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 °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 °C or less to 500 °C or higher; i.e. the dI/dV 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 dI/dV versus V characteristics of the MTJs when T_a 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 T_a 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 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.