Events in Physics

The origin of noise and of magnetic hysteresis in permalloy ring-core fluxgate sensors

6-81.3 Mo permalloy, developed in the 1960s for use in high performance ring-core fluxgate sensors, remains the state-of-the-art for permalloy-cored fluxgate magnetometers. The choices made at that time are reviewed in the context of what are now considered best practices for sensor design. The magnetic properties of 6-81.3, namely magnetocrystalline and magnetoelastic anisotropies and saturation induction are all optimum in the Fe-Ni-Mo system. Recent data suggest that the geometries of typical ring-cores are not optimum, and further suggest a direct relationship between magnetometer noise and domain wall energy. A model is presented for domain walls in magnetically soft, thin foil material, one that provides a basis for relating fluxgate noise power, domain wall energy and magnetostatic energy due to anisotropy. The model predicts the existence of a novel domain configuration for magnetic saturation that requires the presence of shallow, channel-like domains [channel domains], topologically distinct from typical thin foil stripe domains. Transitions between these two states require exothermic magnetic domain wall reconnections which can be identified with Barkhausen jumps, and are the basis for magnetometer noise. The channel domain concept also leads to explanations for remanence, for DC coercivity and for DC hysteresis in an otherwise ideal soft magnetic material. The open loop of DC hysteresis results from the continual cycling between ordinary stripe domain states and channel domain states.