Novel bulk non-rare-earth Mn-Bi permanent magnets processed by severe plastic deformation

Abstract

Novel bulk non-rare-earth Mn-Bi permanent magnets processed by severe plastic deformation An increasing global energy consumption and demand for renewable energy requires permanent magnets with a high energy product. Current high performance hard magnets are mostly based on rare earth transition-metals e.g. SmCo and NdFeB but mining, production and therefore the prize of conventional rare earth elements are often correlated to politic instabilities due to monopolistic situations in the countries of origin. Hence, permanent magnetic materials out of non-rare-earth elements draw interest in many research areas, particularly the concept of exchange coupling is a promising approach to enhance magnetic properties. Although Mn is paramagnetic and Bi is diamagnetic, the low temperature α-MnBi phase shows desirable hard magnetic properties, due to its high magnetic anisotropy and a remarkable property as the coercivity is increasing with increasing temperature. Therefore, α-MnBi is a highly desirable phase for high temperature applications and when exchange coupled with a soft magnetic phase, the composite could even surpass well established hard magnetic materials, in terms of fields of use. Severe plastic deformation (SPD) by high-pressure torsion (HPT) is a technique to produce bulk nanostructured materials. It can also help to evade conventional processing limits of alloying. The resulting sample sizes exhibit dimensions in the millimetre regime but can be increased to several centimetres, which is convenient for further investigations and future applications. In this study, HPT deformed MnBi samples are compared to other processing techniques such as ball milling, inductive melting or arc melting. Furthermore, all processing techniques are properly concatenated prior to HPT deformation. The as-processed samples are subsequently subjected to thermal treatments to investigate the formation and thermal stability of the α-MnBi phase. Results obtained by in-situ annealing synchrotron-XRD measurements indicate a successful preparation of the α-MnBi phase by techniques of SPD and subsequent thermal treatments. The microstructure of as-deformed and annealed samples is further examined by electron microscopy, X-ray diffraction techniques and correlated to magnetic properties. Although the crystal-structure, thermodynamical and mechanical properties differ a lot in Mn and Bi, it is possible to produce novel bulk permanent magnets out of non-rare-earth elements by SPD and subsequent annealing treatments.