Determination of atomic site susceptibility tensors from polarized neutron diffraction data

Polarized neutron diffraction provides information about the magnetization density of each individual crystallographic site. In the present paper the role of the atomic site susceptibility tensor chi(ij) accounting for the magnetic response of individual atoms to an external magnetic field is discussed. The symmetry of this tensor is very similar to that of the tensor u(ij) describing the thermal motion of atoms. By analogy with the atomic displacement parameters (ADPs), atomic susceptibility parameters (ASPs) can be introduced. The six independent ASPs can be determined from polarized neutron flipping ratio measurements and visualized as magnetic ellipsoids which are analogous to the thermal ellipsoids obtained from ADPs. If the local anisotropy is small, these magnetic ellipsoids approximate to spheres with diameters proportional to the induced magnetization. In other cases, anomalous (elongated or flattened) ellipsoids will occur. The ASPs have been determined for the compound Nd3-xS4 which has the Th3P4 structure. They correspond to strongly oblate magnetic ellipsoids; in contrast, in the isomorphous compound Sm3Te4 the magnetic ellipsoids are found to be prolate.