Equivalent magnetic noise of thin film based giant magneto-impedance microsensors

Abstract

The detection limit of thin film giant magneto-impedance (GMI) magnetic sensors has been investigated by measuring their magnetic noise. Samples were patterned by photolithography from sputtered multilayers, in the form of stripes with lengths of 0.5 to 2 mm and widths of 50 to 130 μm. The layer structure [Ni ₈₀ Fe ₂₀ (170nm)/Ti(6nm)] ₃ /Cu(250nm)/[Ti(6nm)/Ni ₈₀ Fe ₂₀ (170nm)] ₃ with enhanced GMI sensitivity was selected. Electronic conditioning circuits, including an oscillator and different detectors, were tested in order to improve the sensitive element performance. The output noise voltage spectral density was measured using a dynamic signal analyzer and the equivalent magnetic noise of the sensor was determined through the measured Fourier transfer function of the device at the working bias field. Filtering and impedance matching strategies were implemented to minimize the equivalent magnetic sensor noise. Presently, a white noise level of 120 pT/√Hz at 2 kHz is obtained.