Careful tunneling studies in high quality $\mathrm{Co}/{\mathrm{Al}}_2{\mathrm{O}}_3/{\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ junctions show a junction magnetoresistance (JMR) of 20.2% and 27.1% at 295 and 77 K, respectively, where the latter is in agreement with Julliere's model. The temperature dependence of the JMR can be explained by the temperature dependence of surface magnetization. The decrease of the JMR with increasing dc bias is intrinsic to ferromagnetic junctions. The strong disagreement with recent theories in the low dc bias region can be attributed to magnetic excitations in these junctions, as seen in inelastic tunneling measurements.

• Received 21 November 1997

DOI:https://doi.org/10.1103/PhysRevLett.80.2941