Enhancement in tunnel magnetoresistance for magnetic tunnel junctions in Mn based Heusler compounds

Magnetic random-access memory is now a mainstream memory offering from semiconductor fabs worldwide. Heusler compounds such as Mn3Ge, which have large bulk perpendicular magnetic anisotropy, low magnetic moment due to its ferrimagnetic structure, and high spin polarization, offer potential pathway to reduction in spin transfer torque switching current. Here, we show using x-ray photoelectron spectroscopy that Mn3Ge is highly prone to oxidation during conventional radio frequency (RF) MgO deposition. Mn oxidation leads to reduction in tunnel magnetoresistance (TMR). The use of high off-axis angle during RF MgO deposition prevents Mn oxidation resulting in higher TMR. We also show that RF MgO growth on conventional CoFeB based electrodes does not need such high deposition angles. A significant enhancement in TMR is observed after thermal anneal of the Heusler layer. Thermal annealing improves ordering within the Mn3Ge layer as the Mn to Ge ratio is unchanged but leads to some interdiffusion along grain boundaries.