THE EXCHANGE INTERACTIONS AND MAGNETIC-BEHAVIOR OF CU(L-ALANINE)(2) -SPECIFIC-HEAT MEASUREMENTS

We report specific heat measurements in the temperature range 0.05 K l ess than or equal to T less than or equal to 1.95 K for the copper-ami no 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-dimensio nal antiferromagnetic spin-chain behaviour. The temperature dependence of the specific heat is compared to that calculated for Heisenberg fi nite spin chains in order to evaluate the magnitude and characteristic s of the exchange interaction. We obtain an antiferromagnetic exchange coupling parameter J(0)/k = (-0.57+/-0.01) K between nearest-neighbou r copper ions. The peak observed at 0.068 K indicates a transition to a phase with three-dimensional magnetic order. The effect of this phas e transition on the specific heat measured at higher temperatures is a nalysed. The experimental results, together with existing magnetic sus ceptibility and EPR data, are discussed considering the crystal struct ure and the superexchange paths connecting the copper ions in Cu(L-ala nine)(2). The nature of the spin chains and the possibility of competi ng exchange interactions between first and second copper neighbours al ong the chains are analysed in order to model the magnetic behaviour o f Cu(L-alanine)(2).