Effect of Electroplated Ni 1– x Fe x Composition on the Field-Induced Anisotropy

Field-induced anisotropy has been proposed as an effective method to produce ferromagnetic rings with radial anisotropy for fluxgate cores. Compared with stress-induced anisotropy, this method easily allows generating a radial anisotropy in electroplated rings. However, it is still not clear what the source of such anisotropy is. It has been suggested that the observed anisotropy could be the effect of the magnetocrystalline anisotropy present in the electroplated film because of the non-ideal stoichiometry of the alloy. If true, this would make impossible to electroplate films with field-induced anisotropy and, simultaneously, low magnetostriction, because this condition occurs at Ni 81 Fe 19 composition where the magnetocrystalline anisotropy vanishes. In order to verify this hypothesis, we electroplated several rings changing the composition of the alloy, with and without radial magnetic field applied. It turned out that the field-induced anisotropy was independent on the composition of the film to a large extend, and it did not show any vanishing trend as we approach Ni 81 Fe 19 composition. This suggests that it is, in fact, possible to obtain field-induced anisotropy and simultaneously low magnetostriction. Finally, we show how such radial field-induced anisotropy reduced the noise of the fluxgate by almost one order of magnitude.