Role of exchange interaction in effecting spin-lattice relaxation: Interpretations of data on Cr3+ in Cu2+xCr2xSn2-2x spinel and dangling bonds in amorphous silicon

The particular role played by the exchange interaction modulated by lattice vibrations (phonons) in effecting spin-lattice relaxation in amorphous mat erials in conjunction with the spin-orbit coupling, along with the role pla yed by the well-known mechanisms, such as coupling to tunneling level state s (TLS) via dipolar and Fermi-contact hyperfine interactions, is investigat ed. From a review of experimental data, it is found that the relaxation rat e as effected by the exchange interaction is two to three orders of magnitu de faster than that due to coupling with TLS centers. In particular, it is deduced that both the exchange interaction and coupling to TLS centers via Fermi-contact hyperfine, or electron-nuclear dipolar interaction, lead, in general, to a quadratic temperature dependency of the relaxation rate at lo w temperatures changing into a linear temperature dependency at intermediat e temperatures. However, when the exchange interaction is strong, the relax ation rate becomes independent of temperature at higher temperatures due to cross relaxation of a spin with an exchange-coupled pair of spins, coupled to it via the exchange interaction. This analysis is then applied to inter pret the spin-lattice relaxation data on (i) Cr3+ ions in the Cu2+xCr2xSn2- 2x spinel samples for x = 0.1 and 0.2 in the semiconductor range (x less th an or equal to 0.5), and (ii) dangling bonds, with spin 1/2, in amorphous s ilicon. [S0163-1829(98)00546-3].