Spin-dependent tunnelling characteristics of fully epitaxial magnetic tunnel junctions with a Heusler alloy Co2MnGe 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 2 MnGe (CMG) lower electrode, a MgO barrier and a Co 50 Fe 50 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 °C. We also found that the d I /d V 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 °C or less to 500 °C or higher; i.e. the d I /d V versus V characteristics of the MTJs with T a 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 d I /d V versus V characteristics of the MTJs when T a was 500 °C or higher. We ascribe the peak structures in the d I /d V 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/Co 50 Fe 50 MTJs with T a of 475 °C or less. We attribute the discontinuous and complete disappearance of these peaks in the d I /d V versus V characteristics to the change in the interface bonding from thermodynamically unstable bonding for T a of 475 °C or less to stable bonding for T a of 500 °C or higher. The significant increase in the TMR ratio with increasing T a 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.