Antisymmetric and anisotropic exchange in ferromagnetic copper(II) layers

The theory of exchange narrowing is extended in systems with large isotropic J₀ to include static spin correlations and antisymmetric exchange. Both contribute to the unusual temperature-dependent, exchange-narrowed EPR linewidth ΔH(T) in CuX₄^_^2- (X=Cl,Br) salts based on ferromagnetic layers of s=1/2 copper(II) sites. Single-crystal EPR data are presented for Pt(NH₃)₄CuCl₄ and for 1-3-propanediammonium-CuCl₄ as a function of temperature and crystal orientation. In both salts, ΔH(T) is shown to be dominated by corrections to J₀, with the antisymmetric exchange larger than the symmetric anisotropic term. Single-crystal EPR provides a novel general approach for finding the magnitude and orientation of the vector →d for antisymmetric exchange. Antisymmetric contributions to ΔH(T) are not enhanced by spin diffusion in low-dimensional systems when the paramagnetic sites are at inversion centers. The different role of static spin correlations for symmetric and antisymmetric perturbations accounts without adjustable parameters for the angular anisotropy of ΔH(T). The residual temperature dependence of the effective exchange frequency is not understood and probably involves various kinds of phonon modulation.