Chemical tuning of the colossal magnetoresistance of ferromagnetic perovskites

The colossal magnetoresistance (CMR) of a series of ferromagnetic L(2/3)Ca(1/3)MnO(3) perovskites is explored for a wide range of lanthanides (L) having different ionic radii. It is shown that the CMR can be tuned through the appropriate size of the lanthanide. The negative magnetoresistance has been modified in this way by two orders of magnitude, reaching Delta R/R values higher than 3 . 10(5)%. We will show that the temperature (T-M) where magnetoresistance is maximum correlates with its intensity (Delta R/R) and we will argue that both effects are simply controlled by the bending of the Mn-O-Mn bond. Above T-M, the enhancement of resistivity when decreasing the temperature can be described by a Variable Hopping model in which the relevant localization length l is found to be reduced when increasing the lattice distortion; inversely l enhances when applying a magnetic field, thus explaining the observation of a negative magnetoresistance. The giant modification of the electrical resistance goes in parallel with an unusually high magnetic polarizability. We argue that the coupling of itinerant charge carriers (holes) and their reduced mobility when they polarize the localized moments, thus forming magnetic polarons, lie at the heart of the CMR.