Abstract
Neutron powder diffraction measurements have shown that, in the body-centered cubic α-Tb2C3 below 33 ± 4°K, two out of four body-diagonally linked Tb arrays become antiferromagnetic linear chains but the two remaining Tb arrays exhibit no ordered moment. The nearest Tb atoms between the different ordered spin arrays are related ferromagnetically. Six alternative ways of choosing two body-diagonal antiferromagnetic arrays are mutually equivalent due to the crystal symmetry but not to the multiplet degeneracy in powder diffractometry. The moments are most likely directed along the face diagonal which lies in the plane formed by two body diagonals parallel to the ordered spin arrays. The next probable moment direction lies longitudinally along any one of the two ordered arrays. The saturation moment per magnetically ordered Tb in these collinear spin structures is very insensitive to the selection of the moment direction and approximates closely the free Tb3+ value of 9 μB. An equivalent multiaxial antiferrostructure is obtained by combining two collinear magnetic structures with the mutually orthogonal face-diagonal moments. In this alternative biaxial structure, all Tb atoms possess the same saturation ordered moment of 9 / 2 μB. A sizeable magnetic diffuse scattering having the modulation of an apparently ferromagnetic short-range order was observed in the magnetically long-range ordered region.
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