Depth-Controlled $\beta$-NMR of ${}^8\mathrm{L}\mathrm{i}$ in a Thin Silver Film

Depth-controlled $\beta$-NMR can be used to probe the magnetic properties of thin films and interfaces on a nanometer length scale. A 30 keV beam of highly spin-polarized ${}^8\mathrm{L}\mathrm{i}^{+}$ ions was slowed down and implanted into a 50 nm film of Ag deposited on a ${\mathrm{S}\mathrm{r}\mathrm{T}\mathrm{i}\mathrm{O}}_3$ substrate. A novel high field $\beta$-NMR spectrometer was used to observe two well resolved resonances which are attributed to Li occupying substitutional and octahedral interstitial sites in the Ag lattice. The temperature dependence of the Knight shifts and spin relaxation rates are consistent with the Korringa law for a simple metal, implying that the NMR of implanted ${}^8\mathrm{L}\mathrm{i}$ reflects the spin suspectibility of bulk metallic silver.

Figure 1

Schematic of the ISAC $\beta$-NMR spectrometer.

Figure 2

(a) The Ag $\beta$-NMR spectrum at 4 K in a field $H_0=0.3\mathrm{T}$ and $H_1\approx 3\mu \mathrm{T}$. The solid line is a fit to a Lorentzian line shape. The inset shows the linewidth (HWHM) of this resonance as a function of temperature. (b) At $H_0=3.0\mathrm{T}$ and $T=145\mathrm{K}$ two signals are resolved. The vertical dashed line indicates the reference frequency in MgO. Typical spectra accumulated $35–100\times {10}^6$ decay events in 10–20 min. The inset shows the octahedral ($O$) and tetrahedral ($T$) interstitial sites and the substitutional site ($S$) in the fcc Ag lattice.

Figure 3

Temperature dependences of (a) shifts with respect to ${}^8\mathrm{L}\mathrm{i}$ in MgO and (b) normalized amplitudes for $H_0=3.0\mathrm{T}$ and $H_1=6\mu \mathrm{T}$. The solid lines are guides to the eye.

Figure 4

Longitudinal spin relaxation spectra recorded in $H_0=3.0\mathrm{T}$ at several temperatures. Inset: Spin-lattice relaxation rate of the component assigned to the $S$ site. The fit of the data to a straight line yields $1/T_1=0.00095{\mathrm{s}}^{-1}{\mathrm{K}}^{-1}T$.