Suppression of magnetic noise in the fundamental-mode orthogonal fluxgate

A new important advantage has been discovered for the fundamental-mode operation of the orthogonal fluxgate employing an amorphous wire. It has been found that a great enough dc bias practically completely suppresses the magnetic noise generated in the fluxgate core by the ac-bias field; the fluxgate resolution becomes limited only by the excess electric noise in the ac-bias current. As a result, the magnetometer resolution increases by a factor of 60 and reaches 10pT/√Hz at frequencies above 2Hz. The suppression of magnetic noise can be explained by excluding magnetization reversals in the fluxgate core. In the fundamental mode, the fluxgate core is kept saturated continuously due to the unipolar bias, and magnetization varies by coherent rotation. Practically no magnetic noise is generated in this case. In the second-harmonic mode, magnetization is reversed by the bipolar bias. This causes nucleating domains and generating intensive magnetic noise in the fluxgate core.