Spin-Lattice Relaxation of Co⁺² Ions in Zinc Formate Dihydrate

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The Co²⁺ ions at two different sites in Zn(HCOO)₂·2H₂O have been studied in their ESR at about 9.37 GHz and between 1.6 and 50 K. The temperature dependence of the spin-lattice relaxation time deduced from the line width depends on the site and can be well explained by the Orbach type process, if the energy difference Δ, between the ground and the first excited Kramers doublet, is 89 and 160 cm⁻¹, respectively. A theoretical calculation based on their anisotropic g-tensors gives 140 and 215 cm⁻¹, if λ=−120 cm⁻¹ is assumed.