Nanometer size effect on magnetic properties 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-field-cooled magnetization (M _ZFC) exhibit a bifurcation just below the Curie temperature, T _C ^dc ≈ 55-64 K, determined from magnetization measurements for all particles studied. The field dependence of M _ZFC peak shows reasonable agreement with both the de Almeida-Thouless H ^2/3 line and the H ² power law. Measurements of ac susceptibility in the temperature range 5-300 K and the frequency range f = 10 Hz to 10 kHz show a sharp peak for both real and imaginary components in the vicinity of T _C ^dc, apparently attributed to the Hopkinson effect. The Curie temperature determined by zeroing the imaginary part of ac susceptibility χ″(T) shows a larger value of T _C ≈ 85-90 K in compliance with T _C of bulk Sm _0.8Ca _0.2MnO ₃ sample. A second small peak in ac susceptibility at T ≈ 11-15 K is seemingly associated with antiferromagnetic or ferrimagnetic ordering. Although for smaller particles both peaks depend on frequency, no shift to higher temperatures with increasing f, characteristic for spin-glass (SG) systems, was observed. Smallest, 23 nm, SCMO particles exhibit "waiting time" dependence in time evolution of M _ZFC, a feature expected for SG. These particles do not show any memory effects in M _ZFC, which is in strong contrast with the usual behavior exhibited by ferromagnetic nanoparticles with strong enough interparticle interaction. The dissimilarity in magnetic properties and dynamic characteristics observed for SCMO and for La _0.8Ca _0.2MnO ₃ nanoparticles is discussed, taking into account a difference in the width of the band and the strength of double exchange and interparticle interactions.