Effect of Stress-Induced Anisotropy on the Noise of Ring-Core Fluxgate

In this paper, we show how an anisotropy perpendicular to the direction of excitation is favorable to obtain low noise in fluxgate sensors. We produced ring cores by electroplating NiFe thin film (6 μm) over a bended copper substrate. After the electroplating process, the sample was released and back-stress-induced anisotropy in the magnetic film due to magnetostriction of the material. As a result, we obtained rings with regions showing hard behavior and orthogonal regions showing soft behavior. Then, we used such rings as core for fluxgate and measured both noise and sensitivity for different orientation of the ring in the pick-up coil. When the pick-up coil covers the area with anisotropy perpendicular to the direction of excitation, the sensitivity is lower due to lower permeability, but finally, the noise is lower. On the other hand, rotating the core by 90°, the anisotropy becomes parallel to the direction of excitation and thus the sensitivity increases. Nevertheless, the noise also increases, showing that such anisotropy parallel to direction of excitation is unfavorable even if it brings higher sensitivity.