Tetragonal distorted Mn3-xGa Heusler compounds: Novel hardmagnetic materials without 4f electrons

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Magnets are vital components of many electromechanical machines and electronic devices. Typical examples are motors and actuators, generators, sound reproduction systems, computer peripherals (disc drives, printers) or as simple magnetic clamps. The principal magnetic materials used today are the Alnico alloys, the ferrites, and the rare-earth (RE) based alloys. For hard magnetic materials, the main requirements are high coercivity, remanence and maximum energy product (BH)max. The wide range of technological applications requires a wide range of permanent magnets with different characteristics of the hysteresis loops and often the final choice is made based on economic considerations. The rare-earth based intermetallic alloys are the most recent and more powerful family of permanent magnets. However, apart from their superior magnetic performance, they have several disadvantages. During their exploration and processing radioactive side-products are generated. Moreover, additional surfactants are necessary to protect the RE from oxidation, which causes higher costs and additional environmental pollution. SE materials have a comparably low Curie temperature (NdFeB, TC=583 K) which also limits their use. Most importantly, RE materials are very expensive and the price for SE materials is currently even more increasing, which makes a substitution for RE materials highly desirable. A clear strategy to overcome these difficulties is to develop new rare earth free magnets with appreciable magnetic properties. Towards this direction substantial attention has been given to Mn3-xGa Heusler alloys which, despite the lack of RE elements exhibit ferrimagnetism and large magneto-crystalline anisotropy. Hence, the advantage of the Mn-Ga materials are their high Curie temperature and considerable magnetic anisotropy combined with a higher chemical stability compared to the RE magnets. Contradicting claims in the literature regarding the coexistence of different crystalline phases in Mn3-xGa alloys call for more thorough studies of the structural, microstructural and magnetic properties of these compounds and to elaborate on the structure-properties-relations. In this project, we plan to explore und understand the origin of the hardmagnetic properties in Mn3-xGa. This will be done by tailoring the preparation process, investigating the (magnetic) structure and microstructure, by systematic substitution of both Mn and Ga, and by relating those properties to the magnetic key properties (remanence, coercive fields, maximum energy product) to assign a new rare earth free permanent magnet with high magnetic performance.