Magnetic properties of Cu(L-aspartato)(H2O)2:: A linear chain antiferromagnet

Specific heat, magnetic susceptibility, and magnetization measurements were performed in polycrystalline samples of the copper complex of the amino acid L-aspartic acid [called Cu(L-asp)]. The specific heat was measured between 3 and 45 K, while de and ac susceptibilities were measured between 1.8 and 300 K. The magnetization data were obtained as a function of an applied field up to 9 T, at various fixed temperatures between 2 and 10 K. The specific heat and magnetic susceptibility curves show peaks at 3.95 K and 6.9 R, respectively, and approach zero at lower and higher temperatures. This behavior cannot be accounted for by considering only the superexchange paths provided by the sigma skeleton of aspartic acid. If this were the case, a uniform J(0) along the asp-Cu-asp-Cu-asp chains would be expected. The data were well fitted, proposing for Cu(L-asp) a behavior characteristic of an alternating linear chain antiferromagnet. Each S = 1/2 copper ion is coupled with an isotropic exchange interaction J(0)/k = (-5.3 +/- 0.1) K with one copper neighbor in the chain, and alpha J(0)/k = (-1.5 +/- 0.1) K with the other [alpha = (0.29 +/- 0.02)]. The average g value is = 2.165 +/- 0.002. Considering the alternating chain model, the susceptibility and magnetization ata suggest also the presence of an additional interchain ferromagnetic coupling, which (within a mean-field approximation) leads to J'/k = (0.7 +/- 0.1) K. Other exchange paths are proposed to explain the data. An important finding is that hydrogen bonds may support relatively large values of J.