High magnetic sensitivity via large-diameter-vortex stability in magnetic tunnel junctions through controlled anisotropy

Magnetic-annealing-induced uniaxial anisotropy is shown to influence vortex formation in magnetic tunnel junction (MTJ) free layers of various diameters, with micromagnetic simulations demonstrating a threshold for single-vortex formation that exhibits an inverse diameter dependence. Angular M-H curves were measured for MTJ multilayers under different annealing conditions, and reveal that a two-step orthogonal magnetic annealing process reduces the uniaxial anisotropy in the free layer from 1040J/m3 to 130J/m3. This reduction in anisotropy allows for vortex formation in MTJ sensors as large as 40μm. Increasing the MTJ diameter leads to an enhancement of magnetic sensitivity up to 2.80%/Oe with the conventional definition, or 1.78%/Oe via an ac sensitivity measurement that verifies the nonhysteretic response. This result demonstrates an eightfold increase in vortex-MTJ sensitivity, compared with standard MTJs that have diameters limited to 5 μm, and this work provides the foundation for highly sensitive vortex-MTJ-based sensors for applications that require a nonhysteretic, ultrasensitive response.