Enhancement of critical temperature in fine La0.7Ca0.3MnO3 particles prepared by mechanochemical processing

Fine La0.7Ca0.3MnO3 powders have been synthesized by mechanochemical processing, involving high-energy ball milling of LaCl3, CaCl2, MnCl2, and Na2CO3 and subsequent heat treatment. Single-phase La0.7Ca0.3MnO3 particles, dispersed in a NaCl matrix, were obtained for heat-treatment temperatures (Tht) of 750 °C or above, with the crystallite size increasing with Tht, from about 20 nm at 750 °C to about 1 μm at 1200 °C. Magnetic measurements showed that as the crystallite size decreases, the saturation magnetization at low temperature decreases from the theoretical value for full polarization of Mn spins, 3.7 μB per Mn ion, while Tc increases from 250 up to 278 K for the crystallite size of ∼20 nm; and that there exists a narrow temperature range near Tc in which the magnetization for a larger crystallite size is smaller in low fields but becomes larger in high fields than for a smaller crystallite size. These observations are discussed in terms of the ferromagnetic transition unique to perovskite manganites, involving the formation and growth of magnetic polarons above Tc.