Equivalent Magnetic Noise of Micro-Patterned Multilayer Thin Films Based GMI Microsensor

The detection limit of thin film giant magneto-impedance (GMI) magnetic sensors has been investigated by measuring their magnetic noise. The GMI sensing elements are multilayers based on Fe₂₀Ni₈₀ Permalloy (Py), deposited by sputtering and patterned by photolithography in the form of 2-mm long and 50- to 130- wide stripes. The multilayered samples had the following structure: [Py(170 nm)/Ti(6 nm)]₃/Cu(250 nm)/[Ti(6 nm)/Py(170 nm)]₃. As electronic conditioning circuits, different configurations of oscillators and detectors were tested in order to determine their influence on the total output voltage sensor noise. The latter 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 operating point. 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.