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In the context of miniaturization, energy conservation, smart devices and IOT, magnetic field sensors based on magnetic tunnel junctions (MTJ) constitute an attractive choice, with small size, very high intrinsic sensitivity and low power consumption. The specific sensor response curve is determined by the magnetization configuration and hysteresis loop behavior of the soft (sensing) layer of the MTJ. A possible implementation is the vortex-based sensor, in which the junction sensing layer magnetization is in a vortex configuration at zero field, consisting of a small central core with out-of-plane magnetization and an in-plane magnetization rotating around the core with a clockwise or counterclockwise direction. Depending on the sensor element geometry, the vortex can be the natural stable spin configuration of the sensor, with the lowest energy at remanent state. This is the case for circular dots of soft ferromagnetic materials with sufficient thickness. Important characteristics of sensors include the sensitivity, the linearity, the measurement range and the hysteresis. Circular vortex-state sensors typically exhibit much lower sensitivity compared to uniform-magnetization sensors but are naturally highly linear and exhibit low hysteresis in a limited field range.
Here we use micro-magnetic simulations to study the effect of sensor shapes, volume defects, and perimeter defects on the magnetic behavior of a NiFe sensing layer, including hysteresis and vortex stability. In the case of circular dots, we show that the application of a large field results in a hysteresis near zero field, which originates from the combined role of volume defects inside the free layer (modeled by vacancies in the free layer) and perimeter defects (modeled by deviations from circular shape). We explore multiple geometries, sizes and thicknesses, and correlate sensor shape asymmetries, sensitivity axis direction, and hysteresis. The detailed vortex behavior following cycles of vortex expulsion and nucleation is also examined for selected asymmetric dot shapes. The role of different kinds of volume and perimeter defects is explored in determining overall sensor properties. A transverse field bias (transverse to sensing direction) is also shown to result in an increase of hysteresis. Finally, we study the effect of the dot aspect ratio on the sensor sensitivity and linearity and show the impact of the direction of the applied field on the extracted parameters.
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