Spin-lattice relaxation in some iridium salts II. Relaxation of nearest-neighbour exchange-coupled pairs

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Abstract
Measurements of the spin-lattice relaxation times at X-band and K-band frequencies have been carried out for transitions within the excited T = 1 state of nearest-neighbour exchange-coupled pairs of (IrCl6)2- complexes in crystals of (NH4)2PtCl6. The temperature dependence of the relaxation time indicates that a two-phonon process dominates at the low concentrations (Ir:Pt = 1.2% and 2.3%) and a calculation is made of the two-phonon relaxation rate occurring through modulation of the crystal field. This gives the observed order of magnitude but predicts larger anisotropy in the relaxation rate than was observed. Other possible mechanisms are also discussed. At higher iridium concentrations (Ir:Pt = 5.0% and 8.5%) we observed a concentration-dependent process with a temperature variation 1/T1 similar exp(-Δ/kT). This is consistent with cross relaxation to larger clusters.

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