Hyperfine fields and anisotropy of the orbital moment in epitaxial Mn5Ge3 films studied by Mn55 NMR

Abstract

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.

Selective modification of the unquenched orbital moment of manganese introduced by carbon dopant in epitaxial Mn5Ge3C0.2/Ge(111)Kalvig1, Jędryka2, Wójcik3 et al. 2020Phys. Rev. BSelf Cite9040View full textAdd to dashboardCite

“…For the field values above the saturation field, the spectrum shifts uniformly towards lower frequencies with a negative slope corresponding to the 55 Mn gyromagnetic ratio whereas the shape of the NMR line remains unchanged. This behavior is analogous to that observed in the pristine sample [21]. The negative slope after saturation means that the dominant contribution to the Mn hyperfine field is due to the Fermi contact term Eq.…”

“…4, the application of an in-plane magnetic field leads to a rapid modification of the NMR lines from both types of Mn II environments. In the case of the Mn II sites that maintained their original nearest neighbors, the field-induced changes to the NMR spectrum resemble those reported for the Mn II NMR line in pristine Mn 5 Ge 3 [21]. A single NMR line observed around 420 MHz in the zero-field spectrum, broadens up in the presence of the in-plane field and develops a doublet structure.…”

“…Our previous study of a pristine Mn 5 Ge 3 film has unambiguously shown that the zero-field 55 Mn NMR signal in the 300-nm film originates from the magnetic domains with magnetization oriented perpendicular to the film plane, i.e., along the hexagonal c axis [21]. In order to verify whether this remains true also in the case of the C-doped sample, we performed the NMR experiment in the presence of an external magnetic field applied perpendicular to the film plane.…”

“…This procedure-called the Panissod protocol-makes it possible to account for the frequency variation of the enhancement factor and correct the NMR signal intensity [20]. [21] where it was shown that the two NMR lines observed in distinctly different frequency ranges represent manganese in the two nonequivalent sites in the crystal lattice: Mn located in the 4(d ) sites (later denoted as Mn I ) gives rise to the quadrupole-split line centered at the frequency of 207.5 MHz, whereas the 6(g) sites (denoted as Mn II ) contribute to the single NMR line centered at 428 MHz [21].…”

“…To describe analytically the NMR frequency ν as a function of the azimuthal angle, we derived the formula (8), using the procedure described in Ref. [21]. For the case of a sample that is magnetically saturated in plane (i.e., the polar angle is θ = 0 • , and the azimuthal angle φ is fixed by the orientation of the external magnetic field), one obtains the following expression:…”