Ordered occupancy of interstitial voids in Mn5Ge3 lattice by the carbon dopant

Abstract

Epitaxial films of Mn5Ge3/Ge are interesting as a potential source of highly spin polarized electrons directly into Ge. Moreover, the high out-of-plane anisotropy of Mn5Ge3 opens up prospect for combining spintronics with data storage. The ferromagnetic Mn5Ge3 compound crystallizes in the hexagonal D88 structure; Mn atoms occupy two magnetically and structurally nonequivalent positions: 4d (MnI) and 6g (MnII). The Curie temperature of 296 K can be increased up to 445 K by doping with carbon [1]. Recent 55Mn NMR studies have shown that the magnetocrystalline anisotropy in Mn5Ge3 originates from the unquenched orbital moment of Mn in both crystallographic positions [2]. Furthermore, the 55Mn NMR study in Mn5Ge3C0.2/Ge(111) film has shown that carbon enters interstitially occupying the 2(b) octahedral voids and reduces the magnetic moment of the MnII atoms in the corners of a host octahedron [3]. To achieve further understanding of the role that carbon plays in this system, we carried out a systematic FMR and 55Mn NMR study on a series of epitaxial 30 nm thick Mn5Ge3Cx/Ge(111) films with the nominal carbon content x varying between 0 and 0.8. The FMR experiment shows that the uniaxial anisotropy constant Ku measured at 4.2 K systematically drops from 5,69x106 erg/cm3 to 8,06x105 erg/cm3 for the nominal concentration of x ≈ 0.6. The 55Mn NMR shows that in the concentration range up to x=0.5 the MnII atoms are found in two distinctly different magnetic states, identified as MnII (no carbon nearest neighbor) and MnII_C (one carbon nn). With increasing carbon concentration the NMR signal intensity is systematically transferred from MnII to MnII_C, and at x=0.5 no MnII NMR signal is observed anymore. On the other hand, no second satellite line is observed in the vicinity of the MnII signal in the entire concentration range, even though there are two available 2(b) positions around the MnII atoms. These two observations implicate that carbon does not enter statistically in the 2(b) voids, but shows a strong preference towards ordering, occupying every second 2(b) void along the hexagonal c-direction. This sets the limit for the uptake of the carbon atoms in the Mn5Ge3 lattice to x=0.5.