BE-03: Magnetic vortex structure in three-dimensional pyramidal Fe nanofilms studied with Kerr microscopy

The distinct characteristics and potential uses of magnetic vortex in ferromagnetic films have garnered significant interest in the field of magnetism [1]. The challenge of maintaining vortex stability in small sizes, which was constrained by patterned films on 2D plane [2], was overcome by implementing films on 3D structures [3]. This breakthrough was achieved through Landau-Lifshitz-Gilbert (LLG) simulation, which indicates a specific feature such as breaking points (BPs) in magnetization-magnetic field (M-H) curves implying the existence of stable vortex in 3D pyramidal shapes [4]. However, there is no experimental research indicating BPs in M-H curves due to the challenges encountered in the method used for sample growth. In our group, experimentally, we successfully produced 3D pyramidal Fe nanofilms using a combination of photolithography of Si substrates and Si surface treatments [5-6] for Fe deposition, resulting in BPs implying vortex formation [7]. Fig.1 shows the schematic of an Fe film on a 3D Si{111} pyramidal structure; 16 µm edge length and 50 nm of Fe thickness were prepared. Fig. 2 shows M normalized by the saturation magnetization (Ms) as a function of H applied in pyramid outer edge direction (|| Si[110]) measured by vibration sample magnetometer (VSM) at room temperature, displaying certain BPs corresponding vortex. To confirm the presence of magnetic vortex, we combined longitudinal magneto-optic Kerr effect (MOKE) microscopy, judging initial magnetization direction denoted by white arrows in Fig. 1. Clearly right (left) oriented magnetization close to the apex on the upper (downer) 3D-Fe pyramidal facet plane, indicating stable vortex is seen; note little information for right and left facets because M should be perpendicular to H. We noticed magnetization direction on 3D-Fe edge area close to 2D Fe oriented to left direction due to magnetic interaction between 3D and 2D Fe area, which would prevent the vortex formation. In the presentation, we will also demonstrate the results for patterned 3D-Fe pyramids without 2D Fe area.References: [1] N. S. Kiselev, A. N. Bogdanov, R. Schafer, et al., J. Phys. D: Appl. Phys. 44, 392001 (2011) [2] R. P. Cowburn, D. K. Koltsov, A. O. Adeyey, et al., Phys. Rev. Lett. 83, 1042−1045 (1999) [3] M. Gavagnin, H. D, Wanzenboeck, D. Belic, et al, ACS nano. 7, 777 (2013) [4] A. Knittel, M. Franchin, T. Fishbacher, et al., New J. Phys. 12, 113048 (2010) [5] A. N. Hattori, S. Takemoto, K. Hattori, et al., Appl. Phys. Express. 9, 085501 (2016) [6] S. Takemoto, A. N. Hattori, K. Hattori, et al., Jpn. J. Appl. Phys. 57, 090303 (2018) [7] A. Irmikimov, L. N. Pamasi, A. N. Hattori, et al., ACS Cryst. Growth Des. 21, 946 (2021)