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Characterization of Advanced Magnetic Materials for Spintronics by means of Scanning Tunnelling Microscopy and Spectroscopy

Final Activity Report Summary - AMMSSTM (Characterization of Advanced Magnetic Materials for Spintronics by means of Scanning Tunneling Microscopy and Spectroscopy)

The outcome of this project has been of outstanding quality both in terms of scientific accomplishments and professional training of the fellow. The main objective of the project was the use of scanning tunnelling microscopy (STM) and spectroscopy (STS) for the identification of magnetic surfaces with technological interest in the field of spintronics.

The major achievements are the following:

- Fe3O4.- The (001) oriented surface of artificially grown epitaxial magnetite has been studied by STM. Antiphase boundaries (APB) are responsible for many interesting magnetotransport phenomena with large relevance in applications. For the first time, we have obtained atomically resolved images of such APB in the same films in which the magnetoresistive response was previously characterised. From our STM studies, a structural model for the APBs can be derived.
- Co adatoms on NaCl/Cu(001). In order to study the magnetic properties of isolated magnetic adatoms by means of STM, an insulating spacer between the atom and the metallic surface is required. We have grown clean NaCl bilayers on Cu(001) and subsequently deposited single Co atoms on top. STS experiments allow us to extract the energy resolved density of states above the Co atoms. These atoms behave as magnetic impurities coupled to a metal, showing a Kondo resonance around the Fermi level. The shape of the Kondo resonance is a measure of the coupling strength between the Co 3d states and the Cu electron gas. We have successfully measured the Kondo line shape of Co on top of the NaCl bilayer, and compared it to the line shape of Co atoms on the bare Cu surface.
- Mn double layer (DL) on W(110). We have found a spin-spiral magnetic ground state on the DL Mn/W(110) making use of spin-polarised STM (SP-STM). This system shows a local magnetisation rotating periodically from out-of-plane to in-plane. This configuration is exploited to show that the total density of states is sensitive to the relative angle between the high symmetry crystallographic axes and the local spin direction (via spin-orbit coupling), thus granting access to magnetic studies without the difficulties arising from magnetically coated tips. Since DL stripes coexist with large Mn monolayer patches, our data further proves that the combined magnetic contrast of both layers can be used to calibrate the sensitivity direction of an arbitrary magnetic tip. These concepts can extend the performances of many standard low-temperature STMs to address fundamental magnetic properties of surfaces.

Controlling the spin direction of individual adatoms. Manipulation of single atoms using a STM tip offers the possibility to build functional structures with precision down to the atomic scale. On the other hand, atomically resolved magnetic contrast can be achieved by means of SP-STM. In this work, we demonstrate the combination of both techniques, which constitutes a major breakthrough in the field of low dimensional magnetism. We used an iron coated tungsten tip to precisely position individual magnetic atoms on a magnetic substrate having well defined local magnetization directions. Here, the substrate plays the role of a magnetic template for the adatoms. SP-STM performed with the same tip on the atomically engineered magnetic nanostructure reveals clear spin contrast, which can be explained on the basis of density functional theory calculations. Thus, this work opens up a novel research direction for engineering spin structures at the atomic scale with simultaneous magnetic characterisation and imaging capability.