Interfaces and Magnetisation Reversal in MnAl-C

Permanent magnets are key elements of modern society. Important application areas are energy conversion including eco-efficient transport, hydro- and wind power. A promising magnetic material is MnAl-C. Although it contains no ferromagnetic elements such as iron, nickel or cobalt, the so-called tau-MnAl-C is ferromagnetic up to high temperatures and has all properties which are prerequisites for high performance permanent magnets. The tau- MnAl-C contains no critical elements and therefore the long term use of this material is environmentally sustainable, in stark contrast to that of rare earth magnets such as Nd-Fe-B. In addition, tau-MnAl-C has a low physical density, which is a significant advantage for transport and aerospace applications. One prerequisite for a good permanent magnet is a high anisotropy: The magnetic moments prefer to be oriented along a certain crystallographic direction. Although the anisotropy of tau-MnAl-C is high for a material without rare earth elements, the resistance to magnetisation reversal, known as coercivity, which has been achieved in practice, is only about 10% of the maximum possible value given by the crystalline anisotropy. This is currently the barrier to the application of MnAl-C as a permanent magnet and can be explained by the microstructure of the material. It contains a range of internal interfaces such as grain and twin boundaries. Previous studies have indicated that these interfaces play a role in the process of magnetisation reversal but the occurring mechanisms and the relative strength of these effects are completely unknown. Understanding the effect of the various interfacial types on the process of magnetisation reversal is the key step in improving the performance of MnAl-C magnets. It will allow researchers and the magnets industry to develop novel processing routes which promote the formation of interfaces which have a beneficial effect on coercivity and supress those which are deleterious. In this project, a novel approach combining state of the art characterisation techniques with cutting edge computer simulations will be used to obtain quantitative information concerning the effect of interfaces on magnetisation reversal in tau-MnAl-C. Previously only small areas of the microstructure have been analysed. In this project electron backscatter diffraction will be used to analyse large areas, yielding accurate populations of the various interface types for the first time. Nanoscale features will be investigated using high resolution transmission electron microscopy. Computer models will be generated from the microstructure data, enabling areas of the microstructure to be reproduced and the effect of interfaces on magnetisation reversal to be computed directly. The application of this approach to magnetic materials is highly novel. Materials with different populations of interfaces will be prepared and studied using this combined method.

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