Influence of a Ta spacer on the magnetic and transport properties of perpendicular magnetic tunnel junctions

Ultrathin Ta layers were inserted in the bottom hard (Co/Pt)/Ta/CoFeB/MgO magnetic electrode of perpendicular magnetic tunnel junctions. The magnetization of the top part of this electrode abruptly falls in-plane when the Ta thickness exceeds 0.45 nm. This results from the balance between the various energy terms acting on this layer (exchange-like coupling through Ta, demagnetizing energy, and perpendicular anisotropy at the CoFeB/MgO interface). For small Ta thicknesses, this insertion leads to a strong improvement of the tunnel magnetoresistance, as long as the magnetization of all layers remains perpendicular-to-plane.