MAGNETIC AND CRYSTALLOGRAPHIC ORDER IN ALPHA-MANGANESE

We have made time-of-flight neutron diffraction measurements on alpha- manganese metal. Powder diffraction measurements were made at 14 tempe ratures between 15 and 305 K, and single crystal measurements were mad e at 15 and 300 K. We found that the crystal structure of alpha-Mn is tetragonal below its Neel point of 100 K, with crystal symmetry $($) o ver bar$$ I42m and magnetic (Shubnikov) symmetry P-l ($) over bar 42(l )c. In agreement with earlier results of Yamada et al., there are six independent magnetic atoms, and we found that their moments are weakly temperature dependent. The onset of magnetic order causes slight chan ges in the atomic positions and in the average atomic elastic constant .