MOLECULES WITH A SPIN ON A TOPOLOGICAL INSULATOR SURFACE: A HYBRID APPROACH TO MAGNETIC TOPOLOGICAL STATES OF MATTER

The properties of topological insulators are mostly dominated by their peculiar topological surface electronic states. Interfacing them with magnetic materials and harnessing their interactions can give rise to interesting quantum and topological phenomena that have inspired novel spintronic applications. Led by researchers from the Catalan Institute of Nanoscience and Nanotechnology, an international team with collaborators from CFM-San Sebastian, ETH Zurich, ISM-Trieste and the ALBA Synchrotron, have demonstrated that it is possible to balance electronic and magnetic interactions between single magnetic ions and topological surface states by coordinating the ions with different organic-ligands, leading to possible new routes to manipulate the electron spin at the interface with a topological insulator.

IM-magnTIs

(a) Ball-and-stick structure of the CoTBrPP molecule and the Bi₂Te₃surface; (b) STM topographic image of the molecular network of CoTBrPP on the surface of Bi₂Te₃; (c) XMCD spectra collected at the BOREAS beamline in an applied magnetic field of 6T. The XMCD signal in the lower panel demonstrates the presence of a magnetic moment on the Co ion.

Cerdanyola del Vallès, 6th May 2020 Interfacing a topological insulator with a magnetic material can give rise to peculiar phenomena, such as the current-induced spin to charge interconversion or the emergence of dissipationless spin polarized edge-localized states, which can be exploited in novel spintronic designs or in electron-spin based quantum information applications. For these phenomena to emerge, one must be able to finely tune the interaction between the spin of the magnetic material and the topological surface states. However, directly interfacing a topological insulator with a magnetic metal commonly leads to a too-strong interaction, inducing either a quenching of the topological surface state, the metal spin or both. Metal-organic molecules, organic molecules hosting a (magnetic) metallic ion, have been envisioned as possible candidates to realize such magnetic/topological interfaces owing to the tunability of the metallic ion-topological insulator interaction via ligand chemistry. In this work published on the ACS Nano journal, an international research team led by scientists from the Catalan Institute of Nanoscience and Nanotechnology (ICN2) have demonstrated for the first time that, by a suitable choice of organic ligands in planar metal-organic molecules, it is possible to prevent the quenching of both the molecular spin and the topological surface state of Bi₂Te₃ thin-films, and further fine-tune the interfacial interaction.

The metal-organic molecules chosen for the experiment are cobalt porphyrin derivative (CoTBrPP) a family of quasi-planar metal-organic molecules with the metallic ion at the molecular center. To characterize the microscopic arrangement of the molecules on the surface of Bi₂Te₃, scientists employed scanning tunneling microscopy (STM) and spectroscopy (STS), suggesting that the interaction between the molecule and the topological insulator was not very strong yet they observed a sizeable interaction of the orbital carrying the molecular spin with the surface. This was confirmed by the observation of an unperturbed topological surface state of the Bi₂Te₃ employing the angular resolved photoemission (ARPES) technique, and further corroborated by density functional theory (DFT) ab-initio calculations of the interfacial electronic states.

In order to sort out the magnetism of the cobalt metallic ion, the research team employed synchrotron-based techniques at the BOREAS beamline of the ALBA Synchrotron, in particular the X-ray magnetic circular dichroism (XMCD) that allows to identify the magnetism of sparse ions with chemical selectivity. In this key experiment they found out that the cobalt ions at the interface with the Bi₂Te₃ preserve their magnetic moment. The tunability of the molecular strategy was demonstrated by showing that using the slightly different phthalocyanine ligand led to a different degree of interactions within the unperturbative regime.

With this multi-technique approach, researchers figured out experimentally and theoretically the complete picture of the electronic and magnetic structure of the CoTBrPP/Bi₂Te₃ interface demonstrating that by employing a properly selected organic ligand it is possible to balance the interface interaction between a magnetic metal-organic molecule and a topological insulator, preserving the most relevant properties of both actors and suggesting that magnetic proximity effects may be induced by the molecules on the topological insulator.

Reference: Marc G. Cuxart, Miguel Angel Valbuena, Roberto Robles, César Moreno, Frédéric Bonell, Guillaume Sauthier, Inhar Imaz, Heng Xu, Corneliu Nistor, Alessandro Barla, Pierluigi Gargiani, Manuel Valvidares, Daniel Maspoch, Pietro Gambardella, Sergio O. Valenzuela & Aitor Mugarza. Molecular Approach for Engineering Interfacial Interactions in Magnetic/Topological Insulator Heterostructures. ACS Nano (2020).
DOI: https://pubs.acs.org/doi/10.1021/acsnano.0c02498