  Using powder neutron diffraction we have discovered an unusual magnetic
order-order transition in the Ising spin chain compound Ca3Co2O6. On lowering
the temperature an antiferromagnetic phase with propagation vector
k=(0.5,-0.5,1) emerges from a higher temperature spin density wave structure
with k=(0, 0, 1.01). This transition occurs over an unprecedented time-scale of
several hours and is never complete.
