Zero-field magnetic structure in CeB6 reinvestigated by neutron diffraction and muon spin relaxation

The zero-field magnetic structure of (CeB6)-B-11 has been revised from neutron powder and single-crystal diffraction including neutron spherical polarimetry. The crystal structure remains cubic in the antiferroquadrupolar (AFQ) ordered state (T-Q=3.3 K, k(Q)=[1/2,1/2,1/2]) and in the antiferromagnetic (AFM) ordered state (T-N=2.3 K, k(1)=[1/4,1/4,0], k(2)=[1/4,-1/4,0], k(1)(')=[1/4,1/4,1/2], k(2)(')=[1/4,-1/4,1/2]) within the precision of the experiment. The model of Effantin [J. Magn. Magn. Mater. 47-48, 145 (1985)] fits our 60-mK high-intensity neutron powder diffraction data rather poorly and therefore a model of the AFM multi-k structure has been developed. It is a 2 k-k(') transverse sine-wave structure with the Ce magnetic moments strictly along [1-10] and [110] and orthogonal arrangement of the nearest moments. Ce atoms located at the z=0 and z=1 layers have significantly different magnetic moment values. In addition there is a modulation of the moment value in each layer. The resulting ordered magnetic Ce moments reach 0.744(16)mu(B), 0.543(16)mu(B) at z=1 and only 0.01mu(B), 0.138(7)mu(B) at z=0 at 60 mK. This complex AFM structure is due to competition between the established AFQ order and the dipolar and octupolar AFM order developing at lower temperatures. The model is consistent with the muSR zero-field results [R. Feyerherm , J. Magn. Magn. Mater. 140-144, 1175 (1995)] and suggests a highly inhomogeneous conduction electron spin polarization and anisotropic RKKY interactions below T-N.