Critical Behavior of La0.7Ca0.3MnO3 Nanoparticles

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 (Tc) of three nanocrystalline La0.7Ca0.3MnO3 samples with average crystallite sizes d = 40, 23, and 16 nm. The analyses obtained the values of critical parameters to be Tc approximate to 261 K, beta = 0.485 + 0.005, gamma = 1.051 +/- 0.094 and delta = 3.1 +/- 0.1 for the sample with d = 40 nm, Tc approximate to 252 K, beta = 0.525 +/- 0.010, gamma = 0.893 +/- 0.139 and delta = 2.7 +/- 0.1 for d= 23 nm, and Tc approximate to 236 K, beta = 0.621 +/- 0.008, gamma = 0.825 +/- 0.007 and delta = 2.2 +/- 0.2 for d = 16 nm. With these critical values, the M(H) data points around Tc of the samples fall into two universal branches of a scaling function M(H,epsilon) = vertical bar epsilon vertical bar(beta)f +/-(H/vertical bar epsilon vertical bar(beta+gamma)), with epsilon = (T -Tc)/ Tc,f(+) for T > Tc and f_ for T < Tc. The results reveal that the crystallite-size reduction of nanoparticles decreases the Tc value. This is ascribed to the decrease of FM doubleexchange interactions between Mn3+ and Mn4+ ions, which is related to the beta 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.