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.
Highlights
Manganese-based magnetic compounds are being investigated for two main reasons: (1) they avoid the need for rare earths, (2) they offer a modest-cost/modest performance alternative to the two dominant hard magnet technologies based on Nd2Fe14B (515 kJ m−3) and Ba(Sr)Fe12O19 (45 kJ m−3).[1]
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
We find that fits based on the tetragonal I4/mmm structure (#139, or D022) give an excellent description of the data, and that by imposing constraints for nominal composition and magnetisation we are able to develop a consistent model for both the cell packing and magnetic ordering
Summary
Manganese-based magnetic compounds are being investigated for two main reasons: (1) they avoid the need for rare earths, (2) they offer a modest-cost/modest performance alternative to the two dominant hard magnet technologies based on Nd2Fe14B (515 kJ m−3) and Ba(Sr)Fe12O19 (45 kJ m−3).[1]. In going from P4/mmm to I4/mmm we pick up an inconvenient selection rule (l = 2n) which is made worse by the occupation of the 4d site in the I4/mmm structure that imposes a further constraint on l: l = 2n, ∀ h, k The result of these constraints is that it is not possible to distinguish the P4/mmm and I4/mmm structures on the basis of any diffraction data, the two structures are formally identical: Any packing of a MnxGa P4/mmm cell can be mapped directly onto an I4/mmm cell yielding the same (doubled cell) crystal structure and precisely the same diffraction pattern. For convenience we have used the I4/mmm cell here, but identical results can be obtained within the P4/mmm cell
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