Hyperfine fields and anisotropy of the orbital moment in epitaxial Mn5Ge3 films studied by 55Mn NMR

55Mn NMR was used to perform the atomic-scale study of the anisotropic properties of Mn5Ge3/Ge(111) epitaxial films with thicknesses between 9 nm and 300 nm. The NMR spectra have been recorded as a function of strong external magnetic field applied in the film plane and perpendicular to it. Two 55Mn NMR resonances have been observed, corresponding to the two manganese sites: 4(d) and 6(g) ) in the hexagonal D88 structure-in zero-field their frequency is centered around 207.5 MHz and 428 MHz, respectively. The anisotropy of 55Mn hyperfine fields between the hexagonal c-direction and the c-plane at both Mn sites was evidenced and attributed to the anisotropic term due to the unquenched Mn orbital momentum. The anisotropy of the orbital contribution to hyperfine fields was determined as 1.52 T in the 4(d) site and up to 2.77 T in the 6(g) site. The 4(d) site reveals a quadrupolar interaction due to the strong electric field gradient (EFG): in this site, which is shown to be oriented along the hexagonal c-axis.