The exchange interactions and magnetic behaviour of Cu(L-alanine)2: specific heat measurements

Abstract

We report specific heat measurements in the temperature range 0.05 K<or=T<or=1.95 K for the copper-amino acid complex Cu(L-alanine)2. The data display a peak at 0.068 K, and a broad maximum at 0.55 K. Above 0.2 K they reflect a one-dimensional antiferromagnetic spin-chain behaviour. The temperature dependence of the specific heat is compared to that calculated for Heisenberg finite spin chains in order to evaluate the magnitude and characteristics of the exchange interaction. We obtain an antiferromagnetic exchange coupling parameter J0/k=(-0.57+or-0.01) K between nearest-neighbour copper ions. The peak observed at 0.068 K indicates a transition to a phase with three-dimensional magnetic order. The effect of this phase transition on the specific heat measured at higher temperatures is analysed. The experimental results, together with existing magnetic susceptibility and EPR data, are discussed considering the crystal structure and the superexchange paths connecting the copper ions in Cu(L-alanine)2. The nature of the spin chains and the possibility of competing exchange interactions between first and second copper neighbours along the chains are analysed in order to model the magnetic behaviour of Cu(L-alanine)2.