Low-field magnetic sensors based on the planar Hall effect

Abstract

Sensitive magnetic field detection devices have been fabricated based on the planar Hall effect. The active material consists of permalloy ultrathin films (6 nm thick) epitaxially grown by molecular beam epitaxy. Uniaxial magnetic anisotropy is induced in the film through ferromagnetic coupling with a Fe/Pd bilayer epitaxially grown on MgO(001). The active layer shows a magnetoresistive ratio ΔR/R=2%. The device gives a sensitivity of 100 V/TA and a minimum detectable field below 10 nT. The detector response is linear over at least four decades. The transverse resistivity is sensitive only to the anisotropic resistivity, and not to the isotropic resistivity term which is highly temperature sensitive. Consequently, the thermal noise at 1 Hz is reduced by four orders of magnitude compared to a similar longitudinal magnetoresistive detector.