A magnetic field sensor based on the planar Hall effect has been developed using epitaxial permalloy (Ni80Fe20) ultrathin films (1-10 nm). The magnetic and magnetotransport properties of these films have been studied in detail. For thicknesses above 5 nm, the resistivity of the permalloy film is below 5μΩ-cm, and its magnetoresistance ratio is 2%. By using the transverse resistivity for detection, we have reduced thermal drift effects by five orders of magnitude. We also make use of a weak uniaxial anisotropy induced in the permalloy through exchange coupling with a 6 nm-thick Fe/Pd multilayer, itself grown directly on the MgO substrate. Magnetic sensors based on these films have been used successfully to detect fields below 10 nT at 1Hz. Since the lateral dimensions of the sensing element are small (<30μm), and because of the ferromagnetic coupling with the Fe/Pd structure, it consists of a single magnetic domain. Sensitivities above 100 V/T-A have been obtained, with deviations from linearity of less than 2% over 4 decades.
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Schuhl, A., Van Dau, F. & Childress, J. Nanotesla Detection Using the Planar Hall Effect. MRS Online Proceedings Library 384, 15–20 (1995). https://doi.org/10.1557/PROC-384-15