Anisotropy, exchange bias, dipolar coupling and magnetoresistive response in Nio–Co–Al2O3–Co magnetic tunnel junctions

We have investigated the behavior of magnetic tunnel junctions (MTJ) with antiferromagnetic/ferromagnetic coupled NiO/Co hard electrode and single Co soft electrode separated by an aluminum oxide barrier and deposited on float glass substrates. We determined conditions for obtaining strong uniaxial anisotropy in the Co layers induced by the deposition mode of NiO (facing target magnetron). Using a fixed position of the sample, we obtained columnar NiO films inducing a well-defined anisotropy to the subsequently deposited Co layer and square hysteresis loops. This process is compared with the deposition under constant magnetic field (1 kOe) and is found to be dominant for controlling the anisotropy. MTJ made with such a process for the bottom electrode show sharp magnetoresistive responses with more than 20% TMR at room temperature along the magnetic easy axis. Weakly anisotropic samples, fabricated by rotating the substrate during NiO sputtering, no applied magnetic field and the same stacking structure exhibit lower TMR values and sensitivity.