Exchange bias phenomena are observed in ${\mathrm{La}}_{0.25}{\mathrm{Ca}}_{0.75}\mathrm{Mn}{\mathrm{O}}_3$ nanoparticles with average sizes ranging from $40\text{to}1000\mathrm{nm}$. It is found that the magnetic hysteresis loops display horizontal and vertical shifts in field-cooled processes. The variations of the exchange bias field $\left(H_{\mathrm{EB}}\right)$ and the coercivity $\left(H_c\right)$ with particle size follow nonmonotonic dependencies and show maxima for particles with diameter around $80\mathrm{nm}$ at $T=5\mathrm{K}$, 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 $H_{\mathrm{EB}}$ is unconspicuous. The linear relationship between $H_{\mathrm{EB}}$ and vertical magnetization shift $\left(M_{\mathrm{EB}}\right)$ further indicates that the characteristics of uncompensated spins play an important role in the variations of $H_{\mathrm{EB}}$ for the manganite.

• Received 29 July 2008

DOI:https://doi.org/10.1103/PhysRevB.78.224408