Paramagnetic Resonance and Spin-Lattice Relaxation for Ytterbium in Yttrium Ethyl Sulfate; Large Anisotropies and Evidence for Electric Dipole Transitions

Paramagnetic resonance is detected for ¹⁷²Yb³⁺ ions diluted in crystals of Y(C₂H₅SO₄)₃ ·{} 9H₂O at temperatures 1.2T4.2 ^∘{}K, and ν=23 GHz, for 0θ70^∘, where θ is the angle between the crystal c axis and →H. We find g_∥=3.328±0.005; g_⊥ is not directly measured, but estimated to be g_⊥≈0.01 from the resonance intensity at θ=0^∘. An observed angular variation of 10² in linewidth can be explained by a c-axis wander throughout the crystal, of order (δθ)ᵣₘₛ≈0.05^∘. A striking angular variation in line intensity, of the form (tan²θ)cosθ over five orders of magnitude, is used to deduce that the observed line is an electric dipole transition rather than the usual magnetic dipole transition, observed only at θ=0^∘. This is further confirmed by placement of the crystal in the cavity in regions of maximum electric or magnetic field. The electric dipole transition comes about by the combined action of the Zeeman perturbation and admixtures of even-parity states into the odd-parity 4f¹³ configuration by odd terms in the C₃ₕ crystal field. The direct spin-lattice relaxation rate is measured by a microwave pulse-recovery method and found to be T1d^-1=134Ttan²θ sec^-1 (T in ^∘{}K) at constant frequency ν=23.11 GHz, which corresponds to T1d^-1=2.4×10^-17H⁵cos³θsin²θcoth(hν2kT) sec^-1 (H in oersteds), the theoretically expected form. At large angles the data indicate a phonon bottle-neck. It was found that the EPR signal could be reduced by optical pumping in the 1- to 3-μ region. An optical pulse-recovery method was used to measure the Raman spin-lattice relaxation rate, T1R^-1=0.0135T⁹ sec^-1. These data are of central importance in the analysis of nuclear spin refrigerators utilizing this unusually anisotropic crystal.