GD-09 Wide-field MOKE microscopy and magnetometry on Cr2Ge2Te6 exfoliated van-der-Waals flakes

Since their discovery [1], ferromagnetic two-dimensional (2D) van-der-Waals (vdW) materials, exfoliated via adhesive tape, are intensively explored [2]. They often have Curie temperatures below room temperature and are characterized by a uniaxial magnetic anisotropy perpendicular to the film plane. As for most magnetic materials, hysteresis measurements and domain research also play crucial roles in the characterization of vdW material. Hysteresis loops are mostly obtained by laser-based magneto-optical Kerr effect (MOKE) magnetometry or by anomalous Hall effect measurements, while for domain imaging primarily Lorentz Transmission Electron Microscopy, Nitrogen-Vacancy Microscopy and X-ray based imaging are applied [2]. Wide-field MOKE microscopy, a common imaging technique for general domain research [3], has hardly been used. Using Cr2Ge2Te6 flakes in the 10 nm thickness range, we explore the potential of this method for the characterisation of low-dimensional crystals. By plotting the Kerr intensity at selectable sample areas as a function of magnetic field, hysteresis loops at specific thicknesses are obtained. Pinched loops with a characteristic shoulder are typically measured, indicating the nucleation of perpendicularly magnetized band and bubble domains. Such domain patterns, however, cannot be seen in the Kerr microscope, probably owed to the resolution limit of cryo-Kerr microscopy (~ 500 nm). The method is nevertheless useful as it detects how the flakes, which usually consist of differently thick areas, are magnetised locally. We further verify a thickness- and light colour dependent sign inversion of the Kerr signal [4] and explain it by a Fresnel-type depth sensitivity concept [5]. Accordingly, the Kerr contrast is governed by the relative phase of the Kerr amplitude that can be freely adjusted by a rotatable compensator. A thickness-dependent magneto-optical Voigt parameter [6] or multilayer interference [4] are not required to explain the sign in-versions. The compensator is thus the decisive optical element in (general) MOKE magnetometry and microscopy on low-dimensional vdW materials. It needs to be appropriately aligned to avoid a cancelation of the Kerr contrast and to maximise the Kerr signal.References: [1] C. Gong et al.: Discovery of intrinsic ferromagnetism in two-dimensional van der Waals crystals. Nature 546, 265–269 (2017). [2] Q.H. Wang et al., The Magnetic Genome of Two-Dimensional van der Waals Materials. ACS Nano, 16, 5, 6960–7079 (2022) [3] R. Schäfer and J. McCord: Magneto-Optical Microscopy. In: Franco, V., Dodrill, B. (eds) Magnetic Measurement Techniques for Materials Characterization. Springer, Cham. (20219, https://doi.org/10.1007/978-3-030-70443-8_9 [4] Z. Ma et al., Micro-MOKE with optical interference in the study of 2D Cr2Ge2Te6 nanoflakes based magnetic heterostructures. AIP Advances 9, 5, 125116 (2019) [5] W. Kuch et al.: Magnetic Microscopy of Layered Structures, Springer (2015) [6] O. Maximova et al.. The Magneto-Optical Voigt Parameter from Magneto-Optical Ellipsometry Data for Multilayer Samples with Single Ferromagnetic Layer. Physics of the Solid State 63(10), 1485–1495 (2021)