Room temperature stabilization and all-electrical manipulation of non-collinear spin structures in metallic multilayers.
This is a scientific research project funded by the European Commission via the Marie Curie fellowship SKDWONTRACK-748006.
This project is based on an international collaboration among three different institutions: the University of Hamburg in Germany, the University of California at Berkeley and the Lawrence Berkeley National Laboratory in the U.S.A.
The initial phase of the project took place in California, where the main research focus was on developing and investigating epitaxial magnetic multilayers hosting non-collinear spin-textures with the spin-polarized low energy electron mycroscope (SPLEEM) available at the Lawrence Berkeley National Laboratory. The project was supervised by Dr. Andreas Schmid.
The second phase of the project is taking place at the NanoScience research center of the University of Hamburg. The project will involve the investigation of non-collinear magnetism in epitaxial thin films at the atomic scale. The in-situ imaging of the magnetic texture will be carried out via spin-polarized scanning tunneling microscopy (SP-STM), in the group of Prof. Roland Wiesendanger.
The constantly increasing energy consumption associated with the functioning of micro-electronic devices calls for the design of more energy efficient technologies. While semiconductor technologies have reached their intrinsic limitations, spintronics offers a new path towards the design of memory and logic devices with high density and low power consumption. However, present spintronic devices such as magnetic-RAMs still suffer from high current density requirements. Magnetic skyrmions (shown below), a new type of magnetic quasi-particles, are fundamentally as well as technologically interesting due to their high stability against annihilation, their potential small size, and their outstanding transport properties. Accordingly, if they are used to encode information (e.g., one skyrmion = one bit), they may allow the design of very fast magnetic data storage and processing devices, with high information density and low power consumption. Particularly promising is the stabilization of skyrmions in multilayer systems, where it seems possible to stabilize magnetic skyrmions at room temperature without any external magnetic field, a key requirement for an actual device. The main objective of the project is the design, investigation and engineering of new magnetic thin film multilayers hosting non-collinear spin-textures, where topologally non-trivial magnetic states can be stabilized. The two main experimental techniques involved in the study are spin-polarized low energy electron microscopy (SPLEEM) and spin-polarized scanning tunneling microscopy (SP-STM).
The initial part of the project has been characterized by the development of epitaxial magnetic multilayers on top of insulating single crystal substrates. The interest in growing metallic multilayers on top of insulating substrates lies in the possibility to explore the magneto-transport properties of the developed materials stacks, such as current-induced skyrmion motion. We successfully developed a recipe for the growth of epitaxial metallic multilayers on top of insulating MgO single crystals . Via spin-polarized low energy electron microscopy (SPLEEM) we were able to explore the type of magnetic texture stabilized in those multilayers. The investigation with the SPLEEM confirmed the presence of magnetic spin-textures with a uni-directional sense of rotation, which is a key requirement for the stabilization of magnetic skyrmions. After the establishment of multilayers hosting non-collinear magnetism, we further developed our stacks so to obtain the stabilization of magnetic skyrmions at room temperature in no external magnetic field. We exploited the interlayer magnetic coupling between two different ferromagnetic thin films separated by a non-magnetic spacer (see figure below) in order to nucleate skyrmions at zero field. After their stabilization we also investigated their topological nature, revealing their handedness (right handed vs left-handed). Interestingly, by tailoring the thickness of the non-magnetic spacer we were able to fine-tune the strength of the coupling between the two magnets, which resulted in the control of the skyrmion size and areal density. The main findings of this part of the project are reported in the following publication: R. Lo Conte et al., Nano Letters 20, 4739-4747 (2020).
Large Dzyaloshinskii-Moriya interaction induced by chemisorbed oxygen on a ferromagnetism surface
G. Chen, A. Mascaraque, H. Jia, B. Zimmermann, M. Robertson, R. Lo Conte, M. Hoffmann, M. A. G. Barrio, H. Ding, R. Wiesendanger, E. G. Michel, Stefan Bluegel, A. K. Schmid, and K. Liu
Science Advances 6 : eaba4924 (2020)
Tuning the properties of zero-field room temperature ferromagnetic skyrmions by interlayer exchange coupling
R. Lo Conte, A. K. Nandy, G. Chen, A. L. F. Cauduro, A. Maity, C. Ophus, Z. Chen, A. T. N'Diaye, K. Liu, A. K. Schmid, and R. Wiesendanger
Nano Letters 20, 4739-4747 (2020)
Roberto earned a B.Sc. and a M.Sc. degree in Engineering Physics from the Politecnico di Milano (Italy) under the supervision of Prof. Marco Finazzi and a M.Sc. degree in Engineering from the Royal Institute of Technology (KTH) of Stockholm (Sweden) under the supervision of Prof. Vladislav Korenivski. Subsequently, he earned his Ph.D. in Physics from the Johannes Gutenberg-University of Mainz (Germany) under the supervision of Prof. Mathias Kläui. After obtaining his doctoral degree, Roberto moved to the U.S.A. to join the group of Prof. Jeffrey Bokor in the EECS Department of the University of California, Berkeley as a post-doctoral researcher. Currently, he is a Marie Curie fellow (as the recipient of a 2016 Marie Skłodowska Curie Individual Fellowship – Global Fellowship) at the Physics Department of the University of Hamburg (Germany), under the supervision of Prof. Roland Wiesendanger. He spent two years between January 2018 and December 2019 working on the Marie Curie project at the SPLEEM Lab of the Lawrence Berkeley National Laboratory (Berkeley Lab) under the supervision of Dr. Andreas Schmid. Since January 2020, Roberto joined Prof. Wiesendanger’s team in Hamburg to work on the investigation of epitaxial magnetic thin films via spin-polarized scanning tunneling microscopy and spectroscopy.