Instrumentation development for studies of magnetic and structural properties of molecular magnets

Abstract

Tetramethyl ammonium manganese trichloride ([CH3)4N][Mn(II)Cl3]) known as TMMC, has been one of the most interesting systems in experimental magnetism due to its highly one-dimensional magnetic Heisenberg behaviour. The focus of this research programme was to study its magnetic and structural properties as a function of pressure. TMMC crystals were prepared by slow evaporation technique at room temperature; however it quickly became apparent that the material is only weakly magnetic and requires a pressure cell with a very low background. This discovery lead to the programme of instrumentation development for studies of weakly magnetic materials and gave a dualistic nature to the project.

The first pressure cell developed was a piston-cylinder type cell for magnetic susceptibility measurements in a Magnetic Properties Measurement System (MPMS®) based on Superconducting Quantum Interference Device (SQUID) technology from Quantum Design, USA. It has been carefully designed in order to reduce the magnetic background. One way in which this has been achieved was through making the pressure cell symmetric with respect to the sample in order to provide an integrable response in the SQUID magnetometer. The cell was made of beryllium copper alloy which has a low background even at low temperature. The use of a multi-layered cylinder with the interference fit method has resulted in the increased strength of the cell and allowed larger sample volume. The use of Lamé equation and finite element method to calculate the change of the cell diameter or cell length as a function of internal pressure enables us to eradicate the need of superconductive manometer. The cell has been successfully tested up to a maximum pressure of 10 kbar. Further development of the cell has resulted in development of an electrical plug for in situ pressure measurement inside the pressure cell. This has been achieved by means of a manganin pressure sensor calibrated to provide pressure reading at any given temperature.

For structural studies, a diamond anvil cell (DAC) was designed to conduct singlecrystal X-ray diffraction measurements at low temperature. The design was based on the well-known Merrill-Bassett DAC and on the design of the miniature DAC which has been developed for use within He-3 system in the Physical Properties Measurement System (PPMS®), Quantum Design. The cell has been tested down to liquid nitrogen temperatures with a cryostream cooling system and has shown a significant improvement compared to the standard pressure cells. The frost formation on the surface of the cell has slowed down significantly compared to the tests on the Merrill-Bassett cell, which led to a better quality diffraction pattern from the sample inside the cell. This result has been achieved due to the high thermal conductivity of the materials used in the construction and the minimisation of the DAC, which was effectively built around the Boehler-Almax diamond anvils.

With the help of some of the high-pressure instruments mentioned above, highpressure properties of TMMC have been studied in this project. The structuremagnetism relationship was established from the results of magnetic and structural measurements under pressure. The magnetic susceptibility data helped to establish the change of the intrachain antiferromagnetic coupling constant as a function of pressure, while X-ray structures of TMMC were refined from ambient pressure to 17 kbar using a synchrotron X-ray diffraction technique. The structure of TMMC at room temperature was confirmed to be hexagonal. However, indirect evidence of the hexagonal-monoclinic structural phase transition was observed at above 17 kbar and room temperature. The combination of the magnetic and structural data has helped to establish that the interaction between high spin d5 metal orbitals (Mn(II)) in facesharing octahedral has a contribution from both direct exchange and superexchange interactions. The power-law relationship developed by Bloch was also observed in this system.