AbstractDue to its negligible spontaneous magnetization, high spin polarization and giant perpendicular magnetic anisotropy, Mn₂Ru_xGa (MRG) is an ideal candidate as an oscillating layer in THz spin-transfer-torque nano-oscillators. Here, the effect of ultrathin Al and Ta diffusion barriers between MRG and MgO in perpendicular magnetic tunnel junctions is investigated and compared to devices with a bare MRG/MgO interface. Both the compensation temperature, T_comp, of the electrode and the tunneling magnetoresistance (TMR) of the device are highly sensitive to the choice and thickness of the insertion layer used. High-resolution transmission electron microscopy, as well as analysis of the TMR, its bias dependence, and the resistance-area product allow us to compare the devices from a structural and electrical point of view. Al insertion leads to the formation of thicker effective barriers and gives the highest TMR, at the cost of a reduced T_comp. Ta is the superior diffusion barrier which retains T_comp, however, it also leads to a much lower TMR on account of the short spin diffusion length which reduces the tunneling spin polarization. The study shows that fine engineering of the Mn₂Ru_xGa/barrier interface to improve the TMR amplitude is feasible.