Non-spherical magnetic moment in MnAlGe

Abstract

The magnetic structure factors of MnAlGe (space group P4/nmm) measured with polarised neutrons have been expressed in terms of the magnetic moment of the Mn atom (site symmetry tetrahedral with tetragonal distortion), the Bessel transforms 〈j_n〉 of the Mn radial functions and the fractional occupancies of the moment density in the various crystal field orbitals. The measured structure factors were least-squares fitted with the theoretical expression involving 〈j_n〉 appropriate to the Mn⁰, Mn⁺ and Mn²⁺ atoms. The best fit was got using Mn⁰ transforms, yielding 1.45μ_B as the Mn magnetic moment. The fractional occupancies of the moment density in the crystal field orbitals A_1g, B_1g E_g and B_2g were obtained. This analysis shows the magnetic moment to be highly non-spherical with a large fractional occupancy (38%) in the A_1g orbital directed along the tetragonal axis while the fractional occupancies of B_1g and B_2g are found to be 31% and 30% respectively. The fractional occupancy of the moment in the E_g orbital directed towards the Ge and Al atoms is very low (1%). The spatially averaged moment density of Mn in MnAlGe is more diffuse than that of Mn I and Mn II in isostructural Mn₂Sb.