Giant magnetoresistance and non-ohmic effects in La0.6Y0.07Ca0.33MnOx thin films

Transport, magnetoresistance and structural properties of La0.6Y0.07Ca0.03MnOx(LYCMO) thin films grown in situ by pulsed laser ablation have been studied. Transmission electron microscopy and X-ray diffraction measurements have shown that the LYCMO films grow epitaxially on (100) LaAlO3 substrates and are cubic with a lattice parameter of 0.3849 run. The as-deposited films exhibited a metal-insulator transition at 130 K and a giant magnetoresistance at 125 K with a MR ratio (ΔR/RH) of 1500% in 6 Tesla magnetic field. By subsequent annealing of LYCMO films at 900 °C for half an hour in an oxygen ambient, this MR ratio was improved to 3200% at 192 K in the presence of the same magnetic field. This value of the MR ratio is the highest so far reported in this temperature regime. We ascribe this colossal magnetoresistance to the improved magnetic exchange between Mn3+ and Mn4+ ions resulting from the suppressed separation between Mn-O layers caused by a smaller sized Y-cation. We also report a non-ohmic response in the LYCMO films which is observed only in the region of the resistance peak and lends support to a conduction mechanism in these materials based on spin-dependent scattering of electrons.