Concentric 360° domain wall nesting in magnetic tunnel junction films: a Lorentz TEM study

Abstract

We describe the formation of an unusual concentric magnetic domain wall pattern in the free layer of a bottom pinned magnetic tunnel junction. Lorentz microscopy reveals that repeated switching of the free layer with a magnetic field applied perpendicular to the exchange bias direction can produce a series of concentric 360 degrees domain wall loops, a phenomenon we refer to as domain wall nesting. We propose two necessary ingredients for the behaviour: (i) inhomogeneities in the grain-by-grain magnetic dispersion that break local symmetry to produce a preferential sense of magnetic rotation upon field switching; and (ii) structural defects that act to pin 360 degrees domain walls. Further control of this behaviour may provide new functionality for future device applications.