Low-Electric-Field Tuned Impurity Conduction in Antiferromagnetic Manganites

Transport measurements for temperatures in the range $4.2~{\rm K}\leq T\leq 300$~K are reported for the semiconducting, antiferromagnetic manganites SrMnO$_3$ and CaMnO$_3$. At low $T$ where impurity conduction predominates, the electrical conductivity and Hall coefficient are found to be strongly electric-field dependent. For SrMnO$_3$, the mobile carrier density is continuously tunable over a range of more than three orders of magnitude in electric fields $F\leq 50$~V/cm. The conductivity and carrier density scale with field $\propto\exp(\sqrt{F})$, indicating Poole-Frenkel field-assisted ionization of bound carriers. The binding energy for SrMnO$_3$ ($\delta \sim 3.5$~meV) implies that electrons are ionized to more mobile states within the energy gap, rather than to the conduction band. This small energy scale correlates with the low-temperature onset of a small ferromagnetic moment in this compound, suggesting that bound electrons form ferromagnetic polarons. Strong electron correlation effects are suggested by the electric-field dependent Hall mobility.