Magnetic and transport properties of granular cobalt films

Simultaneous magnetization and magnetoresistance measurements have yielded the magnetic phase diagram of the granular Co-SiO₂ system. The superparamagnetic-to-ferromagnetic transition temperature changes gradually with composition from ∼80 K at 20 vol% Co to ∼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±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.