Session H15: Focus Session: Spin and Dynamics in Metal, Spin logic and Spin-Based Devices

A novel three-terminal spintronics device utilizing the spin Hall effect

Previous work\footnote{Luqiao Liu \textit{et al.,}arXiv:1110.6846 and Luqiao Liu invited presentation this conference (focus topic 10.1.4)} has established that the spin Hall effect (SHE) in certain thin film metallic layers can generate a transverse spin current large enough to effect, through spin transfer torque (STT), the reversible magnetic switching of an adjacent ferromagnetic layer having perpendicular magnetic anisotropy. Here we discuss a new three-terminal spintronics device that utilizes the SHE induced STT to efficiently and reversibly switch the magnetic orientation of a thin free layer electrode of an MgO magnetic tunnel junction having in-plane magnetization. The low write currents ($\le $ 1mA), large output impedance and good thermal stability (45k$_{B}$T) that has been achieved with this SHE three-terminal device approach, which separates the write and read operations in a manner that is relatively straightforward to fabricate, demonstrate an attractive candidate for application in next generation STT MRAM and non-volatile spin logic circuits.   

Electric field assisted switching in magnetic tunnel junctions

It is of great interest to acquire large effects of electric field on magnetic properties, partly driven by the premise that voltage-controlled magnetization reversal would be far more energy efficient and be compatible with the ubiquitous voltage-controlled semiconductor devices. Normally the effect of electric field in metallic systems is negligible because the electric field can only penetrate into the materials by a few monolayers due to screening by the free electrons. Here we report the pronounced effects of electric field in magnetic tunnel junctions (MTJs) with very thin CoFeB electrodes, where the magnetic anisotropy originates solely from the CoFeB/MgO interfaces. The MTJs have the key structure of Co40Fe40B20(1.2-1.3nm)/MgO(1.2-2nm)/Co40Fe40B20(1.6nm) and the tunneling magnetoresistance in all junctions is in excess of 100{\%}. Due to the redistribution of electrons among the different 3d orbitals of Fe and Co, the perpendicular magnetic anisotropy of the CoFeB electrodes can be significantly modified by an applied electric field. As a result, the coercivity, the magnetic configuration, and the tunneling magnetoresistance of the MTJs can be manipulated by voltage pulses, such that the high and low resistance states of the MTJ can be reversibly controlled by voltages less than 1.5 V in magnitude and with much smaller current densities.   

Nanomagnetic triangles for a non-volatile logic applications

Single domain triangular nanomagnet is a base element in the recently proposed non-volatile logic application [1]. Dependent on the triangle shape and dimensions ``Y'' or ``buckle'' magnetization alignment ground states are defined by configurational anisotropy. In the ``Y'' ground state the local magnetization is aligned to point either towards or away from the triangle vertexes. The three triangle vertexes are used for information storage while switching between different ground states performs logic operations. In this work we study nanomagnetic triangle ground states and switching between them using micromagnetic simulations. We show that triangle shape engineering within the fabrication method tolerance allows maintaining the ``Y'' ground state despite the shape distortions typical for fabrication process. We correlate the height and profile of the energy barrier between the triangle ground states with the triangle shape and dimensions. We discuss ground state switching mechanisms and asses the triangle use for non-volatile logic and memory applications. \\[4pt] [1] A. Kozhanov, C.J. Palmstrom, S.J Allen ``Spin Torque Triad for a non-volatile logic gates'' US Patent pending.   

Ultra low-energy hybrid spintronics and straintronics: multiferroic nanomagnets for memory, logic and ultrafast image processing

We have theoretically shown that multiferroic nanomagnets (consisting of a piezoelectric and a magnetostrictive layer) could be used to perform computing while dissipating $\sim $ few 100 kT/bit (Appl. Phys. Lett. 97,173105, 2010) at clock rates of $\sim $1GHz. They can act as memory elements (Appl. Phys. Lett. \textbf{99}, 063108, 2011), logic gates (Nanotechnology, 22, 155201, 2011, http://arxiv.org/abs/1108.5758v1) and associative memory for higher order computing such as ultrafast image reconstruction and pattern recognition (J. Phys. D: Appl. Phys. 44, 265001 (2011), http://arxiv.org/abs/1109.6932v1). This talk will provide an overview of our research in: \begin{enumerate} \item Theoretical study of stress induced magnetization dynamics in isolated multiferroic nanomagnets (memory) and dipole coupled nanomagnetic arrays laid out in specific geometric patterns to implement a universal logic gate. \item Monte Carlo simulations of the magnetization trajectories in such systems described by the stochastic Landau-Lifshitz-Gilbert (LLG) equation, that show error-free ($>$99.99{\%}) \textit{fast} ($\sim $1 GHz) switching with very low dissipation (few 100kT/bit/magnet). \item Demonstrating that multiferroic nanomagnets possessing biaxial anisotropy could be used for four-state logic and perform image processing applications such as image reconstruction and pattern recognition. \item Experimental fabrication of such devices using e-beam lithography and deposition to create $\sim $ 100 nm diameter elliptical nanostructures and study them with magnetic force microscopy. \end{enumerate}   

Reliable switching in MRAM and multiferroic logic

Low reliable writing in spintronic devices limits their applicability in the automotive and defense industries. Coupling stochastic macromagnetic simulator with quantum transport, we show how greater reliable switching can be achieved in MRAM and multiferroic logic. Using a combination of spin-transfer torque and small applied perpendicular field in MRAM, the error rate can be considerably reduced for a given voltage pulse. In multiferroic logic, strain plays the role of the magnetic field. Information is passed along an array of nanomagnets (NM) (magnetostrictive + piezoelectric layers) through dipole coupling with neighboring NMs. A low voltage applied to the piezoelectric element causes the NM's magnetization to switch to its hard axis. Upon releasing the stress, the magnetization of the NM relaxes to the easy axis, with its final orientation determined by the dipolar coupling with the left NM, thus achieving a low power Bennett clocked computation. In the face of stagnation points along the potential energy landscape, the success rate of the straintronic switching can be controlled with by how fast the stress is removed from the NM. (Funding: DARPA, GRANDIS, NSF-NEB).   

Low-energy information transfer between dipolar-coupled magnetic disks observed by time resolved magnetic soft X-ray microscopy

The coupling between oscillators allows to mutually transfer energy and also to propagate information signals. Utilizing the concept of coupled oscillators, we experimentally demonstrated a new mechanism for energy transfer between spatially separated dipolar-coupled magnetic disks by stimulated vortex gyration. Direct experimental evidence was obtained by state-of-the-art experimental time-resolved soft X-ray microscopy probe. The rate of energy transfer from one disk to the other was derived from the two normal modes' frequency splitting caused by dipolar interaction. This mechanism provides tunable energy transfer rates, low-power input signals and negligible energy loss in the case of negligible intrinsic damping. Coupled vortex-state disks might find applications in future information-signal processing. H. Jung, et al., NPG - Scientific Reports 1 59 (2011); M.-W. Yoo, et al., Phys. Rev. B 82, 174437 (2010); H. Jung, et al., Appl. Phys. Lett. 97, 222502 (2010); Y.-S. Choi, et al., Phys. Rev. B 80, 012402 (2009); P. Fischer, et al., Phys Rev B 83 212402 (2011).   

Four-state straintronics: Ultra low-power collective nanomagnetic computing using multiferroics with biaxial anisotropy

Two-phase multiferroic nanomagnets, consisting of elastically coupled magnetostrictive/piezoelectric layers, can be endowed with four stable magnetization states by introducing biaxial magnetocrystalline anisotropy in the magnetostrictive layer. These states can encode four logic bits. We show through extensive modeling that dipole coupling between such 4-state magnets, combined with stress sequences that appropriately modulate the energy barriers between the stable states through magnetoelastic coupling, can be used to realize 4-state NOR logic (J. Phys. D: Appl. Phys. 44, 265001 (2011)) as well as unidirectional propagation of logic bits along a ``wire'' of nanomagnets (arXiv:1105.1818). As very little energy is consumed to ``compute'' in such a system, this could emerge as an ultra-efficient computing paradigm with high logic density. We show, by solving the Landau-Lifshitz-Gilbert (LLG) equation, that such nanomagnet arrays can be used for ultrafast image reconstruction and pattern recognition that go beyond simple Boolean logic. The image processing attribute is derived from the thermodynamic evolution in time, without involving any software. This work is supported by the NSF under grant ECCS-1124714 and VCU under PRIP.   

GMAG Dissertation Award Talk: All Spin Logic -- Multimagnet Networks interacting via Spin currents

Digital logic circuits have traditionally been based on storing information as charge on capacitors, and the stored information is transferred by controlling the flow of charge. However, electrons carry both charge and spin, the latter being responsible for magnetic phenomena. In the last few decades, there has been a significant improvement in our ability to control spins and their interaction with magnets. All Spin Logic (ASL) represents a new approach to information processing where spins and magnets now mirror the roles of charges and capacitors in conventional logic circuits. In this talk I first present a model [1] that couples non-collinear spin transport with magnet-dynamics to predict the switching behavior of the basic ASL device. This model is based on established physics and is benchmarked against available experimental data that demonstrate spin-torque switching in lateral structures. Next, the model is extended to simulate multi-magnet networks coupled with spin transport channels. The simulations suggest ASL devices have the essential characteristics for building logic circuits. In particular, (1) the example of an ASL ring oscillator [2, 3] is used to provide a clear signature of directed information transfer in cascaded ASL devices without the need for external control circuitry and (2) a simulated NAND [4] gate with fan-out of 2 suggests that ASL can implement universal logic and drive subsequent stages. Finally I will discuss how ASL based circuits could also have potential use in the design of neuromorphic circuits suitable for hybrid analog/digital information processing because of the natural mapping of ASL devices to neurons [4]. \\[4pt] [1] B. Behin-Aein, A. Sarkar, S. Srinivasan, and S. Datta, ``Switching Energy-Delay of All-Spin Logic devices,'' \textit{Appl. Phys. Lett.}, 98, 123510 (2011).\\[0pt] [2] S. Srinivasan, A. Sarkar, B. Behin-Aein, and S. Datta, ``All Spin Logic Device with Inbuilt Non-reciprocity,'' \textit{IEEE Trans. Magn}., 47, 10 (2011).\\[0pt] [3] S. Srinivasan, A. Sarkar, B. Behin-Aein and S. Datta, ``Unidirectional Information transfer with cascaded All Spin Logic devices: A Ring Oscillator,'' \textit{IEEE Device Research Conference} (2011).\\[0pt] [4] A. Sarkar, S. Srinivasan, B. Behin-Aein and S. Datta, ``Multimagnet networks interacting via spin currents'' \textit{IEEE} \textit{International Electron Devices Meeting} 2011. (to appear).   

``Spin inverter'' as building block of All Spin Logic devices

All-spin logic (ASL) represents a new approach to information processing where the roles of charges and capacitors in charge based transistors are played by spins and magnets, without the need for repeated spin-charge conversion. In our past work, we have presented numerical simulations based on a coupled spin transport and Landau Lifshitz Gilbert model showing that ring oscillators and logic circuits with intrinsic directionality [IEEE Trans. Magn. 47,10, 4026, 2011; Proc. IEDM, 2011)] can be implemented by manipulation of spins in magnetic nanostructures. The aim of this talk is (1) to identify a basic ASL unit that can be interconnected to build up spin circuits analogous to the way transistors are interconnected to build conventional circuits and (2) to present a compact model for this basic unit that can be used to design and analyze large scale spin circuits. We will show that this basic ASL unit is a one-magnet ``spin inverter'' with gain that can be cascaded to accomplish a spin circuit implementation of almost any logic functionality~   

Ultra low-power straintronics with multiferroic nanomagnets: magnetization dynamics, universal logic gates and associated energy dissipation

Stress induced magnetization dynamics of dipole coupled multiferroic nanomagnet arrays is modeled by solving the Landau-Lifshitz-Gilbert (LLG) equation. We show that in such multiferroic nanomagnets, consisting of magnetostrictive layers elastically coupled to piezoelectric layers, the single domain magnetization can be rotated by a large angle ($\sim $ 90$^{\circ})$ in $\sim $ 1 ns if a tiny voltage of a few tens of millivolts is applied across the piezoelectric layer [Nanotechnology, 22, 155201, 2011, Appl. Phys. Lett. 99, 063108, 2011]. Arrays of such multiferroic nanomagnets can be laid out in specific geometric patterns to implement combinational and sequential logic circuits by exploiting inter-magnet dipole coupling and Bennett clocked with specific stress cycles to propagate logic bits and implement dynamic logic. In this work, we theoretically demonstrate logic propagation in and fan-out characteristics of a universal NAND gate and discuss energy dissipation in the magnet and in the external clock. We show that this energy dissipation can be 3 orders of magnitude more energy-efficient than current CMOS technology for a reasonable clock speed of 1 GHz. This work is supported by the NSF under grant ECCS-1124714.   

Hybrid spintronics and straintronics: A paradigm for ultra-low-energy computing

We have shown in the past that the magnetization of a two-phase multiferroic single-domain nanomagnet can be electrically switched (flipped) with very little energy dissipation at low temperatures. This heralds a new energy-efficient magnetic logic and memory technology [Appl. Phys. Lett., \underline {99}, 063108, 2011, Phys. Rev. B, \underline {83}, 224412, 2011, Nanotechnology, \underline {22}, 155201, 2011]. Here, we extend our low-temperature result to room temperature where thermal noise can cause switching failures and increase average energy dissipation and switching delay. Using Monte Carlo simulations of switching trajectories described by the stochastic Landau-Lifshitz-Gilbert (LLG) equation, we show that even at room temperature, nearly error-free \textit{fast} switching is possible with very low dissipation. The energy dissipated to switch an appropriately designed nanomagnet with $>$ 99.99{\%} probability at room temperature is only $\sim $400 kT for a switching delay of sub-nanosecond. This is enabled by the complex interplay between the in-plane and out-of-plane excursions of the magnetization vector which \textit{aids} switching. This work is supported by the NSF under grant ECCS-1124714.