Spin-wave velocities, density of magnetic excitations, and NMR relaxation in iron pnictides

We perform an analysis of the experimentally known temperature dependence of the staggered magnetization in the antiferromagnetic phase. This analysis allows us to put an upper limit on the unknown value of the spin-wave velocity along the stripes of equal spin direction (spin stripes). The velocity is about ten times smaller than the velocity perpendicular to the spin stripes. The strongly anisotropic spin-wave dispersion implies a high density of low-energy magnetic excitations. We demonstrate that this high density strongly enhances the As-75 NMR spin-lattice relaxation via the Raman scattering of magnons. We derive the polarization dependence of this relaxation channel and find very good agreement with experimental data. The high density of low-energy magnetic excitations deduced from our phenomenological analysis supports the scenario that ferropnictides are close to a quantum phase transition.