Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session E19: Ultrafast Magnetism and SwitchingFocus
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Sponsoring Units: GMAG DMP FIAP Chair: Ezekiel Johnston-Halperin, Ohio State University Room: LACC 308A |
Tuesday, March 6, 2018 8:00AM - 8:36AM |
E19.00001: Ultrafast Magnetization Manipulation Using Single Femtosecond Light and Hot-Electron Pulse Invited Speaker: Stephane Mangin Current induced magnetization manipulation is a key issue for spintronic applications. This manipulation must be fast, deterministic and non-destructive in order to function in device applications. Therefore, single electric pulse driven deterministic switching of the magnetization at the picosecond timescale represents a major step towards the future developments of ultrafast spintronic systems. Here we have studied the ultrafast magnetization dynamics in engineered Gdx[FeCo]1-x based structures to compare the effect of femtosecond laser and hot-electron pulses. We demonstrate that a single femtosecond hot-electron pulse causes deterministic magnetization reversal in either Gd-rich and FeCo-rich alloys similarly to a femtosecond laser pulse. In addition, we show that the limiting factor of such manipulation for perpendicular magnetized films arises from formation of a multi-domain state due to dipolar interaction. By performing time resolved measurements under various magnetic fields, we demonstrate that the same magnetization dynamics is observed for both light and hot-electron excitation, and that the full magnetization reversal takes place within 40 ps. The efficiency of the ultrafast current induced magnetization manipulation is optimized thanks to the ballistic transport of hot-electrons before reaching the GdFeCo magnetic layer. |
Tuesday, March 6, 2018 8:36AM - 8:48AM |
E19.00002: Surface-plasmon Opto-magnetic Field Enhancement for All-optical Magnetization Switching Aveek Dutta, Deesha Shah, Bradlee Beauchamp, Vladimir Shalaev, Alexandra Boltasseva, Alexander Kildishev, Ernesto Marinero All-optical magnetization switching, based on the inverse Faraday effect, has been shown to be an attractive method for achieving magnetization switching at ps speeds. Successful magnetization reversal in thin films has been demonstrated by using circularly polarized light. However, a method for all-optical switching of on-chip nanomagnets in high density memory modules has not been described. In this work we propose to use plasmonics, with CMOS compatible plasmonic materials, to achieve on-chip magnetization reversal in nanomagnets. Plasmonics allows light to be confined in dimensions much smaller than the diffraction limit of light. This in turn, yields higher localized electromagnetic field intensities. In this work, through simulations, we show that using localized surface plasmon resonances, it is possible to couple light to nanomagnets and achieve significantly higher opto-magnetic field values in comparison to free space light excitation. |
Tuesday, March 6, 2018 8:48AM - 9:00AM |
E19.00003: Critical Behavior within 20fs Drives the Out-of-Equilibrium Laser-induced Magnetic Phase Transition in Nickel Wenjing You, Phoebe Tengdin, Cong Chen, Xun Shi, Dmitriy Zusin, Yingchao Zhang, Christian Gentry, Adam Blonsky, Mark Keller, Peter Oppeneer, Henry Kapteyn, Zhensheng Tao, Margaret Murnane Ferromagnets undergo a ferromagnetic to paramagnetic phase transition when the material is slowly heated above the Curie temperature under equilibrium conditions. However, to date it is not known how fast such phase transitions can proceed under out-of-equilibrium excitation conditions, nor what the connection is between out-of-equilibrium and equilibrium phase transitions. By combining time- and angle-resolved photoemission with time-resolved transverse magneto-optical Kerr spectroscopy, we show that critical behavior governs the ultrafast magnetic phase transition in nickel. For laser fluences that transiently drive the electron temperature above the Curie temperature, we find that the heat capacity of the electron and spin system diverges. Very surprisingly, the spin system absorbs sufficient energy within 20 fs to subsequently proceed through the phase transition, which defines a new timescale in the process of ultrafast demagnetization in ferromagnets. Demagnetization and the collapse of the exchange splitting then occur on longer timescales of ~176 fs. Our results connect the out-of-equilibrium material behavior to the strongly coupled equilibrium behavior, and also establish that the transient electron temperature dominantly dictates the magnetic response. |
Tuesday, March 6, 2018 9:00AM - 9:12AM |
E19.00004: Correlating EUV TMOKE and ARPES measurements to understand the temporal and spatial length scales underlying ultrafast demagnetization in nickel Phoebe Tengdin, Wenjing You, Cong Chen, Xun Shi, Dmitriy Zusin, Yingchao Zhang, Christian Gentry, Adam Blonsky, Mark Keller, Peter Oppeneer, Henry Kapteyn, Zhensheng Tao, Margaret Murnane Time-resolved magneto-optical Kerr effect spectroscopies spanning the M-shell absorption edges of magnetic materials provide powerful probes of the spin dynamics. However, to date, the disparate dynamics that occur at different depths in a laser-heated material were not disentangled. Here we show that this spatial averaging masks the true behavior of the material, where parts of the material at the surface can go through the ferromagnetic-paramagnetic phase transition, while deeper layers will not. We quantitatively compare depth sensitive time-resolved transverse magneto-optical Kerr effect measurements with time- and angle-resolved photoemission spectroscopy surface measurements on nickel. We show that the first critical behavior of the material occurs when the electron temperature as measured directly through ARPES and transient reflectivity reaches the Curie temperature, followed by a second critical behavior when the lattice temperature reaches the Curie temperature. These observations coupled with knowledge of the probing depth of each method, completely explain the results obtained from both experiments. |
Tuesday, March 6, 2018 9:12AM - 9:48AM |
E19.00005: Picosecond electrical excitation of ultrafast magnetization dynamics in ferro- and ferrimagnetic metals Invited Speaker: Richard Wilson When electrons in a magnetic metal are driven far from equilibrium via ultrafast heating of the electrons, the magnetic order undergoes radical changes within tens of femtoseconds due to massive flows of energy and angular momentum between electrons, spins, and phonons. In ferrimagnetic metals such as GdFeCo, ultrafast optical heating can deterministically reverse the magnetization in less than a picosecond. In this talk, I describe our experimental work to gain a better understanding of how energy is exchanged between electrons, phonon, and spins in a magnetic metal following ultrafast heating. We use time-resolved measurements of the magneto-optic Kerr effect to record the response of ferro- and ferri-magnetic metals to heating via ultrafast optical or electrical pulses. Picosecond electrical pulses are generated with photoconductive Auston switches. By comparing the magnetic dynamics that result from electrical vs. optical heating, we identify differences in the rate of energy transfer to phonons from thermal vs. nonthermal electrons. We also find that both optical and electrical heating are effective for ultrafast switching of ferrimagnetic metals. We observe deterministic, repeatable ultrafast reversal of the magnetization of a GdFeCo thin film with a single sub-10 ps electrical pulse. The magnetization reverses in ~10 ps, which is more than one order of magnitude faster than other electrically controlled magnetic switching mechanisms. |
Tuesday, March 6, 2018 9:48AM - 10:00AM |
E19.00006: Simulating Ultrafast Switching of Magnetic Order using TDDFT Peter Elliott, John Dewhurst, Sam Shallcross, Sangeeta Sharma, Eberhard K Gross Time dependent density functional theory (TDDFT) has recently[1] been applied to study magnetization dynamics in periodic systems. We identified two mechanisms that can cause changes in the local moment: 1) spin-flips mediated by the spin-orbit interaction[1,2] which contributes to ultrafast demagnetization in simple ferromagnets like Ni,Fe,Co and 2) inter sublattice spin transfer due to optical transitions induced by the applied laser[3]. In this work we look at systems with anti-ferromagnetic coupling and show how the inter sublattice spin transfer can actually cause a switch in the magnetic ordering from anti-ferromagnetic to ferromagnetic. |
Tuesday, March 6, 2018 10:00AM - 10:12AM |
E19.00007: Abstract Withdrawn
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Tuesday, March 6, 2018 10:12AM - 10:24AM |
E19.00008: Electronic correlation and quantum memory effects in the ultrafast laser induced charge and spin dynamics in bulk Ni Shree Ram Acharya, Volodymyr Turkowski, Talat Rahman We analyze the roles of electronic correlations and memory effects in the ultrafast charge and spin dynamics in bulk ferromagnetic Ni excited by laser pulses. The studies are performed by using the Time-Dependent Density-Functional Theory+Dynamical Mean-Field Theory (TDDFT+DMFT) approach [1;2], where the effects of strong correlations are taken into account by means of the exchange-correlation (XC) kernel obtained from the DMFT local-in-space charge susceptibility. It is shown that the TDDFT+DMFT results for the time-dependent ultrafast demagnetization in Ni is much closer to the experimental data as compared to the ones obtained with TDLDA that strongly underestimates the change of the magnetization. We argue that the main reason for a good agreement between the TDDFT+DMFT and experimental data is due to inclusion of the memory effects through the frequency-dependent DMFT XC kernel. |
Tuesday, March 6, 2018 10:24AM - 10:36AM |
E19.00009: The role of interface roughness on laser-induced helicity-dependent THz-emission in thin film Co/Pt bilayers Rajasekhar Medapalli, Guanqiao Li, Rostislav Mikhaylovskiy, Fred Spada, Thomas Silva, Theo Rasing, Alexey Kimel, Eric Fullerton
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Tuesday, March 6, 2018 10:36AM - 10:48AM |
E19.00010: Simulation of Ultrafast Spin-Currents in Optically-Excited Magnetic Multilayers Dennis Nenno, Michael Choquer, Marius Weber, Rudolf Binder, Hans Christian Schneider Optically excited spin-currents in magnetic heterostructures can induce magnetization dynamics even in non-excited layers [Alekhin et al. PRL 119, 017202 (2017)]. In this context, the hot carrier transport behavior is often characterized as superdiffusive [Battiato et al., PRB 86, 024404 (2012)]. We present a bottom-up approach to electron transport after ultrashort laser-excitation in magnetic multilayers, using the Particle-In-Cell method for the Boltzmann transport equation [Nenno et al., PRB 94, 115102 (2016)]. Simulating the induced dynamics in the whole slab, including transmission coefficients and ab-initio material data. From the calculations, we extract typical transport coefficients and clarify the contribution of secondary carrier generation in the transition from ballistic to diffusive transport behavior. |
Tuesday, March 6, 2018 10:48AM - 11:00AM |
E19.00011: Time-dependent Liouville density functional investigation of
ultrafast demagnetization Guoping Zhang, Yihua Bai, Thomas George Laser-induced ultrafast demagnetization in ferromagnetic 3d |
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