Magnetic anisotropy of Co/h-BN heterostructures: First principles calculations

Magnetic anisotropy (MA) is an important characteristic of magnetic films for the application of spintronics devices such as magnetoresistive random access memories (MRAMs) and magnetic field sensors. Perpendicular MA is more suitable for MRAM, while in-plane MA is better for magnetic field sensor applications. Hexagonal boron nitride (h-BN) has recently attracted much attention as an efficient tunnel barrier for magnetic tunnel junctions with hexagonal symmetry. In this study, the magnetic anisotropy of a Co thin film deposited on h-BN is theoretically investigated using first-principles calculations. It is observed that a substantial perpendicular magnetocrystalline anisotropy (MCA) of the order of 1 mJ/m² is achieved for the film comprising 1 $\sim$ 5 Co atomic layers. The buckled structure of the h-BN layer at the interface is found to enhance the perpendicular MCA. The magnetic dipole energy is also calculated and is found to cause the in-plane MA for the films comprising 2 $\sim$ 5 Co atomic layers. The film comprising a monoatomic Co layer is found to be perpendicularly magnetised with an anisotropy constant of 1.68 mJ/m² and a voltage-controlled MA coefficient of -175 fJ/Vm, which are of the same order as those for the Fe/MgO film. In the case of in-plane magnetised films comprising 2 $\sim$ 5 Co atomic layers, the MA energy is found to be independent of the in-plane angle, which is a preferable characteristic for sensor applications. The results provide fundamental insights into the MA of Co/h-BN heterostructures and a foundation for developing voltage-controlled MRAM and magnetic field sensors based on this material.