Second- to first-order transition in two coupled antiferromagnetic rings

We numerically investigate an S=1/2 spin model, in which two dimerized antiferromagnetic rings interact with each other ferromagnetically. It is shown that the order of the magnetoelastic transition is strongly affected by the interring coupling J<Subscript>⊥</Subscript> and there may exist a critical J<Subscript>⊥</Subscript> <Superscript>*</Superscript> dividing the first-order transition and the continuous transition. Copyright EDP Sciences/Società Italiana di Fisica/Springer-Verlag 2006