THE LOW-TEMPERATURE ANTIFERROMAGNETIC STRUCTURE OF MN5SI3 REVISED IN THE LIGHT OF NEUTRON POLARIMETRY

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 angstrom and it is related to the hexagonal cell of the paramagnetic phase by b almost-equal-to 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 +/-[theta = 70(1)-degrees, phi = 93(1)-degrees] and the remainder have 1.85(9)mu(B) at +/-[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.