ELECTRON-PARAMAGNETIC-RESONANCE AND SPIN-LATTICE RELAXATION IN 2-DIMENSIONAL SYSTEMS

The anisotropies of the EPR linewidth and g-factor were investigated i n two-dimensional molecular composites of the type [NH3-R-NH3]MX(4). M easurements were performed both in single crystals and powders over th e temperature range 4.2-290 K. The spin-lattice relaxation time T-1 wa s measured using the modulation method, as a function of temperature. The samples exhibit different structures and coupling interactions, ac cording to the nature of the halogen X, the metal M and the organic ra dical R. We have analysed the influence of these parameters on spin be havior by studying the samples [NH3-(CH)(n)-NH3]MX(4) with M = Mn, Cu; X = Cl, Br, and n = 2, 3, 4, 5. When R is constituted by molecules wi th unsaturated bonds, these materials can be considered as excellent m atrices for selective polymerisation reactions by irradiation or therm al processing. We have performed EPR measurements on the heated comple x of propargylamine and cadmium chloride [HC=C-CH2-NH3]2CdCl4. The obt ained data are interpreted taking into account the strong exchange int eraction and the various coupling interactions in the samples. The the rmal dependences of T-1 are explained by means of the Bloembergen and Wang three-reservoir model. The data show spin diffusion when the meta l is manganese, and an antisymmetric exchange interaction modulated by phonons in the case of copper. The nature of the halogen plays an imp ortant role in spin dynamics and namely in spin-lattice relaxation. Th e results obtained on [HC=C-CH2-NH3]2CdCl4 after heating under vaccum show the creation of many paramagnetic centers due to the vanishing of triple bonds and the occurrence of a strong exchange interaction.