NMR of nuclei near rare-earth ions in yttrium ethyl sulfate

Nuclear magnetic resonances are detected for protons adjacent to rare-earth impurities in yttrium ethyl sulfate (YES) crystals at $T<\mathrm{}4.2\mathrm{}$ K. The near-proton resonances are shifted several hundred gauss away from the usual bulk proton NMR and are two to four orders of magnitude weaker, depending on the impurity concentration. A sensitive and versatile rf spectrometer used to detect these weak signals is described in detail. The magnitude of the NMR shift is shown to be approximately described by a point dipolar field of the paramagnetic impurity, even for protons only 3 Å from the impurity. However, field-angle dependences of these nearest proton splittings reveal significant deviations from the simple dipolar interaction which most likely arise from the finite extent of the impurity wave function. A frequency dependence of the splitting gives the components of the local field parallel and perpendicular to the external field direction. The intensities of the near-nuclei magnetic resonances (NNMR) in YES: Yb decrease rapidly above 3.8 K. The observed temperature dependence is quantitatively explained for $1.5 \mathrm{K}<T<\mathrm{}4.2\mathrm{} \mathrm{K}$ by including time-dependent fluctuations in the local field. A general NNMR criterion is experimentally established: lifetime broadening of the near-nuclei resonances occurs whenever the electron $〈S_z〉\mathrm{}\left(t\right)\mathrm{}$ correlation time $\tau$ is shorter than the inverse dipolar NMR linewidth; i.e., when $\tau \lesssim {10}^{-5\mathrm{}}$ sec for abundant ${}_{}{}^1{}_{}{}^{}\mathrm{H}$ and ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$ spins. From the near proton splittings in YES: Yb near $\theta =90\mathrm{}°$, a large distribution in the value of the elusive $g_{\perp }$ factor was discovered, giving ${\overline{g}}_{\perp }=0.01\mathrm{}$. Comparisons of NNMR spectra for several rare-earth ions in YES and the extension of this method to other crystals are discussed.