Patent ID: 8334147

Claim:
A magnetic sensor comprising: A planar array of multi-state sensor elements, wherein said multi-state sensor elements are MTJ cells, wherein each MTJ cell includes a fixed layer of spatially fixed magnetization direction and a free layer of movable magnetization direction, wherein said free layer has an easy axis that determines two low energy states of stable magnetic equilibrium corresponding to the magnetization of said free layer being oriented in either of two directions along said easy axis and wherein said free layer has a hard axis that defines two intermediate energy states of unstable equilibrium when the magnetization of said free layer is oriented in either of two directions along said hard axis, wherein a resistance of said MTJ cell is a function of the relative alignments of said free layer magnetization and said fixed layer magnetization and wherein differences of said resistances are measurable and a magnetic field within said array for placing said element into one of said two intermediate energy states of unstable equilibrium; and a magnetic field within said array for maintaining said element in said at least one intermediate energy state of unstable equilibrium; whereby while said element is in said unstable equilibrium state but not being maintained therein, said element will transition from said unstable equilibrium state to one of said two stable equilibrium states by interacting with an external perturbative magnetic field produced by a bound magnetic particle; and wherein a transition between each of the two low energy states produces a maximal resistance variation by which the presence of said bound particle is determined.