Co − MgF 2 granular films were deposited on glass substrates by rf co-sputtering at room temperature (RT). The influence of the Co volume fraction, fv, of these granular films on the tunneling magnetoresistance (TMR) and magnetic properties was studied systematically. In a magnetic field of 1.2 T, the TMR value at RT initially increases gradually with decreasing fv, reaches its maximum value of −8% for fv=0.38, and then decreases. The corresponding magnetization curves indicate a change from ferromagnetism to superparamagnetism. A minimum in the coercivity, Hc, (11 Oe) is obtained in the Co50(MgF2)50 granular film which also has a large zero field resistivity. This magnetically soft granular film may consequently be a good candidate for high frequency applications. These variations of the TMR and magnetic properties can be ascribed to gradual changes in the film microstructure with decreasing fv, from interconnected metallic Co grains to nano-scaled Co particles dispersed in a crystallized insulating MgF2 matrix. The zero field cooled (ZFC) and field cooled (FC) curves for the samples were obtained in the temperature range 5–300 K in various magnetic fields. The peak in the ZFC curve shifts gradually towards lower temperature with increasing applied magnetic field and, with increasing fv, the peak temperature decreases more quickly with increasing field. The latter indicates that the magnetic interactions between grains become stronger, consistent with model predictions.

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