Detection of neutron scattering from phase IV of Ce0.7La0.3B6:: A confirmation of the octupole order

We have performed a single crystal neutron scattering experiment on Ce0.7La0.3B6 to microscopically investigate the order parameter of phase IV. Below the phase transition temperature 1.5 K of phase IV, weak but distinct superlattice reflections at the scattering vector K = (h/2, h/2, l/2) (h, l = odd number) have been observed for the first time by neutron scattering. The intensity of the superlattice reflections is stronger for high scattering vectors, which is quite different from the usual magnetic form factor of magnetic dipoles. This result directly evidences that the order parameter of phase IV has a complex magnetization density, consistent with the recent experimental and theoretical prediction in which the order parameter is the magnetic octupoles T-beta with Gamma(5) symmetry of the point group Oh. Neutron scattering experiments using short wavelength neutrons, as done in this study, could become a general method to study the high-rank multipoles in f electron systems.