Colossal magnetoresistance of ferromagnetic manganites: Structural tuning and mechanisms.

The amplitude $\ensuremath{\Delta}\ensuremath{\rho}(T,H)/\ensuremath{\rho}$ and temperature ${T}_{M}$, where the colossal magnetoresistance (CMR) response of ${L}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{MnO}}_{3}$ manganites are maximum, are found to be controlled by the radius of the lanthanide $({L}^{3+})$ which modifies the bending of the Mn-O-Mn bond. Increasing the bond distortion lowers ${T}_{M}$ and enhances $\ensuremath{\Delta}\ensuremath{\rho}(T,H)/\ensuremath{\rho}$. 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 $\ensuremath{\rho}\left(H\right)$ follows an $\ensuremath{\approx}{\mathrm{BM}}^{2}\left(H\right)$ 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.