We have performed a neutron diffraction experiment on Pr-doped Ce0.5Pr0.1La0.4B6, in which an antiferromagnetic octupole order with q = ( 1/2, 1/2, 1/2 ) could be anticipated by analogy with Ce0.7La0.3B6. Contrary to this natural expectation, we detected an unambiguous magnetic peak at q = ( 1/4, 1/4, 1/2 ), which is the same q-vector frequently realized in the magnetic ordered phases of RB6 (R=rare earth) compounds. No significant signal was observed at q = (1/2, 1/2, 1/2 ) at zero magnetic field. This result shows that the normal antiferromagnetic dipole moment is also one of the competing multipole order parameters in the CexLa1－xB6 system. The relevant order parameters are close in energy and can be tuned by a weak perturbation.

This work was supported by Grants-in-Aid for Scientific Research (Nos. 21204456, 21102515, and 2430087) from JSPS and MEXT.