Unconventional bias dependence of tunnel magnetoresistance induced by the Coulomb blockade effect

In conventional magnetic tunnel junctions (MTJs), the tunnel magnetoresistance (TMR) monotonically decreases with increasing bias voltage, which limits the bias voltage range for the operation of MTJs. In our study, using double-barrier MTJs composed of Fe/MgO/Fe/gamma -Al2O3 grown on a Nb-doped SrTiO3 substrate, we demonstrate unconventional bias dependences of the TMR, in which the TMR ratio increases with increasing bias voltage. We reveal that this behavior originates from the sharp giant resistance peak near zero bias likely induced by the Coulomb blockade effect via Fe impurities in gamma -Al2O3, which are diffused from the Fe layer. The observed TMR ratio is 23% at a bias voltage of -4 V at 3.5 K, which is a very high value in this large bias voltage range. Our results offer a novel way to improve the bias voltage dependence of TMR.