Noise in small magnetic systems—applications to very sensitive magnetoresistive sensors

Abstract

Reduction for 1/ f noise (or random telegraph noise) is a crucial issue for small magnetic sensors which is strongly related to structural properties and magnetic configuration. We show how it is possible to eliminate magnetic noise at low frequency in GMR/TMR sensors by a combination of cross anisotropies, window frame shapes and suitably designed magnetoresisitive stack. These sensors are superior to almost all existing field and flux sensors. Results are presented on a mixed sensor, where a superconducting loop acts as a flux-to-field transformer to the GMR sensor. This device is suitable for detection of biomagnetic signals, such as in magnetocardiography or in magnetoencephalography. Measurements on niobium-based and YBCO-based sensors are presented, leading to sensitivity of 30 fT/√Hz at 77 K for small samples. Sensitivity lower than 1 fT/√(Hz) is expected with appropriate design and use of TMR or CMR layers, which makes these a powerful alternative to SQUIDs.