Antiferroquadrupole order and magnetic field induced octupole in CeB 6

Abstract

We have studied the antiferroquadrupole ordered phase of CeB 6 in magnetic fields by resonant x-ray diffraction. By analyzing the significant change in the energy spectrum on reversing the field direction along [1̄10], we have deduced field dependencies of the antiferro components of magnetic dipole, electric quadrupole, and magnetic octupole moments which are simultaneously induced in the Ce 4f orbital with a propagation vector (12,12,12). The data treatments are based on theoretically calculated spectral functions. The existence of the field-induced octupole is also concluded for other field directions. We also show direct evidence for the formation of a linear-combination-type antiferroquadrupole order parameter in magnetic fields, which is expressed as αO yz+βO zx+γO xy and changes continuously with the field direction (α,β,γ). A possibility of observing the quadrupolar fluctuation is also pointed out. © 2012 American Physical Society.