D.Sc. / Please refer to full text to view abstract

M.Sc. (Theoretical Physics) / Please refer to full text to view abstract

M.Sc. / When a pure chromium single crystal is cooled through its Néel temperature, it undergoes a paramagnetic to a multi-wave vector incommensurate spin-density-wave magnetic transition. Should a chromium single crystal be cooled through its Neel temperature in the presence of a strong enough magnetic field, 4-5 T, then all the spin-density-wave vectors that occur as it undergoes the paramagnetic to incommensurate spin-density-wave magnetic transition, are forced to be aligned parallel to the direction of the applied magnetic field, producing what is called a single spin-density-wave wave vector state in the crystal. The single spin-density-wave wave vector state will remain in the crystal when the magnetic field is removed providing the crystal is not heated above its Neel temperature. If the crystal when in the single spin-density-wave wave vector state is orientated so that the single spin-density-wave wave vector is aligned perpendicular to the direction of an applied magnetic field, then the spin polarisation vectors of the magnetic moments will align themselves so that they are perpendicular to the applied magnetic field and perpendicular to the single wave vector, producing what is called a single spin-densitywave wave vector and single spin polarisation vector state in the crystal. There is a difference in value between the elastic constants measured when a chromium single crystal is in the single spin-density-wave wave vector state to the elastic constants measured when the crystal is in the multi-wave vector spin-density-wave state.When pure chromium is alloyed with other materials the topology of the Fermi surface is changed resulting in some of the alloys being able to undergo paramagnetic to commensurate spin-densitywave magnetic transitions, as well as being able to undergo commensurate spin-density-wave to incommensurate spin-density wave transitions. This study makes use of a magnetic field of strength 4.5 Tin an attempt to produce a single spin-density-wave wave vector state in a Cr + 0.3 at.% Ru and a Cr + 1.6 at.% Si single crystals. Both single crystals undergo paramagnetic to commensurate spin-density-wave transitions when cooled through their, respective Neel temperatures, as well as the Cr + 0.3 at.% Ru single crystal being able to undergo a commensurate spin-density-wave to an incommensurate spin-density-wave transition if cooled further, down to 77 K, well below it's Neel temperature. The effects of the applied magnetic field on the Cr + 0.3 at.% Ru and Cr + 1.6 at.% Si single crystals was determined by measuring the elastic constants of the respective crystals.

Ph.D. / A comprehensive experimental study of the effects of the spin-density-wave (SDW) on the physical properties of antiferromagnetic , doped with Mn and V, and Cr-Re alloys is reported. The purpose of the study is twofold: c c Al Cr − 1 (i) To gain insight in the anomalous behaviour of the magnetic phase diagram reported for the binary Cr-Al system. (ii) To investigate SDW effects on the anharmonic behaviour of the lattice vibrations of Cr alloys with a member of the transition metals of group-7 in the periodic table. The investigation entails the following measurements: thermal expansion in the temperature range 77 – 450 K for all the specimens, velocity of sound in the temperature range 4 – 300 K for the Cr-Al-V alloys, ultrasonic wave velocity for the Cr-Re alloys as a function of applied pressure (up to 0.242 GPa) at different temperatures and electrical resistivity in the temperature range 77 – 450 K for the Cr-Re alloys. Concentration-temperature magnetic phase diagrams of the (Mn, V) alloy systems were constructed from the measurements. Alloying with Mn, to increase the electron concentration, is observed to drive an incommensurate (I) SDW alloy towards a commensurate (C) SDW state. This results in a triple point, where the ISDW, CSDW and paramagnetic (P) phases coexist on the magnetic phase diagram. A hysteretic first-order ISDW-CSDW/CSDWISDW phase transition line is then observed on the phase diagram for Mn concentrations above the triple point concentration. Adding V, in order to decrease the electron concentration, to an ISDW alloy is found to have the opposite effect. It drives such a system deeper into the region of the ISDW c c Al Cr − 1 c c Al Cr − 1 phase. A CSDW alloy is, on the other hand, driven towards the triple point by addition of V, instead of Mn. c c Al Cr − 1 Theoretical analysis of the magnetic phase diagrams of the (Mn, V) systems confirms a previous suggestion that the Al impurity acts as an electron acceptor in the Cr matrix for c c Al Cr − 1 2 < c at.% Al, as opposed to an electron donor for at.% Al. 2 > c The high-pressure ultrasonic studies on the Cr-Re alloys were used to construct their pressure-temperature ( T p − ) magnetic phase diagrams. Applying hydrostatic pressure to a CSDW Cr-Re alloy induces a hysteretic first-order CSDW-ISDW phase transition at a certain critical pressure, resulting in a triple point on the phase diagram. An interesting aspect of the observations on the Cr-Re alloys is the suggestion of a new phase line, separating pressureinduced and temperature-induced ISDW phases, on the T p − T p − phase diagram. Acoustic-mode Grüneisen parameters, which quantify the lattice anharmonicity, were calculated for the Cr-Re alloys from the high-pressure ultrasonic measurements. The results indicate exceptionally large interactions between the SDW and the long-wavelength longitudinal phonons in Cr-Re alloys. These effects are particularly large in the vicinity of the Néel phase transition temperature. Interactions of the SDW with the shear mode phonons are on the other hand relatively much smaller. The work on the Cr-Re alloys is considered to finally complete studies of the lattice anharmonicity of Cr alloy systems with elements of all the important groups of the periodic table. It now paves the way for developing microscopic theories to explain the unique behaviour of the magneto-elasticity of dilute Cr alloys. / Prof. H.L. Alberts Dr. A.R.E. Prinsloo

No description available.

Thesis (Ph. D.)--Florida State University, 2005. / Advisor: Dr. James S. Brooks, Florida State University, College of Arts and Sciences, Dept. of Physics. Title and description from dissertation home page (viewed Sept. 15, 2005). Document formatted into pages; contains xiii, 120 pages. Includes bibliographical references.

D.Phil. / Spin-density-wave (SDW) effects were studied in diluted Cr-Si and Cr-Ga alloy single cristals. Ga and Si impurities in Cr are respectively from groups 3 and 4 non-magnetic non-transitional elements from the periodic table. Both influence the magnetic behaviour of Cr in a special way. The alloying of Cr with Si and Ga impurities influences the magnetic phase transition temparatures, namely the Néel temparature (Tn) and the transition temparature (T1c) for the incommensurate to commensurate (C) SDW transition, in a very complex way. The magnetic phase diagrams of both Cr-Si and Cr-Ga show a triple phase where the paramegnetic (P), CSDW and ISDW phases co-exist.

Ph.D. / Spin—density—wave (SDW) effects are investigated in four dilute Cr—Ir alloy single crystals. The Ir concentrations in these crystals were chosen to cover all four magnetic phases existing on the magnetic phase diagram of the Cr—Ir system. Thermal expansion, electrical resistivity, elastic constants and ultrasonic attenuation are studied as functions of temperature and alloy concentration. The elastic constants are also studied as a function of applied hydrostatic pressure. The SDW effects in the Cr—Ir system are compared with those in other Cr alloys. The full temperature—concentration and temperature—pressure magnetic phase diagrams of the Cr—Ir system are determined. Existing theories are used to discuss the observations. The following major observations are made: Magnetoelastic interactions in Cr—Ir alloys are very large, resulting in well defined magnetic anomalies in the elastic constants and thermal expansion at all magnetic phase transition temperatures. Elastic constant measurements as a function of temperature appear to be a very sensitive tool to determine the magnetic phase transition temperatures of the Cr—Ir alloy system. Below TN of Cr—Ir alloys, where TN is the transition temperature from the incommensurate transverse spin—density-wave (TISDW) magnetic phase to the paramagnetic phase, elastic constant and thermal expansion measurements show the existence of hysteresis effects, which are probably due to a redistribution of antiferromagnetic domains. These hysteresis effects are the first evidence of such effects in Cr alloys. Spin fluctuation effects are shown to exist to temperatures well above the Neel temperatures of the Cr—Ir alloys. Analyses of the data for electrical resistivity measurements of Cr—Ir alloys show that the fraction of the electron and hole Fermi surface sheets that nests is roughly the same in the ISDW and CSDW (commensurate spin—density—wave) phases, making the resistivity anomaly near the ISDW—CSDW transition temperature very small or non—existing. Measurements of elastic constants as a function of applied pressure at different constant temperatures are shown to be a very powerful tool for an investigation of the interaction of the SDW with the acoustic phonons in dilute Cr—Ir alloys. It is found that the SDW in Cr—Ir alloys couples mainly with the longitudinal—mode acoustic phonons. Coupling to the shear—mode phonons is relatively small. Empirical correspondence is found between the temperature—concentration and temperature— pressure magnetic phase diagrams of the dilute Cr—Ir alloy system by using a linear scaling between pressure and concentration. Existing thermodynamic models fit the experimental results for the elastic constants and magnetovolume of the Cr—Ir alloys well. The main features of the temperature—concentration and temperature—pressure magnetic phase diagrams of the Cr—Ir alloy system are resonably well predicted by existing microscopic theories.

M.Sc. / Spin-density-wave (SDW) effects are investigated in a Cr + 1.9 at.% Fe alloy single crystal, where the Fe concentration lies just below the triple point found in the temperatureconcentration magnetic phase diagram of the Cr-Fe alloy system. The crystal is expected to undergo a commensurate (C) SDW to an incommensurate (I) SDW phase transition at a temperature Tc, and an ISDW-P (paramagnetic) phase transition at the Neel temperature, TN. The magnetoelastic properties and the anharmonic behaviour of this crystal were studied with the aid of velocity of sound measurements as function of temperature and pressure. Electrical transport measurements were carried out using the standard fourprobe method. In order to determine the various phases present in the crystal a preliminary neutron-diffraction study was also done. Fe belongs to the group-8 magnetic transition metals, possessing localized magnetic moments. The SDW effects in the Cr + 1.9 at.% Fe crystal are therefore compared with that of Cr-Ru and Cr-Ir alloys, as Ru and Ir also belong to the group-8 transition metals, however these impurities are nonmagnetic. The following observations were made: The longitudinal mode elastic constants and the bulk modulus show a prominent change in the slope at Tc, and a sharp, deep minimum at TN. For the c' shear propagation mode peaks were seen at Tcl as well as TN and the c4 4 propagation mode showed no anomalies at either phase transition temperatures. The longitudinal ultrasonic wave velocities for the cL propagation mode were measured as a function of temperature at different constant pressures. TN obtained from these measurements varies linearly with increasing pressure. High-pressure ultrasonic wave velocity measurements were taken at various constant temperatures in the range of 230 K to 350 K for the C L, c44 and c' propagation modes of the Cr + 1.9 at.% Fe alloy single crystal. This was used to determine the pressure derivatives of the second order adiabatic elastic constants (acu /ap). The pressure derivatives of the second order adiabatic elastic constant are shown to be a very powerful tool for investigating the interaction of the SDW with the acoustic phonons in the Cr-Fe crystal. II The long-wavelength acoustic-mode Gitmeisen parameters, calculated from (acu/ap), showed that the SDW in the Cr + 1.9 at.% Fe alloy single crystal couples mainly with the longitudinal acoustic phonons. Coupling to the shear modes is relatively small. The mean acoustic-mode GrOneisen parameter shows a small maximum between Tc, and TN. It increases on heating through TN, reaching a large maximum value above TN, and then decreases with further increase in the temperature. The electrical resistivity was measured_in the temperature region of 4 Kt() 900 K in order to obtain the nonmagnetic component of the resistivity at all temperatures. Only the Neel phase transition was observed in these measurements with no resistivity anomalies taking place at -Va. The experimental results on the resisitivity were analyzed according the model of Chui et al.. The magnetic component of the electrical resistivity was calculated from the model with and without the inclusion of the effects of resonant impurity scattering of the conduction electrons by the local impurity states lying in the SDW energy gap. The magnetic contributions were found to be appreciable above TN, even up to temperatures as high as 1.5TN. The neutron-diffraction experiments show that the Cr + 1.9 at.% Fe crystal remains in the ISDW phase at all temperatures below TN. This is an unexpected result as a CSDW-ISDW phase transition is expected at To, the temperature of the observed anomaly in elastic constant and thermal expansion measurements on the crystal

M.Sc. / The aim of this study is to show that there exists a parallelism between the effect of the concentration of the itinerant electrons per atoom, and the applied hydrostatic pressure, p, on the magnetic phase diagram of a Cr + 0.2 at.% Ir alloy. This Cr-Ir alloy was chosen, because it contains all possible magnetic phases that can exist in a Cr alloy.