Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session H9: Focus Session: Low Dimensional Magnetism |
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Sponsoring Units: DCOMP DMP GMAG Chair: Kirill Belashchenko, University of Nebraska-Lincoln Room: LACC 153A |
Tuesday, March 22, 2005 8:00AM - 8:36AM |
H9.00001: Magnetic Properties of Quantum Corrals Invited Speaker: One of the iconal experiments in nanotechnology has been the fabrication of a circular arrangement of 48 Fe atoms on a Cu(111) surface and the direct observation, by Scanning Tunneling Microscopy, of charge oscillations within the circle. Now, the remarkable developments in spin-polarized STM make the observing of spatial variations in the magnetic density a distinct possibility and therefore an attractive new area of research. We present 'first principles' calculations for electronic and magnetic properties of surface states confined by a circular quantum corral built of magnetic adatoms (Fe) on a Cu(111) surface. The calculations are performed fully relativistically using the embedding technique within the Korringa-Kohn-Rostoker method. We show the 'Friedel like' oscillations of charge and magnetization densities and the possibility of the appearance of spin-polarized states. In order to classify the peaks in the calculated density of states with orbital quantum numbers the problem is analyzed in terms of a simple quantum mechanical circular well model. This model is also used to an estimate the behavior of the magnetization and energy with respect to the radius of the circular corral. [Preview Abstract] |
Tuesday, March 22, 2005 8:36AM - 8:48AM |
H9.00002: Noncollinear magnetism at ferromagnetic/anti\-ferro\-magnetic interfaces: canting and half-metallicity K. Nakamura, T. Ito, A.J. Freeman We present first principles determinations of noncollinear magnetic structures at the exchange bias FM/AFM interfaces by using the FLAPW method\footnote{Wimmer, Krakauer, Weinert and Freeman, PRB 24, 864(1981)} including intra-atomic noncollinear magnetism\footnote{Nakamura, Freeman, Wang, Zhong and Fernandez-de-Castro, PRB 65, 12402 (2002); 67, 14420 (2003)} to treat the magnetic complexity at interfaces involved in the spin-flop coupling. First, we demonstrate exchange bias at the Co/FeMn interface, in which we find that the Fe moments in the FeMn layer reorient away from their directions in bulk FeMn so as to be parallel to the Co moment direction, which induces an out-of-plane magnetic anisotropy. The results appear to support and confirm recent experimental XMCD findings that rule out spin-flop coupling as the mechanism for the exchange bias in this system. Next, we determine magnetic structures at the CrSe/MnSe and CrTe/MnTe interfaces, in order to investigate exchange-biased half-metallic ferromagnets. The results show that the Cr moments in the FM layer lie perpendicular to the Mn moments in the AFM layer but that the Mn moments strongly cant to induce net moments at the AFM interfaces. Importantly, we find that the canting of the moments tends to retain the half-metallic state at the FM/AFM interface. [Preview Abstract] |
Tuesday, March 22, 2005 8:48AM - 9:00AM |
H9.00003: Effect of spin-orbit interaction on a magnetic impurity in the vicinity of a surface Laszlo Szunyogh, Gergely Zarand, M. Carmen Munoz, Balazs L. Gyorffy The most successful explanation to the Kondo effect in thin films relies on the assumption that the dynamics of magnetic impurities is frozen due to surface-induced anisotropy effects. Combining a simple tight-binding calculation for the semi-infinite geometry of the host metal with a description of the coupling of the spin of magnetic atom with its neighborhood by means of Abrikosov's pseudofermion technique, we study the spin-orbit induced anisotropy in two models: {\em (i)} a $J=3/2$ impurity with strong local spin-orbit coupling and {\em (ii)} a $S=3/2$ impurity embedded into a host with spin-orbit coupling. We find that in both cases the anisotropy energy is an oscillating function of the distance $d$ from the surface with an amplitude that decays as 1/$d^2$. This observation is clearly supported by an asymptotic analysis of the spectral function of the host. Furthermore, numerical estimates suggest that the first kind of mechanism gives rise to an anisotropy energy of the desired order of magnitude. A simple physical interpretation of the effect emerges naturally. [Preview Abstract] |
Tuesday, March 22, 2005 9:00AM - 9:12AM |
H9.00004: Ab-initio theory of Kerr angles in magnetic multilayer systems Irene Reichl, Andras Vernes, Peter Weinberger Ab-initio magneto-optic Kerr rotation and ellipticity angles are presented for the system Cu$_4$/Ni$_8$/Cu$_n$/Ni$_9$/Cu(100) and for Fe$_n$/Au(100) in the vicinity of the reorientation transition, with $n$ varying between 2 and 10. For the Cu/Ni system it will be shown that the occurring oscillations in the Kerr angles with respect to the spacer thickness resemble closely those for the experimentally recorded interlayer exchange coupling energy. In the case of Fe/Au a very accurate description of the magneto-optical properties can be given as the preferred magnetization turns from perpendicular to in-plane. The obtained theoretical results are in very good agreement with available experimental data. [Preview Abstract] |
Tuesday, March 22, 2005 9:12AM - 9:24AM |
H9.00005: Ab initio studies of non-collinear magnetic ordering of Cr dimers on Mo(110) Yang Wang, Ruqian Wu Small clusters may display innovative properties in ultra small scales, and are very promising for applications in the future. Cr has anti-ferromagnetic ordering in its bulk, described by a spin density wave with a wavelength incommensurate with the lattice constant. In thin films, Cr films are anti-ferromagnetic on most substrates. Here we study magnetic ordering, magnetic anisotropy and spin dynamics of a Cr dimer on Mo (110), smallest possible magnetic recording unit. The~locally self-consistent multiple scattering (LSMS) method for spin-dynamics and the~full potential linearized augmented plane wave (FLAPW)~method~for magnetic anisotropy are adopted in the calculations. We found various peculiar features in the free and supported Cr dimers and explained our results from electronic structures. [Preview Abstract] |
Tuesday, March 22, 2005 9:24AM - 9:36AM |
H9.00006: Calculation of the free energy of NiFe2O4 nanopoarticles by Monte Carlo simulation Chenggang Zhou, T. C. Schulthess, D. P. Landau Magnetic properties of nanoparticles are of great current interest in light of possible applications to high density magnetic storage media. Finite size and surface effect are important for magnetic nanoparticles and differentiate them from their bulk counterparts. We use Monte Carlo simulation to study a model of NiFe$_{2}$O$_{4}$ nanopoarticles proposed by Kodama and Berkowitz [1]. The Hamiltonian of the nanoparticle contains superexchanges between magnetic ions modeled by Heisenberg spins, and surface/bulk anisotropy terms. A continuous version of the Wang- Landau algorithm [2] is used to calculate the joint density of states \textit{$\rho $}($M$, $E)$ efficiently. From \textit{$\rho $}($M$, $E)$, we can directly evaluate the free energy of the particle, and many other physical quantities. A hysteresis loop for particles with surface disorder and surface anisotropy is observed, in agreement with previous studies [1]. We found that such a hysteresis loop is the result of interplay between surface disorder and surface anisotropy. Compared with micromagnetic modeling, our Monte Carlo simulation treats the thermodynamic effects properly and is capable of calculating physical quantities at all temperatures and magnetic fields with very limited CPU time. [1] R. H. Kodama, et. al. Phys. Rev. Lett. \textbf{77}, 394 (1996); Phys. Rev. B 59, 6321 (1999). [2] C. Zhou, et al., in preparation. [Preview Abstract] |
Tuesday, March 22, 2005 9:36AM - 9:48AM |
H9.00007: Theoretical study on the magnetic structure of ultrathin Fe on Cu(100) Laszlo Udvardi, Laszlo Szyunyogh, Peter Weinberger Thin films of Fe on Cu (100) surface exhibit rich variety of magnetic and structural phases. As it is accepted by most of the experimental work at large thickness a bcc phase is formed and below 4 atomic layers a ferromagnetic fct structure is realized. In the present paper the magnetic structure of the intermediate region between 4 and 10 monolayers has been studied by means of an extended Heisenberg model. All the parameters appearing in the model, namely the exchange, anisotropic symmetric-exchange and the on-site uniaxial anisotropy has been determined from first principle by a method described in Ref.[1]. The ground state and the finite temperature behavior of the thin film has been investigated by Monte Carlo simulations. The results of magneto optical Kerr effect and depth-resolved x-ray dichroism measurements suggested that the ground state of the system is a spin density wave. The averages of the layer resolved magnetization resulted by our simulations qualitatively agree the experimental results. We were also able to reproduce the different shape of the magnetization curve for films consist of odd and even number of atomic layers. [1] L. Udvardi, L. Szunyogh, K. Palotas, and P. Weinberger, Phys. Rev. B 68, 104436 (2003) [Preview Abstract] |
Tuesday, March 22, 2005 9:48AM - 10:00AM |
H9.00008: Effects of Magnetic Impurities in Quasi-One-Dimensional Magnets Munehisa Matsumoto, Hajime Takayama In the low-dimensional gapped magnets such as CuGeO$_3$ and PbNi$_2$V$_2$0$_8$, doping effects of non-magnetic impurities have been studied and novel order-by-disorder phenomena such as the non-magnetic-impurity-induced phase transitions have been found. Recently, the effects of magnetic impurities was experimentally investigated in the Haldane material PbNi$_2$V$_2$O$_8$ and the impurity-induced transition temperature was found to have strange non-monotonic dependence on the magnitude of the impurity spin. Motivated by this recent progress, we study the non-magnetic and magnetic impurity effects in the gapped quasi-one-dimensional system, namely, the weakly coupled spin-1 antiferromagnetic Heisenberg chains on a simple cubic lattice. We utilize the quantum Monte Carlo method with the continuous-time loop algorithm and determine numerically the impurity-induced transition temperature for the systems with non-magnetic impurities and also with the magnetic impurities with spin magnitude $S=1/2$, $3/2$, $2$, and $5/2$. Our simulation results are compared to the experimental data and the picture for the impurity-induced ordered state is discussed. [Preview Abstract] |
Tuesday, March 22, 2005 10:00AM - 10:12AM |
H9.00009: Computational Electronic Structure of Photoinduced Spin Transitions in Molecular Photoswithches Teepanis Chachiyo, Jorge H. Rodriguez Minimum Energy Crossing Points (MECPs) are the energy barriers associated with radiationless spin-crossover. Understanding the mechanisms of such spin-forbidden processes is of great fundamental and practical interest. Methods of locating MECPs have been developed by several authors with, in general, a poor convergence behavior which increases the computational cost. We present a new method based on spin density functional theory (SDFT) that exhibits fast logarithmic convergence and is suitable for large molecular systems. Spin photo-switchable compounds are becoming a growing area of research since, in principle, it is possible to exploit their spin degrees of freedom to store bits of information. For instance, the photoswitch $[Fe(ptz)_6](BF_4)_2 \;(ptz=1-propyltetrazole)$ has a singlet (S=0) ground state and, upon optical excitation, becomes trapped in a quintet state (S=2) as long as T $<$ 50K. We applied our MECP methodology to study its geometrical and spin dynamics during spin crossover. Our results are useful for elucidating the geometric rearrangements and microscopic mechanisms that control the lifetime and the critical temperature of the meta-stable high spin (quintet) state. For example, the MECP-SDFT calculations showed that the transition from singlet ground state to a triplet ``intermediate'' state is accompanied by an unusually large 0.3{\AA} bond length elongation of the iron axial ligands. [Preview Abstract] |
Tuesday, March 22, 2005 10:12AM - 10:24AM |
H9.00010: Ab Initio Calculation of Zero Field Splitting Parameters of Iron Complexes and Molecular Magnets Fredy Aquino, Jorge H. Rodriguez Zero-Field Splittings (ZFS) in metal complexes arise from the combined action of crystalline fields acting on the metal valence shells and spin-orbit coupling, a relativistic effect. The Ab-Initio calculation of Zero-Field Splitting parameters, $H_{ZFS} = D[S_z^2+\frac{1}{3}S(S+1)]+E(S_x^2-S_y^2)$, of metal-containing molecules is a challenging computational problem of practical relevance to biology and molecular-based nanotechnology. Metal-containing molecular magnets, in particular, have been postulated as potentially useful highly dense memory storage devices depending on the magnitudes and signs of their ZFS parameters (i.e., magnetic anisotropies). We present results based on a computational methodology which treats the non-relativistic electronic structure of magnetic molecules within the framework of spin density functional theory (SDFT) and adds the relativistic effects of spin-orbit coupling via second order perturbation theory (PT). This combined SDFT-PT approach has allowed us to compute from first principles the ZFS parameters of iron-containing complexes with a fairly good degree of accuracy. We present results for some biologically relevant iron complexes which serve as models for metal centers in proteins and which display fairly large ZFS. The computed parameters depend strongly on the molecular geometries. Some functionals within the generalized gradient approximation of SDFT yield fairly satisfactory results. Supported by NSF. [Preview Abstract] |
Tuesday, March 22, 2005 10:24AM - 10:36AM |
H9.00011: Review of calculation of magnetic hyperfine Interactions for a molecular system Ho-Ki Jeong, Junho Jeong Magnetic hyperfine interactions related to the nuclear and the electron spins of a molecular system consist of dipole-dipole interaction and the Fermi contact interaction. And it installed in the Gaussian program is to use to analyze NMR, ESR, $\mu $SR, and EPR experimental data. However, the Fermi contact term, important effect in a molecule, of magnetic hyperfine interaction has physical and mathematical problems on the vector potential definition. By contrasting the definitions of vector potential derived by the electromagnetic field theory with by the dipole-dipole interaction theory, it will be proved that the Fermi contact term cannot exist mathematically and physically. Most theoretical scientists have mistake is to choose wrong initial coordinates. [Preview Abstract] |
Tuesday, March 22, 2005 10:36AM - 10:48AM |
H9.00012: Quantum Percolation in Two-dimensional antiferromagnets Rong Yu, Tommaso Roscilde, Stephan Haas The interplay of geometric randomness and strong quantum fluctuations is one of the most exciting topics in quantum many-body physics. Recently the problem whether a system of two-dimensional Heisenberg antiferromagnets on a square lattice can be driven through a quantum phase transition by either bond or site dilution attracts lots of research interests. Both experimental result and numerical analysis give evidence that the system with homogeneous site and bond dilution is driven through a classical geometric percolation transition. In this work, we show that non-homogeneous bond dilution introduces a different scenario -- the percolative quantum phase transition. The quantum percolation is investigated by Quantum Monte Carlo simulations. A new phase characterized by an infinite percolating network with vanishing antiferromagnetic order parameter appears as an intermediate phase between the geometric disordered phase and the antiferromagnetic ordered phase. [Preview Abstract] |
Tuesday, March 22, 2005 10:48AM - 11:00AM |
H9.00013: An Ab Initio Study of H2 Interaction with Pu (100) Surface M. N. Huda, A. K. Ray We present here a detailed study of hydrogen molecule adsorption on a plutonium (100) surface, using the generalized gradient approximation to density functional theory. All calculations were done at the non-spin-polarized and spin-polarized levels of theory using the DMol3 suite of programs [1]. Weak molecular adsorptions with a layer by layer alternate spin arrangement of the plutonium atoms have been observed and the vertical approaches at the center site are found to be the most favorable molecular adsorption sites, with slightly higher adsorption energies for non-spin-polarized cases. The most favorable dissociation channel needs activation energies of 0.551eV and 0.778eV at the non-spin-polarized and spin-polarized levels, respectively, with considerably higher adsorption energies compared to molecular adsorptions. The 5f electrons are more localized in spin polarized cases than the non-spin polarized counterparts. For dissociative adsorption, the ionic part plays an important role in H-Pu bonding, and the Pu 5f - H 1s hybridization is rather weak. Also, adsorption of hydrogen pushes the top of 5f band further away from Fermi level. [1] B. Delley, J. Chem. Phys. \textbf{92}, 508 (1990); J. Chem. Phys. \textbf{113}, 7756 (2000). [Preview Abstract] |
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H9.00014: NMR studies of quantum spin liquids using high magnetic fields: SrCu$_{2}$(BO$_{3})_{2}$ and BaCuSi$_{2}$O$_{6}$ Raivo Stern, I. Heinmaa, P. Kuhns, A. Reyes, W. Moulton, M. Horvatic, C. Berthier, C. Batista, M. Jaime, T. Kimura, H. Dabkowska, B. D. Gaulin We present the results of Nuclear Magnetic Resonance (NMR) experiments on the quasi-2D spin gap systems SrCu$_{2}$(BO$_{3})_{2}$ and BaCuSi$_{2}$O$_{6}$ in continuous magnetic fields H of up to 44 T. Using $^{11}$B spectra and spin-lattice relaxation times T$_{1}$ we explore the spin structure on the so-called 1/3 magnetization plateau of the Shastry-Sutherland model material SrCu$_{2}$(BO$_{3})_{2}$. In ancient Han purple - BaCuSi$_{2}$O$_{6}$ -- we observe with help of $^{29}$Si and $^{63,65}$Cu T$_{1}$ the closure of the singlet gap at H$_{c1}$ = 23.4 T down to temperatures T $\sim $ 40 mK. While entering the suggested Bose-Einstein Condensate phase for higher fields [M. Jaime \textit{et al}., Phys. Rev. Lett, \textbf{93}, 087203 (2004)] we discover dramatic changes in $^{29}$Si line shape suggesting incommensurate (and complicated) spin modulation in the ordered phase. [Preview Abstract] |
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