Half-metallic spin polarized electron states in the chimney-ladder higher manganese silicides MnSi1-x (x=1.75-1.73) with silicon vacancies

The electronic and optical properties of the higher manganese silicides, known as the Nowotny "chimney-ladder" phases, are investigated by means of first-principles density-functional calculations and compared with the experimental results. According to our calculations, Mn4Si7 is an intrinsic semiconductor and a nonmagnetic material, while Mn11Si19 and Mn15Si26 are metals and half-metallic weak ferromagnets, in agreement with recent experimental findings. The role of intrinsic lattice defects, namely vacancies, common for such open crystal structures, is clarified. The vacancies in Mn4Si7 lead to a metallization of the electronic spectrum and to a magnetic state of half-metallic nature. In Mn11Si19 and Mn15Si26 an effect of vacancies is that the average magnetic moment per Mn atom is almost four times larger than that in stoichiometric higher manganese silicides. The origin of this magnetic moment enhancement is explained by the variation of the Mn coordination numbers. The calculated optical spectra are in good agreement with available experimental data. We find a relatively high (1.2 degrees) polar magneto-optical Kerr effect in Mn11Si19 and Mn15Si26 in the infrared region 0.1-0.3 eV. It is shown that such magneto-optical peaks are formed by the low energy plasmons.