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

Description: This dissertation is concerned with the interplay of long-range order and anisotropy in the tetragonal RNi{sub 2}Ge{sub 2} (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. This work has correlated a strong Fermi surface nesting to the magnetic ordering in the RNi{sub 2}Ge{sub 2} compounds. Generalized susceptibility, {chi}{sub 0}(q), calculations found nesting to be responsible for both incommensurate ordering wave vector in GdNi{sub 2}Ge{sub 2}, and the commensurate structure in EuNi{sub 2}Ge{sub 2}. A continuous transition from incommensurate to commensurate magnetic structures via band filling is predicted. The surprisingly higher T{sub N} in EuNi{sub 2}Ge{sub 2} than that in GdNi{sub 2}Ge{sub 2} is also explained. Next, all the metamagnetic phases in TbNi{sub 2}Ge{sub 2} with an applied field along the c 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.
Date: May 10, 1999
Creator: Islam, Z.
Partner: UNT Libraries Government Documents Department

Heat capacities, magnetic properties, and resistivities of ternary RPdBi alloys where R = La, Nd, Gd, Dy, Er, and Lu

Description: Over the past four and a half decades research on the rare earths, their compounds, and their alloys has yielded significant insights into the nature of materials. The rare earths can be used to systematically study a series of alloys or compounds. Magnetic ordering, crystalline fields, spin fluctuations, the magnetocaloric effect, and magnetostriction are a small sample of phenomena studied that are exhibited by the rare earth family. A significant portion of research has been conducted on the abundant RM{sub 2} and RM phases, where R is the rare earth and M is a transition metal. The natural progression of science has led to the study of related RMX ternary phases, where X is either another transition metal or semimetal. There are now over 1,000 known RMX phases. The focus of this study is on RPdBi where R = La, Nd, Gd, Dy, Er, and Lu. Their heat capacities, magnetic properties, and resistivities are studied.
Date: May 1, 1996
Creator: Riedemann, T.M.
Partner: UNT Libraries Government Documents Department

Crystal field and exchange interactions in DyT{sub 4}Al{sub 8} (T = Fe and Mn)

Description: The authors investigated the magnetic excitations in polycrystalline samples of hard magnet related compounds DyFe{sub 4}Al{sub 8} and DyMn{sub 4}Al{sub 8} by neutron spectroscopy. For both compounds the magnetic spectra at energies below 40 meV are dominated by the response of the Dy{sup 3+} ions. In DyMn{sub 4}Al{sub 8} they observed pure crystal-field transitions within the Dy{sup 3+} J = 15/2 ground multiplet ID which is split into 8 doublets under the low point-group symmetry of 4/mmm. In DyFe{sub 4}Al{sub 8} the Dy crystal-field-split states are perturbed by the molecular fields of the ordered Fe sublattice.
Date: December 31, 1994
Creator: Loewenhaupt, M.; Tils, P.; Hahn, W. & Loong, C.K.
Partner: UNT Libraries Government Documents Department

Magnetic and magneto-topical properties of RMn{sub 6}Sn{sub 6} single crystals where R = Gd, Tb, Dy, Ho, Er, Tm, Lu

Description: Single crystals of the ternary compounds RMn{sub 6}Sn{sub 6}, where R = Gd, Tb, Dy, Ho, Er, Tm, and Lu, have been prepared by the flux growth technique. All of the compounds have the HfFe{sub 6}Ge{sub 6} type crystal structure (space group P6/mmm), and the variation of the lattice parameters with lanthanide element exhibits the lanthanide contraction as well as evidence of crystal field effects. Measurements on the magnetic properties of TbMn{sub 6}Sn{sub 6} indicate that it orders ferrimagnetically at 450K. It undergoes a change in the easy axis from close to the c axis at low temperatures to in plane at higher temperatures. This transition occurs at 310K in zero DC field and can be shifted to lower temperatures by a field applied parallel to the a direction and to higher temperatures by a field applied parallel to the c direction. HoMn{sub 6}Sn{sub 6} orders ferrimagnetically at 400K, and it also undergoes a change in the easy axis between 150 and 225K with the moments located in the basal plane at high temperatures. The magnetic properties of ErMn{sub 6}Sn{sub 6} are more complex than the first two compounds. A magnetic phase diagram has been mapped out which shows that in a zero DC field the erbium sublattice orders antiferromagnetically at 65K and the manganese sublattice orders antiferromagnetically at 350K. The structure of the room temperature magneto-optic spectra for RMn{sub 6}Sn{sub 6} with R = Gd-Tm are all similar. The maximum Kerr rotation found was 0.225 degrees in TbMn{sub 6}Sn{sub 6}, and the magnitude of the maximum Kerr rotations scaled across the series as a function of the fraction of saturation. This, combined with the temperature dependence, indicates that the Kerr rotation is primarily a function of the manganese moment and is independent of the lanthanide element present. The main ...
Date: February 12, 1999
Creator: Clatterbuck, D.M.
Partner: UNT Libraries Government Documents Department

The time evolution of the first order magnetic phase transition in Dy-4%Y alloy

Description: The first order magnetic transition of spiral to ferromagnetic structure of Dy-4%Y alloy has been investigated by means of Synchrotron X-ray diffraction. The development of the ferromagnetic phase could be studied by X-ray diffraction measurement for the observation of the hexagonal to orthorhombic crystal distortion accompanying the first order magnetic transition. Below {Tc}, the ferromagnetic and spiral phases coexist and the development of the ferromagnetic phase exhibits a time dependent behaviour and is proportional to logt except in the early stage of the phase transition. 6 refs., 2 figs.
Date: January 1, 1991
Creator: Tajima, K. (Keio Univ., Yokohama (Japan). Dept. of Physics); Akimitsu, J. (Aoyama Gakuin Univ., Tokyo (Japan). Dept. of Physics); Akimitsu, M. (Shibaura Inst. of Tech., Tokyo (Japan)); Rebelsky, L.; Gruebel, G. & Shirane, G. (Brookhaven National Lab., Upton, NY (USA))
Partner: UNT Libraries Government Documents Department

Magnetic and Moessbauer studies on GdCo/sub 3/B/sub 2/ and DyCo/sub 3/B/sub 2/

Description: Magnetization and Moessbauer studies have been carried out on GdCo/sub 3/B/sub 2/ and DyCo/sub 3/B/sub 2/. These compounds are magnetically ordered with Curie temperatures of 56/sup 0/ and 21/sup 0/K respectively. The Co atoms are either nonmagnetic or carry a small moment in these compounds. The saturation moment of DyCo/sub 3/B/sub 2/ at 5/sup 0/K is smaller than the Dy/sup 3 +/ free-ion value. From /sup 161/Dy Moessbauer studies, the measured hyperfine magnetic field at the Dy site is also observed to be smaller than the free-ion value. /sup 155/Gd Moessbauer measurements in GdCo/sub 3/B/sub 2/ reveal the presence of large crystalline electric fields at the rare earth site. This causes the moment and the hyperfine field at the Dy site in DyCo/sub 3/B/sub 2/ to be reduced from its free-ion value.
Date: October 1, 1984
Creator: Malik, S. K.; Umarji, A. M. & Shenoy, G. K.
Partner: UNT Libraries Government Documents Department

Oxygen stabilized rare-earth iron intermetallic compounds

Description: A new, oxygen-stabilized intermetallic compound was identified in sintered, pre-alloyed rare-earth iron powder samples. Its composition corresponds to formula R$sub 12$Fe$sub 32$O$sub 2$ and its crystal structure belongs to space group Im3m. The presence of these compounds was observed, so far, in several R--Fe--O systems, with R = Gd, Tb, Dy, Ho, Er, and Y. (auth)
Date: October 1, 1975
Creator: Dariel, M.P.; Malekzadeh, M. & Pickus, M.R.
Partner: UNT Libraries Government Documents Department

Effects of solutes on damage production and recovery in zirconium

Description: Dilute zirconium-based alloys and pure zirconium were irradiated at 10 K with spallation neutrons at IPNS. Four types of alloys - Zr-Ti, Zr-Sn, Zr-Dy and Zr-Au - each with three concentration levels, were used. Low-temperature resistivity damage rates are enhanced by the presence of any of the four solutes. The greatest enhancement was produced by Au while the least by Dy. Within each alloy group, damage production also increased but at a decreasing rate, with increasing concentration. Post-irradiation annealing experiments, up to 400 K, showed that all four solutes suppress recovery due to interstitial migration, indicative of interstitial trapping by the solutes. Vacancy recovery is also suppressed by the presence of Sn, Dy or Au. The effect of Ti is to shift this stage to lower temperature. No clear correlation between the results with solute size was detected.
Date: April 1, 1986
Creator: Zee, R.H.; Birtcher, R.C.; MacEwen, S.R. & Abromeit, C.
Partner: UNT Libraries Government Documents Department

Development of instrumentation for magnetic nondestructive evaluation

Description: The use of failure-prone components in critical applications has been traditionally governed by removing such components from service prior to the expiration of their predicted life expectancy. Such early retirement of materials does not guarantee that a particular sample will not fail in actual usage. The increasing cost of such life expectancy based operation and increased demand for improved reliability in industrial settings has necessitated an alternate form of quality control. Modern applications employ nondestructive evaluation (NDE), also known as nondestructive testing (NDT), as a means of monitoring the levels and growth of defects in a material throughout its operational life. This thesis describes the modifications made to existing instrumentation used for magnetic measurements at the Center for Nondestructive Evaluation at Iowa State University. Development of a new portable instrument is also given. An overview of the structure and operation of this instrumentation is presented. This thesis discusses the application of the magnetic hysteresis and Barkhausen measurement techniques, described in Sections 1.3.1 and 1.3.2 respectively, to a number of ferromagnetic specimens. Specifically, measurements were made on a number of railroad steel specimens for fatigue characterization, and on specimens of Damascus steel and Terfenol-D for materials evaluation. 60 refs., 51 figs., 5 tabs.
Date: September 23, 1991
Creator: Hariharan, S.
Partner: UNT Libraries Government Documents Department

Experimental study of induced staggered magnetic fields in dysprosium gallium garnet (DGG)

Description: Neutron diffraction techniques have been used to study induced staggered magnetic field effects in DGG. The application of a uniform magnetic field at temperatures much greater than the Neel temperature induces a significant amount of antiferromagnetic order. The temperature and field dependences of this effect are in good agreement with recent theoretical predicions.
Date: January 1, 1979
Creator: Steiner, M.; Corliss, L.M.; Hastings, J.M.; Blume, M.; Giordano, N. & Wolf, W.P.
Partner: UNT Libraries Government Documents Department

Magnetic hyperfine fields in Dy/sub 1-x/Y/sub x/Al/sub 2/

Description: /sup 161/Dy Moessbauer spectroscopy was carried out at 4.2 K on Dy/sub 1-x/Y/sub x/Al/sub 2/ for 0 less than or equal to x less than or equal to 0.6. For x=0 we find B=580 T and f/sub Q/=2810 MHz, close to free-ion values. For x > 0 both B and f/sub Q/ decrease weakly. We obtain (..delta..B)/sub n/ = -14 T, (..delta..f/sub Q/)/sub n/ = -110 MHz and (..delta..B)/sub nn/ = + 1.6 T, (..delta..f/sub Q/)/sub nn/ = +4 MHz per replaced nearest and next-nearest Dy/sup 3 +/ neighbor, respectively.
Date: January 1, 1982
Creator: Hessel, A.; Kalvius, G.M.; Shenoy, G.K.; Zinn, W. & Wiedemann, W.
Partner: UNT Libraries Government Documents Department

Metastable phases produced by ion implantation in metals

Description: Experimental evidence for the formation of metastable phases in metals using ion implantation is reviewed. Ion channeling and transmission electron microscopy are the main experimental techniques which have been used to investigate these materials. For heavy metallic implanted species at low implanted concentrations (less than or equal to 1 at. percent), the materials are generally substitutional solid solutions, often exceeding equilibrium solid solubilities. At higher concentrations both metastable solid solutions and amorphous structures can be produced. Examples from the Ag : Cu, Ta : Cu, Dy : Ni, and Au : Pt systems are shown to illustrate specific points. A thermal spike-type mechanism has been proposed to explain these behaviors.
Date: January 1, 1977
Creator: Borders, J.A.
Partner: UNT Libraries Government Documents Department

Fundamental magnetic studies of iron-rare-earth-metalloid alloys

Description: Objective was to understand strongly magnetic rare earth-transition metal compounds and alloys, with potential use as hard or semi-hard permanent magnet materials. Efforts were focused on three areas: search for Fe-rich new phases and compounds with high magnetization, anisotropy, and Curie temperature; use of rapid solidification (melt-spinning) to produce fine grain and high coercivity; and relation of hard magnetic properties to microstructure and domain structure. Coercivities of up to 10 to 15 kOe have been produced.
Date: January 1, 1990
Creator: Hadjipanayis, G.C.
Partner: UNT Libraries Government Documents Department

Terfenol: A study of the phase equilibrium diagram and the solidification process

Description: Terfenol is a rare earth-iron alloy that was first developed at the Naval Ordinance Laboratory because of its rare magnetostrictive properties. Terfenol is composed of terbium and dysprosium combined with iron in a composition Tb{sub x}Dy{sub 1{minus}x}Fe{sub 2}, where x{approximately}0.3. The objective of this work was to determine the growth characteristics of Terfenol and its dependence on solidification rate, temperature gradient, and stoichiometry. Specific goals of this work were to verify the phase equilibria that is currently accepted for the systems DyFe{sub 2} and TbFe{sub 2}, and establish the phase equilibria near the composition Tb{sub 0.3}Dy{sub 0.7}Fe{sub 2}; establish that Terfenol grows directly from the liquid and that the reaction is occurring under metastable conditions; evaluate whether or not Terfenol can be grown under plane front conditions with a new radiofrequency float zone apparatus, and; determine whether or not <111> seeded crystals can be grown and <111> single crystals produced by elimination of dendrites employing growth methods capable of achieving high gradient/solidification rate ratios.
Date: December 7, 1993
Creator: Anderson, M.
Partner: UNT Libraries Government Documents Department