Magnetic Properties of Granular Nickel Films

Granular films, about 1 $\mu$ thick and ranging in composition from 20-90-vol% nickel were prepared by cosputtering nickel and quartz. Electron micrography and diffraction show the specimens to be composed of nickel particles (∼ 50 Å in diameter) embedded in a matrix of Si${\mathrm{O}}_2$. The electrical resistivity increased from about ${10}^{-4\mathrm{}}$ Ω cm at 90-vol% Ni to about ${10}^{-2\mathrm{}}$ Ω cm at 50 vol%. Measurements were made of the magnetoresistance and the magnetic field dependence of the Kerr rotation as a function of temperature and composition of the Ni-Si${\mathrm{O}}_2$ system. These measurements provide evidence that the granular Ni-Si${\mathrm{O}}_2$ system can exist in three possible magnetic phases. At sufficiently high temperatures each nickel grain is paramagnetic. At lower temperatures the ensemble is superparamagnetic with each grain being ferromagnetic but with the magnetic moments of neighboring grains weakly correlated. At sufficiently low temperature the ensemble of grains becomes ferromagnetic, the moment of each grain being highly correlated with those of its neighbors. Our magnetoresistance measurements show that the electron tunneling probability is spin dependent.