A magnetic sensor for identifying small magnetic particles bound to a substrate includes a regular, planar orthogonal array of MTJ cells formed within or beneath that substrate. Each MTJ cell has a high aspect ratio and positions of stable magnetic equilibrium along an easy magnetic axis and positions of unstable magnetic equilibrium along a hard magnetic axis. By initializing the magnetizations of each MTJ cell in its unstable hard-axis position, the presence of even a small magnetic particle can exert a sufficient perturbative strayfield to tip the magnetization to its stable position. The magnetization change in an MTJ cell can be measured after each of two successive opposite polarity magnetizations of a bound particle and the presence of the particle thereby detected.