Arrott plots (H/M = a(T) + b(T)M^{2) of sample have been analyzed. It showed that a(T) changed from positive to negative values at different temperatures in the field ranges of H = 0-10, 10-30, and 30-50 kOe, indicating that the critical behavior could not be described with a single model under different applied fields. Through the modified Arrott plots method, the Kouvel-Fisher method, and the critical isotherm analysis, we determined the values of the critical exponents for La_1.2Sr_1.8Mn₂O₇around its magnetic phase transition over different magnetic field ranges. The critical exponent β value is found to be 0.501, 0.417, and 0.371 under field ranges of H = 0-10, 10-30, and 30-50 kOe, respectively. This means that the β value depends strongly on the strength of the applied field, shifting from the value approaching that of the mean field model (β = 0.5) to the 3-D-Heisenbeg model (β = 0.365). Meanwhile, its y value is quite stable (γ = 0.973-1.074), almost independent on the choice of field fitting range. In addition, using the reduced temperature ε = (T - T )/T and the obtained critical exponents, almost M(H, T) data measured near T obey the scaling equation M(H, e) = eβ f_±(H/eβ+γ), where f₊and f_-are regular analytic functions corresponding to data at T > T_Cand T <; T_C, respectively.}