Low voltage I – V characteristics in magnetic tunneling junctions

In 1975, Julliere proposed a simple model for tunneling between two ferromagnetic metals, assuming that the spin is conserved in the tunneling process and that the tunneling current is proportional to the density of states of each spin at the ferromagnetic electrodes. A magnetoresistance ~MR! effect then appears when one compares the resistance for cases in which the magnetization of the electrodes is antiparallel ~AP! and parallel ~P!. However, it is only quite recently that tunneling ferromagnetic samples have been shown to produce the large magnetoresistance effect ~25%–30%! found, for example, in Refs. 2–4. In addition, a remarkable dependence of the junction conductance on the voltage bias (V) has been observed at low voltages ~of the order of a few hundred millivolts!. In experiments reported in Refs. 3 and 4, the junction resistance drops significantly with applied voltage, with a peak at zero bias ~called the zero-bias anomaly! that is more pronounced for AP alignment. The effect is also temperature dependent, with the peak being less sharp at room temperature. Finally, it is found that the junction magnetoresistance ~JMR! has a large decrease with the voltage, up to 60% at 0.5 V in some cases. Early theoretical calculations of tunneling currents with applied voltages by Simmons showed variations of the conductance that are of the order of that observed in Refs. 3 and 4, but his theory yields no structure at zero bias. Scattering from surface magnons has been proposed as a mechanism for randomizing the tunneling process and opening the spin-flip channels that reduce the MR. While the above phenomenon may explain the JMR behavior in the immediate vicinity of the zero-bias peak ~for voltages smaller than ;40 mV), estimations of magnon scattering cross sections show that the effect is too small to account for the sharp drop in resistance in the whole range of 400– 500 mV observed in Refs. 3 and 4. In fact, inelasticelectron tunneling spectroscopy ~IETS! measurements at low temperature showed peaks which can unambiguously be associated with one-magnon spectra at very small voltages ~from 12 to 20 mV, with tails up to 40 mV!. Also, early experiments with spin polarized photoelectrons that tunnel from a ferromagnet through the surface barrier indicate that