Strong current dependence of resistivity in CaMnO$_3$

The perovskite manganite CaMnO$_3$ (CMO) has a G-type antiferromagnetic ground state with N\'eel temperature $T_N$=125K. Prior transport measurements in the magnetic and paramagnetic phases$^{a}$ establish that CMO is a $n$-type semiconductor with $n\sim 10^{18}-10^{19} {\rm cm}^{-3}$ (from native defects like oxygen vacancies) and modestly heavy (large- polaron) mass, $m^{\ast}\sim 10m_0$. Here we report transport measurements on single crystal and polycrystalline CMO which reveal a strong current dependence of the resistivity ($\rho$) at low temperatures where $\rho >10^6\ \Omega\ {\rm cm}$ and impurity-band conduction predominates. For example, at 30~K, $\rho$ decreases by an order of magnitude for small current densities ($J<100\ \mu$A/cm$^2$), indicating that the effect is not associated with Joule heating. The possible role of spin-polarized hopping in this phenomenon will be discussed.\hfill \vskip .05in \noindent $^a$ J. L. Cohn, C. Chiorescu, and J. J. Neumeier, Phys. Rev. B {\bf 72}, 024422 (2005).