High-Resolution Magnetic Domain Wall Imaging by DPC STEM

Permanent magnets are utilized in many applications such as the main power motor in electric vehicles. It is known that the coercivity, the resistance to changes in magnetization, of magnets can be significantly improved by suppressing the formation of magnetization reversal nuclei and preventing the growth of the reversal nuclei[1]. Micromagnetic simulations predicted that the local change in magnetic properties such as magnetocrystalline anisotropy is crucial to prevent the growth of the reversal nuclei[2]. Thus, the coercivity may be improved by optimizing microstructures consisting of the phases with different magnetic properties. The existence of various phases in microstructures has been reported in high coercive Nd-Fe-B permanent magnets[3], but the key microstructural features to improve the coercivity are still not well understood. It is thus essential to identify one-by-one relationship between microstructural features and their magnetic properties. One of the methods to directly measure the local magnetic properties is to measure the magnetic domain wall (DW) width. DW width varies with the change in the ratio of exchange stiffness and magnetocrystalline anisotropy where the DW is located. The larger DW width indicates larger exchange stiffness or smaller magnetocrystalline anisotropy. However, direct and precise measurement of DW width requires high spatial resolution, since DW is a nanometer-sized magnetic structure.<br/>Differential phase contrast scanning transmission electron microscopy (DPC STEM)[4] is an imaging technique of local electromagnetic fields inside specimens. Combined with recently developed magnetic-field-free atomic resolution STEM [5], DPC STEM now enables visualization of nanoscale magnetic structures in magnetic free environment. Furthermore, one can obtain structural information in the same field of view through other STEM techniques such as high-angle annular dark field images and electron energy loss spectroscopy (EELS).<br/>In this study, we directly measured DW width in La-substituted Nd-Fe-B magnets by DPC STEM. In this magnet, Nd is non-uniformly substituted by La, and the magnetocrystalline anisotropy is expected to locally fluctuate with Nd concentration. To confirm the correlation of DW width and Nd concentration, we measured DW width by DPC and analyzed Nd composition by EELS at the same area. The results show the DW width increases with the decrease in Nd concentration. Details will be reported in the presentation.<br/>1) O. Gutfleisch, J. Phys. D: Appl. Phys. 33 (2000).<br/>2) G. Hrkac, et al. Appl. Phys. Lett. 23 (2010).<br/>3) T. G. Woodcock, et al. Acta Mater. 77 (2014).<br/>4) N. Shibata <i>et al</i>., Acc. Chem. Res. 50 (2017).<br/>5) N. Shibata <i>et al</i>., Nat. Commun. 10 (2019).