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 T_N, the crystal undergoes a hexagonal to monoclinic transition at the distortion temperature T_d. 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.

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Temperature dependent high-resolution x-ray diffraction measurements were used to characterize the magneto-elastic ferromagnetic transition of (Fe,Mn)2(P,Si,B) compounds. Across the transition, apart from a change in lattice parameters across the transition, the internal coordinates of Mn and Fe also change. This intrinsic degree of freedom allows Fe in the tetrahedral coordination to decrease the two interatomic distances with the 2c position and increase the two distances with the two 1b position, while the Fe–Mn distance remains constant. For Mn in the square based pyramidal coordination, all interatomic distances effectively remain constant. Electron density plots show that for second-order transitions, the observed changes are smaller and continuously extending over a wide temperature range in the ferromagnetic and paramagnetic states, due to short-range order. This study shows that the mechanism behind the phase transition in Fe2P-based materials is an isostructural transition that is equal for both first- and second-order transitions.@en

In this thesis, the boundary conditions for the development of giant magnetocaloric materials are investigated. The magnetocaloric effect is found in magnetic materials, when they are subjected to an external magnetic field. This leads to abrupt magnetization changes that cause a temperature change in the material. Materials based on Fe2P show giant temperature changes around room temperature and are especially suited for cooling applications. This is due to the large magnetization change that can be realized with the application of a relatively small magnetic field around the ferro- to paramagnetic phase transition. This transition is of a first order, giving rise to latent heat and rise to ‘‘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...@en

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.

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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.

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Magnetic refrigeration near room temperature is considered as environmentally benign alternative for the current compressor based cooling technology. Two materials that are based on ferromagnetic transition metal compounds are considered as the most promising candidates to be used in real world applications. Here we discuss the main features of these materials.@en

Chromium (Cr), manganese (Mn) and carbon (C) are well known alloying elements used in technologically important alloy steels and advanced high strength steels. It is known that binary Crc(x) and MnCx carbides can be formed in steels, but in this study we reveal for the first time that Cr and Mn were found combined in novel ternary cementite type (Cr, Mn)C carbides. Electron diffraction experiments showed that Cr, Mn and C formed two distinct carbide phases possessing orthorhombic and monoclinic crystal structures. Density functional theory calculations were performed on these phases and excellent agreement was found between calculations and experiments on the lattice parameters and relative atomic positions. The calculations showed that the combination of Mn and Cr resulted in a very high thermodynamic stability of the (Cr, Mn)C carbides, and that local structural relaxations are associated with carbon additions. Possible implications of these ternary carbides for novel applications in steel design and manufacturing are discussed.@en