Abstract
Exchange bias phenomena are observed in La0.25Ca0.75MnO3 nanoparticles with average sizes ranging from 40 to 1000 nm. It is found that the magnetic hysteresis loops display horizontal and vertical shifts in field-cooled processes. The variations of the exchange bias field (H-EB) and the coercivity (H-c) with particle size follow nonmonotonic dependencies and show maxima for particles with diameter around 80 nm at T=5 K, which can be mainly ascribed to the changes in uncompensated surface spins with nanoparticle size. The peak position for H-c shifts to larger particle size at higher temperature while that for H-EB is unconspicuous. The linear relationship between H-EB and vertical magnetization shift (M-EB) further indicates that the characteristics of uncompensated spins play an important role in the variations of H-EB for the manganite.