SPIN DYNAMICS OF AN S = 1/2 ANTI-FERROMAGNETIC HEISENBERG CHAIN AT LOW-TEMPERATURES

Photon spin-lattice relaxation data in magnetic crystals can be interpreted in terms of two-spin correlation functions of the electron spin system. In this paper we present experimental results on the compound CuSO4.5H2O, for temperatures well below J/k and fields up to values well above the saturation field. A theoretical explanation of these results has been obtained using the Bulaevskii approximation of the Heisenberg model. This reduces the calculation of the required spectral densities of spin autocorrelation functions to the calculation of the corresponding quantities for the XY-model. In this paper a theory starting from the correct spin-cyclic hamiltonian is developed for T = 0. The observed field dependence of the relaxation rate is well explained. The theory reproduces the absolute magnitude of the relaxation rates within about 35%. This result is satisfactory in view of the simplified point dipolar model that has been used for the interaction between the electron and proton spins. © 1979.