Spin-dependent tunnelling characteristics of fully epitaxial magnetic tunnel junctions with a Heusler alloy Co2MnGe thin film and a MgO barrier

Abstract

We investigated the spin-dependent tunnelling characteristics of fully epitaxial magnetic tunnel junctions (MTJs) consisting of a Heusler alloy Co2MnGe (CMG) lower electrode, a MgO barrier and a Co50Fe50 upper electrode, which were fabricated as a function of Ta, where Ta is the temperature at which the MTJ trilayer was in 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 Ta was increased from 475 to 500 °C. We also found that the dI/dV versus V characteristics of fabricated MTJs for the parallel (P) and antiparallel (AP) magnetization configurations changed discontinuously and markedly with increasing Ta from 475 °C or less to 500 °C or higher; i.e. the dI/dV versus V characteristics of the MTJs with Ta of 475 °C or less exhibited distinct peak structures at V ∼ 0.22 V for P and at V ∼ −0.38 and 0.27 V for AP, where the bias voltage (V) was defined with respect to the CMG lower electrode. On the other hand, these structures were not observed in the dI/dV versus V characteristics of the MTJs when Ta was 500 °C or higher. We ascribe the peak structures in the dI/dV versus V 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/Co50Fe50 MTJs with Ta of 475 °C or less. We attribute the discontinuous and complete disappearance of these peaks in the dI/dV versus V characteristics to the change in the interface bonding from thermodynamically unstable bonding for Ta of 475 °C or less to stable bonding for Ta of 500 °C or higher. The significant increase in the TMR ratio with increasing Ta 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.