Stress-Induced Anisotropy in Electroplated FeNi Racetrack Fluxgate Cores

Magnetic anisotropy plays an important role in the behavior of a fluxgate core. It is desired to have an anisotropy orthogonal to the direction of the excitation field to move from one saturated state to the opposite saturated state without abrupt change of magnetization. In this paper, we present a method for electroplating a Permalloy film in the shape of a racetrack core with built-in anisotropy. The core is electroplated under bending and later released so that the resulting stress on the film generates the anisotropy. We show how the BH loop of the film changes as we increase the bending radius. Moreover, we demonstrate that electroplating under bending extends the frequency range before the inductance drops. Therefore, the fluxgates based on such cores can be used for higher frequencies (at the expense of lower sensitivity given by lower permeability).