Alternating antiferromagnetic and ferromagnetic exchange interactions in the S = 1 Heisenberg chain. Theory and magnetic properties

We focus on the magnetic properties of the S = 1 Heisenberg chain with alternating antiferromagnetic and ferromagnetic exchange interactions J1 and J2. The magnetic behavior of this system is calculated as a function of the alternation parameter α = J2/|J1|, from a general numerical procedure based on closed spin chains of increasing length. These theoretical results are fitted to rational unified expressions, which are subsequently used to describe the magnetic behavior of a nickel (II) complex, [Ni(bipy)(N3)2]n, exhibiting an alternating chain structure with a dominant ferromagnetic exchange.