Microscale sensor systems based on AMR sensors

Magnetic field sensors are essential components of several industrial, biomedical and consumer electronics applications. They make use of different physical principles to probe magnetic fields, which results in different sensor properties in terms of sensitivity, linearity, field range, power consumption and costs. Among the existing types, solid-state magnetic field sensors are particularly attractive as they are prone to miniaturization and can offer a high sensitivity in a relatively compact footprint. Sensors based on the anisotropic magnetoresistance (AMR) are particularly attractive owing to their relatively simple fabrication process (which makes their downscaling straightforward) and to their robust structure (which allows them to be fabricated on a wide variety of substrates).
Here, we discuss the development of sensor systems based on the detection and reconstruction of the motion of permanent magnets via properly designed AMR sensors. The basic general structure of this class of sensor systems consists of two main parts: a set of AMR sensors located at the bottom and a small permanent magnet on top, embedded within a deformable membrane and therefore capable of moving relative to the magnetic sensors as a result of the external solicitation generated by the physical observable of interest.
For the AMR sensors, high-aspect ratio stripes of Permalloy (Py, i.e., Ni80Fe20) are considered, so that their easy axis of magnetization lies along their longitudinal direction due to shape anisotropy. The stripes are typically arranged in Wheatstone bridges, which ensures output stability over a large range of temperatures as well as zero voltage output in the absence of an external magnetic field. The response of the AMR sensors can be linearized by employing the so-called barber-pole biasing scheme, which consists in covering the Py stripes with highly conductive shorting bars oriented at 45° with respect to their longitudinal axis. Through a proper choice of the spacing between the barber poles, this structure forces the current to flow from one bar to the next at an angle of 45° with respect to the easy axis of the Py stripes while leaving their magnetization unaffected, thereby linearizing their response.
Micromagnetic simulations based on finite-difference methods have been performed to study the resistance variation and the corresponding output voltage response of the barber pole-biased AMR sensor system with the assumption that a uniform electrical current flows across the Py stripes. The static properties of the latter have been investigated via Mumax3 using standard parameters for the Py (saturation magnetization Ms = 800×103 A/m and exchange constant A = 1.3×10-12 J/m). The magnetic field generated by the permanent magnet is computed by means of analytical expressions implemented in the Magpylib Python package and has been used as external magnetic field input in the micromagnetic simulations to compute the sensor response. Finite-element calculations (Ansys Maxwell) have been instead applied to simulate the electric current distribution within the Py stripes in the barber-pole configuration.
System-specific analytical models have been developed for the reconstruction of the magnet motion in different dimensionalities. For each of them, a different number and arrangement of AMR sensors is required. The relationship between magnet position and AMR sensor outputs – as expressed by the models – is linear within limited motion ranges (typically from sub-mm to few mm), whose amplitude can be tailored for each specific application via suitable magnetic system design.
Experimental fabrication and validation of the simulated results are ongoing at the time of writing.
The concept presented here – combined with state-of-the-art CMOS-compatible micro- and nanofabrication techniques (both for sensor and magnet manufacture) – holds potential for the realization of a wide spectrum of easy-to-fabricate, miniaturized and low-cost sensors (e.g., tactile, pressure, flow, acceleration, etc.), suitable for probing a broad variety of physical observables and for integration into microscale (e.g., MEMS) devices.