Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session V33: Cooperative Phenomena- Experiment, Spin Dynamics and Materials |
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Sponsoring Units: GMAG Chair: Shireen Adenwalla, University of Nebraska-Lincoln Room: E143 |
Thursday, March 18, 2010 8:00AM - 8:12AM |
V33.00001: Incommensurate Spin Wave Structures in FeP and FeAs Efrain E. Rodriguez, Mark A. Green, Pawel Zajdel, Shanta R. Sahar, Kevin Kirshenbaum, Nicholas P. Butch, Johnpierre Paglione Determining the magnetic ground states in ternary and quaternary iron pnictides is relevant towards understanding the superconductivity displayed in some phases. We investigate the magnetic structure and properties of the simple binary phosphide and arsenide of iron in order to better understand the properties of iron pnictides. The crystal structure of FeP and FeAs is orthorhombic and related to the hexagonal NiAs-type structure. Both FeP and FeAs order magnetically below 300 K and their magnetic structures reported in the past literature consist of simple spirals incommensurate with the chemical lattice and moments directed transverse to the propagation vector of the spin wave. With neutron powder diffraction data, we explore other spin wave structures with representational analysis of its crystal symmetry. We further compare the different models with transport and magnetic susceptibility measurements on single crystalline samples. Overall, we study the constraints posed from both the nature of the magnetic phase transitions and their crystal symmetries to find the spin structures that best describe the magnetic ground states of FeP and FeAs. [Preview Abstract] |
Thursday, March 18, 2010 8:12AM - 8:24AM |
V33.00002: Magnetic transition in the 1D spin-chain system SrCo$_{2}$V$_{2}$O$_{8}$ Ben-Li Young, C.S. Lue, Zhangzhen He We report the $^{51}$V NMR study on the quasi-one-dimensional spin system SrCo$_{2}$V$_{2}$O$_{8}$. Incommensurate antiferromagnetic transition in this material is revealed from the splitting of the spectrum peak from the paramagnetic phase. When applying magnetic fields, spin reorientation is observed before entering the paramagnetic phase for fields both parallel and perpendicular to the c axis. Similar spin reorientation is also observed if by varying the temperature across the Neel temperature. We have obtained the $^{51}$V NMR spectra for three different field angles (0$^{\circ}$, 45$^{\circ}$, 90$^ {\circ}$) between the c axis. A detailed analysis of this antiferromagnetic structure will be discussed. [Preview Abstract] |
Thursday, March 18, 2010 8:24AM - 8:36AM |
V33.00003: Magnetic structures of Tm$_{2}$PdSi$_{3}$ studied by neutron diffraction in zero and finite magnetic fields Fei Tang, Matthias Frontzek, Jens-Uwe Hoffmann, Michael Loewenhaupt $R_{2}$PdSi$_{3}$ ($R$ = rare earth) compounds crystallize in hexagonal AlB$_{2}$ structure (P 6/mmm) with rare earth ions on Al positions and Pd/Si ions on B positions. A crystallographic superstructure caused by Pd and Si ordering is observed, which is closely related to a generic magnetic structure observed in finite magnetic fields from earlier studies. However, the mechanism leading to the generic magnetic structure is yet not clear although crystal electric field effects are supposed to play an essential role. Neutron diffraction measurements have been performed on a Tm$_{2}$PdSi$_{3}$ single crystal on the flat cone diffractometer E2 at HZB. In zero field at 0.4 K (well below $T_{N}$ = 2 K) a magnetic structure with propagation vector \textbf{\textit{$\tau $}} = (1/8, 1/8, 1/16) was identified. In addition, extra magnetic satellites have been observed which can only be explained if starting from the (nuclear) superstructure reflections. Evolution of the magnetic structures in magnetic fields applied along 001 has been followed up to 4 T. The generic magnetic structure found in other $R_{2}$PdSi$_{3}$ ($R$ = Tb, Ho and Er) compounds was also observed in Tm$_{2}$PdSi$_{3}$ in finite magnetic fields. In this contribution we will present the neutron diffraction data and their interpretation and discuss the relation between the observed magnetic structures and the crystallographic superstructure. [Preview Abstract] |
Thursday, March 18, 2010 8:36AM - 8:48AM |
V33.00004: A high-resolution x-ray resaonant exchange scattering study of the magnetic structure of PrNi$_2$Ge$_2$ J. C. Lang, Z. Islam, G. Srajer, P. C. Canfield, L. Bouchanoire, S. Brown The zero-field magnetic structure of PrNi$_2$Ge$_2$ has been determined using high-resolution x-ray resonant magnetic scattering at the Pr L$_2$ edge. Magnetic susceptibility measurements had previously indicated the presence of two distinct ordering transitions at T$_N$=20.4K and T$_t$=7.7K respectively. Below TN, magnetic Bragg peaks were observed of the form q=(0,0,q$_z$) consistent with Fermi-surface nesting vector in RNi$_2$Ge$_2$ series compounds. q$_z$ reaches a value of 0.8128$\pm$0.0002 c$^*$ for temperatures below $\sim$12K. Intensity measurements as a function of scattering wave vector Q are consistent with moments aligned along the c axis for all temperatures. No obvious changes in intensity or wave vector were observed at the T$_t$ transition. [Preview Abstract] |
Thursday, March 18, 2010 8:48AM - 9:00AM |
V33.00005: Non-magnetic Ground State of a Pr-based Caged Compound PrRu$_{2}$Zn$_{20}$ Takahiro Onimaru, Keisuke Matsumoto, Yukihiro F. Inoue, Kazunori Umeo, Yuta Saiga, Toshiro Takabatake, Yoshitaka Matsushita, Kazue Nishimoto, Ryuji Tamura Recently there has been considerable interest in rare-earth based compounds with caged structures, because they show a variety of ground states originating from the caged structures, for example, heavy-fermion superconductivity in PrOs$_{4}$Sb$_{12}$, a phase transition attributed to scalar-type multipolar degrees of freedom in PrFe$_{4}$P$_{12}$, etc. We have studied Pr-based caged compounds of PrRu$_{2}$Zn$_{20}$ crystallizing in the cubic CeCr$_{2}$Al$_{20}$-type structure, where a Pr-ion is encapsulated in a cage formed of sixteen zinc atoms. In analogy of the filled-skutterudite structure, the large coordination number of the Pr-ion, CN=16, suggests weak crystalline electric field (CEF) effect and strong hybridization of the cage and $f$ electrons of the Pr-ion. In PrRu$_{2}$Zn$_{20}$, the magnetic susceptibility obeys the Curie-Weiss law above 25 K, suggesting the trivalent state of the Pr-ion suffering from weak CEF effect. A Schottky peak of the specific heat appearing at 2.5 K is the manifestation of a first excited state located around 7 K above the CEF ground state. No phase transition has been observed down to 0.4 K, indicating the non-mangnetic singlet ground state. [Preview Abstract] |
Thursday, March 18, 2010 9:00AM - 9:12AM |
V33.00006: Nonlinear Response in strongly diluted $LiH_x Y_{1-x} F$ Michael Schmidt, Thomas Rosenbaum, Daniel Silevitch, Gabriel Aeppli $LiH_x Y_{1-x} F$ is a physical manifestation of the Ising model in transverse field, where substitution of Ho ions with magnetically inert Y generates random, internal transverse fields at the spin sites. In the low-temperature, small x regime, the spatial anisotropy of the dipolar interaction, disorder, and the random fields compete to balance tendencies towards glassy and spin liquid behavior and emphasize the effects of the nonlinear response. By measuring the ac magnetic susceptibility of single crystals with x=0.045 in both internal and external transverse fields, we explore the nonlinear dynamics and the stability of the glassy state. [Preview Abstract] |
Thursday, March 18, 2010 9:12AM - 9:24AM |
V33.00007: ABSTRACT WITHDRAWN |
Thursday, March 18, 2010 9:24AM - 9:36AM |
V33.00008: Microstructural, magnetic anisotropy, and magnetic domain structure correlations in $L$1$_{0}$ FePd thin films J. R. Skuza, C. Clavero, B. Wincheski, W. Chen, O. Amponsah, N. Noginova, R. A. Lukaszew Understanding microstructural, magnetic anisotropy, and magnetic domain structure correlations in materials with large perpendicular magnetic anisotropy (PMA) is of fundamental interest and it is also important in many technological applications such as next generation magneto-recording media and spin-transfer torque devices. $L$1$_{0}$ ordered phases in some binary alloys (FePd, FePt, MnAl) have PMA due to chemical ordering that can be controlled with adequate thin film deposition parameters. A detailed study on epitaxial FePd thin films grown by dc magnetron sputter deposition on MgO(001) substrates will be presented. XRD, MFM, SQUID magnetometry, and FMR were used to investigate structure-property correlations in these films. A quantitative analysis and correlation of the strong PMA to magnetic domain structure in these FePd thin films was accomplished with good agreement using an analytical energy model[1] and builds on previous work that had only correlated magnetic domain structure with film thickness.[2],[3] [1] C. Kooy and U. Enz, Philips Res. Reports \textbf{15}, 7 (1960). [2] V. Gehanno \textit{et al.}, J. Magn. Magn. Mater. \textbf{172}, 26 (1997). [3] V. Gehanno \textit{et al.}, Phys. Rev. B \textbf{55}, 12552 (1997). [Preview Abstract] |
Thursday, March 18, 2010 9:36AM - 9:48AM |
V33.00009: Surface Acoustic Wave Induced Magnetic Switching S. Davis, A. Baruth, S. Adenwalla We report on the use of Surface Acoustic Waves (SAW) to switch the magnetization direction of lithographically patterned 40um by 10um cobalt rectangles between two titanium inter-digital transducers (IDTs) on Y-cut LiNbO$_{3}$. Easy and hard axis magnetization loops measured using the magneto-optical Kerr effect (MOKE) show the expected in-plane shape anisotropy. After magnetic saturation along the long easy axis, the magnetic field is turned off and the IDT's are excited at the fundamental resonance frequency, 91.5 MHz, producing a SAW that travels across the patterned Co magnetic structure producing a fast time dependent mechanical strain parallel to the short hard axis of the Co. Magneto-elastic coupling results in a rotation of the magnetization into the hard axis direction, measured by in-plane MOKE measurements along the hard axis direction. Both dc MOKE and high frequency MOKE show, as expected, a definite turn on voltage followed by an asymptotic approach to saturation. Support from NSF MRSEC (DMR-0820521), UCARE, and NFC-Minnesota. [Preview Abstract] |
Thursday, March 18, 2010 9:48AM - 10:00AM |
V33.00010: Interaction of surface acoustic waves in the GHz frequency regime with ferromagnetic thin films R. Huber, S. Neusser, D. Grundler, M. Weiler, S.T.B. Goennenwein, M. Schneider, P. Boeni We report GHz spectroscopy on ferromagnetic(FM)/piezoelectric hybrid systems consisting of FeCoV or Co on LiNbO substrates, where surface acoustic waves (SAWs) propagate through the FM film. Thereby we study the dynamic coupling of mechanical motion and magnetization in the GHz frequency regime. The 50 nm thick FM films are deposited between two nanopatterned interdigital transducers (IDTs). Using a vector network analyzer connected to the IDTs we detect the variations in amplitude $A$ and phase $\Phi $ of the SAWs when applying in-plane magnetic fields $H$ under different angles $\alpha $. [1] When the SAW frequency is intentionally below the ferromagnetic resonance (FMR) frequency we observe a hysteretic behavior of $A$ and $\Phi $ when varying $H$. Depending on the magnetic anisotropy we find either a fourfold symmetry (FeCoV) or a much more complex behavior (Co) of $\Phi $ vs $\alpha $. We explain this by magnetoelastic effects. When SAW and FMR frequencies are almost degenerate, no clear hysteresis is observed for $A$. [1] R. Huber et al., Solid State Comm., in press. [Preview Abstract] |
Thursday, March 18, 2010 10:00AM - 10:12AM |
V33.00011: Effect of size on spin-transfer-driven ferromagnetic resonance in nanoscale magnetic tunnel junctions Zhongming Zeng, Kwun Hung Cheung, Hongwen Jiang, Ilya Krivorotov, J.A. Katine Recently spin-transfer-driven ferromagnetic resonance (ST-FMR) has been explored to investigate the spin dynamics in various magnetic structures. ~For ST-FMR in a magnetic tunnel junction, an RF current at ferromagnetic resonance generates a sizable DC voltage across the tunnel junction [1], which provides a simple, yet effective means, to electrically detect the spin dynamics of the nanoscale free layer magnetization. In this talk, we present our measurements of ST-FMR for a set of MgO-based nanopillar magnetic tunneling junctions of different pillar aspect-ratios and sizes ranging from 65nm x 50nm to 170nm x 90nm. These data provide useful information to analyze the spin dynamics properties, such as damping and resonance conditions. Specifically, we found a strong dependence of the FMR condition on the cell size. ~Pronounced asymmetry in the spectra shape was observed for larger cells. ~The results may shed some light on boundary conditions for dipole-exchange spin waves in thin-film nanomagnets.[1]A. A. Tulapurkar et al., Nature 438, 339 (2005). [Preview Abstract] |
Thursday, March 18, 2010 10:12AM - 10:24AM |
V33.00012: Local excitation of ferromagnetic resonance and its spatially-resolved detection by using an open-ended radio frequency probe Toshu An, Nobuhito Ohnishi, Toyoaki Eguchi, Yukio Hasegawa A local excitation of ferromagnetic resonance (FMR) and its detection were performed by using an open-ended radio frequency (RF) probe demonstrating the potential of the local probe for spatially resolved FMR microscopy. The RF probe is composed of a coaxial cable transmitting broadband RF waves over 10 GHz and a sharp tip attached to its end. As a magnetic sample, a 10 mm-diameter and 1 mm-thickness polycrystalline YIG (yttrium iron garnet) disk was used. The probe is set so that the tip is nearly contacted with a sample, and the reflection spectrum (S$_{11})$ was measured by using a vector network analyzer. Under in-plane static magnetic field of 550 Oe, by scanning the RF probe on the YIG disk, spatially dependent three FMR signals were detected at 2.79, 3.03 and 3.14 GHz. All of them showed strong spatial dependences over the sample, and these detected FMR signals are well explained by the magnetostatic waves, such as magneto static backward volume wave (MSBVW) and magnetostatic surface wave (MSSW). The used RF probe showed an ability of local prober of magnetic resonance signal, and its spatial resolution limit will be discussed. [Preview Abstract] |
Thursday, March 18, 2010 10:24AM - 10:36AM |
V33.00013: Time-resolved terahertz spectroscopy and Hall measurement on chromium vanadium thin films Amir Farahani, Saeid Kamal, Eric Fullerton, J. Steven Dodge We have measured the low-frequency dynamical conductivity of Cr$_{1-x}$V$_x$ thin films through the quantum phase transition at $x\approx 0.35$ using terahertz time-domain spectroscopy. From Drude model fits we have determined the plasma frequency $\omega_p$ of samples over concentrations $x=0-0.1$ and temperatures 10--300 K. We have compared these to the Hall resistance R$_H$ on the same samples and found that both reveal the opening of the spin-density wave gap. At high temperatures $\omega_p^2\propto 1/R_H$, but as the temperature is lowered below 75 K, $1/R_H$ falls more rapidly than $\omega_p^2$. We will relate these observations to a theoretical model based on a realistic Fermi surface. [Preview Abstract] |
Thursday, March 18, 2010 10:36AM - 10:48AM |
V33.00014: Influence of growth field on Py, Fe3O4, and Py/Cr/Fe3O4 spin-valves Priyanga Jayathilaka, Chris Bauer, Daryl Williams, Casey W. Miller Thin films of Ni80Fe20, Fe3O4, as well as Py/Cr/Fe3O4 spin valves, have been grown with and without magnetic fields applied during the deposition, and their magnetotransport properties have been studied at room temperature. The applied field induces an anisotropy in both single layer films, which causes notable differences in their anisotropic magnetoresistance. In the spin valve system, the applied field enables the parallel and antiparallel states to be more well-defined, which reveals a possible giant magnetoresistance in the system. The origin of this signal is likely the interaction of electrons that have been polarized by spin-dependent reflection from the Cr/Fe3O4 interface with the Ni80Fe20 interface [Preview Abstract] |
Thursday, March 18, 2010 10:48AM - 11:00AM |
V33.00015: Electronic structure of Fe/MgO and Fe/vacuum interfaces: QSGW theory Sergey Faleev, Oleg Mryasov Magnetic tunnel junctions (MTJs) and, in particular, Fe/MgO/Fe MTJ have attracted much attention recently due to discovery of large magneto-resistance. Here we study electronic structure of Fe/MgO and Fe/vacuum interfaces using recently developed Quasiparticle Self-Consistent GW (QSGW) theory [1]. We show that electronic structure of Fe/MgO calculated within the QSGW allows one to resolve the so-called `zero-bias anomaly' contradiction between experimental results and earlier LDA/DFT predictions for Fe/MgO/Fe MTJs. For Fe/MgO and Fe/vacuum systems the QSGW results are in a good agreement with the tunneling conductance measurements [2], but in contract with earlier LDA/DFT calculations [3,4]. Presented results show that accurate of electronic structure beyond LDA/DFT is necessary to describe correctly transport properties of MgO based MTJs. [1]. S. V. Faleev, M. van Schilfgaarde, and T. Kotani, Phys. Rev. Lett. \textbf{93}, 126406 (2004) ; M. van Schilfgaarde, T. Kotani, and S. V. Faleev, , Phys. Rev. Lett. \textbf{96}, 226402 (2006). [2]. P.-J. Zermatten, G. Gaudin, G. Maris, M. Miron, A. Schuhl, et.al., Phys. Rev. B \textbf{78}, 033301 (2008). [3]. W. H. Butler, X.-G. Zhang, and T. C. Schulthess, and J. M. MacLarenm$,$ Phys. Rev. B\textbf{ 63}, 054416 (2001). [4]. K. D. Belashchenko, J. Velev, and E. Y. Tsymbal, Phys. Rev. B \textbf{72}, 140404(R) (2005). [Preview Abstract] |
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