Antiferromagnetic order and the structural order–disorder transition in the Cd6Ho quasicrystal approximant

Abstract

It has generally been accepted that the orientational ordering of the Cd₄ tetrahedron within the Cd₆ R quasicrystal approximants is kinetically inhibited for R = Ho, Er, Tm and Lu by steric constraints. Our high-resolution X-ray scattering measurements of the Cd₆Ho quasicrystal approximant, however, reveal an abrupt (first-order) transition to a monoclinic structure below T _S = 178 K for samples that have ‘aged’ at room temperature for approximately one year, reopening this question. Using X-ray resonant magnetic scattering at the Ho L ₃-edge we have elucidated the nature of the antiferromagnetic ordering below T _N = 8.5 K in Cd₆Ho. The magnetic Bragg peaks are found at the charge forbidden H + K + L = 2n + 1 positions, referenced to the high-temperature body-centred cubic structure. In general terms, this corresponds to antiferromagnetic arrangements of the Ho moments on adjacent clusters in the unit cell as previously found for Cd₆Tb.