AC biased sub-nano-tesla magnetic field sensor for low-frequency applications utilizing magnetoimpedance in multilayer films

A sensitive magnetic sensor based on the measurement of reflected incident power from magnetoimpedance (MI) multilayer-film element incorporated into 50 Ω matching network has been produced. The film system was fabricated by radio-frequency sputtering having two outer magnetic layers (NiFe), insulation layers (AlO₃), and inner conductor Au. For optimized layer parameters, an MI ratio of 135%, sensitivity 15%/Oe, and linearity within -2.5 and 5 Oe was obtained at a frequency of 434 MHz. An ac sense magnetic field applied to the MI element results in a small amplitude modulation signal due to the impedance variation, which can be measured on the carrier signal. For the sense field frequency of 1 kHz, the resolution of 3.73×10^-7 Oe was achieved. To improve the field resolution at lower frequencies (<100 Hz), the use of an ac bias technique has been proposed. For a sense field of 20 and 10 Hz with 5 kHz ac bias, field resolutions of 4.78×10^-6 Oe and 5.27×10^-6 Oe, respectively, have been obtained.