Analytical and experimental study of the improved magnetic field sensitivity for nanocrystalline soft magnetic alloy and coil laminate with different layers

To lower the operation frequency of the giant magnetoimpedance (GMI) sensor, the nanocrystalline soft magnetic alloy (i.e., FeCuNbSiB) with embedded coil laminate is investigated, specifically the enhanced inductance variations vs dc magnetic field are studied to evaluate the magnetic field detection performance. It consists of multiple repeated sandwich structure (i.e., soft magnetic alloy/planar coil/soft magnetic alloy, abbreviated as MCM). Both the coils' self-inductance and electromagnetic coupling among different coils are significantly increased by covered soft magnetic ribbons, and this improves magnetic field sensitivity at low frequency. By considering the demagnetizing field's effect on the permeability, the performances of laminates with different MCM layers are theoretically investigated. Meanwhile various samples are fabricated and studied with experiments. It is found that (i) the maximum inductance ratio (i. e.,Ind_Ra = (L(H) - L(H-max))/L(H-max)) vs dc magnetic field is larger when the current directions of neighboring coils in the multilayer are opposite compared to that with the same direction. (ii) When the MCM layers are increased, the inductance ratio first increases and reaches the maximum value at the MCMCM structure due to the enhanced inductance contributed by multiple magnetic layers, and then gradually decreases with further increased MCM layers due to the increased demagnetizing field by neighboring magnetic layers. Additionally, by connecting a capacitor with the laminate, the inductance variation of laminate vs external dc magnetic field is changed into the resonant frequency variation. The investigated multilayer magnetic sensor can provide the sensitivity between 2.66 kHz/Oe and 3.56 kHz/Oe depending on the number of MCM layers, which is higher than most magnetic sensors using frequency tuning methods. This study plays an exploratory role at the design of ferromagnetic multilayer for magnetic sensor at low frequency.