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Title: The temperature dependent phases of some 3d transition element Heusler alloys
Authors: Maw, Toby
Issue Date: 2012
Publisher: © Toby Maw
Abstract: The full characterisation of the compound series Cr2VX (X = Al, Si, Ga, Ge) is reported. X-ray and neutron diffraction measurements were performed along with electrical resistivity and magnetisation measurements on each compound. It is found that Cr2VAl crystallises in the disordered Heusler A2 structure with space group Im3 ̅m and this structure remains stable between 5K and 1223K. Contrary to this, the other compounds in the series crystallise in a partially ordered A15 structure with space group Pm3 ̅n. These compounds are shown to behave more like pseudo binary systems of composition X3Y such as Cr3X and V3X than Heusler systems of composition X2YZ. Each compound acts metallically and has magnetic properties consistent with those of weak Pauli paramagnetic materials. It is suggested that the size of the unit cell is the dominant factor for these compounds whether they crystallise into either the A2 or the A15 structure. This is investigated with the high temperature characterisation of the compound series Cr2VGa1-xAlx (x = 0, 0.25, 0.5, 1). Structural phase transformations are identified and characterised using a combination of high temperature neutron diffraction and electrical resistivity measurements. The results of these measurements show that the phase transformations involve displacive, order disorder and diffusion based processes. Furthermore, they reveal the existence of a metastable intermediate phase with a tetragonal unit cell and space group P4 ̅2c. The compound series Cr2-xFexVAl (x = 0, 0.5, 1.5, 2) was investigated to better understand the nature of the magnetic properties observed within this group of alloys. The magnetisation of each compound was measured with a SQUID magnetometer. At high temperature, weak Pauli paramagnetic behaviour retained for each compound. However, at low temperature an up-turn can be seen in the magnetisation of the compounds containing Fe such that their susceptibility has a logarithmic relationship with temperature. Neutron diffraction measurements show that the Fe rich and Cr rich compounds form the ordered L21 and disordered A2 Heusler structures, respectively. However, no evidence was found of a structural change at low temperatures to activate the change in magnetic response. Finally, the structural ordering of the compound series Ni2-xMn1+xGa is explored using high temperature electrical resistivity measurements. Increasing Mn content is shown to decrease the order disorder transition temperatures due to increased levels of atomic disorder. This affects a sample s state of atomic order after heat treatment, which is usually performed by quenching samples from 800ºC. Therefore, it is an important influence on the compound s reported magnetic and martensitic transitions particularly for Mn rich samples.
Description: A Doctoral Thesis. Submitted in partial fulfillment of the requirements for the award of Doctor of Philosophy of Loughborough University.
URI: https://dspace.lboro.ac.uk/2134/9641
Appears in Collections:PhD Theses (Physics)

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