The low-temperature antiferromagnetic structure of Mn5Si3revised in the light of neutron polarimetry

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Abstract
The magnetic structures previously proposed for the low-temperature antiferromagnetic phase (AF1) of the intermetallic compound Mn5Si3 have been revised in the light of the results of neutron polarimetry in conjunction with unpolarized-neutron single-crystal integrated-intensity measurements. At 4.2 K the commensurate magnetic unit cell is orthorhombic with a=6.889, b=11.901 and c=4.805 AA and it is related to the hexagonal cell of the paramagnetic phase by b approximately= square root 3a. The hexagonal cell contains four Mn1, six Mn2 and six Si atoms. The non-collinear magnetic structure of the AF1 phase is stable below 66 K and has monoclinic symmetry. At 4.2 K the Mn1 atoms have moments of 1.20(5) mu B inclined parallel and antiparallel to the polar direction theta =116(1) degrees , phi =105(1) degrees , where theta is measured from (001) and phi from (010). One third of the Mn2 atoms carry no moment, one third have moments of 2.30(9) mu B at +or-( theta =70(1) degrees , phi =93(1) degrees ) and the remainder have 1.85(9) mu B at +or-( theta =21(1) degrees , phi =11(7) degrees ). The non-collinearity is attributed to topological frustration and the wide variation in the magnetic moments to the combined effects of Mn moment instability, frustration and single-ion anisotropy.

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