Nuclear-Magnetic-Resonance Studies of Ferroelectricity in Normal and Irradiated Rochelle Salt

The ferroelectric properties of Rochelle salt have been studied by means of two new techniques based on the ${\mathrm{Na}}^{23}$ NMR. In the first technique, the separation between a pair of ${\mathrm{Na}}^{23}$ quadrupolar satellites is used as a measure of the spontaneous polarization of the crystal. Using this method, the effects of electric fields and $\gamma$ irradiation on the polarization have been studied. The irradiation caused the spontaneous polarization to decrease in magnitude, but to persist over a wider temperature range than in an unirradiated crystal. In the second technique, a pair of quadrupolar satellites was identified as arising from oppositely polarized domains. The relative intensities of these satellites provides a measure of the fraction of the crystal polarized in the forward and reverse directions. Under the influence of an applied electric field, the relative intensities of the two satellites change. By studying these changes versus applied electric field, "quasistatic" hysteresis loops have been obtained which have a very low coercive field (25 V/cm). When the crystal is irradiated with $\gamma$ rays, these hysteresis loops split into double loops. Finally, domain dynamics were studied by measuring the relative intensities of these satellites versus time after an electric field had been switched. The time required for polarization reversal was observed to depend on the magnitude of the applied electric field.