Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session L9: Focus Session: Exchange Interactions and Magnetization |
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Sponsoring Units: DCOMP DMP GMAG Chair: Renat Sabiryanov, University of Nebraska-Omaha Room: LACC 153A |
Tuesday, March 22, 2005 2:30PM - 2:42PM |
L9.00001: L3/L2 Branching Ratio for Rare Earth Compounds Yongbin Lee, Jong-Woo Kim, Alan Goldman, Bruce Harmon Variation of the L$_{3}$/L$_{2}$ intensity ratio for rare earth compounds-- the so called branching ratio (BR) is an outstanding subjects in the field of magnetic X-ray scattering. In X-ray absorptions not influenced by magnetism, the ratio is about 2, which can be explained by the statistical number of 2p core electrons. However, in X-ray circular dichroism(XMCD) and X-ray resonant magnetic scattering (XRMS), BRs greater than 10 and less than 0.1 have been observed. We show by first principles calculation for hcp heavy rare earth metals that the 4f-5d exchange interaction and the spin-orbit interaction in the 5d band states are the key to understand the variation of BR. In this talk, we will also discuss quadrupole transitions, crystal fields, and hybridization of 5d states with the empty, highly polarized 4f states and the effects on the BR. We will compare our results with XRMS data obtained for RENi2MnGe2 samples, and describe general systematics. [Preview Abstract] |
Tuesday, March 22, 2005 2:42PM - 2:54PM |
L9.00002: Orbital magnetization as a ground-state bulk property Raffaele Resta, Davide Ceresoli, Timo Thonhauser, David Vanderbilt The magnetic dipole moment of any finite sample is well defined, while it becomes ill defined in the thermodynamic limit, due to the unboundedness of the position operator. Effects due to surface currents and to bulk magnetization are not easily disentangled. The corresponding electrical problem, where surface charges and bulk polarization appear as entangled, has been solved about one decade ago by the modern theory of polarization, based on a Berry phase. We follow a similar path here, providing a bulk expression for orbital magnetization for any lattice-periodical, though time-reversal breaking, Hamiltonian. We therefore limit ourselves to cases where the macroscopic (i.e. cell-averaged) magnetic field vanishes. For crystalline insulators we express the bulk magnetization in terms of Wannier functions, and we then transform the expression into a Brillouin-zone integral involving the occupied Bloch orbitals. The gauge-invariance of the final expression is explicitly shown. Interestingly, the final expression remains well-defined even for metals, but it is not yet clear whether it is correct in that case. [Preview Abstract] |
Tuesday, March 22, 2005 2:54PM - 3:06PM |
L9.00003: Tight-binding calculations of the orbital magnetization in chiral insulators Timo Thonhauser, Davide Ceresoli, David Vanderbilt, Raffaele Resta We present tight-binding calculations of the orbital magnetization in chiral insulators. Our investigations focus on two-dimensional periodic systems with broken time-reversal symmetry and zero Chern number, and on finite samples cut from such systems. Time-reversal symmetry is broken by threading magnetic fluxes through parts of the unit cell in such a way that the net magnetic field remains zero. Results for the calculated magnetization as a function of the flux show that, in the limit of large but finite systems, the orbital magnetization converges to its bulk value as computed in k-space using the formulation of the previous abstract. We also investigate the surface bandstructure, obtaining insight about the role of edge states in the circulation of the current. Possible extensions to non-zero Chern number will also be discussed. [Preview Abstract] |
Tuesday, March 22, 2005 3:06PM - 3:18PM |
L9.00004: Quantum magnetic short-range order in the paramagnetic state R.Y. Gu, V.P. Antropov A specific quantum magnetic short-range order above the critical temperature for ferromagnets is identified and analyzed. Using quantum Heisenberg model calculations with various exchange interactions parameters of different spins, we found that the quantum spin effect significantly contribute to the magnetic short-range order at rather high temperatures. For a system of small spins such a large short-range order can appear without the abnormal susceptibility which has been previously reported. The influence of this quantum effect on the critical temperature and spin wave spectrum will be discussed. We have also studied the fluctuations of the magnitude of the magnetic moment, and believe that they can also be responsible for the observed Curie-Weiss behavior of the susceptibility above the Curie temperature in real metals. [Preview Abstract] |
Tuesday, March 22, 2005 3:18PM - 3:30PM |
L9.00005: Magnetism in FeAl, Ni$_3$Al and Ni$_3$Ga within DFT: Importance of asymmetry in the Exact Exchange potential Sangeeta Sharma, Kay Dewhurst, Claudia Ambrosch-Draxl, Clas Persson We have calculated the magnetic properties of FeAl, Ni$_3$Al and Ni$_3$Ga with exact exchange DFT within the all-electron full-potential linearized augmented-plane-wave method, including core-valence interactions. The correct ground state for these materials is obtained in all cases: non-magnetic for FeAl and Ni$_3$Ga and ferromagnetic in Ni$_3$Al with a magnetic moment of 0.20 $\mu_B$ per formula unit, which is in excellent agreement with experiments. Both LDA and GGA fail to produce the correct magnetic ground state of all three compounds. This failure has been the subject of several investigations in the past. {\it Ad hoc} corrections to the LDA have been used to obtain the correct ground state for these materials, but are either not parameter free (LDA$+U$) or include dynamical variables (spin fluctuations) which are closer in spirit to the quasi-particle picture. We attribute the success of exact exchange to the strong asymmetry in the exchange potential. This should be a desirable feature for next generation approximate functionals. [Preview Abstract] |
Tuesday, March 22, 2005 3:30PM - 3:42PM |
L9.00006: Application of the spin cluster methods for the description of the thermodynamics of the Heisenberg model. German Samolyuk, Vladimir Antropov We present a general formulation of the spin cluster methods applied to the calculation of thermodynamics of the Heisenberg model in terms of renormalized fields describing interaction between a cluster and its environment. The results of our calculations demonstrate that a pair cluster approximation reproduces Monte-Carlo and spin-dynamic results for the Curie temperature with a rather high accuracy. Such non-mean field systems as systems with the frustrated interactions and systems with a small number of nearest neighbors are investigated. Both classical and quantum Heisenberg model results are obtained. We discuss the general applicability and the enormous computational advantages of this approach. This work was supported by the Office of Basic Energy Sciences of the U.S. Department of Energy under Contract No. W-7405-ENG-82. [Preview Abstract] |
Tuesday, March 22, 2005 3:42PM - 4:18PM |
L9.00007: Magnetism of nano-alloys with Stoner interaction enhanced elements Invited Speaker: Rapidly decreasing grain size of data storage media stimulate research targeted on understanding the factors controlling performance of magnetic metallic alloys at sub 8 nm lengths scale. We discuss a few specific examples where it is necessary to consider atomic scale finite size effects and correspondingly to develop a quantitative model of magnetic interactions. We present results of modeling motivated by the advances in preparation and characterization of L1$_{0}$ FePt, FePd, and CoPt alloys as well research on the meta-magnetic transformation in FeRh to be usedthe Heat Assisted Magnetic Recording (HAMR) recording process. The use of a multi-scale modeling approach which combines a microscopic model of the magnetic interactions and statistical modeling/theory techniques enables us to investigate the thermomagnetic process for 2-8 nm nano- particles. The microscopic model of the magnetic interactions is calibrated with measurements of the temperature dependent magnetic properties for nano-particulate and granular FePt thin films. The analysis of these experimental results uncovers the mechanism of the large magnetic anisotropy as being dominated by the two-ion contribution. In the case of FeRh, isotropic exchange interactions mediated by the induced moment of Rh atoms is found to increase with thermal fluctuations. This, along with a large Stoner intra-atomic exchange parameter results in an unusual M (T) dependence. We show that proposed microscopic mechanism of the AF-FM transformation explains the well established observations. The proposed model of isotropic and anisotropic magnetic inter-atomic interactions mediated by the Stoner intra-atomic interaction enhanced elements Pt, Pd and Rh is shown to be capable of explaining the unsusual finite size and temperature dependent magnetic properties of these nano- alloys. [Preview Abstract] |
Tuesday, March 22, 2005 4:18PM - 4:30PM |
L9.00008: Phonon dynamics of the Heisenberg chain with finite-frequency phonons. Franz Michel, Hans Gerd Evertz Since the discovery of the inorganic Spin-Peierls compound CuGeO$_3$, one dimensional spin systems coupled to phonons have been studied intensively. While static properties are well understood, the dynamic phonon behavior is still unclear. We have studied the phonon dynamics of the spin $\frac{1}{2}$ Heisenberg chain with bond phonons, around the structural phase transition which occurs as a function of spin-phonon coupling. We have employed Quantum Monte Carlo simulations based on stochastic series expansion (SSE), at almost zero and at finite temperature. The dynamic properties have been obtained by mapping the SSE to a continuous time path integral. At zero temperature we find that the quantum phase transition is of the central peak type as inferred before by Sandvik et al[1]. The renormalisation of the main phonon branch, however, depends strongly on the phonon frequency. As a function of temperature at fixed coupling, we find both a central peak for lower and phonon softening for higher spin-phonon coupling. This behavior is similar to the 3 dimensional case. [1] A. W. Sandvik et al., Phys. Rev. Lett. 83, 195 (1999) [Preview Abstract] |
Tuesday, March 22, 2005 4:30PM - 4:42PM |
L9.00009: Exchange interaction and bonding in CuO from first-principles Alessio Filippetti, Vincenzo Fiorentini The understanding of the chemistry of Cu-O interactions is an oustanding open issue in solid state physics in view of its relevance for high-T$_{c}$ superconductors, whose basic units are Cu-O chains or layers. Despite the clear experimental evidence that Cu-Cu spin coupling is strongly antiferromagnetic in the insulating parent compounds and in the doped superconducting phase, a consistent and realistic picture of bonding and magnetic ordering in real Cu-O compounds is still missing. Here we apply a self-interaction-free density-functional approach to investigate the complex interplay of bonding and magnetism in CuO. The puzzling apparently one-dimensional is explained by the presence of a single, spin-polarized d$_{x}^{2}$ character, which induces an antifferromagnetic ordering built up by ferromagnetic planar chains compensating, with period 2, along the $z$ axis. Despite the apparent one-dimensionality, the analysis of the exchange interactions reveals as many as 5 relevant parameters and shows that only a fully three-dimensional view provides a detailed understanding of magnetic ordering and low-energy excitations. [Preview Abstract] |
Tuesday, March 22, 2005 4:42PM - 4:54PM |
L9.00010: Spin gap in coupled quarter-filled Hubbard ladders Bernhard Edegger, Hans Gerd Evertz, Reinhard Noack We present DMRG calculations in quarter-filled ladder systems coupled to the lattice. Single and coupled ladders are considered, with model parameters obtained from first-principles band-structure calculations for $\alpha^\prime$-$NaV_2O_5$. The relevant Holstein coupling to the lattice causes zig-zag lattice distortions, concurrently with the formation of a charge ordered-state. The coupling to the lattice drastically reduces the critical nearest neighbor Coulomb repulsion $V$ and changes the critical exponent of the corresponding quantum phase transition. We find excellent agreement with experimental data for lattice distortion, charge gap, and charge order parameter at a $V^*$ which agrees with previous estimates. The spin gap extrapolates to zero on a single ladder and on two coupled ladders, in spite of a dimerization of spins in chain direction. A finite spin gap appears only on a system with periodicity of {\em four} ladders, as observed experimentally in $\alpha^\prime$-$NaV_2O_5$. The gap is of the right size at $V^*$ but, surprisingly, vanishes again at larger charge order. [Preview Abstract] |
Tuesday, March 22, 2005 4:54PM - 5:06PM |
L9.00011: Collective modes in a Half-Metallic Ferromagnet Raul Chura, Kevin Bedell By using the Theory of Spin-polarized Fermi liquids, we study the spin dynamics of a model for a Half-metallic ferromagnet. We formulate the corresponding kinetic equation and solve it to determine the collective modes of the model. We calculate the velocities of these modes and also study the associated dispersion relations and spin stiffness. We compare the results obtained through this approach with the ones obtained through the application of the Green’s functions method. We discuss the results in the context of the currently available experimental data. [Preview Abstract] |
Tuesday, March 22, 2005 5:06PM - 5:18PM |
L9.00012: The Truncated Polynomial Expansion Monte Carlo Algorithm for Spin-fermion Models: Application to Diluted Magnetic Semiconductors and Manganites Cengiz Sen, Gonzalo Alvarez, Elbio Dagotto A system of fermions coupled to classical fields is common to a wide range of strongly correlated electron problems where the fermionic operators appear in the Hamiltonian involving only quadratic terms. A conventional approach to solve these kinds of models is by diagonalizing the fermions in the one-electron sector at finite temperature for a given configuration of classical fields. However, this results in a high computational cost as the computational complexity grows with the 4-th power of the size of the system. The Truncated Polynomial Expansion Monte Carlo Algorithm (TPEM), developed by N. Furukawa and Y. Motome (J. Phys. Soc. Jpn. \textbf{73}, (2004) 1482), replaces the exact diagonalization of the one-electron sector in these models and has a complexity that is linear with the size of the system. In this talk, I will discuss the performance and reliability of the method as well as the parallelization of the algorithm. I will also show novel applications of the TPEM to disordered systems in the context of diluted magnetic semiconductors and to finite Hund coupling models for manganites (G. Alvarez \textit{et al.}, submitted to Computer Physics Communications). I will discuss how the TPEM can drastically improve the study of those systems. [Preview Abstract] |
Tuesday, March 22, 2005 5:18PM - 5:30PM |
L9.00013: Flat bands on partial line graphs Shin Miyahara, Kenn Kubo, Hiroshi Ono, Yoshihiro Shimomura, Nobuo Furukawa We introduce a systematic method to construct a lattice structure partial line graph. In the tight binding models on the partial line graphs, a flat band emerges on all over k-space. This method can be applied to any two- and three-dimensional systems. In addition to that, there is a large room to modify the lattice as the tight binding model on it has a flat band, i.e. we have many degrees of parameters, for examples, the on-site energy and~the hopping amplitudes.~We show several examples of the partial line graphs. It is expected that our method is also useful in giving a guide line for synthesizing materials with flat bands. [Preview Abstract] |
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