Electron spin resonance lineshifts in paramagnetic copper-amino acid complexes

Abstract

The angular variation of the gyromagnetic factor measured by electron spin resonance (ESR) in single crystals of several copper-amino acid complexes (CAC) does not follow the second-order tensorial dependence expected for spins 1/2. Fourth-order contributions with magnitudes of up to delta g=0.01 have been clearly distinguished from the ESR data. CAC have two copper positions per unit cell differing in the orientation of their anisotropic gyromagnetic tensors. The ESR spectra show a single, exchange collapsed line for both sites. Linewidth and magnetic susceptibility measurements indicate exchange interactions 0.2 K > mod J mod >0.6 K, and a low-dimensional magnetic behaviour which is supported by the structural information. The observed anomalous contribution to the gyromagnetic factor is examined in terms of the theory of Kubo and Tomita, and the authors analyse three types of contributions to the shifts. One arises from the difference between the Zeeman interactions for the two sites, and is produced when the non-secular part of this term is modulated by the exchange interaction. The dipole-dipole interaction between copper ions gives rise to two additional contributions. One is due to the non-secular terms, while the other arises because of the low symmetry of CAC, and becomes important in magnetic low-dimensional systems. Only the secular part of this last term is studied, giving a contribution that depends on the orientation of the microwave field. Although the theoretical results obtained within the standard scheme give contributions with the right order of magnitude, the overall agreement with their experimental results is poor, suggesting that a more detailed analysis is required.