ELECTRONIC TRANSPORT IN DISORDERED ION-BOMBARDED YTTRIUM-IRON-GARNET

Epitaxial thin films of ferrimagnetic yttrium iron garnet Y3Fe5O12, grown on Gd3Ga5O12(111) substrates, have been irradiated with heavy ion beams in a high-energy range where damage is due to ionizations and electronic excitations. Measurements of the film d.c. conductivity, with a two-probe technique, were then performed on fully disordered paramagnetic films as a function of temperature down to 90 K for the more conducting samples (around 10(-4) OMEGA-1 cm-1 at 300 K). The thermal dependence of conductivity data is rather well fitted by an exp (-T0/T)1/4 law up to room temperature, with very high T0 values (10(9)-10(10) K). In this case, the variable-range hopping (VRH) model yields very small densities of localized states at the Fermi level, between 10(16) and 10(17) eV-1 cm-3, values which are questionable for such highly damaged solids. The relevance of the VRH model in this case is discussed with respect to other current models, such as multiphonon-assisted polaron hopping conduction in a disordered medium.