A Nonvolatile Spintronic Memory Element with a Continuum of Resistance States

A continuum of stable remanent resistance states is reported in perpendicularly magnetized pseudo spin valves with a graded anisotropy free layer. The resistance states can be systematically set by an externally applied magnetic field. The gradual reversal of the free layer with applied field and the field-independent fixed layer leads to a range of stable and reproducible remanent resistance values, as determined by the giant magnetoresistance of the device. An analysis of first-order reversal curves combined with magnetic force microscopy shows that the origin of the effect is the field-dependent population of up and down domains in the free layer.