Vasile Caciuc

Interface-engineered templates for molecular spin memory devices

The use of molecular spin state as a quantum of information for storage, sensing and computing has generated considerable interest in the context of next-generation data storage and communication devices, opening avenues for developing multifunctional molecular spintronics. Such ideas have been researched extensively, using single-molecule magnets and molecules with a metal ion or nitrogen vacancy as localized spin-carrying centres for storage and for realizing logic operations. However, the electronic coupling between the spin centres of these molecules is rather weak, which makes construction of quantum memory registers a challenging task. In this regard, delocalized carbon-based radical species with unpaired spin, such as phenalenyl, have shown promise. These phenalenyl moieties, which can be regarded as graphene fragments, are formed by the fusion of three benzene rings and belong to the class of open-shell systems. The spin structure of these molecules responds to external stimuli (such as light, and electric and magnetic fields), which provides novel schemes for performing spin memory and logic operations. Here we construct a molecular device using such molecules as templates to engineer interfacial spin transfer resulting from hybridization and magnetic exchange interaction with the surface of a ferromagnet; the device shows an unexpected interfacial magnetoresistance of more than 20 per cent near room temperature. Moreover, we successfully demonstrate the formation of a nanoscale magnetic molecule with a well-defined magnetic hysteresis on ferromagnetic surfaces. Owing to strong magnetic coupling with the ferromagnet, such independent switching of an adsorbed magnetic molecule has been unsuccessful with single-molecule magnets. Our findings suggest the use of chemically amenable phenalenyl-based molecules as a viable and scalable platform for building molecular-scale quantum spin memory and processors for technological development.

Design of the local spin polarization at the organic-ferromagnetic interface.

By means of ab initio calculations and spin-polarized scanning tunneling microscopy experiments the creation of a complex energy dependent magnetic structure with a tailored spin-polarized interface is demonstrated. We show this novel effect by adsorbing organic molecules containing π(p(z)) electrons onto a magnetic surface. The hybridization of the out-of-plane p(z) atomic-type orbitals with the d states of the metal leads to the inversion of the spin polarization at the organic site due to a p(z)-d Zener exchange-type mechanism. As a key result, we demonstrate the possibility to selectively and efficiently inject spin-up and spin-down electrons from a ferromagnetic-organic interface, an effect which can be exploited in future spintronic devices.

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.

The Backside of Graphene: Manipulating Adsorption by Intercalation

The ease by which graphene is affected through contact with other materials is one of its unique features and defines an integral part of its potential for applications. Here, it will be demonstrated that intercalation, the insertion of atomic layers in between the backside of graphene and the supporting substrate, is an efficient tool to change its interaction with the environment on the frontside. By partial intercalation of graphene on Ir(111) with Eu or Cs we induce strongly n-doped graphene patches through the contact with these intercalants. They coexist with nonintercalated, slightly p-doped graphene patches. We employ these backside doping patterns to directly visualize doping induced binding energy differences of ionic adsorbates to graphene through low-temperature scanning tunneling microscopy. Density functional theory confirms these binding energy differences and shows that they are related to the graphene doping level.

JuNoLo - Jülich nonlocal code for parallel post-processing evaluation of vdW-DF correlation energy

Abstract Nowadays the state of the art Density Functional Theory (DFT) codes are based on local (LDA) or semilocal (GGA) energy functionals. Recently the theory of a truly nonlocal energy functional has been developed. It has been used mostly as a post-DFT calculation approach, i.e. by applying the functional to the charge density calculated using any standard DFT code, thus obtaining a new improved value for the total energy of the system. Nonlocal calculation is computationally quite expensive and scales as N 2 where N is the number of points in which the density is defined, and a massively parallel calculation is welcome for a wider applicability of the new approach. In this article we present a code which accomplishes this goal. Program summary Program title: JuNoLo Catalogue identifier: AEFM_v1_0 Program summary URL: http://cpc.cs.qub.ac.uk/summaries/AEFM_v1_0.html Program obtainable from: CPC Program Library, Queens University, Belfast, N. Ireland Licensing provisions: Standard CPC licence, http://cpc.cs.qub.ac.uk/licence/licence.html No. of lines in distributed program, including test data, etc.: 176 980 No. of bytes in distributed program, including test data, etc.: 2 126 072 Distribution format: tar.gz Programming language: Fortran 90 Computer: any architecture with a Fortran 90 compiler Operating system: Linux, AIX Has the code been vectorised or parallelized?: Yes, from 1 to 65536 processors may be used. RAM: depends strongly on the problems size. Classification: 7.3 External routines: • FFTW ( http://www.tw.org/ ) • MPI ( http://www.mcs.anl.gov/research/projects/mpich2/ or http://www.lam-mpi.org/ ) Nature of problem: Obtaining the value of the nonlocal vdW-DF energy based on the charge density distribution obtained from some Density Functional Theory code. Solution method: Numerical calculation of the double sum is implemented in a parallel F90 code. Calculation of this sum yields the required nonlocal vdW-DF energy. Unusual features: Binds to virtually any DFT program. Additional comments: Excellent parallelization features. Running time: Depends strongly on the size of the problem and the number of CPUs used.

Absence of edge states in covalently bonded zigzag edges of graphene on Ir(111).

The zigzag edges of graphene on Ir(111) are studied by ab initio simulations and low-temperature scanning tunneling spectroscopy, providing information about their structural, electronic, and magnetic properties. No edge state is found to exist, which is explained in terms of the interplay between a strong geometrical relaxation at the edge and a hybridization of the d orbitals of Ir atoms with the graphene orbitals at the edge.

Long-range magnetic coupling between nanoscale organic–metal hybrids mediated by a nanoskyrmion lattice

The design of nanoscale organic-metal hybrids with tunable magnetic properties as well as the realization of controlled magnetic coupling between them open gateways for novel molecular spintronic devices. Progress in this direction requires a combination of a clever choice of organic and thin-film materials, advanced magnetic characterization techniques with a spatial resolution down to the atomic length scale, and a thorough understanding of magnetic properties based on first-principles calculations. Here, we make use of carbon-based systems of various nanoscale size, such as single coronene molecules and islands of graphene, deposited on a skyrmion lattice of a single atomic layer of iron on an iridium substrate, in order to tune the magnetic characteristics (for example, magnetic moments, magnetic anisotropies and coercive field strengths) of the organic-metal hybrids. Moreover, we demonstrate long-range magnetic coupling between individual organic-metal hybrids via the chiral magnetic skyrmion lattice, thereby offering viable routes towards spin information transmission between magnetically stable states in nanoscale dimensions.

Probing the Magnetic Exchange Forces of Iron on the Atomic Scale

Applying magnetic exchange force microscopy with an Fe-coated tip, we experimentally resolve the atomic-scale antiferromagnetic structure of the Fe monolayer on W(001). On the basis of first-principles calculations, using an Fe nanocluster as a tip, we determine the distance dependence of the magnetic exchange forces. Significant relaxation of tip and sample atoms occurs, which depend sensitively on the local magnetic configuration. This shifts the onset of magnetic interactions toward larger separations and facilitates their observation. Implementing a multiatom tip in the calculations and accounting for relaxation effects are crucial to obtain the correct sign and distance dependence of the magnetic exchange interaction. By comparison with our calculations, we show that the experimentally observed contrast is due to a competition between chemical and magnetic forces.

Structure and Growth of Hexagonal Boron Nitride on Ir(111)

Using the X-ray standing wave method, scanning tunneling microscopy, low energy electron diffraction, and density functional theory, we precisely determine the lateral and vertical structure of hexagonal boron nitride on Ir(111). The moiré superstructure leads to a periodic arrangement of strongly chemisorbed valleys in an otherwise rather flat, weakly physisorbed plane. The best commensurate approximation of the moiré unit cell is (12 × 12) boron nitride cells resting on (11 × 11) substrate cells, which is at variance with several earlier studies. We uncover the existence of two fundamentally different mechanisms of layer formation for hexagonal boron nitride, namely, nucleation and growth as opposed to network formation without nucleation. The different pathways are linked to different distributions of rotational domains, and the latter enables selection of a single orientation only.

Ab initio and semi-empirical van der Waals study of graphene–boron nitride interaction from a molecular point of view

We have performed a systematic semi-empirical and ab initio van der Waals study to investigate the bonding mechanism of benzene (C(6)H(6)), triazine (C(3)N(3)H(3)) and borazine (B(3)N(3)H(6)) adsorbed on graphene and a single boron nitride (BN) sheet. The two semi-empirical approaches used to include the van der Waals (vdW) interactions in our density functional theory (DFT) calculations suggest that the strength of the molecule-surface interaction corresponds to a strong physisorption with no net charge transfer between the molecules and the corresponding substrates. This observation is strengthened by the use of first-principles non-local correlation vdW-DF functionals which provide a sound physical basis to include vdW interactions in DFT calculations. In particular we have employed two flavors of vdW-DF functionals which enabled us to determine the role of the non-local correlation effects in the molecule-surface bonding mechanism which cannot be assessed by using only semi-empirical vdW methods. Our study also reveals that the strength of the molecule-surface interaction can be influenced by the electronegativity of the B, C and N atoms.