Bio-sensor with hard-direction field

A method for detecting the presence of a bound magnetic particle comprising:

• providing a substrate on which is formed a planar array of multi-state magnetic field sensors wherein each sensor has two measurably distinguishable low energy stable equilibrium states and an unstable intermediate energy state into which said sensor can be placed and within which said sensor can be maintained by activation of a maintenance mechanism, wherein a transition from said unstable intermediate state to either of said low energy stable equilibrium states can be induced by the presence of a perturbative magnetic field and wherein each sensor is proximate to a site on said substrate at which a magnetic particle can be bound;
• then
• placing and maintaining each of said of magnetic field sensors in said unstable intermediate state;
• and
• prior thereto or subsequent thereto, binding at least one magnetic particle to a site;
• then
• contacting said magnetic particle with a substantially uniform external magnetic field directed substantially perpendicularly to said substrate whereby said magnetic particle is magnetized with a first polarity and produces a strayfield corresponding to said polarity;
• then
• deactivating said maintenance mechanism, whereby said magnetized magnetic particle causes a proximate magnetic field sensor to transition to one of said measurably distinguishable stable equilibrium states in accord with said strayfield while remaining magnetic field sensors relax to a common equilibrium state;
• then
• determining the state of each magnetic field sensor;
• then
• again placing and maintaining each of said of magnetic field sensors in said unstable intermediate state;
• and
• again contacting said magnetic particle with an external magnetic field whereby said magnetic particle is magnetized with a second polarity that is opposite to said first polarity and produces a strayfield corresponding thereto;
• then
• deactivating said maintenance mechanism, whereby said magnetized magnetic particle causes a proximate magnetic field sensor to transition to one of said measurably distinguishable stable equilibrium states in accord with said strayfield while remaining magnetic field sensors relax to said common stable equilibrium state;
• then
• determining the state of each magnetic field sensor;
• whereby,
• only said proximate sensor will have been determined to have transitioned to two different stable equilibrium states, by which can be inferred the presence of said bound magnetic particle.