Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session V42: Magnetization Dynamics |
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Sponsoring Units: GMAG Chair: Manfred Fiebig, Max Born Institute Room: LACC 150B |
Thursday, March 24, 2005 11:15AM - 11:27AM |
V42.00001: Ab-initio theory of pump-probe experiments Andras Vernes, Peter Weinberger By linearizing the density of both the pump- and probe-excited states and neglecting the overlap between femtosecond laser pulses, the Kubo response theory is extended to describe pump-probe experiments in the visible optical regime. In this scheme second order responses are included, although it is formally a linear theory and therefore all obtained expressions can be implemented straightforwardly via a Green's function approach. In particular, already the time-dependent zeroth order dynamic conductivity as obtained by means of the spin-polarized relativistic screened Korringa-Kohn-Rostoker method for fcc Ni(100) predicts a demagnetization process of about 100 fs after the impact of the probe pulse, which is in reasonably good agreement with available experimental data. [Preview Abstract] |
Thursday, March 24, 2005 11:27AM - 11:39AM |
V42.00002: Ultrafast magnetization Dynamics of Antiferromagntic NiO M. Fiebig, N.P. Duong, T. Satoh The dynamics of antiferromagnetic (AFM) compounds is substantially different from that of ferromagnets: Magnetic switching is facilitated by the absence of a macroscopic magnetic moment, so that spin reversal within 10-100~fs was predicted, and the discreet states of AFM dielectrics can lead to long coherence times. We investigated the magnetization dynamics of the AFM exchange-bias compound NiO by optical pump/probe experiments using second harmonic generation as probe of the AFM order. A photoinduced ultrafast reorientation of the Ni$^{2+}$ spins within 100~fs was observed and detected by quantum beating between the hard- and easy-axis state of the crystal. The lifetime of the photoinduced state is 1~ns and limited by spin-lattice relaxation. However, a second laser pulse can trigger an ultrafast return of the system into the ground state so that controlled switching of an AFM order parameter was demonstrated for the first time. [Phys.\ Rev.\ Lett.\ {\bf 93}, 117402 (2004)] [Preview Abstract] |
Thursday, March 24, 2005 11:39AM - 11:51AM |
V42.00003: Terahertz Spectrocopy of Ultrafast Demagnetization in iron David Hilton, Richard Averitt, Chad Meserole, Greg Fisher, David Funk, Antoinette Taylor We use ultrafast terahertz transmission and emission spectroscopy to study ultrafast demagnetization in ferromagnetic iron. We show that the emitted THz pulse results from both the demangetization process and the time-dependent induced conductivity of the sample. Using the experimentally determined time-dependent optical conductivity, we determine that ultrafast demagnetization proceeds with a $1.7\,\mathrm {ps} \pm 0.5\,\mathrm{ps}$ lifetime in single crystal ferromagnetic films at room temperature. [Preview Abstract] |
Thursday, March 24, 2005 11:51AM - 12:03PM |
V42.00004: Time resolved optical studies of coherent spin waves in Fe/AlGaAs (001) Haibin Zhao, Diyar Talbayev, Qiguang Yang, Gunter Luepke, Aubrey Hanbicki, Connie Li, Olaf van't Erve, George Kioseoglou, Berry Jonker Coherent magnetization precessions are generated by ultrafast optical excitation in epitaxial Fe films grown on AlGaAs (001) over a wide range of applied magnetic fields. In a 10-nm thick Fe film, uniform magnetization precessions are excited along the in-plane easy axis [100], as well as along the hard axis [1-10]. At low excitation, the magnetic anisotropy constants and damping parameters are determined from the temporal evolution of the coherent magnetization precession. Standing spin waves as well as uniform precession are excited in a 50-nm thick Fe film, allowing simultaneous determination of anisotropy constants and exchange coupling stiffness. The comparison of spin wave strength between the two films reveals the significant influence of uniaxial magnetic anisotropy on the transient magnetic torque. This might be useful to achieve precession switching in nanomagnets where the uniaxial anisotropy is dominant. [Preview Abstract] |
Thursday, March 24, 2005 12:03PM - 12:15PM |
V42.00005: Ultrafast Laser -Induced Absorption Changes in Half-Metallic CrO2 Hailong Huang, Keoki Seu, Anne Reilly, William Egelhoff, Yagil Kadmon Half-metals are ferromagnetic materials in which the electrons are fully spin polarized at the Fermi level. They are important materials seeing application as spin injectors and other spintronics devices. Chromium dioxide (CrO2) is a simple half-metal which has shown experimentally the highest degree of spin polarization. Here we present our recent ultrafast optical study of CrO2. An ultrafast pump-probe technique was applied to investigate the laser-induced absorption change of the material. A dramatic change in induced absorption was observed near the phase transition temperature. The induced absorption was also dependent on the probe beam polarization relative to the crystal c axis. The experimental results are compared with the similar phenomena observed in half-metallic manganites[1]. We have also investigated the wavelength dependence of the absorption change. We will discuss the results based on the calculated band structure by previous researchers[2]. [1]. Y. H. Ren et al. J. Appl. Phys., 91, 7514 (2002) [2]. M. A. Korotin et al., Phys. Rev. Lett., 80, 4305 (1998) [Preview Abstract] |
Thursday, March 24, 2005 12:15PM - 12:27PM |
V42.00006: Ultrafast laser excitation of coherent spin waves in exchange-biased IrMn/Co Keoki Seu, Hailong Huang, Anne Reilly Recent experiments have shown that coherent magnetization oscillations could be set up by ultrafast laser pulses in any magnetic film by manipulation of the anisotropy(1). These experiments are in analogy with FMR, but with the benefit of direct access to the time domain, sub-micron spatial resolution and straightforward in-situ application. We have produced coherent spin waves in exchange-biased Co systems by ultrafast laser pump-probe magnetooptical Kerr effect (MOKE). A static magnetic field is applied at an angle from the pinning axis and ultrafast photoexcitation is used to spontaneously decouple the antiferromagnetic/ferromagnetic system through electron heating(2). We have observed single-frequency oscillations which depend weakly on exchange bias field strength and correspond to ferromagnetic resonance frequencies. The damping of these oscillations are strongly dependent on the exchange bias field strength indicating that the exchange bias field plays a role in the damping. We have measured these oscillations and damping as a function of the angle between the exchange bias field and applied field, and correlated these results to FMR predictions. References: 1 M. van Kampen et al., Phys. Rev. Lett. 88 227201 (2002) 2 G. Ju et al., Phys Rev Lett 82 3705 (1999), Phys Rev. B., 62 1171 (2000) [Preview Abstract] |
Thursday, March 24, 2005 12:27PM - 12:39PM |
V42.00007: FMR Study of Ferromagnetic Stability of Nb/Ni Multilayers and Trilayers Wentao Xu, S. Kryukov, Lance De Long, Chengtao Yu, M. Pechan, E. Navarro, J. Villegas, E. Gonzalez, Jose Vicent Ni(y)[Nb(x)/Ni(y)]$_{z}$ multilayers (ML) with z = 5, 8, and x = 23, 10 nm, and y = 2.5, 3.5, 5 nm, and Nb(x)/Ni(y)/Nb(x) and Ni(y)/Nb(x)/Ni (y) trilayers with x = 23, 200 nm and y = 5 nm, were investigated via FMR above and below the superconducting (SC) transition temperature. Absorption peaks in the SC state of ML broadened and shifted in applied DC field compared to the normal state; however, for y = 2.5 nm ML, the low- temperature resonances were enhanced due to the instability of ferromagnetism in thin Ni layers. The x = 10 nm ML exhibited stronger FMR than the x = 23 nm ML, indicating significant interlayer coupling exists between Ni layers in the SC state. Trilayer samples exhibited additional sharp resonances at low field, as well as a broad feature at higher field, in the SC state. [Preview Abstract] |
Thursday, March 24, 2005 12:39PM - 12:51PM |
V42.00008: Broadband ferromagnetic resonance in epitaxial magnetic thin films and bilayers Xiaobin Zhu, D. LaGarde, Z. Liu, M. Freeman, G. Woltersdorf, A. Omosendz, B. Kardasz, B. Heinrich Broadband ferromagnetic resonance is used to study spin dynamics in epitaxial Au(20)/Fe(16)/GaAs(100), Au(20)/Cr(20)/Fe(16)/GaAs, and Au(15)/Fe(30)/Au(21)/Fe(15)/GaAs structures, where the numbers represent the atomic layers. Angular and external bias field dependence of resonance frequency for single magnetic layers could be explained by considering the coexistence of fourfold crystalline anisotropy and a uniaxial anisotropy. The damping parameters are obtained through fitting the experimental data. For Fe film covered with Au, $\alpha $ (0.0037) is consistent with bulk material. However for the film covered with Cr, substantial faster decay of the precessional amplitude is observed [1]. In the magnetic bilayers, the probe beam probes both magnetic layers. Through Fourier transforming the time-domain signals, two resonance frequencies were found: one corresponds to the top layer and the other one corresponds to the bottom layer. The bias field dependence of the resonance frequency for each thin layer however cannot simply be explained by the behavior of each isolated layer, which suggests there exists magnetic coupling, i.e. weak exchange coupling or dynamic dipolar coupling. [1] B. Heinrich et al, PRL 90, 187601 (2003) [Preview Abstract] |
Thursday, March 24, 2005 12:51PM - 1:03PM |
V42.00009: Spin pumping and magnetization dynamics in ferromagnet-Luttinger liquid junctions Cristina Bena, Leon Balents We study spin transport between a ferromagnet with time-dependent magnetization and a conducting carbon nanotube or quantum wire, modeled as a Luttinger liquid. The precession of the magnetization vector of the ferromagnet due for instance to an outside applied magnetic field causes spin pumping into an adjacent conductor. Conversely, the spin injection causes increased magnetization damping in the ferromagnet. We find that, if the conductor adjacent to the ferromagnet is a Luttinger liquid, spin pumping/damping is suppressed by interactions, and the suppression has clear Luttinger liquid power law temperature dependence. We apply our result to a few particular setups. First we study the effective Landau-Lifshitz-Gilbert (LLG) coupled equations for the magnetization vectors of the two ferromagnets in a FM-LL-FM junction. Also, we compute the Gilbert damping for a FM-LL and a FM-LL-metal junction. [Preview Abstract] |
Thursday, March 24, 2005 1:03PM - 1:15PM |
V42.00010: Current induced dynamics in thin ferromagnets Shaffique Adam, Mikhail L. Polianski, Piet W. Brouwer Recent theoretical work by Polianski and Brouwer, Phys. Rev. Lett. 92, 026602 (2004), and experimental work by Oezyilmaz, Kent, Sun, Rooks, and Koch, Phys. Rev. Lett. 93,176604 (2004) demonstrate that a sufficiently strong unpolarized electric current induces transverse spin-wave instabilities in thin, asymmetric ferromagnetic nanojunctions for only one polarity of the current. Here we present analytic and numerical calculations for the ferromagnet dynamics in the linear and non-linear regimes. [Preview Abstract] |
Thursday, March 24, 2005 1:15PM - 1:27PM |
V42.00011: Nonlinear Theory of Microwave Generation by Spin-Polarized Current Andrei Slavin, Pavel Kabos An approximate analytic theory of microwave spin wave generation by spin-polarized direct current in magnetic nano-contacts magnetized in an arbitrary direction is developed. For sufficiently large density of spin-polarized current the damping in the magnetic ``free'' layer is compensated and a quasi-uniform precession of magnetization about the direction of the \textit{internal }bias magnetic field \textbf{H} (which differs from the direction of the \textit{external} bias field \textbf{H}$_{e}$ applied to the ``free'' layer) is excited. The precession amplitude is subsequently limited by the positive nonlinear dissipation of the same precession. With the increase of the current magnitude $I $the angle of precession increases, making precession \textit{nonlinear}, and \textit{reducing the projection M}$_{z}$ of the precessing magnetization vector on the axis of precession ($z$-axis). This reduction of $M_{z}$ is responsible for the observed frequency shifts of the generated microwave oscillations and for the limitation of their amplitudes. Due to the influence of demagnetizing fields in the ``free'' layer the nonlinear frequency shifts have different magnitudes and signs for different orientations of the external bias field \textbf{H}$_{e}$. The theory gives good qualitative and even partly quantitative explanation of the majority of recent experimental results on microwave generation by direct current in nano-contacts. [Preview Abstract] |
Thursday, March 24, 2005 1:27PM - 1:39PM |
V42.00012: Time-Domain Spin Filter in Resonant Tunneling Structures Roberto Romo, Sergio Ulloa Solutions of the time-dependent Schr\"{o}dinger's equation are used to explore the spin-dependent tunneling in resonant structures from a dynamical point of view. The transmission of the spin-polarized electrons incident from the left occurs in such a way that they appear at the right edge of the system at time scales that depend on their particular spin orientation. This effect provides us with a mechanism for spin filtering in the time domain. [Preview Abstract] |
Thursday, March 24, 2005 1:39PM - 1:51PM |
V42.00013: Energy-resolved inelastic electron scattering off magnetic impurities Markus Garst, Leonid Glazman, Peter W\"{o}lfle We study inelastic scattering of energetic electrons off a Kondo impurity. If the energy $E$ of the incoming electron (measured from the Fermi level) exceeds the Kondo temperature $T_K$ significantly then the differential inelastic cross-section $\sigma (E,\omega)\equiv d\sigma (E)/d\omega$, characterising scattering of an electron with a given energy transfer $\omega$, is well-defined. We show that $\sigma (E,\omega)$ factorizes into two parts dependent on $E$ and $\omega$, respectively. The $E$--dependence is logarithmically weak and is due to the Kondo renormalization of the effective coupling. We are able to relate the $\omega$--dependent factor to the spin-spin correlation function of the magnetic impurity. Using this relation, we find two different regimes in the $\sigma$ {\it vs.} $\omega$ dependence: the cross-section grows as $\sigma\propto\omega$ at $\omega \ll T_K$, and upon reaching a maximum at $\omega\sim T_K$, starts falling off as $\sigma\propto [\omega\ln^2(\omega/T_K)]^{-1}$. At finite temperature the ``scattering gap'' for small $\omega$ is filled and only a broad peak at zero energy transfer remains for $T > T_K$. We also find $\sigma (E,\omega)$ in the presence of a magnetic field. The differential inelastic scattering cross section determines the relaxation of hot electrons injected in a metal with magnetic impurities. [Preview Abstract] |
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V42.00014: Theory of the Spin-EPR shift Prasanta Misra, Gouri Tripathi, R.K. Das We use a Green's function approach and include the effects of an external magnetic field, spin-orbit interaction and the interaction between the conduction electron spin and local magnetic moments(c-l interaction)to derive a theory for the spin-contribution to the Electron-Paramagnetic Resonance (EPR) shift. This contribution is proportional to the effective g-factor, density of states and the strength of the c-l interaction. We also consider inter-band effects for application to semiconductors. The EPR shift can shed light on the interaction between conduction electrons and/or holes (in case of semiconductors) with local magnetic moments of the magnetic ions embedded either periodically or otherwise in an electronic system. We calculate the spin-EPR shift(Ps) at the Mn2+ ion in the diluted magnetic semiconductor Pb1-xMnxTe. Ps shows large anisotropy due to the spin-orbit interaction. In addition to the dominant two-band interaction, contributions due to far bands are also included in our calculation. There is good agreement with the experimental results. [Preview Abstract] |
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