Interaction of nuclear spins with a time-dependent electric field in ferroelectric KNbO3

The effect of transitions in the nuclear spin system on the motion of the electric polarization in ferroelectric potassium niobate is studied theoretically. The equations of motion of the polarization are derived with inclusion of the interaction of the polarization with the nuclear spins and employed to calculate the contribution from nuclear transitions to the dielectric susceptibility of the crystal. This contribution has resonance maxima at the frequencies of nuclear transitions and is proportional to the square of the background dielectric susceptibility of the crystal. For the ferroelectric, this contribution is far larger than for earlier studied nonferroelectric dielectrics. Numerical estimates show that, in potassium niobate, the contribution from the nuclei to the dielectric susceptibility is of the same order of magnitude as that to the magnetic susceptibility.