Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session J32: Focus Session: Damping and Spin Relaxation |
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Sponsoring Units: GMAG DMP FIAP Chair: Casey Miller, University of South Florida Room: Morial Convention Center 225 |
Tuesday, March 11, 2008 11:15AM - 11:51AM |
J32.00001: Spin-orbit damping in transition metals Invited Speaker: Magnetization dynamics are routinely described with the Landau-Lifshitz-Gilbert (LLG) equation. However, it is expected that the LLG equation fails to properly describe the large amplitude dynamics that occur during magnetization reversal. Improving switching speeds in nanoscale devices by tailoring materials requires both a qualitative understanding of the relaxation processes that contribute to damping and the ability to quantitatively calculate the resulting damping rates. We consider small amplitude LLG damping in transition metals as a prelude to approaching the more complicated mechanisms expected in complete reversal events. LLG damping rates in pure transition metal systems have non-monotonic temperature dependencies that have been empirically shown by Heinrich et al. [1] to have one part proportional to the conductivity and one part proportional to the resistivity. Kambersky [2] postulated that both contributions result from a torque between the spin and orbital moments. We have conducted first-principles calculations that validate this claim for single element systems [3]. Our calculations for Fe, Co, and Ni both qualitatively match the two trends observed in measurements and quantitatively agree with the observed damping rates. We will discuss how the spin-orbit interaction produces two contributions to damping with nearly opposite temperature dependencies and compare calculations of the damping rate versus resistivity with experimental results. \newline [1] B. Heinrich, D.J. Meredith, and J.F. Cochran, J. Appl. Phys., 50(11), 7726 (1979). \newline [2] V. Kambersky, Czech. J. Phys. B, 26, 1366 (1976). \newline [3] K. Gilmore, Y.U. Idzerda, and M.D. Stiles, Phys. Rev. Lett., 99, 027204 (2007). [Preview Abstract] |
Tuesday, March 11, 2008 11:51AM - 12:03PM |
J32.00002: Non-local damping effects in Py-Cu-FeMn trilayers Eric R.J. Edwards, Lei Wen, Zeenath Reddy Tadisina, Claudia K.A. Mewes, Subhadra Gupta, Tim Mewes We report a non-local enhancement of the Gilbert damping in Py-Cu-FeMn trilayers for in-plane ferromagnetic resonance measurements. We observe a 1/t dependence, t being the thickness of the permalloy, of the slope of the linewidth vs. frequency measurements indicating non-local contributions to the damping. In view of experimental verification of the adiabatic spin pump theory [1] in ferromagnetic heterostructures [2], these results suggest parallels in exchange biased systems. With the thickness of the non-magnetic layer, Cu, fixed below its spin-diffusion length, we understand this effect to be non-local resulting from the injection of spins by the precessing magnetization at the Py-Cu interface and subsequent scattering at the Cu-FeMn interface. \newline [1] Tserkovnyak et al., Rev. Mod. Phys. \textbf{77}, 1375 (2005). \newline [2] Heinrich et al. Phys. Rev. Lett. \textbf{90}, 187601 (2003) [Preview Abstract] |
Tuesday, March 11, 2008 12:03PM - 12:15PM |
J32.00003: Gilbert Damping in Single-Crystalline Ni/MgO(001) Kevin J. Smith, R. Ale Lukaszew, Anne Reilly, Gunter L\"{u}pke The dynamical properties of Nickel and Nickel alloys are of great interest in spintronic applications, as these materials exhibit low coercivity and significant magnetoresistance, however, the interplay of the various damping mechanisms, such as two magnon scattering and local resonance, is not well understood. The frequency, $\omega$, and Gilbert damping, $\alpha$, behavior of uniform spin precession on Ni/MgO(001) are studied over a wide range of external field angles and magnitudes using the Time-Resolved Magneto-Optical Kerr effect (TR-MOKE). The damping parameter shows a strong dependence on the magnetocrystalline anisotropy, suggesting a tuneability of $\alpha$ over the range of 0.09 to 0.5. Two separate trends in $\alpha$ vs. $\omega$ are observed depending on the magnitude and direction of the applied field when applied in-plane, indicating the presence of competing damping mechanisms. We further investigate these properties in measurements in which the external field is applied normal to the sample plane, thereby minimizing the role of two magnon scattering. [Preview Abstract] |
Tuesday, March 11, 2008 12:15PM - 12:27PM |
J32.00004: Inhomogeneous Gilbert damping from disorder and electron-electron interactions Ewelina Hankiewicz, Giovanni Vignale, Yaroslav Tserkovnyak We present a unified theory of Gilbert damping in itinerant ferromagnets at order $q^2$ ($q$ being the wave vector of the spin modulation) including electron-electron interactions and electron-impurity scattering -- the idea being that these interactions are much stronger than the spin-orbit interaction, which controls the damping in the homogeneous case ($q=0$). We show that Gilbert damping can be expressed in terms of the spin conductivity, which can be calculated straightforwardly (no vertex corrections involved) leading to a Drude-like formula in which the inverses of the disorder and interaction scattering times enter in a simple additive form. We also make contact with earlier theories of the transverse spin susceptibilities and with the spin-pumping picture of the Gilbert damping. [Preview Abstract] |
Tuesday, March 11, 2008 12:27PM - 12:39PM |
J32.00005: FMR Investigations on Ni-Co thin films Justin Baize, Steven Michalski, Roger D. Kirby, Mircea Chipara, David J. Sellmyer Magnetic interactions between thin films of Co and Ni spaced by a conducting, non-magnetic film (Pt) are analyzed by utilizing a Bruker ELEXSYS - EPR spectrometer operating in the X-band (9 GHz). Ferromagnetic resonance spectra have been recorded at room temperature in the out-of-plane configuration, for different orientation of the external applied film relative to the plane of the sample. The effect of the metallic film (Pt) thickness on the interaction between Ni and Co is investigated. For most orientations, the ferromagnetic line of such multilayers is the result of a convolution between the lines of Ni and Co films and has been fitted by a superposition of two Lorentzian lines. The outcome of the interaction between the two magnetic layers on the position and the width of ferromagnetic resonance lines is analyzed in detail. [Preview Abstract] |
Tuesday, March 11, 2008 12:39PM - 12:51PM |
J32.00006: Determination of ferromagnetic resonance of thin films with coplanar waveguide Xin Fan, Takahiro Moriyama, Rong Cao, John Xiao Determination of ferromagnetic resonance of thin films with coplanar waveguide X. Fan, T. Moriyama, R. Cao, John Q. Xiao Department of Physics and Astronomy, University of Delaware, Newark Delaware 19716 USA Recently, Coplanar Waveguide (CPW) has been applied to determine Ferromagnetic Resonance (FMR) of magnetic thin film$^{1}$ due to its simple geometry and broadband nature. Compared to the conventional method using reflection in a resonant cavity, CPW should be interpreted by the transmission line theory, taking into account of both transmission and reflection. It has been shown that FMR linewidth extracted from S-matrix after four port calibration $^{2}$ differs from that extracted from transmission only, which neglects the impedance mismatch effect. However, the four port calibration is rather complex and tedious. In this presentation, we introduce a new method to extract FMR spectrum without performing four port calibration. We use both transmission and reflection signals, and consider the impedance mismatch, We will demonstrate the difference between these two methods are negligibly small. 1. Y. Ding, T. J. Klemmer, T. M. Crawford, J. Appl. Phys. 96, 2969, 2004 2. C. Bilzer, T. Devolder, P. Crozat, J. Appl. Phys. 101, 074505, 2007 [Preview Abstract] |
Tuesday, March 11, 2008 12:51PM - 1:03PM |
J32.00007: Magnetostatic micro-resonators. Alexander Kozhanov, Zach Griffith, Mark Rodwell, Jim Allen, Dok Won Lee, Shan Wang, Ajey Jacob 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 transmission and reflection resonances in micron size resonators coupled to coplanar waveguides. Ferromagnetic CoZrTa films were sputtered onto Si wafers covered by SiO$_{2}$ and lithographically patterned into stripes and crosses of varying length and width. Magnetostatic waves were excited and detected by overlaying coupling loops patterned as shorted coplanar waveguides. Transmission and reflection S-parameters of fabricated structures were measured in the frequency range (0-50)GHz. Transmission and reflection resonances strongly dependent on the geometry of the ferromagnetic device and applied magnetic field are observed. The results are modeled as standing magneto static waves in micro-resonators. We discuss effect of biasing magnetic fields, approaches to enhanced coupling to the magnetostatic resonators, magnetostatic wave interferometers and magnetization controlled magnetostatic wave switching in junctions. This work is supported by the Nanoelectronics Research Initiative (NRI) - Western Institute of Nanoelectronics (WIN) [Preview Abstract] |
Tuesday, March 11, 2008 1:03PM - 1:15PM |
J32.00008: Measurements of spin diffusion length in gold with point contact Andreev reflection spectroscopy Muhammad Faiz, Raghava Panguluri, Benjamin Balke, Sabine Wurmehl, Claudia Felser, Andre Petukhov, Boris Nadgorny Spin diffusion length, $L_{s}$, is of fundamental importance for spin dependent transport and spintronic devices. So far, most of the measurements of $L_{s}$ in non-magnetic metals have been done in the lateral non-local geometry, with the chemical potential difference characterizing the spin imbalance. In our approach the spin diffusion length was measured directly with point contact Andreev reflection spectroscopy. A spin polarized current was injected from a ferromagnetic electrode, Co$_{2}$Mn$_{0.5}$Fe$_{0.5}$Si Heulser alloy, into Au films of variable thickness. The spin current, which gradually decays with the increased thickness of the film, was measured with a superconducting Nb tip. We developed a phenomenological theory which allowed us to determine the values of $L_{s}$ in such a system. We found $L_{s}$ to be on the order of 250 nm at 4 K, comparable to the results obtained by other techniques. Similar results were obtained with a Gd single crystal. [Preview Abstract] |
Tuesday, March 11, 2008 1:15PM - 1:27PM |
J32.00009: A single minority spin-flip detector Y. Hancock A family of inhomogeneous Hubbard models is shown to have multi-level spin-switching properties. The generic structure of the device is NNNMMMNNN, which defines a linear (quasi-0D) cluster, having open boundary conditions. Within the context of the Hubbard model, N refers to a nonmagnetic (U=0) atom and M is a magnetic atom having finite value of the Hubbard U. The model is solved by numerically exact diagonalization. Localized spin-switching is obtained as a function of the electron filling and is activated by application of an external magnetic field. Arbitrary numbers of nonmagnetic atoms at the edges of the chain, relative to the number of magnetic atoms in the system, lead to a range of spin-switching signals. One potential application to be demonstrated is that of a single minority spin-flip detector. [Preview Abstract] |
Tuesday, March 11, 2008 1:27PM - 1:39PM |
J32.00010: Anisotropic spin exchange between electrons mediated by spin-orbit interaction Suhas Gangadharaiah, Jianmin Sun, Oleg Starykh It is well known that the exchange coupling between electrons in the presence of Rashba spin-orbit interaction (SOI) is anisotropic. In particular, due to the SOI, electron spins are not conserved during a tunneling process as a result anisotropic exchange terms of the Dzyaloshinkii-Moriya (DM) type are generated. In this talk, we re-visit and clarify the role of spin-orbit interaction in lowering the symmetry of the exchange coupling between spins. We point out that the exchange Hamiltonian, despite of its anisotropic appearance, retains spin-rotational ($SU(2)$) invariance to the second order in the spin-orbital coupling. We argue that spin- rotational symmetry is broken only in the fourth order in SOI coupling. To capture this, we calculate the exchange problem along the lines of the Heitler-London approach [Preview Abstract] |
Tuesday, March 11, 2008 1:39PM - 1:51PM |
J32.00011: Noise Spectrum Signature of Large Jumps in the Magnetization of a Finite System Near a Phase Transition Zhi Chen, Clare Yu It is well known that a finite-size spin system can undergo thermally driven flips of the magnetization of the system as a whole. But what is an experimentally measurable signal of this? We show that the low frequency noise spectrum of the magnetization has a distinct signature of these total magnetization flips which as particularly evident just below the phase transition temperature. To see this, we studied the magnetization and energy spectra of the 2D Ising model by using Monte Carlo simulations. We find that at $T_C$ the noise power is a power law in the frequency where the power is given by the critical exponents. As the frequency decreases for a finite system just below $T_C$, the magnetization noise spectrum crosses over to $f^{-2}$. We show that this is due to large jumps in the entire magnetization. Finally at the lowest frequencies, the noise spectrum saturates at a frequency that depends on the system size. The method we used can be applied without much modification to quantify the contribution of jumps to the dynamics of other systems. [Preview Abstract] |
Tuesday, March 11, 2008 1:51PM - 2:03PM |
J32.00012: Alkanethiol capping-induced changes in the magnetoresistance of Co Brad Knaus, Samir Garzon, Thomas M. Crawford We demonstrate that chemisorption of a $\sim $ 1 nm thick dodecanethiol (C$_{12}$H$_{25}$SH) self-assembled monolayer on the surface of an Au film alters the magnetotransport of an underlying Co film. Giant paramagnetism has been previously detected in Au thin films and nanoparticles capped with alkanethiols via SQUID magnetometry. By observing the impact of Au-thiol magnetism on a ferromagnetic thin film we avoid background subtraction and the influence of impurities. After thiolation, significant changes are observed in the anisotropic magnetoresistance (AMR) and planar Hall effect (PHE) from Co/Au bilayers (30 nm/5-60 nm) patterned into 1 x 5 mm$^{2}$ bars. Driven with nearly perpendicular external fields, we observed domain nucleation shifts of $\sim $0.65 T and changes in the FWHM of the AMR. We have also measured differences in the MR as a function of ambient exposure time presumably due to oxidation effects known to occur in alkanethiols. Effects of surface scattering were eliminated with the introduction of a 12nm Al$_{2}$O$_{3}$ insulation barrier between Co and Au. We have calculated that the effects of magnetostriction are too small by more than two orders of magnitude to explain our observations. We therefore hypothesize that Au-thiol magnetization acts as a source of magnetic field which biases the underlying Co film. [Preview Abstract] |
Tuesday, March 11, 2008 2:03PM - 2:15PM |
J32.00013: The role of uncompensated spins in exchange biasing Hans J. Hug, Iris Schmid, Peter Kappenberger, Sara Romer The origin of the exchange bias (EB) effect has been traced back to the existence of pinned uncompensated spins (UCS) in the antiferromagnet (AFM) or at its interface. However, the understanding of the underlying mechanism is still clouded by contradictory reports: For example, both a parallel as well as an antiparallel orientation of the UCS relative to the magnetization direction of the ferromagnet (FM) were reported for similar FM/AFM systems. Here different magnetization histories in magnetometry and high resolution magnetic force microscopy measurements are used advantageously to demonstrate the co-existence of pinned UCS that are parallel and antiparallel to the cooling field in metallic (IrMn) and oxidic (CoO) EB systems. We further conclude that the EB effect is mainly a result of pinned interfacial UCS, which are antiparallel to the FM spins [1]. In further experiments, the distribution of density of the UCS were imaged on the length scales of single grains. A surprisingly strong fluctuation of the local UCS density (UCSD) was observed. A correlation between the UCSD and the local exchange field was performed. Clearly, a high UCSD results in a high local exchange bias field. Regions with an anti-biasing effect were found. Using grain-boundary engineering, exchange-biased materials without such regions could be fabricated that showed a substantially increased exchange bias effect. \newline [1] Schmid et al. EPL, 81 (2008) 17001 [Preview Abstract] |
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