Magnetoresistance of the double-tunnel-junction Coulomb blockade with magnetic metals

We have studied the junction magnetoresistance (JMR) and the differential junction magnetoresistance (DJMR) for double tunnel junctions with magnetic metals in the Coulomb blockade regime. Spikes are seen in both the JMR and the DJMR vs voltage curves. They occur at those places where the current increases by a step. In all cases the large bias limit can be obtained by adding the resistances of each of the junctions in series. The JMR is positive in all the cases we studied, whereas the DJMR can be positive or negative as a function of the voltage. Moreover, the relative variation of the DJMR as a function of the voltage is larger than the variation of the JMR with the voltage.