Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session J16: Focus Session: Molecular Nanomagnets/Devices |
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Sponsoring Units: GMAG DMP Chair: Stefano Carretta, University of Parma Room: 318 |
Tuesday, March 19, 2013 2:30PM - 2:42PM |
J16.00001: Modification of Molecular Spin Crossover in Ultra-Thin Films Daniel Dougherty, Alex Pronschinske, Yifeng Chen, Arrigo Calzolari, Geoff Lewis, David Shultz, Marco Buongiorno-Nardelli Iron (II) spin crossover compounds exhibit a strong connection between molecular spin state and electronic structure that make them exciting candidates for highly tunable materials for spintronic applications. The spin crossover phenomenon is often extremely sensitive to crystal packing effects that may be modified in device environments compared to bulk materials. We report evidence for dramatic modification of spin crossover in bilayer films of Fe[(H$_{\mathrm{2}}$Bpz$_{\mathrm{2}})_{\mathrm{2}}$bpy] on Au(111) compared to bulk behavior. Scanning Tunneling Microscopy, spectroscopy, and local conductance mapping show spin-state coexistence in bilayer films of Fe[(H$_{\mathrm{2}}$Bpz$_{\mathrm{2}})_{\mathrm{2}}$bpy] on Au(111) that is independent of temperature between 130 K and 300 K due to the unique packing constraints of the bilayer film that promote deviations from bulk behavior. Local density of states measured for different spin states show that high-spin molecules have a smaller transport gap than low-spin molecules in agreement with density functional theory calculations. In addition, aggregation of spin states into ``like-spin'' domains is observed. [Preview Abstract] |
Tuesday, March 19, 2013 2:42PM - 2:54PM |
J16.00002: Electronic and transport properties of Fe-based spin crossover complexes from first principles Yifeng Chen, Marco Buongiorno Nardelli Using calculations from first principles, we studied the electronic and transport properties of the Fe(II) spin crossover (SCO) compound Fe[H$_2$B(pz)$_2$]$_2$(bpy). The magnetic transition has been imposed by constrained magnetization calculations and the computed electronic structure agrees with available experimental data. The unique bilayer configuration achievable by vacuum evaporation on Au(111) in experiments, is modeled by a $\pi$-stacking dimer structure that is used for the interpretation of STM and transport data. Our results explain the meandering spinodal decomposition of the spin domains of the bilayer films and the conductive properties of the system. In particular, we found the high-spin configuration to be more conductive than the low-spin case, in agreement with experimental measurements of corresponding currents through disordered thin films. The spin-switchable electronic transport properties of this kind of Fe(II) SCO compound systems provide viable proofs for future switchable molecular spintronic devices and applications. [Preview Abstract] |
Tuesday, March 19, 2013 2:54PM - 3:06PM |
J16.00003: Complex Materials for Molecular Spintronics Applications: Cobalt Bis(dioxolene) Valence Tautomers, from Molecules to Polymers Marco Buongiorno Nardelli, Arrigo Calzolari, Yifeng Chen, Daniel Dougherty, David Shultz Using first principles calculations we predict a complex multifunctional behavior in cobalt bis(dioxolene) valence tautomeric compounds. Molecular spin-state switching is shown to dramatically alter electronic properties and corresponding transport properties. This spin state dependence has been demonstrated for technologically-relevant coordination polymers of valence tautomers as well as for novel conjugated polymers with valence tautomeric functionalization. As a result these materials are proposed as promising candidates for spintronic devices that can couple magnetic bistability with novel electrical and spin conduction properties. Our findings pave the way to the fundamental understanding and future design of active multifunctional organic materials for spintronics applications. [Preview Abstract] |
Tuesday, March 19, 2013 3:06PM - 3:42PM |
J16.00004: Electronic read-out of a single nuclear spin using a molecular spin transistor Invited Speaker: Franck Balestro Thanks to recent advances of nanofabrication techniques, molecular electronics devices can address today the ultimate probing of electronic transport flowing through a single molecule. Not only this electronic current can show signatures of the molecular quantum levels but it can also detect the magnetic state of the molecule. As a consequence, an entirely novel research field called \textit{molecular spintronics} in which quantum magnetism of molecular systems can be interfaced to nanoelectronics is now emerging. One of the recent challenges of this field was to probe by this current, not the only spin state of an electron, but the state of a single nuclear spin. Such an achievement was experimentally unimaginable a few years ago. Indeed, the magnetic signal carried by a single nuclear spin is a thousand times less than that of a single electron spin ... Using a Single Molecular Magnet (TbPc2) as a molecular spin transistor in a three terminals configuration, the experiment consists in measuring the current changes when ones sweep the external magnetic field applied to the molecule. When the magnetic spin of the molecule changes its quantum state, a change of current is recorded. Because of the well-defined relationship that exists between the electron spin and nuclear spin carried by the nuclei of the Terbium atom, it is possible to perform the electronic read-out of the electronic spin state which, in turn give information on the state of a single nuclear spin. Application of this effect for quantum information manipulation and storage can be envisioned, as the observation of energy level lifetimes on the order of tens of seconds opens the way to coherent manipulations of a single nuclear spin.\\[4pt] Reference:\\[0pt] ``Electronic read-out of a single nuclear spin using a molecular spin transistor,'' R. Vincent, S. Klyatskaya, M. Ruben, W. Wernsdorfer, F. Balestro, Nature, Vol. 488, p.357, (2012). [Preview Abstract] |
Tuesday, March 19, 2013 3:42PM - 3:54PM |
J16.00005: Quantum Dot Spin Valves Controlled by Single Molecule Magnets Fatemeh Rostamzadeh Renani, George Kirczenow We explore theoretically for the first time the properties of a new class of spintronic nano-devices in which the electrical resistance of a non-magnetic quantum dot contacted by non-magnetic electrodes is controlled by transition metal-based single molecule nanomagnets (SMMs) bound to the dot. Although the SMMs do not lie directly in the current path in these devices, we show that the relative orientation of their magnetic moments can strongly influence on the electric current passing through the device. If the magnetic moment of one of the SMMs is reversed by the application of a magnetic field, we predict a large change in the resistance of the dot, i.e., a strong spin valve effect. The mechanism is resonant conduction via molecular orbitals extending over the entire system. The spin valve is activated by a gate that tunes the transport resonances through the Fermi energy. Detailed results will be presented for the case of Mn$_{12}$ SMMs bound to a gold quantum dot. [Preview Abstract] |
Tuesday, March 19, 2013 3:54PM - 4:06PM |
J16.00006: The effect of current-induced spin switching in the presence of quantum tunneling of magnetization Maciej Misiorny, J\'{o}zef Barna\'{s} Knowledge of transport properties of individual large-spin ($S> 1/2$) atoms/molecules exhibiting magnetic anisotropy is of key importance from the point of view of information processing technologies [1]. The ultimate aim is to incorporate such objects as functional elements of spintronic devices, with the objective of employing spin-polarized currents to control the magnetic state of the system. In particular, for an atom/molecule with the predominant \emph{`easy-axis' uniaxial} magnetic anisotropy this allows for switching the system's spin between two metastable states [2,3]. However, the \emph{uniaxial} component of magnetic anisotropy, underlying the magnetic bistability, is frequently accompanied by the \emph{transverse} one, whose presence manifests, e.g., as quantum tunneling of magnetization (QTM). Here, we show that not only does QTM induce an effective energy barrier for the spin switching, but also its effect on the transport reveals as an additional signal in transport characteristics. Furthermore, we propose how to experimentally investigate QTM by means of the STM inelastic transport spectroscopy. [1] M. Mannini et al., Nature Mater. 8, 194 (2009); [2] M. Misiorny and J. Barna\'{s}, Phys. Rev. B 75, 134425 (2007); [3] S. Loth et al., Nature Phys. 6, 340 (2010). [Preview Abstract] |
Tuesday, March 19, 2013 4:06PM - 4:18PM |
J16.00007: Vibrational properties of single-molecule magnet Fe4 Michael Warnock, Kyungwha Park, Yoh Yamamoto A single-molecule magnet (SMM) Fe$_4$ consists of four Fe ions interacting through O anions via antiferromagnetic superexchange coupling, with the total ground-state spin of $S=5$. The SMM Fe$_4$ has a magnetic anisotropy energy of $16$ K, and its ground-state spin multiplet is well separated from the first excited spin multiplet. A recent experimental effort demonstrated that SMMs Fe$_4$ can be deposited on various substrates with magnetic cores intact and that individual Fe$_4$ molecules can be bridged between electrodes. SMMs Fe$_4$ deposited on substrates or in contact with electrodes revealed interesting magnetic and transport properties. Electronic and spin degrees of freedom of SMM Fe$_4$ may be coupled to vibrational degrees of freedom. Such coupling can affect various properties of SMM Fe$_4$. Here we present our calculation of vibrational spectra (Raman and infrared) of SMM Fe$_4$ using density-functional theory (DFT) within simple harmonic oscillator approximation. We identify normal modes and compare our calculated result with available experimental data. [Preview Abstract] |
Tuesday, March 19, 2013 4:18PM - 4:30PM |
J16.00008: Cotunneling signatures of spin-electric coupling in frustrated triangular single-molecule magnets Javier Nossa, Carlo Canali The ground state (GS) of frustrated (antiferromagnetic) triangular single-molecule magnets is characterized by two total-spin $S =$ 1$/$2 doublets with opposite chirality. According to a group theory analysis [M. Trif \textit{et al.}, Phys. Rev. Lett. \textbf{101}, 217201 (2008)] an external electric field can efficiently couple these two chiral spin states, even when the spin-orbit interaction (SOI) is absent. The strength of this coupling, $d$, is determined by an off-diagonal matrix element of the dipole operator, which can be calculated by \textit{ab-initio} methods [M. F. Islam \textit{et al.}, Phys. Rev. B \textbf{82}, 155446 (2010)]. In this work we propose that Coulomb-blockade transport experiments in the cotunneling regime can provide a direct way to determine the spin-electric coupling strength. Indeed, an electric field generates a $d$-dependent splitting of the GS manifold, which can be detected in the inelastic cotunneling conductance. Our theoretical analysis is supported by master-equation calculations of quantum transport in the cotunneling regime. We employ a Hubbard-model approach to elucidate the relationship between the Hubbard parameters $t$ and $U$, and the spin-electric coupling constant $d.$ This allows us to predict the regime in which the coupling constant $d$ can be extracted from experiment. [Preview Abstract] |
Tuesday, March 19, 2013 4:30PM - 4:42PM |
J16.00009: ABSTRACT WITHDRAWN |
Tuesday, March 19, 2013 4:42PM - 4:54PM |
J16.00010: Spin moment distributions in Cr-based antiferromagnetic rings Cr7M (M$=$Ni and Cd) studied by $^{53}$Cr NMR Yuji Furukawa, Cecilia Casadei, Lorenzo Bordonali, Ferdiando Borsa, Grihore Timco, Richard Winpenny Recent progress in synthesizing molecular magnets offers the opportunity to investigate magnetic properties of the system composed of small number of magnetically coupled spins. In this study, we have investigated magnetic properties of Cr-based antiferromagnetic (AF) ring Cr7M (M$=$Ni and Cd)). The ancestor of Cr7M is a well-known AF ring Cr8 with a spin single S$=$0 ground state due to AF interaction (J $\sim$ 16K) between nearest neighbor Cr$^{3+}$ (s$=$3/2) spins. A substitution of one of eight Cr$^{3+}$ ions with Ni$^{2+}$ (s$=$1) or Cd$^{2+}$ (s$=$0) leads to destroy the coherence of spin singlet ground state in Cr8. As a result, the Cr7M has a magnetic ground state with total spin S$_{\mathrm{T}}=$1/2 and S$_{\mathrm{T}}=$3/2 for Cr7Ni and Cr7Cd, respectively. In the magnetic ground state, local spin moments will appear on each Cr$^{3+}$ ion. In order to investigate the details of spin moments distributions on Cr ions in the systems, we have carried out $^{53}$Cr-NMR measurements in Cr7M in its magnetic ground state at low temperature. Based on the $^{53}$Cr-NMR results, we will discuss differences in distributions of the spin moments in Cr7M systems in its magnetic ground state. [Preview Abstract] |
Tuesday, March 19, 2013 4:54PM - 5:06PM |
J16.00011: Spin dynamics in atomically assembled antiferromagnets Sebastian Loth Antiferromagnetic materials possess ordered magnetic states that have vanishing magnetization. We used a low-temperature scanning tunneling microscope to construct few-atom antiferromagnets. Even-numbered arrays of antiferromagnetically coupled atoms were found to have no net spin. Their shapes can be defined precisely by atom manipulation avoiding uncompensated magnetic moments at the nanoparticle's edge. We use such spin-compensated atomic arrays to study the intrinsic dynamics of nanoscale antiferromagnets [1]. For chains of more than four atoms we observe two Neel-ordered ground states and frequent switching between them. The spontaneous switching rates depend strongly on the number of coupled atoms and we observed magnetic tunneling of the Neel vector for the smallest structures. In arrays with ten or more atoms the residence time in each state can exceed many hours but current-induced switching proceeds at nanosecond speed. These properties enable a model demonstration of dense magnetic data storage that uses antiferromagnets as memory elements. [1] S. Loth, S. Baumann, C. P. Lutz, D. M. Eigler and A. J. Heinrich, Science 335, 196 (2012). [Preview Abstract] |
Tuesday, March 19, 2013 5:06PM - 5:18PM |
J16.00012: Spin dynamics of molecular nanomagnets unraveled at atomic scale by four-dimensional inelastic neutron scattering Paolo Santini, Michael Baker, Tatiana Guidi, Stefano Carretta, Jacques Ollivier, Hannu Mutka, Hans Guedel, Grigore Timco, Eric McInnes, Giuseppe Amoretti, Richard Winpenny Molecular nanomagnets (MNMs) have been test-beds for addressing several elusive but important phenomena in quantum dynamics, but to this point it has been impossible to determine the spin dynamics directly. We show that recently-developed inelastic-neutron-scattering instrumentation, yielding the cross-section in vast portions of reciprocal space, enables two-spin dynamical correlation functions of MNMs to be directly determined without assuming an underlying model Hamiltonian. We use the Cr$_8$ antiferromagnetic ring as a benchmark to demonstrate the potential of this approach which allows us, for example, to examine how a quantum fluctuation propagates along the ring or to test the degree of validity of the Neel-vector-tunneling framework [1]. This result opens remarkable perspectives in the understanding of the quantum dynamics in several classes of MNMs. [1] M. Baker et al., Nature Physics in press (doi:10.1038/nphys2431) [Preview Abstract] |
Tuesday, March 19, 2013 5:18PM - 5:30PM |
J16.00013: Diamagnetic Exciton Properties in Asimetrical Quantum Dot Molecules Nelson Ricardo Fino Puerto, Hanz Ramirez, Angela Camacho Beltran The magnetic properties of nanostructures like quantum dots and rings are the subject of intense research. In particular, magnetic control of coupled quantum dots has become subject of interest. By using a first order perturbation approach, and within the effective mass approximation, we calculate magnetic field dependent electronic structures of confined excitons and trions in vertically coupled quantum dots. With these results we study the photoluminescence spectra of neutral and charged excitons in these structures that are coupled via magnetic field in the Faraday configuration (quantum dot molecules QDM). In this work study this spectra around three charge configurations: neutral exciton (X), positive trion (X$^{\mathrm{+}})$ and negative trion (X), where the charged can be distributed over any of the dots in the basis of the optically active excitons and tunneling electron through the interdot barrier. Also we study different different ratios between the dots, that allow the appearance of crossings and anticrossings in the behavior of the energy with respect to the magnetic field. [Preview Abstract] |
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