Realization of Room-Temperature Ferromagnetic NiO Flakes with Perpendicular Anisotropy via Morphology Control.

The regulation of perpendicular magnetic anisotropy (PMA) is key to the development of spintronic devices. As a traditional antiferromagnetic insulator with in-plane spin orientation, NiO films have been widely used for exchange bias and field-free spin-orbit torque (SOT) switching applications. However, the growth of a single-crystalline NiO film with PMA remains a big challenge. In this study, high-quality NiO (111) thin flakes with room-temperature ferromagnetism were achieved by chemical vapor deposition. The growth of the thin (thickness: ∼4.0 nm) flakes highly depends on temperature; as temperature increases, homostructures of nanoparticles or nanopyramids on flakes would form due to the enhanced diffusion rate and the requirement to reduce the strain energy. The flakes and homostructures show different magnetic performance, of which the pure flakes exhibit ferromagnetic behavior with a magnetic easy axis of off-plane direction, while the perpendicular anisotropy and magnetic moment attenuate for the flake-particle homostructure, and finally turn to antiferromagnetic order for flake-pyramids. Density functional theory (DFT) calculations further confirm the oxygen vacancy as the main source of the net magnetic moment. This work provides a facile method to prepare high-quality NiO flakes and homostructures and supplies a regulation strategy for the magnetic anisotropy of NiO, which may open novel potential applications in the field of magnon spintronics.