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

摘要

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 T-a, where T-a 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 T-a was increased from 475 to 500 degrees 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 T-a from 475 degrees C or less to 500 degrees C or higher; i.e. the dI/dV versus V characteristics of the MTJs with T-a of 475 degrees C or less exhibited distinct peak structures at V similar to 0.22V for P and at V similar to -0.38 and 0.27V 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 T-a was 500 degrees 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 T-a of 475 degrees 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 T-a of 475 degrees C or less to stable bonding for T-a of 500 degrees C or higher. The significant increase in the TMR ratio with increasing T-a from 475 to 500 degrees 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.