Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session L18: Ultrafast Magnetization DynamicsFocus
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Sponsoring Units: GMAG DMP FIAP Chair: Emrah Turgut, Cornell University Room: 317 |
Wednesday, March 16, 2016 11:15AM - 11:27AM |
L18.00001: Tabletop soft x-ray magnetic circular dichroism measurements using circularly polarized high harmonic sources T Fan, R Knut, C Hernández García, D Hickstein, D Zusin, C Gentry, F Dollar, C Mancuso, C Hogle, J Ellis, K Dorney, D Legut, K Carva, P Oppeneer, O Shpyrko, E Fullerton, O Kfir, O Cohen, D Milosevic, A Becker, A Jaron Becker, T Popmintchev, M Murnane, H Kapteyn, P Grychtol X-ray magnetic circular dichroism (XMCD) allows for the extraction of the orbital and spin contributions to the magnetization and its interaction with phononic and electronic degrees of freedom on fs time and nm length scales, with element-specificity. However, to date, circular soft x-ray beams were restricted to large-scale x-ray facilities. These facilities have great advantages of high peak and average powers, but have limited access and temporal resolution. In this work, we present the first direct tabletop approach for generating bright circularly polarized light exceeding 160 eV. This makes it possible to implement XMCD on the tabletop for the first time, allowing to probe not only the 3d ferromagnets, but also the 4f rare earth materials with element-specificity. We demonstrate the stability, circularity and brightness of our high harmonic source by extracting the magneto-optical coefficients near the $N$ edge of Gd (145 eV), as well as at the $M$ edges of Fe (52eV), for an out-of-plane magnetized Gd/Fe multilayer sample thus enabling ultrafast studies of magnetization dynamics. [Preview Abstract] |
Wednesday, March 16, 2016 11:27AM - 11:39AM |
L18.00002: Localization of Fe d-states in Ni-Fe-Cu alloys and implications for ultrafast demagnetization Tom Silva, Ronny Knut, Erna Delczeg-Czirjak, Justin Shaw, Hans Nembach, Patrik Grychtol, Dmitriy Zusin, Christian Gentry, Emrah Turgut, Henry Kapteyn, Margaret Murnane, Dario Arena, Olle Eriksson, Olof Karis Ni$_{0.8}$Fe$_{0.2}$ (Py) and Py-Cu exhibit intriguing ultrafast demagnetization behavior, where the Ni magnetic moment shows a delayed response relative to the Fe. To unravel the mechanism responsible for this behavior, we have studied Py-Cu alloys for a wide range of Cu concentrations using X-ray magnetic circular dichroism (XMCD). The magnetic moments of Fe and Ni are found to respond very differently to Cu alloying: Fe becomes a strong ferromagnet in Py, with the magnetic moment largely unaffected by Cu alloying. In contrast, the Ni magnetic moment decreases continuously with increasing Cu concentration. Ab-initio calculations corroborate these results and we discuss the electronic structure in the framework of virtual bound states (VBSs). Fe exhibits VBSs in the minority band that lie approximately 1 eV above the Fermi level in pure Py, and which move closer to the Fermi level upon Cu alloying. A strong interaction between the VBSs and electrons above the Fermi level enhances the formation of magnons at Fe sites. This mechanism is consistent with a demagnetization delay between Fe and Ni, as found experimentally. [Preview Abstract] |
Wednesday, March 16, 2016 11:39AM - 11:51AM |
L18.00003: Time Resolved X-ray Magnetic Circular Dichroism at the Linac Coherent Light Source W. Schlotter, D. Higley, E. Jal, G. Dakovski, E. Yuan, J. MacArthur, A. Lutman, K. Hirsch, P. Granitzka, Z. Chen, G. Coslovich, M. Hoffman, A. Mitra, A. Reid, P. Hart, H.-D. Nuhn, H. Duerr, E. Arenholz, P. Shafer, P. Dennes, J. Joseph, L. Guyader, A. Tsukamoto We demonstrate ultrafast time resolved X-ray Magnetic Circular Dichroism on optically switchable GdFeCo thin film samples. This method extends the element specificity of time resolved x-ray absorption spectroscopy to characterize the evolution of electron spin and orbital angular momenta. These measurements were enabled by a recent upgrade at the Linac Coherent Light Source (LCLS) to generate circularly polarized x-rays. Additionally these measurements were enhanced by new detection systems that benefit all x-ray absorption spectroscopy experiments performed in transmission. Consequently static XMCD data are in excellent agreement with similar measurements at synchrotron light sources. The LCLS is an x-ray free electron laser user facility accessible via a peer-reviewed proposal process. [Preview Abstract] |
Wednesday, March 16, 2016 11:51AM - 12:27PM |
L18.00004: Spin pumping in electrodynamically coupled magnon-photon systems Invited Speaker: Lihui Bai The electronics industry is quickly approaching the limitation of Moore's Law due to Joule heating in high density-integrated devices. To achieve new higher-speed devices and reduce energy consumption, researchers are turning to spintronics where the intrinsic spin, rather than the charge of electrons, is used to carry information in devices. Advances in spintronics have led to the discovery of giant magnetoresistance (GMR), spin transfer torque etc. Another subject, cavity electrodynamics, promises a completely new quantum algorithm by studying the properties of a single electron interacting with photons inside of a cavity. By merging both spintronics and cavity electrodynamics, a new cutting edge field called Cavity Spintronics is forming, which draws on the advantages of both subjects to develop new spintronics devices utilizing light-matter interaction. In this work, we use electrical detection, in combination with microwave transmission, to investigate both resonant and nonresonant magnon-photon coupling in a microwave cavity at room temperature. Spin pumping in a dynamically coupled magnon-photon system is found to be distinctly different from previous experiments. Characteristic coupling features such as modes anticrossing, linewidth evolution, peculiar line shape, and resonance broadening are systematically measured and consistently analyzed by a theoretical model set on the foundation of classical electrodynamic coupling. Our experimental and theoretical approach paves the way for pursuing microwave coherent manipulation of pure spin current via the combination of spin pumping and magnon-photon coupling. [Preview Abstract] |
Wednesday, March 16, 2016 12:27PM - 12:39PM |
L18.00005: Ultrafast magnetization dynamics in heterogeneous granular FePt media Patrick Granitzka, Alexander Reid, Emmanuelle Jal, TianMin Liu, William Schlotter, Padraic Shafer, Virat Metha, Olav Hellwig, Yukiko Tahakashi, Eric Fullerton, Joachim Stohr, Hermann Durr Granular FePt in the L1$_0$ phase is a key material for future magnetic data storage devices, supporting stable magnetic domains less than 10 nm in diameter. To switch the magnetization of magnetically hard materials like FePt, new writing techniques are needed such as Heat Assisted Magnetic Recording (HAMR). However, it is not known how HAMR works on the fundamental length and time scales of magnetization in FePt. Here we investigate the nanoscale aspects of magnetization dynamics in FePt HAMR with fs X-ray pulses from the Linac Coherent Light Source at Stanford using resonant X-ray diffraction. We show that while many spins display switching in a magnetic field following a fs duration optical excitation. The remaining spins do not switch. Surprisingly the ratio of spins that switch to spins that do not, stays constant over a large fluence range. Furthermore we observe that the spin reservoir which displays heat assisted magnetic recording is quenched homogeneously over the size distribution of grains, while the spins that do not follow the field display a length-scale dependent quenching. [Preview Abstract] |
Wednesday, March 16, 2016 12:39PM - 12:51PM |
L18.00006: A simple and effective theory for all-optical helicity-dependent spin switching Guoping Zhang, Yihua Bai, Thomas F George All-optical helicity-dependent spin switching (AOS) represents a new frontier in magnetic recording technology, where a single ultrafast laser pulse, without any assistance from an external magnetic field, can permanently switch spin within a few hundred femtoseconds. By contrast, the existing theory does rely on an artificial magnetic field to switch spins. Here we develop a microscopic spin switch theory, free of any artificial field, and demonstrate unambiguously that both circularly and linearly polarized lights can switch spins faithfully. Our theory is based on the Hookean theory, but includes two new elements: spin-orbit coupling and exchange interaction. We predict that left (right) circularly polarized light only flips (flops) spin, a symmetry constraint that strongly favors ferrimagnetic orderings over ferromagnetic ones, with the allowable exchange interaction within 10 meV, consistent with all prior theories. The effect of the laser amplitude is highly nonlinear: If it is too weak, AOS does not occur, but if too strong, the spin cants; a compromise between them produces a narrow spin reversal window as observed experimentally. We envision that our model can be easily extended to describe spin frustrated systems and multiferroics, where the light-spin interaction [Preview Abstract] |
Wednesday, March 16, 2016 12:51PM - 1:03PM |
L18.00007: A fast time-dependent density functional theory method for the simulation of ultrafast demagnetization induced by laser Zhanghui Chen, Lin-Wang Wang We present a fast real-time time-dependent density functional theory method to investigate the ultrafast spin dynamics induced by laser. The Hamiltonian considers non-collinear magnetic moment, spin-orbital coupling and electron-laser interaction. An accelerated method with leapfrog prediction of charge matrix is used to solve the time-evolving equation. The investigation of Ni bulk found that the spin demagnetization consists of one time-lag stage and one fast demagnetization stage followed by one slow demagnetization stage. The time-lag and fast stages are mainly affected by the spin-electron interaction and their interactions with photons while the slow stage is affected by phonon-related interaction. Demagnetization appears only when spin-oribital coupling effect is considered. We further demonstrated how to manipulate the spin dynamics by changing laser fluence, duration and wavelength. [Preview Abstract] |
Wednesday, March 16, 2016 1:03PM - 1:15PM |
L18.00008: A comparative study of laser-induced demagnetization dynamics in Fe, Co, and Ni Maithreyi Gopalakrishnan, Christian Gentry, Dmitriy Zusin, Patrik Grychtol, Ronny Knut, Justin Shaw, Hans Nembach, Stefan Mathias, Martin Aeschlimann, Peter Oppeneer, Claus Schneider, Henry Kapteyn, Margaret Murnane Even twenty years after the discovery of ultrafast demagnetization of ferromagnetic materials induced by a femtosecond laser pulse there is still an ongoing debate about the mechanisms that drive the process. Surprisingly, a comprehensive study that compares demagnetization dynamics in different materials on equal footing is lacking. Yet, the scientific community would greatly benefit from such study. We fill this gap by performing a systematic comparison of ultrafast demagnetization behavior in Iron, Cobalt and Nickel, the simplest itinerant ferromagnets, under a wide range of pump fluences. In this experiment, we utilize a tabletop broadband extreme ultraviolet source to probe magnetization dynamics at the M$_{2,3}$ absorption edges of these three elements using the transverse magneto-optical Kerr effect. The obtained data can be used to inform theory and, thereby, assist in resolving the remaining questions about the micro- and macroscopic mechanisms behind ultrafast laser-induced magnetization dynamics in materials. [Preview Abstract] |
Wednesday, March 16, 2016 1:15PM - 1:27PM |
L18.00009: Ultrafast demagnetization, spin-dependent Seebeck effect, and thermal spin transfer torque in Pt/TbFe/Cu and Pt/TbFe/Cu/Fe thin films Johannes Kimling, Birgit Hebler, Judith Kimling, Manfred Albrecht, David G. Cahill We investigate diffusive spin currents in Pt(20nm)/TbFe(10nm)/Cu(100nm) and Pt(20 nm)/TbFe(10nm)/ Cu(100nm)/Fe(3nm) stacks using time-resolved magneto-optic Kerr effect (TRMOKE) and time-domain thermoreflectance measurements. Our experiments are based on two hypothesis: (1) fast changes of magnetization due to laser excitation are transferred into spin accumulation, e.g., via electron-magnon scattering; the generated spin accumulation drives a diffusive spin current into adjacent normal metal layers; (2) electronic thermal transport through the ferromagnetic layer injects a spin current into adjacent normal metal layers, based on the spin-dependent Seebeck effect. [1] We excite the Pt layer with ps-laser pulses. Resulting diffusive spin currents generate nonequilibrium magnetization in the Cu layer (sample I) and induce a precession of the magnetization of the Fe layer via spin transfer torque (sample II). Both responses are probed using TRMOKE. Prior experiments used [Co(0.2nm)/Pt(0.4nm)]$_{\mathrm{x5}}$/Co(0.2nm) instead of TbFe. [1] The ferrimagnetic TbFe layer with introduces two major modifications: (1) slow demagnetization behavior, and (2) large thermal resistance. Hence, thermal spin transfer torques can be observed on significantly longer time scales. [1] G.-M. Choi, C.-H. Moon, B.-C. Min, K.-J. Lee, and D. G. Cahill, Nature Physics 11, 576 (2015). [Preview Abstract] |
Wednesday, March 16, 2016 1:27PM - 1:39PM |
L18.00010: Electronic properties of solids excited with intermediate laser power densities. Fausto Sirotti Intermediate laser power density up to about 100 GW/cm2 is below the surface damage threshold is currently used to induce modification in the physical properties on short time scales. The absorption of a short laser pulse induces non-equilibrium electronic distributions followed by lattice-mediated equilibrium taking place only in the picosecond range. The role of the hot electrons is particularly important in several domains as for example fast magnetization and demagnetization processes [1], laser induced phase transitions, charge density waves. Angular resolved photoelectron spectroscopy measuring directly energy and momentum of electrons is the most adapted tool to study the electronic excitations at short time scales during and after fast laser excitations. The main technical problem is the space charge created by the pumping laser pulse. I will present angular resolved multiphoton photoemission results obtained with 800 nm laser pulses [2] showing how space charge electrons emitted during fast demagnetization processes can be measured. $~$ \newline \textbf{[1]}\underline {N. Beaulieu et al. } Journal of Electron Spectroscopy and Related Phenomena, 2013, 189 Supp: 40--45\textbf{ \newline [2] }\underline {F. Sirotti et al. }Physical Review B. 2014, 90(3): art.n\textdegree 035401 [Preview Abstract] |
Wednesday, March 16, 2016 1:39PM - 1:51PM |
L18.00011: Ultrafast Study of Dynamic interfacial Exchange Coupling in Ferromagnet/Oxide/Semiconductor Heterostructures Yu-Sheng Ou, Yi-Hsin Chiu, Nicholas Harmon, Patrick Odenthal, Matthew Sheffield, Michael Chilcote, Roland Kawakami, Michael Flatté, Ezekiel Johnston-Halperin Time-resolved Kerr/Faraday rotation (TRKR/TRFR) is employed to study GaAs spin dynamics in the regime of strong and dynamic exchange coupling to an adjacent MgO/Fe layer. This study reveals a dramatic, resonant suppression in the inhomogeneous spin lifetime (T2*) in the GaAs layer. Further investigation of the magnetization dynamics of the neighboring Fe layer, also using TRKR/TRFR, reveals not only the expected Kittel-dispersion but also additional lower frequency modes with very short lifetime (~65 ps) that are not easily observed with conventional ferromagnetic resonance (FMR) techniques. These results suggest the intriguing possibility of resonant dynamic spin transfer between the GaAs and Fe spin systems. We discuss the potential for this work to establish GaAs spin dynamics as an efficient detector of spin dissipation and transport in the regime of dynamically-driven spin injection in ferromagnet/semiconductor heterostructures. [Preview Abstract] |
Wednesday, March 16, 2016 1:51PM - 2:03PM |
L18.00012: Heisenberg vs. Stoner: Ultrafast magnon generation and exchange renormalization in the course of laser-induced demagnetization Dmitriy Zusin, Emrah Turgut, Patrik Grychtol, Ronny Knut, Dominik Legut, Justin Shaw, Hans Nembach, Thomas Silva, Stefan Mathias, Martin Aeschlimann, Claus Schneider, Karel Carva, Peter Oppeneer, Henry Kapteyn, Margaret Murnane In this work, we access the microscopic mechanisms responsible for the ultrafast magnetization dynamics of ferromagnets following a femtosecond laser excitation. Using a tabletop high-harmonic source of extreme ultraviolet light, we perform magneto-optical pump-probe spectroscopic studies across the M$_{\mathrm{2,3}}$ absorption edge of Cobalt with time, energy and angle resolution. This novel approach allows us to extract the time-dependent resonant magneto-optical properties of the Cobalt sample. In combination with ab-initio calculations of the density of states and the magneto-optical response, this gives us indirect access to ultrafast dynamics of the band structure. A comparison of our theoretical simulations with the experimental measurements suggests a variety of demagnetization mechanisms, which include ultrafast magnon excitations, enhanced electron temperature and transient renormalization of exchange splitting. [Preview Abstract] |
Wednesday, March 16, 2016 2:03PM - 2:15PM |
L18.00013: Studies of Ultrafast Demagnetization in Ferromagnetic Metal Alloys using TDDFT Peter Elliott, Kevin Krieger, John Kay Dewhurst, Sangeeta Sharma, E.K.U. Gross Time dependent density functional theory (TDDFT) has recently[1,2] been applied to study magnetization dynamics in periodic systems. In particular it was found for short intense pulses, a significant source of demagnetization is spin-flips mediated by the spin-orbit interaction. In this work, we perform TDDFT simulations for the case of bulk Heusler compounds under the same conditions, and find a similar loss of the global magnetic moment can occur. Furthermore, we also see local loss of moment due to transfer of moment from one sublattice to another during the optical excitation process. This is then followed by the spin-orbit mediated demagnetization in certain cases. Additionally we will analyze the spin-current densities to better understand the various processes at work. [1] Laser induced ultrafast demagnetization: an ab-initio perspective, K. Krieger, J.K. Dewhurst, P. Elliott, S. Sharma, E.K.U. Gross, J. Chem. Theory and Comput. 11, 4870 (2015). [2] Demonstration of Optimal Control of Laser Induced Spin-Orbit Mediated Ultrafast Demagnetization, P. Elliott, K. Krieger, J. K. Dewhurst, S. Sharma, E. K. U. Gross, submitted (2015). [Preview Abstract] |
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