Progress towards the optimization of the signal-to-noise ratio in giant magnetoimpedance sensors

Melt extracted CoFeSiBNb soft magnetic wires were current and stress annealed in order to optimize their giant magnetoimpedance (GMI) response. Optimal results were obtained with a 75 mA dc current circulating for 15 min in a wire under a 100 MPa tensile stress. Stress annealed samples were found to be much more stable with respect to further applied stresses than the wires which were only current-annealed. A rough estimate of thermally induced magnetic noise is proposed. The results suggest that the magnetic noise in a GMI sensing element can contribute a significant part of its intrinsic noise. Magnetic field sensors were built using these wires driven in a modified Colpitts configuration. The transfer curves and noise characteristics of the sensors were investigated as a function of tensile and torsional stresses. Traction and torsion induced anisotropy modified the transfer curves and the noise voltage accordingly. An input-referred noise spectral density in the 1–10 pT/Hz1/2 range was observed above 1 kHz. The observed noise voltage appears to have an intrinsic contribution which is consistent with the calculated magnetic noise of the wires.