Dr. Christian Teichert

Abstract:

Since the first reports on intrinsically magnetic two-dimensional (2D) materials in 2017 [1,2], the price-to-pay for accessing their monolayers is the lack of ambient stability. We discovered in a mineral aggregate – mainly composed of hematite, magnetite, and chalcopyrite – soft layers of which macroscopic flakes easily could be peeled off that stuck to a permanent magnet. Employing mechanical exfoliation, we succeeded in thinning and transferring micrometer sized – mainly hexagonally shaped – flakes to SiO2 substrates. Energy dispersive x-ray spectroscopy (EDS) revealed magnesium and silica as major components of the flakes. Raman spectroscopy indicated the presence of hydroxide groups, pointing towards talc, a hydrated magnesium phyllosilicate mineral, namely talc (Mg3Si4O10(OH)2). Long-term EDS and Raman spectroscopy revealed that in the flakes about 10 % of the Mg atoms are substituted by Fe which clusters into about 20 nm regions according to scanning transmission electron microscopy. With atomic force microscopy, a minimum flake thickness of 1 nm was determined indicating cleavage down to a talc monolayer. Combined magnetic force microscopy (MFM) measurements in external out-of-plane fields up to 0.5 T and SQUID magnetometry measurements imply that the 2D Fe-rich talc exhibits weak ferromagnetic behavior [3]. The flakes are showing long-term stability under ambient conditions in contrast to the 2D magnets reported so far. Besides iron-rich talc, we investigate also ultrathin flakes of exotic minerals like minnesotaite (Mg3Si4O10(OH)2) as well as iron rich micas like biotite and annite [4]. As another approach towards iron-rich 2D talc flakes, we used ion implantation of iron-free talc single crystals and subsequent mechanical exfoliation.

Work has been supported by FWF, Austria via 2020 START program (grant # Y1298 N) and has been performed in collaboration with A. Matković, M. Z. Khan, K.-P. Gradwohl, L. Ludescher, M. Kratzer, M. Zimmermann, R. Bakker, J. Raith (all Montanuniversität Leoben), O. E. Peil, L. Romaner (all Materials Center Leoben), C. Gammer (Erich Schmid Institute of Materials Science, Leoben), E. Fisslthaler, D. Knez, F. Hofer (Graz University of Technology), J. Genser, A. Lugstein (Vienna University Of Technology), A. Sharma, O. Selyshchev, D.R.T. Zahn, G. Salvan (TU Chemnitz), O. Selyshchev, S. Valencia, F. Kronast (all Helmholtz-Zentrum Berlin), and U. Kentsch, N. Klingner, G. Hlawacek (all Helmholtz-Zentrum Dresden-Rossendorf).