(ESICMM-G8 Symposium on Next Generation Permanent Magnets, Tsukuba, 2015)
Advances on MnBi based hard magnets and recent XMCD results on high performance permanent magnets
Yu-Chun Chen, Eberhard Goering, Sapana Tripathi, Gisela Schütz, Helmut Kronmüller
Max Planck Institute for Intelligent Systems, Stuttgart, Germany
Abstract:
Figure 1. RT magnetization curve of oriented MnBi
resin-bonded sample, which exhibits nearly rectangular
shape with both high coercivity and remanence.
In order to improve the quality of hot compacted and sparc plasma sintered MnBi bulk samples, we optimized the starting MnBi powder. We achieved high performance magnetic properties, which are above the reported maximum energy product (BH)max values (~ 17.7 MGOe). This high-performance MnBi powder could be easily mass produced with traditional ball milling technique leading to small particle size about 0.1~5 µm. The as-prepared powder was then mixed and oriented with molten resin in the presence of high magnetic fields. Figure 1 shows the room temperature hysteresis curve of the anisotropic bonded magnet, which exhibits nearly rectangular shape with both high remanence and coercivity. Besides, the aligned powder was also shaped into bulk samples by hot compaction and spark plasma sintering. It is found that the thermal stability of SPS-compacted magnets is relatively better probably due to unique microstructure (submitted to APL). The physical properties of the as-milled powders and compacted samples were analyzed using x-ray diffraction (XRD), scanning electron microscopy (SEM), differential scanning calorimetry (DSC) and superconducting quantum interference device (SQUID).
Following our previous report at the G8 meeting in Darmstadt, we have confirmed our XMCD results on anisotropic MnBi samples. Indeed, we have shown the presence of a strong Tz term and a temperature dependence of the orbital moment anisotropy. These findings provide the basis for a microscopic understanding of the unusual MCA in this system.
In addition, we performed element specific XMCD investigations in NdFeB system. We found a remarkable reduced orbital moment in the hard magnetic samples with respect to the Hunds rules or a soft magnetic reference system. This result is completely unexpected, providing a new microscopic model to understand hard magnetic properties in those systems. Analogous behavior of SmCo based hard magnets suggest a more general basic phenomenon behind.