GMR biosensor with enhanced sensitivity
View Patent ↗A sensor array comprising a series connection of parallel GMR sensor stripes provides a sensitive mechanism for detecting the presence of magnetized particles bonded to biological molecules that are affixed to a substrate. The adverse effect of hysteresis on the maintenance of a stable bias point for the magnetic moment of the sensor free layer is eliminated by a combination of biasing the sensor along its longitudinal direction rather than the usual transverse direction and by using the overcoat stress and magnetostriction of magnetic layers to create a compensatory transverse magnetic anisotropy. By making the spaces between the stripes narrower than the dimension of the magnetized particle and by making the width of the stripes equal to the dimension of the particle, the sensitivity of the sensor array is enhanced.
1. A GMR sensor stripe formed as a lamination of horizontal planar layers of substantially identical horizontal shape and having a free layer bias point along a length dimension that is stable with respect to hysteresis, comprising:
a pinned layer having a magnetic moment;
a non-magnetic electrically conducting spacer layer formed on said pinned layer;
a free layer having a magnetic moment and characterized by magnetic anisotropies along said length dimension and along a perpendicular width dimension, wherein said free layer has a magnetostriction that is adjustable by variations in dimension and composition of said free layer, wherein said free layer is formed on said spacer layer and weakly coupled magnetically to said pinned layer across said spacer layer; and
a capping layer formed on said free layer;
and wherein said layers have a length dimension that is greater than said width dimension, creating, thereby, a shape induced magnetic anisotropy in said length dimension of said free layer, maintaining a bias point of said free layer magnetic moment in a stable position along said length dimension while said free layer in a quiescent state not acted upon by external fields; and
dielectric material contiguously surrounding said GMR sensor stripe
placing said free layer under tensile or compressional stresses along said width dimension whereby a stress-induced anisotropy is produced in said width dimension as a result of adjusting said magnetostriction thereby canceling effects of said shape induced anisotropy in directions away from said length dimension and permitting sensitive response of said free layer magnetic moment when said free layer is not in a quiescent state, and
wherein the magnetic moments of said pinned and free layers are aligned parallel to said length dimension with said stable bias point when said sensor is in a quiescent state yet whereby said free layer magnetic moment is responsive to external magnetic fields when said sensor is not in a quiescent state.
2. The GMR sensor stripe of claim 1 wherein said pinning layer and said free layer are weakly magnetically coupled across said spacer layer and wherein the strength of said coupling is less than the shape magnetic anisotropy in said free layer.
3. The GMR sensor stripe of claim 1 wherein said free layer is formed of CoFe or NiFe has a thickness between approximately 10 and 100 angstroms.
4. The GMR sensor stripe of claim 1 wherein said pinned layer is formed of CoFe or NiFe and has a thickness between approximately 10 and 100 angstroms.
5. The GMR sensor stripe of claim 1 wherein said spacer layer is formed of Cu and has a thickness between approximately 10 and 20 angstroms.
6. The GMR stripe of claim 2 wherein the weakness of said coupling allows a maximum response of said free layer magnetic moment to small external magnetic fields.
7. The GMR sensor of claim 1 wherein said free layer has a horizontal length dimension of between approximately 10 microns and 20 microns and a width dimension between approximately 1 micron and 5 microns.