Very-Low-Frequency Excitations in Frustrated Two-DimensionalS=12Heisenberg Antiferromagnets

Muon spin resonance and ${}^7\mathrm{Li}$ NMR relaxation measurements in frustrated two-dimensional $S=\frac{1}{2}$ Heisenberg antiferromagnets on a square lattice are presented. It is found that, in both ${\mathrm{Li}}_2{\mathrm{VOSiO}}_4$ and ${\mathrm{Li}}_2{\mathrm{VOGeO}}_4$, spin dynamics at frequencies orders of magnitude below the Heisenberg exchange frequency are present. These dynamics are associated with the motions of walls separating coexisting collinear domains with a magnetic wave vector rotated by $90°$.