Magnetic properties of La_0.7Ca_0.3MnO₃ manganite bulk and nanometer-sized crystals

AC and DC magnetic measurements in temperature interval 5-300 K and at external magnetic field H ≤ 5 T were employed to probe and compare magnetic ordering in bulk and nanometer-sized (15 ± 2 nm) La_0.7Ca _0.3MnO₃ single crystals. The respective Curie temperatures (T_C) ∼ 239 K and 233 K, close to each other are observed in bulk and nano samples. While, their low T spontaneous magnetizations are strongly different, being about 3.6 and 1.2 μ_B/f.u., respectively. AC susceptibility (χ) of a bulk sample, measured at H_dc = 0, shows a maximum of χ(T) at T ∼ T_C. In a contrast, χ(T) of the nano-sample demonstrates a broad maximum far below its T_C. In the nano-sample both real and imaginary parts of χ are frequency dependent within the 5-280 K range. In a contrary, the frequency dependence is observed only for imaginary part of χ in bulk crystal. It is concluded that the similar ferromagnetic ground state exists in both considered samples; however, it is strongly frustrated in agglomerated nano particles due to surface magnetic disorder and interparticle interactions.