Magnetic skyrmions are topologically stable configuration with particle-like properties. Due to their efficient current-induced manipulation, they could act as carrier of information in futur devices. They where initially observed at low temperature and by applying an intense magnetic field in bulk materials exhibiting broken inversion symmetry[1-2] and in epitaxial ultrathin films with interfacial Dzyaloshinskii-Moriya interaction[3-4]. An intense research effort during the past few years has led to their stabilization at room temperature[5] but still by applying an external magnetic field. A promising strategy to overcome this last obstacle is to exploit the interlayer exchange coupling in magnetic heterostructure to produce an internal effective magnetic field Bint[6-7]. Here, we demonstrate that magnetic skyrmions with a mean diameter around 60 nm can be stabilized at room temperature and zero external magnetic field in an exchange-biased Pt/Co/Ni80Fe20/Ir20Mn80 multilayerstack[8]. This is achieved through an advanced optimization of the multilayer-stack composition in order to balance the different magnetic energies controlling the skyrmion size and stability. Magnetic imaging is performed both with magnetic force microscopy and scanning nitrogen-vacancy magnetometry, the latter providing unambiguous measurements at zero external magnetic field. Simulation of the PL quenching image are also achieved and the results, in agreement with experimental data, allows us to extract informations such as skyrmion's size. These results establish exchange-biased multilayer stacks as a promising platform toward the effective realization of memory and logic devices based on magnetic skyrmions.References: [1] S. Mühlbauer et al., Science323, 915 (2009) [2] X. Z. Yu et al., Nature (London) 465, 901 (2010) [3] S. Heinze et al., Nature Phys.7, 713 (2011) [4] N. Romming et al., Science341, 636 (2013) [5] W. Jiang et al., Science349, 283 (2015) [6] G. Chen et al., Appl. Phys. Lett.106, 242404 (2015) [7] Y. Guang et al., Nat. Commun.11, 949 (2020) [8] K. Gaurav Rana et al., Phys. Rev. Applied 13, 044079 (2020)

Room-Temperature Skyrmions at Zero Field in Exchange-Biased Ultrathin Films

Even if magnetic van der Waals crystals where known since decades, two-dimensional (2D) magnetism has been obtained experimentally only a few years ago[1]. Since its discovery in Cr2Ge2Te6 and CrI3, an intense research effort has started, triggered by their high potential in studying 2D magnetic states[2-4]. A wide range of new physical properties emerging from the reduced dimensionality can be explored and exploited like the control of magnetic properties through gating. New devices based on van der Waals magnetic heterostructures have been proposed, such as spin-filter magnetic tunnel junctions based on CrI3 heterostructures, showing a tunneling magnetoresistance higher than their counterparts based on conventionally grown magnetic thin films. The potential of 2D magnets in topological spintronics is also under investigation, with for example recent works on the stabilization of skyrmions in Fe3GeTe2. However, in order to use van der Waals magnets as building blocks for relevant spintronics devices, a magnetic material with a Curie temperature (TC) above room temperature is required. Unfortunately, most of the available van der Waals magnets have a low intrinsic TC. Even though a 2D magnetic state at room temperature can be achieved through gating for example, it remains a challenging task and only few materials are reported to have a TC above 300K down to the monolayer limit, like MnSex, VSe2 and more recently for few-layers CrTe2[5]. Here, we report our work on CrTe2 from bulk to 40nm thin mechanically exfoliated flakes using Nitrogen-Vacancy (NV) magnetometry, a well-suited technique for the study of 2D magnets[6]. We find an in-plane ferromagnetic order at room temperature and we extract a value for the saturation magnetization Ms of 24.5 ± 3.0 kAm-1 at room temperature.References: [1] N. Samarth, Nature 546, 216 (2017) [2] K. S. Burchet al., Nature 563, 47 (2018) [3] X. Z. Cheng Gong, Science 363, 6428 (2019) [4] M. Gibertini et al., Nature Nanotechnology 14, 408 (2019) [5] A. Purbawati et al., ACS Appl. Mater. Interfaces (2020) [6] L. Thiel et al., Science 364, 973 (2019)

Room-temperature ferromagnetism in CrTe2 thin flakes imaged by single spin magnetometry

I3: Skyrmion Imaging

technical paper

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Florentin Fabre

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Room-Temperature Skyrmions at Zero Field in Exchange-Biased Ultrathin Films

Room-temperature ferromagnetism in CrTe2 thin flakes imaged by single spin magnetometry

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