Critical Behavior of La_0.7Ca_0.3MnO₃ Nanoparticles

Abstract

We used the modified Arrott plot method to analyze magnetic-field dependences of magnetization, M(H), around the ferromagnetic-paramagnetic (FM-PM) phase-transition temperature (T_C) of three nanocrystalline La_0.7Ca_0.3MnO₃ samples with average crystallite sizes d = 40, 23, and 16 nm. The analyses obtained the values of critical parameters to be T_C ≈ 261 K, β = 0.485 ± 0.005, γ = 1.051 ± 0.094 and δ = 3.1 ± 0.1 for the sample with d = 40 nm, T_C ≈ 252 K, β = 0.525 ± 0.010, γ = 0.893 ± 0.139 and δ = 2.7 ± 0.1 for d = 23 nm, and T_C ≈ 236 K, β = 0.621 ± 0.008, γ = 0.825 ± 0.007 and δ = 2.2 ± 0.2 for d = 16 nm. With these critical values, the M(H) data points around T_C of the samples fall into two universal branches of a scaling function M(H, ε) = |ε|^βf_±(H/|ε|^β+γ), with ε = (T − T_C)/T_C, f₊ for T > T_C and f₋ for T < T_C. The results reveal that the crystallite-size reduction of nanoparticles decreases the T_C value. This is ascribed to the decrease of FM double-exchange interactions between Mn³⁺ and Mn⁴⁺ ions, which is related to the β change from 0.485 for d = 40 nm to 0.621 for d = 16 nm, corresponding to the change in FM order from the long-range type to the short-range one.