Optical CMR in manganites

Colossal magnetoresistance (CMR) and metal insulator transition in the perovskite manganites are still puzzling phenomena. We focussed on epitaxial films of (La1-yPry)0.7Ca0.3MnO3 with y=0-0.7, where a phase separation scenario with coexisting ferromagnetic metallic and charge-ordered insulating phases is believed to be the origin of the CMR. Using reflectivity measurements, we observed changes in the optical conductivity with external magnetic field in the visible range. This optical CMR peaks at the metal insulator transition temperature TMI as does the CMR. Furthermore the magnetic field behaviour resembles the field dependence of the resistance. Moreover optical CMR shows a pronounced spectral dependence for photon energies E=1.2-4 eV. This indicates that with optical CMR we probe the short range order of the Mn-spins. Since this optical technique generates information on the volume of our samples, we get further insight to the CMR-effect, proving the percolative nature of phase transition by modelling the results within random resistor networks.