Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session W34: Focus Session: Nanomagnetism -- Atomic Size Structures |
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Sponsoring Units: DMP GMAG Chair: Pietro Gambardella, Universitat Autònoma de Barcelona Room: E144 |
Thursday, March 18, 2010 11:15AM - 11:27AM |
W34.00001: Spin-dependent quantum interference within a single Co nanostructure D. Sander, H. Oka, S. Wedekind, G. Rodary, L. Niebergall, P. Ignatiev, V. Stepanyuk, J. Kirschner We present results of a combined experimental and theoretical study of spin-polarized electron confinement on individual nm small Co islands on Cu(111). Spin-polarized low-temperature STM in magnetic fields is used to identify and prepare parallel (P) and anti-parallel (AP) states of the magnetization orientation between a Co island and the magnetic tip of the STM. We find a pronounced spatial modulation of the differential conductance within one island, which is ascribed to electron confinement. The analysis of these modulation patterns for P and AP magnetization states between tip and island reveals a strong variation between both states. Maps of the asymmetry of the differential conductance, (GAP-GP)/(GAP+GP), show strong spatial variations, where the contrast depends on the gap voltage. These results are ascribed to a spatial modulation of the spin-polarization within the Co islands. In conjunction with theory we conclude that the modulation of the spin- polarization, and its variation with energy, can be described by the relative magnitudes of the spin-resolved density of states, where the spatial modulation is mainly due to electron confinement of majority electrons. [Preview Abstract] |
Thursday, March 18, 2010 11:27AM - 11:39AM |
W34.00002: A model for spin sensitive dissipation in atomically resolved probe microscopy experiments on magnetic surfaces Giuseppe Ernesto Santoro, Franco Pellegrini, Erio Tosatti We describe a mechanism of energy dissipation relevant for experiments of single spin manipulation, such as the recently introduced magnetic exchange force microscopy. By considering a driven damped two-state system in the overdamped regime, we show how the spin-phonon coupling in a magnetic system can lead to hysteresis effects and thus to a spin-dependent dissipation channel of relevant magnitude down to low frequencies. We use standard real time path-integral techniques to derive simple rate equations for the spin-boson model that can be numerically integrated to investigate the frequency and temperature dependence of the dissipated energy. [Preview Abstract] |
Thursday, March 18, 2010 11:39AM - 11:51AM |
W34.00003: Ligand control of the spin states of single Mn atom on lead oxide film P. Cheng, T. Zhang, S.H. Ji, Y.S. Fu, X. Chen, X.C. Ma, J.F. Jia, Q.K. Xue Controlling the spin states of individual magnetic atoms and molecules is of fundamental interests and important for atomic- scale technology. Using low temperature scanning tunneling microscopy and single molecule manipulation technique, we demonstrate that the spins of an Mn atom can be manipulated by bonding it to organic ligands (4, 4'- biphenyldicarbonitrile). This chemical reaction at single molecular level was performed on thin lead oxide film, which acts as a decoupling layer from metal substrate. The spin states of the resulting magnetic complex were probed by inelastic electron tunneling spectroscopy. Different from a single Mn atom, which shows the Zeeman splitting in the tunneling spetra, a complex formed by one Mn atom and one organic ligand exhibits zero field splitting, indicating a magnetic anisotropy of 0.58 meV. When two organic ligands are bonded to an Mn atom, the spin of the Mn atom is fully quenched. [Preview Abstract] |
Thursday, March 18, 2010 11:51AM - 12:03PM |
W34.00004: Temperature and size dependence of antiferromagnetism in Mn nanostructures P. Sessi, N.P. Guisinger, J.R. Guest, M. Bode We report on variable-temperature STM investigations of the spontaneous long-range magnetic order of Mn monolayer nanostructures epitaxially grown on stepped W(110). The measurements reveal that the onset of the antiferromagnetic order is closely related to the Mn nanostructure width along the [001] direction, with a decreasing N{\'e}el temperature as we move from a 2D toward a quasi-1D system. In contrast, lateral confinement along the [$1\overline{1}0$] direction seems to play a less important role. The results are discussed in terms of anisotropic exchange coupling and of boundary effects, both potentially stabilizing long-range magnetic order in nanostructures confined in the [$1\overline{1}0$] direction. [Preview Abstract] |
Thursday, March 18, 2010 12:03PM - 12:15PM |
W34.00005: STM studies of spin excitations in few-atom cobalt clusters David Gohlke, Taeyoung Choi, Jay Gupta The understanding of spin interaction between atoms is essential to the production of nanoscale magnetic materials. We use low-temperature (5 K) scanning tunneling microscopy and spectroscopy to study the magnetic properties of few-atom cobalt clusters. The clusters are built with atomic manipulation on one-monolayer thick copper nitride (Cu$_2$N) islands, which serve to decouple the clusters from the underlying Cu(100) or Cu(111) substrate. Inelastic electron tunneling spectroscopy reveals a variety of spin-flip excitations at low energies ($<$ 20meV). These transitions are visible due to the mediation of the insulating Cu$_2$N layer, and spatially vary along dimers and larger multimers. In particular, we examine the effects of the anisotropic copper nitride lattice on the spin-coupling between the constituent cobalt atoms. http://www.physics.ohio-state.edu/\~{}jgupta/ [Preview Abstract] |
Thursday, March 18, 2010 12:15PM - 12:27PM |
W34.00006: Analysis of the Kondo Effect in Low Coordinated Ferromagnets Carlos Untiedt, M. Reyes Calvo Recently we reported the observation of Kondo Physics in one-atom contacts between pure ferromagnetic metals such as Fe, Co, or Ni [Nature 458, 1150 (2009)]. Using a Scanning Tunneling Microscope (STM) or Electromigrated Break Junctions we fabricated atomic contacts on these ferromagnetic materials and Fano-Kondo resonances where found in the conductance with the characteristic behavior of a Kondo system. One of the advantages of our setup configuration is the capability of the STM to study and analyze hundreds of atomic contacts. This has given us the unique opportunity of performing statistics on the Fano parameters of our conductance curves. Here we show a complete analysis of the Fano parameters of a Kondo system consisting in a one-atom contact with a localized magnetic moment interacting with the conduction electrons. A statistical analysis shows the dependence of the Kondo system with the different degrees of couplings to the environment. We will compare the cases of Ni, Fe and Co which could reflect differences in the localization of the magnetic moment. [Preview Abstract] |
Thursday, March 18, 2010 12:27PM - 12:39PM |
W34.00007: Skyrmionic like spin-texture of Rashba electron scattering at magnetic adatoms deposited on the Au(111) surface Samir Lounis, Andreas Bringer, Stefan Bl\"ugel Surfaces are an inversion asymmetric environment. In combination with the spin-orbit interaction, surface electrons experience a Rashba effect, which leads to spin-split surface states [1]. Having an adatom on such a surface, surface states scatter at it. Interferences are created from which, surprisingly, the fingerprints of spin-orbit coupling cannot be seen with a scanning tunneling microscope (STM) [2]. Instead of a single adatom, Walls and Heller [3] proposed to use a corral of atoms to create extra spin-orbit coupling related modulations in the charge density. Resting on multiple scattering theory, we propose to visualize such effects using a spin-polarized STM considering either a single magnetic adatom or a corral of magnetic adatoms.\\[4pt] [1] S.~Lashell, B.A.~McDougall, E.~Jensen, Phys.\ Rev.\ Lett.{\bf 77}, 3419 (1996).\\[0pt] [2] L.~Petersen and P.~Hedeg\aa rd, Surf.\ Sci.\ {\bf 49}, 459 (2000).\\[0pt] [3] J.D.~Walls and E.J.~Heller, Nano Letters {\bf 7}, 3377 (2007). [Preview Abstract] |
Thursday, March 18, 2010 12:39PM - 12:51PM |
W34.00008: Density functional calculations of the absorption and magnetic properties of oxygen molecule on Au(110) Ruqian Wu, Yanning Zhang, Juexian Cao, Ying Jiang, Wilson Ho Studies of the Kondo effect in molecular magnets and molecular junctions capture extensive attention of experimentalists and theorists. Our low temperature STM experiments revealed fascinating features of O$_{2}$ on the reconstructed Au(110) surface. O$_{2}$ forms alternating zig-zag and ying-yang rows on Au(110) and displays sizeable Kondo effect. Density functional calculations were performed to understand the driving force for the unusual structure and magnetic properties. Using the fixed triplet state of O$_{2}$, the optimized atomic structure resembles the experimental topography well. The magnetization of O$_{2}$ is stable with regular density functional calculations, 1.97 $\mu _{B}$ from. We will discuss the pattern of Kondo clouds in view of molecular orbitals and density of states. Extensive experimental and theoretical data allow comprehensive understanding of the essence of spin polarization and spatial Kondo effect in molecular systems. [Preview Abstract] |
Thursday, March 18, 2010 12:51PM - 1:03PM |
W34.00009: Origin of the spin-polarization of magnetic STM tips Paolo Ferriani, Cesar Lazo, Stefan Heinze Spin-polarized (SP-) STM is a powerful tool to resolve atomic scale magnetic structures that relies on the use of magnetic tips. A long debate developed about the origin of the tip spin-polarization in the vacuum region around the Fermi level, which is the crucial feature to achieve magnetic contrast. Whether it comes from the strongly spin polarized d-electrons or from the slowly decaying s-electrons is not yet settled. To clarify this issue, we performed density functional theory calculations of magnetic STM tips composed of Fe or Cu with a 3d transition-metal apex atom. Surprisingly, the local density of states in the vacuum region stems from majority s-electrons, resulting in positive spin-polarization, although minority d-electrons dominate at the apex atom. Interestingly, majority and minority states exhibit different orbital characters at the Fermi level. We interpret these findings as the consequence of reduced symmetry at the tip apex. This effect should also be observable on single magnetic atoms on a surface, which is confirmed by SP-STM measurements of a Co atom on Mn/W(110). [Preview Abstract] |
Thursday, March 18, 2010 1:03PM - 1:15PM |
W34.00010: Quantum size effects in competing charge and spin orderings of dangling-bond wires on Si(001) Jun-Hyung Cho, Ji Young Lee, Zhenyu Zhang Using spin-polarized density functional theory calculations, we investigate the competition between charge and spin orderings in dangling-bond (DB) wires of increasing lengths fabricated on an H-terminated Si(001) surface. For wires containing less than 10 DBs as studied in recent experiments, we find antiferromagnetic (AF) ordering to be energetically much more favorable than charge ordering. The energy preference of AF ordering shrinks in an oscillatory way as the wire length increases, and preserves its sign even for DB wires of infinite length. The oscillatory behavior can be attributed to quantum size effects as the DB electrons fill discrete quantum levels. The predicted AF ordering is in startling contrast with the prevailing picture of charge ordering due to Jahn-Teller distortion or Peierls instability for wires of finite or infinite lengths, respectively. [Preview Abstract] |
Thursday, March 18, 2010 1:15PM - 1:27PM |
W34.00011: Determination of rotatable and frozen antiferromagnetic spins in exchange biased CoO/Fe/Ag(001) system J. Wu, J. Park, Wondong Kim, E. Arenholz, M. Liberati, Chanyong Hwang, Z.Q. Qiu X-ray Magnetic Circular and Linear Dichrisms are applied to epitaxially grown CoO/Fe/Ag(001) system to obtain both the ferromagnetic Fe and antiferromagnetic CoO hysteresis loops. The observation of the antiferromagnetic CoO hysteresis loop in the exchange-biased CoO/Fe bilayer illuminates the existence of two types of CoO spins (rotatable and frozen) in response to the Fe magnetization reversal by an external magnetic field. The absolute amounts of the rotatable and frozen spins are determined as a function of the CoO thickness to give a direct comparison to the exchange bias field of the Fe film. We find a surprising result that the exchange bias is well established before the CoO spins are frozen. This result suggests that the current theoretical models need to exam the common assumption that frozen antifferomagnetic spins are a necessary condition for the exchange bias. By using 2ML NiO as a probe layer inside the CoO film, we also show that the rotatable/frozen spin ratio is uniform for each layer of the CoO film, ruling out the formation of CoO domain wall in the mormal direction of the film. [Preview Abstract] |
Thursday, March 18, 2010 1:27PM - 1:39PM |
W34.00012: Magnetic anisotropy of CoO/Fe films grown on vicinal Ag(001) J. Park, J. Wu, A. School, A. Doran, E. Arenholz, W. Kim, Chanyong Hwang, Z. Q. Qiu CoO/Fe films were grown epitaxially on Ag(001) vicinal surfaces with the steps parallel to Ag [110] axis. Magnetic hysteresis loop measurement at room temperature shows that as the CoO thickness increases to establish the antiferromagnetic order, it introduces a magnetic anisotropy to the Fe ferromagnetic layer to enhance the Co coercivity. However, we find that the step-induced uniaxial magnetic anisotropy in the Fe film is not affected by the CoO antiferromagnetic order. This result shows that the CoO only imprints a 4-fold magnetic anisotropy to the Fe film. [Preview Abstract] |
Thursday, March 18, 2010 1:39PM - 1:51PM |
W34.00013: Magnetism of Fe-Pt surface alloy formed on Pt(110) Chanyong Hwang, W.D. Kim, Y.S. Park, M.B. Hossain, C.G. Kim Ordered Fe-Pt surface alloys(local (2$\times$1) and (2 $\times$2) phase) are formed on Pt(110) surface. Scanning tunneling microscope and core-level photoemission spectroscopy have been applied to characterize the atomic structure of these surface alloys. Surface magnetism has been probed with the magneto-optic Kerr effect. These two phases are ferromagnetic and it has been confirmed by the first principles calculation. The surface of these surface alloys is very interesting since the hysteresis obtained by the SMOKE shows the atomistic evidence of magnetic dipolar interaction on magnetization reversal. The position of two adjacent off-axis Fe atoms and the direction of its spin form a critical angle, where the magnetic dipolar interaction is zero. In addition to this magnetic dipolar interaction, role of the exchange interaction and induced ferromagnetic order in Pt atom will be discussed. [Preview Abstract] |
Thursday, March 18, 2010 1:51PM - 2:03PM |
W34.00014: Ni spin switching induced by interfacial spin frustration in FeMn/Ni/Cu(001) Z. Q. Qiu, J. Wu, J. Choi, A. Scholl, A. Doran, E. Arenholz, Chanyong Hwang FeMn/Ni/Cu(001) bilayer films are grown epitaxially and investigated by photoemission electron microscopy and magneto-optic Kerr effect. We find that as the FeMn overlayer changes from paramagnetic to antiferromagnetic states, it switches the ferromagnetic Ni spin direction from out-of-plane to in-plane directions of the film. This phenomenon reveals the mechanism of creating a magnetic anisotropy in the Ni film by the antiferromagnetic order of the FeMn film. We argue that this antiferromagnetic order induced anisotropy comes from the out-of-plane spin frustration at the FeMn-Ni interface. [Preview Abstract] |
Thursday, March 18, 2010 2:03PM - 2:15PM |
W34.00015: Magnon softening in a ferromagnetic monolayer Andrea Taroni, Anders Bergman, Lars Bergqvist, Johan Hellsvik, Bj\"orgvin Hj\"orvarsson, Olle Eriksson Recent progress in experimental techniques have allowed the first ever observation of the magnon dispersion spectrum of a single ferromagnetic monolayer of Fe on W(110) [1]. The measurements revealed magnon energies that are small in comparison to the bulk and surface Fe(110) excitations. More surprisingly, these energies are also much smaller than those predicted theoretically [2, 3]. This discrepancy, related to a strong magnon softening, raises the possibility that 1 ML Fe/W(110) may not be a simple itinerant ferromagnet, as generally assumed. Prompted by these developments, we have examined the Fe/W(110) system through a combination of first principles calculations and detailed atomistic spin dynamics simulations [4]. We focused on the dispersion of the spin waves parallel to the [001] direction. Our results compare favourably with the experimental data [1], and correctly reproduce the drastic softening of the magnon spectrum, with respect to the bulk. We thus resolve a discrepancy between theory and experiment, and demonstrate the predictive power of the atomistic spin dynamics approach. [1] Prokop et al PRL 102, 177206 (2009) [2] Muniz and Mills, PRB 66, 174417 (2002) [3] Costa et al, PRB 78, 054439 (2008) [4] Skubic et al, JPCM 20, 315203 (2008) [Preview Abstract] |
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