Instrumentation development for magnetic and structural studies under extremes of pressure and temperature
The study of the magnetic and structural properties of matter under extreme conditions is a fast developing field. With the emergence of new techniques and innovative instruments for measuring physical properties, the need for compatible pressure generating devices is constantly growing. The work described in this thesis is focused on development, construction and testing of several high pressure (HP) cells of novel design. One of the cells is intended for single crystal X-ray diffraction (SXD) studies at low temperature (LT) and the other three HP devices are designed for a Magnetic Property Measurement System (MPMS), two of which are suitable for dc susceptibility studies and the other one is aimed at high frequency ac susceptibility measurements. HP crystallographic studies are routinely carried out in diamond anvil cells (DAC) at room temperature while ambient pressure SXD studies are often conducted at LT to reduce atomic vibrations and obtain more precise structural data as well as to study LT phases. Combining HP with LT gives access to a whole new area on the phase diagrams but due to the size of the existing DACs this is generally achieved by cooling down the cells inside a cryostat and it is mainly possible at synchrotrons where dedicated facilities exist. A miniature DAC which can be used with commercially available laboratory cry-flow cooling systems and achieves pressures in excess of 10 GPa has been developed. The design of the pressure cell is based on the turnbuckle principle and therefore it was called TX-DAC. Its dimensions have been minimised using Finite Element Analysis (FEA) and the final version of the cell weighs only 2.4 g. The cell is built around a pair of 600 μm culet Boehler-Almax anvils which have large conical openings for the diffracted beam. The TX-DAC is made of beryllium copper (BeCu) alloy which has good thermal conductivity and allows quick thermal equilibration of the cell. The MPMS from Quantum Design is the most popular instrument for studies of magnetic properties of materials. It is designed to measure ac and dc magnetic susceptibility of sample with detectable signals as low as 10-8 emu. The MPMS has a sample chamber bore of 9 mm in diameter and this puts a constraint on the dimensions of the pressure cells. However, several types of clamp piston-cylinder cells and DACs have been designed for the MPMS. The former are used for measurements at pressure up to 2 GPa and the later can be used for studies at higher pressure. Taking advantage of the turnbuckle principle, a DAC (TM-DAC) and a piston-cylinder cell (TM-PCC) for dc magnetic studies were built. They allow HP measurements to be performed at the full sensitivity of MPMS. Both pressure cells are made of BeCu and their small dimensions combined with symmetrical design is the key to an ideal background signal correction. The TM-DAC is 7 mm long and 7 mm in diameter, it weighs 1.5 g and with 800 μm culet anvils it can generate a sample pressure of 10 GPa. Inherently the sample volume is limited to approximately 10-3 mm3 and the signal corresponding to this volume of some weakly magnetic material remains below the sensitivity of the MPMS. This constraint led us to the development of the TM-PCC – a piston-cylinder variant of the turnbuckle design. With a 4 mm3 sample volume it allows the study of weakly magnetic samples in the range 0-1.9 GPa. The TM-PCC uses two zirconia pistons of 2.5 mm in diameter; it is 10 mm long, 7 mm in diameter and weights 2.7 g. Conventional metallic pressure cells perform well in dc mode however in ac susceptibility measurements, the Eddy currents set in the cells’ body lead to a screening effect which can significantly obscure the signal from the sample. This problem was solved by designing a composite piston-cylinder cell made with Zylon fibre and epoxy resin. The sample is located in the middle of the cell in the 2.5 mm bore and the pressure is transmitted through zirconia pistons. Keeping the metallic parts away from the sample resolves any interference issue. The composite cell performs well in a pressure range of 0-1 GPa. The performance of the pressure cells developed within this project is illustrated by studies of various systems at high pressure.