Effects of high-energy heavy ion bombardment on the electrical conductivity of yttrium iron garnet

Epilayers of yttrium iron garnet Y3Fe5O12 on a (111)-Gd3Ga5O12 substrate were irradiated with 50 MeV 32S, 50 MeV 63Cu, and 235 MeV 84Kr ion beams. Film thicknesses were always smaller than the mean projected ranges of the incoming ions to avoid implantation effects in the layers. Dc-resistivity and saturation magnetization of ion-bombarded films were then measured at room temperature as a function of ion fluence ranging between 1011 and 1016 ions cm-2. Resistivity was found to decrease when fluence is increased above a threshold (≤1013 ions cm-2). The threshold fluence increases as the ion energy loss is reduced. 84Kr resistivity data level off above 3x1012 ions cm-2 after a two-order-of-magnitude decrease, whereas 63Cu and 32S data decrease by seven orders of magnitude according to a nearly parabolic law versus fluence without any saturation up to 1016 ions cm-2. The saturation of damage observed with 84Kr bombardment is interpreted by the overlap of paramagnetic disordered track cores, while in the case of 63Cu and 32S bombardment, such a picture is not valid. These two different behaviours might be interpreted by a change in the morphology of the extended defects produced by electronic excitations above a threshold of electronic stopping power. (orig.)