Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session W32: Focus Session: Theory and Simulation of Spin-Dependent Effects and Properties II |
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Sponsoring Units: GMAG DCOMP DMP Chair: Aldo Romero, Cinvestav Querétaro Room: 336 |
Thursday, March 19, 2009 11:15AM - 11:27AM |
W32.00001: The Ising model for the bcc, fcc and diamond lattices; a comparison Per H\aa kan Lundow, Klas Markstr\"om, Anders Rosengren A large scale Monte Carlo simulation study of the Ising model for the simple cubic lattice was recently performed (Adv. Phys. \textbf{56}, 653--755 (2007)). We have complemented that with a study of the bcc, fcc and diamond lattices. Both the canonical and microcanonical ensembles were employed. We present estimates of the critical temperature and other quantities in the critical region. An analysis of the critical behaviour suggests distinct high- and low-temperature exponents, especially for the specific heat, as was obtained also for the simple cubic lattice. This discrepancy is briefly discussed. [Preview Abstract] |
Thursday, March 19, 2009 11:27AM - 11:39AM |
W32.00002: Thermodynamics of magnetic systems from first principles: gWL-LSMS Markus Eisenbach, G. Malcolm Stocks, Don M. Nicholson, Thomas Schulthess Density Functional Calculations have proven to be a useful tool to study the ground state of many materials. For finite temperatures the situation is less ideal an one is often forced to rely on models with parameters either fitted to first principles or experimental results. This approach is especially unsatisfactory in inhomogeneous systems, nano particles or other systems where the model parameters should vary significantly from one site to another. Here we describe a possible solution to this problem by combining classical thermodynamic Monte Carlo calculations - The Wang-Landau method in this case [F Wang and DP Landau, PRL 86, 2050 (2001)] - with a first principles electronic structure calculation, specifically our locally selfconsistent multiple scattering code. The combined code shows superb scaling behavior on massively parallel computers and first tests on Fe systems provide a proof of principle. [Preview Abstract] |
Thursday, March 19, 2009 11:39AM - 11:51AM |
W32.00003: Quantum Monte Carlo calculates bulk properties and magnetic ordering in iron William D. Parker, John W. Wilkins Quantum Monte Carlo (QMC) models electronic systems with high accuracy but its computational demands limit wider use. Few QMC calculations exist for solid-state systems and none comparing the energetic ordering of different spin configurations (magnetic states). Density-functional calculations with generalized-gradient-approximation exchange-correlation give correct magnetic ordering and accurate lattice constants and bulk moduli for bcc iron. However, the predicted cohesive energy differs from experiment by 0.5-1.0 eV. QMC-calculated bulk elastic properties for pure iron in the bcc, hcp and fcc phases compare with properties in the ferromagnetic, antiferromagnetic and nonmagnetic spin configurations. [Preview Abstract] |
Thursday, March 19, 2009 11:51AM - 12:27PM |
W32.00004: Current-driven vortex oscillations in metallic nanocontacts Invited Speaker: In this paper, we performed full micromagnetics simulations of metallic nano-contacts from the TUNAMOS consortium, by solving the Landau Lifshitz Gilbert Slonctewski equation simultaneously with quasi-static Maxwell equations. We take into account the spatially inhomogeneous current distribution flowing through the magnetic free layer and consequently use the Oersted field generated by this current for the magnetization dynamics. The system we simulated was a trilayer CoFe 3$.$5 nm/Cu 3nm/NiFe 4nm stack. The saturation magnetization of the free layer is taken to be the same as the experimental value \textit{Ms }=1$.$1 T, and a GMR ratio of 1{\%} is used. We account for the inhomogeneous current distribution flowing through the free layer by computing the local current density from the local angle between the free and fixed layer magnetizations. The Oersted field is computed with the Biot-Savart law from this current distribution [2], and an asymmetric Slonczewski term for the spin transfer is used [3]. We observe that the additional spin torque drives the vortex out of the contact area and towards a stable orbit around the contact. These simulations reveal that the oscillations observed are related to the large-amplitude translational motion of a magnetic vortex. In contrast to the nanopillar geometry in which the vortex core precesses within the confining part of the Oersted field [1], the dynamics here correspond to an orbital motion \textit{outside }the contact region. This behavior can be likened to planetary orbital motion under the influence of a gravitational field; the spin-transfer torque leads to a centripetal motion of the vortex core, which is counterbalanced by the attractive potential provided by the Oersted field. Good quantitative agreement between the simulation and experimental frequencies is achieved [4]. \\[4pt] [1] V. S. Pribiag et al., Nat. Phys. \textbf{3}, 498 (2007) \\[0pt] [2] O. Ertl \textit{et al.}, J. Appl. Phys. \textbf{99}, 08S303 (2006). \\[0pt] [3] J. Xiao, A. Zangwill, and M. D. Stiles, Phys. Rev. B \textbf{70}, 172405 (2004). \\[0pt] [4] Q. Mistral, M. van Kampen, G. Hrkac, et al. PRL \textbf{100, }257201 (2008) [Preview Abstract] |
Thursday, March 19, 2009 12:27PM - 12:39PM |
W32.00005: Monte Carlo simulation of incommensurate helical ordering in a frustrated FCC lattice of Heisenberg spins Seongweon Park, Ch. M. Sullivan, G. Schneider, T.M. Giebultowicz The zincblende structure of MnSe can be stabilized in thin films and is expected to exhibit Type III FCC antiferromagnetic ordering. The expected magnetic order is indeed observed in MnSe/ZnSe superlattices where the MnSe layers experience compressive strain. However, in the MnSe/ZnTe system, in which MnSe layers experience tensile strain, the Mn spins form incommensurate helical structures \footnote{T.M.Giebultowicz et al, Phys Rev B, 46, 12076-12079 (1992)}. Mean field theory can explain the basic mechanism leading to helical ordering but cannot explain details such as the temperature dependence of the pitch of the helical ordering. We report results of Monte Carlo simulations using classical 3D Heisenberg spins and ``free'' boundary conditions. The simulations were performed for a range of systems with different sizes (including ``bulk'' thickness) and exchange constants. The change of helical pitch with temperature is correctly reproduced in our results and our data indicate that it is at least partially a finite-thickness effect. We compare our results with earlier simulations using XY spins and ``self-determined'' boundary conditions\footnote{M. Collins and W.M. Saslow, Phys Rev B 53, 8533-8538 (1996)}. [Preview Abstract] |
Thursday, March 19, 2009 12:39PM - 12:51PM |
W32.00006: Dynamical magnetizations of nanomagnets with strong magnetic anisotropy Bang-Gui Liu We developed a non-equilibrium Monte Carlo method to investigate dynamical spins and magnetizations of nanomagnets with strong magnetic anisotropy and applied it to Co spin chains on Pt surface and a composite spin system (Phys. Rev. B 73, 174418; Phys. Rev. Lett. 96, 217201; Front. Phys. China 2, 424). Here we report on our exploration for universal dynamical magnetic properties of spin chains and single-layered nanomagnets with strong magnetic anisotropy. Furthermore, we investigate representative systems composed of sub-10nm nanomagnets with large uniaxial anisotropy for magnetic data storage, finding various magnetization memory effects and aging effect in such single systems of the interacting nanomagnets with the same easy axis, and study the exchange bias and training effect observed in composite films and heterostructures. This method is proved to be effective and reliable in simulating dynamical magnetism in nanomagnets with strong magnetic anisotropy. [Preview Abstract] |
Thursday, March 19, 2009 12:51PM - 1:03PM |
W32.00007: Tailoring Effective Exchange Interactions via Domain Walls in Coupled Heisenberg Rings Vanita Srinivasa, Jeremy Levy The nature of the exchange coupling variation in an antiferromagnetic spin-1/2 system can be used to tailor its ground-state properties. In particular, dimerized Heisenberg rings containing domain walls have localized states which can serve as ``flying spin qubits'' when the domain walls are moved (PRB \textbf{76}, 094411 (2007)). We show theoretically that, when two of these rings are coupled, the movement of the domain walls leads to modulation of the effective exchange interaction between the qubits. Appropriately chosen configurations of domain walls can give rise to ferromagnetic effective exchange. We describe how these spin rings may be used as basic building blocks to construct quantum spin systems whose properties are tunable by virtue of the exchange variation within the rings. [Preview Abstract] |
Thursday, March 19, 2009 1:03PM - 1:15PM |
W32.00008: Electron magnetism of antiferromagnetic conductors. Revaz Ramazashvili Essential momentum dependence of the electron g-tensor in an antiferromagnet turns the common Zeeman term into a spin-orbit coupling. I will discuss some of the remarkable experimental consequences of this phenomenon. The predictions may be relevant to antiferromagnetic conductors from chromium to electron- and hole-doped cuprates, borocarbides, pnictides, organic and heavy fermion materials. [Preview Abstract] |
Thursday, March 19, 2009 1:15PM - 1:27PM |
W32.00009: On the Electric-Field-Controlled Surface Ferromagnetic Transition in Metals Igor V. Ovchinnikov, Kang L. Wang It is widely believed that in metals, unlike in the dilute magnetic semiconductors, the control of the ferromagnetic ordering by an external voltage is hardly achievable. We compare the two materials and show that there is no obvious reason why metals are less preferable for this phenomenon. Similar effect in metals, however, has a different physical picture and should be identified as a voltage-induced surface ferromagnetic transition. We study its properties within the theory of the surface critical phenomena and discuss possible difficulties on the way to its experimental realization. [Preview Abstract] |
Thursday, March 19, 2009 1:27PM - 1:39PM |
W32.00010: Effect of fluctuations on effective Hamiltonians of anisotropic frustrated pyrochlore antiferromagnets. Paul McClarty, Michel Gingras The rare earth pyrochlore magnets R$_{2}$Ti$_{2}$O$_{7}$ exhibit a wide range of puzzling features. Tb$_{2}$Ti$_{2}$O$_{7}$, a weakly Ising-like antiferromagnet, is a cooperative paramagnet down to, at least, 50 mK despite having a -20 K Curie-Weiss temperature. Er$_{2}$Ti$_{2}$O$_{7}$, which has magnetic ions with a strong easy plane anisotropy, has a transition to an ordered phase but the origin of a long-range ordered state with discrete broken symmetry is not understood. Recent experimental work has also uncovered interesting field-induced phases in both of these materials. We construct effective Hamiltonians, derived from microscopic models, for these two frustrated antiferromagnets by considering the effects of quantum fluctuations out of the classical ground states of these models to assess the stability of these states, the nature of the excitations and possible mechanisms of degeneracy breaking. [Preview Abstract] |
Thursday, March 19, 2009 1:39PM - 1:51PM |
W32.00011: Ferromagnetic spin coupling as the origin of $0.7$ anomaly in quantum point contacts Karan Aryanpour, Jong E. Han We study one-dimensional itinerant electron models with ferromagnetic coupling to investigate the origin of $0.7$ anomaly in quantum point contacts (QPC). Linear conductance calculations using the Kubo formula from the quantum Monte Carlo (QMC) technique for spin interactions of different spatial range suggest that $0.7\times(2e^{2}/h)$ anomaly results from a strong interaction of low-density conduction electrons to ferromagnetic fluctuations formed across the potential barrier. The conductance plateau results due to the strong incoherent scattering at high enough temperatures when the electron traversal time through the gate voltage barrier matches the time-scale of dynamic ferromagnetic excitations (magnons). In addition, our model also captures the correct evolution of the anomalous plateau as a function of temperature and Zeeman magnetic field. [Preview Abstract] |
Thursday, March 19, 2009 1:51PM - 2:03PM |
W32.00012: Intrinsic Perturbation of the Landau Levels in Metals and Semiconductors at Low Temperatures Ayodeji Awobode The de Haas--van Alphen effect in non-superconducting metals and semiconductors at very low temperatures is proposed as a test of an intrinsic perturbative term which appears in the Landau equation sequel to the modification of the Pauli equation. Corrections to the frequency (or period) of the de Haas--van Alphen oscillation in metals is calculated and shown to depend on the Fermi energy and the measured anomalous part of the electron magnetic moment. Precision measurement of the magneto-optical properties which arise from the motion of electrons in binary semiconductors placed in a weak magnetic field is also proposed as a means of observing very small changes in the. [Preview Abstract] |
Thursday, March 19, 2009 2:03PM - 2:15PM |
W32.00013: Variation of magnetic H field in closed loop magnetic circuits: problems with the standard equation Esaindang Umenei, Eugene Melikhov, David Jiles We have developed a reliable method for calculating the variation of magnetic field H in closed circuits. This offers advantages over standard numerical Finite Element Modeling which requires meshing of the spatial domain. Such calculations can consume enormous computational resources and time. Analytical models work much faster but are only applicable in restricted cases. The well known ``standard model'' for the relationship between current I and magnetic field H derived from Ampere's Law is$H=\frac{N\,I}{L}$, where $N$ I is magnetomotive force and $L$ is the length of the magnetic path. However, this formula fails to describe the variation in magnetic field with position. In fact H is usually inhomogeneous around a closed path unless special precautions have been taken to ensure uniformity. In order to describe the magnetic field around a closed circuit we have introduced extensions to the standard formula for a finite coil in a closed circuit. This includes parameters for location and shape of core to enhance the accuracy. This analytic model produces fast and accurate predictions for the variation of H with position. Results are comparable with FEM calculations that take much longer to generate. [Preview Abstract] |
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