Hyperfine fields and anisotropy of the orbital moment in epitaxial Mn_5Ge_3 films studied by ~(55)Mn NMR

摘要

~(55)Mn NMR was used to perform the atomic-scale study of the anisotropic properties of Mn_5Ge_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 Ad site and up to 2.77 T in the 6g site. The Ad 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.