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.

Material performance is significantly governed by grain boundaries (GBs), a typical crystal defects inside, which often exhibit unique properties due to the structural and chemical inhomogeneity. Here, it is reported direct atomic scale evidence that oxygen vacancies formed in the GBs can modify the local surface oxygen dynamics in CeO2, a key material for fuel cells. The atomic structures and oxygen vacancy concentrations in individual GBs are obtained by electron microscopy and theoretical calculations at atomic scale. Meanwhile, local GB oxygen reduction reactivity is measured by electrochemical strain microscopy. By combining these techniques, it is demonstrated that the GB electrochemical activities are affected by the oxygen vacancy concentrations, which is, on the other hand, determined by the local structural distortions at the GB core region. These results provide critical understanding of GB properties down to atomic scale, and new perspectives on the development strategies of high performance electrochemical devices for solid oxide fuel cells.

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