Session N16: Focus Session: Molecules on Surfaces

Kondo-like Resonances in the high spin MnPc. Atomic and Molecular Theoretical Approach

In recent years, Kondo$-$like resonances have been measured by different experimental groups in the 3$\backslash$2 high spin Manganese Phthalocyanine (MnPc) on different kinds of surfaces [1,3]. With the aim to understand these resonances we have performed Dynamical Mean Field Theory calculations based on models extracted from Density Functional Theory calculations and Green's function formalism [4,5]. Two types of models are considered: one based on atomic d orbitals and one based on frontier molecular orbitals which contain the spin of the molecule.\\[4pt] [1] Ying-Shuang Fu et. al. Phys. Rev. Lett. 99, 256601. 2007.\\[0pt] [2] K. J. Franke et. al. Science 332, 940. 2011.\\[0pt] [3] A. Str\'{o}zecka et. al. Phys. Rev. Lett. 109, 147202. 2012.\\[0pt] [4] ANT.G03. www.alacant.dfa.ua.es. \\[0pt] [5] D. Jacob et. al. Phys. Rev. B. 82, 195115. 2010.   

DFT$+$U studies of atomic scale magnetism: A curious case study for future spintronic devices

Atomic scale magnetism attracts interest due to both its possible application to nanoscale spintroic devices, and due to its inherent interest as a source of basic quantum mechanical interactions. We work together with the local Scanning Tunneling Microscopy (STM) team to match our calculations to experiment, and in the process learn much which can't be measured with the STM. In particular we use DFT$+$U to calculate the properties of magnetic atoms on nanolayers of insulator on top of a metal such as silver. In this talk we report the results of detailed calculations of singles and dimers of Mn on MgO/Ag. As time permits we may include our calculations of other magnetic adatoms for comparison. We find that the local interactions are very different for the three stable binding sites on this surface, both for charge and spin densities. Using on onsite Hubbard U parameter which we determine from first principles, we are able to study the variability of the magnetic moment between the binding sites, as well as determine the lowest energy binding site. The magnetic adatoms affect the surrounding interface layer in unexpected ways. We are able to obtain interesting insights which help us understand how magnetism propagates along surfaces as well as between interfaces.   

High magnetic anisotropy of magnetic atoms on thin MgO films on Ag(001)

High quality thin films of magnesium oxide on silver (MgO/Ag(001)) are obtained by magnesium evaporation in an oxygen atmosphere. MgO is often used as insulating layer in magnetic tunnel junctions. Therefore, the interplay of magnetic atoms with MgO is of interest e.g. to the hard drive industry. We characterize the thin films by a combination of scanning tunneling microscopy (STM) and atomic force microscopy (AFM). In particular, we determine the thickness of the deposited layers by a combined use of the two tools. We find that single transition metal atoms, such as Iron and Cobalt, deposited on the thin oxide film show inelastic tunneling steps at higher voltages compared to other insulating layers. The inelastic tunneling spectroscopy (IETS) is used to detect the discrete spin excitations of these atoms. The measured IETS steps indicate high magnetic anisotropies.   

Magnetic excitations of molecular spins on a superconductor

Single magnetic molecules on a superconducting substrate present a model system to study the influence of a local magnetic moment on the superconducting state at the atomic scale. The magnetic moment of the adsorbate interacts with the Cooper pairs by exchange coupling and tends to break them apart [1]. Signatures of this interaction are localized states in the superconducting gap, which can be probed by tunneling spectroscopy [2,3]. On the other hand, the quasi-free electrons in the substrate screen the localized spin via the Kondo effect. The delicate balance between these phenomena determines the resulting ground state of the system. Using scanning tunneling spectroscopy, we show that the interaction of paramagnetic molecules with a superconducting lead surface is very sensitive to the details of the atomic scale surrounding [4]. Depending on the interaction strength, the magnetic moment is able to perturb the Cooper pairs, or the superconducting state is unaffected.\\[4pt] [1] H. Shiba, Prog. Theor. Phys. 40, 435 (1968)\\[0pt] [2] A. Yazdani, et al. Science 275, 1767 (1997)\\[0pt] [3] S.-H. Ji, et al. Phys. Rev. Lett. 100, 226801 (2008)\\[0pt] [4] K. J. Franke, G. Schulze, J. I. Pascual, Science 332, 940 (2011)   

Probing superexchange coupling in atomically fabricated d-metal complexes

Magnetic coupling between transition metal atoms that are linked through ligand p-orbitals relies on the virtual exchange of electrons between neighboring sites. The characteristics of the resulting superexchange coupling rely on a complex interplay between electron hopping and Coulomb interaction. In this talk I will review recent experiments on individual superexchange coupled d-metal atoms placed inside a covalent surface network. By using low temperature scanning tunneling microscopy, Fe atoms may be positioned in a Cu$_{2}$N lattice with atomic precision, and their quantum-magnetic properties probed. Our experiments reveal novel insights into the resulting p-orbital mediated magnetic coupling, that are of importance in the fields of molecular magnetism and strongly correlated transition metal oxides.   

Exploring the magnetic properties of metallophthalocyanines on a thin insulator

The scaling of electrical components to the atomic-scale limit has led to a great deal of interest in molecular electronics. Further proposals outline the use of magnetic molecules in new applications in information technology and spintronics. Since the electronic and magnetic properties of a molecule can be modified by interactions with the surfaces on which they are deposited, understanding these changes is of significant importance. Here we present studies of metal-doped pthalocyanine (MPc) molecules deposited on the thin insulator copper nitride (Cu$_2$N). FePc molecules have been shown to display a large magnetic anisotropy on copper oxide, which is also a thin insulator [1]. Using STM imaging and theoretical calculations we investigate how the interaction of the surface with the molecule varies and how this can affect the charge transport through the molecules. Through the application of a magnetic field and both elastic and inelastic spectroscopy, we gain access to the magnetic states of the molecule. In addition, by imaging the molecules at different bias voltages, we are able to probe the different molecular orbitals and explore how they are modified by interactions with the surface.\\[4pt] [1] N. Tsukahara et al., Phys. Rev. Lett. 102, 167203 (2009)   

Spin-resolved measurements of single molecular magnets on graphene

The use of magnetic molecules opens a gateway to a flexible design of novel spintronic devices to store, manipulate, and read spin information at the nanoscale. Crucial is the precise knowledge of molecular properties at the interface towards an electrode. Progress in this field relies on resolving and understanding the physics at the relevant interfaces. In particular the role of individual molecular constituents and the impact of the atomic environment on molecular properties determine device relevant parameters, such as conductance and spin polarization. Recently, the incorporation of a graphene sheet to electronically decouple molecules from a ferromagnetic surface has been addressed by surface averaging high-resolution electron energy loss spectroscopy. Here, we applied spin-polarized scanning tunneling microscopy to resolve the physics of the molecule-graphene-ferromagnet interface. The analysis focuses on different phthalocyanine molecules adsorbed on cobalt-intercalated graphene on Ir(111). The phthalocyanine constitutes of an organic macrocyclic ligand and can be functionalized with various metal ions in order to modify, e.g. the molecular spin state. We will discuss the spin-dependent transport from magnetic surfaces through such molecules. In particular, the spin polarization of molecular frontier orbitals is resolved with sub-molecular spatial resolution and the variations in the lifetimes of different orbitals are discussed.   

A first-principles study of a single-molecule magnet Mn12 adsorbed on Bi(111)

Recently, elemental Bi and Bi-based alloys have attracted a lot of attention due to unique quantum properties of their surface states induced by strong spin-orbit coupling. A single-molecule magnet Mn12 is known to be a prototype molecular magnet with significant magnetic anisotropy caused by spin-orbit coupling. Despite a great effort to fabricate monolayers of single-molecule magnets on various substrates, there are few studies of single-molecule magnets on strongly spin-orbit coupled substrates. Here we present our theoretical study of electronic and magnetic properties of single-molecule magnets Mn12 adsorbed on a strongly spin-orbit coupled semi-metallic Bi surface without any linker molecules. This work was motivated by a recent low-temperature scanning tunneling microscopy (STM) experiment where individual single-molecule magnets Mn12 were grafted on Bi. We apply density-functional theory (DFT) including on-site Coulomb repulsion U and self-consistent spin-orbit coupling, to two adsorption geometries of Mn12 on Bi. We compare our calculated electronic and magnetic properties of the Mn12 molecule on Bi with those of an isolated Mn12.   

STM Studies of Mn$_{12}$-Ph

Mn$_{12}$-Ph displays tunneling of quantized magnetization below 3K. In other Mn$_{12}$ ligand variants this magnetic behavior can alter the electronic behavior of the molecule making them good candidates for a molecular logic gate or q-bit. Mn$_{12}$O$_{12}$(C$_6$H$_5$COO)$_{16}$ (referred to as Mn$_{12}$-Ph) has a Mn$_{12}$ core with 16 Phenyl ligands and is deposited via spray injection onto surfaces of highly oriented pyrolytic graphite (HOPG) and other surfaces. We report Mn$_{12}$-Ph in isolation, resembling single molecules with metallic core atoms and organic outer ligands. The local tunneling current observed within the molecular structure shows a strong bias voltage dependence, which is distinct from that of the surface. Further, evidence of internal inhomogeneity in the local density of states has been observed with high spatial resolution, and this inhomogeneity appears to be due to localized metallic behavior. These results facilitate magneto-metric studies of single molecule magnets in isolation. As compared to bulk crystal studies, our experiments allow the specific investigation of atomic sites in individual molecules.   

Magnetic Relaxation in Iron Chains of Phthalocyanine Thin Films

Self-assembled iron chains are formed in metallo-organic thin films based on the small iron phthalocyanine molecule. The chains are grown parallel to the substrate and the mean chain length is controlled via deposition parameters from 30 -- 300 nm. The strong intra-chain coupling with weak inter-chain coupling leads to ferromagnetic behavior below the critical temperature. After application of a magnetic saturation field, the remanent magnetic moment is not stable when measured over time scales of 10$^{\mathrm{4}}$ s. The magnetic relaxation can be fit to a stretched exponential function, which yields the mean relaxation time and a stretch exponent. The temperature-dependent peak of the relaxation time occurs at lower temperatures for shorter iron chains that also have smaller coercivities. This means that by templating iron phthalocyanine thin films both magneto-crystalline anisotropy and inter-grain interactions can be selected.   

Binding Structures of Diatomic Molecules to Co-Porphyrins on Au(111) Studied by Scanning Tunneling Microscopy

Axial bindings of diatomic molecules to metalloporphyrins involve in the dynamic processes of biological functions such as respiration, neurotransmission, and photosynthesis. The binding reactions are also useful in sensor applications and to control molecular spins in metalloporphyrins for spintronic applications. Here, we present the binding structures of diatomic molecules to surface-supported Co-porphyrins studied using scanning tunneling microscopy. Upon gas exposure, three-lobed structures of Co-porphyrins transformed to bright ring shapes on Au(111), whereas H2-porphyrins of dark rings remained intact. The bright rings are explained by the structures of reaction complexes where a diatomic ligand, tilted away from the axis normal to the porphyrin plane, is under precession. Our results are consistent with previous bulk experiments using X-ray diffraction and nuclear magnetic resonance spectroscopy.   

Molecule-induced Spin Rotation of Photoelectrons from FePc on Fe(110)

We have studied sub-monolayers to multi-layers of iron phthalocyanine (FePc) adsorbed on $\sim$ 10-20 monolayer epitaxial films on Fe(110) on W(110). We find that the spin-resolved photoemission changes rapidly as a function of coverage and the initial (majority spin axis along [110] rotates by $\sim$ 30 degrees for sub-monolayer coverage and then becomes unpolarized at $\sim$ 1 monolayer (ML). The coverage is determined by work function measurements which show that the initial work function of clean Fe(110) of 5.0 eV decreases monotonically to a value of $\sim$ 3.8 eV at a coverage that we assign as $\sim$ 1 monolayer of FePc. These values were determined from the measurements of the photoelectron spectrum using the low-energy vacuum-level cutoff of a biased sample. Our spin-resolved data for clean Fe(110) show highly spin-polarized photoelectrons from the Fermi energy to values about 3.5 eV below the Fermi energy for an applied B-field along [110] both for majority-spin and minority-spin electrons. The polarization is about 60{\%} at -3.2 eV below E-Fermi. For 0.13 ML adsorbed FePc the spin polarization is somewhat reduced and is rotated from [110] towards [100] in the plane of the sample. We interpret this rotation as due to a strong coupling of the orbital moment of FePc with the conduction electrons of the Fe substrate.