Specific heat and susceptibility of the 1-dimensional [formula omitted] Heisenberg antiferromagnet Cu(Pyrazine) (NO 3) 2. evidence for random exchange effects at low temperatures

Abstract

Magnetic susceptibility and specific heat data are presented on the S = 1 2 antiferromagnetic Heisenberg chain Cu(pyrazine) (NO 3) 2 in the range 0.05 K < T < 18 K. The intrachain exchange is obtained as J/ k B = -5.20(5) K by a theoretical fit to data. No evidence for a transition to a 3-d ordered state could be detected down to the lowest temperatures (corresponding to k B T/| J| ≈ 0.01). The specific heat data provide a beautiful demonstration of the predicted linear chain low-temperature behaviour, c/ R = k B T/3| J|, over one decade in temperature. Superimposed upon the behaviour for the uniform antiferromagnetic chain, the data show contributions that can be interpreted in terms of a random exchange model. It is argued that these arise from random lattice imperfections (defects) which apparently break up the chains into weakly coupled finite segments with an average length of about 200 lattice sites. The segments with an odd number of spins act as a system of highly diluted, weakly interacting, spins 1 2 . The contributions due to defects may predominate the behaviour of the susceptibility and (to a lesser extent) the specific heat at very low temperatures, in case the interchain interactions are sufficiently small. In particular, the random defects will suppress the occurence of 3-d ordering between the chains. Furthermore, they lead to random exchange phenomena similar as observed in other quasi 1-d antiferromagnetic systems. It is suggested that many of the known examples of random exchange antiferromagnets can be explained in terms of the random defect model.