Research Area C: Molecular- and atomic-based NANO-SPINTRONICS
Integration of nanocomposites and tailor-made organic and organometallic compounds on the molecular scale into electronic devices is tantamount to a significant progress in modern circuit design as it adds a new degree of functionality in terms of chemical, optical or mechanical properties. Likewise, the use of magnetic molecules opens a gateway to a flexible design of spintronic devices based on the generation, transport, and detection of a spin current as well as on storing, manipulating, and reading spin information at the nanoscale level. A central issue towards the development of nanoscale spintronic devices is the precise knowledge of the magnetic properties of adsorbed molecules in dependence of the local atomic environment. The goal is to exploit the advantages of molecular and nanoparticle systems for nanoscale spintronic elements in multifunctional devices.
In molecular paramagnets composed of different magnetic sites, the spin states are strongly influenced by through bond interactions depending on the electronic nature of the bridge linking the different subunits. In spintronics, it is crucial to control spin coupling between the subunits by switching, e.g., from a strong to a weak coupling. In addition, it would be most desirable to switch as fast as possible. Hence, the Heck and Prosenc groups will employ light induced changes in bonding to trigger communication between spin systems of attached molecular paramagnets. As optical switches organic molecules and complexes will be used which change their geometry or spin state upon irradiation. The transition between individual molecules and molecular solids will be investigated in the Bachmann and Nielsch groups, where magnetic metalloporphyrins will be grafted onto the inner walls of ordered silica nanotubes. In this manner, they will create stacks of flat molecular units assembled in close proximity to one another. The weak electrical conduction of those stacked structures will sensitively depend on geometric, chemical, and magnetic effects. Subsequently, each nanotube of molecular paramagnets will be embedded in a larger nanotube of ferromagnetic iron oxide: Fe3O4 tubes will be designed to generate well-controlled remanent magnetic fields, which will affect the orientation of the individual paramagnets. The changes in electrical conductivity of the porphyrin stacks will be measured depending on the strength of the externally applied magnetic field. The molecular devices will be studied in solution by SQUID (Nielsch), EPR-, and NMR-spectroscopy (Heck, Nielsch, Prosenc), and - even more important - in contact with a surface, i.e. metals and semiconductors, by using STM (Hoffmann, Wiesendanger), or insulators, e.g. SiO2 tubes, by using AFM techniques (Schwarz).
The electron density distribution in the molecular devices will be studied by modern high resolution X-ray structure analysis in collaboration with DESY and GKSS (HARWI II beamline). Magnetic structure elucidation of complexes and nanostructured materials will be performed by neutron scattering at GKSS.
In addition to the molecular approach, the Weller and Klinke groups will investigate and optimize the attachment of defined ferromagnetic FePt, CoPt, and NiPt nanoparticles on delocalized sp2 carbon surfaces. The synthesis of the nanoparticles will take place in the presence of carbon nanotubes (CNTs), or graphene to yield a composite material, i.e. nanoparticles attached to carbon systems. Individual objects will be contacted by lithographic techniques in field-effect transistor configuration. Spin-polarized electrons will be transported through the sp2 carbon material. The contacts will locally be addressed by scanning probe methods to overcome the limits of spatial resolution of the techniques used so far. Based on and in extension of the ongoing projects within the SFB 668 and GrK 611 the transversal transport will be addressed and the environment will be tailored by local manipulation. The experimental investigations will be guided by ab-initio calculations including correlation effects and spin-orbit coupling (Lichtenstein) accounting for the atomistic structure of the Au/ferromagnetic nanoparticle/graphene junctions. These calculations will reveal the microscopic mechanisms which are effective when the electrons pass through nanoparticles (Rashba effect) and guide the way for optimizing the spin injection process.
Since probing the magnetism of single atoms and molecules on surfaces is still challenging, a major effort has to be made in order to further develop scanning probe techniques with spin sensitivity at the molecular level (Wiesendanger, Hoffmann). Based on the first SP-STM experiments demonstrating successfully the local read-out of molecular spin states the present project aims at controlling the spin state of magnetic molecules on conductive substrates by external stimuli such as light (Heck, Prosenc). The Schwarz group intends to study individual magnetic atoms and molecules deposited on insulating surfaces, where hybridization effects are absent, by magnetic exchange force microscopy (MExFM). Magnetic properties of individual atoms and molecules will be characterized by recording local magnetization curves by SP-STM or MExFM. In addition, spin flip processes will be analyzed by inelastic tunneling processes in case of SP-STM or by the energy of dissipation during MExFM experiments.
The Hoffmann group intends to map the magnetic response of magnetic molecules and atoms to modifications of the atomic environment. The capabilities of STM will be utilized to laterally manipulate single molecules and single atoms so as to control the distance between magnetic objects. Hence, experimental access is obtained to coupling strengths or coupling ranges, to achieve site specific contacting of molecules to investigate magnetic coupling through organic ligands, or to tune the electronic environment of magnetic objects in well-defined atomic geometries with peculiar electronic states. The magnetic response of the molecules or atoms will be locally monitored by SP-STM. As the process of molecule and atom manipulation is still rather time consuming, an automated atom manipulator will be set up based on the interdisciplinary cooperation of groups from the Departments of Physics (Wiesendanger) and Informatics (Zhang). In particular, methods of robotic navigation and sensor-based manipulation skills will be combined with atomic-resolution structuring and imaging based on STM methods.
In the Wiesendanger group a new four-probe STM system has recently been installed to contact nanoscale objects and to measure the transversal rather than the vertical transport. It is intended to combine local transport measurements through one- and two-dimensional self-assembled molecular structures formed of magnetic molecules and spin sensitivity by using magnetic electrodes for contacting. The controlled formation of low-dimensional structures of magnetic molecules has already been demonstrated within the framework of the GrK 611 and the SFB 668. Such self-assembled structures will be addressed on weakly and non-conductive substrates to access spin-dependent transport through molecular structures (Schwarz, Hoffmann, Wiesendanger). In close collaboration with the Heck and Prosenc groups strategies will be developed to guide efficient magnetic coupling through ligands.
A constant feedback will be maintained between experiment and theory. The main goal is the development of a first-principles many-body theory for molecular and atomic-based spintronics. It has to include charge, orbital, and spin fluctuations together with electron-phonon interactions in correlated nanocontacts. Phonon-mediated tunneling into graphene systems will be studied in close collaboration between the Lichtenstein and Weller groups. Here, new models of spin systems with novel types of long-range order allowing for coupled piezoelectric response will be used. The theoretical understanding of microscopic mechanisms of coupling between magnetism and lattice deformation will guide the design of novel materials based on conceptually new principles. The computer programs will be developed and optimized for high-performance computing clusters in collaboration of the theory and informatics groups.
The theoretical treatment of molecular spintronics will be based on the accurate description of magnetic properties from first-principles, including the coherent description of non-collinear magnetism, spin-orbit coupling and eventually the presence of a strong local Coulomb interaction (Lichtenstein group). The full frequency dependence of spin susceptibilities will be obtained by the recently developed continuous time Quantum Monte-Carlo scheme (Lechermann, Lichtenstein).
For calculations of ground and excited spin states of paramagnetic complexes proposed here geometries will be optimized first by density functional theory (DFT) methods (Prosenc, Lichtenstein) and subsequently by multireference (DDCI2) calculations (Prosenc, Lechermann) including spin-orbit and spin-spin coupling. New methods for the simulation of nanoparticles on carbon surfaces and for porphyrin complexes on Fe3O4 tubes will be developed using a combination of quantum chemical calculations for the particle and semi-empirical calculations for the surface. The adsorption energy of the particle will be corrected by an optimized van-der-Waals term (Prosenc, Lichtenstein). The more accurate state functions obtained will be introduced into many-particle calculations of the spin-dependent transport and magnetization dynamics (Chudnovskiy, Lichtenstein). As a major goal we will bring together quantum-chemical approaches and condensed-matter many-body approaches.
Within this initiative calculations of paramagnetic complexes in high spin states and molecular excitations have to be performed by multireference techniques. It is intended to install the position of a Professor (W2) who leads a research group for theoretical chemistry of paramagnetic compounds and their interaction with matter.
In conclusion, in the research area C groups that have demonstrated successful cooperation within the SFB 668 and the GrK 611 will undertake a joint effort to advance molecular-based spintronics, particularly in view of the role of the local environment (surfaces, leads) and external stimuli (e.g. light, magnetic fields). As a visionary goal we aim at a sound understanding of spin-dependent transport through single molecules in dependence of their structural, electronic, and spin states and to apply this knowledge to develop multifunctional nanoscale spintronic devices.
