55 Mn NMR was used to perform the atomic-scale study of the anisotropic properties of Mn 5 Ge 3 /Ge(111) epitaxial films with thicknesses between 9 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 55 Mn NMR resonances have been observed, corresponding to the two manganese sites 4d and 6g, in the hexagonal D8 8 structure; in zero field their frequency is centered around 207.5 and 428 MHz, respectively. The anisotropy of 55 Mn 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 4d site and up to 2.77 T in the 6g site. The 4d site reveals a quadrupolar interaction due to the strong electric field gradient: V zz = 5.3 × 10 19 V/m 2 in this site, which is shown to be oriented along the hexagonal c axis.

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55 Mn NMR was used to investigate the effect of carbon doping on the local magnetic anisotropy in Mn 5 Ge 3 epitaxial films (space-group P6 3 /mcm). It was found that carbon enters interstitially in the vicinity of the 6(g) crystallographic positions, occupying the 2(b) octahedral voids. The magnetic properties of the Mn atoms located in the corners of a host octahedron are strongly modified by the presence of carbon. Their magnetic moment is reduced by 0.7μ B with respect to the pristine Mn 5 Ge 3 film and the anisotropy of their orbital moment, measured as a difference between its value along the hexagonal c direction and on the c plane is reduced to 0.058μ B , whereas, in the pristine Mn 5 Ge 3 films, it oscillated every 60 • between 0.151μ B and 0. These changes are responsible for a significant decrease in magnetocrystalline anisotropy, which was reported to drop by an order of magnitude upon doping the Mn 5 Ge 3 films with carbon.

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