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

摘要

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 T_N, with propagation vectors (0 0 $\sim$ 0.758) and (0 0 $\sim$ 0.75), respectively. Below T_t T_N, 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 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 $\sim$ 0.793) at T_N. There is an ordered component along in the Gd compound below T_N but above T_t. Below T_t, 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 T_N in EuNi₂Ge₂ than that in GdNi ₂Ge₂ is also explained.; Next, all the metamagnetic phases in TbNi₂Ge₂ with an applied field along the 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.