Interlayer exchange coupling in the free layers of perpendicular magnetic tunnel junctions*

Perpendicular magnetic tunnel junctions (pMTJ) are composite heterostructures that have the capacity to retain a state with zero energy consumption and therefore are examined for their capacity to be used in devices such as magnetic random access memory (MRAM). A pMTJ stack usually contains several layers of ferromagnetic and nonmagnetic materials. Free layers consisting of multiple interfaces are used in recent pMTJs to enhance thermal stability and switching speed.  We investigated the interlayer exchange coupling in CoFeB/NM/CoFeB structures with typical NM materials such as MgO, Ru, Ta, Mo and Ir. FM or AF coupling can be obtained by controlling the NM thickness and/or annealing condition. The interlayer coupling is characterized by a magneto optical Kerr effect (MOKE) system, where a photo elastic modulator and lock-in amplifier are employed to enhance the signal-to-noise ratio. Large TMR above 200% have been obtained in these pMTJs.