Instrumentation development for magneto-transport and neutron scattering measurements at high pressure and low temperature
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.