Topologically protected vortex structures for low-noise magnetic sensors with high linear range

Abstract

Micromagnetic sensors play a key role in a variety of industries, including the automotive industry, where they are used, for example, for speed and position detection. The adoption of emerging magnetoresistive sensor technology such as anisotropic magnetoresistance, giant magnetoresistance and tunnel magnetoresistance sensors is driven principally by their enhanced sensitivity and improved integration capabilities compared with conventional Hall effect sensors. At the heart of such sensors is a microstructured ferromagnetic thin-film element that transduces the magnetic signal, but these elements often exhibit a nonlinear hysteresis curve and the performance of the sensors is limited by magnetic noise. Here, we examine the origin of magnetic noise in magnetoresistive sensors and show that a topologically protected magnetic vortex state in the transducer element can be used to overcome these limitations. Using analytic and micromagnetic models, we find that the noise is due mainly to irreproducible magnetic switching of the transducer element at external fields that are close to the Stoner–Wohlfarth switching field. Then, using a flux-closed vortex configuration, we develop a giant magnetoresistance sensor layout that, compared to existing state-of-the-art sensors, has lower magnetic noise, a linear regime that is around an order of magnitude higher and negligible hysteresis.