term and therefore lacks an orbital contribution to its magnetic
moment and is also much less likely to undergo a significant
degree of canting.
Application of the Curie–Weiss law to the high temperature
( > 100 K) data gives a remarkably large value of the Weiss
constant (q = 2585(3) K) which is consistent with the low
value of m at room temperature compared to the spin only value
of 3.87 mB. This is indicative of strong antiferromagnetic
coupling as the lack of first-order angular momentum (vide
supra) for the singlet ground term rules out the possibility that
such a large deviation is due to single ion anisotropy.
Comparison with typical values we have observed for q ≈ 250
K in Co–O–Co bridged compounds with similar bridging angles
shows that this strong interaction must be mediated by sulfur
bridges. The greater covalency of Co–S bond vs. the Co–O bond
leads to greater delocalisation of spin density and therefore
stronger superexchange.10 Since the Co–S superexchange
lattice is comprised of isolated chains we should expect to see a
broad maximum in the susceptibility vs. temperature graph
coinciding with short-range ordering. Mean Field Theory is not
a particularly good model for low dimensional systems but can
give us a qualitative estimate of the exchange coupling; when it
is combined with an expression11 for the temperature at which
the susceptibility maximum should occur we obtain the result
Tmax = 1.9¡q¡. The susceptibility maximum will therefore occur
at a much higher temperature than could be measured.
We wish to thank EPSRC and the EU ERASMUS program
for funding.
Fig. 3 The one-atom superexchange pathways within a single layer.
range ordering (Fig. 4). The magnetisation does not begin to
saturate before 5 K but it is clear from its small magnitude that
this is not a simple ferromagnetic state. As there is only one
cobalt environment we may eliminate ferrimagnetism as a
possibility and the source of the spontaneous magnetisation
must therefore be a small degree of canting in an essentially
antiferromagnetic system. This was confirmed by the variation
of magnetisation with temperature for the ordered phase in
different applied fields. The sample magnetisation is higher in
lower measuring fields, which is characteristic of spin canting.
Hysteresis measurements (Fig. 5) show a small coercive field of
45 G and that the sample magnetisation is not saturated at 5 T.
The ferromagnetic interlayer interaction is a dipolar one across
a relatively large gap of approximately 13 Å and is therefore
small and only becomes significant at low temperatures. Canted
antiferromagnetism is usually due to single ion anisotropy
therefore high spin, octahedral Co(II) with large spin–orbit
coupling due to its 4T1g ground term is a good candidate for this
effect. Trigonal bipyramidal high spin Co(II) has a 4A1 ground
Notes and references
†
Crystal data: C7H4O2SCo, M = 211.09, monoclinic, space group C2/c,
a = 27.890(2), b = 7.6926(6), c = 6.7497(5) Å , b = 103.895(2)°, V =
1405.8(2) Å3, Z = 8, Dc = 1.995 g cm23, m = 2.669 mm21, l = 0.6904
Å (Daresbury Station 9.8), crystal dimensions 0.12 3 0.10 3 0.01 mm.
Total number of observed [F2 > 2s(F2)] and independent reflections 1865,
1505 (Rint = 0.0217). Full-matrix least-squares on F2 gives R1 = 0.0385
and wR2 = 0.0974.
b1/b111339a/ for crystallographic data in CIF or other electronic format.
‡ Magnetisation measurements: Data were recorded on a Quantum design
MPMS-XL magnetometer. Temperature scans were recorded between 4.5
and 300 K using a cooling and measuring field of 500 G. Magnetisation
hysteresis was studied between 5 T at 5 K. Sample diamagnetism was
corrected for by employing Pascal’s constants.10
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Fig. 4 The thermal evolution of the magnetic moment.
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Fig. 5 Expansion of the magnetic hysteresis at 4.5 K in the low field region.
Insert: field sweep between 5 T.
CHEM. COMMUN., 2002, 1050–1051
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