Frank Matthes

Accessing 4f-states in single-molecule spintronics

Magnetic molecules are potential functional units for molecular and supramolecular spintronic devices. However, their magnetic and electronic properties depend critically on their interaction with metallic electrodes. Charge transfer and hybridization modify the electronic structure and thereby influence or even quench the molecular magnetic moment. Yet, detection and manipulation of the molecular spin state by means of charge transport, that is, spintronic functionality, mandates a certain level of hybridization of the magnetic orbitals with electrode states. Here we show how a judicious choice of the molecular spin centres determines these critical molecule-electrode contact characteristics. In contrast to late lanthanide analogues, the 4f-orbitals of single bis(phthalocyaninato)-neodymium(III) molecules adsorbed on Cu(100) can be directly accessed by scanning tunnelling microscopy. Hence, they contribute to charge transport, whereas their magnetic moment is sustained as evident from comparing spectroscopic data with ab initio calculations. Our results showcase how tailoring molecular orbitals can yield all-electrically controlled spintronic device concepts.

A Closer Look Into Magnetism: Opportunities With Synchrotron Radiation

The unique properties of synchrotron radiation, such as broad energy spectrum, variable light polarization, and flexible time structure, have made it an enormously powerful tool in the study of magnetic phenomena and materials. The refinement of experimental techniques has led to many new research opportunities, keeping up with the challenges put up by modern magnetism research. In this contribution, we review some of the recent developments in the application of synchrotron radiation and particularly soft X-rays to current problems in magnetism, and we discuss future perspectives.

Electronic structure, surface morphology, and topologically protected surface states of Sb2Te3 thin films grown on Si(111)

We have performed a combined spectroscopy and microscopy study on surfaces of Sb2Te3/Si(111) thin films exposed to air and annealed under ultra-high vacuum conditions. Scanning tunneling microscopy images, with atomic resolution present in most areas of such processed surfaces, show a significant amount of impurities and defects. Scanning tunneling spectroscopy reveals the bulk band gap of ∼170 meV centered ∼65 meV above the Fermi level. This intrinsic p-type doping behavior is confirmed by high-resolution angle-resolved photoemission spectra, which show the dispersions of the lower Dirac cone and the spectral weight of the bulk valence bands crossing the Fermi level. Spin-polarized photoemission revealed up to ∼15% in-plane spin polarization for photoelectrons related to the topologically protected Dirac cone states near the Fermi level, and up to ∼40% for several states at higher binding energies. The results are interpreted using ab initio electronic structure simulations and confirm the robustness of ...

Spin-polarized photoemission spectroscopy of the MgO/Fe interface on GaAs(100)

We studied the electronic band states at the Fe/MgO interface with spin-polarized photoemission using excitation energies between 35 and 60 eV. Epitaxial Fe films on GaAs(100) were covered with 0.5, 1, and 1.5 ML thick MgO films. The photoemission spectra revealed a spin-dependent attenuation for photoelectrons related to direct transitions from initial bulk Fe bands having minority or majority spin character.

STRUCTURAL INTEGRITY OF SINGLE BIS(PHTHALOCYANINATO)-NEODYMIUM(III) MOLECULES ON METAL SURFACES WITH DIFFERENT REACTIVITY

Magnetic molecules are auspicious candidates to act as functional units in molecular spintronics. Integrating molecules into a device environment providing mechanical support and electrical contacts requires their deposition as intact entities onto substrates. Thermal sublimation is a very clean deposition process that, however, thermally decomposes molecules of insufficient stability leading to the deposition of molecular fragments. Here, we show that the molecule-surface interaction of chemisorbed molecules affects the intramolecular bonding and can lead depending on the surface reactivity to either molecular decomposition or enhanced stability. We study the integrity of single bis(phthalocyaninato)-neodymium(III) molecules (NdPc2) deposited by sublimation on differently reactive surfaces, namely Au(111), Cu(100), and two atomic layers of Fe on W(110), on the single molecular level by scanning tunneling microscopy (STM) and spectroscopy. We find a strongly substrate-dependent tendency of the NdPc2 molecules to decompose into two Pc molecules. Surprisingly, the most reactive Fe/W(110) surface shows the lowest molecular decomposition probability, whereas there are no intact NdPc2 molecules at all on the least reactive Au(111) surface. We attribute these findings to substrate-dependent partial charge transfer from the substrate to the Pc ligands of the molecule, which strengthens the intramolecular bonding mediated predominantly by electrostatic interaction.

Photoemission study of the Fe(001)∕MgO interface for varying oxidation conditions of magnesium oxide

The nature of interface bonding essentially determines the spin polarization of tunneling currents through ferromagnet/insulator interfaces. We focus on the influence of ultrathin off-stoichiometric MgO coverages on the spin-polarized electronic structure of the (001) surface of bcc-Fe. By applying a combination of low-energy spin-resolved and core-level photoelectron spectroscopy, we show that the MgO stoichiometry has a direct impact on the interfacial spin polarization. The results underline the importance of controlling the chemical composition and bonding at ferromagnet/oxide interfaces to further optimize spin-dependent tunneling devices.

Spin polarization at ferromagnet-insulator interfaces: The important role of stoichiometry in MgO/Fe(001)

The electronic structure of ferromagnet-insulator interfaces plays a key role in spin-dependent transport processes by determining the spin polarization of the tunneling charge carriers. Employing spin-polarized, angle-resolved photoelectron spectroscopy we studied the spin-resolved electronic structure in the model system MgO/Fe(001) and observed a surprisingly multifaceted influence of the MgO stoichiometry. In particular, oxygen-deficient MgO barriers are found to strongly enhance the interfacial spin polarization. The results highlight the important role of the specific nature of the oxygen bonding and its relevance for the tunnel magnetoresistance effect.

Magnetic surface domain imaging of uncapped epitaxial FeRh(001) thin films across the temperature-induced metamagnetic transition

The surface magnetic domain structure of uncapped epitaxial FeRh/MgO(001) thin films was imaged by in-situ scanning electron microscopy with polarization analysis (SEMPA) at various temperatures between 122 and 450 K. This temperature range covers the temperature-driven antiferromagnetic-to-ferromagnetic phase transition in the body of the films that was observed in-situ by means of the more depth-sensitive magneto-optical Kerr effect. The SEMPA images confirm that the interfacial ferromagnetism coexisting with the antiferromagnetic phase inside the film is an intrinsic property of the FeRh(001) surface. Furthermore, the SEMPA data display a reduction of the in-plane magnetization occuring well above the phase transition temperature which, thus, is not related to the volume expansion at the phase transition. This observation is interpreted as a spin reorientation of the surface magnetization for which we propose a possible mechanism based on temperature-dependent tetragonal distortion due to different the...

Observation and implications of magnetic domains in lateral spin valves

Co/Cu/Co lateral spin valves (LSV), with Co being the topmost layer, are in situ prepared and measured under ultrahigh vacuum conditions. The clean process yields a non-local spin signal of 0.9 mΩ. Scanning electron microscopy with polarization analysis (SEMPA) reveals domain structures in both magnetic electrodes that depend on the LSV dimensions. The spin signal correlates to SEMPA images as well as the anisotropic magnetoresistance of both Co magnets, revealing a strong impact of multi-domain states on the spin signal.