A Series of Weak Ferromagnets Based on a Chromium–Acetylide–TTF Type Complex: Correlation of the Structures and Magnetic Properties and Origin of the Weak Ferromagnetism

The
crystal structures and magnetic properties of a series of new
weak ferromagnets containing a chromium–acetylide–tetrathiafulvalene
(TTF) type complex, [CrCyclam­(CC-5-methyl-4′5′-ethylenedithio-TTF)2]2+ ([1]2+), were investigated.
The six new isostructural weak ferromagnets [1]­[BF4]2(PhF)2(MeCN), [1]­[ClO4]2(PhF)2(MeCN), [1]­[ReO4]2(PhCl)2(MeCN), [1]­[ClO4]2(PhBr)3, [1]­[ReO4]2(PhBr)3, and [1]­[ClO4]2(PhI)3 contain ferrimagnetic chain
structures of [1]2+∞ with
different interchain distances that are dependent on the sizes of
the anions and solvent molecules. Magnetic measurements of the salts
revealed that the weak ferromagnetic transition temperature gradually
increases from 14.5 to 26.0 K as the interchain distance decreases
from 3.997(2) to 3.803(2) Å, while the remanent magnetization
at 2 K decreases from 0.0215 to 0.0079 μB. The observed
magnetic properties and crystal structures suggest that the weak ferromagnetism
originates from the single-ion anisotropy of [1]2+, where a stronger interchain antiferromagnetic interaction
not only causes a higher transition temperature but also suppresses
the noncollinear canted spin alignment.