Crystal structure and magnetism of the MnxGa (1.15 ≤ x ≤ 2.0) rare-earth-free permanent magnet system

Abstract

Room temperature neutron powder diffraction has been used to investigate the chemical structure and magnetic ordering of a series of tetragonal (I4/mmm #139) MnxGa (1.15 ≤ x ≤ 2.0) alloys. Initially (x < 1.5) the excess Mn goes on the 2b site with vacancies appearing at the 2a site. For x > 1.5 Mn also appears on the 2a site. The manganese atoms on the 4d site carry an almost constant moment of 2.16(6) μB/Mn. The loss of magnetisation seen with increasing Mn content is shown to be the result of large (∼3"μB/Mn), antiparallel Mn moments on the 2b, and later 2a sites, and not to a reduction of the Mn moment on the 4d sites.