Title: Critical behavior in magnetic phase diagrams of uranium compounds Author: Bc. Petr Opletal Department: Department of Condensed Matter Physics Supervisor: RNDr. Jan Prokleška, Ph.D., Department of Condensed Matter Physics Abstract: This work is focused on study of phase diagrams and related critical effects in the pseudoternary UCo1-xRuxAl compound. Three single crystals of nominal composition UCo0.99Ru0.01Al, UCo0.995Ru0.005Al and UCo0.9975Ru0.0025Al have been prepared by Czochralski method in triarc furnace. Quality of single crystals was checked by EDX analysis, Laue method and XRPD. For UCo0.99Ru0.01Al Curie temperature was determined as TC = 16 K. UCo0.995Ru0.005Al shows transition from paramagnetic to ferromagnetic phase at TC= 4.5 K and at higher temperatures we observe a metamagnetic transition of first order with a critical field HC = 0.04 T. UCo0.9975Ru0.0025Al is paramagnetic to low temperatures with metamagnetic transition of first order and critical field HC = 0.55 T. Experiments in hydrostatic pressure on UCo0.995Ru0.005Al and UCo0.9975Ru0.0025Al showed decay of ferromagnetism and increase of critical field. This behavior is similar to evolution of magnetism in UCoAl and URhAl in applied hydrostatic pressure. Keywords: UCoAl, URuAl, ferromagnetism, metamagnetism

High pressure, high magnetic field and low temperature techniques are required to investigate magnetic transitions and quantum critical behaviour in different ferromagnetic materials to elucidate how novel forms of superconductivity and other new states are brought about. In this project, several instruments for magneto-transport and neutron scattering measurements have been designed and built. They include inserts for a dilution refrigerator and pressure cells for resistivity, magnetic susceptibility and inelastic neutron scattering measurements. The technical drawings of the low temperature inserts and pressure cells were produced with Solid Edge computer-aided software and the performance and safety assessments were evaluated with the ANSYS finite element analysis package. The pressure cells developed include diamond anvil cells, piston cylinder cells and some auxiliary equipment. Pressure effects on the physical properties such as the electrical resistivity and magnetic ordering of some ferromagnetic materials were studied with the equipment developed. A two-axis rotating stage was developed and deployed with a dilution refrigerator combined within a superconducting magnet to measure various physical properties as a function of the orientation of the sample with respect to applied field at sub-Kelvin temperature. The rotating stage is made of Beryllium Copper (BeCu) alloy. In order to avoid the entanglement of the wires, custom-designed “flexi cables” - copper tracks printed on a Kapton foil with a yield of nearly 100% - to work with the rotating stage were manufactured. The performance of the rotating stage has been demonstrated by a quantum oscillation in the electrical resistivity study of a high field ferromagnetic superconductor URhGe. A miniature diamond anvil cell based on the turnbuckle principle has been designed. The cell, made of BeCu alloy, is 7mm in length and 7mm in diameter. It has been shown to reach a maximum pressure of 10 GPa with diamond anvils with 800 μm culets. The small dimensions of the cell allow it to fit into the existing sample environment such as Physical Properties Measurement System (PPMS) and Magnetic Properties Measurement System (MPMS) from Quantum Design, USA, and onto the customized two-axis rotating stage built for the dilution fridge. It also thermalizes rapidly allowing rapid cooling and heating during the experiments. The cell can be used to make both resistivity and magnetic susceptibility measurements. To ensure the hydrostaticity of the pressure around the sample in the turnbuckle cell, a gearbox was designed for cryogenic loading of liquid argon and room temperature gas loading of either helium or argon at a loading pressure of up to 0.3 GPa. Pressure effects on the Curie temperature of a PrNi ferromagnet were studied in a diamond anvil cell. Four-probe resistance measurements under pressures up to 9 GPa were carried out in a PPMS. The possibility of tuning the physical properties of the material by altering the pressures has been demonstrated. By analysing the results of the electrical resistivity measurements under pressures, it was concluded that the Curie temperature of PrNi increases with pressure at the rate of 0.85 K per GPa. The quantity ∆(δρ/δτ)which reflects some part of the entropy change also increases with pressure. The expected quantum critical point has not been observed in this material up to 9 GPa. A large volume high-pressure piston-cell for inelastic neutron scattering measurements has been designed and can reach a pressure of up to 1.8 GPa with a sample volume in excess of 400 mm3. The dimension of the part of the cell exposed to the neutron beam has been optimized to minimize the attenuation of the neutron beam. The novel design of the piston seal also eliminates the use of a sample container, which makes it possible to accommodate larger samples and reduces the absorption. The pressure in the cell is measured by a manganin pressure gauge placed next to the sample. The performance of the cell was illustrated by an inelastic neutron scattering study of UGe2.

This study investigated the critical behaviour of ternary mixtures of DOPC and DPPC, with cholesterol. The properties of model membranes such as these are studied in order to provide insight into aspects of complex biological systems. Experiments were performed using the Jeener echo, a static solid-state NMR technique, however no information about the critical phenomena was obtained. Conversely, the sideband linewidths measured from 2H MAS NMR are sensitive to temperature and dependent upon the phase behaviour. By fitting the linewidth data to an equation from Suwelack et al. (J. Chem. Phys., 1980; 73(6):2559-2569), the critical temperature and the critical exponent for the correlation length of the system were calculated. The critical exponent values obtained from these samples ranged between νc = 0.65 and νc = 1.2, which encompasses the critical exponents for both the 2D and 3D Ising models within error. The universality class for these model membranes cannot be unambiguously assigned yet.