Spin-lattice relaxation times in amorphous materials as effected by exchange interactions, tunneling level states (TLS) centres, and fractons

The processes of spin-lattice relaxation (SLR) in amorphous materials are r eviewed. In particular, the mechanisms involving exchange interaction, tunn eling level states (TLS) centers and fractons predominant in effecting elec tron SLR at low temperatures in amorphous materials are reviewed. It has be en deducded that the former two lead to quadratic temperature dependences a t very low temperatures and linear temperature dependences at intermediate temperatures of the relaxation rate in amorphous materials. As for relaxati on due to fractons in fractal state, the theories proposed by Alexander, En tin-Wohlman and Orbach (Phys. Rev. B 32 (1985) 6447; Phys. Rev. B 33 (1986) 3935; J. Phys. Lett. 46 (1985) 1555) are reviewed and the temperature depe ndences of fracton-assisted relaxation rate in the fractal state for Kramer s and non-Kramers ions are examined. (C) 1998 Elsevier Science B.V. All rig hts reserved.