Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session K22: Focus Session: Magnetization Dynamics |
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Sponsoring Units: GMAG FIAP DMP Chair: Paul Crowell, University of Minnesota Room: Baltimore Convention Center 319 |
Tuesday, March 14, 2006 2:30PM - 2:42PM |
K22.00001: Time-domain diffracted magnetooptic Kerr probing of magnetization dynamics of patterned magnet arrays Xiaobin Zhu, Fabian Giesen, Zhigang Liu, Mark Freeman, Hiroyuki Akinaga We probe the ultrafast magnetooptic Kerr effect in the diffracted spots of a patterned array of magnets, which serve as diffraction grating. This technique, diffracted MOKE, provides valuable information, as the off-specular spots contain the finite spatial frequencies of the magnetic moment configuration [1-3]. In this talk, we will present the D-MOKE study in the time domain of a 2 $\mu $m sized Permalloy square array and a 1 $\mu $m sized Permalloy disk array. The time-domain measurements are performed using a time resolved scanning Kerr microscopy in its spectroscopic mode [4]. Due to spatial averaging, we find that time-domain curve shows faster damping compared with individual magnets. We also find that the sensitivity of the Kerr signal is enhanced at the first order. Through Fourier transformation of the time-domain data, and through comparison with micromagnetic simulation, information about magnetic normal modes in individual magnets is extracted. [1] M. Grimsditch, \textit{et al.}, J. Phys.: Condens. Matter \textbf{16} R275 (2004); [2] R. Antos\textit{, et al}., Appl. Phys. Lett., \textbf{86}, 231101 (2005); [3] N. Kida, \textit{et al.}, Phys. Rev. Lett. \textbf{94}, 077205 (2005). [4] W. K. Hiebert, \textit{et al.}, Phys. Rev. lett.,79, 1134 (1997). [Preview Abstract] |
Tuesday, March 14, 2006 2:42PM - 2:54PM |
K22.00002: Ultrafast Laser Spin Wave Measurement of Temperature Dependent Magnetic Anisotropy of Half-Metallic Chromium Dioxide Anne Reilly, Hailong Huang, Keoki Seu Recent work has shown that the all-optical ultrafast laser production and detection of spin waves can provide useful measurements of anisotropy in magnetic thin films[1,2]. A pump laser pulse momentarily affects the magnetic anisotropy, perturbing the magnetization, which relaxes towards a new equilibrium by means of coherent oscillation. The coherent oscillation is detected by the magnetoopitcal Kerr effect. We present a study of the temperature dependence of the magnetocrystalline anisotropy of half-metallic CrO2 thin films from temperatures 10 K to 363 K. At higher temperatures, the oscillations are approximately single frequency. The anisotropy constants obtained by analyzing the oscillation frequencies as a function of applied fields using the Landau Lifshitz Gilbert equation agrees with measurements made by other techniques, particularly ferromagnetic resonance. Interesting features of the temperature dependence, as compared to ferromagnetic metals such as Co, Ni and Fe, will be highlighted. 1. M. Van Kampen et al., Phys. Rev. Lett. 88, 227201 (2002) 2. D. Talbayev et al., Appl. Phys. Lett., 86, 182501 (2005) [Preview Abstract] |
Tuesday, March 14, 2006 2:54PM - 3:06PM |
K22.00003: Interface magnetization switching and precession in Fe/AlGaAs (001) Gunter Luepke, Haibin Zhao, Diyar Talbayev, Aubrey Hanbicki, Connie Li, Berend Jonker Understanding of interface magnetic properties is a key for efficient spin-polarized injection in semiconductor spintronic devices. Here, we have measured the reversal process of the Fe interface layer magnetization in Fe/AlGaAs (001) using magnetization-induced second harmonic generation (MSHG), and compared it with the bulk magnetization obtained from magneto-optic Kerr effect (MOKE). The switching characteristics are distinctly different -- single step switching occurs at the interface layer, while two-jump switching occurs in the bulk Fe for the magnetic field orientations employed. The different switching processes lead to a deviation angle of 40-85$^{o}$ between interface and bulk magnetization, which may result from reduced exchange interaction and different magnetic anisotropies at the interface. We also use time-resolved MSHG to investigate the coherent magnetization precession at the interface, and compare with the bulk spin precession obtained from time-resolved MOKE. The different switching behaviors are further revealed in the precession dynamics. The field dependence of precession frequency provides a quantitative analysis of magnetic anisotropies of the interface layer. [Preview Abstract] |
Tuesday, March 14, 2006 3:06PM - 3:18PM |
K22.00004: Excitation of single-frequency spin waves in exchange-biased IrMn/Co by ultrafast laser repinning Keoki Seu, Hailong Huang, Anne Reilly We have excited and detected coherent spin waves in exchange-biased IrMn/Co using the all-optical ultrafast laser pump probe technique. The laser pump produces changes in the exchange biasing which launches spin waves which are detected by the magnetooptical Kerr effect. The oscillations are single frequency and can be described according to ferromagnetic resonance equations. Unlike previous work in these systems [1-3], oscillations have been detected when the applied magnetic field is along any direction relative to the pinning axis, including directly along the easy axis, and in fields larger than the magnetic saturation. This is contrary to energetic arguments and predictions from the Landau Lifshitz Gilbert equation[1]. Our suggested mechanism for these oscillations is a destruction and re-pinning of the exchange-bias interaction, introducing a `kick' required for the magnetization to precess. Evidence of this ultrafast laser repinning will be presented. 1 Ganping Ju, et al., Phys. Rev. B. 62 1171 (2000). 2 M. C. Weber, et al., Eur. Phys. J. B. 45 243 (2005). 3 Ganping Ju, et al., Phys. Rev. Lett. 82 3705 (1999). [Preview Abstract] |
Tuesday, March 14, 2006 3:18PM - 3:30PM |
K22.00005: Optically induced magnetization dynamics in manganite films studied by time-resolved magneto-optical Kerr effect 1. Diyar Talbayev, Haibin Zhao, Gunter Luepke, Adyam Venimadhav, Jun Chen, Qi Li Magnetization precession is the fastest way to switch magnetic memory elements. We studied optically-induced magnetization dynamics in epitaxial La$_{2/3}$Sr$_{1/3}$MnO$_{3}$ (LSMO) and La$_{2/3}$Ca$_{1/3}$MnO$_{3}$ (LCMO) films on different substrates. Both uniform precession of magnetization and spinwaves were observed. The precession frequency is governed by magnetic anisotropy that depends on the strain state of the films. In LSMO, compressive and tensile strain induces uniaxial normal-to-plane anisotropy. Similar behavior was observed in tensile-strained LCMO. However, a large in-plane anisotropy was discovered in LCMO on NdGaO$_{3}$. The field-dependent relaxation times of the decaying precession are in 100 -- 1400 ps range. The corresponding Gilbert damping parameter is field-dependent and decreases sharply with increasing field in LSMO films. The research was supported in part by the National Science Foundation and the Department of Energy. [Preview Abstract] |
Tuesday, March 14, 2006 3:30PM - 3:42PM |
K22.00006: Laser induced ultrafast magnetization dynamics in CoPt multilayer A. Barman, S. Wang, H. Schmidt, A. Berger We study femtosecond laser induced magnetization dynamics in CoPt multilayer structures in the presence of an externally applied magnetic field. The dynamics revealed three different time regimes. The first few hundred femtoseconds show an instantaneous demagnetization, followed by a quick recovery within next few picoseconds. Subsequently, an oscillatory motion comes into play whose frequency is field dependent and attributed to the precessional motion of magnetization. The sudden heating with laser pulses causes a rapid change in the anisotropy of the sample that results in a change in the equilibrium orientation of magnetization and triggers a precession. The precessional dynamics completely decays within 100ps leaving it to a slower recovery of magnetization. Analysis of the time-resolved magnetization shows a single precessional mode at higher fields, while several modes become apparent at lower fields. The dependence of different frequency modes and their damping on the applied field are studied to understand the dynamics in more detail. The effects of the anisotropy field distribution and different multilayer compositions on the dynamic response will also be discussed. [Preview Abstract] |
Tuesday, March 14, 2006 3:42PM - 4:18PM |
K22.00007: Ultrafast coherent control of Spin- and magnetization dynamics Invited Speaker: The manipulating of the electron spin is not only relevant for magnetic storage but may also lead to the development of novel electronic devices with new characteristics (so-called spintronics). Therefore, the investigations of the physical mechanisms underlying the manipulation of electron spin in ferromagnets, semiconductors and hybrid ferromagnet/semiconductor structures constitute at present an exciting area of research. Due to the fact that in antiferromagnets no angular momentum is associated with the order parameter, spin dynamics in these materials is intrinsically much faster than in ferromagnets, expanding the area of spin-dynamics even more $^{[1]}$. Femto-second laser excitation opens the way to excite magnetic systems on a time scale much shorter than fundamental time scales such as spin-lattice relaxation or precession times. This has already lead to surprising and exciting results like changes in magnetization on a sub-picoscond time scale $^{[2]}$. Fs laser pulses can also be used to generate short magnetic field pulses, that allow coherent control of the magnetization dynamics $^{[3]}$ . Recent progress in this area will be discussed, demonstrating in particular the use of time resolved magneto-optical methods to investigate the static and dynamic properties of magnetically ordered structures and the possibility of direct spin manipulation with optical fields$^{[4,5]}$. [1] A. V. Kimel, A. Kirilyuk, A. Tsvetkov, R. V. Pisarev, and Th. Rasing, Nature 429 850 (2004). [2] \textit{Spin Dynamics in Confined Magnetic Structures I-II}, edited by B. Hillebrands and K. Ounadjela (Springer-Verlag, Berlin, 2002-2003). [3] Th. Gerrits, H. A. M. van den Berg, J. Hohlfeld, L. B\"{a}r, and Th. Rasing, Nature 418, 509 (2002). [4] A. Kimel, A. Kirilyuk, P.A. Usachev, R.V. Pisarev, A.M. Balbashov and Th. Rasing, Nature 435, 655 (2005) [5] F. Hansteen, A.V. Kimel, A. Kirilyuk and Th. Rasing, PRL 95, 047402-1 (2005). \textbf{Acknowledgements} This work was partially supported by the European IST network SPINOSA, the RTN network DYNAMICS, the Russian Foundation for Basic Research (RFBR), Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO) as well as Stichting voor Fundamenteel Onderzoek der Materie (FOM). [Preview Abstract] |
Tuesday, March 14, 2006 4:18PM - 4:30PM |
K22.00008: Ferromagnetic Resonance of a Single Nanoscale Magnet Driven by an RF Spin-Transfer Torque. Jack Sankey, P.M. Braganca, I.N. Krivorotov, A.G.F Garcia, D.C. Ralph, R.A. Buhrman By applying a spin-polarized RF current through Py/Cu/Py$_{0.65}$Cu$_{0.35}$ spin-valve nanopillar devices approximately 100 nm in cross section, we have excited ferromagnetic resonance modes in the Cu-doped Py ``free'' layer. We detect the resonance via a DC voltage generated through mixing of the RF current and the oscillating magnetoresistance from the precessing magnetization. We find two different regimes of behavior. For small applied DC currents, we observe simple ferromagnetic resonance. The frequency-widths of the resonant peaks are related to the intrinsic magnetic damping, with the damping decreasing approximately linearly with DC current. In this regime we can also characterize the nonlinearities of the resonances from peak-shape analysis at larger RF currents. For DC currents large enough to excite spontaneous magnetic precession, we find a second regime in which the applied RF current produces phase locking, with a distinctive line shape. In both regimes, we observe dynamical modes not seen previously in DC-driven dynamical measurements. [Preview Abstract] |
Tuesday, March 14, 2006 4:30PM - 4:42PM |
K22.00009: FMR spectroscopy of individual nanopillars Gregoire de Loubens, Vladimir V. Naletov, Olivier Klein We have performed FMR spectroscopy on perpendicularly magnetized hybrid nanopillars (lateral sizes range between 500 nm and 2 $\mu$m) using a Magnetic Resonance Force Microscope. The sensitivity of this technique allows us to measure an individual sample. Firstly, 100 nm thick Permalloy (Py) disks sandwiched by Cu have been studied. It is found that the lowest energy mode is an edge mode localized at surfaces instead of being the uniform precession. The study of the dynamics in the non linear regime shows that the damping of this edge mode decreases with increasing power, which favors its excitation at high power level compare to the other modes. Secondly, Py100/Cu10/Py10 (nm) devices showing GMR effect have been measured and we have studied the spectral changes induced by high DC current densities J flowing through them. FMR spectra at J=0 are compared to those of single magnetic layer disks and qualitatively interpreted. The influence of the Oersted field and of the temperature increase when J$\neq$0 is quantified and a small effect of spin transfer is extracted. [Preview Abstract] |
Tuesday, March 14, 2006 4:42PM - 4:54PM |
K22.00010: Ferromagnet dynamics in a driven spin valve Joern N. Kupferschmidt, Shaffique Adam, Piet W. Brouwer The magnetoresistance of a ferromagnet/normal-metal/ferromagnet trilayer depends on the relative orientation of the two magnetic moments. We analyze out-of-plane precession of the magnetization for a soft ferromagnetic layer in such a geometry, where the system is driven by an alternating charge current. We consider the effect of both spin-torque and spin-pumping on the magnetization dynamics and find that these have signatures in the magnetoresistance. [Preview Abstract] |
Tuesday, March 14, 2006 4:54PM - 5:06PM |
K22.00011: Strong interactions between a nanomagnet and a microcavity mode \"Oney O. Soykal, M.E. Flatt\'e We consider the interaction of a nanomagnet with a single mode of a microcavity in a fully quantum treatment. We derive the interaction Hamiltonian from Maxwell's equations by introducing the fully quantized forms of the spherical electromagnetic solutions. For a nanomagnet acting as a macrospin, within a cavity roughly 1 mm$^3$ in volume, the magnet-microwave mode coupling is $\sim 10^{-7}$eV. Strong coupling can therefore be seen if the quality factor of the cavity exceeds 1000. [Preview Abstract] |
Tuesday, March 14, 2006 5:06PM - 5:18PM |
K22.00012: Magnetic permeability and dielectric permittivity of ferrite materials in millimeter waves. Konstantin Korolev, Lakshmi Subramanian, Mohammed Afsar Magneto-optical approach for the measurements of complex magnetic permeability and dielectric permittivity of solid and powdered strontium ferrite materials have been performed in the frequency range of 34-120 GHz. Free-space quasi-optical millimeter wave spectrometer equipped with a backward wave oscillator as a tunable source of coherent radiation provides the transmittance spectra in transverse magnetic field up to 7.5 kOe. Frequency dependences of dielectric and magnetic parameters of strontium ferrites have been calculated by matching theoretical curves to the experimental transmittance spectra. Shift of the ferromagnetic resonance to higher frequencies and the broadening of the zone of strong absorption in transverse magnetic field have been observed for solid ferrite materials. A relation between the specific gravity and the saturation of magnetization for these materials has been found. The shift of ferromagnetic resonance frequency vs. specific gravity has been observed. The correlations between complex dielectric permittivity and magnetic permeability and density of the samples have been observed. [Preview Abstract] |
Tuesday, March 14, 2006 5:18PM - 5:30PM |
K22.00013: Multiphoton antiresonance in large-spin systems Christian Hicke, Mark Dykman We show that multiphoton resonance in a $S>1$ spin system is accompanied by anticrossing of the responses in resonating states. This anticrossing accompanies the standard anticrossing of quasienergy levels. It leads to antiresonance: the ratio of the vibration amplitude to the modulation amplitude in one of the states becomes much less and in the other state much larger than in the weak-modulation limit. The response anticrossing can be observed by adiabatically sweeping the modulation frequency through multiphoton resonance. The shape and overall width of the dip/peak of the response as function of frequency strongly depend on the modulation amplitude. The effect has no analog in two-level systems and is sensitive to the parameters of the spin. It is most pronounced when the spin Hamiltonian in the absence of driving is $H_0= \omega_0 S_z + \gamma S_z^2$. In this case several states experience multiphoton resonance at a time. The higher-order terms in $S_z^2$ lead to smearing of the antiresonance. We also study the response of the spin when the modulation is sharply turned on. The antiresonance leads to oscillations of the response with the multiphoton Rabi frequency. Their amplitude strongly depends on the modulation amplitude. The effect is compared with multiphoton antiresonance in a nonlinear oscillator [1]. [1] M. I. Dykman and M. V. Fistul, Phys. Rev. B {\bf 71}, 140508(R) (2005). [Preview Abstract] |
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