Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session B32: Cooperative Phenomena (incl. Spin Structures, Spin Waves, Phase Transitions) I |
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Sponsoring Units: GMAG Chair: Paul Haney, National Institute of Standards and Technology Room: 336 |
Monday, March 16, 2009 11:15AM - 11:27AM |
B32.00001: Point Contact Andreev Reflection Spin Spectroscopy in MnSi Single Crystals Pushkal Thapa, Raghava Panguluri, F. Carbone, D. van der Marel, A.A. Nugroho, B. Nadgorny We have carried out a detailed investigation of magnetic and spin transport properties of MnSi, an itinerant helimagnet with the weak Dzyaloshinski-Moria interaction. MnSi single crystals were grown in the form of a rod by the floating zone technique and we used the center of the rod which is clean and free from any impurities for all our measurements. Magnetization measurements suggest the critical temperature, T$_{c}$ of $\sim $ 29 K with the saturation magnetic moment of $\sim $ 0.4$\mu _{B}$/Mn at 2 K. A T$^{2}$ -- dependence of the resistance is observed near T$_{c}$, indicating that the transport is governed by spin fluctuations with a residual resistivity $\sim $ 20 $\mu \Omega $.cm. We used the point contact Andreev reflection (PCAR) technique with electrochemically etched superconducting niobium tips to observe the effects of spin polarization in a variable magnetic field as the magnetic structure of MnSi gradually transformed from helical - to conical - to weakly ferromagnetic. [Preview Abstract] |
Monday, March 16, 2009 11:27AM - 11:39AM |
B32.00002: High Pressure studies of the magnetic phase transition in MnSi Sergei Stishov, Alla Petrova, Vladimir Krasnorusski, Thomas Lograsso New measurements of AC magnetic susceptibility and DC resistivity of a high quality single crystal MnSi were carried out at high pressure making use of helium as a pressure medium. The form of the AC magnetic susceptibility curves at the magnetic phase transition suddenly changes upon helium solidification. This implies strong sensitivity of magnetic properties of MnSi to non hydrostatic stresses and suggests that the early claims on the existence of a tricritical point at the phase transition line are probably a result of misinterpretation of the experimental data. At the same time resistivity behavior at the phase transition does not show such a significant influence of helium solidification. The sharp peak at the temperature derivative of resistivity, signifying the first order nature of the phase transition in MnSi successfully survived helium crystallization and continued the same way to the highest pressure. [Preview Abstract] |
Monday, March 16, 2009 11:39AM - 11:51AM |
B32.00003: Ultrasonic studies of the magnetic phase transition in MnSi Alla Petrova, Sergei Stishov We report results of the ultrasonic studies of a single crystal of MnSi in the temperature range 4-150 K, performed using digital pulse echo techniques. All the elastic constants and their combinations except for $c_{44}$ reveal profound rounded dips at temperature ${\approx}$ 29.6 K with discontinuities at the low temperature sides of the dips at ${\approx}$ 28.9 K, which corresponds to the magnetic phase transition in MnSi. Both of these features are beautifully correlate with the heat capacity, thermal expansion and resistivity data, obtained with the same single crystal of MnSi, but the sharp peaks of the mentioned quantities are replaced by the modest discontinuities in the elastic properties at the phase transition. The sound attenuation displays a double peak structure and looks like an almost exact copy of the corresponding curves, characterizing behavior of the heat capacity, thermal expansion and resistivity in the vicinity of the phase transition in MnSi. All the features observed correspond to a first order phase transition at 28.9 K. [Preview Abstract] |
Monday, March 16, 2009 11:51AM - 12:03PM |
B32.00004: High-Energy Magnon Dispersion and Multimagnon Continuum in La$_2$CuO$_4$ Neil Headings, Stephen Hayden, Radu Coldea, Toby Perring We report high-energy neutron scattering measurements of the magnetic excitations in the S=1/2 antiferromagnet La$_2$CuO$_4$. Measurements were made using the MAPS time-of-flight spectrometer at the ISIS spallation source. Previous measurements found evidence for higher order (cyclic) exchange couplings. We find evidence for significant corrections to linear spin-wave (SW) theory including these higher-order exchange constants. In particular, the intensity of the spin wave pole deviates strongly from that predicted by SW theory near the $\mathbf{Q}=(1/2,0)$ position. We also find evidence for a multi-magnon continuum. [Preview Abstract] |
Monday, March 16, 2009 12:03PM - 12:15PM |
B32.00005: Collective spin-excitations in Cu L and O K edge Resonant Inelastic X-ray Scattering on Sr$_{14}$Cu$_{24}$O$_{41}$. J. Schlappa, T. Schmitt, F. Vernay, V.N. Strocov, B. Thielemann, H.M. Ronnow, J. Mesot, B. Delley, L. Patthey, V. Ilakovac Using Resonant Inelastic X-ray Scattering (RIXS) at the Cu$ L_{3}$ and O $K$ edge we studied the spin-ladder/spin-chain compound Sr$_{14}$Cu$_{24}$O$_{41}$ [1,2]. We observe collective spin-excitations from the ladders, which we assign to two-Triplon modes [3]. At the Cu $L_{3}$ resonance the dispersion of the modes was mapped out, obtaining excellent sensitivity over the full first Brillouin-zone, in contrast to Inelastic Neutron Scattering [4]. At the O $K$ edge site-sensitive hole-excitation studies of the chain- and ladder-subsystems were performed, giving insight into the character of the holes. [1] T. Vuletic et al., Physics Reports 428, 169-258 (2006). [2] A. Kotani and S. Shin, Rev. Mod. Phys. 73, 203 (2001). [3] K. P. Schmidt and G. S Uhrig. Mod. Phys. Rev. Lett. 90, 227204 (2003). [4] S. Notbohm et al., Phys. Rev. Lett. 98, 027403 (2007). [Preview Abstract] |
Monday, March 16, 2009 12:15PM - 12:27PM |
B32.00006: Antiferromagnetic instability in Sr$_3$Ru$_2$O$_7$: stabilized and revealed by dilute Mn impurities Muhammed Hossain, B. Bohnenbuck, Y.-D. Chuang, E. Cruz, H.-H. Wu, L.H. Tjeng, I.S. Elfimov, Z. Hussain, B. Keimer, G.A. Sawatzky, A. Damascelli X-ray Absorption Spectroscopy (XAS) and Resonant Elastic Soft X-ray Scattering (RESXS) studies have been performed on Mn-doped Sr$_3$Ru$_2$O$_7$, both on the Ru and Mn L-edges, to investigate the origin of the metal insulator transition. Extensive simulations based on our experimental findings point toward an intrinsic antiferromagnetic instability in the parent Sr$_3$Ru$_2$O$_7$ compound that is stabilized by the dilute Mn impurities. We show that the metal-insulator transition is a direct consequence of the antiferromagnetic order and we propose a phenomenological model that may be applicable also to metal-insulator transitions seen in other oxides. Moreover, a comparison of Ru and Mn L-edge data on 5\% Mn doped system reveals that dilute Mn impurities are generating much more intense signal than Ru which is occupying 95\% of the lattice sites. This suggests the embedding of dilute impurities as a powerful mean to probe weak and, possibly, spatially inhomogeneous order in solid-state systems. In collaboration with: Y. Yoshida (AIST), J. Geck, D.G. Hawthorn (UBC), M.W. Haverkort, Z. Hu, C. Sch\"{u}\ss ler-Langeheine (Cologne), R. Mathieu, Y. Tokura, S. Satow, H. Takagi (Tokyo), J.D. Denlinger (ALS). [Preview Abstract] |
Monday, March 16, 2009 12:27PM - 12:39PM |
B32.00007: Collapse of the Hyperfine Magnetic Field at the Ru site in GdRu$_2$ and HoRu$_2$ D. Coffey, M. DeMarco, P.-C. Ho, T. Sayles, M. B. Maple, J. W. Lynn, Q. Huang The M\"{o}ssbauer Effect(ME) is frequently used to investigate magnetically ordered systems. One usually assumes that the magnetic order induces a hyperfine magnetic field, $H_{hyper}$, at the ME active site. This is the case in the ruthenates where the temperature dependences of $H_{hyper}$ at $^{99}$Ru sites track the magnetic order. This is not the case in GdRu$_2$ and HoRu$_2$. Specific heat, magnetization, and susceptibility show that there is ferromagnetic order below 93K in GdRu$_2$. Neutron diffraction data reveal that HoRu$_2$ orders ferromagnetically at 15.30(4) K with an ordered moment of 7.98(8) $\mu_B$. However there is no evidence of a correspondingly large $H_{hyper}$ in the $^{99}$Ru ME in either material. {\it Ab initio} calculations shows that spin polarization occurs only on the rare earth sites with $ H_{hyper} < 5T$ on the Ru sites. The results are compared with the corresponding calculations for ferromagnetic SrRuO$_3$. [Preview Abstract] |
Monday, March 16, 2009 12:39PM - 12:51PM |
B32.00008: Neutrons Scattering Study of Magnetic Correlations in Y$_{0.7}$La$_{0.3}$VO$_3$ Sung Chang, Jiaqiang Yan, Robert McQueeney \textit{R}VO$_3$ ($R =$~rare earth) perovskite vanadites exhibit multiple orbital and spin orderings and provide a unique opportunity to study the spin-orbital-lattice coupling of $\pi$-bonding \textit{t} electrons. The nature of the orbital order in these vanadites has been a matter of significant controversy, particularly with respect to whether the different orbital-ordered phases of YVO$_3$ and LaVO$_3$ are best described by a novel orbital-Peierls model or more traditional Jahn-Teller interactions. Here we report on a neutron scattering study of the magnetic correlations in Y$_{0.7}$La$_{0.3}$VO$_3$, which may be expected to depend sensitively on the orbital degrees of freedom. The results are discussed in terms of the interplay between magnetism and orbital order. [Preview Abstract] |
Monday, March 16, 2009 12:51PM - 1:03PM |
B32.00009: ABSTRACT WITHDRAWN |
Monday, March 16, 2009 1:03PM - 1:15PM |
B32.00010: Possible Observation of Electromagnons in BiFeO3 Crystal Pauline Rovillain, Yann Gallais, Alain Sacuto, Ricardo Lobo, Doroth\'ee Colson, Delphine Lebeugle, Rogerio De Sousa We show the first optical observation via inelastic light scattering of sharp low energy modes in multiferroic BiFeO$_{3}$. We have detected two different modes ($\Psi $ and $\Phi $ modes) and interpreted them as the optical fingerprint of the cycloidal magnetic order. The energies of the $\Phi $ modes are equally spaced and exhibit a linear dependence as a function of the mode index (n) down to zero while the energy sequence of the $\Psi $ modes is not equally spaced and exhibit a gap at low energy. Our experimental data are in remarkable agreement with the theory of de Sousa and Moore who pointed out that these modes could possibly be observed due to coupling between spins and lattice degrees of freedom. These experimental findings offer a unique opportunity for optical probing of the electromagnetic excitations of multiferroics. [1] Cazayous et al., Phys. Rev. Lett. 101, 097003 (2008). [Preview Abstract] |
Monday, March 16, 2009 1:15PM - 1:27PM |
B32.00011: Magnon Sidebands and Spin-Charge Coupling in Bismuth Ferrite Probed by Nonlinear Optical Spectroscopy Amit Kumar, M. Ramirez, S. Denev, N. Podraza, X. Xu, R. Rai, Y-H. Chu, J. Seidel, L. Martin, J. Ihlefeld, J. Klug, M. Bedzyk, O. Auciello, D. Schlom, R. Ramesh, J. Orenstein, J. Musfeldt, V. Gopalan The interplay between spin waves (magnons) and electronic structure in materials leads to the creation of additional bands associated with electronic energy levels, called magnon sidebands which are difficult to probe due to their smaller energy scales (meV). Linear light absorption and scattering techniques at low temperatures are traditionally used to probe these sidebands. We show that optical second harmonic generation (SHG) can successfully probe the magnon sidebands at room temperature and up to 723K in bismuth ferrite, associated with large wave-vector multi-magnon excitations which linear absorption studies have thus far been unable to resolve. Polarized light studies and the temperature dependence of these sidebands reveal a spin-charge coupling interaction between the spontaneous polarization ($P_{s}$) and antiferromagnetic order parameter, $L$ in bismuth ferrite, that persists with short range correlation well into the paramagnetic phase. [Preview Abstract] |
Monday, March 16, 2009 1:27PM - 1:39PM |
B32.00012: Aging in Co/Cr Superlattices T. Mukherjee, M. Pleimling, Ch. Binek Aging phenomena are observed in various systems brought into non-equilibrium and subsequently showing slow relaxation dynamics. Magnetic specimens with well defined interactions and dimensions can serve as model systems for universal aspects of aging. Magnetic thin films provide access to a wide range of microscopic parameters. Superlattice structures allow tuning the intra and inter-plane exchange and enable geometrical confinement of the spin fluctuations. We use Co/Cr thin film superlattices to study magnetic aging. The static and dynamic properties are affected via the Co and Cr film thicknesses. T$_{C}$ of the Co films is reduced from the bulk value by geometrical confinement. Non-ergodic behavior sets in at a tunable temperature $T^{\ast }$ in a range of some 100K above zero. Cr provides antiferromagnetic coupling between the Co films. Non-equilibrium spin states are set via low field cooling in 5mT in-plane magnetic field to below $T^{\ast }$. Next various in-plane magnetic set fields of some 10-100 mT are applied and the sample is exposed to the latter for various waiting times $t_{w}$, respectively. After removing the field, relaxation of the magnetization is recorded via longitudinal Kerr-magnetometry. The relaxation data are analyzed by scaling plots revealing universal aspects of aging. Financial support by Teledyne-Isco, NRI, and NSF through EPSCoR, Career DMR-0547887, and MRSEC. [Preview Abstract] |
Monday, March 16, 2009 1:39PM - 1:51PM |
B32.00013: Exchange-coupling modified spin wave spectra in the perpendicularly magnetized permalloy nanodot chain arrays Jian Dou, Sarah C. Hernandez, Chengtao Yu, Michael J. Pechan, Liesl Folks, Jordan A. Katine, Matthew J. Carey Spin wave spectra in exchange coupled nanoscale dot chain arrays were studied using ferromagnetic resonance. The dot chain arrays, with dot diameters of 300 nm and thicknesses of 40 nm, coupled via permalloy bridges of width ranging from 0 to 60 nm, were fabricated using e-beam lithography. In the perpendicularly magnetized isolated dots, multiple sharp ferromagnetic resonant peaks were observed\footnote{G.N.Kakazei et al, Appl. Phys. Lett. \textbf{85}, 443 (2004)}, which is associated with the quantized in-plane wave vector due to the finite dot radius. These spectrum lines shift to higher fields for samples with wider bridges due to the increasing effective demagnetizing factor. Additional higher order spin wave modes were observed as satellite peaks near the resonance peaks at both higher and lower fields, with larger separation between adjacent spin wave peaks for wider bridge samples. These extra spin wave modes, associated with the inter-dot exchange coupling,will be described in detail. This work is supported by US Dept. of Energy at MU. [Preview Abstract] |
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