Colossal Magnetoresistance of Ferromagnetic Manganites: Structural Tuning and Mechanisms

The amplitude $\Delta \rho (T,H)/\rho$ and temperature $T_M$, where the colossal magnetoresistance (CMR) response of $L_{1-x}{\mathrm{Ca}}_x{\mathrm{MnO}}_3$ manganites are maximum, are found to be controlled by the radius of the lanthanide $\left(L^{3+}\right)$ which modifies the bending of the Mn-O-Mn bond. Increasing the bond distortion lowers $T_M$ and enhances $\Delta \rho (T,H)/\rho$. Enhanced CMR arises from (1) a shift to lower temperatures of $T_M$, (2) a reduced mobility of the doping holes, and (3) an increase of the coupling between itinerant and localized electrons. The resistivity $\rho \left(H\right)$ follows an $\approx {\mathrm{BM}}^2\left(H\right)$ law and the parameter $B$ is also tuned by the Mn-O-Mn bond angle. The narrowing of the electronic bandwidth is the fundamental parameter controlling the observed CMR.