Magnetic properties of compacted $50\mathrm{nm}$ ${\mathrm{CaMnO}}_{3-\delta }$ (CMO) nanoparticles have been investigated. Measurements of ac-susceptibility exhibit upon cooling two magnetic transitions at $T\sim 270\mathrm{K}$ accompanied by a small spontaneous magnetic moment and a para-antiferromagnetic (AFM) transition at $T_{\text{N}}\sim 120\mathrm{K}$, observed previously in bulk CMO. Asymmetric magnetization hysteresis loops observed in applied magnetic fields $H\le 90\mathrm{kOe}$ are attributed to an exchange coupling between the antiferromagnetic core and the ferromagnetic (FM) shell of the CMO nanoparticles. This work provides the observation of exchange bias effect in manganite nanoparticles with inverted AFM-core–FM-shell structure, as compared to the typical FM-core–AFM-shell. Effects of surface and exchange anisotropy are also discussed.

• Received 9 September 2007

DOI:https://doi.org/10.1103/PhysRevB.77.054410