Nuclear-spin relaxation of ${}^{207}\mathrm{Pb}$ in ferroelectric powders

Motivated by a recent proposal by Sushkov and co-workers [Phys. Rev. A 72, 034501 (2005); 73, 022107 (2006)] to search for a $P,T$-violating Schiff moment of the ${}^{207}\mathrm{Pb}$ nucleus in a ferroelectric solid, we have carried out a high-field nuclear magnetic resonance study of the longitudinal and transverse spin relaxation of the lead nuclei from room temperature down to $10\mathrm{K}$ for powder samples of lead titanate (PT), lead zirconium titanate, and a PT monocrystal. For all powder samples and independently of temperature, transverse relaxation times were found to be $T_2\approx 1.5\mathrm{ms}$, while the longitudinal relaxation times exhibited a temperature dependence, with $T_1$ of over an hour at the lowest temperatures, decreasing to $T_1\approx 7\mathrm{s}$ at room temperature. At high temperatures, the observed behavior is consistent with a two-phonon Raman process, while in the low-temperature limit, the relaxation appears to be dominated by a single-phonon (direct) process involving magnetic impurities. We discuss the implications of the results for the Schiff-moment search.

Figure 1

(Color online) Pulse sequences used for measuring $T_1$ and $T_2$ relaxation times. In both cases, a train of $\pi ∕2$ pulses was used to saturate the magnetization, followed by a recovery period. The $T_1$ measurement used a spin echo readout to avoid coil ringing artifacts and several values of $T_R$ were used. $T_2$ measurements used a Hahn echo for several values of $\tau$. The $\pi ∕2$ pulse widths were in the range of $10–13\mu \mathrm{s}$.

Figure 2

(Color online) Representative magnetization recovery curves for $\mathrm{Pb}\mathrm{Ti}{\mathrm{O}}_3$ (a) and $\mathrm{Pb}{\mathrm{Zr}}_{0.52}{\mathrm{Ti}}_{0.48}{\mathrm{O}}_3$ (b) powders. Two fits are shown for each data set: a single exponential and a stretched exponential. The PT sample is $P1$ (see text).

Figure 3

(Color online) The experimentally determined longitudinal relaxation rates vs temperature. The data were fit to different functions above and below $50\mathrm{K}$ (see text).

Figure 4

(Color online)An example of data and exponential-decay fit used to extract the $T_2$ values.