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

Abstract

The processes of spin-lattice relaxation (SLR) in amorphous materials are reviewed. In particular, the mechanisms involving exchange interaction, tunneling level states (TLS) centers and fractons predominant in effecting electron SLR at low temperatures in amorphous materials are reviewed. It has been deducded that the former two lead to quadratic temperature dependences at very low temperatures and linear temperature dependences at intermediate temperatures of the relaxation rate in amorphous materials. As for relaxation due to fractons in fractal state, the theories proposed by Alexander, Entin-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 dependences of fracton-assisted relaxation rate in the fractal state for Kramers and non-Kramers ions are examined.