Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session A8: Focus Session: Magnetodynamics: Generation and Application |
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Sponsoring Units: GMAG Chair: Jeffrey Grossman, Massachusetts Institute of Technology Room: 104 |
Monday, March 3, 2014 8:00AM - 8:36AM |
A8.00001: Spin transfer torque excited spin-waves in metal--magnetic insulator bilayer Invited Speaker: Yan Zhou We develop a self-consistent theory for current-induced spin-wave excitations in normal metal-magnetic insulator bilayer structures. We compute the spin-wave dispersion and dissipation, including dipolar and exchange interactions in the magnet, the spin diffusion in the normal metal, as well as the surface anisotropy, spin-transfer torque, and spin pumping at the interface. We find that (1) the spin-transfer torque and spin pumping affect the surface modes more than the bulk modes; (2) spin pumping inhibits high-frequency spin-wave modes, thereby redshifting the excitation spectrum; (3) easy-axis surface anisotropy induces a new type of surface spin wave, which reduces the excitation threshold current and greatly enhances the excitation power. We propose that the magnetic insulator surface can be engineered to create spin-wave circuits utilizing surface spinwaves as information carriers. [Preview Abstract] |
Monday, March 3, 2014 8:36AM - 8:48AM |
A8.00002: Phonon Diodes and Transistors from Magneto-acoustics Sophia Sklan, Jeffrey Grossman The creation of non-reciprocal phononic systems holds the promise of allowing computers that would process thermal or acoustic (rather than electronic) signals. By sculpting the magnetic field applied to magneto-acoustic materials (which couple phonons to a magnetic field, typically due to effects like magnon-phonon coupling in yttrium iron garnet), phonons can be used for information processing in analogy with photonic computing. Using a combination of analytic and numerical techniques, we demonstrate designs for diodes (isolators) and transistors that are independent of their conventional, electronic formulation. We analyze the experimental feasibility of these systems, including the sensitivity of the circuits to likely systematic and random errors. [Preview Abstract] |
Monday, March 3, 2014 8:48AM - 9:00AM |
A8.00003: Ultra low-power hybrid spintronics-straintronics clocked with Surface Acoustic Waves (SAW) Mohammad Salehi Fashami, Supriyo Bandyopadhyay, Jayasimha Atulasimha The study of magnetization dynamics in magnetostrictive materials triggered with surface acoustic waves (SAWs) is of great interest not only from a fundamental point of view, but also for potential applications in energy efficient nanomagnetic computing. In this presentation, we model magnetization dynamics in dipole coupled arrays of nanomagnets clocked by acoustic waves. Specifically, this theoretical work demonstrates the feasibility of sequential logic devices such as flip-flops by showing that NAND gates and information propagation with cross-over of nanomagnet ``wires'' can be implemented and synchronously clocked with surface acoustic waves. [Preview Abstract] |
Monday, March 3, 2014 9:00AM - 9:12AM |
A8.00004: Surface Acoustic Wave induced magnetization change in Perpendicular anisotropy Co/Pt multilayers Uday Singh, Shireen Adenwalla We have demonstrated the ability of focused surface acoustic waves (FSAW) to control the magnetization direction of Co/Pt multilayers microstructure (4 x 5 $\mu$m) with perpendicular anisotropy. The strain wave generated by the FSAW results in large values of periodic compressive and tensile strain at the focal spot. The magnetoelasticity of Co results in changes in the magnetization easy axis with strain. To switch the magnetization from out of plane to in-plane requires tensile strain of more than 1{\%}. These large strains are obtained using annular interdigital transducers (AIDT) fabricated on 128$^{\circ}$ Y-Cut LiNbO$_{3}$, with a fundamental resonance frequency of 87.95MHz. We have mapped the strain distribution at the focal center using optical reflectivity and a knife edge, which selects for reflections above the specular edge. An array of Co/Pt multilayers was patterned at the focal center using e-beam lithography. In the region of highest strain, we observe magnetization changes in the Co/Pt multilayers excited by FSAW. We will discuss both dc and rf measurements of the changes in magnetization. [Preview Abstract] |
Monday, March 3, 2014 9:12AM - 9:24AM |
A8.00005: Dipole-exchange modes in transversely magnetized ferromagnetic stripes Rodrigo Arias, Zheng Duan, Ilya Krivorotov We present a theory of dipole-exchange modes in transversely magnetized ferromagnetic stripes of rectangular cross sections: a comparison is made with experimental results on Permalloy stripes. The model applies to very thin stripes (of the order of the exchange length): the magnetization is considered uniform over their thickness, and we consider modes of long wavelength along the longitudinal direction of the stripes. An applied magnetic field saturates the stripes along the transverse direction, and we also consider the effect of the exchange and dipolar fields. Under these assumptions we obtain the frequencies and shapes of the modes either considering free or pinned boundary conditions. We obtain good agreement with measurements of the frequency spectra in Permalloy nano wires of several rectangular cross sections: this happens for modes with appreciable amplitude throughout the samples. There is frequency disagreement for edge modes due to limitations of the model, since the effects of roughness, corners and imperfections at the edges of the samples are quite relevant in this case. [Preview Abstract] |
Monday, March 3, 2014 9:24AM - 9:36AM |
A8.00006: Ultra-low-energy analog straintronics using multiferroic composites Kuntal Roy Multiferroic devices, i.e., a magnetostrictive nanomagnet strain-coupled with a piezoelectric layer, are promising as binary switches for ultra-low-energy digital computing in beyond Moore's law era [Roy, K. Appl. Phys. Lett. \underline {103}, 173110 (2013), Roy, K. et al. Appl. Phys. Lett. \underline {99}, 063108 (2011), Phys. Rev. B \underline {83}, 224412 (2011), Scientific Reports (Nature Publishing Group) \underline {3}, 3038 (2013), J. Appl. Phys. \underline {112}, 023914 (2012)]. We show here that such multiferroic devices, apart from performing digital computation, can be also utilized for analog computing purposes, e.g., voltage amplification, filter etc. The analog computing capability is conceived by considering that magnetization's mean orientation shifts gradually although nanomagnet's potential minima changes abruptly. Using tunneling magnetoresistance (TMR) measurement, a continuous output voltage while varying the input voltage can be produced. Stochastic Landau-Lifshitz-Gilbert (LLG) equation in the presence of room-temperature (300 K) thermal fluctuations is solved to demonstrate the analog computing capability of such multiferroic devices. [Preview Abstract] |
Monday, March 3, 2014 9:36AM - 9:48AM |
A8.00007: Efficient room-temperature Spin Hall nano-oscillator Andrei Zholud, Sergei Urazhdin \newline Spin current injected into a ferromagnet exerts a spin torque on the magnetization, modifying its dynamical damping. Complete compensation of damping by spin current can result in magnetization auto-oscillations, as was demonstrated for in-plane point-contact spin Hall oscillator devices utilizing Pt spin Hall material as a source of spin current and permalloy (Py) as active magnetic layer [1]. Electronic spectroscopy has demonstrated microwave generation by oscillations of magnetization at cryogenic temperatures, but this microwave generation decreases with increasing temperature and disappears at room temperature[2]. We will describe a new device geometry that decouples spin transport from the magnetic configuration by separate patterning of the spin Hall Pt layer and the active Py layer. We demonstrate that this device geometry can operate at smaller driving dc currents for microwave generation that persists up to room temperature. We discuss the physical mechanisms that affect the temperature- and geometry-dependent performance of spin Hall nano-oscillators.\newline [1] V. Demidov, S. Urazhdin and S.O. Demokritov, Nature Mater. 9, 984 (2010) [2] R.H. Liu, W.L. Lim, and S. Urazhdin, Phys. Rev. Lett. 110, 147601 (2013) [Preview Abstract] |
Monday, March 3, 2014 9:48AM - 10:00AM |
A8.00008: Ferromagnetic cross junction based spin wave logic device Alexander Kozhanov Spin wave based signal processing/logic devices have long history of development and exploration. Typically the spin wave phase is used to encode the input information. Spin wave interference is used to produce the device output in form of the spin wave amplitude. Electronic amplitude-to-phase signal converter is required to build a logic gate capable of providing necessary fan-out. In case of destructive interference the phase information is lost and a ``new'' wave should be excited at the next logic stage. In this work we demonstrate the spin wave interference in ferromagnetic CoTaZr cross and propose a spin wave logic device based on this structure. Two neighboring arms of the cross serve as the device inputs. For the certain input wave phase offsets the interference is constructive in one output arm of the cross while destructive in another and vice versa thus resulting in a phase controlled spin wave switching. The output waves in the cross arms have different phase offsets dependent on the input wave phase offset. By merging the spin waves scattered into the cross output arms the device output is formed with a wave phase following the OR/NOR logic operation. We model local spin wave scattering in the cross center and discuss the effect of the local spin wave modes in the cross junction on the proposed device operation. [Preview Abstract] |
Monday, March 3, 2014 10:00AM - 10:12AM |
A8.00009: Experiments on straintronic nanomagnetic logic with two-state elliptical and four-state diamond and concave magnetostrictive nanomagnets Noel D'Souza, Mohammad Salehi Fashami, Supriyo Bandyopadhyay, Jayasimha Atulasimha Experimental work on strain-induced magnetization switching of single-domain magnetostrictive nanomagnets grown on a bulk \textless 001\textgreater PMN-PT substrate is demonstrated through Magnetic Force Microscopy (MFM) studies. Low-moment MFM probes are used in order to minimize tip-induced magnetization switching of the nanomagnets. Voltages are applied along the length of the PMN-PT substrate ($d_{33}$ mode) to generate the required strain in the magnetostrictive nanomagnet. Domain switching is then investigated in uniaxial (two-state) i) isolated, ii) dipole-coupled, and iii) an array of nanomagnets to implement NAND logic. Subsequent theoretical studies focus on four-state magnetostrictive nanomagnets (diamond- and concave-shaped). The magnetization characteristics of these shapes, particularly the switching coherence, are examined for various criteria (size, concavity depth, thickness, etc.) with the conclusion that concave nanomagnets are the ideal shape for coherent and reliable magnetization switching in future magnetoelectric devices. Experimental results of magnetic field- and stress-induced switching in these concave nanomagnets on a bulk PMN-PT substrate are also presented. [Preview Abstract] |
Monday, March 3, 2014 10:12AM - 10:24AM |
A8.00010: Large-area patterned substrates for micromagnetic actuation of superparamagnetic microbeads Minae Ouk, Geoffrey Beach Superparamagnetic microbeads (SBs) are widely used to capture biological entities in a fluid environment. Chip-based magnetic actuation provides a means to transport SBs in lab-on-a-chip technologies. This is usually accomplished using the stray field from patterned magnetic microstructures [1], or domain walls in magnetic nanowires [2]. However, lithographic patterning over a large area is costly and impractical using conventional techniques such as electron beam lithography. Here we use a simple floating-transfer technique [3] for large-area self-assembly of polystyrene microspheres on a Si wafer to produce lithographic masks texturing a substrate. Hexagonal patterns are used as lift-off and etching masks to create magnetic dot and anti-dot arrays in CoFe thin films, with a size and spacing that can be tuned via sphere diameter and RIE etch time. Using a rotating magnetic fields, we show that these magnetically-patterned substrates can transport SBs across large distances on the wafer surface, opening the possibility to augment or replace microfluidic actuation for long distance transport. [1] B. Yellen, et al., Lab Chip, 7, 1681 (2007) [2] E. Rapoport and G. S. D. Beach, APL 100, 082401 (2012) [3] X. Ye and L. Qi, Nano Today 6, 608 (2011) [Preview Abstract] |
Monday, March 3, 2014 10:24AM - 10:36AM |
A8.00011: Spin gated transistors for reprogrammable logic Chiara Ciccarelli, Fernando Gonzalez-Zalba, Andrew Irvine, Richard Campion, Liviu Zarbo, Brian Gallagher, Andrew Ferguson, Tomas Jungwirth, Joerg Wunderlich In spin-orbit coupled magnetic materials the chemical potential depends on the orientation of the magnetisation [1,2]. By making the gate of a field effect transistor magnetic, it is possible to tune the channel conductance not only electrically but also magnetically. We show that these magnetic transistor can be used to realise non-volatile reprogrammable Boolean logic. The non-volatile reconfigurable capability resides in the magnetization-dependent band structure of the magnetic stack. A change in magnetization orientation produces a change in the electrochemical potential, which induces a charge accumulation in the correspondent gate electrode. This is readily sensed by a field-effect device such as standard field-effect transistors or more exotic single-electron transistors. We propose circuits for low power consumption applications that can be magnetically switched between NAND and OR logic functions and between NOR and AND logic functions. [1] J. Wunderlich et al., PRL 97, 077201 (2006) [2] C. Ciccarelli, et al., Appl. Phys. Lett. 101, 122411(2012) [Preview Abstract] |
Monday, March 3, 2014 10:36AM - 10:48AM |
A8.00012: Electric-field modulation of the phase shift for spin waves Tianyu Liu, Xufeng Zhang, Hong Tang, Michael E. Flatt\'{e} An electric field has been predicted to manipulate the phase of spin waves in yttrium iron garnet (YIG) through the spin-orbit interaction, which couples the electric field with the gradient of the magnetization [1,2]. We have observed an electric-field-dependent phase shift in the propagation of surface spin waves in a YIG waveguide. In addition to the spin-orbit effect there is a stronger effect on the phase shift due to the change of the magnetization of the YIG due to the applied electric field (a magnetoelectric effect). The contributions of the two effects can be distinguished by varying the direction of the electric field relative to the YIG magnetization. \\[4pt] [1] T. Liu and G. Vignale, Phys. Rev. Lett. 106, 247203 (2011).\\[0pt] [2] T. Liu and G. Vignale, Journal of Applied Physics 111, 083907-083907-6 (2012). [Preview Abstract] |
Monday, March 3, 2014 10:48AM - 11:00AM |
A8.00013: Fast Deterministic Bipolar Switching in Orthogonal Spin Torque Devices via the Control of the Relative Spin Polarizations Junbo Park, Daniel C. Ralph, Robert A. Buhrman We model 100 ps pulse switching dynamics of orthogonal spin transfer (OST) devices that employ an out-of-plane polarizer (OPP) and an in-plane polarizer (IPP). Simulation results indicate that increasing the spin polarization ratio, $C_{P\, }= \quad P_{IPP}/P_{OPP}$, results in deterministic switching of the free layer without over-rotation (360 degree rotation). By using spin torque asymmetry to realize an enhanced effective $P_{IPP}$, we experimentally demonstrate this behavior in OST devices. Modeling predicts that decreasing the effective demagnetization field can substantially reduce the minimum $C_{P}$ required to attain deterministic bipolar switching, while retaining low critical switching current, $I_{p\, }=$ 500 $\mu $A. [Preview Abstract] |
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