High-temperature dipolar local field correlations in two-dimensional exchange-coupled systems

Abstract

High-temperature local field correlations are needed to calculate the angular variation of the electron spin resonance (ESR) linewidth of exchange-coupled systems in the paramagnetic phase. In low-dimensional magnetic systems, these time correlations are usually calculated by appealing to arbitrary intermediate functions to interpolate between short- and long-time regimes. In this work, the authors calculate the correlation function of the dipolar field in a two-dimensional (2D) square lattice of spins 1/2, for several orientations of an external magnetic field. Arbitrary interpolation functions are avoided by assuming that the normal modes of the spin correlations behave diffusively as soon as quantum coherence is broken. The effects of interlayer exchange in an actual system are considered by introducing a cut-off in the 2D diffusion. The angular variation of the resulting ESR linewidth is analysed as a function of the 'quantum coherence time' tau 0 and the cut-off time tau c. Comparison with experimental data in layered Cu-amino acid crystals allows the authors to estimate tau 0<or=0.5h(cross)/J and tau c approximately=200h(cross)/J, where J is the in-layer exchange.