Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session C41: Spin Dynamics IFocus
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Sponsoring Units: GMAG DMP Chair: Yu-Sheng Ou, University of Delaware Room: BCEC 209 |
Monday, March 4, 2019 2:30PM - 3:06PM |
C41.00001: Competing spin transfer and dissipation at Co/Cu(001) interfaces on femtosecond timescales Invited Speaker: Andrea Eschenlohr Spin dynamics driven by optical excitation with femtosecond (fs) laser pulses offers fascinating possibilities in the ultrafast manipulation of spin-dependent microscopic processes in magnetically ordered materials and heterostructures. In particular, fs spin currents are very promising for future ultrafast spintronics applications. However, the role of the interface in fs spin dynamics is to date largely unexplored. By means of a combined experimental-theoretical approach, we identify the fundamental microscopic processes during optically induced, fs charge and spin transfer at a model epitaxial ferromagnet/paramagnet interface for technologically relevant ferromagnetic heterostructures. A comparison of fs time-resolved, interface-sensitive magneto-optical experiments with ab initio time-dependent density functional theory on the Co/Cu(001) interface demonstrates that the ultrafast spin dynamics originates from spin-dependent charge transfer, including resonantly excited minority spin back-transfer from Cu to Co. Already on timescales below 100 fs, this fs spin transfer competes with dissipation of spin angular momentum mediated by spin-orbit coupling [1]. |
Monday, March 4, 2019 3:06PM - 3:18PM |
C41.00002: Domain dynamics of a ferromagnetic CoFe/Ni multilayer in response to ultrafast optical pumping Dmitriy Zusin, Ezio Iacocca, Löic Le Guyader, Adam Blonsky, Alex Reid, William F Schlotter, TianMin Liu, Daniel J Higley, Phoebe Tengdin, Christian Gentry, Sheena Patel, Anatoly Shabalin, Nelson Hua, Stjepan Hrkac, Hans T. Nembach, Justin Shaw, Mark A Hoefer, Henry C Kapteyn, Margaret Mary Murnane, Eric Fullerton, Hermann Durr, Thomas Silva We studied a response of nanoscale magnetic domains in a 50 nm thick [Co90Fe10/Ni] multilayer to a femtosecond laser pulse by use of time-resolved soft X-ray resonant magnetic scattering (RMS) at the Linear Coherent Light Source. By collecting X-ray diffraction at high scattering angles, we observe the 1st, 3rd and 5th order RMS intensity peaks and thus capture the dynamics of fine magnetic features. We observe a rapid shift in the position of all diffraction orders towards smaller scattering angles and a simultaneous reduction of their magnitude. Surprisingly, higher orders start to decline relative to the 1st order only after a few picoseconds. With our data, we develop a method to extract real-space domain dynamics. We find that the dynamics across the domains are non-uniform, with smaller domains demagnetizing stronger than their larger neighbors. The domain walls remain relatively unchanged and start to broaden only after a few picoseconds. Using our models we attribute this behavior to a combined effect of non-uniform spin transport and a gradient in the absorption of the laser pump throughout the sample. |
Monday, March 4, 2019 3:18PM - 3:30PM |
C41.00003: Simulations of all optical switching using TDDFT Peter Elliott, J. Kay Dewhurst, Sangeeta Sharma, E.K.U. Gross Time dependent density functional theory (TDDFT) is an ab-initio method for studying the magnetization dynamics induced by strong laser pulses. We have previously applied TDDFT to study ultrafast demagnetization in simple ferromagnets[1,2] and optical inter sublattice spin transfer (OISTR)[3] where we found we could switch the magnetic ordering from AFM to a transient FM state[4]. In this work, we simulate RE-TM2 compounds in the C15 Laves geometry and ask whether OISTR plays a role in the all optical switching observed in such materials. |
Monday, March 4, 2019 3:30PM - 3:42PM |
C41.00004: High harmonic generation in ferromagnetic thin films as a tool to probe spin and charge dynamics Guoping Zhang, Mingsu Si, Mitsuko Murakami, Yihua Bai, Thomas F George High-order harmonic generation (HHG) in solids has garnered worldwide attention [1-3]. The materials include metals, semiconductors, insulators, graphene, fullerenes, and topological insulators. However, this has never been explored for magnetic materials. This study represents the first investigation in the field. We show that HHG in Fe(110) and Fe(001) monolayers carries spin information. If a HHG signal is dispersed into the crystal momentum space, the harmonic peak can be assigned to a single transition, a key step to spin-resolved band structure detection. Different from the optical counterpart, the spin HHG only has even orders. Our findings thus predicts a new frontier of magneto-high-order harmonic generation [4]. |
Monday, March 4, 2019 3:42PM - 3:54PM |
C41.00005: Absence of Correlation between Damping and Crystal Quality in Epitaxial Fe Behrouz Khodadadi, Min Gao, Jie-Fang Li, Dwight D Viehland, Satoru Emori Most studies on ferromagnetic relaxation in metallic thin films are performed on polycrystalline structures, which makes it difficult to compare the experimental findings with theoretical calculations. Also it is unclear how microstructure – in addition to electronic properties – would affect damping in such polycrystalline metallic thin films. Our experimental study investigates the interplay between structure and damping in epitaxial thin films of pure Fe, grown on single-crystal MgO and MgAl2O4 substrates by magnetron sputtering. Our structural characterization by X-ray diffraction indicates significantly higher crystalline quality for Fe/MgAl2O4 as evidenced by pronounced Laue oscillations and an order of magnitude narrower rocking curve, compared to Fe/MgO. However, we find no difference in the value of Gilbert damping parameter α from broadband ferromagnetic resonance measurements up to 50 GHz, i.e., both Fe/MgAl2O4 and Fe/MgO consistently exhibit α ≈ 0.003. Our experimental observations are contrary to the expectations that structural disorder should impact ferromagnetic relaxation, and may provide insight into the fundamental mechanism of damping in metallic ferromagnets. |
Monday, March 4, 2019 3:54PM - 4:06PM |
C41.00006: Spin wave generator via oscillating vortex-antivortex core pairs at zero external field Liang-Juan Chang, Shang-Fan Lee We demonstrate that the radiation of spin waves can be generated from an oscillating vortex-antivortex core pair. The emission of spin waves was produced by the oscillation of nanoscale magnetic vortex-antivortex core pair in a NiFe disk-film composite structure. The sample consists of a 20-nm-thick, 4 micron long and 1 micron wide NiFe magnonic waveguide, located at the gap of a single-end GS coplanar waveguide circuit, and magnetic disk with thickness 50 nm and diameter 500 nm on top. The vortex cores in the disk was excited by an out of plane radio frequency magnetic field. The dynamic behaviors of the magnetization were studied using a micro-focused Brillouin light scattering spectroscopy (BLS) setup. In addition to the discrete ferromagnetic resonance (FMR) signals above external dc saturation magnetic field, we observed clear signals at zero magnetic field where vortex cores are present. We have found an ultra- flexible mechanism for consistent excitation of propagating spin waves. The frequencies of the spin waves excited are coherently tunable by the driving frequencies from 5 GHz up to 15 GHz. The short wavelength down to 80 nm for 15 GHz was observed. |
Monday, March 4, 2019 4:06PM - 4:18PM |
C41.00007: Ramsauer-Townsend Resonance for Spin Waves Pablo Borys, Naser Qureshi, Oleg Kolokoltsev Spin waves are fluctuations about the stable configuration in a ferromagnet. As spin waves are free from Joule heating, they are interesting for technological applications. Controlling spin waves propagating in solids relies on the predefined, constant magnetic parameters. However, recently, it has been shown that it is possible to have a dynamic variation of these parameters. Using a laser, thermal landscapes are created in the magnetic medium that results in a modulation of the saturation magnetization[1]. |
Monday, March 4, 2019 4:18PM - 4:30PM |
C41.00008: Exchange Stiffness in pMTJ Free Layers by Magnetometry and Spin-Torque Ferromagnetic Resonance Jamileh Beik Mohammadi, Jinting Hang, Bartek Kardasz, Georg Wolf, Mustafa Pinarbasi, Andrew D Kent Exchange interactions play a critical role in setting the scale of micromagnetic structure and the spin-wave excitation spectrum in nanometer scale magnetic elements. Specifically, in perpendicularly magnetized free layers of magnetic tunnel junction (pMTJ) nanopillars, exchange, magnetic anisotropy and dipolar interactions determine the spin-wave modes and magnetization reversal pathways. Here we report studies of the exchange stiffness A in composite CoFeB free layers in pMTJ (extended) thin films. Vibrating sample magnetometry (VSM) was used to determine the free layer magnetization versus temperature, which is fit to Bloch’s law to determine A. We further perform field modulated spin-transfer FMR (ST-FMR) measurements on 30 to 80nm diameter nanopillars to infer the confined spin-wave modes, providing, in principle, an independent means of determining A. We discuss the correspondence between these methods and the nature of the spin-transfer excited spin-wave modes in such pMTJ nanopillars. |
Monday, March 4, 2019 4:30PM - 4:42PM |
C41.00009: Dynamics of 1D Magnetic Topological Structures Martyna Sedlmayr, Nicholas Sedlmayr, Jamal Berakdar, Vitalii Dugaev In one dimensional ferromagnetic wires domains of collinear ferromagnetic order can be separated by different types of topological, and non-topological, domain walls. Domains of opposite orientation are typically separated by either Bloch or Néel domain walls, whereas domains of the same orientation can be separated by magnetic impurities, or by a one dimensional equivalent of a skyrmion which may be found in quasi-one dimensional wires. We consider the different current induced dynamics and stability of these various magnetic deformations. We will go on to make a thorough investigation of the different magnetoresistance which can be caused by these various forms of domain walls, paying particular attention to the one dimensional skyrmion. |
Monday, March 4, 2019 4:42PM - 4:54PM |
C41.00010: New spin excitations in metals Vladimir Antropov, Alex Wysocki, Manh Cuong Nguyen, Andrey Kutepov, Cai-Zhuang Wang, Kai-Ming Ho The spin excitations in metals have been studied using density functional methods. First, we used linear response method with well converged basis set to obtain the spin excitations in para- and ferromagnetic phases of different d metals at all q-vectors and frequencies. Our calculations revealed the existence of new nearly localized in real space spin excitations in metals and we discuss their relation to the existing experiments. We further analyzed the spin correlations in these metals by using the fluctuation dissipation ratio and a corresponding quantum spin ‘noise’. The very large dynamic spin short range order has been identified and will be shown in paramagnetic Pd. These observed unusual features of spin dynamics and the exchange coupling will be discussed using the analysis of microscopical equations of motion for the charge, current and spin densities in the frame of corresponding density functional approach. |
Monday, March 4, 2019 4:54PM - 5:06PM |
C41.00011: On and Off-Resonance Spin Wave/Surface Acoustic Wave Coupling Measured Using Brillouin Light Scattering Katherine E. Nygren, Joseph D. Schneider, Qianchang Wang, Dominic Labanowski, Sayeef Salahuddin, Greg Carman, Kristen S. Buchanan Surface acoustic waves (SAWs) have a much longer propagation distance than most spin waves, especially in metallic ferromagnets. If spin waves can be effectively coupled to SAWs they can travel farther and SAW/spin wave coupling also offers new opportunities for energy efficient spin wave generation. Here we have used an interdigital transducer (IDT) on a piezoelectric substrate to produce a SAW at 1.8 GHz and we have studied the coupling of the SAW to spin waves in a 20 nm thick Ni thin film using Brillouin light scattering (BLS). A strong signal is observed at the driving frequency f that varies in intensity as a function of the applied magnetic field. At fields where f overlaps with the spin wave manifold an additional peak is observed at 2f that is not present at higher fields, which indicates that nonlinear process are active. Measurements were also performed at multiple locations on the nickel pad to characterize how the spin wave decays with distance. |
Monday, March 4, 2019 5:06PM - 5:18PM |
C41.00012: Spin wave excitations of magnetic metalorganic materials Roberto Pérez, Johan Hellsvik, Richard Geilhufe, Martin Månsson, Alexander Balatsky The Organic Materials Database (OMDB) is an open database at Nordita that is hosting about 25,000 electronic band structures, density of states and other properties for synthesized 3-dimensional organic crystals. The web interface of the OMDB offers various search tools for the identification of novel functional materials such as band structure pattern matching and density of states similarity search. In this work we extend the OMDB to include magnetic excitation properties. For inelastic neutron scattering we focus on the dynamical structure factor S(Q, ω) which contains information on the excitation modes of the material. We introduce a new dataset containing atomic magnetic moments and Heisenberg exchange parameters for which we calculate the spin wave spectra and dynamic structure factor with linear spin wave theory and atomistic spin dynamics. We thus develop the materials informatics tools to identify topological magnon spectra such as Dirac crossings within the class of organic molecular crystals, and reveal mechanisms for the topological protection of the crossings. |
Monday, March 4, 2019 5:18PM - 5:30PM |
C41.00013: Terahertz-frequency magnetization oscillations in an uncompensated ferrimagnet under a spin-transfer torque Ivan Lisenkov, Roman Khymyn, Johan Akerman, Nian Xiang Sun, Boris Ivanov Current of spin-polarized electrons can interact with materials exhibiting a long-range magnetic order via a spin-transfer torque (STT). An STT of a sufficient magnitude may compensate damping and the spins come into a self-sustained precession [1]. In ferromagnetic materials the precession frequency is limited to tens of GHz by the value of the external magnetic field. In materials with more than one magnetic sublattice, the strong exchange interaction speeds up the precession frequency up to several THz [2], but due to magnetic compensation, in antiferromagnetic materials the THz frequency output power is small [3]. In our simulations we show that in easy-plane unbiased nearly compensated GdFeCo thin layers: (i) the precession frequency can be in over of THz and controlled by current, (ii) the precession of the Neel vector is conical, where the cone angle is defined by the non-compensation of the sublattices. The conical precession excites an AC spin-current, which can be converted to AC voltage by inverse spin-Hall effect. |
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