Observation of the Influence of Electron-Pair Spin Flips on Nuclear Thermal Relaxation

We report measurements on the thermal relaxation rate of water-of-hydration protons in magnetically dilute La₂{Mg}₃(NO₃)₁₂·{}24H₂O:Nd³⁺. The observed dependence of the proton relaxation rate on the angle between the applied magnetic field and the crystal symmetry axis exhibits a complicated structure which is shown to be consistent with that arising from processes in which a proton gives up its Zeeman energy to a pair of electrons, such that the Zeeman energy for the 3-spin system is conserved. These observations constitute direct evidence for the influence of electron-pair flips on proton thermal relaxation. Estimates made of the relative strength of such multiple-spin processes are shown to correspond to observed data.