Spin dynamics and ordering of a cuprate stripe antiferromagnet

In ${\mathrm{La}}_{1.48}{\mathrm{Nd}}_{0.4}{\mathrm{Sr}}_{0.12}{\mathrm{CuO}}_4$ the ${}^{139}\mathrm{La}$ and ${}^{63}\mathrm{Cu}$ nuclear quadrupole resonance relaxation rates and signal wipe-out upon lowering temperature are shown to be due to purely magnetic fluctuations. They follow the same renormalized classical behavior as seen in neutron data, when the electronic spins order in stripes, with a small spread in spin stiffness (15% spread in activation energy). The La signal, which reappears at low temperatures, is magnetically broadened and experiences additional wipe out due to slowing down of the Nd fluctuations.