Session Y13: Focus Session: Low-Dimensional and Molecular Magnetism - Nanomagnetism on Surfaces - Magnetic Adatoms and Clusters

Magnetism of single-vacancy defects in graphene and boron-nitride nanoflakes

In this work we have used the hexagonal zigzag graphene and boron-nitride nanoflakes as a simple systems for studying the new class of magnetic materials obtained by structural vacancies in nonmagnetic s-p nanostructures. We have shown that for these systems, it is possible to predict the total spin moment from a electron counting analysis. Employing DFT calculations based on the LCAO approximation and the Fixed Spin Moment method, we have determinate the ground state spin multiplicity and the spin magnetic distribution for these structures. We have found that the ground state multiplicity of graphene nanoflakes is triplet, corresponding to a spin magnetic moment of $M=2\mu_{\beta}$. Analyzing the spin orbital distribution we have determinate that the spin-polarized for the graphene nanoflakes is equally distributed in the $sp^{2}$ and $p_{z}$ orbitals. For the boron-nitride nanoflakes we have obtained a quartet state ($M=3\mu_{\beta}$) in the case of a boron vacancy, and a doublet state ($M=1\mu_{\beta}$) for a nitrogen vacancy. We have found that for the boron-vacancy the spin-polarized is mainly localized on the $sp^{2}$ orbitals of nitrogen atoms. In contrast, for the nitrogen-vacancy the spin-polarized is concentrated at the $p_{z}$ orbitals of boron atoms.   

Dynamical magnetic excitations in adatoms and dimers on metallic surfaces

There is hardly any method which has shaped nanoscience and nanotechnology more profoundly than the scanning tunneling microscope. Such a tool is used nowadays to probe spin-excitations in nano-objects[1,2,3,4]. A key quantity describing these excitations is the transverse dynamical magnetic susceptibility that we calculate using the Korringa-Kohn-Rostoker Green function method within the framework of time-dependent density functional theory[5]. The behavior of adatoms and dimers will be discussed and comparison to experimental works will be provided when available. \\[4pt] [1] C. F. Hirjibehedin {\it et al.}, Science 317, 1199 (2007)\newline [2] T. Balashov {\it et al.}, Phys. Rev. Lett. 102, 257203 (2009)\newline [3] A. A. Khajetoorians {\it et al.}, Phys. Rev. Lett. 106, 037205 (2011)\newline [4] B. Chilian {\it et al.}, Arxiv:1108.2443\newline [5] S. Lounis {\it et al.}, Phys, Rev, Lett. 105, 187205 (2010); Phys. Rev. B 83, 035109 (2011)   

Real Space Observation of Inelastic Kondo Effect and Interorbital Spin-Coupling in Molecule-Metal Contacts

We present a comparative scanning tunneling spectroscopy study of four different types of MPc complexes (M = Fe, Co, Ni, Cu) adsorbed on the Ag(100) surface. Their magnetic properties are studied via the Kondo interaction with the substrate. Whereas the spectra of FePc and CoPc near the Fermi level is featureless, CuPc and NiPc show a Kondo resonance arising from the interaction of a ligand spin with conduction electrons. The spin at the organic macrocycle is induced by charge transfer from the Ag substrate. In CuPc, the coexistence of ion and ligand spin gives rise to interorbital coupling and spin excitations. The latter are observed via inelastic tunneling, where the Kondo interaction appears coupled to spin and vibrational excitations. By using the tip as a mobile electron we find that each type of excitation occupy mutually exclusive regions within the molecule, and result in different spin relaxation dynamics, reflecting the need of an atomic control of the molecule-metal interface to obtain reproducible transport properties. Finally, we study the influence of intermolecular interactions on the electronic and magnetic properties by creating artificial clusters in a controlled manner by manipulation of individual molecules.   

Emerging magnetic stability in atomically assembled spin arrays

Magnetic stability is usually created by the interaction of a large ensemble of atomically small magnetic moments that are themselves unstable. We make use of the Scanning Tunneling Microscope's ability to move individual atoms and construct arrays of interacting spins. Owed to their smallness, the magnetic states of these spin arrays are quantized and we probe their energies by inelastic electron tunneling spectroscopy [1]. To gain access to the equally important dynamical properties we employ an all-electronic pump-probe measurement scheme with which we follow the evolution between the spin states at nanosecond speed [2]. The combination of energetic and dynamical information allows identification of the relevant spin interaction and spin relaxation mechanisms at the atomic level. We design arrangements of atoms that suppress quantum tunneling of magnetization and drastically stabilize different spin configurations. Tracing the emergence of magnetic stability in the progression from individual atoms to arrays of spins points to new avenues for spintronic applications at atomic dimensions. \\[4pt] [1] A. J. Heinrich, J. A. Gupta, C. P. Lutz, D. M. Eigler, Science 306 466 (2004).\\[0pt] [2] S. Loth, M. Etzkorn, C. P. Lutz, D. M. Eigler, A. J. Heinrich, Science 329 1628 (2010).   

Magnetocrystalline anisotropy of 3d transition metal atoms on graphen

Graphene has attracted most attention in the field of condensed matter physics, chemistry and material science since the first day when it was produced in lab. It's an ideal material for two-dimensional electron gas (2DEG). Most intriguingly, the electronic and magnetic properties can be easily engineered by decorating with external elemental atoms ranging from nonmetal to transition metal. Especially, graphene is attractive for spintronics due to its long spin life time and high mobility. So far, ultrathin Co films have been deposited on graphene which exhibit perpendicular magnetic anisotropy. In this work, first-principles calculations are performed to systematically study the magnetic properties of graphene decorated by 3d transition metal atoms with several covering patterns. We find that Fe/graphene always exhibits in-plane anisotropy regardless the coverage, while Mn/graphene and Co/graphene tend to have perpendicular easy axis for most range of coverage. The spin moments of Ni atoms are largely quenched for Ni/graphene. Moreover, the Mn atoms on graphene prefer ferromagnetic exchange coupling which are totally different from the pure counterpart without graphene support. The strong hybridization between 3d orbitals of transition metal atoms and pi states of graphene are responsible for the modification of magnetic properties. \textbf{Acknowledgement.} This work was supported by DOE Grant DE-FG02-05ER46237.   

Exploring Kondo Phenomena in Physisorbed Nitric Oxide

The NO molecule is a S=1/2 system that can be physisorbed on gold surfaces. As there are presently no data indicating whether or not physisorbed NO retains its spin and displays an observable Kondo effect, we investigate that question by means of our ab-initio based DFT+NRG approach [1]. DFT calculations for NO/Au(111) confirm that at low coverage the on-top adsorption site is the most stable, with the NO molecule forming an angle of approximately 60 degrees with the surface normal. Spin-polarized DFT calculations reveal that the molecule retains one unpaired electron in an antibonding $\pi$ orbital that hybridizes, albeit moderately, with the surface. Based on these ingredients, we discuss the possibility of observing a zero-bias Kondo anomaly in scanning tunneling spectroscopy above NO/Au(111). The influence of gold surface states and rovibronic motion of the molecule are also investigated. [1] P. Lucignano et al., Nature Mat. 8, 563 (2009).   

Magnetic Interaction between Surface Engineered Rare-earth Atomic Spins

We report an ab initio study of rare-earth adatoms (Gd) on a surface, where it has been demonstrated previously that the STM can build and manipulate spin-coupled transition-metal atoms on such a surface one atom at a time, and have their spin excitation measured to be antiferromagnetic. The present work is the first attempt of studying rare-earth spin-coupled adatoms, the geometry effect of spin coupling, and the underlying mechanism of ferromagnetic coupling. The exchange coupling between Gd atoms on the surface is calculated to be antiferromagnetic in one geometry and ferromagnetic in another, by considering their collinear spins and using the PBE+U exchange correlation. We also find the Gd dimer in these two geometries is similar to nearest-neighbor and next-nearest-neighbor Gd atom pairs in GdN bulk. We analyze how much direct exchange, superexchange, and RKKY interactions contribute to the exchange coupling for the ferromagnetic arrangement by additional first-principles calculations of alternative model systems. Our calculations also show that the Gd spin of these structures is 7/2, the same as that of a GdN bulk.   

Atom-by-atom engineering and atomic magnetometry of tailored nanomagnets with SP-STM

Nanomagnets, namely arrays of a few exchange coupled atomic magnetic moments, possess a rich variety of magnetic properties and are explored as constituents of nano-spintronics technologies. They have been realized as magnetic clusters or molecular nanomagnets. Individual nanomagnets, built from magnetic atoms adsorbed onto a nonmagnetic surface (adatoms) coupled by Ruderman-Kittel-Kasuya-Yosida (RKKY) exchange, exhibit a high level of versatility resulting from distance-dependent interactions. Here, we combine spin-resolved scanning tunneling microscopy (SP-STS), atom manipulation and simulations to tailor nanomagnets ranging from linear chains to complex two-dimensional arrays and perform magnetometry in an atom-by-atom fashion. Distinct ground states of each chain, depending on even or odd numbers of constituent atoms, and magnetic frustration within the arrays have been observed directly. Our work demonstrates real space access to the magnetic states of tailored nanostructures providing an approach to tackling open fundamental questions in magnetism.   

First-principles study of magnetic exchange interactions in scanning probe microscopy

In the last years, spin-polarized scanning tunneling microscopy (SP-STM) has been established as a technique to resolve complex magnetic structures down to the atomic scale. More recently, it has even become possible to detect the exchange interactions between tip and sample by magnetic exchange force microscopy (MExFM). However, the interpretation of such measurements is non-trivial, especially on the atomic scale. Here, we use density functional theory in order to study the effect of exchange interactions in SP-STM and MExFM measurements. First, we demonstrate the occurrence of a spin-valve effect for single Co and Cr atoms on Fe islands on W(110) contacted by an SP-STM tip as a result of the spin-dependent orbital symmetry of the states in the vicinity of the Fermi energy [1]. We find that the exchange interaction between tip and adsorbed atoms affects the magnetoresistance in the tunneling regime. Second, we explain the quantitative measurement of the exchange interaction across a vacuum gap using MExFM applied to an Fe monolayer on W(001) [2]. We show how the chemical tip composition influences the magnitude and distance dependence of the exchange forces.\\[4pt] [1] M. Ziegler et al., New J. Phys. 13, 085011 (2011).\\[0pt] [2] R. Schmidt et al., Phys. Rev. Lett. 106, 257202 (2011).   

Evidence of Kondo effect in organic radical nanoassemblies

The outstanding spatial resolution of low temperature (LT) scanning tunneling microscopy (STM) and spectroscopy (STS) enables to probe the frontier orbital electronic structure of single magnetic molecules and clusters adsorbed on substrates. Here, we study self-aligned nanostructures of (spin-1/2) hydrocarbon radicals on a metal surface by means of LT-STM and STS. Pronounced involvement of surface state electrons is observed in the frontier molecular orbital (MO) resonances. An empty hybrid state closely above the substrate Fermi level exhibits the characteristic properties of surface Kondo effect reported for similar systems in the literature. By identifying three electronic states as hybrids of molecular orbitals and surface state electrons (two of them directly related to the Kondo effect), we are able to present a modified picture of the surface Kondo effect. It is based on a valence-bond model, where the bonding state represents Kondo's virtual bound state and the antibonding state is the so called 'Kondo resonance' reported in STM studies of the surface Kondo effect. Furthermore, double occupation of the originally singly unoccupied MO by tunneling electrons leads to the third state well above the Fermi level due to Coulomb repulsion as described by the Anderson model.   

Spin inelastic electron transport through magnetic nanostructures

Recent experimental advances in scanning tunneling microscopy make the measurement of the conductance spectra of isolated and magnetically coupled atoms on nonmagnetic substrates possible. Notably, these spectra are characterized by a competition between the Kondo effect and spin-flip inelastic electron tunneling. In particular they include Kondo resonances and a logarithmic enhancement of the conductance at voltages corresponding to magnetic excitations, two features that cannot be captured by second order perturbation theory in the electron-spin coupling. We have now derived a third order analytic expression for the electron-spin self-energy, which can be readily used in combination with the non-equilibrium Green's function scheme for electron transport at finite bias. We demonstrate that our method is capable of semi-quantitative description of the competition between Kondo resonances and spin-flip inelastic electron tunneling at a computational cost significantly lower than that of other approaches. The examples of Co and Fe on CuN are discussed in detail. We also explain the theoretical origin of the conductance assymetry that is present for both spin and non-spin polarized STM tips in the experimentally determined spectra of these atoms.   

Real-space imaging of inelastic Kondo effect in a single O$_{2}$ molecule

Inelastic Kondo effect of the single O$_{2}$ molecule physisorbed on Ag(110) surface is investigated in real space using a low-temperature scanning tunneling microscope (STM) at 10 K. The O$_{2}$ molecule carries an unpaired spin, as supported by density functional theory, showing a Kondo resonance at the Fermi energy. The coupling between the vibrations and the unpaired electron in the O$_{2}$ molecule results in the inelastic Kondo effect, which is manifested as striking side peaks at finite biases in the dI/dV spectra, in clear contrast to the normal vibrational inelastic tunneling spectroscopy (IETS). Spectroscopic imaging shows that two vibrational modes are coupled to the Kondo resonance with different strengths, which arises from the symmetry match between the Kondo state and the vibrational modes.   

Scanning Tunneling Microscopy and Spectroscopy of Fe[H$_2$B(Pz)$_2$]$_2$(bipy) Spin-Crossover Complex on Au(111)

Spin crossover compounds have externally-tunable magnetic moments that will be useful for spintronic applications if they result in correspondingly tunable electronic properties. In order to assess this electronic tunability, we have performed variable temperature scanning tunneling microscopy and spectroscopy on a known Fe(II) spin crossover compound, Fe[H$_2$B(Pz)$_2$]$_2$(bipy), thermally-evaporated onto a single crystal Au(111) surface. We report on the surface assembly of this molecule in few layer films including imaging with submolecular resolution. In addition, we use STS to probe the local density of electronic states across a temperature range spanning the low spin to high spin transition and compare with first principles electronic structure calculations.   

Frustration in the Magnetic Molecules W$_{72}$V$_{30}$ and Pr$_{13}$ Probed by NMR and Tunnel-diode Resonator

The magnetic molecules Pr$_{13}$ and W$_{72}$V$_{30}$ have been studied by DC and AC magnetic susceptibilities and NMR. These molecules exhibit geometrical frustration resulting from antiferromagnetic intramolecular coupling of nearest neighbor paramagnetic ions. In Pr$_{13}$, 12 Pr$^{III}$ ions (S=1) interact along edge sites of an icosahedron centered about an additional Pr$^{III}$ ion. For W$_{72}$V$_{30}$, 30 V$^{IV}$ ions (S=1/2) interact along corner sharing sites of a near perfect icosidodecahedron, a spherical representation of a 2-D Kagome Lattice. Characterizing the DC magnetic response requires a distribution of exchange constants for both molecules at low temperatures. Results are compared to similar frustrated systems which feature a variety of magnetic phenomena - metamagnetic phase transitions, magnetic hysteresis without anisotropy, spin glass behavior, and superparamagnetism.   

Unification of ultrafast, laser-driven and slow, phonon-driven spin-flip scenarios in the three-magnetic-center cluster Ni$_3$Na$_2$

Recently the use of the three-magnetic-center Ni$_3$Na$_2$ on a fictitious, inert surface as a prototypical system for ultrafast, laser-driven spin manipulation and logic functionalization was reported [1]. Here we extend this investigation of spin dynamics on the same system by including vibronic motion and recalculating all electronic states for each structural distortion. The first immediate finding is that the system exhibits an unexpectedly rich set of magnetic phases (geometry sets with different orientations of the highly localized spin-density). Exploiting those phases allows us (a) to establish the combination of spin dynamics and phonons as an unprecedentedly accurate sensor of the bond length between metallic centers, and (b) to present for the first time a unified picture of ultrafast (fs), laser-driven and slow (ps), phonon-driven spin dynamics in molecular magnets.\\[4pt] [1] W. H\"{u}bner, S. Kersten, G. Lefkidis, Phys. Rev. B {\bf 79}, 184431 (2009).