Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session B16: Focus Session: Spin Transport and Orbital Polarization Effects |
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Sponsoring Units: GMAG DCOMP DMP Chair: Adnan Rebei, Seagate Research Room: Colorado Convention Center Korbel 4F |
Monday, March 5, 2007 11:15AM - 11:27AM |
B16.00001: Spin-dependent electronic transport through magnetic molecules Carsten Timm, Florian Elste Electronic transport through magnetic molecules has recently received considerable attention. This is partly motivated by the idea to integrate spintronics with molecular electronics. This talk highlights a number of interesting effects we predict for tunneling through single magnetic molecules and molecular monolayers weakly coupled to metallic leads. The results are obtained in a rate-equation approach which treats the intra-molecular interactions exactly and works also for situations far from equilibrium (large bias voltage). Effects to be discussed include fingerprints of magnetic excitations seen in inelastic tunneling beyond the sequential-tunneling approximation, very slow spin relaxation, giant spin amplification, and negative differential conductance at high temperatures. [Preview Abstract] |
Monday, March 5, 2007 11:27AM - 11:39AM |
B16.00002: Spin-polarized transport through the endohedral fullerene Gd@C$_{82}$ Laxmidhar Senapati, Steven C. Erwin We investigate theoretically the spin-polarized electron transport through an endohedral metallofullerene Gd@C$_{82}$ sandwiched between magnetic Fe contacts. Using density-functional theory and the Landauer-Buttiker formalism, we demonstrate that the total current depends on the relative orientation of the Gd spin and the two Fe-contact spins. Specifically, the current varies by 15-20{\%} as the orientation of the Gd spin moment is changed with respect to that of Fe contacts. We also studied the effect of changing the strength of the coupling between the Fe contacts and the fullerene molecule. We find that strong coupling leads to metallic current-voltage characteristics, while weak coupling leads to Coulomb blockade. [Preview Abstract] |
Monday, March 5, 2007 11:39AM - 11:51AM |
B16.00003: Magnetic phenomena, spin orbit effects, and electron transport in Pt nanowire contacts Alexander Smogunov, Andrea Dal Corso, Erio Tosatti We present a first-principles DFT study of the electronic, magnetic, and transport properties of short monatomic Pt nanowire contacts. For an infinite tipless Pt wire a fully relativistic calculation, including spin-orbit effects, yields a ferromagnetic ground state already for the unstressed wire [1]. We found that short 3-atom and 5-atom stressed wires in contact with nonmagnetic Pt leads remain locally magnetic, with the magnetization parallel to the wire axis, owing to orbital magnetism. Ballistic conductance of these nanocontacts is calculated using the scattering-based method [2]. Preliminary results indicate a ballistic conductance for a stressed 5-atom wire of about 2.0 $G_0$ ($G_0 = 2e^2/h$ is the conductance quantum) for parallel magnetization, 2.3 $G_0$ for perpendicular magnetization, and 2.4 $G_0$ in the nonmagnetic case. The former is in closest agreement with experimental values reported in break junctions [3].\newline [1] A. Delin and E. Tosatti, Phys. Rev. B {\bf 68}, 144434 (2003); R. Weht, A. Smogunov, A. Delin, A. Dal Corso, and E. Tosatti, in preparation. \newline [2] A. Smogunov, A. Dal Corso, and E. Tosatti, Phys. Rev. B {\bf 70}, 045417 (2004); A. Dal Corso, A. Smogunov, and E. Tosatti, Phys. Rev. B {\bf 74}, 045429 (2006).\newline [3] R.H.M. Smit {\it et al.}, Phys. Rev. Lett. {\bf 87}, 266102 (2001). [Preview Abstract] |
Monday, March 5, 2007 11:51AM - 12:27PM |
B16.00004: Progress in ab initio methods for spin transport Invited Speaker: Numerical simulations have an important role in spintronics, here one envisages the use of the spin as well as the electron charge for electronic applications. In this talk I will present our code {\it Smeagol} [1] which combines the non-equilibrium Green function formalism with density functional theory and it has been specifically designed for magnetic devices. With {\it Smeagol} I will first investigate the possibility of large ballistic magnetoresistance in nickel point contacts, addressing the effects of local exchange and correlation functionals as well as the possible presence of oxygen impurities. Then I will describe an attempt to integrate the fields of spin- and molecular-electronics by constructing spin-valves using organic molecules. I will demonstrate that it is possible to obtain different transport behaviour, large magnetoresistance [1] as well as current rectification and spin-diode effects by simply selecting the molecule and the anchoring groups. Finally I will show how {\it Smeagol} is a valuable tool for simulating spin-polarised STM images. \begin{thebibliography}{99} \bibitem{smeagol} A. R. Rocha, {\it et al.}, Phys. Rev. B {\bf 70}, 085414 (2006), Nature Materials, {\bf 4}, 335 (2005). \end{thebibliography} [Preview Abstract] |
Monday, March 5, 2007 12:27PM - 12:39PM |
B16.00005: Calculations of spin-disorder resistivity from first principles Aleksander Wysocki, Kirill Belashchenko, Julian Velev, Mark van Schilfgaarde Spin-disorder resistivity of Fe and Ni is studied using the noncollinear density functional theory. The Landauer conductance is averaged over random disorder configurations and fitted to Ohm's law. The distribution function is approximated by the mean-field theory. The dependence of spin-disorder resistivity on magnetization in Fe is found to be in excellent agreement with the results for the isotropic s-d model. In the fully disordered state, spin-disorder resistivity for Fe is close to experiment, while for fcc Ni it exceeds the experimental value by a factor of 2.3. This result indicates strong magnetic short-range order in Ni at the Curie temperature. We suggest that the analysis of calculated and measured spin-disorder resistivity provides a powerful method to extract information on the temperature dependence of the magnetic short-range order parameter in ferromagnetic metals. [Preview Abstract] |
Monday, March 5, 2007 12:39PM - 12:51PM |
B16.00006: Size-dependent alternation of magnetoresistive properties in atomic chains R. Tugrul Senger, Engin Durgun, Haldun Sevincli, Hatem Mehrez, Salim Ciraci Spin-polarized electronic and transport properties of carbon atomic chains are investigated when they are capped with transition-metal (TM) atoms like Cr or Co, using density functional theory. The magnetic ground state of the TM-C$_n$-TM chains alternates between the ferromagnetic (F) and antiferromagnetic (AF) spin configurations as a function of $n$. The desirable AF state is obtained for only even-$n$ chains with Cr; conversely only odd-$n$ chains with Co have AF ground states. We present a simple model that can successfully simulate these variations, and the induced magnetic moments on the carbon atoms. Depending on the relative strengths of the spin-dependent couplings and the on-site energies of the TM atoms there induces long-range spin polarizations on the carbon atoms which mediate the exchange interaction. When connected to appropriate electrodes these atomic chains display a strong spin-valve effect. Analysis of electronic and magnetic properties of these atomic chains, and the indirect exchange coupling of the TM atoms through carbon chain will be presented. $^1$E. Durgun \textit{et al.}, J. Chem. Phys. \textbf{125}, 121102 (2006). $^2$E. Durgun \textit{et al.}, Phys. Rev. B \textbf{74} (\textit{in press}). [Preview Abstract] |
Monday, March 5, 2007 12:51PM - 1:03PM |
B16.00007: T-dependent matrix elements in x-ray magnetic circular dichroism Yongbin Lee, Bruce Harmon, Alan Goldman, Jonathan Lang Dramatic changes in the Er L$_{2}$ and L$_{3}$ XMCD spectra in Er$_{2}$Fe$_{17}$ as a function of temperature have been investigated with detailed experiments and first principles calculations. This study seeks to understand the fundamental mechanisms governing the spectral shape and magnitude of the L$_{2}$ and L$_{3}$ XMCD spectra as a step toward developing XMCD as a quantitative probe for rare earths similar to its effectiveness for transition metals via the use of sum rules (which do not work for rare earths). The calculations simulate the key thermal physics by evaluating the spin polarized band structures obtained with the 4f moment on the Er atom constrained to values of 0, 1, 2, and 3 Bohr magnetons. Both the theory and our experiments, performed at the Advanced Photon Source, show the XMCD L$_{2}$-edge spectrum changes sign as the temperature is lowered, and the L$_{3}$-edge spectrum also shows systematic and significant changes. We will discuss the effects of dipole matrix elements, spin-orbit coupling, hybridization between 5d-3d orbital, and magnetic anisotropy on the XMCD spectra. Quadrupole transitions and core hole effects will be also discussed. [Preview Abstract] |
Monday, March 5, 2007 1:03PM - 1:15PM |
B16.00008: Orbital Polarization in Itinerant Magnets Igor Solovyev The correct description of the orbital magnetism is one of the longstanding problems in the density functional theory (DFT). One possible solution is to extend DFT by considering explicit dependence of the exchange-correlation energy on the orbital degrees of freedom. Since the angular momentum operator does not commute with electrostatic potential, it is not an observable except a small atomic region where this potential is nearly spherical. Hence, the orbital magnetism is an atomic property, and we inevitably have to deal with the problem of on-site Coulomb interactions and screening of these interactions in solids.$^1$ For itinerant systems, this screening can be evaluated in the random-phase approximation (RPA), by considering the strong-coupling limit. Then, the orbital polarization can be computed as the self-energy correction in the static version of the GW method, without any adjustable parameters.$^2$ This opens a formal way for combining the spin itineracy in the local-spin-density approximation (LSDA) with the atomic orbital magnetism. RPA can be further improved by restoring the spin polarization of LSDA through the local- field corrections. Numerical applications reveal a remarkable improvement for the orbital magnetization and magnetocrystalline anisotropy energies of transition metals and actinide compounds. $^1$ I.~V.~Solovyev~\textit{et al.}, Phys.~Rev.~Lett.~\textbf{80}, 5758 (1998). $^2$ I.~V.~Solovyev, Phys.~Rev.~Lett.~\textbf{95}, 267205 (2005). [Preview Abstract] |
Monday, March 5, 2007 1:15PM - 1:27PM |
B16.00009: Magnetic circular dichroism and the orbital magnetization of ferromagnets Ivo Souza, David Vanderbilt The spontaneous magnetization of ferromagnets has both spin and orbital contributions, ${\bf M}={\bf M}_{\rm spin}+{\bf M}_{\rm orb}$, which can be separated out via gyromagnetic measurements. Recently\footnote{D. Ceresoli, T. Tonhauser, D. Vanderbilt, and R. Resta, {\it Phys. Rev. B} {\bf 74}, 024408 (2006).} it was found that, when expressed as a bulk property of the Bloch electrons, the orbital magnetization itself consists of two terms, ${\bf M}_{\rm orb}= \widetilde{\bf M}_{\rm LC}+\widetilde{\bf M}_{\rm IC}$, which can be loosely interpreted as the localized and itinerant contributions, respectively. Interestingly, $\widetilde{\bf M}_{\rm LC}$ and $\widetilde{\bf M}_{\rm IC}$ are separately gauge-invariant, which raises the possibility that they may be independently measurable. We show that indeed they are related to the magnetic circular dichroism (MCD) spectrum by a subtle sum rule. MCD, the difference in absorption between left- and right-circularly-polarized light, is given by $\sigma_{{\rm A},\alpha\beta}^{(2)}(\omega)$, the absorptive part of the antisymmetric conductivity. We derive the following sum rule for the interband contribution: $\int_0^\infty \vec\sigma_{\rm A}^{(2)}(\omega)d\omega= (2\pi e c/\hbar)\big( \widetilde{\bf M}_{\rm LC}-\widetilde{\bf M}_{\rm IC}\big)$, where $\vec\sigma_{\rm A}^{(2)}(\omega)$ is a pseudo-vector. Hence, by combining the results of gyromagnetic and magneto-optical experiments, $\widetilde{\bf M}_{\rm LC}$ and $\widetilde{\bf M}_{\rm IC}$ can in principle be measured independently. [Preview Abstract] |
Monday, March 5, 2007 1:27PM - 1:39PM |
B16.00010: Alternative approach to ab-initio NMR spectra for periodic systems Timo Thonhauser, Arash Mostofi, Nicola Marzari, David Vanderbilt, Raffaele Resta We propose a novel finite-differences approach for computing the NMR response in periodic solids that is based on the theory of orbital magnetization\footnote{T.~Thonhauser, D.~Ceresoli, D.~Vanderbilt, and R.~Resta, Phys. Rev. Lett. {\bf 95}, 137205 (2005).} recently introduced by some of us. Instead of obtaining the shielding tensor from the response to an external magnetic field, we derive it directly from the orbital magnetization appearing in response to a microscopic magnetic dipole. This procedure has an established parallel in the case of electric fields, where Born effective charges are often obtained from the polarization induced by a sublattice displacement instead of the force induced by an electric field. Among the advantages of the present approach are its simplicity and its applicability to situations in which linear-response theory would be cumbersome. [Preview Abstract] |
Monday, March 5, 2007 1:39PM - 1:51PM |
B16.00011: Orbital magnetization in a supercell framework: Single {\bf k}-point formula Davide Ceresoli, Raffaele Resta The position operator ${\bf r}$ is ill-defined within periodic boundary conditions: owing to this, both the macroscopic (electric) polarization and the macroscopic orbital magnetization are nontrivial quantities. While the former has been successfully tamed since the early 1990s, the latter remained a long-standing unsolved problem. A successful formula within DFT for crystalline systems has been recently found.\footnote{ D. Ceresoli, T. Thonhauser, D. Vanderbilt, R. Resta, Phys. Rev. B {\bf 74}, 024408 (2006).} The formula is based on a Brillouin-zone integration, which is discretized on a reciprocal-space mesh for numerical implementation. We find here the single ${\bf k}$-point limit, useful for large enough supercells, and particularly in the framework of Car-Parrinello simulations for noncrystalline systems. We validate our formula on the test case of a crystalline system, where the supercell is chosen as a large multiple of the elementary cell. Rather counterintuitively, even the Chern number (in 2d) can be evaluated using a single ${\bf k}$-point diagonalization. [Preview Abstract] |
Monday, March 5, 2007 1:51PM - 2:03PM |
B16.00012: Orbital magnetism of mesoscopic integrable system Ming Lou, Slava Serota In the mesoscopic regime, magnetic properties (such as orbital magnetism) are sensitive to whether the corresponding classical dynamics is chaotic or integrable. Non-interacting electron gas in a rectangular box is proposed as a ``generic'' model to study orbital magnetism of integrable system. We derived the exact energy level correlation function for this system, including the perturbation by magnetic field. Combining the exact correlation function and Imry's formalism, we calculated the orbital magnetic susceptibility and discussed the field dependence at$T\to 0$ and temperature dependence at $B\to 0$. As a result, the susceptibility $\chi \sim \left| {\chi _L } \right|\sqrt {k_F L} \left\{ {\begin{array}{l} \log \left( {{\phi _0 } \mathord{\left/ {\vphantom {{\phi _0 } \phi }} \right. \kern-\nulldelimiterspace} \phi } \right)\mbox{ }(T\to 0,\phi <<\phi _0 ) \\ \log \left( {{\sqrt {\Delta E_F } } \mathord{\left/ {\vphantom {{\sqrt {\Delta E_F } } T}} \right. \kern-\nulldelimiterspace} T} \right)\mbox{ }(B\to 0,T<<\sqrt {\Delta E_F } ) \\ \end{array}} \right.$, where \textit{$\chi $}$_{L}$ is the Landau susceptibility, $k_{F}$ the is Fermi vector, $L$ is the rectangle's side, \textit{$\phi $}$_{0}$ is the magnetic flux quantum, $\Delta $ is the mean level spacing, and $E_{F}$ is the Fermi energy. For high temperature and large field, the mesoscopic part of susceptibility exponentially vanishes and only the bulk Landau diamagnetism is left. The logarithmic divergence at zero field and zero temperature is consistent with previous numerical calculations and is a manifestation of pronounced non-self-averaging properties of integrable systems. [Preview Abstract] |
Monday, March 5, 2007 2:03PM - 2:15PM |
B16.00013: Orbital Ordering in Cs$_{2}$AgF$_{4}$ - an electronic structure study Deepa Kasinathan, Klaus Koepernik, Ulrike Nitzsche, Helge Rosner The argentate Cs$_{2}$AgF$_{4}$, first synthesized in 1974 has many similarities to the high-T$_{c}$ cuprates, featuring AgF$_{2}$ sheets in place of CuO$_{2}$ sheets. While the undoped cuprates are antiferromagnetic, this argentate is ferromagnetic with a T$_{Curie}$ of about 15 K. Density functional calculations in the proposed tetragonal structure produce an itenerant half-metallic ferromagnet. Recent inelastic neutron scattering experiments have suggested an orthorhombic structure that allows an orbitally ordered ferromagnetic ground state. We performed electronic structure calculations using the LDA+U method on this system and were able to obtain an orbitally ordered ground state, not only for the newly proposed orthorhombic lattice but also for the orginal tetragonal lattice by constraining the bravais lattice and allowing the change of the atomic basis. In specific, very small changes in the position of the in-plane fluorine atoms already trigger an orbital ordering. Our calculated energy scale shows that this orbitally ordered state should be stable for all temperatures, consistent with the experiment. [Preview Abstract] |
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