The interplay of long-range magnetic order and single-ion anisotropy in rare earth nickel germanides

This dissertation is concerned with the interplay of long-range order and anisotropy in the tetragonal RNi2Ge2 (R = rare earth) family of compounds. Microscopic magnetic structures were studied using both neutron and x-ray resonant exchange scattering (XRES) techniques;The magnetic structures of Tb, Dy, Eu and Gd members have been determined using high-quality single-crystal samples. TbNi2Ge2 and DyNi2Ge2 order in an amplitude modulated structure below TN, with propagation vectors (0 0 ∼ 0.758) and (0 0 ∼ 0.75), respectively. Below Tt \u3c TN, the structures are equal moment commensurate , described by three modulations, (0 0 ¾) and its third harmonic (0 0 ¼), (½ ½ 0) and (½ ½ ½), respectively. Whereas the Tb compound is uniaxial with moments along the c&d4; axis in both phases, its Dy neighbor has an ordered in-plane component. XRES is the probe of choice for the study of neutron-opaque Eu and Gd members. While EuNi2Ge2 has a simple antiferromagnetic state, (0 0 1), with moments in the basal plane, GdNi2Ge 2 orders with an incommensurate modulation, (0 0 ∼ 0.793) at TN. There is an ordered component along c&d4; in the Gd compound below TN but above Tt. Below Tt, Gd moment is locked to the basal plane;This work has correlated a strong Fermi surface nesting to the magnetic ordering in the RNi2Ge2 compounds. Generalized susceptibility, c0 (q), calculations found nesting to be responsible for both incommensurate ordering wave vector in GdNi2Ge 2, and the commensurate structure in EuNi2Ge 2. A continuous transition from incommensurate to commensurate magnetic structures via band filling is predicted. The surprisingly higher TN in EuNi2Ge2 than that in GdNi 2Ge2 is also explained;Next, all the metamagnetic phases in TbNi2Ge2 with an applied field along the c&d4; axis have been characterized with neutron diffraction measurements. A mixed phase model for the first metamagnetic structure consisting of fully-saturated as well as reduced-moment Tb ions is presented. The moment reduction may be due to moment instability which is possible if the exchange is comparable to the low-lying CEF level splitting and the ground state is a singlet. In such a case, certain Tb sites may experience a local field below the critical value needed to reach saturation