Powder neutron diffraction studies of structure and magnetic Ce ordering in the strongly correlated electron compoundCe0.75La0.25B611

By means of powder neutron diffraction investigations, chemical structure and magnetic Ce ordering of Ce{sub 0.75}La{sub 0.25}{sup 11}B{sub 6} have been investigated at temperatures down to 105 mK. In the antiferromagnetic low-temperature phase III Ce{sub 0.75}La{sub 0.25}{sup 11}B{sub 6} resembles pure CeB{sub 6}, but has a lower Neel temperature T{sub N} between 0.89 and 1.25 K. Similar to the recently determined magnetic ordering in pure CeB{sub 6} (model C), one obtains the compared to CeB{sub 6} smaller ordered magnetic Ce moments {mu}{sub Ce(1)}=0.53(1) {mu}{sub B} at z=0 and {mu}{sub Ce(2)}=0.12(1) {mu}{sub B} at z=1/2 with respect to the magnetic unit cell at 105 mK. Presumably this magnetic ordering, which is characterized by both propagation vectors k=[1/4,1/4,0] and k{sup '}=[1/4,1/4,1/2], is caused by competition between magnetic dipole and multipolar ordering. At 0.89 K the magnetic moments are reduced to {mu}{sub Ce(1)}=0.31(1){mu}{sub B} and {mu}{sub Ce(2)}=0.08(1){mu}{sub B}. At 1.25 K in phase IV with presumably multipolar Ce ordering no significant magnetic Bragg peaks associated with magnetic dipole Ce moments were observed, supporting the octupole model of Kubo and Kuramoto for this phase.