Films consisting of fine Ni grains (∼50 Å) dispersed in ${\mathrm{SiO}}_2$ exhibit a large negative magnetoresistance. The effect is accounted for by the field dependence of the magnetic exchange energy, associated with tunneling of electrons between neighboring grains whose magnetic moments are not parallel.

• Received 24 May 1976

DOI:https://doi.org/10.1103/PhysRevLett.37.1429