Magnetostatic modes in highly polarized solid helium-three

We report the observation of an unusual series of spin-wave modes seen in continuous-wave NMR measurements on long cylindrical crystals of highly polarized solid ${}_{}{}^3{}_{}{}^{}\mathrm{He}$. When a magnetic field and field gradient of appropriate magnitude are placed along the cylinder axis, the NMR spectrum breaks up into a series of discrete peaks whose spacing is geometric in mode number, with the spacing going to zero at an accumulation point. We show that this behavior is consistent with magnetostatic waves in the highly polarized medium.