Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session P18: Spin-Dynamics in Patterned Films and DevicesFocus
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Sponsoring Units: GMAG DMP FIAP Chair: Volker Sluka, New York University Room: 317 |
Wednesday, March 16, 2016 2:30PM - 3:06PM |
P18.00001: Electrically driven magnetization dynamics in yttrium iron garnet Invited Speaker: Matthias Benjamin Jungfleisch Creation and manipulation of magnetization states by spin-orbital torques are important for novel spintronics applications. Magnetic insulators were mostly ignored for this particular purpose, despite their low Gilbert damping, which makes them outstanding materials for magnonic applications and investigation of nonlinear spin-wave phenomena. Here, we demonstrate the propagation of spin-wave modes in micro-structured yttrium iron garnet (Y$_{\mathrm{3}}$Fe$_{\mathrm{5}}$O$_{\mathrm{12}}$,YIG) stripes. Spin waves propagating along the long side of the stripe are detected by means of spatially-resolved Brillouin light scattering (BLS) microscopy. The propagation distance of spin waves is determined in the linear regime, where an exponential decay of 10 $\mu $m is observed\footnote{ M. B. Jungfleisch \textit{et al.}, J. Appl. Phys.~\textbf{117}, 17D128~(2015).}. We also explored the possibility of driving magnetization dynamics with spin Hall effects (SHE) in bilayers of YIG/Pt microstructures. For this purpose we adopted a spin-transfer torque ferromagnetic resonance (ST-FMR) approach. Here a \textit{rf} charge current is passed through the Pt layer, which generates a spin-transfer torque at the interface from an oscillating spin current via the SHE. This gives rise to a resonant excitation of the magnetization dynamics. In all metallic systems the magnetization dynamics is detected via the homodyne anisotropic magnetoresistance of the ferromagnetic layer. However, since there is no charge flowing through ferromagnetic insulators there is no anisotropic magnetoresistance. Instead, we show that for the case of YIG/Pt the spin Hall magnetoresistance can be used. Our measured voltage spectra can be well fitted to an analytical model evidencing that the ST-FMR concept can be extended to insulating systems\footnote{ J. Sklenar \textit{et al.}, Phys. Rev. B, \textit{in press,} arXiv:1505.07791 [cond-mat.mes-hall] (2015).}. Furthermore, we employ spatially-resolved BLS spectroscopy to map the ST-FMR driven spin dynamics. We observe the formation of a strong, self-localized spin-wave intensity in the center of the sample\footnote{ M. B. Jungfleisch \textit{et al.}, arXiv:1505.07791 [cond-mat.meshall] (2015).}. This spin-wave `bullet' is created due to nonlinear cross coupling of eigenmodes existing in the magnetic system, which is confirmed by micromagnetic simulations. [Preview Abstract] |
Wednesday, March 16, 2016 3:06PM - 3:18PM |
P18.00002: Spin Hall Control of Magnetization in a Perpendicularly-Magnetized Magnetic Insulator Chi-Feng Pai, Andy Quindeau, Astera Tang, Mehmet Onbasli, Maxwell Mann, Lucas Caretta, Caroline Ross, Geoffrey Beach Spin Hall effect (SHE)-induced spin-orbit torque (SOT) has been shown to be an efficient mechanism to control the magnetization in magnetic heterostructures. Although numerous works have demonstrated the efficacy of SOT in manipulating the magnetization of ferromagnetic metals (FM), SOT-controlled switching of ferromagnetic insulators (FMIs) has not yet been observed. In this work we show that spin Hall currents in Pt and Ta can generate SOTs strong enough to control the magnetization direction in an adjacent thulium iron garnet FMI film with perpendicular magnetic anisotropy. We find that dc current in the heavy metal (HM) generates an out-of-plane effective field in the FMI consistent with an antidamping torque whose magnitude is comparable to that observed in all-metallic systems. Spin Hall magnetoresistance (SMR) measurements reveal a large spin-mixing conductance, which implies considerable spin transparency at the metal/insulator interface and explains the observed strong current-induced torque. Our results show that charge currents flowing in a HM can be used to both control and detect the magnetization direction in a FMI electrically. [Preview Abstract] |
Wednesday, March 16, 2016 3:18PM - 3:30PM |
P18.00003: Spin transfer torque switching in MTJ arrays with nanoengineered uniaxial anisotropy Ilyas A. H. Farhat, E. Gale, M. Abi Jaoude, A. F. Isakovic Enhancing Magnetic Tunnel Junction (MTJ) energy efficiency is the key to embed it in low power applications. We report a detailed study on the behavior of the analytical expression of switching current density as a function of geometrica and magnetic parameters, for both I-MTJs and P-MTJs. Our study shows that the current model requires some modifications to improve the match between the model and the experiment. We also show under which criteria a scaledown of MTJ devices can help in reduction of current density. We then used the device model of MTJ to study the power performance of MTJ device, proposing a roadmap to lower switching power of the device. Comparisons between ours and data for similar devices in literature, combined with the above analysis, suggests the need for qualitatively different model, and for this purpose, we explored the variations of the effective energy density model [1], which may explain the device behavior better. [1] T. Taniguchi et al., Phys. Rev. B 87, 054406 (2013). [Preview Abstract] |
Wednesday, March 16, 2016 3:30PM - 3:42PM |
P18.00004: Vortex Gyrotropic Motion in patterned Ferromagnetic Dots Junjia Ding, Pavel Lapa, Trupti Chair, Chrystian Posada, Axel Hoffmann, Valentine Novosad A vortex state consists of a large region of in-plane curling magnetization and a small core region with out-of-plane magnetization. The gyrotropic oscillation frequency of the vortex core is known to be weakly dependent to the core position (which is adjustable by changing the applied field) and can only be efficiently tuned by changing the dimension of the dots. Here, we demonstrated that the vortex gyrotropic frequency can be stepwise tuned by introducing a vortex barrier to a regular ferromagnetic dot. Systematical investigations of the dynamic response of the engineered dots have been performed as a function of the outer dot diameter, barrier diameter and the barrier profile using both microwave absorption spectroscopy and micromagnetic simulation. We found that the vortex frequency is mostly dependent on the outer diameter of the dot when the core is outside the barrier, while it is more rely on the dimension of the barrier when the core is inside the barrier. This approach certainly gives several additional freedoms to adjust the vortex gyrotopic frequency and opens extra perspectives for spintronic applications. [Preview Abstract] |
Wednesday, March 16, 2016 3:42PM - 3:54PM |
P18.00005: Dynamics of spin valves investigated using Magneto-Optical Kerr Effect Spectroscopy Christopher Stevens, Jagannath Paul, Prasenjit Dey, Casey Miller, Stephen McGill, Denis Karaiskaj Through an all-optical approach, we are investigating the spin dynamics in different spin torque based structures. Using pump-probe Time-Resolved Magneto-Optical Kerr Effect (TR-MOKE) spectroscopy, we are able to monitor the ultrafast magnon propagation on a sub-picosecond timescale as well as the longer lived oscillations and demagnetization. This represents a recent efforts to realize magnon induced spin torque using an all optical method. [Preview Abstract] |
Wednesday, March 16, 2016 3:54PM - 4:06PM |
P18.00006: The shot noise like feature of the magnetic 1/f noise in CoFeB/MgO/CoFeB magnetic tunnel junctions. Liang Liu, Jiasen Niu, Huiqiang Guo, Jian Wei, D. L. Li, J. F. Feng, X. F. Han, X.-G. Zhang, J. M. D. Coey The magnetic field dependent 1/f noise in magnetic multilayers and magnetic tunnel junctions (MTJs) is conventionally considered as resistance fluctuations (S$_{\mathrm{R}})$, for which an applied current (I) is merely used to convert S$_{\mathrm{R}}$ to measurable voltage fluctuations (S$_{\mathrm{V}} \quad =$ I$^{\mathrm{2}}$S$_{\mathrm{R}})$. From S$_{\mathrm{R}}$ and magnetoresistance, magnetization fluctuations can be inferred obeying the fluctuation-dissipation relation (FDR), thus comes the name magnetoresistive noise. However, we find that 1/f noise in CoFeB/MgO/CoFeB MTJs is better described by S$_{\mathrm{I}}$/I, instead of S$_{\mathrm{V}}$/I$^{\mathrm{2}}$, particularly near the magnetic reversal fields of the reference layer and the free layer, the latter of which has not been previously investigated in detail. More surprisingly, the bias dependence resembles that of shot noise. These findings call for further investigation on FDR for magnetic noise in MTJs, especially in the far from equilibrium state with high bias and possible contribution from collective magnon excitations. [Preview Abstract] |
Wednesday, March 16, 2016 4:06PM - 4:18PM |
P18.00007: Coherent spin-transfer precession switching in orthogonal spin-torque devices Colm Ryan, Graham Rowlands, Daniele Pinna, Li Ye, Laura Rehm, Volker Sluka, Andy Kent, Thomas Ohki We present experimental results in concert with macrospin simulations of the switching characteristics of orthogonal spin-transfer devices incorporating an out-of-plane magnetized polarizing layer and an in-plane magnetized spin valve device at cryogenic temperatures. Switching at 3.4K between parallel and anti-parallel spin-valve states is investigated for current pulses with varying durations from 0.1 to 1.4ns to observe the averaged response of the time dependent dynamics of the spin-transfer induced precession of the magnetization. We demonstrate high speed switching at short pulse lengths, down to 100ps, and also observe ensemble decoherence effects with longer pulses. The results show that even at cryogenic temperatures finite temperature noise is still important in the dynamics of precessional switching. [Preview Abstract] |
Wednesday, March 16, 2016 4:18PM - 4:30PM |
P18.00008: Effects of spin relaxation on trap-assisted tunneling through ferromagnetic metal-oxide-semiconductor structures Viktor Sverdlov, Siegfried Selberherr A signal measured within a three-terminal setup at room temperature [1,2] is attributed to the spin injection from a ferromagnetic electrode into n-silicon; however, its amplitude is orders of magnitude larger than predicted by theory [3]. The reasons for this discrepancy are heavily debated [3-6], with trap-assisted resonant tunneling [4] and spin-dependent magnetoresistance gaining recognition. However, effects of spin relaxation important at room temperature were not considered in [4]. To elucidate the role of spin relaxation and coherence, corresponding Lindblad terms are added to the equation for the density matrix evolution of spin on a trap coupled to ferromagnetic contacts. Fast spin relaxation suppresses the magnetoresistance modulation. Interestingly, strong decoherence at fixed spin lifetime results in a more pronounced magnetoresistance modulation and in a narrower magnetoresistance linewidth as a function of the perpendicular magnetic field. 1.S.P.Dash {\it et al.}, Nature {\bf 462},491 (2009). 2.C.Li {\it et al.}, Nature Commun.{\bf 2}, 245 (2011). 3.R.Jansen, Nature Mater.{\bf 11}, 400 (2012). 4.Y.Song and H.Dery, PRL {\bf 113}, 047205 (2014). 5.A.Spiesser {\it et al.}, PRB {\bf 90}, 205213 (2014). 6.K.-R.Jeon {\it et al.}, PRB {\bf 91}, 155305 (2015). [Preview Abstract] |
Wednesday, March 16, 2016 4:30PM - 4:42PM |
P18.00009: Thermally reliable clocked non-volatile spin wave logic device Sourav Dutta, Dmitri Nikonov, Sasikanth Manipatruni, Ian Young, Azad Naeemi The possibility of utilizing spin waves for information transmission and computation has been an area of active research due to the unique ability to manipulate the amplitude and phase of the spin waves for building complex logic circuits. Here, we present a comprehensive scheme for building a thermally reliable clocked non-volatile spin wave logic device [1,2] (SWLD) by introducing a charge-to-spin converter that translates information from electrical domain to spin domain, exploiting the magneto-electric effect for spin wave transmission, detection and non-volatile memory, utilizing the phase of the spin wave as information token, ensuring phase-dependent deterministic switching of the magnetoelectric spin wave detector in the presence of thermal noise via compensation of demagnetization and a novel clocking scheme that ensures sequential transmission of information in a cascaded SWLD and non- reciprocity. [1] S. Dutta et. al., “Non-volatile clocked spin wave interconnect for beyond-cmos nanomagnet pipelines,” Scientific Reports 5 (2015). [2] S. Dutta et. al., “Phase-dependent deterministic switching of magnetoelectric spin wave detector in the presence of thermal noise via compensation of demagnetization ,” Applied Physics Letters (accepted 2015). [Preview Abstract] |
Wednesday, March 16, 2016 4:42PM - 4:54PM |
P18.00010: Magnetotransport properties of Co$_{90}$Fe$_{10}$/Cu/Ni$_{80}$Fe$_{20}$ pseudo-spin-valve with out-of-plane tilted magnetic field Linqiang Luo, Nam Dao, Salinporn Kittiwatanakul, Stuart Wolf, Jiwei Lu The giant magnetoresistance (GMR) effect of a pseudo spin valve made of Co$_{90}$Fe$_{10}$/Cu/Ni$_{80}$Fe$_{20}$ has been investigated, with a magnetic field applied perpendicularly tilted to the sample plane. Without using a pinning layer, the magnetic separation of the free and fixed layers is uniquely achieved by utilizing perpendicular fields due to different anisotropy energies between Ni$_{80}$Fe$_{20\, }$and Co$_{90}$Fe$_{10}$. The magneto-transport measurements are carried out by Van der Pauw method in current-in-plane geometry at room temperature. By tilting the magnetic field at different angles from out-of-plane, the GMR plateau's width can be tuned. A plateau width of about 2000 Oe is observed at tilted angle 0.5$^{o}$, which opens a significantly larger window for high-resistance states comparing with a plateau width of 10 Oe for in-plane fields. With the out-of-plane tilted fields, the orientation of the magnetic moments can be tuned continuously out of the sample plane, and the relative orientation between Ni$_{80}$Fe$_{20}$ and Co$_{90}$Fe$_{10}$ can also be tuned by the tilted angle, enabling us to precisely control the sample's states for current-induced spin dynamics study that is very difficult in the case of in-plane applied magnetic fields. [Preview Abstract] |
Wednesday, March 16, 2016 4:54PM - 5:30PM |
P18.00011: Spin-torque ferromagnetic resonance in arbitrarily magnetized thin films Invited Speaker: Joseph Sklenar The spin Hall effect (SHE) in non-magnetic metals can be used to generate spin-transfer-torque (STT), subsequently inducing ferromagnetic resonance (FMR) in magnetic thin films; this experimental method is termed spin-torque ferromagnetic resonance (ST-FMR). Most ST-FMR experiments that are reported have an applied magnetic field in the plane of the sample and the research focuses on material combinations that have large and efficient STT. The most common way ST-FMR signals are detected is through an anisotropic magnetoresistance (AMR) rectification process. In this work we will present ST-FMR results in thin films where the magnetization has both an in-plane and out-of-plane component. The arbitrary magnetization direction is achieved by tipping the applied magnetic field out of the sample plane. We find that when the material system is a permalloy/Pt bilayer, ST-FMR signals are not mirror-symmetric upon magnetic field reversal \footnote{J. Sklenar \textit{et al}, Submitted}. This is because the combination of both a STT from the bulk SHE and the Oersted field-like torque from the device do not drive the dynamics in the same manner when the field is reversed. We interpret our results in the Py/Pt experiment by extending an already established ST-FMR lineshape model to describe the general case of arbitrarily magnetized films. We compare and contrast our Py/Pt experiment with another system we measured, a Py/MoS$_2$ bilayer. For the Py/MoS$_2$ system, in-plane experiments suggest that a large STT is present and are comparable to what is observed for the more traditional Py/Pt system \footnote{W. Zhang, J. Sklenar \textit{et al}, Submitted}. On the other hand, the out-of-plane experiment for the Py/MoS$_2$ system is qualitatively very different from Py/Pt. Our results suggest that ST-FMR experiments for arbitrarily magnetized magnetic films are useful in characterizing STT generated from interface rather than bulk effects. Work at Northwestern was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Materials Science and Engineering Division under grant number DE-SC0014424. Work at Argonne was supported by the U.S. Department of Energy, OS, Materials Science and Engineering Division. Lithography was carried out at the Center for Nanoscale Materials, which is supported by DOE, OS-BES under Contract No. DE-AC02-06CH11357. [Preview Abstract] |
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