Spin diffusion in the quasi-two-dimensional magnetic system bis[1,2-bis(2-methoxyethoxy)ethane] sodium biphenylide

Abstract

The exchange narrowed E.S.R. line of single crystals of the quasi-two-dimensional magnetic system bis[1,2-bis(2-methoxyethoxy)ethane] sodium biphenylide (NaBp . 2Tg) is studied at 9·4 GHz as a function of temperature from 295 to 1·2 K. At 77 K the linewidth has the characteristic (3 cos² ϑ-1)² dependence and the resonance line is non-lorentzian except for the magic angle ϑ=cos^-1 (1/✓3). These facts are interpreted in terms of spin diffusion in this two-dimensional magnetic system. At 1·2 K the two-dimensional anomalies are lost and the linewidth and lineshape behave as expected for a three-dimensional magnetic system. The temperature dependence of the linewidth and the linewidth anisotropy indicates that at 1·5 K a crossover takes place from two-dimensional to three-dimensional behaviour. Susceptibility measurements from 270-1·1 K point to a ferromagnetic coupling at high temperatures and an antiferromagnetic coupling at low temperatures. From magnetization measurements it could be concluded that at about 1·1 K the spins in the magnetic planes are antiferromagnetically ordered.