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 lanthanide s (L) having different ionic radii. It is shown that the CMR can be tu ned through the appropriate size of the lanthanide. The negative magne toresistance 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 t he temperature (T-M) where magnetoresistance is maximum correlates wit h its intensity (Delta R/R) and we will argue that both effects are si mply controlled by the bending of the Mn-O-Mn bond. Above T-M, the enh ancement of resistivity when decreasing the temperature can be describ ed by a Variable Hopping model in which the relevant localization leng th l is found to be reduced when increasing the lattice distortion; in versely l enhances when applying a magnetic field, thus explaining the observation of a negative magnetoresistance. The giant modification o f the electrical resistance goes in parallel with an unusually high ma gnetic polarizability. We argue that the coupling of itinerant charge carriers (holes) and their reduced mobility when they polarize the loc alized moments, thus forming magnetic polarons, lie at the heart of th e CMR.