Magnetic sensors for nanotesla detection using planar Hall effect

Abstract

We have fabricated sensitive magnetic sensors based on the planar Hall effect with a field resolution below 10 nT. The sensor response is linear over at least four decades. Due to the transverse measurement configuration, the noise associated with thermal drift at 1 Hz is reduced by four orders of magnitude compared to a similar longitudinal magnetoresistive sensor. The active material consists of permalloy ultrathin films (60 Å thick) on top of a Fe/Pd bilayer epitaxially grown on MgO (001) by molecular beam epitaxy. A process-induced uniaxial magnetic anisotropy is built into the Fe/Pd bilayer, which then ensures that the magnetization at zero field is aligned with the current regardless of the magnetic history of the device. Optimized devices exhibit a sensitivity of 100 V T −1 A −1. In order to shift the range of operation towards lower fields, it is shown that a process-induced uniaxial anisotropy can be built into FeNi layers grown on Si(100) substrates. Planar Hall-effect sensors fabricated with such material exhibit sensitivities up to 300 V T −1 A −1.