XRM2024 - Thu09K - "3D Imaging of Magnetic Domains in Nd2Fe14B using Scanning Hard X-Ray Nanotomography"

3D Imaging of Magnetic Domains in Nd₂Fe₁₄B using Scanning Hard X-Ray Nanotomography 

Srutarshi Banerjee¹⁾, Doga Gürsoy¹⁾, Junjing Deng¹⁾, John Fullerton¹⁾, M. Kahnt²⁾, M. Lynn³⁾, M. Kramer³⁾, D. Haskel¹⁾, Jörg Strempfer^1),*

(1)   Argonne National Laboratory, Lemont, 60439, U.S.A.
(2)   MAX IV Laboratory, Lund University, 22100 Lund, Sweden 
(3)   Ames National Laboratory, Ames, IA 50011, USA 
*Corresponding Author: strempfer@anl.gov

Keywords: Imaging, Magnetic, Dichroic, STXM

Nanoscale structural and electronic heterogeneities are prevalent in condensed matter physics. Investigating these heterogeneities in three dimensions (3D) has become an important task for understanding the emergent properties in magnetic materials [1,2]. We present a combination of circular dichroic scanning transmission X-ray microscopy (STXM) with vector tomography to reconstruct the 3D magnetic domains [3]. For the reconstruction and visualization of the 3D magnetization, a novel computational imaging framework has been developed. The tomographic reconstruction algorithm is implemented entirely in Python, eliminating the need to use commercial software, and enhancing transparency to ensure reproducibility of the results. In addition, a coordinate descent-based algorithm for alignment of the sample between the different orientations is introduced and the alignment of the object is an inherent iterative part of the magnetization reconstruction algorithm [4].

We demonstrate the applicability of our algorithm by reconstructing the three components of the vectorial magnetization in the magnetic domains of two nano-fabricated cylindrical pillars of Nd₂Fe₁₄B. The experiment was conducted at the diffraction end station [5] of the NanoMAX beamline [6] at the MAX IV synchrotron radiation source at the Nd L₂ absorption edge to obtain maximum circular dichroic absorption contrast. We retrieved the 3D magnetization vector fields from the pillars with a resolution of close to 120 nm, which corresponds to the focal spot size of the X-ray beam used in the STXM experiment. The use of high-quality single crystal Nd₂Fe₁₄B, free of defects or grain boundaries, allows a direct comparison of the reconstructed magnetic domains with micromagnetic simulations.

The reconstruction pipeline presented here is implemented at the POLAR beamline at the upgraded Advanced Photon Source at Argonne National Laboratory, where highly focused coherent beams with variable polarization in the energy range from 2.7 to 27 keV are available [7].

Research conducted at MAX IV, a Swedish national user facility, is supported by the Swedish Research council under contract 2018-07152, the Swedish Governmental Agency for Innovation Systems under contract 2018-04969, and Formas under contract 2019-02496. This research used resources of the Advanced Photon Source; a U.S. DOE Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. Sample growth and focused ion milling was performed at Ames Laboratory which is supported by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences, Materials Science and Engineering Division. Ames Laboratory is operated for the U.S. DOE by Iowa State University under contract No. DE-AC02-07CH11358.

References:

[1] Donnelly, C., and Scagnoli, V., (2020) J Phys Condens Matter, 32, 213001
[2] Donnelly, C. et al., (2017) Nature 547, 328
[3] Banerjee, S. et al., (2024) J. Synchrotron Radiat. 31, 877
[4] Gürsoy, D. et al., (2017) Scientific Reports, 7, 11818.
[5] Carbone, D. et al, (2022) Journal of Synchrotron Radiation, 29, 876–887
[6] Johansson, U. et al., (2021) Journal of Synchrotron Radiation, 28, 1935–1947
[7] Strempfer, J. et al., (2022) J Phys Conf Ser 2380, 012038