Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session S50: Molecular Magnets: STM, Ad-atoms, InterfacesFocus
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Sponsoring Units: GMAG DMP Chair: Cyrus Hirjibehedin, University College London Room: 397 |
Thursday, March 16, 2017 11:15AM - 11:27AM |
S50.00001: SP-STM study of layered magnetic material Fe3GeTe2 Giang Nguyen, Jewook Park, Saban Hus, Qiang Zou, Zheng Gai, Jinhwan Lee, Renlong Liu, Changgu Lee, An-Ping Li 2D magnets have attracted great research interest due to their great potentials for spintronic application. With this regard, magnetic layered 2D materials have offered great opportunities for creating exfoliated 2D magnets. Here we report a recent study on magnetic 2D materials of Fe3GeTe2 cleaved in situ under ultra-high vacuum (UHV) using Variable-Temperature Spin-Polarized Scanning Tunneling Microscopy (VT-SP-STM). Atomic resolution images and scanning tunneling spectroscopy are acquired with both non-magnetic (W) tips and ferromagnetic (Ni) tips. Interestingly, two kinds of magnetic domain structure are observed at liquid nitrogen temperature. These domains show temperature-dependent magnetic phase transitions at \textasciitilde 180 K and \textasciitilde 250 K, respectively, which are further corroborated by SQUID magnetometry measurements. The microscopic origins of these transition will be discussed. This research was conducted at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy. [Preview Abstract] |
Thursday, March 16, 2017 11:27AM - 11:39AM |
S50.00002: Universal mechanism for electron paramagnetic resonance of individual adatoms Jose Lado, Alejandro Ferron, Joaquin Fernandez-Rossier We propose a new universal mechanism that makes it possible to drive an individual atomic spin using a spin polarized scanning tunnel microscope (STM) with an oscillating electric signal. We show that the combination of the distance dependent exchange with the magnetic tip and the electrically driven mechanical oscillation of the surface spins permits to control their quantum state. Based on a combination of density functional theory and multiplet calculations, we show that the proposed mechanism is essential to account for the recently observed electrically driven paramagnetic spin resonance (ESR) of an individual Fe atom on a MgO/Ag(100) surface. Our findings set the foundation to deploy the ESR-STM quantum sensing technique to a much broader class of systems. [Preview Abstract] |
Thursday, March 16, 2017 11:39AM - 11:51AM |
S50.00003: Unraveling surface enabled magnetic phenomena in low dimensional systems Milos Baljozovic, Jan Girovsky, Jan Nowakowski, Md Ehesan Ali, Harald Rossmann, Thomas Nijs, Elise Aeby, Sylwia Nowakowska, Dorota Siewert, Gitika Srivastava, Christian Wäckerlin, Jan Dreiser, Silvio Decurtins, Shi-Xia Liu, Peter M. Oppeneer, Thomas A. Jung, Nirmalya Ballav Molecular spin systems with controllable interactions are of both fundamental and applied importance. These systems help us to better understand the fundamental origins of the interactions involved in low dimensional magnetic systems and to put them in the framework of existing models towards their further development. Following our first observation of exchange induced magnetic ordering in paramagnetic porphyrins adsorbed on ferromagnetic Co surface we showed that magnetic properties of such molecules can be controllably altered upon exposure to chemical and physical stimuli [1]. In our most recent work it was shown that a synthetically programmed co-assembly of Fe and Mn phthalocyanines can also be realized on diamagnetic Au(111) surfaces where it induces long-range 2D ferrimagnetic order, at first glance in conflict with the Mermin-Wagner theory. Here we provide evidence for the first direct observation of such ordering from STM/STS and XMCD data and from DFT$+$U calculations demonstrating key role of the Au(111) surface states in mediating AFM RKKY coupling of the Kondo underscreened magnetic moments [2]. [1] Ballav, N. et al. JPCL, 4, 2303 (2013). [2] Girovsky, J. et al. Second Stage Rev. Nat Commun [Preview Abstract] |
Thursday, March 16, 2017 11:51AM - 12:03PM |
S50.00004: Tailoring decoherence in nanomagnets by geometrical design Fernando Delgado, Joaquín Fernández-Rossier Magnetic atoms on surfaces suffer relaxation and decoherence [1], which limit their possible applications in both classical storage and quantum computation. Kondo exchange interaction is usually the dominant source of relaxation. Hence, for a single magnetic impurity, the product of density of states at the Fermi level and the Kondo coupling controls relaxation and decoherence together with the renormalization of the magnetic anisotropy. Here we show that in the case of small arrays of magnetic adatoms, which can be build by STM manipulation, relaxation and decoherence are controlled in addition by the product of Fermi wavenumber and inter-spin distance, giving place to interesting interference phenomena similar to those appearing in optics [2]. This is nothing else that the dissipative counterpart of the RKKY oscillation. In addition, we explore different configurations to reduce the spin decoherence of antiferromagnetic spin arrays opening a route to engineer spin relaxation and decoherence in atomically designed spin structures. [1] F. Delgado and J. Fernández-Rossier, arXiv: 1609.03389.\\ [2] F. Delgado and J. Fernández-Rossier, arXiv: 1608.07462. [Preview Abstract] |
Thursday, March 16, 2017 12:03PM - 12:15PM |
S50.00005: Magnetic Anisotropy of an Fe-Porphyrin Complex on Au(111) Surface Weihua Wang, Bing Liu, Huixia Fu, Shuya Xing, Sheng Meng, Jiandong Guo By a combined study of low temperature scanning tunnelling microscopy (STM) and density functional theory (DFT) calculations, we have investigated the magnetic properties of an Fe-TPyP complex ($i$-FeTPyP) in the initial stage of metalation reaction on Au(111) substrate. The inelastic electron tunneling spectroscopy of $i$-FeTPyP showed typical zero-field excitation energy of 18 meV for the first excited state. Modeling the spin excitation energy in magnetic fields by spin Hamiltonian gave an easy-axis anisotropy perpendicular to the molecular plane. DFT calculations reveal that the Fe atom in $i$-FeTPyP is lifted from Au substrate and surrounded by elongated Fe-N bonds, and has an orbital angular momentum of \textbf{\textit{L}}$=$2 and spin angular momentum of \textbf{\textit{S}}$=$2. The orbital angular momentum not only contributes to the large magnetic anisotropy by spin-orbital coupling interaction, but also the in-plane orbital motion causes the easy-axis anisotropy, in agreement with experimental results. Our experiment demonstrate a new method to achieve large magnetic anisotropy by ligand fields realized in on-surface metalation reaction, and again highlights the crucial role of ligand field in determining the magnetic property of 3$d$ magnetic atoms. [Preview Abstract] |
Thursday, March 16, 2017 12:15PM - 12:27PM |
S50.00006: Tunable Finite-Sized Iron Chains to Control Magnetic Relaxation in Phthalocyanine Thomas Gredig, Matthew Byrne The magnetic dynamics of low-dimensional iron ion chains have been studied using iron phthalocyanine thin films. The deposition temperature limits the average crystal size in the range from 40 nm to 110 nm, allowing for tunable control of the chain length. Using a method common for single chain magnets, the magnetic relaxation time for each chain length is determined from temporal remanence data fit to a stretched exponential form in the temperature range below 5 K, the onset for magnetic hysteresis. Scaling the remanence by its relaxation time generates a temperature-independent master curve to fit the spin reversal energy barrier and single spin relaxation time. The energy barrier of 95K is found to be independent of the chain length. In contrast, the single spin relaxation time increases with longer chains. Both results are interpreted with the Glauber-Ising framework in the regime where the correlation length exceeds the chain length. We show that thin films provide the nano-architecture to control magnetic relaxation and a testbed to study finite-size effects in low-dimensional magnetic systems. [Preview Abstract] |
Thursday, March 16, 2017 12:27PM - 12:39PM |
S50.00007: Protected giant magnetic anisotropy in transition-metal adatoms on defected tungsten disulfide monolayer Jie Li, Hui Wang, Ruqian Wu Giant magnetic anisotropy, especially in systems with magnetic units protected, is very important for the development of spintronics and quantum computing devices. Through systematic first-principles calculations, we identified that Ir@D-WS2 and Os@D-WS2 may have magnetic anisotropy energies up to 40 meV even when they are covered by graphene, sufficient to frustrate the thermal fluctuation at room temperature. Moreover, the magnetic anisotropy of Os@D-WS2 can be enhanced by 300{\%} when an external electric field is applied in a range of 0.5 to -0.5 V/Å. This finding of giant magnetic anisotropy in a well protected materials opens an vista of the development of data storage and spintronic devices. [Preview Abstract] |
Thursday, March 16, 2017 12:39PM - 12:51PM |
S50.00008: Magnetic properties of rare earth single atoms on metal substrates Aparajita Singha, Romana Baltic, Fabio Donati, Christian W\"{a}ckerlin, Jan Dreiser, Luca Persichetti, Pietro Gambardella, Stefano Rusponi, Harald Brune The interaction of individual rare earth (RE) atoms with single-crystal surfaces leads to magnetic ground and excited states that determine their magnetic properties, e.g., magnetic relaxation time, total magnetic moment, zero-field splitting, and magnetic anisotropy energy. We present a systematic study of several RE elements (Dy, Ho, Er, and Tm) on different metal surfaces (Pt(111), Cu(111), Ag(100), and Ag(111)). Using x-ray absorption spectroscopy and magnetic circular dichroism we reveal two $4f$ configurations, i.e., $4f^{n}$ and $4f^{n-1}$, where $n$ corresponds to the free atom occupation. We identify two factors governing the valency of these adatoms: (a) the ionization potential of the $4f$ elements and (b) the substrate density of states at the Fermi level. Magnetization loops at $2.5$ K reveal that all RE adatoms are paramagnetic, i.e., their magnetic relaxation is faster than about $10$ s. Comparison of our experimental spectra with multiplet calculations identify the role of the crystal field in determining the magnetic quantum levels of RE adatoms. [Preview Abstract] |
Thursday, March 16, 2017 12:51PM - 1:03PM |
S50.00009: Magnetic Spatial Profile Across a Molecular / Metal Interface J. Shoup, C. Kinane, S. Langridge, F. Al Ma'Mari, M. Rogers, O. Cespedes, B. Kirby, J. Borchers, D.A. Arena The recent discovery of ferromagnetism originating at the interface between molecular carbon (C60) and diamagnetic or paramagnetic transition metals (TM) points to a new path for development of ferromagnetic materials. Muon spin resonance ($\mu$-SR) confirms that the ferromagnetic spins originate at the C60/TM interface, but $\mu$-SR can lack the depth resolution to localize the spins and determine the spin decay length away from the interface. We performed polarized neutron reflectometry (PNR) measurements on a C60/TM superlattice to attempt to determine the magnetic scattering length density (SLD) in our sample. A challenge in the PNR measurements is the extremely small bulk magnetization in the sample ($\leq$ 50 emu per cm$^3$ of Cu). At magnetic saturation the PNR spin asymmetry (SA), where SA = (S - S) / (S + S), exhibits a small oscillatory variation. The SA oscillation amplitude appears to track the sample magnetization; the SA measured at remanence over a smaller perpendicular momentum transfer ($Q_z$) range seems to converge towards zero. Modeling currently underway will combine structural parameters obtained from laboratory-based x-ray reflectivity (XRR) over a wide $Q_z$-range with the PNR results to arrive at a consistent structural / magnetic depth profile. [Preview Abstract] |
Thursday, March 16, 2017 1:03PM - 1:15PM |
S50.00010: Adiabatic Spin Pump through an Antiferromagnetic Molecular Magnet Jie Gu, Xiangguo Li, Hai-Ping Cheng, X-G Zhang We propose an adiabatic spin pump through an antiferromagnetic molecular magnet connected to two nonmagnetic electrodes. This molecule has spin-dependent charging energies, which open a new possible way of generating spin currents without requiring a magnetic field, ferromagnetic leads, or spin-orbit coupling. Making use of the quantum master equation approach combined with full counting statistics, we show that under certain conditions, applying periodically oscillating voltages to this molecular magnet can generate highly polarized spin currents and pure spin currents. Negative differential resistance (NDR) is also predicted. [Preview Abstract] |
Thursday, March 16, 2017 1:15PM - 1:27PM |
S50.00011: Electronic Transport in Gd and Eu Atomic-Size Contacts Carlos Untiedt, Bernat Olivera, Carlos Salgado, Jose. L. Lado, Amin Karimi, Elke Scheer, Joaquin Fernandez-Rossier, Juan J. Palacios Here we explore whether and how the local moments influence electronic transport properties at the atomic scale on Gd and Eu. Both Eu and Gd are known to have local moments associated with their f-electrons. These coexist with itinerant s and d bands that account for their metallic character. We have studied their conductance when only few atoms form the junction between bulk electrodes made out of the very same material. Thousands of measurements show that both metals have an average lowest conductance, attributed to an atom-size contact, below the quantum of conductance. In the case of Eu, unlike other metals, a strong dependence of the atomic conductance on the macroscopic configuration of the contacts is observed. Our DFT for both metals show f bands fully spin polarized and insulating. s−p bands are dominant for transport, where d orbitals seem to have a relevant contribution in some cases. On another hand, the strong variability of the atomic conduction of Eu could be explained as a consequence of magnetic disorder due to the f- character of magnetism in this material. [Preview Abstract] |
Thursday, March 16, 2017 1:27PM - 1:39PM |
S50.00012: Enhanced Magnetoresistance in Molecular Junctions by Geometrical Optimization of Spin-Selective Orbital Hybridization Soumyajit Sarkar, David Rakhmilevitch, Ora Bitton, Leeor Kronik, Oren Tal Molecular junctions based on ferromagnetic electrodes allow the study of electronic spin transport near the limit of spintronics miniaturization. However, these junctions reveal moderate magnetoresistance that is sensitive to the orbital structure at their magnetic tip - molecule interfaces. The key structural parameters that should be controlled in order to gain high magnetoresistance have not been established, despite their importance for efficient manipulation of spin transport at the nanoscale. Here, we show that single-molecule junctions based on nickel electrodes and benzene molecules can yield a significant anisotropic magnetoresistance of up to $\sim$200\% near the conductance quantum G$_{\rm 0}$. The measured magnetoresistance is mechanically tuned by changing the distance between the electrodes, revealing a nonmonotonic response to junction elongation. These findings are ascribed with the aid of first-principles calculations to variations in the metal–molecule orientation that can be adjusted to obtain highly spin-selective orbital hybridization. Our results demonstrate the important role of geometrical considerations in determining the spin transport properties of metal–molecule interfaces. [Preview Abstract] |
Thursday, March 16, 2017 1:39PM - 1:51PM |
S50.00013: Storing Information in Single Atom Magnets Fabian Natterer, Kai Yang, William Paul, Philip Wilke, Taeyoung Choi, Thomas Greber, Andreas J. Heinrich, Chris P. Lutz In a Gedankenexperiment about shrinking the size of a magnetic bit, the single atom magnet is the natural limit. Previous experimental efforts reached a size of few atoms per individually addressable magnetic bit, but a recent report of magnetic remanence for ensembles of holmium (Ho) atoms on magnesium oxide (MgO) promised a path toward stable magnetic bits at the atomic limit. It remained unclear, however, how to access the individual magnetic centers. Here we demonstrate the reading and writing of individual Ho atoms on MgO, and show that they independently retain their magnetic information over several hours. We read the Ho states by tunnel magnetoresistance and write with current pulses using a scanning tunneling microscope. We prove magnetic origin of the long-lived states by single-atom electron spin resonance (ESR) and measure a large magnetic moment of (10.1$+$/-0.1) Bohr magnetons. The high magnetic stability combined with electrical reading and writing shows that single atom magnetic memory has become a physical reality. [Preview Abstract] |
Thursday, March 16, 2017 1:51PM - 2:03PM |
S50.00014: A fully first–principles approach to the molecular Kondo problem María soriano, Juan José Palacios, David Jacob There has been a great effort in recent years to understand the emerging Kondo-like resonances in different magnetic molecules such as MnPc. Theoretical approaches based on atomic models have proven to be very useful for the study of this phenomenon when the magnetic moment is essentially localized on a magnetic atom [1,2]. Nevertheless the Kondo effect can arise in pure carbon-based systems as has been demonstrated experimentally in fullerenes and carbon nanotubes [3]. In this communication we present a fully first principles approach to the molecular Kondo problem based in a combination of atomistic calculation and a multiorbital Anderson model where the orbitals are not atomic but molecular orbitals. That allows us to address this problem in systems with delocalized spin states, such as fullerenes, and in general in all kind of organic molecules such as radicals, where the atomic and conventional models fail. This model is fully obtained from Density Functional Theory calculation in combination with Green's functions methodologies [4,5]. [1] Phys. Rev. Lett. 109, 147202 (2012); [2] Phys. Rev. B 88, 134417 (2013); [3] Nature 434, 484. (2005); [4] ANT.G03. www.alacant.dfa.ua.es; [5] Phys. Rev. B. 82, 195115 (2010). [Preview Abstract] |
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