Proton Spin-Lattice Relaxation in a Quasi-Two-Dimensional, Ferromagnetic Heisenberg System: (CH₃NH₃)₂CuCl₄

Abstract

The spin dynamics of a quasi-two-dimensional, ferromagnetic Heisenberg system (CH₃NH₃)₂CuCl₄ (T_c=8.91 K) are studied by measuring the proton spin-lattice relaxation rate in the temperature range between 4.2 and 300 K. In the ferromagnetic state, the relaxation rate is explained by the two-magnon process. The relaxation rate in the paramagnetic state includes an effect of the rotation of the CH₃- or NH₃-groups and a temperature-dependent magnetic contribution peculiar to a ferromagnetic system. A weak frequency dependence observed at room temperature as well as an analysis of the absolute rate indicate an existence of a two-dimensional long-time tail in the spin correlation function for the above system.