Synthetic Antiferromagnetic MgO/CoFeB/Ta(x)/CoFeB/MgO Structures With Perpendicular Magnetic Anisotropy

We have studied the exchange coupling between two ultrathin CoFeB (1.2 nm) layers separated by a Ta spacer with a thickness varying from 0.5 nm to 3.5 nm. These structures show well-defined perpendicular anisotropy due to the MgO stabilization effect after post annealing. The magnetic coupling varies from parallel to antiparallel, depending upon the Ta thickness, and an oscillatory period 1.1 nm with maximum coupling strength (J) ~ 0.02 erg/cm² is revealed. A simulation, based on coherent rotation, is carried out for a comparison with the magnetic hysteresis curves. The variation of both anisotropy constant and exchange coupling strength are included to analyze the experimental results and the main characteristic of the magnetic reversal shows a spin-flip transition, which corresponded to the antiparallel coupled condition with comparable J and K. The change of the magnetic anisotropy due to thicker cap layer and the influence on the magnetic reversal will be also discussed.