Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session D41: Cooperative Phenomena: Magnetic Properties and Spin Dynamics |
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Sponsoring Units: GMAG Chair: David Pappas, NIST, Boulder Room: LACC 150A |
Monday, March 21, 2005 2:30PM - 2:42PM |
D41.00001: The magnetic short range order in magnets Vladimir Antropov Following our prediction of the strong magnetic short range order (MSRO) above the Curie temperature in the itinerant magnets we consider the influence of this MSRO on several important observable properties. Using the density functional spin dynamics we analyze the spin wave like exitations appearance in the paramagnetic state and discuss the conditions of their persistence at the high temperature. The first principles dynamic correlation function S(q,w) at the finite temperature in different materials is discussed. A giant antiferromagnetic MSRO in pure Cr is found, and both transversal and longitudinal MSRO have been identified. Such MSRO strongly affects all observable properties. The validity of our spin dynamic approach at high temperatures is questioned. The conditions to regulate such MSRO to improve magnetic properties of materials are analysed. We also discuss what kind of new experiments should be performed to estimate a degree of MSRO. [Preview Abstract] |
Monday, March 21, 2005 2:42PM - 2:54PM |
D41.00002: Magnetic Excitations of Stripes Daoxin Yao, Erica Carlson, David Campbell Competing tendencies in electronic systems with strong correlations can lead to spontaneous nanoscale structure, pattern formation, and even long-range spatial order. There has been continued interest in various ``stripe'' phases of electrons, as well as more recent interest in possible ``checkerboard'' patterns. New experimental techniques allow for the extraction of detailed and reproducible neutron scattering spectra in copper oxide superconductors and related nickelate compounds. We discuss the magnetic excitations of well-ordered stripe phases, including the high energy magnetic excitations of recent interest and possible connections to the ``resonance peak'' in cuprate superconductors. Using a suitably parametrized Heisenberg model and spin wave theory, we study a variety of possible stripe configurations, including vertical, diagonal, staircase, and zigzag stripes. We calculate the expected neutron scattering intensities as a function of energy and momentum. Constant energy cuts at high energy often reveal a square-like scattering pattern, and occasionally a circular pattern. Bond-centered stripes have weight gathered near (pi,pi) at low energy, indicating that only part of the spin wave cone is expected to be resolvable experimentally. In addition, we present a litmus test for experimentally distinguishing bond-centered stripes from site-centered stripes using low energy data. [Preview Abstract] |
Monday, March 21, 2005 2:54PM - 3:06PM |
D41.00003: Strain-dependent magnetization dynamics in La$_{0.67}$Sr$_{0.33}$MnO$_3$ studied by time-resolved magneto-optical Kerr effect Diyar Talbayev, Haibin Zhao, Gunter L\"upke, Jun Chen, Qi Li We observed the uniform magnetization precession in the ferromagnetic state of La$_{0.67}$Sr$_{0.33}$MnO$_{3}$ films grown on different substrates. The precession was induced by an optical pump pulse and probed by a time-delayed pulse using the Kerr rotation. We recorded the time-evolution of magnetization in the 50-300 K temperature range in magnetic fields of up to 1.3 T. The field dependence of the precession frequency was found to follow the predictions of Kittel's mean field theory, and the frequencies agree with those recorded in microwave absorption experiments. The observed precession frequencies depend on the film substrate, and thus on the amount of strain induced in the film. To account for the strain differences, we introduce the magnetic anisotropy that ranges from easy plane to easy-out-of-plane axis. Precession frequencies displayed no or very little temperature dependence. The field-dependent relaxation times of the exponentially decaying precession are in 100 -- 1400 ps range. The corresponding Gilbert damping parameter decreases with increasing field and is about 0.0003 for all studied temperatures. The research was supported in part by the National Science Foundation and the Department of Energy. [Preview Abstract] |
Monday, March 21, 2005 3:06PM - 3:18PM |
D41.00004: Low-frequency dynamics of magnetostatically coupled vortices K. Yu. Guslienko, K. Buchanan, S.D. Bader, V. Novosad The low-frequency dynamics of magnetostatically coupled vortices trapped in submicron magnetic dots have been investigated analytically and numerically. The system of interest consists of two identical disk-shaped ferromagnetic (F) elements with a single vortex ground state separated by a non-magnetic spacer (N). This allows for a detailed investigating of the effects of interlayer magnetostatic interactions on the low frequency spin excitation mode predicated for translational motion of the vortex core [K.Yu. Guslienko et al., J. Appl. Phys. 91, 8037 (2002).]. The proposed model is based on the equations of motion for the vortex center positions. The vortex restoring force was calculated analytically, taking into account the magnetostatic interactions assuming a realistic ``surface charges free'' spin distribution. For the tri-layer F/N/F dot (opposing chiralities, same polarization) we predict the existence of two excitation modes with size-dependent eigenfrequencies, one of which has a low frequency in the MHz range. The calculated vortex core trajectories depend on the vortex topological charges (vorticity, polarization and chirality). Analytical results for all combinations of core polarizations and magnetization chiralities will be compared with numerical simulations. [Preview Abstract] |
Monday, March 21, 2005 3:18PM - 3:30PM |
D41.00005: Tunable Magnetic Thermal Hysteresis for Total Moment in Fe and Co/Gd Multilayers Sezen Demirtas, Maria R. Hossu, Robert E. Camley, Hector C. Mireles, Ali R. Koymen We investigated the magnetic thermal hysteresis in Fe/Gd and Co/Gd multilayers for the first time by SQUID magnetometry. Total magnetic moment of the antiferromagnetically coupled sublayers show a bow-tie shape with respect to temperature indicating magnetic superheating and supercooling. The width of the ``bow-tie'' figures can be \textit{tuned} to have values of 200K or higher by decreasing the external field strength and/or thickness of the multilayers. The results are in excellent agreement with theoretical calculations. [Preview Abstract] |
Monday, March 21, 2005 3:30PM - 3:42PM |
D41.00006: Surface roughness effects on the magnetostatic modes of cylindrical nanowires Rodrigo Arias A new formulation of the method [1] for finding the magnetostatic modes of ferromagnetic nanowires by use of the extinction theorem is presented. Integral equations along the periphery of the sample resulting from the latter method [1] allow in principle to find the spectrum and the form of the magnetostatic modes propagating along the principal axis (the magnetization is parallel to it) of a cylindrical nanowire of uniform and arbitrary cross sectional shape, without resort to a basis set. The new formulation is technically better since it is independent of the evaluation of the infinite medium Green's functions (upon which the method relies) at arbitrary points inside or outside the sample. This new approach allowed to develop a perturbation theory in order to find the magnetostatic eigenfrequencies and eigenmodes of cylinders whose cross sections differ slightly from an elliptical shape. These results are then used to study the effect of surface roughness on the long wavelength, lowest lying surface modes of a cylinder of elliptical cross section. Numerically one can also determine the eigenfrequencies and shape of the eigenmodes of cylinders whose cross sections are not so close to elliptical. [1] "Magnetostatic modes in ferromagnetic nanowires", Rodrigo Arias and D.L. Mills, Phys. Rev. B, Vol. 70 (9), 094414 (2004). [Preview Abstract] |
Monday, March 21, 2005 3:42PM - 3:54PM |
D41.00007: Study of the Magnetic Structure in Gd5Ge4 L. Tan, J.W. Kim, A. Kreyssig, R. Mcqueeney, D. Wermeille, B. Sieve, A. Goldman, S. Budko, Thomas Lograsso, V. Pecharsky, K. Gschneidner, D. Schlagel Gd$_{5}$Ge$_{4}$ - Gd$_{5}$Si$_{4}$~based compounds show promising applications due to magneto-caloric, magneto-strictive, and magneto-resistance properties. Gd$_{5}$Ge$_{4}$ crystallizes in the orthorhombic space group \textit{Pnma}, and orders antiferromagnetically below the temperature $T_{N}$~$\sim $~ 127~K. The details of the magnetic structure, however, have not been determined because of the strong neutron absorption of natural Gd. We have employed X-ray Resonant Magnetic Scattering (XRMS) to elucidate the details of the magnetic structure. The magnetic unit cell is the same as the chemical unit cell. Azimuthal scan of the (0 3 0) magnetic peak indicates that the main magnetic moment component lies along the \textbf{\textit{c}} axis. From the azimuthal scan and the $Q$ dependence of the magnetic scattering, all three atomic sites are in the same magnetic space group \textit{Pnm'a}. No clear component of magnetic moment along the \textbf{\textit{a}} direction was seen. [Preview Abstract] |
Monday, March 21, 2005 3:54PM - 4:06PM |
D41.00008: Structure of Short Range Magnetic Order in the half-doped layered perovskite La$_{1.5}$Sr$_{0.5}$CoO$_{4}$ A.T. Savici, I. Zaliznyak, G. Gu, J. Tranquada, R. Erwin We have investigated the short range magnetic ordering in La$_{1.5}$Sr$_{0.5}$CoO$_{4}$ by elastic neutron scattering. The magnetic scattering is diffuse, with broad peaks characteristic of short range correlations. Such scattering is often described by replacing the delta-functions in the Bragg scattering cross- section by the corresponding multi-dimensional Lorentzians. However, this approach is not quite satisfactory. First, because it extends the small-q Orstein-Zernike approximation beyond the region of its validity and second, because the multi-dimensional Lorentzians cannot be properly normalized. On the other hand, in the case of a disorder introduced by the linear topological defects such as magnetic disclinations, the scattering is described by a simple expression in the form of the factorized ``lattice Lorentzians'' (I. A. Zaliznyak, S.-H. Lee, in Y. Zhu (Ed.), Modern Techniques for Characterizing Magnetic Materials, Kluwer Academic, New York (2005)).We compare our data with both expressions and show that it is inconsistent with a system of linear magnetic disclinations. [Preview Abstract] |
Monday, March 21, 2005 4:06PM - 4:18PM |
D41.00009: Time-resolved spin dynamics studies of ferromagnetic thin films grown by molecular beam epitaxy Vladimir Alexandru Stoica, Roberto Merlin, Rosa Alejandra Lukaszew, Roy Clarke Results are presented for pump-probe experiments that are using ultrafast laser excitation and detection of coherent spin waves in ferromagnetic thin films. Spin precession around the magnetic field is excited by an intense laser pulse whereas a weaker pulse monitors the real-time spin dynamics by means of detecting the magneto-optical Kerr effect (MOKE) changes. The phase, amplitude, frequency and damping of the spin waves are experimentally determined for magnetic field values between 500 and 5000 Oe. The thickness dependency of the magnetic anisotropy is estimated for the case polycrystalline and epitaxial ferromagnetic films. Materials considered under the present study (Ni, Co and Fe) are grown by molecular beam epitaxy (MBE) on semiconducting and insulating single crystal substrates. [Preview Abstract] |
Monday, March 21, 2005 4:18PM - 4:30PM |
D41.00010: Spin dynamics in the stripe phase of the cuprate superconductors Brian Andersen, Per Hedegard Within a model that supports stripe spin and charge order coexisting with a d-wave superconducting phase, we study the self-consistently obtained electronic structure and the associated transverse dynamical spin susceptibility. In the coexisting phase of superconducting and static stripe order, the resulting particle-hole continuum can strongly damp parts of the low-energy spin wave branches. This provides insight into recent inelastic neutron scattering data revealing the dispersion of the low-energy collective magnetic modes of lanthanum based cuprate superconductors. [Preview Abstract] |
Monday, March 21, 2005 4:30PM - 4:42PM |
D41.00011: Spin Dynamics of a Canted Antiferromagnet in a Magnetic Field Randy Fishman The spin dynamics of a canted antiferromagnet with a quadratic spin-wave dispersion near ${\bf q} =0$ possesses a unique signature. When the anisotropy gap is negligible, the spin-wave stiffness $D_{sw}({\bf q},B) = (\omega_{\bf q}-B)/q^2$ depends on whether the limit of zero field or zero wavevector is taken first. Consequently, $D_{sw} $ is a strong function of magnetic field at a fixed wavevector. Even in the presence of a sizeable anisotropy gap, the field dependence of the extrapolated ${\bf q} = 0$ gap energy $\Delta_0(B)$ distinguishes a canted antiferromagnet from a phase-separated mixture containing both ferromagnetic and antiferromagnetic regions. For a ferromagnet, $d\Delta_0 /dB =1$ whereas for a canted antiferromagnet, $d\Delta_0/dB > 1$. Calculations performed for a generalized Villain model with additional anisotropy terms are used to demonstrate these ideas. These results are used to demonstrate that the ``ferromagnetic'' regions in Pr manganites are actually canted. [Preview Abstract] |
Monday, March 21, 2005 4:42PM - 4:54PM |
D41.00012: Spin-Flop Transition in the Stripe Phase of $\bf La_{1.875}Ba_{0.125}CuO_4$ Markus Huecker, Gen D. Gu, John M. Tranquada The static magnetic properties of the spin stripe phase in $\rm La_{1.875}Ba_{0.125}CuO_4$ were studied by means of magnetization measurements. Two magnetic transitions associated with the stripe order were observed. At the low temperature structural transition, which induces the charge stripe order, the spin dimensionality changes from Heisenberg to XY. Below the spin stripe ordering temperature a spin-flop in the spin stripes is observed for a strong magnetic field applied parallel to the $\rm CuO_2$-planes, i.e. parallel to the XY- plane. The field and temperature dependence of the spin susceptibility and the spin stripe structure is analyzed. {\it This work was supported by the U.S. Department of Energy, Division of Material Science, Contract No. DE-AC02-98CH10886.} [Preview Abstract] |
Monday, March 21, 2005 4:54PM - 5:06PM |
D41.00013: Magnetic Profiles at Engineered Perovskite LSMO Heterointerfaces J.J. Kavich, J.W. Freeland, R.H. Kodama, M.P. Warusawithana, J.N. Eckstein One of the current challenges presented by manganites is the poor spin polarization of tunnel currents in spintronic devices due to degradation of the magnetic order at interfaces. The systems investigated are perovskite La$_{1-x}$Sr$_{x}$MnO$_{3}$(300{\AA}) with a thin cap layer of STO(8{\AA}) and a similar structure with a modified LaMnO$_{3}$(8{\AA}) /STO(8{\AA}) cap grown by ozone-assisted ALL-MBE. Detailed fitting of x-ray resonant magnetic scattering (XRMS) spectra allows direct characterization and comparison of the interface magnetic profiles. The XRMS indicates a smoothly varying profile of $\sim $40 {\AA} depth with dramatically reduced surface magnetization and a reversible evolution as a function of temperature. Comparison of the magnetic profiles shows that changing the capping layer (equivalent to reducing the interface hole doping) has a negligible effect on the reduced magnetic order. Use of the Advanced Photon Source was supported by the U.S. Department of Energy, Office of Science, under Contract No. W-31-109-Eng-38. [Preview Abstract] |
Monday, March 21, 2005 5:06PM - 5:18PM |
D41.00014: Ballistic magnon transport in the antiferromagnet Nd2CuO4 Louis Taillefer, Shiyan Li, C.H. Wang, X.H. Chen The in-plane thermal conductivity of the antiferromagnetic insulator Nd$_{2}$CuO$_{4}$ was measured down to 50 mK. In zero magnetic field, the system is in a noncollinear state with gaps in all magnon branches so that only phonons carry heat. In an in-plane field of 10 T, a spin-flop transition causes the system to be in a collinear state, in which significant magnon transport has been observed above 2 K. We show that below 0.5 K these magnons travel ballistically, with a thermal conductivity that varies as $T^{3}$ which confirms the existence of acoustic magnons and allows us to extract their velocity. At higher temperature, we can extract the temperature dependence of: 1) magnon scattering of acoustic phonons; 2) phonon scattering of acoustic magnons. [Preview Abstract] |
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D41.00015: In-plane spin wave modes in permalloy antidot arrays – observation and analysis Chengtao Yu, Michael Pechan, Gary Mankey Previously, we have reported demagnetization field induced localized modes[1] in-plane at 35 GHz ferromagnetic resonance, and dipolar-exchange governed lateral standing spin waves out-of-plane at 9.7 GHz in permalloy antidots. Here we present in-plane investigations at 9.7 GHz on various hole arrays (hole diameter 1.5$\mu$m; hole lattice 3$\mu$m x 3, 4, 5, and 7$\mu$m). In addition to the two main localized modes, which arise from regions confined by holes along the long axis and short axis (region A and B, respectively), spin wave manifolds pertinent to each peak are identified. Owing to the confinement imposed by the holes as well as the demagnetization field, region A and B exhibit distinct resonance geometry. For instance, for field along short axis, region A and B are in Damon-Esbach and magnetostatic backward volume mode geometry respectively, with the spin wave vectors determined by hole separations along long and short axis. This is reversed with field along long axis. The dispersion of the observed spin waves is analyzed accordingly. Supported by US DOE FG02-86ER45281 (MU) and NSF DMR-0213985 (UA). $^{1}$Chengtao Yu, Michael J. Pechan, G. J. Mankey, Appl. Phys. Lett. \textbf{83,} 3948 (2003). [Preview Abstract] |
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