Topological ferrimagnetism and superparamagnetic-like behaviour in a disordered homometallic coordination network.

Crystals of a cobalt(II) 3-amino-1,2,4-triazolate; Co(2)Cl(C(2)N(4)H(3))(3), 1 can be formed from a hydrothermal synthesis. X-Ray crystallography shows an extended 3-dimensional network structure within a hexagonal space group with a=9.9655(7) and c=7.7523(7) A. The data reveal an orientational disorder to the ligand. Structural considerations suggest that the ligand orientation is strongly correlated in 1-dimension, but the nature and length scale of the 2-dimensional order is not obvious. The structure is discussed in terms of three key structural models, two of which are crystallographically ordered, while the third is disordered in 2-dimensions. The effect of ligand orientation on the network topology has profound effects on the expected magnetic behaviour, with the two fully ordered models having either antiferromagnetic or ferrimagnetically ordered ground states. The magnetism of 1 is complex. It shows an antiferromagnetically coupled paramagnetic phase above approximately 5.3 K. Below approximately 4.5 K we see a ferrimagnetically ordered state (H(coer)=50 Oe, M(rem)=1830 Oe cm(3) mol(-1), M(sat) = 3450 Oe cm(3) mol(-1) at 2 K). In the intermediate temperature range 4.5 to 5.3 K we see unusual behaviour with evidence of rapid, low-energy relaxation of the magnetisation. The unusual relationship between the structure and the magnetism of 1 is discussed in detail.