Magnetic Interactions in Dysprosium-Antimonide and Holmium-Antimonide

This thesis presents the first systematic investigation of the magnetization of single crystals of DySb and HoSb for all the principal magnetic field orientations relative to the crystal axes, and for temperatures ranging from well above to well below the antiferromagnetic ordering (Neel) temperatures (DySb 9.2(DEGREES)K, HoSb 5.2(DEGREES)K). The complete magnetic phase diagrams of these compounds have been determined here, for which there have only been piecemeal reports in the literature. While it has been known that strong quadrupole-quadrupole couplings are needed to explain certain magnetoelastic properties of these compounds above their Neel temperatures (T(,N)), this thesis presents the first direct evidence for such interactions from magnetization data. It further provides the first quantitative determination, for any rare-earth monopnictide, of the magnitude and symmetry of the strongly-temperature-dependent, strongly-anisotropic quadrupolar coupling over a temperature range above T(,N). Presented here are the first neutron diffraction studies of the field-induced magnetic state for DySb. For a magnetic field parallel to the <100> axis, the relative strengths of the nuclear and magnetic reflections are consistent with the HoP structure, which others have shown is stabilized by strong dipolar or quadrupole-quadrupole interactions, or by a combination of both. A simple energy argument for the necessity of such interactions in DySb and HoSb is presented here, based on the magnetization data below T(,N). The computer calculations we needed to make in determining the expected magnetization for a given effective magnetic field, based on crystal field theory, are straight -forward extensions of previous work, and are presented here in sufficient detail for a comprehension of the method. The formalism for the analysis of the paramagnetic-state magnetization data was recently developed for the study of polycrystalline PrAg, and is applied here for the first time to single -crystal data. The introduction of the quadrupole-quadrupole terms in a self-consistent set of calculations used to simulate the experimental data is novel in its format. Using only the rotation properties of quantum mechanical operators, and two parameters representing the quadrupole-quadrupole couplings at a given temperature obtained independently from magnetization data for two different field orientations, we have calculated the magnetization for a third field orientation and show that it compares very closely with the measured magnetization. While it is impossible to determine the exact mechanisms for the effective quadrupole-quadrupole interactions observed here, it is shown that more than one mechanism must pertain. Portions of the data for both compounds and of the analysis for DySb have been published in brief elsewhere. This work was performed in part in the Solid State Science Division of Argonne National Laboratory, while a Laboratory Graduate Thesis Program Participant. This program was coordinated by the Argonne Center for Educational Affairs, and supported by the U.S. Department of Energy.