Quantitative Surface-Magnetic Imaging by SEMPA to Measure the Dzyaloshinskii–Moriya Interaction and Investigate Three-Dimensional Magnetic Topologic Structures

Scanning electron microscopy with polarization analysis (SEMPA) is a versatile tool for high-resolution magnetic surface imaging. The capability of synchronous vector imaging is especially useful for accurately determining the in-plane angle of the local magnetization vector. Domain walls in ultrathin films with DMI and out-of-plane magnetization show a thickness-driven continuous transition from Néel to Bloch orientation, which is due to the 1/d reduction in DMI strength that is competing with the dipolar energy of the wall’s magnetic charge. By measuring the DW angle and modeling the dipolar energy the DMI strength can be determined. We present examples for epitaxial, in-situ grown layers of Co/Pt(111) and Co/Ir(111) [1,2], where due to the absence of DMI in the ferromagnet/vacuum interface exactly the DMI of one epitaxially ordered interface can be measured. While UHV conditions are required in SEMPA, there is in fact no strict limitation to imaging in-situ prepared, uncapped magnetic layers. After covering an ex-situ sputtered sample with 1 nm of Pt imaging is still possible, however at reduced magnetic contrast [3]. This enables surface imaging of sputtered multilayer stacks with varied couplings that can host three-dimensional magnetic structures. Of special interest is this for imaging synthetic antiferromagnets [4], where due to the high surface sensitivity of less than one nanometer no contrast reduction from the AF coupled second layer occurs. The combination of high-resolution surface in-plane imaging with micromagnetic simulation and additional, less surface-sensitive microscopy methods allows for successful investigation of such structures. By tilting the sample or using additional techniques like MFM the out-of-plane component of the magnetization gets accessible and thus chiral objects like skyrmions or merons can be identified.<br/><br/>[1] E.C. Corredor, et al. Phys. Rev. B <b>96</b>, 060410(R), (2017).<br/>[2] F. Kloodt-Twesten, et al. Phys. Rev. B <b>100</b>, 100402(R), (2019).<br/>[3] S. Kuhrau, et al. Appl. Phys. Lett. <b>113</b>, 172403, (2018).<br/>[4] T. Dohi et al, Nat. Commun. <b>10</b>, 5153 (2019).