Weak canted-spin ferromagnetism

Fdd2 fi form (Banister et al. 1996). The result is that in the a structure neighbouring chains are antiparallel, while in they are all parallel. Due to these symmetry considerations the a modification cannot exhibit a so-called spin-canting mechanism (Carlin 1989) which can account for the transition from a low-dimensional antifer-romagnet to a weakly ferromagnetic state. However, these restrictions are absent in the P modification, and this mechanism, based to a large extent on the structural distinction between the two polymorphs, has been used to account for the fact that this p form is the first example of an open-shell molecule to exhibit spontaneous magnetization above liquid helium temperature (36 K). 

A completely separate phenomenon that happens also to occur in this compound is canted antiferromagnetism (also known as weak ferromagnetism). In the nominally antiferromagnetic state (i.e. below the critical field), anisotropy creates a situation in which antiparallel alignment of the spins is imperfect. Because the spins are vector quantities, a small nonzero angle between them results in incomplete cancellation (Figure 3.24b). This gives rise to a net moment that, in the case of [FeCp 2][DCNQ], is manifest as a tiny hysteresis loop centred about zero. 

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