Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session W23: Focus Session: MAG.THY IV / ab initio Studies |
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Sponsoring Units: GMAG DMP DCOMP Chair: Renat Sabiryanov, University of Nebraska, Omaha Room: Baltimore Convention Center 320 |
Thursday, March 16, 2006 2:30PM - 3:06PM |
W23.00001: Ab initio Study of Mirages and Magnetic Interactions in Quantum Corrals Invited Speaker: We present the state of the art ab initio studies of mirages and magnetic interactions in quantum corrals. Our results demonstrate that quantum corrals could permit to manipulate the exchange interaction between magnetic adatoms on metal surfaces at large distances. We show that the spin-polarization of surface-state electrons can be projected to a remote location by quantum states of corrals. Our study gives a clear evidence that the 'spin-polarization transfer' takes place in a mirage experiment of Manoharan et al.,[2]. We find that the spin-polarization of surface-state electrons on transition metal surfaces[3] can be manipulated by quantum corrals. Our results reveal that an atomic motion in quantum corrals could be strongly affected by the quantum confinement of surface-state electrons. 1. V. S. Stepanyuk, L. Niebergall, W. Hergert, P. Bruno, Phys. Rev. Lett. 94, 187201 (2005). 2. H.C. Manoharan, C.P. Lutz, D.M. Eigler, Nature 403, 512 (2000). 3.L. Diekh\"{o}ner, M.A. Schneider, A.N. Baranov, V.S. Stepanyuk, P. Bruno, K. Kern Phys. Rev. Lett. 90, 236801 (2003). [Preview Abstract] |
Thursday, March 16, 2006 3:06PM - 3:18PM |
W23.00002: Noncollinear magnetism in antiferromagnetic manganese chalcogenides Kohji Nakamura, Toru Akiyama, Tomonori Ito, A.J. Freeman* Metastable zincblende compounds of transition-metal pnictides and chalcogenides have recently become the subject of much attention due to their unique properties exhibiting combinations of magnetism and semiconductivity. Here we investigate magnetism in the antiferromagnetic (AFM) transition-metal chalcogenides, namely MnSe and MnTe, by using the FLAPW method.\footnote{Wimmer, {\it et al.}, PRB 24, 864(1981)} Assuming a collinear magnetic structure, we demonstrate that the AFM structure consisting of alternating Mn (001) spin-up and spin-down planes is favored over the ferromagnetic state, since the majority-spin $d$-bands are completely filled and so achieve the half-filling state that leads to the superexchange interaction. However, with FLAPW calculations that now treat full noncollinear magnetism,\footnote{Nakamura, {\it et al.}, PRB 65, 12402 (2002); 67, 14420 (2003)} we find that the lowest energy state is a noncollinear AFM structure --- the so-called AFM type III structure --- which relaxes frustration in the AFM Mn moment alignment on the fcc sublattice, a result that agrees with neutron experiments.\footnote{Samarth, {\it et al.}, PRB 44, R4701 (1991)} *Supported by NSF MRSEC through the NU MRC. [Preview Abstract] |
Thursday, March 16, 2006 3:18PM - 3:30PM |
W23.00003: Embedded Clustering and Metastable Magnetism in Transition-Metal doped III-Nitrides Cui Xiangyuan, Julia Medvedeva, Arthur J. Freeman, Bernard Delley, Catherine Stampfl From extensive density-functional theory calculations [1] we find that Cr atoms in GaN prefer to form embedded clusters, occupying Ga sites [2]. Significantly, for larger than 2-Cr- atom clusters, states containing antiferromagnetic coupling with net spin in the range 0.06-1.47 $\mu_{\rm B}$/Cr are favored. Similar behavior is found for Mn:GaN, and Cr:AlN and Mn:AlN. We show that various configurations may coexist leading to a strong dependence of the magnetic properties on the growth conditions. This elucidates many puzzling observations such as the 5 (20-30) times lower value of the measured magnetic moment on Cr (Mn) as compared to the theoretically predicted one for the isolated dopants. In addition to the expected ground high spin (HS) states for isolated Mn and Fe in GaN (4 $\mu_{\rm B}$/Mn and 5 $\mu_{\rm B}$/Fe), metastable low spin (LS) states (0 $\mu_{\rm B}$/Mn and 1 $\mu_{\rm B}$/Fe) are found. The transition between the HS and LS states corresponds to an intra-ionic electron transfer between the $t_2$ and $e$ orbitals, accompanied by a spin-flip process.\\ $[1]$ B. Delley, J. Jchem. Phys. {\bf 113}, 7756 (2000).\\ $[2]$ X.Y. Cui, $et\, al.$, Phys. Rev. Lett. Dec. 2005. [Preview Abstract] |
Thursday, March 16, 2006 3:30PM - 3:42PM |
W23.00004: Optimized Effective Potential Method for Non-Collinear Magnetism Claudia Ambrosch-Draxl, Sangeeta Sharma, John K. Dewhurst, Nicole Helbig, Stefan Kurth, Eberhard K. U. Gross, Sam Shallcross, Lars Nordstr\"om A description of non-collinear magnetism in the framework of spin-density functional theory is presented for an exact exchange energy functional which depends explicitly on two-component spinor orbitals. The equations or the effective Kohn-Sham scalar potential and magnetic field are derived within the optimized effective potential framework. We have implemented this formalism within the full-potential linearized augmented planewave method, with an unconstrained magnetization density. Our calculations for Co and Fe show that the overestimation of moments seen in previous work was an artifact of the decoupled equations used. We further demonstrate, with the example of a magnetically frustrated Cr monolayer, how intra-atomic non-collinearity may be underestimated by local functionals. [Preview Abstract] |
Thursday, March 16, 2006 3:42PM - 3:54PM |
W23.00005: Spin susceptibility calculation based on the QP self-consistent GW method Takao Kotani, Mark van Schilfgaarde Recently we have developed the quasi-particle self-consistent $GW$ method (QPsc$GW$) based on the full-potential LMTO method. The method is designed to determine the best independent-particle picture. The most significant impact is that QPsc$GW$ covers rather wide-range of materials, including semiconductor and transition metal oxides with acceptable accuracy [1,2]. In contrast to LDA$+U$, QPsc$GW$ can provide reasonable description of the $d$ band position relative to the $sp$ band without any free parameters. The $d$ band position critically affects the exchange coupling between magnetic ions. We are now developing the method to calculate the dynamic spin susceptibility based on the QPsc$GW$ method. We will show results for elemental transition metals and zincblende(ZB)-type of materials including magnetic ions. We have found that the ferromagnetic phase of ZB-MnAs is stable, contrary to the LDA result. In addition, we will show analysis of the exchange-coupling between magnetic ions. [1] Mark van Schilfgaarde, Takao Kotani, and Sergey V. Faleev, cond-mat/0510408 [2] Sergey V. Faleev, Mark van Schilfgaarde, and Takao Kotani, PRL93, 126406 (2004) [Preview Abstract] |
Thursday, March 16, 2006 3:54PM - 4:06PM |
W23.00006: A band theory for magnetic cuprates based on self-interaction free local density approximation Vincenzo Fiorentini, Alessio Filippetti The pseudo-SIC approach is based on an approximate form of self-interaction corrected (SIC) Kohn-Sham Equations. We overview the functionalities of this method applied to cuprates, which are prototypes of difficult materials for standard local-spin density functional theories such as LSDA (or even GGA). Indeed, theories based on local exchange-correlation potentials fail to predict the correct spin-polarized ground-state solution expected for the low-magnetization state (S=1/2) of the Cu(I) ions, thus describing these systems as metallic and nonmagnetic. Here we present our results for a series of relevant cases, including CuO, Cu$_2$O, CuGeO$_3$, and YBa$_2$Cu$_3$O$_{6+x}$, showing that the pseudo-SIC is capable to correct the gross failures of LSDA, restoring the expected S=1/2 electronic ground state and an overall satisfying description of the chemistry and the electronic and magnetic properties of these systems. Furthermore, since the pseudo-SIC is designed to work for metals as well as for insulators we can approach the challenging task of studying by first-principles the insulating-metal transition in doped Mott insulators. We will consider the example of Mn-doped CuO, where Mn-doping induces a simultaneous insulating-to-metal and antiferromagnetic-to-ferromagnetic phase transition. [Preview Abstract] |
Thursday, March 16, 2006 4:06PM - 4:18PM |
W23.00007: Interplay of Vacancy Defects and Magnetism in Carbon Structures Yiming Zhang, Saikat Talapatra, Swastik Kar, Robert Vajtai, Saroj Nayak, Pulickel Ajayan Magnetic properties of diamond and graphite with vacancy defects have been studied using spin-polarized plane-wave basis density functional theory. Various scenarios of vacancy defects are investigated in these two allotropic configurations. The calculation shows that the vacancy defect concentration and nearby bonding structure is critical to determine the induced magnetism. The total magnetism start to decrease after vacancy accumulation reach the interacting configuration, in both diamond and graphite. We also shows that foreign species like nitrogen close to the vacancy is able to further enhance the magnetic moment in graphite. [Preview Abstract] |
Thursday, March 16, 2006 4:18PM - 4:30PM |
W23.00008: The effect of disorder and short-range correlations on ferromagnetism in dilute magnetic semiconductors B. Moritz, K. Mikelsons, J. Moreno, M. Jarrell, R. S. Fishman We use the Dynamical Cluster Approximation (DCA) and double exchange model, coupling spin one-half holes to magnetic impurities, to study the ferromagnetic transition in semiconductors doped with transition metal magnetic ions. Our approach includes the effect of local dynamics as well as short-range correlations between the magnetic impurities. We systematically incorporate the effect of disorder in the impurity positional configurations with a new algorithm, based on the DCA, specific to dilute systems. This new algorithm serves as a replacement for the Traveling Cluster Approximation and Coherent Potential Approximation. We focus on the appearance of the impurity band and the development of the magnetization for a range of coupling strengths and hole and impurity concentrations. In addition, we discuss the effect of impurity clustering on the hole mobility and the ferromagnetic transition temperature. We conclude that the successful design of spintronic nanostructures based on ferromagnetic semiconductors must include an understanding and careful analysis of disorder and spatial correlations. [Preview Abstract] |
Thursday, March 16, 2006 4:30PM - 4:42PM |
W23.00009: Spatial correlations, spin-orbit coupling, and ferromagnetism in Ga(Mn)As K. Mikelsons, B. Moritz, S. Kancharla, J. Moreno, R.S. Fishman, M. Jarrell The self-consistent Dynamical Cluster Approximation (DCA) is used to study the effect of strong spin-orbit coupling in models of GaMnAs. Both heavy and light carrier bands, degenerate at the $\Gamma$-point, are included using the spherical approximation. Local dynamics as well as short-range spatial correlations are studied using the DCA, adapted for impurity systems in the dilute limit. The critical temperature for ferromagnetism is obtained for different arrangements of magnetic impurities and a range of coupling strengths and carrier concentrations. These calculations clearly demonstrate the suppression of the ferromagnetic transition temperature when one accounts for spatial correlations between impurities and the reduction in saturation magnetization due to the strong spin-orbit coupling. [Preview Abstract] |
Thursday, March 16, 2006 4:42PM - 4:54PM |
W23.00010: Structural, electronic and magnetic properties of Mn-doped GaAs(110) surface Alessandro Stroppa, Maria Peressi First principles total-energy pseudopotential calculations have been performed to investigate structural, electronic---including scanning tunneling microscopy (STM) images---and magnetic properties of the (110) cross-sectional surface of Mn-doped GaAs. We have considered configurations with Mn in interstitial positions in the uppermost surface layers with Mn surrounded by As (Int$_{As}$) or Ga (Int$_{Ga}$) atoms. The presence of Mn on the GaAs(110) surface originates strong local distortion in the underlying crystal lattice, with variations of interatomic distances up to 8\%. In both cases, Int$_{As}$ and Int$_{Ga}$, the surface electronic structure is half-metallic (or \emph{nearly} half metallic) with details strongly dependent on the local Mn environment. The atoms surrounding the Mn impurity show an induced polarization resulting in a ferromagnetic Mn--As and antiferromagnetic Mn--Ga configuration respectively in the two cases. The simulation of the STM images show very different patterns of the impurity region in the two cases, suggesting that they could be easily discerned by STM analysis. We have also simulated STM images of Mn interstitials pairs on surface. The comparison of the simulated images with recent experimental cross-sectional STM images of Mn $\delta$-doped GaAs is discussed. [Preview Abstract] |
Thursday, March 16, 2006 4:54PM - 5:06PM |
W23.00011: Structures and magnetic properties of Cr-doped GaN nanotubes Qian Wang, Qiang Sun, Puru Jena, Yoshiyuki Kawazoe The electronic and magnetic properties of Cr-doped GaN nanotubes are investigated theoretically from first principles using the generalized gradient approximation (GGA) as well as LSDA+U method. We have shown that GaN single wall nanotube, which was generated from GaN wurtzite crystal undergoes large structural relaxation and resemble the structure of carbon (9,0) single wall nanotubes. In addition, it is stable at room temperature. Cr-doped GaN single wall and multi-wall nanotubes are ferromagnetic with each Cr atom carrying a magnetic moment of about 2.67 \textit{$\mu $}$_{B. }$ This ferromagnetic coupling is mediated by the neighboring N atoms which are weakly polarized and carry a magnetic moment of -0.18 \textit{$\mu $}$_{B}$. These results are not sensitive to the tube diameter, Cr concentration, and the level of correlation. Thus, Cr doped GaN nanotubes may be a robust system for applications in spintronics. [Preview Abstract] |
Thursday, March 16, 2006 5:06PM - 5:18PM |
W23.00012: Ferromagnetism in Mn doped GaN Nanowires Qiang Sun, Qian Wang, Puru Jena Using density functional theory and generalized gradient approximation for exchange and correlation potential we show that the magnetic coupling of Mn atoms in the nanowires, unlike that in the thin film, is ferromagnetic in spite of the thickness of the wire and the contraction of the Mn-Mn and Mn-N bond distances. This ferromagnetic coupling, brought about due to the confinement of electrons in the radial direction and the curvature of the Mn-doped GaN nanowires' surface, is mediated by N as is evidenced from the overlap between Mn 3$d$ and N 2$p $states. The Mn atoms prefer to occupy the nearest neighbor positions on the outer surface of the wire and carry a magnetic moment ranging from 0.56 to 3.5 \textit{$\mu $}$_{B}$/atom depending on the thickness of the wire. Calculations of the anisotropic energy show that the magnetic moment orients preferably along the [10$\overline 1 $0] direction while the wire axis points along the [0001] direction. The flexibility of both controlling the magnetic coupling and the magnetic moment by choosing the dimensionality and the size of the wire may be useful in practical applications. The results are in agreement with the recent experimental data which show that Mn-doped GaN nanowire can be ferromagnetic without the presence of other defects. [Preview Abstract] |
Thursday, March 16, 2006 5:18PM - 5:30PM |
W23.00013: First-principles calculation of Mn Atoms on the CuN/Cu(100) Surface Chiung-Yuan Lin, Barbara Jones, Andreas Heinrich The electronic structure is calculated using GGA+U for one and two Mn atoms on a single CuN layer coated on the Cu(100) surface. This unique insulator-metal junction surface prevents the Mn spins from being screened by the conduction electrons and at the same time allows experimentalists to pass tunneling electrons through the Mn atoms to flip their spins. Our spin-density analysis shows that Mn atoms in such a surface preserve their atomic spins S=5/2. This result agrees with a recent STM measurement on such systems. Electron-density change and surface relaxation due to the Mn atoms are also analyzed. [Preview Abstract] |
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