Ferromagnetic dislocations in antiferromagnetic NiO

Crystal lattice defects often degrade device functionality, but engineering these defects may have value in future electronic and magnetic device applications. For example, dislocations--one-dimensional lattice defects with locally distinct atomic-scale structures--exhibit unique and localized electrical properties and can be used as a template for producing conducting nanowires in insulating crystals. It has also been predicted that spin-polarized current may flow along dislocations in topological insulators. Although it is expected that the magnetic properties of dislocations will differ from those of the lattice, their fundamental characterization at the individual level has received little attention. Here, we demonstrate that dislocations in NiO crystals show unique magnetic properties. Magnetic force microscopy imaging clearly reveals ferromagnetic ordering of individual dislocations in antiferromagnetic NiO, originating from the local non-stoichiometry of the dislocation cores. The ferromagnetic dislocations have high coercivity due to their strong interaction with the surrounding antiferromagnetic bulk phase. Although it has already been reported that nanocrystals of rock-salt NiO show ferromagnetic behaviour, our study characterizes the ferromagnetic properties of individual lattice defects. We discuss the origin of the unexpected ferromagnetism in terms of the physical properties of the atomic-scale core structures of single dislocations, and demonstrate that it is possible to fabricate stable nanoscale magnetic elements inside crystalline environments composed of these microstructures.