M.F.J. Boeije
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We have investigated the crystallographic and magnetic properties of (Mn,Fe)3-δGa alloys. The hexagonal phase is stable between 600 and 700 °C and can be stabilized by quenching to room temperature. Mn3Ga is reported to be off-stoichiometric, but we show that using melt-spinning the stoichiometric compound is attainable. Below the antiferromagnetic transition temperature TN, the crystal undergoes a hexagonal to monoclinic transition at the distortion temperature Td. This gives rise to an in-plane rotation of the magnetic moments that is accompanied by a simultaneous increase in magnetization in a magnetic field of 1 T. Fe substitution for Mn removes the monoclinic distortion. Substitutional Fe weakens the antiferromagnetism and a paramagnetic to ferromagnetic transition is observed. The Mn sublattice couples antiparallel throughout the series. Substitution of Ga with Si preserves the monoclinic distortion.
‘‘giant’’ effects. After earlier studies investigated the relation between microscopic and macroscopic properties of these materials, the focus of this thesis is on the electronic factors that play a role in the stability and phase transitions of these compounds. After all, when the mechanism behind these phase transitions is clear, is it easier to search for new materials that show similar phase transitions. Two strategies are possible: elucidating the mechanism of Fe2P-based materials or investigating materials that show similar phase transitions. The latter is described in the next paragraph... ...
‘‘giant’’ effects. After earlier studies investigated the relation between microscopic and macroscopic properties of these materials, the focus of this thesis is on the electronic factors that play a role in the stability and phase transitions of these compounds. After all, when the mechanism behind these phase transitions is clear, is it easier to search for new materials that show similar phase transitions. Two strategies are possible: elucidating the mechanism of Fe2P-based materials or investigating materials that show similar phase transitions. The latter is described in the next paragraph...
We present a hybrid method to inspect the phase stability of compounds having a CaCu5-type crystal structure. This is done using 2D stability plots using the Miedema parameters that are based on the work function and electron density of the constituent elements. Stable compounds are separated from unstable binary compounds, with a probability of 94%. For stable compounds, a linear relation is found, showing a constant ratio of charge transfer and electron density mismatch. DFT calculations show the same trend. Elements from the s,d,f-block are all reliably represented, elements from the p-block are still challenging.
Magnetic cooling is a highly efficient refrigeration technique with the potential to replace the traditional vapor compression cycle. It is based on the magnetocaloric effect, which is associated with the temperature change of a material when placed in a magnetic field. We present experimental evidence for the origin of the giant entropy change found in the most promising materials, in the form of an electronic reconstruction caused by the competition between magnetism and bonding. The effect manifests itself as a redistribution of the electron density, which was measured by X-ray absorption and diffraction on MnFe(P,Si,B). The electronic redistribution is consistent with the formation of a covalent bond, resulting in a large drop in the Fe magnetic moments. The simultaneous change in bond length and strength, magnetism, and electron density provides the basis of the giant magnetocaloric effect. This new understanding of the mechanism of first order magneto-elastic phase transitions provides an essential step for new and improved magnetic refrigerants.