Experimental evidence for spin diffusion in the quasi-two-dimensional Heisenberg paramagnet ${\left({\mathrm{C}}_2{\mathrm{H}}_5\mathrm{N}{\mathrm{H}}_3\right)}_2$Mn${\mathrm{Cl}}_4$

High-temperature spin dynamics in the quasi-two-dimensional Heisenberg system ethylammonium-manganesechloride have been investigated by EPR with very high sensitivity. Two different experimental methods are presented which can be used to analyze the resonances of the respective memory spectrum occurring at the Larmor frequency ${\omega }_0$ and at $2{\omega }_0$. Around ${\omega }_0$ the memory spectrum is obtained from the absorption and dispersion part of the transversal susceptibility. The resonance behavior at $2{\omega }_0$ can be obtained directly from a satellite structure at half EPR field which has been observed in the transversal and in the longitudinal susceptibility. For both resonances a logarithmic shape is determined which—as theory predicts—is due to spin diffusion. There is some experimental evidence for a decoupling of the 4-spin correlation function for long times. The spin-diffusion constant is evaluated to be $\frac{D}{a^2}=13\mathrm{}\pm 2$ K.