Ferromagnetic-insulator-ferromagnetic tunneling has been measured in CoFe $/$A${\mathrm{l}}_2$${\mathrm{O}}_3$ $/$Co or NiFe junctions. At 295, 77, and 4.2 K the fractional change in junction resistance with magnetic field, $\Delta R/R$, is 11.8%, 20%, and 24%, respectively. The value at 4.2 K is consistent with Julliere's model based on the spin polarization of the conduction electrons of the magnetic films. $\Delta R/R$ changes little with a small voltage bias, whereas it decreases significantly at higher bias $(>0.1\mathrm{}\mathrm{V})$, in qualitative agreement with Slonczewski's model. These junctions have potential use as low-power field sensors and memory elements.

• Received 29 November 1994

DOI:https://doi.org/10.1103/PhysRevLett.74.3273