Magnetic properties of compacted ${\text{La}}_{0.8}{\text{Ca}}_{0.2}{\text{MnO}}_3$ manganite nanoparticles with average particle size of 18 and 70 nm and Curie temperatures $T_{\text{C}}\approx 231\text{K}$ and $T_{\text{C}}\approx 261\text{K}$, respectively, have been investigated. The relative volume of the ferromagnetic phase has been estimated to be 52% for ensembles of 18 nm particles and 92% for 70 nm particles. It was found that applied hydrostatic pressure enhances $T_{\text{C}}$ of ${\text{La}}_{0.8}{\text{Ca}}_{0.2}{\text{MnO}}_3$ nanoparticles at a rate $d{T}_{\text{C}}/dP\approx 1.8–1.9\text{K}/\text{kbar}$, independently on the average particle size. Pronounced irreversibility of magnetization below $T_{\text{irr}}\approx 208\text{K}$ and strong frequency dependent ac susceptibility below $T_{\text{C}}$ for smaller 18 nm particles have been observed. 18 nm particles have also shown aging and memory effects in zero-field-cooled (ZFC) and field-cooled magnetization. These features indicate the appearance of spin-glass-like state, partially reminiscent the behavior of ${\text{La}}_{1-x}{\text{Ca}}_x{\text{MnO}}_3$ crystals, doped below the percolation threshold $x<x_{\text{C}}=0.225$. In contrast, ensembles of larger 70 nm particles have shown insignificant irreversibility of magnetization only and no frequency dependence of ac susceptibility, similarly to the behavior of ${\text{La}}_{1-x}{\text{Ca}}_x{\text{MnO}}_3$ crystals with $x>x_{\text{C}}$. The temperature of the ZFC magnetization maximum for 18 nm particles decreases with increasing magnetic field and forms a critical line with an exponent $1.89\pm 0.56$. The results suggest that superspin-glass features in ensembles of interacting 18 nm particles appear along with superferromagnetic-like features.

• Received 8 December 2009

DOI:https://doi.org/10.1103/PhysRevB.81.134440