Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session T32: Focus Session: Gilbert Damping and Non-local Spin Injection |
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Sponsoring Units: GMAG DMP FIAP Chair: Gerrit E. W. Bauer, Technical University Delft Room: 336 |
Wednesday, March 18, 2009 2:30PM - 2:42PM |
T32.00001: Anisotropic Damping in Single-Crystalline Ni/MgO(001) studied by the Time-Resolved Magneto-Optical Kerr effect (TR-MOKE) Kevin Smith, A. Lukaszew, J. Skuza, C. Clavero, K. Yang, A. Reilly, G. L\"{u}pke The damping behavior of uniform spin precession in single- crystalline Ni/MgO(001) of various thicknesses from t = 10 nm to t = 60 nm is investigated in the time domain using TR-MOKE over a wide range of external field parameters and temperatures. Planar measurements indicate that the effective Gilbert damping parameter, $\alpha_{\mathrm{eff}}$, is coupled to the magnetocrystalline anisotropy, as $\alpha_ {\mathrm{eff}}$ ranges from 0.05 near the hard axis to 0.10 near the easy axis. Previous experiments by other groups using FMR [1] and TR-MOKE on polycrystalline samples [2] have placed the intrinsic value of the damping at 0.045. When the field is applied normal to the film surface, $\alpha_{\mathrm {eff}}$ increases to as high as 0.3 when the angle of the magnetization, $\phi_{\mathrm{M}}$, is greater than 45 degrees out of plane. These results are discussed in terms of various models of extrinsic damping mechanisms, such as two magnon scattering. [1] S. Bhagat et al. Phys. Rev. B, \textbf{10} 179 (1974) [2] J. Walwoski et al. J. Phys. D: Appl. Phys. \textbf{41} 164016 (2008) [Preview Abstract] |
Wednesday, March 18, 2009 2:42PM - 2:54PM |
T32.00002: Scattering Theory of Gilbert Damping Arne Brataas, Yaroslav Tserkovnyak, Gerrit E.W. Bauer Magnetization relaxation is a collective many-body phenomenon that remains intriguing despite decades of theoretical and experimental investigations. It is important in topics of current interest since it determines the magnetization dynamics in magnetic memory devices and state-of-the-art magnetoelectronics experiments on current-induced magnetization dynamics [1]. We study the magnetization dynamics of a single domain ferromagnet in contact with a thermal bath by scattering theory. We recover the Landau-Lifshitz-Gilbert equation and express the Gilbert damping tensor in terms of the scattering matrix [2]. Dissipation of magnetic energy equals energy current pumped out of the system by the time-dependent magnetization, with separable spin-relaxation induced bulk and spin-pumping generated interface contributions. In linear response, our scattering theory for the Gilbert damping tensor is equivalent with the Kubo formalism [1] M. Stiles and J. Miltat, Top. Appl. Phys. 101, 225 (2006), and references therein. [2] A. Brataas, Y. Tserkovnyak, and G. E. W. Bauer, Phys. Rev. Lett. 101, 037207 (2008). [Preview Abstract] |
Wednesday, March 18, 2009 2:54PM - 3:06PM |
T32.00003: Physical damping processes in Co-Cr granular films. Sangita Kalarickal, Pavol Krivosik, Nan Mo, Carl Patton, Stella Wu Recent ferromagnetic resonance (FMR) results on metallic ferromagnetic alloy films have shown that a simple one parameter Gilbert damping description is inadequate for most systems. New FMR results have been obtained on Co-Cr granular films with the columnar microstructure amenable to perpendicular media applications. The nominal 17.3 GHz FMR field and linewidth vs. the out-of-plane field angle was measured and analyzed for a 16 nm thick granular film with a relatively low effective anisotropy field of 1 kOe and a nominal grain size of 8 nm or so. The analysis reveals a three component linewidth comprised of a small Gilbert term with an $\alpha $-value of 0.003 that is consistent with intrinsic processes, a large two magnon term that derives from the grain-to-grain anisotropy variations, and an inhomogeneity broadening term due to anisotropy dispersion and grain size variations. [Preview Abstract] |
Wednesday, March 18, 2009 3:06PM - 3:18PM |
T32.00004: Effect of rapid thermal annealing on ferromagnetic resonance line width of CoFeB thin films Yaping Zhang, Xin Fan, Weigang Wang, Xing Chen, Chaoying Ni, Rong Cao, John Xiao Magnetization dynamics has attracted much attention recently due to their implication on magnetic recording and storage applications. CoFeB material is one of most common magnetic layers used in MgO based magnetic tunnel junctions (MTJs). Unlike traditional thermal treatments, a giant Tunneling Magnetoresistance can be archived by rapid thermal annealing (RTA) [1]. We show the effect of RTA on magnetization dynamics. CoFeB thin film, subjected to RTA at 380 \r{ }C for various time, was investigated by ferromagnetic resonance (FMR) measurement. It is found that FMR linewidth reaches a minimum at 60 second annealing, after which the linewidth increases with annealing time. A clear trend of decreasing of uniaxial anisotropy and increasing of cubic anisotropy with annealing time indicates that competition between these anisotropies plays an important role in linewidth evolution. [1] WG. Wang et al., Appl. Phys. Lett. 92, 152501 (2008). [Preview Abstract] |
Wednesday, March 18, 2009 3:18PM - 3:30PM |
T32.00005: Calculation of intrinsic damping in half metals Chunsheng Liu, Claudia K.A. Mewes, Mairbek Chshiev, Tim Mewes, William H. Butler The extended Hueckel tight binding method in combination with Kambersky's torque correlation model [1] is used to calculate the precessional magnetization relaxation in half-metallic systems. In Kambersky's model damping is described by a combination of spin-flip excitations and orbital excitations. An analytical expression of the transition matrix element which represents scattering events within a single band (intraband) and between different bands (interband) respectively [2] can be obtained within the TB scheme, which enables a better understanding of the damping mechanisms in half-metallic structures. Due to the absence of spin-flip scattering in half-metallic systems, the Gilbert damping rate of half-metals is expected to be much smaller than that of metals. Using this approach we calculated the damping for different half-metallic structures. The minimum intrinisic relaxation rate $\lambda $ was calculated to be, 3.2 MHz, 1.1 MHz, 0.13 MHz, for the Heusler structures Co$_{2}$MnGe , Co$_{2}$MnSi and the Rutile structure CrO$_{2}$ respectively. The damping rates for these half-metallic materials are much lower than that of bcc Fe, as we anticipated from the analytical analysis. References: [1] V. Kambersky, Czech. J. Phys. B 26, 1366 (1976). [2] K. Gilmore, Y.U. Idzerda and M.D. Stiles, Phys. Rev. Lett. 99, 027204 (2007). [Preview Abstract] |
Wednesday, March 18, 2009 3:30PM - 3:42PM |
T32.00006: Unidirectional Damping in Exchange Biased Systems Matthew Bradford, Hwachol Lee, Eric Edwards, Zeenath Tadisina, Claudia Mewes, Subhadra Gupta, Tim Mewes We report on the investigation of the angular dependence of the damping parameter in thin film NiFe, exchange biased by a layer of FeMn. By using a broadband ferromagnetic resonance technique (shorted waveguide), the resonant field and linewidth were determined as a function of the in-plane angle and the microwave frequency. We find that the effective damping parameter, as extracted from the frequency dependent linewidth data, shows a unidirectional anisotropy, displaying a sinusoidal behavior with respect to the in-plane angle. The effective damping parameter is minimal when the field during FMR measurements is applied parallel to the exchange bias direction and maximal for antiparallel alignment. These experiments in conjunction with thickness dependent measurements suggest that uncompensated spins at the ferromagnet/antiferromagnet interface are responsible for the increased magnetization relaxation observed in these structures. [Preview Abstract] |
Wednesday, March 18, 2009 3:42PM - 3:54PM |
T32.00007: The origin of intrinsic Gilbert damping Mark C. Hickey, Jagadeesh S. Moodera The damping of magnetization, represented by the rate at which it relaxes to equilibrium, is successfully modeled as a phenomenological extension in the Landau-Lifschitz-Gilbert equation. This is the damping torque term known as Gilbert damping and its direction is given by the vector product of the magnetization and its time derivative. Here we derive the Gilbert term from first principles by a non-relativistic expansion of the Dirac equation. We find that the Gilbert term arises when one calculates the time evolution of the spin observable in the presence of the full spin-orbital coupling terms, while recognizing the relationship between the curl of the electric field and the time varying magnetic induction. [Preview Abstract] |
Wednesday, March 18, 2009 3:54PM - 4:30PM |
T32.00008: Theory of Magnetization Relaxation in Conducting Ferromagnets Invited Speaker: The advent of technologically promising spintronics devices has revived efforts to achieve a microscopic understanding of magnetization damping in magnetic metals and semiconductors. In absence of an electric current, magnetization relaxation is described by the Gilbert parameter $\alpha$, whose quantitative prediction relies on electronic structure calculations that treat disorder in an approximate fashion. We assess the reliability of these studies by using simple models where disorder may be treated exactly. Transport currents modify the magnetization damping. We associate this change with the non-adiabatic spin transfer torque, which is characterized by a dimensionless parameter $\beta$. We derive a concise analytical expression for $\beta$ that can be applied to real materials. We also discuss the counterparts of $\alpha$ and $\beta$ in out-of-equilibrium superconductors viewed as easy-plane ferromagnets in particle-hole space. [Preview Abstract] |
Wednesday, March 18, 2009 4:30PM - 4:42PM |
T32.00009: Nonlocal Spin Valves With Very Short Injector Detector Distances Andrew McCallum, Mark Johnson Nonlocal spin valves with a center to center distance of 42 nm between spin injector and detector have been fabricated. This distance is much less than the injector detector spacing in previously made nonlocal spin valves and is much shorter than the spin diffusion length of the Cu used as a channel. Nonlocal resistance changes of up to 2.6 $m\Omega$ were seen in these devices at room temperature. From this data it was determined that the average spin polarization of the ferromagnetic interfaces is between 4.3\% and 5.9\% at room temperature. The nonlocal resistance changes of these devices are much less sensitive to changes in temperature, as determined by measurements at liquid nitrogen temperature, than nonlocal spin valves with longer injector detector distances. [Preview Abstract] |
Wednesday, March 18, 2009 4:42PM - 4:54PM |
T32.00010: Temperature dependence of the non-local spin signal in Cu-based lateral spin-valves M.J. Erickson, C. Leighton, P.A. Crowell We report on measurements of the $T$ dependence of the non-local spin signal in lateral metallic spin valves, focusing on the limit of transparent ferromagnet (FM) / normal metal (Cu) interfaces. Devices with channel width 250 nm and contact widths $\approx $100 nm (Ni$_{80}$Fe$_{20}$ or Co) were fabricated using in-situ shadow masking. We employed high purity sources in UHV, enabling one-shot deposition with no air exposure of the interface. Multiple contact separations (250 -- 800 nm) were fabricated on a single substrate to facilitate measurement of the spin diffusion length ($\lambda _{s})$. NiFe/Cu devices with 250 nm contact separation show a maximum non-local transresistance of 420 $\mu \Omega $. Analysis of Hanle effect measurements yields spin lifetimes $\approx $ 8 ps at low $T$ which compare well to those extracted from the measured $\lambda _{s}$ (300 nm) and resistivity (1.5 $\mu \Omega $cm), demonstrating consistency of our analysis. We observe a qualitatively different $T$ dependence of the non-local signal depending on the relative sizes of the contact separation and $\lambda _{s}$. When the separation becomes comparable to $\lambda _{s}$ we observe a maximum in the non-local spin signal at 35 -- 85 K, with strongly thickness dependent magnitude. These measurements of spin lifetime, resistivity, and $\lambda _{s}$ vs $T$ allow a quantitative comparison with the conductivity mismatch model. Work supported by the NSF MRSEC program. [Preview Abstract] |
Wednesday, March 18, 2009 4:54PM - 5:06PM |
T32.00011: Bias-independent spin signals in a tunnel-junction-based non-local spin valve Xiaojun Wang, Han Zou, L.E. Ocola, R. Divan, Yi Ji A pure spin current can be generated in the non-magnetic component of a non-local spin valve (NLSV). It has been demonstrated recently that the pure spin current can be used for spin transfer torque and spin-Hall effects. A high spin current density is desirable for realizing these effects, and therefore a large d.c. bias current will be applied. It is essential to maintain high degree of spin polarization at a high bias current. It has been previously reported that the spin polarization decreases drastically in a tunnel-junction-based CoFe/Al/NiFe NLSV. The goal of this study is to investigate the dependence of spin signals upon a d.c. bias current in tunnel-junction-based Co/Cu/Co NLSV's. Submicron Co/Cu/Co NLSV's are fabricated by e-beam lithography combined with angle deposition. A layer of 2 nm Al$_{2}$O$_{3}$ is deposited at the Co/Cu interface to form a tunnel barrier. A spin signal $>$ 1m$\Omega $ is observed at room temperature (RT). A d.c. current up to 1.0mA is applied at both 4.2 K and RT. No change of spin signal is observed for an injection current density $>$ 10$^{6}$ A/cm$^{2}$. [Preview Abstract] |
Wednesday, March 18, 2009 5:06PM - 5:18PM |
T32.00012: Separating the Contributions of Spin Injection Efficiency and Spin Diffusion Length in Non Local Spin Valves Mikhail Erekhinsky, Felix Casanova, Amos Sharoni, Ivan K. Schuller Non Local Spin Valves (NLSV) are unique devices which permit studies of spin transport related phenomena at the nanoscale. Two important parameters determine the signal in NLSV: a) the effective polarization of injected current from the ferromagnet (FM) through the interface, and b) the spin diffusion length of the non-magnetic metal (NM). We performed a systematic study of the NLSV signal for different device lengths as a function of NM thickness, composition and temperature. By fitting the near-exponential decay of the signal with distance we can separate the effects of polarization and NM spin diffusion length. We will discuss the contributions of surface effects on NM spin diffusion length, and FM/NM interface on efficiency of injection. In addition, we show the importance of adjacent FM electrodes in a multi-terminal spintronics device. [Preview Abstract] |
Wednesday, March 18, 2009 5:18PM - 5:30PM |
T32.00013: Control of Spin Injection by DC Current in Transparent Lateral Spin Valves Amos Sharoni, Felix Casanova, Mikhail Erekhinsky, Ivan K. Schuller Lateral spin valves can be classified according to the type of interfaces (tunnel or transparent) between the ferromagnetic electrodes and the normal material. The tunnel barrier yields a large spin signal, but the maximum spin-polarized current was reported to decrease strongly with applied bias. In transparent interfaces the maximum current density through the interface is much larger, which is beneficiary for some applications, but the effect on spin signal was not measured. To address this issue, we prepared metallic lateral spin valves with excellent transparent interfaces. In addition, instead of AC lock-in techniques commonly used to measure these devices, we perform DC measurements, which enables us to measure directly the effects of the current direction and magnitude on the spin signal. We compared the injection of majority spins from the ferromagnet into a non-magnetic metal (NM) with the reversed process where minority carriers are left in the NM. We were also able to study the effect of joule heating and identified the origin of voltage backgrounds usually observed in these devices. [Preview Abstract] |
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