Elliptical Planar Hall Effect (PHE) Sensors

Elliptically patterned soft ferromagnetic thin films behave effectively as single magnetic domains due to shape-induced magnetic anisotropy aligned along the long axis of the ellipse. For a given film composition, the anisotropy field is proportional to the ratio of the film thickness to the short axis of the ellipse, provided that the aspect ratio of the ellipse is sufficiently large (typically ≥ 6).

These patterned ellipses form the basis of magnetic sensors operating via the planar Hall effect. In such devices, an electrical current is driven along the long axis of the ellipse, while the transverse voltage measured across the short axis is proportional to an externally applied magnetic field along that direction. For magnetic fields significantly smaller than the anisotropy field, the sensor response is linear.

Key Capabilities of Elliptical PHE Sensors

Elliptical planar Hall effect sensors offer a unique combination of performance, robustness, and design flexibility:

• Ultra-high magnetic resolution, reaching the pico-Tesla range

• Simple fabrication, based on a single ferromagnetic layer

• Excellent magnetic stability, resulting from shape anisotropy and soft magnetic materials

• Tailorable magnetic anisotropy, controlled by the ellipse dimensions and orientation

• Spatially resolved sensing, enabling magnetic field measurements at multiple locations within a single ellipse

• High performance on flexible substrates, without significant degradation

• Dual functionality as strain gauges, owing to their sensitivity to mechanical deformation

Unique Sensor Architectures

Gradiometers

The ability to probe the magnetic field at multiple locations within a single device enables compact and efficient gradiometric configurations. This capability simplifies sensor arrays and improves common-mode noise rejection.

Flexible Sensors

Elliptical PHE sensors fabricated on flexible substrates such as Kapton retain high sensitivity, achieving pico-Tesla-level resolution. This enables conformal sensor placement and facilitates combined magnetic and strain-sensing applications.

Vector Magnetic Field Sensing

By engineering the shape-induced magnetic anisotropy, sensors sensitive to different magnetic field components can be integrated on the same chip. This approach also allows simultaneous implementation of sensors optimized for different magnetic field ranges.

Representative Configurations and Magnetic Resolution

• PHE sensor with magnetic flux concentrators:
  5 pT/√Hz @ 10 Hz

• Single elliptical PHE sensor:
  24 pT/√Hz @ 50 Hz

• PHE sensor array (4 ellipses):
  16 pT/√Hz @ 100 Hz

• Gradiometer configuration:
  26 pT/mm/√Hz @ 50 Hz

• Flexible PHE sensor:
  better than 200 pT/√Hz @ 10 Hz