Bulletin of the American Physical Society
APS March Meeting 2018
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session L22: Spin Dynamics, Damping and Domain WallsFocus

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Sponsoring Units: GMAG DMP FIAP Chair: Emrah Turgut, Cornell Univ Room: LACC 402A 
Wednesday, March 7, 2018 11:15AM  11:27AM 
L22.00001: Spin dynamics modulated by collective magnetization reversal in artificial spin ice Matthias Benjamin Jungfleisch, Joseph Sklenar, Junjia Ding, Jungsik Park, John Pearson, Valentyn Novosad, Peter Schiffer, Axel Hoffmann Magnon spintronics aims at developing novel functional devices that combine magnonic and electronic spin transport phenomena. In particular, magnetic metamaterials such as artificial spin ice and magnonic crystals offer unique possibilities in magnon spintronics \footnote{M. B. Jungfleisch et al., Phys. Rev. B 93, 100401(R) (2016).} We recently demonstrated the detection of Oerstedfield driven spin dynamics in connected artificial spinice networks made of bilayer Ni_{80}Fe_{20}/Pt with different lattice parameters by dc electrical means \footnote{M. B. Jungfleisch et al., Appl. Phys. Lett. 108, 052403 (2016).}. 
Wednesday, March 7, 2018 11:27AM  11:39AM 
L22.00002: Intrinsic Damping Phenomena from Quantum to Classical Magnets: An abinitio Study of Gilbert Damping in Pt/Co Bilayer. Farzad Mahfouzi, Nicholas Kioussis A fully quantum mechanical description of the precessional damping is presented in the framework of the Keldysh Green function approach. We demonstrate that $\alpha_{GD}$ in the quantum case does not diverge in the ballistic regime due to the finite size of the total spin, $S$. In the limit of $S\rightarrow\infty$ we show that the formalism recovers the torque correlation expression for $\alpha_{GD}$ which we decompose into spinpumping and spinorbital torque correlation contributions. The formalism is generalized to take into account a self consistently determined dephasing mechanism which preserves the conservation laws and allows the investigation of the effect of disorder. We employed the formailsm to calculate the intrincic Gilbert damping constant of Pt/Co bilayer system. The dependence of $\alpha_{GD}$ on Pt thickness and disorder strength is calculated and the spin diffusion length of Pt and spin mixing conductance of the bilayer are determined and compared with experiments. 
Wednesday, March 7, 2018 11:39AM  11:51AM 
L22.00003: Spintransfer torque and domain wall dynamics induced by triplet supercurrent Rina Takashima, Satoshi Fujimoto, Takehito Yokoyama We theoretically study spintransfer torques induced by a spintriplet supercurrent in a magnet in proximity to a triplet superconductor. We show that spintriplet correlations change the spintransfer process and realize new types of torques, which are analogous to the adiabatic and nonadiabatic torques (beta term), without extrinsic spinflip scattering. Remarkable advantages compared to conventional spintransfer torques are highlighted in domainwall manipulation. Oscillatory motions of a domain wall do not occur for a small Gilbert damping, and the threshold current density to drive its motion becomes zero in the absence of extrinsic pinning potentials due to the nonadiabatic torque controlled by the triplet correlations. 
Wednesday, March 7, 2018 11:51AM  12:27PM 
L22.00004: Fast domain wall motion induced by antiferromagnetic spin dynamics at the angular momentum compensation temperature of ferrimagnets Invited Speaker: KabJin Kim Antiferromagnetic spintronics is an emerging research field which aims to utilize antiferromagnets as core elements in spintronic devices. A central motivation toward this direction is that antiferromagnetic spin dynamics is expected to be much faster than ferromagnetic counterpart because antiferromagnets have higher resonance frequencies than ferromagnets. Recent theories indeed predicted faster dynamics of antiferromagnetic domain walls (DWs) than ferromagnetic DWs. However, experimental investigations of antiferromagnetic spin dynamics have remained unexplored mainly because of the immunity of antiferromagnets to magnetic fields. Furthermore, this immunity makes fielddriven antiferromagnetic DW motion impossible despite rich physics of fielddriven DW dynamics as proven in ferromagnetic DW studies. Here we show that fast fielddriven antiferromagnetic spin dynamics is realized in ferrimagnets at the angular momentum compensation point T_{A}. Using rareearth–3dtransition metal ferrimagnetic compounds where net magnetic moment is nonzero at T_{A}, the fielddriven DW mobility remarkably enhances up to 20 km s^{−1}T^{−1}. The collective coordinate approach generalized for ferrimagnets and atomistic spin model simulations show that this remarkable enhancement is a consequence of antiferromagnetic spin dynamics at T_{A}. Our finding allows us to investigate the physics of antiferromagnetic spin dynamics and highlights the importance of tuning of the angular momentum compensation point of ferrimagnets, which could be a key towards ferrimagnetic spintronics [1]. 
Wednesday, March 7, 2018 12:27PM  12:39PM 
L22.00005: Semiclassical dynamics of spin density waves GiaWei Chern, Kipton Barros, Zhentao Wang, Hidemaro Suwa, Cristian Batista We present a theoretical framework for equilibrium and nonequilibrium dynamical simulation of quantum states with spindensitywave (SDW) order. Within a semiclassical adiabatic approximation that retains electron degrees of freedom, we demonstrate that the SDW order parameter obeys a generalized LandauLifshitz equation. With the aid of an enhanced kernel polynomial method, our linearscaling quantum LandauLifshitz dynamics (QLLD) method enables dynamical SDW simulations with $N \simeq 10^5$ lattice sites. Our realspace formulation can be used to compute dynamical responses, such as dynamical structure factor, of complex and even inhomogeneous SDW configurations at zero or finite temperatures. Applying the QLLD to study the relaxation of a noncoplanar topological SDW under the excitation of a short pulse, we further demonstrate the crucial role of spatial correlations and fluctuations in the SDW dynamics. 
Wednesday, March 7, 2018 12:39PM  12:51PM 
L22.00006: Semiclassical methods on magnetization dynamics. Bangguo Xiong, Hua Chen, Xiao Li, Qian Niu We formulate the magnetization dynamics in slowly perburbed crystals using the semiclassical description of electrons, and systematically study electric/magnetic fieldinduced effects. The geometric phase effects are found important in the magnetization dynamics. In the presence of electric field, spin orbital torques is reformulated in the language of Berry curvature. As a complete first order theory, electric field influence on damping and gyromagnetic ratio is also studied as nonequilibrium electronic contribution. In the presence of magnetic field, we find correction on gyromagnetic ratio from the instrinsic electronic contribution in terms of second chern form. This correction contains a combine effect of charge pumping and spin transfer torque in inhomegeneous system, motive force and spin orbital torque and a new term due to density of states correction. Our theory unifies those seperated discussions and gives new contribution. 
Wednesday, March 7, 2018 12:51PM  1:03PM 
L22.00007: Superfluidinspired spintronics Pramey Upadhyaya, Yuxiang Liu, Se Kwon Kim, Yaroslav Tserkovnyak Superfluidity, i.e. coherent flow of a physical quantity in the absence of dissipation, is a fascinating condensed matter phenomena, which presents opportunities for constructing energyefficient information processing devices. Easyplane magnets are known to support such superfluidlike transport of spin, which have remained largely unexplored for information processing applications due to the absence of efficient knobs to control magnets. More recently, the ability to engineer spinorbit interaction in magnetic systems, has emerged as a universal energyefficient knob to control magnetic order via electrical and thermal means. In this talk, we will present how this “spinorbitronic” control of magnets allows for uncovering a new class of spinbased phenomenon and devices, inspired from the wellestablished superfluidlike phenomenon observed in charged superfluids (i.e. superconductors). Firstly, we demonstrate the possibility of manipulating domain walls by coherent spin currents transported in spin superfluids. Secondly, we will demonstrate an electrically tunable magnetic phaseslip oscillator inspired from onedimensional superconducting Josephson junctions. 
Wednesday, March 7, 2018 1:03PM  1:15PM 
L22.00008: Anisotropic Damping in Exchange Bias Systems Alison Farrar, Jamileh Beik Mohammadi, Tim Mewes, Claudia Mewes Current experimental broadband investigations of the magnetization dynamics have shown that the damping mechanism in exchange bias systems exhibits a strong unidirectional contribution, caused in part by twomagnon scattering. However, a detailed analysis strongly suggests the presence of a previously undescribed unidirectional relaxation mechanism [1]. To describe this phenomenon theoretically we use the formalism of an anisotropic Gilbert damping tensor that takes the place of the (scalar) Gilbert damping parameter in the LandauLifshitzGilbert equation of motion. Each component of the symmetric tensor can depend implicitly on the magnetization direction. While for single crystals the anisotropy of the damping tensor is expected to be small, making experimental confirmation difficult, the broken symmetry in exchange bias systems provides an excellent testing ground to study the modified magnetization dynamics as well as the dynamic response under the influence of unidirectional damping. 
Wednesday, March 7, 2018 1:15PM  1:27PM 
L22.00009: Low Gilbert Damping in Perpendicularly Magnetized W/CoFeB/MgO Films with High Thermal Sustainability Dustin Lattery, Delin Zhang, Jie Zhu, Jianping Wang, Xiaojia Wang To advance the field of spintronics, it is imperative to study worthwhile materials to improve performance. For technologies such as magnetoresistive random access memory (MRAM), the recent focus has been on materials with perpendicular magnetic anisotropy (PMA) due to their reduced critical switching current and higher memory density. Interfacial PMA structures such as NM/CoFeB/MgO (where NM is a nonmagnetic metal) have been shown to be an effective structure. One of the most common interfacial PMA structures uses Ta, but annealing at temperatures exceeding 350 °C causes the Ta/CoFeB/MgO structures to breakdown. In this work, we developed and demonstrated a PMA structure of W/CoFeB/MgO that can sustain annealing temperatures up to 400 °C. The magnetization dynamics were captured with a timeresolved magnetooptical Kerr effect (TRMOKE) technique. It was found that the Gilbert damping in W/CoFeB/MgO samples reached a minimum at an annealing temperature of 350 °C. Additionally, the damping of the W/CoFeB/MgO structure annealed at 400 °C increased to only ~0.02. This suggests that W/CoFeB/MgO, with enhanced thermal stability, are viable alternatives to Ta/CoFeB/MgO for spintronic devices. 
Wednesday, March 7, 2018 1:27PM  1:39PM 
L22.00010: Ferromagnetic resonance measurements of low magnetization damping in Co_{2}FeAl films Timothy Peterson, W. Peria, Tao Qu, Anthony McFadden, R. Victora, Chris Palmstrom, Paul Crowell Halfmetallic Heusler compounds are predicted to have low intrinsic damping due to the suppression of spinflip scattering. Here, we measure the damping constant in epitaxial MgO(001)/Co_{2}FeAl Heusler films using broadband (040 GHz) ferromagnetic resonance (FMR). For inplane FMR, the linewidth (~hundreds of Oe) is a nonlinear function of frequency and anisotropic, smaller for magnetization along CFA<110> and larger for magnetization along CFA <100>. A much narrower linewidth (~tens of Oe) is found for outofplane FMR, constraining the intrinsic damping value α≤103. The inplane behavior of the FMR linewidth is attributed to an anisotropic twomagnon scattering mechanism. We follow the Krivosik’s approach (P. Krivosik et al. JAP 101, 083901 (2007)) to fit our inplane linewidth data with a selfconsistent twomagnon linewidth calculation and obtain an inhomogeneity correlation length of ~70nm with an intrinsic damping α≤103, which is consistent with the outofplane measurement. We discuss how the presence of low intrinsic damping enhances the twomagnon linewidth, explaining common reports of large twomagnon FMR linewidths in Heusler compounds. 
Wednesday, March 7, 2018 1:39PM  1:51PM 
L22.00011: Co_{25}Fe_{75} Thin Films with Ultralow Total Damping Eric Edwards, Hans Nembach, Justin Shaw We demonstrate ultralow total Gilbert damping of 0.0013 for Co_{25}Fe_{75} thin films grown on a Si/SiO2 substrate by dc magnetron sputtering. While ultralow intrinsic damping in this material has previously been reported, spinpumping contributions from the seed and capping layers increased the total damping of the material from the intrinsic value. We show that by engineering the seed and capping layers to minimize the spinpumping contribution we can significantly reduce the total damping. As an additional figure of merit, the minimum fullwidth halfmaximum resonance linewidth of 1 mT at a resonance frequency of 10 GHz can be achieved in Co_{25}Fe_{75} films of only 12nm thickness. Ferromagnetic resonance measurements taken in the inplane geometry exhibit a slightly enhanced total damping parameter of 0.002, which is of great importance for magnonics applications. We further characterize the structural and morphological character of the films grown with many different seed layer stacks to understand the optimum conditions for minimization of both the total magnetic damping as well as the inhomogeneous linewidth broadening. We find great sensitivity of the qualitative behavior of the inplane resonance linewidth as a function of film structure and morphology. 
Wednesday, March 7, 2018 1:51PM  2:03PM 
L22.00012: Measurement of domain wall dynamics in response to ultrafast optical pumping by use of coherent Xray scattering at the Linac Coherent Light Source Dmitriy Zusin, TianMin Liu, Loic Le Guyader, Alexander Reid, William Schlotter, Daniel Higley, Phoebe Tengdin, Christian Gentry, Adam Blonsky, Sheena Patel, Anatoly Shabalin, Nelson Hua, Stjepan Hrkac, Ezio Iacocca, Hans Nembach, Justin Shaw, Mark Hoefer, Henry Kapteyn, Margaret Murnane, Eric Fullerton, Hermann Durr, Thomas Silva We used coherent Xray resonant magnetic scattering at the Linac Coherent Light Source to study the dynamic response of a nanoscale magnetic domain network in a [Co90Fe10/Ni] multilayer after ultrafast optical pumping. The signal to noise was sufficient for us to observe both the first and third order magnetic diffraction rings produced by the domain network, allowing us to interrogate how ultrafast optical pumping affects details of the domain wall structure. We found that the first and third order diffraction rings exhibit different dynamics. Our results suggest that the magnetization dynamics of magnetic features depends on their size, and that the ultrafast dynamics of the domains and domain walls are different when subject to ultrafast optical pumping. We will present an analysis of the data in terms of ultrafast generation of spin currents that flow between the domains. 
Wednesday, March 7, 2018 2:03PM  2:15PM 
L22.00013: Spin waves in doped graphene with inplane magnetic fields Matthew Anderson, Carsten Ullrich Plasmonics in graphene is a subject of much current interest, with many prospects for novel applications. However, the dynamics of collective spin excitations and spin waves in graphene has been much less explored. Here, we study the spinwave excitations of itinerant electrons in doped graphene in the presence of inplane magnetic fields. We calculate the spinwave dispersions using timedependent densityfunctional methods within a standard tightbinding approach. We treat dynamical exchangecorrelation effects using the STLS approach, generalized for systems with noncollinear spins. 
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