Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session P1: Recent Advances in Magnetization Dynamics |
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Sponsoring Units: DCMP Chair: Jonathan Sun, IBM T.J. Watson Research Center Room: Colorado Convention Center Four Seasons 2-3 |
Wednesday, March 7, 2007 11:15AM - 11:51AM |
P1.00001: Imaging fast spin dynamics at the nanoscale with soft x-ray microscopy Invited Speaker: Nanoscale and multicomponent magnetic systems are attracting both fundamental interest and are widely used in technological applications such as high density magnetic storage and sensor devices. The challenge to modern magnetic microscopies is to image magnetic microstructures in such specimens with high spatial and time resolution and elemental specificity. Magnetic soft X-ray microscopy is a novel technique combining a spatial resolution down to currently 15nm, elemental sensitivity due to X-ray magnetic circular dichroism used as huge magnetic contrast mechanism and a sub-ns time resolution limited by the current time structure of the synchrotron radiation used as source for circularly polarized soft X-rays. We report on recent results and achievements in magnetic soft X-ray microscopy obtained at the full-field soft X-ray microscopy beamline 6.1.2 (XM-1) located at the Advanced Light Source in Berkeley CA. Magnetization reversal processes at the grain level in a nanogranular CoCrPt system were studied with 15nm spatial resolution to obtain insight into spin fluctuations on a fundamental length scale. The inherent elemental sensitivity of XMCD contrast allows e.g. in (coupled) multilayered magnetic systems to explore their microscopic magnetization reversal process with layer resolution. Spin dynamics in magnetic nanostructures can be addressed by a stroboscopic pump and probe scheme utilizing the inherent time structure of synchrotron radiation, where the pump is a fast electronic pulse launched into a waveguide structure to excite the spin dynamics of a magnetic nanoelement. Varying the delay time between the pump and the probing x-ray flash one can follow the time development of e.g local spin and vortex dynamics and relaxation phenomena, but also spin-torque driven domain wall displacements with sub-ns time resolution. Current developments of X-ray optics aim to achieve better than 10nm spatial resolution. At upcoming high brillant ultrast X-ray sources snapshots of spin dynamics with fs time resolution recorded with magnetic soft X-ray microscopy can be foreseen. Many thanks to D.-H. Kim, B. Mesler, W. Chao, R. Oort, E. Anderson, G. Meier, R. Eiselt, M. Bolte, M.-Y. Im, S.-C. Shin, S. Mangin, E. Fullerton. [Preview Abstract] |
Wednesday, March 7, 2007 11:51AM - 12:27PM |
P1.00002: The Spin Transfer Torque Critical Current in Magnetic Nanopillars Invited Speaker: Spin transfer in magnetic nanopillar has become a major focus of experimental research since Slonczewski and Berger's seminal theoretical work in 1996. A spin current has been demonstrated to switch the magnetization direction of a small magnet at a specific current density, as well as to induce microwave excitations. However, there are basic questions about the factors that control the critical current density for magnetization dynamics. For instance, in Slonczewski's model, spin angular momentum transfer occurs at ferromagnetic/non-magnetic interfaces and competes with bulk magnetization damping. This model predicts a critical current that scales linearly with ferromagnet layer thickness and extrapolates to zero in the limit of zero thickness. In this talk I will present experiments on Co (10 nm) /Cu (10 nm) /Co ($t$) nanopillars in which the Co free-layer thickness, $t$, has been varied from $2$ to $5$ nm. The critical current has been studied at low-temperature as a function of applied magnetic field perpendicular to the plane of the layers. The critical current decreases linearly with decreasing free-layer thickness, but extrapolates to a finite critical current in the limit of zero thickness, while the junction magnetoresistance is independent of thickness [1]. The limiting current is in agreement with that expected due to a spin-pumping contribution to the magnetization damping. It is also consistent with our FMR studies of Co films, which indicate an enhancement of the magnetization damping in ultra-thin ($<4$ nm thick) Co layers due to spin-pumping [2]. Finally, I will discuss more recent studies of nanopillars with Ni/Co multilayer free layers. In these experiments, the role of the magnetic easy plane anisotropy can be explored, as this anisotropy varies with the number of Ni/Co interfaces within a fixed film thickness [3]. \\ 1. W. Chen, et al., Phys. Rev. B 74, 144408 (2006) \\ 2. J-M. Beaujour et al., Phys. Rev. B 74, 214405 (2006) \\ 3. J-M. Beaujour et al., cond-mat/0611027 [Preview Abstract] |
Wednesday, March 7, 2007 12:27PM - 1:03PM |
P1.00003: Very low damping in epitaxial Fe and Fe1-xVx films Invited Speaker: |
Wednesday, March 7, 2007 1:03PM - 1:39PM |
P1.00004: Spin Momentum Transfer and Oersted Field Induce a Vortex Nano-Oscillator in Thin Ferromagnetic Film Devices Invited Speaker: A nonlinear model of spin-wave excitation involving a point contact in a thin ferromagnetic film that includes the Oersted magnetic field contribution is presented. We consider the case of an external dc field applied perpendicular to the film plane. The two-dimensional vectorial model reduces to an exact one-dimensional equation of motion. Large-amplitude vortex modes are computed, which represent a fundamental shift in the geometrical understanding of spin transfer nano-oscillators. Odd symmetry forces the magnetization to be pinned in the center of the point contact. Using the spin transfer efficiency as a single fitting parameter, the calculated dependence of frequency on current and contact size is in good agreement with recent experimental data. These vortex states are geometrically very different from previously computed cylindrical modes that exhibit even symmetry when the Oersted field is ignored. [Preview Abstract] |
Wednesday, March 7, 2007 1:39PM - 2:15PM |
P1.00005: Fingerprinting Magnetic Nanostructures by First Order Reversal Curves Invited Speaker: Realistic systems of magnetic nanostructures inevitably have \textit{inhomogeneities}, which are manifested in distributions of magnetic properties, mixed magnetic phases, different magnetization reversal mechanisms, etc. The first order reversal curve (FORC) method [1-3] is ideally suited for ``fingerprinting'' such systems, both qualitatively and quantitatively. Here we present recent FORC studies on a few technologically important systems. In arrays of Fe nanodots [4], as the dot size decreases from 67 to 52nm, we have observed a vortex state to single-domain transition. Despite subtle changes in the major hysteresis loops, striking differences are seen in the FORC diagrams. The FORC method also gives quantitative measures of the magnetic phase fractions and vortex nucleation and annihilation fields. Furthermore, with decreasing temperature, it is more difficult to nucleate vortices within the dots and the single domain phase fraction increases. In exchange spring magnets [3], we have investigated the reversibility of the soft and hard layers and the interlayer exchange coupling. In FeNi/polycrystalline-FePt films, the FeNi and FePt layers reverse in a continuous process via a vertical spiral. In Fe/epitaxial-SmCo films, the reversal proceeds by a reversible rotation of the Fe soft layer, followed by an irreversible switching of the SmCo hard layer. As the SmCo partially demagnetizes, the Fe layer still remains reversible, as revealed by second order reversal curves (SORC). The exchange coupling between the two layers can be extracted as a function of the SmCo demagnetization state. These results demonstrate that FORC is a powerful method for magnetization reversal studies, due to its capability of capturing magnetic inhomogeneities, sensitivity to irreversible switching, and the quantitative phase information it can extract. Work done in collaboration with J. E. Davies, R. K. Dumas, J. Olamit, C. P. Li, I. V. Roshchin, I. K. Schuller, O. Hellwig, E. E. Fullerton, J. S. Jiang, S. D. Bader, J. Wu, C. Leighton, H. G. Katzgraber, C. R. Pike, R. T. Scalettar, G. T. Zimanyi, and K. L. Verosub. \newline \newline [1] C. R. Pike, et al, JAP \textbf{85}, 6660 (1999). \newline [2] H. G. Katzgraber, et al. PRL \textbf{89}, 257202 (2002). \newline [3] J. E. Davies, et al, PRB \textbf{70,} 224434 (2004); APL \textbf{86,} 262503 (2005); PRB \textbf{72}, 134419 (2005). \newline [4] K. Liu, et al., APL. \textbf{81}, 4434 (2002). [Preview Abstract] |
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