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

磁体是许多机电机器和电子设备的重要部件。典型的例子是电动机和执行器，发电机，声音再现系统，计算机外围设备（磁盘驱动器，打印机）或简单的磁夹。今天使用的主要磁性材料是铝镍钴合金、铁氧体和稀土（RE）基合金。对于硬磁材料，主要要求是高矫顽力、剩磁和最大磁能积（BH）max。广泛的技术应用需要各种具有不同磁滞回线特性的永磁体，并且通常基于经济考虑做出最终选择。稀土基金属间化合物合金是最新和更强大的永磁体家族。然而，除了它们的上级磁性能之外，它们还有几个缺点。在其勘探和加工过程中会产生放射性副产品。此外，需要额外的表面活性剂来保护RE免于氧化，这导致更高的成本和额外的环境污染。SE材料具有非常低的居里温度（NdFeB，TC=583 K），这也限制了它们的使用。最重要的是，RE材料非常昂贵，并且SE材料的价格目前甚至更高，这使得非常需要RE材料的替代品。克服这些困难的一个明确的策略是开发新的稀土自由磁体具有可观的磁性能。朝着这个方向，大量的注意力已经给予了Mn 3-xGa Heusler合金，尽管缺乏RE元素，但其表现出亚铁磁性和大的磁晶各向异性。因此，与RE磁体相比，Mn-Ga材料的优点是它们的高居里温度和相当大的磁各向异性以及更高的化学稳定性。文献中关于Mn 3-xGa合金中不同晶相共存的相互矛盾的说法要求对这些化合物的结构、微观结构和磁性进行更彻底的研究，并详细说明结构-性能-关系。在这个项目中，我们计划探索和理解Mn 3-xGa中硬磁性质的起源。这将通过定制制备过程，研究（磁性）结构和微观结构，通过Mn和Ga的系统替代，以及通过将这些特性与磁性关键特性（剩磁，矫顽场，最大能量积）相关联来完成，以分配具有高磁性能的新的稀土自由永磁体。

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.