Low-temperature transport and magnetization measurements, reported for the antiferromagnets $\mathrm{Sr}\mathrm{Mn}{\mathrm{O}}_3$ and $\mathrm{Ca}\mathrm{Mn}{\mathrm{O}}_3$, provide a means for distinguishing self-trapped from bound magnetic polaron (MPs) and reveal new insight into the role of impurity states on MP formation in oxides. The data identify an impurity band of mobile states separated by energy $\delta$ from electrons bound in Coulombic potentials. Very weak electric fields are sufficient to excite bound electrons to the impurity band, increasing the mobile carrier concentration by more than three orders of magnitude. The data indicate that bound MPs become stable only for $k_{B}T⪡\delta$.

• Received 5 July 2007

DOI:https://doi.org/10.1103/PhysRevB.76.020404