Spin Correlation Functions in Cu(NH3)4PtCl4

The angular dependence of the exchange-narrowed Lorentzian EPR line in Cu${\left(\mathrm{N}{\mathrm{H}}_3\right)}_4$Pt${\mathrm{Cl}}_4$, a crystal with magnetically equivalent square planar Cu${\left(\mathrm{N}{\mathrm{H}}_3\right)}_4$ ${}^{++}$sites, is shown to provide a direct evaluation of several high-temperature Fourier components of the spin autocorrelation function $C\left(t\right)=4\mathrm{}〈S_i^z\mathrm{}\left(t\right)\mathrm{}{S}_i^z\mathrm{}\left(0\right)\mathrm{}〉$ and of $C^2\mathrm{}\left(t\right)\mathrm{}$. The angular dependence of the "10/3" effect observed at $X$ and $Q$ bands supports the Blume-Hubbard model for spin correlation functions. The enhanced $\omega =0\mathrm{}$ Fourier components are consistent with largely one-dimensional exchange in Cu${\left(\mathrm{N}{\mathrm{H}}_3\right)}_4$Pt${\mathrm{Cl}}_4$ and the $\omega =0\mathrm{}$ component of the four-spin correlations $〈S_i^z\mathrm{}\left(t\right)\mathrm{}{S}_{i+1\mathrm{}}^z\mathrm{}\left(t\right)\mathrm{}{S}_{i+1\mathrm{}}^z\mathrm{}\left(0\right)\mathrm{}{S}_i^z\mathrm{}\left(0\right)\mathrm{}〉$ satisfies a numerical bound due to Carboni and Richards for finite one-dimensional chains. The method is applicable to any exchange-coupled paramagnetic crystals with magnetically equivalent sites and provides a direct test for theoretical models of spin correlation functions.