On Wednesday 28^th August 2024, we have the pleasure to welcome in SPINTEC Hans Josef Hug from EMPA and Univ. of Basel (*), Switzerland. He will give us a seminar at 14:00 entitled :
Nanoscale Magnetic Field Quantification Using In-Vacuum Magnetic Force Microscopy

Place : IRIG/SPINTEC, auditorium 445 CEA Building 10.05 (access to CEA requires an entry authorization. Request it before August 21^th at admin.spintec@cea.fr)

video conference : https://univ-grenoble-alpes-fr.zoom.us/j/98769867024?pwd=dXNnT3RMeThjYStybGVQSUN0TVdJdz09
Meeting ID: 987 6986 7024
Passcode: 025918

Abstract : This study explores the capabilities of a magnetic force microscope (MFM) operated in a vacuum environment for precise measurement of magnetic fields at the nanoscale. The MFM maps shifts in resonance frequency, arising from the effective derivative of the total force component acting along the cantilever’s normal. This requires separating magnetic components—created by the interaction of the tip’s magnetic moment with the sample’s stray field—from non-magnetic forces like electrostatic and van der Waals forces. Differential measurement strategies, essential for this separation, necessitate precise tip-sample distance control without physical contact to prevent tip wear [1].
The magnetic force-induced frequency shift results from the convolution of the tip’s magnetic moment distribution with the stray field’s derivative, integrated over the tip’s oscillation path. The stray field’s intensity decays exponentially with distance and inversely with spatial wavelength. By calibrating the tip’s response to specific field wavelengths [2,3], we can deconvolve the spatial integration arising from the tip’s magnetic moment distribution, and distortion and averaging effects caused by the canted oscillation path of the tip, and the decay of the different spatial components of the stray field with distance from the sample surface. Moreover, for samples with perpendicular anisotropy, the z-direction of the magnetic moment distribution can be obtained.
However, deconvolution fidelity, especially at higher spatial frequencies of the stray field, depends on signal-to-noise ratio of the frequency shift data. MFM measurements performed under moderate vacuum conditions and with optimized cantilevers significantly enhance measurement sensitivity [4] —by two orders of magnitude compared to ambient conditions—thus improving deconvolution and allowing for reduced tip magnetic moments, thus minimizing alterations in the sample’s micromagnetic state.
Recent results obtained on ferro- and ferrimagnetic multilayer samples exhibiting interfacial Dzyaloshinkii-Moriya interaction will be discussed [5]. The deconvolution of the measured frequency shift contrasts permitted obtaining skyrmion magnetization patterns with radii as small as 20 nm in samples with reduced saturation magnetization. Additionally, MFM results on patterned samples designed for biomedical applications will be shown, demonstrating the technique’s versatility and potential for broad scientific and technological impacts.
References:

1. X. Zhao, J. Schwenk, A. O. Mandru, M. Penedo, M. Bacani, M. A. Marioni, and H. J. Hug, Magnetic Force Microscopy with Frequency-Modulated Capacitive Tip–Sample Distance Control, New Journal of Physics 20, 013018 (2018).
2. P. J. A. van Schendel, H. J. Hug, B. Stiefel, S. Martin, and H.-J. Güntherodt, A Method for the Calibration of Magnetic Force Microscopy Tips, Journal of Applied Physics 88, 435 (2000).
3. Y. Feng, A.-O. Mandru, O. Yıldırım, and H. J. Hug, Quantitative Magnetic Force Microscopy: Transfer-Function Method Revisited, Phys Rev Appl 18, 024016 (2022).
4. Y. Feng, P. M. Vaghefi, S. Vranjkovic, M. Penedo, P. Kappenberger, J. Schwenk, X. Zhao, A.-O. Mandru, and H. J. Hug, Magnetic Force Microscopy Contrast Formation and Field Sensitivity, J Magn Magn Mater 551, 169073 (2022).
5. A. O. Mandru, O. Yıldırım, R. Tomasello, P. Heistracher, M. Penedo, A. Giordano, D. Suess, G. Finocchio, and H.-J. Hug, Coexistence of Distinct Skyrmion Phases Observed in Hybrid Ferromagnetic/Ferrimagnetic Multilayers., Nature Communications 11, 6365 (2020).

(*) Empa, Swiss Federal Laboratories for Materials Science and Technology, 8600 Dübendorf, Switzerland and Department of Physics, University of Basel, 4056 Basel, Switzerland