Magnetic and transport properties of granular cobalt films

Abstract

Simultaneous magnetization and magnetoresistance measurements have yielded the magnetic phase diagram of the granular Co-Si${\mathrm{O}}_{2}$ system. The superparamagnetic-to-ferromagnetic transition temperature changes gradually with composition from \ensuremath{\sim}80 K at 20 vol% Co to \ensuremath{\sim}280 K at 43 vol% Co. The results confirm the theory of spindependent tunneling in granular magnetic metals and yield the value of $JP=(1.0\ifmmode\pm\else\textpm\fi{}0.3)$ meV. Here $P$ is the spin polarization of the tunneling electrons and $J$ their exchange-coupling constant. This quantity has not been measured previously for cobalt. A quantitative comparison shows that the molecular-field theory does not provide an adequate description of the magnetization of granular cobalt.