Symmetric Response Magnetoresistance Sensor With Low 1/f Noise by Using an Antiphase AC Modulation Bridge

A new high-sensitive magnetic sensor which is referred to a symmetric response magnetoresistance (SRMR) sensor is proposed. This sensor uses giant magnetoresistance (GMR) elements which have symmetric field response. An ac magnetic field with frequency
$f$
is applied to the GMR elements in order to modulate a small detecting magnetic field. Due to the symmetric response of the MR element, a carrier signal of the ac magnetic field is converted to
$2f$
and the detecting signal is converted to
$f$
. By using a full bridge circuit, where two pairs of GMR elements have opposite phase of the applied ac field, the
$2f$
components can be eliminated at the differential output terminal of the bridge. Since this system converts the frequency range of the detecting magnetic field to higher one, its noise level, which is dominated by the 1/
$f$
noise, can be decreased. In this study, the SRMR sensor with the antiphase ac modulation bridge was fabricated and investigated. The effect of noise reduction and carrier signal suppression were confirmed by a circuit simulation based on the fabricated GMR element. A full-bridge circuit using four dual-spin-valve GMR elements was fabricated. It was confirmed that the carrier signal (
$2f$
components) was reduced to less than the signal level (
$f$
components). An increase in the noise level was found at the frequency range near
$f$
, which may be due to slight asymmetry shown in the magnetoresistance (MR) response.