Alexei Bagrets

Giant magnetoresistance through a single molecule

Magnetoresistance is a change in the resistance of a material system caused by an applied magnetic field. Giant magnetoresistance occurs in structures containing ferromagnetic contacts separated by a metallic non-magnetic spacer, and is now the basis of read heads for hard drives and for new forms of random access memory. Using an insulator (for example, a molecular thin film) rather than a metal as the spacer gives rise to tunnelling magnetoresistance, which typically produces a larger change in resistance for a given magnetic field strength, but also yields higher resistances, which are a disadvantage for real device operation. Here, we demonstrate giant magnetoresistance across a single, non-magnetic hydrogen phthalocyanine molecule contacted by the ferromagnetic tip of a scanning tunnelling microscope. We measure the magnetoresistance to be 60% and the conductance to be 0.26G(0), where G(0) is the quantum of conductance. Theoretical analysis identifies spin-dependent hybridization of molecular and electrode orbitals as the cause of the large magnetoresistance.

Organometallic Benzene-Vanadium Wire: A One-Dimensional Half-Metallic Ferromagnet

Using density functional theory we perform theoretical investigations of the electronic properties of a freestanding one-dimensional organometallic vanadium-benzene wire. This system represents the limiting case of multidecker Vn(C6H6)(n+1) clusters which can be synthesized with established methods. We predict that the ground state of the wire is a 100% spin-polarized ferromagnet (half-metal). Its density of states is metallic at the Fermi energy for the minority electrons and shows a semiconductor gap for the majority electrons. We find that the half-metallic behavior is conserved up to 12% longitudinal elongation of the wire. Ab initio electron transport calculations reveal that finite size vanadium-benzene clusters coupled to ferromagnetic Ni or Co electrodes will work as nearly perfect spin filters.

Atomically Wired Molecular Junctions: Connecting a Single Organic Molecule by Chains of Metal Atoms

Using a break junction technique, we find a clear signature for the formation of conducting hybrid junctions composed of a single organic molecule (benzene, naphthalene, or anthracene) connected to chains of platinum atoms. The hybrid junctions exhibit metallic-like conductance (~0.1-1G0), which is rather insensitive to further elongation by additional atoms. At low bias voltage the hybrid junctions can be elongated significantly beyond the length of the bare atomic chains. Ab initio calculations reveal that benzene based hybrid junctions have a significant binding energy and high structural flexibility that may contribute to the survival of the hybrid junction during the elongation process. The fabrication of hybrid junctions opens the way for combining the different properties of atomic chains and organic molecules to realize a new class of atomic scale interfaces.

Single molecule magnetoresistance with combined antiferromagnetic and ferromagnetic electrodes.

The magnetoresistance of a hydrogen-phthalocyanine molecule placed on an antiferromagnetic Mn(001) surface and contacted by a ferromagnetic Fe electrode is investigated using density functional theory based transport calculations and low-temperature scanning tunneling microscopy. A large and negative magnetoresistance ratio of ~50% is observed in combination with a high conductance. The effect originates from a lowest unoccupied molecular orbital (LUMO) doublet placed almost in resonance with the Fermi energy. As a consequence, irrespective of the mutual alignment of magnetizations, electron transport is always dominated by resonant transmission of Mn-majority charge carries going through LUMO levels.

Magnetoresistance of atomic-sized contacts: An ab initio study

Institute of Microelectronics, NCSR ”Demokritos”, GR-15310 Athens, Greece(Dated: February 2, 2008)The magnetoresistance (MR) effect in metallic atomic-sized contacts is studied theoretically bymeans of first-principle electronic structure calculations. We consider three-atom chains formedfrom Co, Cu, Si, and Al atoms suspended between semi-infinite Co leads. We employ the screenedKorringa-Kohn-Rostoker Green’s function method for the electronic structure calculation and eval-uate the conductance in the ballistic limit using the Landauer approach. The conductance throughthe constrictions reflects the spin-splitting of the Co bands and causes high MR ratios, up to 50%.The influence of the structural changes on the conductance is studied by considering different geo-metrical arrangements of atoms forming the chains. Our results show that the conductance throughs-like states is robust against geometrical changes, whereas the transmission is strongly influencedby the atomic arrangement if p or d states contribute to the current.

Evaluation of conduction eigenchannels of an adatom probed by an STM tip

Ballistic conductance through a single atom adsorbed on a metallic surface and probed by a scanning tunneling microscope (STM) tip can be decomposed into eigenchannel contributions, which can be potentially obtained from shot noise measurements. Our density functional theory calculations provide evidence that transmission probabilities of these eigenchannels encode information on the modifications of the adatoms local density of states caused by its interaction with the STM tip. In the case of open shell atoms, this can be revealed in nonmonotonic behavior of the eigenchannels transmissions as a function of the tip-adatom separation.

Local Current Density Calculations for Molecular Films from Ab Initio

We present a formalism relying on density functional theory for the calculation of the spatially continuous electron current density j(r) and induced magnetic fields B(r) in molecular films in dc transport. The proposed method treats electron transport in graphene ribbons containing on the of order 10(3) atoms. The employed computational techniques scale efficiently when using several thousand CPUs. An application to transport through hydrogenated graphene will be presented. As we will show, the adatoms have an impact on the transmission function not only because they introduce additional states but also because their presence modifies the geometry of the carbon host lattice (lattice relaxation).

Ab initio simulations of scanning-tunneling-microscope images with embedding techniques and application to C58-dimers on Au(111)

We present a modification of the standard electron transport methodology based on the (non-equilibrium) Greens function formalism to efficiently simulate STM-images. The novel feature of this method is that it employs an effective embedding technique that allows us to extrapolate properties of metal substrates with adsorbed molecules from quantum-chemical cluster calculations. To illustrate the potential of this approach, we present an application to STM-images of C58-dimers immobilized on Au(111)-surfaces that is motivated by recent experiments.

Ab Initio Transport Calculations for Functionalized Graphene Flakes on a Supercomputer

We present ab initio transport studies of large graphene flakes focusing on the local current density j(r) as it arises from a dc-transport measurement. Such ab initio transport calculations for sufficiently large flakes can be successfully tackled only using well scaling ab initio packages capable for transport calculations in thin film geometries. We employ the FHI-aims /AitransS packages to study the effect of disorder on the local current density in graphene flakes, in particular, the effect of chemical functionalization on mesoscopic fluctuations of the current density. Simulating graphene flakes with several thousands of atoms, we clearly see the qualitative effects of quantum interference and mesoscopic fluctuations in such systems. We also discuss the parallelization and optimization techniques, which we implemented into the transport module AitransS to allow efficient ab initio transport calculation on Cray XE6 and XC40 supercomputers.

Spintronics with single molecules

We demonstrate that with the help of Scanning Tunneling Microscopy (STM), spintronic functions can be realized with single molecules. First, spin transport across single organic molecules was investigated and a molecular giant magnetoresistance (GMR) junction was realized. For this, single phthalocyanine molecules (Pc) were contacted by two ferromagnetic electrodes or by an antiferromagnetic and a ferromagnetic electrode. As substrates, ferromagnetic Co nano-islands grown Cu(111) or antiferromagnetic Mn films on Fe(100) were used, onto which the Pc molecules were deposited. The magnetic state of the substrate was determined by spin-polarized STM with Co or Fe tips. Then, the tip of the STM was approached in a controlled way to contact the molecule. Below 0.4 nm distance, an attractive interaction between the tip and the molecule leads to a jump to contact of one of the side groups of the molecule and to a well defined molecular junction. Through the contacted molecule, a GMR of 60% was observed in case of Co substrates and Co tips. In case of Mn surfaces and Fe tips, a negative GMR of -50% was seen. These results are explained on basis of ab initio calculations showing a selective hybridization of the molecular states with states of the electrodes. Second, we demonstrate that single spin-crossover molecules can be switched between a non-magnetic and a magnetic state reversibly and deterministically by the application of local tunneling currents and that the lifetimes of both states exceed practical measuring times of STM. Thus, we demonstrate a magnetic memory device containing a single magnetic molecule and which can be read and written entirely by electric currents.