Magnetization and susceptibility of ion-irradiated granular magnetite films

Abstract

Porous granular films of magnetite (Fe${}_{3}$O${}_{4}$) with grains of \ensuremath{\sim}3 nm in size were prepared using a state-of-the-art nanocluster deposition system. The films are initially superparamagnetic but become magnetized following Si${}^{2+}$ ion irradiation. A significant increase in the grain size and a dramatic change in the microstructure are observed. There are dipolar interactions between the nanoparticles in both the unirradiated and irradiated films. The in-phase alternating current magnetic susceptibility of the unirradiated film shows a blocking temperature of \ensuremath{\sim}150 K, depending on frequency. A broadened Verwey transition for the irradiated film occurs at \ensuremath{\sim}75 K, above which the susceptibility exhibits unusual behavior: a nearly linear decrease with decreasing temperature. There are irreversible domain rotations in the irradiated film during zero-field cooling and warming cycles between 10 and 300 K. The observed behavior of the irradiated granular films is quite distinct from that of metallic nanostructures after irradiation, and is due to the dramatic change in microstructures.