Pretransitional short-range ordering in a triangular lattice of Ising spin chains

Sr2Ca2CoMn2O9 is made of Ising spin chains distributed on a triangular lattice, with antiferromagnetic coupling between them. In spite of a unique interchain distance, this compound exhibits two peculiarities making it deviate from the textbook case of geometrical frustration expected in ideal triangular lattices: first, there are three subclasses of chains shifted from each other along the chain direction; second, these chains fall into two categories which differ by some of the intrachain distances between spins. These features are found to confer a character of centered honeycomb lattice on this system of spin chains. Experimental investigation of the magnetism of Sr2Ca2CoMn2O9 reveals a peculiar pretransitional regime, which is manifested by a frequency-dependent peak of susceptibility. We put this behavior in relation with the formation of short-range one-dimensional ferromagneticlike segments taking place a few degrees above the TN. This phenomenon is itself regarded as a precursor effect of the long-range ordering, induced by the particular topology of the spin system.