Current-driven magnetization switching was observed in the nanoring-shaped magnetic tunnel junctions (NR-MTJs) with key stack layers of both spin-valve-type antiferromagnetic/ferromagnetic/insulator/ferromagnetic and sandwich-type hard ferromagnetic/insulator/soft ferromagnetic structures. We successfully fabricated a series of ring-shaped MTJs with different ring-outer diameters of between 80nm and 4μm and different ring width of between 25nm and 2μm. Tunneling magnetoresistance ratio between 20% and 80% with different thickness of thin Al–O barrier was measured at room temperature as we apply a magnetic field or a pulsed current. When the electric current density exceeds a critical value of the order of 6×106A∕cm2, the magnetization of the two free and reference magnetic rings can be switched back and forth between parallel and antiparallel onion states. The experiments show that the spin transfer torque plays a main switching role in the magnetization reversal and the current-induced circular magnetic field plays an assisted-switching role in such NR-MTJs.
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Han et al. (2008) studied this question.