Picotesla field sensor design using spin-dependent tunneling devices

Pinned spin-dependent tunneling devices were fabricated and tested in a mode suited for low-field sensing. The basic structure of the devices was NiFeCo125/${\mathrm{Al}}_2{\mathrm{O}}_3$ 25/CoFe70/Ru9/CoFe70/ FeMn125 (in Å). This structure had a tunneling resistivity of $\text{110\hspace{0.17em}}{\mathrm{M\Omega \hspace{0.17em}\mu m}}^{\mathrm{2}}$ and exhibited a 20% magnetoresistance when a field was swept along the easy direction of the soft electrode. High sensitivity, low hysteresis operation was achieved by applying a bias field orthogonal to the easy axis. A sensitivity of 3%/Oe with negligible hysteresis was observed using this mode of operation. A sensor using 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 1 pT ${\text{(10}}^{\text{−8}}\hspace{0.17em}\mathrm{Oe})$ is possible assuming achievable values for the tunneling resistivity, device size, bias level, and sensitivity.