Anomalous magnetic behavior of Sm_0.8Ca_0.2MnO ₃ nanoparticles

Magnetic properties of compacted Sm_0.8Ca_0.2MnO ₃ (SCMO) particles with average particle size of 23-100 nm, prepared by the glycine-nitrate method, have been investigated. It was found that the relative volume of the ferromagnetic phase decreases with decreasing particle size. Curves of field cooled and zero filed cooled magnetization (M _ZFC) exhibit a bifurcation just below the Curie temperature (T _C ≈ 55-64 K) for all particles studied. The field dependence of M_ZFC peak follows de Almeida-Thouless line. Both features are characteristic of spin-glasses (SG). Measurements of ac-susceptibility in the temperature range 5-300 K and the frequency range f =10 Hz-10 kHz show a sharp peak for both real and imaginary components in the vicinity of T_C, apparently attributed to the Hopkinson effect. A second small peak is seemingly associated with antiferromagnetic or ferrimagnetic ordering. Though, for smaller particles both peaks depend on frequency, no shift to higher temperatures with increasing f , characteristic for SG systems, was observed. The dissimilarity in magnetic properties and dynamic characteristics observed for SCMO and for La_0.8Ca_0.2MnO₃ nanoparticles is discussed, taking into account a difference in the width of the band and the strength of double exchange and interparticle interactions.