Spin-dependent tunnelling characteristics of fully epitaxial magnetic tunnel junctions with a Heusler alloy Co@d2MnGe thin film and a MgO barrier

We investigated the spin-dependent tunnelling characteristics of fully epitaxial magnetic tunnel junctions (MTJs) consisting of a Heusler alloy Co@d2MnGe (CMG) lower electrode, a MgO barrier and a Co@@d50@Fe@@d50@ upper electrode, which were fabricated as a function of @iT@da, where @iT@da is the temperature at which the MTJ trilayer was @@iin situ@ annealed right after deposition of the upper electrode. We found that the tunnel magnetoresistance (TMR) ratio increased discontinuously and significantly from 92% at room temperature (RT) (244% at 4.2 K) to 160% at RT (376% at 4.2 K) when @iT@da was increased from 475 to 500'C. We also found that the d@iI/d@iV versus @iV characteristics of fabricated MTJs for the parallel (P) and antiparallel (AP) magnetization configurations changed discontinuously and markedly with increasing @iT@da from 475'C or less to 500'C or higher; i.e. the d@iI/d@iV versus @iV characteristics of the MTJs with @iT@da of 475'C or less exhibited distinct peak structures at @iV 60.22 V for P and at @iV 6 -0.38 and 0.27 V for AP, where the bias voltage (@iV) was defined with respect to the CMG lower electrode. On the other hand, these structures were not observed in the d@iI/d@iV versus @iV characteristics of the MTJs when @iT@da was 500'C or higher. We ascribe the peak structures in the d@iI/d@iV versus @iV characteristics to the existence of peak structures in the interfacial density of states at the CMG electrode-MgO barrier interface arising from possible thermodynamically unstable interface bonding in CMG/MgO/Co@@d50@Fe@@d50@ MTJs with @iT@da of 475'C or less. We attribute the discontinuous and complete disappearance of these peaks in the d@iI/d@iV versus @iV characteristics to the change in the interface bonding from thermodynamically unstable bonding for @iT@da of 475'C or less to stable bonding for @iT@da of 500'C or higher. The significant increase in the TMR ratio with increasing @iT@da from 475 to 500'C is attributed to the increase in the interfacial spin polarization at the Fermi level associated with the change in the spin-dependent interfacial density of states.