Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session A33: Focus Session: Magnetization and Spin Dynamics I |
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Sponsoring Units: GMAG DMP Chair: Michael Flatté, University of Iowa Room: E143 |
Monday, March 15, 2010 8:00AM - 8:12AM |
A33.00001: Element-selective magnetization dynamics probed using ultrafast soft x-ray beams S. Mathias, C. La-O-Vorakiat, M. Siemens, M. Murnane, H. Kapteyn, M. Aeschlimann, P. Grychtol, R. Adam, C. Schneider, J. Shaw, H. Nembach, T. Silva We use few-femtosecond soft x-ray pulses from high-harmonic generation to extract elementally-specific demagnetization dynamics of a compound material for the first time. Using a geometry where high-harmonic beams are reflected from a magnetized Permalloy grating, we observe large changes in the reflected soft x-ray intensity of up to 6\% at the M absorption edges of Fe (54eV) and Ni (67eV) when the magnetization is reversed. In a second experiment, an ultrashort laser pulse is used to destroy the magnetic alignment, which allows us to measure the fastest, elementally-specific, demagnetization dynamics to date, with 55 fs time-resolution. Both Fe and Ni demagnetize on the same timescales in this strongly exchange coupled material. The use of high harmonics for probing magnetic materials with nanometer spatial resolution, elemental specificity, and femtosecond-to-attosecond time resolution will be discussed. [Preview Abstract] |
Monday, March 15, 2010 8:12AM - 8:24AM |
A33.00002: Time Resolved X-ray Imaging of Magnetic Dynamics in Perpendicularly Magnetized Nanopillars David Bernstein, Keng Chou, Roopali Kukreja, Bj\"{o}rn Br\"{a}uer, Tolek Tyliszczak, Jordan Katine, Joachim St\"{o}hr, Yves Acremann Spin Transfer Torque (STT) provides a novel means to manipulate magnetic bits. Previous time-resolved scanning transmission x-ray microscopy (STXM) experiments have shown that in-plane magnetized nanomagnets switch via vortex motion through or virtual vortex motion around a metallic pillar[1]. Here we present results for perpendicularly magnetized samples. It has been suggested that such samples have a smaller critical switching current than those magnetized in-plane[2]. We find that the samples switch via domain wall propagation across the sample. Pillars of 300nm width switch from anti-parallel to parallel over 700 ps when subjected to 2 ns pulses, yielding a domain wall speed of approximately 400 m/s once the domain wall has formed. The reset pulse revealed a slower propagation, taking 1.3 ns, perhaps due to the difference in spin-polarization reflected by and transmitted through the polarizing layer. \\[4pt] [1] J.P. Strachan, \textit{et. al.} Phys. Rev. Lett. 100, 247201 (2008)\\[0pt] [2] S. Mangin, \textit{et. al}, Appl. Phys. Lett. 94, 012502 (2009) [Preview Abstract] |
Monday, March 15, 2010 8:24AM - 8:36AM |
A33.00003: Separating spin-dependent interaction mechanisms in femtosecond optical pulse excitation of ferromagnetic metals Vladimir Stoica, Christian Schlepuetz, Lynn Endicott, Donald Walko, Yuelin Li, Eric Landahl, Roy Clarke Coherent spin dynamics phenomena in ferromagnetic metals can be excited and observed using femtosecond lasers. Several interaction mechanisms are presently considered as the dominant driving force in influencing the spin direction trough optical pulses. These include ultrafast heating and the spin-photon, spin-orbit, and spin-lattice coupling. We employ epitaxial films and multilayers that are suitable for optical studies of the role of magnetic anisotropy combined with picosecond X-ray diffraction observation of the lattice dynamics. By using temporal and spatial separation of the excitation and detection mechanisms, we are able to show that coherent excitations are possible without the need of direct spin-photon interaction. We demonstrate that when diminishing the heating of the lattice, one can study the spin interaction with elastic waves. At large laser fluences, we observe anisotropic nonlinear behavior that is related with the magnetic field dependency of the lattice relaxation. [Preview Abstract] |
Monday, March 15, 2010 8:36AM - 9:12AM |
A33.00004: Femtosecond magnetization dynamics using the inverse Faraday effect Invited Speaker: |
Monday, March 15, 2010 9:12AM - 9:24AM |
A33.00005: Optical Control of Ultrafast Spin-wave Relaxation by Magnetic Anisotropy in a Ferromagnet Kevin Smith, Y. Fan, R.A. Lukaszew, J.R. Skuza, C. Clavero, K. Yang, O. Amponsah, N. Noginova, A. Reilly, G. Lupke Ultrafast switching of magnetization requires a detailed understanding and control of spin-wave excitation and relaxation, which is important in spintronic applications such as magnetic logic and memories. To this end, there has been a recent wealth of activity in utilizing ultrafast lasers and the Time-Resolved Magneto-Optical Kerr Effect (TR-MOKE) to generate and study spin wave dynamics. However, there is still an incomplete understanding of damping and reports have shown that TR-MOKE in particular, while capturing the precession frequency quite well, often yields stronger damping than that seen in ferromagnetic resonance (FMR) experiments. In this talk, we demonstrate that an ultrafast pump pulse can control the effective damping by interacting with magnetic anisotropy in 10 nm thick Ni(001)/MgO(001) thin-films and that TR-MOKE and FMR are inconsistent for geometries in which the magnetization is pulled away from the easy axis. We thus introduce a novel optically mediated decay mechanism: pump-induced anisotropic damping (PIAD). [Preview Abstract] |
Monday, March 15, 2010 9:24AM - 9:36AM |
A33.00006: Spin wave scattering in the ferromagnetic cross Alexander Kozhanov, Dok Won Lee, Shan X. Wang, Ajey Jacob, S. James Allen Small scale magnetostatic wave devices are potentially important for on-chip filters for communication systems and more exotic gated spin wave devices. We describe experimental results that measure spin wave scattering in the thin film CoTaZr ferromagnetic cross. We fabricated the CoTaZr cross on the Si/SiO$_{2}$ wafer. Shorted coplanar waveguides placed over the cross arms were used to excite and detect the spin waves. Spin waves were excited in one of the arms of the cross. Scattered spin waves were detected in the other 3 arms with use of the vector network analyzer (frequency range 0-20GHz, in-plane magnetic fields 0-2000 Oe). Measurements of the fabricated structure show a strong dependence of the spin wave scattering direction and scattered spin wave amplitude on the external magnetic field amplitude and on the angle at which the field is applied. We discuss effect of biasing magnetic fields on the spin wave scattering and approaches to an effective spin wave switch based on the fabricated structure. [Preview Abstract] |
Monday, March 15, 2010 9:36AM - 9:48AM |
A33.00007: Ferromagnetic resonance (FMR) in ferromagnet/normal metal bilayer system Taewan Noh, Danwen Song, Venkat Chadrasekhar We have performed ferromagnetic resonance measurements in ferromagnet/normal metal bilayers. Our sample was made by depositing thin film of gold on top of permalloy. With a GHz ac magnetic field applied parallel to the sample, we measured the transmission while sweeping the dc field. In addition to the ferromagnetic resonance coming from the permalloy film, we observed additional structure which we associate with a finite magnetization in the normal metal film. [Preview Abstract] |
Monday, March 15, 2010 9:48AM - 10:24AM |
A33.00008: Ferromagnetic resonance linewidth and damping in perpendicular-anisotropy magnetic multilayers thin films Invited Speaker: Transition metal ferromagnetic films with perpendicular magnetic anisotropy (PMA) have ferromagnetic resonance (FMR) linewidths that are one order of magnitude larger than soft magnetic materials, such as pure iron (Fe) and permalloy (NiFe) thin films. We have conducted systematic studies of a variety of thin film materials with perpendicular magnetic anisotropy to investigate the origin of the enhanced FMR linewidths, including Ni/Co and CoFeB/Co/Ni multilayers. In Ni/Co multilayers the PMA was systematically reduced by irradiation with Helium ions, leading to a transition from out-of-plane to in-plane easy axis with increasing He ion fluence [1,2]. The FMR linewidth depends linearly on frequency for perpendicular applied fields and increases significantly when the magnetization is rotated into the film plane with an applied in-plane magnetic field. Irradiation of the film with Helium ions decreases the PMA and the distribution of PMA parameters, leading to a large reduction in the FMR linewidth for in-plane magnetization. These results suggest that fluctuations in the PMA lead to a large two magnon scattering contribution to the linewidth for in-plane magnetization and establish that the Gilbert damping is enhanced in such materials ($\alpha \simeq 0.04$, compared to $\alpha \simeq 0.002$ for pure Fe) [2]. We compare these results to those on CoFeB/Co/Ni and published results on other thin film materials with PMA [e.g., Ref. 3]. \newline [1] D. Stanescu \textit{et al.}, J. Appl. Phys. \textbf{103}, 07B529 (2008). \newline [2] J-M. L. Beaujour, D. Ravelosona, I. Tudosa, E. Fullerton, and A. D. Kent, Phys. Rev. B RC \textbf 80, 180415 (2009). \newline [3] N. Mo, J. Hohlfeld, M. ulIslam, C. S. Brown, E. Girt, P. Krivosik, W. Tong, A. Rebel, and C. E. Patton, Appl. Phys. Lett. \textbf{92}, 022506 (2008). \newline \newline *Research done in collaboration with: A. D. Kent, New York University, D. Ravelosona, Institut d'Electronique Fondamentale, UMR CNRS 8622, Universit\'{e} Paris Sud, E. E. Fullerton, Center for Magnetic Recording Research, UCSD, and supported by NSF-DMR-0706322. [Preview Abstract] |
Monday, March 15, 2010 10:24AM - 10:36AM |
A33.00009: Ferromagnetic resonance of individual magnetic bilayer microwire structures Snorri Ingvarsson, Mustafa Arikan, Yat-Yin Au Ferromagnetic resonance measurements were done on individual magnetic bilayer microwires. The magnetic layers are separated by a nonmagnetic spacer layer of Cu. Magnetic precession is excited by current running both above the wire structure and through its cross section. Two magnetic precession modes, namely the acoustic (in-phase precession of the two magnetic layers) and the optical (out-of-phase) mode, were clearly revealed. Their frequency dependencies on applied magnetic field were measured, fitted with simple theoretical predictions and explained in the context of interlayer magnetostatic dipolar coupling. The effectiveness of exciting the two precession modes by injected microwave electric current was studied against different layer dimensions. The results are highly relevant to fast switching of small magnetic devices relying on Amperian field generated by in-plane current inside the devices. [Preview Abstract] |
Monday, March 15, 2010 10:36AM - 10:48AM |
A33.00010: ABSTRACT WITHDRAWN |
Monday, March 15, 2010 10:48AM - 11:00AM |
A33.00011: Theoretical foundation of the time-resolved magneto-optical Kerr effect for femtosecond magnetism Guoping Zhang, Wolfgang H\"ubner, Georg Lefkidis, Yihua Bai, Thomas F. George Laser-induced femtosecond magnetism or femtomagnetism opens a new frontier for a faster magnetic storage device, but to probe such a rapid magnetization change challenges the existing experimental and theoretical wisdom. We establish a new paradigm through a first-principles investigation in ferromagnetic nickel [1]. We show that the time-resolved optical and magnetic responses energetically follow their respective optical and magneto-optical susceptibilities; as a result, the one-to-one correspondence between them sensitively depends on the incident photon energy. For a shorter laser pulse, a delay of 10 fs in the magnetic signal with respect to the optical one is revealed through a phase-sensitive polarization-magnetization diagram; for a longer pulse, such a delay diminishes and the correlation can be established unambiguously [2].\\[4pt] [1] Zhang {\it et al}, Nature Physics {\bf 5}, 499 (2009);\\[0pt] [2] Zhang, Phys. Rev. Lett {\bf 101}, 187203 (2008). [Preview Abstract] |
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