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