Picotesla field sensor design using spin-dependent tunneling devices

Pinned spin-dependent tunneling devices were fabricated and tested in a mod e suited for low-field sensing. The basic structure of the devices was NiFe Co125/Al(2)O(3)25/CoFe70/Ru9/CoFe70/ FeMn125 (in Angstrom). This structure had a tunneling resistivity of 110 M Omega mu m(2) and exhibited a 20% magn etoresistance when a field was swept along the easy direction of the soft e lectrode. High sensitivity, low hysteresis operation was achieved by applyi ng a bias field orthogonal to the easy axis. A sensitivity of 3%/Oe with ne gligible hysteresis was observed using this mode of operation. A sensor usi ng this type of material was designed to achieve a minimum resolvable field in the picotesla range. The sensor consists of a bridge with four elements , each having 16 tunnel junctions in series. A signal-to-noise ratio of 1:1 at pT(10(-8) Oe) is possible assuming achievable values for the tunneling resistivity, device size, bias level, and sensitivity. (C) 1998 American In stitute of Physics.