Electric field gradient and magnetic hyperfine field in the bulk and surfaces of α-MnAsα-MnAs compound

We use the highly accurate all-electron full potential (linearized) augmented plane-wave plus local orbital method in the framework of the density functional theory to study the electronic and magnetic properties, electric field gradients, and magnetic hyperfine fields in the bulk and free (0 0 0 1) surfaces of the hexagonal MnAs. In the bulk calculations, we use the local spin density (LSDA) and the generalized gradient approximations (GGA) to the exchange-correlation functional without and with the Hubbard U correction while surface calculations are performed by using the GGA and GGA+U. We perform a convenient volume and c/a optimization to obtain the ground state properties of the hexagonal MnAs. A supercell approach is applied to calculate the effective Hubbard U parameter for the Mn 3d electrons in bulk MnAs within both LSDA and GGA. We argue that the behavior of the electric field gradient at the site of Mn is mainly controlled by the Mn 3d electrons. The effects of the hydrostatic pressure on the magnetic, hyperfine field, and electric field gradient properties are calculated and discussed in terms of the orbital hybridizations and asymmetry factors. The As terminated surface is shown to be the stable ideal termination of the hexagonal MnAs in the (0 0 0 1) direction. The surface effects on the atomic magnetic moments, electric field gradients, and hyperfine fields, in the absence and presence of the relativistic spin–orbit correction, are calculated and discussed by considering the surface behavior of the orbital hybridizations and asymmetry factors.