Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session A30: Focus Session: Nanomagnetic Devices I |
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Sponsoring Units: GMAG DMP Chair: Alexander Khitun, University of California, Riverside Room: 206B |
Monday, March 2, 2015 8:00AM - 8:12AM |
A30.00001: Magnonic Holographic Memory Alexander Khitun, Alexander Kozhevnikov, Frederick Gertz, Yuri Filimonov Collective oscillation of spins in magnetic lattice known as spin waves (magnons) possess relatively long coherence length at room temperature, which makes it possible to build sub-micrometer scale holographic devices similar to the devices developed in optics. In this work, we present a prototype 2-bit magnonic holographic memory. The memory consists of the double-cross waveguide structure made of Y$_{3}$Fe$_{2}$(FeO$_{4})_{3}$ with magnets placed on the top of waveguide junctions. Information is encoded in the orientation of the magnets, while the read-out is accomplished by the spin waves generated by the micro-antennas placed on the edges of the waveguides. The interference pattern produced by multiple spin waves makes it possible to build a unique holographic image of the magnetic structure and recognize the state of the each magnet. The development of magnonic holographic devices opens a new horizon for building scalable holographic devices compatible with conventional electronic devices. [Preview Abstract] |
Monday, March 2, 2015 8:12AM - 8:24AM |
A30.00002: Calculating a parameter space to smoothly transport magnetically-trapped suspended superparamagnetic microbeads with electric-field domain wall control Brenda McLellan, Mark Nowakowski, Jeffrey Bokor, Cheng-yen Liang, Joshua Hockel, Kyle Wetzlar, Scott Keller, Hyunmin Sohn, Gregory Carman, Anthony Young, Andrew Doran, Matthew Marcus, Mathias Klaui, Robert Candler We demonstrate the capture and electrically-driven piecewise transport of superparamagnetic microbeads trapped in a magnetostatic potential energy well produced by the magnetic domain walls of Ni microrings on a [Pb(Mg$_{1/3}$Nb$_{2/3})$O$_{3}$]$_{0.66}$-[PbTiO$_{3}$]$_{0.34}$ (PMN-PT) substrate. Here I present micromagnetic simulations that illustrate the formation of field-initialized domain walls in Ni microrings and calculate the approximate force of attraction experienced by superparamagnetic microbeads near the domain walls. This force is estimated as a function of the ring geometry, bead diameter, and distance from the domain wall, and provides an upper bound for the strain-mediated, electrically-induced domain wall velocity that can be implemented to smoothly transport coupled microbeads within a fluidic environment. These results provide an initial estimate for important technological parameters and set a foundation for the optimization of this microfluidic magnetic control scheme. H. Sohn, M. Nowakowski, et al. submitted, 2014. [Preview Abstract] |
Monday, March 2, 2015 8:24AM - 8:36AM |
A30.00003: Stochastic simulations of switching error in magneto elastic and spin-Hall effect based switching of nanomagnetic devices Md Mamun Al-Rashid, Supriyo Bandyopadhyay, Jayasimha Atulasimha Switching of single domain multiferroic nanomagnets with electrically generated mechanical strain [1] and with spin torque due to spin current generated via the giant spin Hall effect [2] are two promising energy-efficient methods to switch nanomagnets in magnetic computing devices. However, switching of nanomagnets is always error-prone at room temperature owing to the effect of thermal noise. In this work, we model the strain-based and spin-Hall-effect-based switching of nanomagnetic devices using stochastic Landau-Lifshitz-Gilbert (LLG) equation and present a quantitative comparison in terms of switching time, reliability and energy dissipation.\\[4pt] [1] J. Atulasimha, {\&} S. Bandyopadhyay, Applied Physics Letters,~97(17), 173105, 2010.\\[0pt] [2] L. Liu, C. F. Pai, Y. Li, H.W. Tseng, D. C. Ralph, {\&} R. A. Buhrman Science, 336(6081), 555-558, 2012. [Preview Abstract] |
Monday, March 2, 2015 8:36AM - 8:48AM |
A30.00004: Preliminary experiments on SAW based magnetization switching of nanomagnets Vimal Sampath, Noel D'Souza, Supriyo Bandyopadhyay, Jayasimha Atulasimha Magnetization rotation in micron-sized ferromagnetic elements, using Surface Acoustic Waves (SAW), has been demonstrated experimentally [1] while the use of SAW to lower the energy dissipation in switching of nanomagnets with spin transfer torque has been studied theoretically [2]. Furthermore, SAW can be used to ``Bennett clock'' an array of nanomagnets in nanomagnetic logic without requiring lithographic contacts to individual nanomagnets [3]. We report preliminary experiments on use of SAW to switch magnetostrictive Co nanomagnets grown on bulk 128 Y-cut lithium niobate. Switching is studied by imaging the nanomagnets' magnetic states with Magnetic Force Microscopy (MFM) before and after the SAW waves interact with them. Switching of single, isolated nanomagnets of various sizes, and dipole coupled nanomagnets implementing a Boolean NOT gate, is studied. \\[4pt] [1] S. Davis, A. Baruth {\&} S. Adenwalla, App. Phys. Lett., \underline {97}, 112904 (2010). \\[0pt] [2] A. K. Biswas, S. Bandyopadhyay {\&} J. Atulasimha, Appl. Phys. Lett.,~\underline {105}, 072408~(2014). \\[0pt] [3] J. Atulasimha {\&} S. Bandyopadhyay, App. Phys. Lett.,~\underline {97}, 173105 (2010). [Preview Abstract] |
Monday, March 2, 2015 8:48AM - 9:00AM |
A30.00005: Magneto-elastic artificial neurons with extremely low energy dissipation Ayan K. Biswas, Md Mamun Al-Rashid, Jayasimha Atulasimha, Supriyo Bandyopadhyay We present a detailed analysis of artificial step transfer function neurons and binary weight synapses implemented with magneto-tunneling junctions whose soft layers are magnetostrictive nanomagnets switched with voltage generated mechanical strain. These devices are more energy-efficient than CMOS-based neurons or so-called spin neurons that are based on magnets switched with spin-polarized current [1]. We studied their switching dynamics using stochastic Landau-Lifshitz-Gilbert simulations for two different geometries (elliptical and cylindrical) of the magnetostrictive nanomagnet. Our study revealed that while the step transition (firing) of the magnetic neuron is always very sharp at 0 K, the threshold is significantly broadened at room temperature, regardless of geometry and regardless of whether the magnet is switched with strain or spin-polarized current. While this could preclude some applications, the extreme energy-efficiency of these neurons makes them nearly ideal for use in certain types of neuromorphic computation. [1] M. Sharad, et al., IEEE Trans. Nanotechnol., \underline {11}, 843 (2012). [Preview Abstract] |
Monday, March 2, 2015 9:00AM - 9:12AM |
A30.00006: Three dimensional magnetic abacus memory Shilei Zhang, Jingyan Zhang, Alexander Baker, Shouguo Wang, Guanghua Yu, Thorsten Hesjedal Stacking nonvolatile memory cells into a three-dimensional matrix represents a powerful solution for the future of magnetic memory [1,2]. However, it is technologically challenging to access the individual data in the storage medium if large numbers of bits are stacked on top of each other. Here we introduce a new type of multilevel, nonvolatile magnetic memory concept, the magnetic abacus [3]. Instead of storing information in individual magnetic layers, thereby having to read out each magnetic layer separately, the magnetic abacus adopts a new encoding scheme which envisages a classical abacus with the beads operated by electron spins. It is inspired by the idea of second quantization, dealing with the memory state of the entire stack simultaneously. Direct read operations are implemented by measuring the artificially engineered `quantized' Hall voltage [4], representing a count of the spin-up and spin-down layers in the stack. This concept of `second quantization of memory' realizes the 3D memory architecture with superior reading and operation efficiency, thus is a promising approach for future nonvolatile magnetic random access memory. [1] Parkin, S. S. P. et al. Science 320, 190 (2008). [2] Lavrijsen, R. et al. Nature 493, 647 (2013). [3] Zhang, S. L. et al. Sci. Rep. 4, 6109 (2014). [4] Zhang, S. L. et al. Sci. Rep. 3, 2087 (2013). [Preview Abstract] |
Monday, March 2, 2015 9:12AM - 9:48AM |
A30.00007: Controlling Magnetization using Spin Orbit Torque Invited Speaker: Sayeef Salahuddin Recently it has been shown that spin orbit coupling (SOC) and/or broken inversion symmetry in vertical heterostructures can generate accumulation of spins when a charge current is flowing through them. In doing so, it can exert a torque on an adjacent magnet [1,2]. Indeed, high Z metals (Ta, Pt, W, \textit{etc.}) with strong SOC have been used to inject spin currents into adjacent ferromagnetic layers and thereby to induce magnetic switching, oscillation, domain wall movement etc. SOC physics promises to significantly reduce the required current for current induced magnetic switching for next generation data-storage applications. In this presentation we shall discuss some of our recent work on SOC induced control of magnets with perpendicular magnetic anisotropy (PMA). A current flowing in-plane presents interesting symmetry problems with respect to a PMA magnet. We shall discuss how these symmetry relations can be utilized for switching of and domain wall movement in the PMA magnets [3]. In addition to storage applications, we shall also discuss possibility of exploiting SOC for spintronic logic applications [4].\\[4pt] [1] Miron, I. M. \textit{et al}. Perpendicular switching of a single ferromagnetic layer induced by in-plane current injection. \textit{Nature} \textbf{476,} 189-193 (2011).\\[0pt] [2] Liu, L. Q.~\textit{et al}.~Spin-torque switching with the giant spin Hall effect of tantalum.\textit{ Science}~\textbf{336}, 555--558~(2012).\\[0pt] [3] D. Bhowmik, et al., Deterministic Domain Wall Motion Orthogonal To Current Flow Due To Spin Orbit Torque, arXiv:1407.6137v1\\[0pt] [4] D. Bhowmik, L. You, S. Salahuddin, Spin Hall effect clocking of nanomagnetic logic without a magnetic field., \textit{Nat. Nanotechnol.} \textbf{9}, 59--63 (2014). [Preview Abstract] |
Monday, March 2, 2015 9:48AM - 10:00AM |
A30.00008: Bridging amount of spin-glasses over ferromagnetic/antiferromagnetic thin films and bit-cell dispersion of exchange bias in corresponding TA-MRAM devices Kamil Akmaldinov, Clarisse Ducruet, Jeremy Alvarez-Herault, Vincent Baltz For thermally-assisted magnetic random access memories (TA-MRAM), lowering bit-cells dispersions of exchange bias is necessary. In this study, we prove that spin-glass-like phases (SG) spread over the ferromagnetic/antiferromagnetic (F/AF) storage layer are the main cause of such distributions once the film is nanofabricated into a device. In particular, we show that the less the SG, the lower the bit-cell dispersion. More precisely, the amount of SG was varied from sample to sample by sputtering various AFs: IrMn, FeMn and their alloys [1]. Blocking temperature distributions were measured to quantify the amount of SG at the wafer level [2]. The wafers were then patterned to obtain 1kb devices and all the cells were tested electrically. Finally, the resulting loop shift cumulative distribution functions accounting for the bit-cell dispersions were correlated to the initial amount of SG. In addition to bridging the gap between fundamental SG and a technological application, we also demonstrated that blocking temperature distributions are a versatile method to qualify TA-MRAM production batches before processing [3]. [1] K. Akmaldinov, et al, J. Appl. Phys. 115, 17B718 (2014) [2] V. Baltz, et al, Phys. Rev. B 81, 052404 (2010) [3] K. Akmaldinov et al, to be published. [Preview Abstract] |
Monday, March 2, 2015 10:00AM - 10:12AM |
A30.00009: Electromechanical Switching of the Magnetization in Nanomagnets Reem Jaafar, Eugene Chudnovsky We demonstrate the possibility of switching the magnetization by a mechanical kick generated by, e.g., a pulse of the electric field applied to a multiferroic nanoparticle or to a piezoelectric coupled to a magnetic particle that is free to rotate. The effect is based upon the observation that the mechanical rotation is equivalent to the magnetic field in the coordinate frame of the particle. This removes the symmetry argument on the way of reversing the magnetic moment by the electric field as the latter is used to generate rotation which provides the effective magnetic field acting on the magnetic moment. Analytical and numerical results will be reported. [Preview Abstract] |
Monday, March 2, 2015 10:12AM - 10:24AM |
A30.00010: Strain-induced Electric Field Switching of Magnetic Anisotropy in Ferromagnetic/Ferroelectric Interface Dorj Odkhuu, P. V. Ong, T. Tsevelmaa, N. Kioussis Multifunctionality of the magnetoelectric materials, simultaneous electric and magnetic orders, would offer a great opportunity in memory applications, in which switching the magnetization direction with an electric field is the main prerequisite. Ab initio calculations were carried out to reveal the importance of an epitaxial strain on magnetoelectric effects, possibly the spin reorientation of magnetization by ferroelectric polarization, in the interface between ferroelectric and ferromagnetic films. As a generic example, we show that the compressive strains larger than 1{\%} that imposed to the in-plane lattice of SrTiO$_{3}$ (001) underneath the Fe (001) overlayers result in a phase transition in magnetocrystalline anisotropy (MCA) from an in-plane to perpendicular magnetization with polarization reversal. A considerably large sensitivity of MCA with ferroelectric polarization is also found in the strained Fe/SrTiO$_{3}$ (001), a factor of greater than those of well-studied multiferroic heterostructures. This switching of magnetization pertains to a competition of spin-orbit coupling states between $t_{2g}$ bands, driven by the mutual mechanisms of the electrostatic screening with the spin-polarized carriers and the orbital hybridization at the interface. [Preview Abstract] |
Monday, March 2, 2015 10:24AM - 10:36AM |
A30.00011: Strain control of magnetization in TbFe$_{2}$ Ritika Dusad, M.D. Johannes, Craig J. Fennie Magnetostrictive materials change their shape upon application of strain and can be used as actuators and sensors. In this work, we perform a computational analysis of a highly magnetostrictive compound, TbFe$_{2}$, to understand how the lattice and magnetization couple. We use Density Functional Theory (DFT) to investigate the magnitude and direction of the metallic moment as a function of pressure. The localized nature of Tb f-electrons classify this compound as ``strongly-correlated'' and necessitate the simultaneous use of spin-orbit coupling to treat magnetostriction and the DFT$+$U methodology to capture the physics of the f-electrons. Although, the energy scales involved in spin-lattice interactions are extremely small, we were able to correctly reproduce the correct magnetic ground state and the experimentally observed ferrimagnetic coupling between Tb and Fe atoms in TbFe$_{2}$. The easy axis in TbFe$_{2}$ points along one of its body diagonals, which makes the shape of the crystal rhombohedral. Switching of magnetization between the easy axes requires the magnetization to pass through one of the [100] directions. In our study we show that by applying isotropic strain on TbFe$_{2}$ crystal, we can decrease the energy barrier between [111] and [100] magnetization directions of the crystal. [Preview Abstract] |
Monday, March 2, 2015 10:36AM - 10:48AM |
A30.00012: Spin Optodynamics in Magnetic Solids Tianyu Liu, Xufeng Zhang, Hong X. Tang, Michael E. Flatt\'{e} Coherent couplings between cavity photons and spin ensembles, such as cold atoms [1] and nanomagnets [2], have been studied theoretically before. By virtue of magneto-optic interactions, we propose here to use a photonic cavity made of magnetic crystal to study the intrinsic coupling rate ($g_0$) that describes the effect of a single photon on the cavity. Cavity bistability due to multistatic magnetization is identified, and with clever choosing of driving fields, one can realize coherent amplification/damping of spin-wave amplitudes, which is superior than the incoherent methods, such as the one using spin transfer torque. Our theory has great potential in developing all optical control of magnonics.\\[4pt] [1] N. Brahms and D. M. Stamper-Kurn, Phys. Rev. A {\bf 82}, 041804 (2010).\\[0pt] [2] \"{O} O. Soykal and M. E. Flatt\'e, Phys. Rev. Lett. {\bf 104}, 077202 (2010). [Preview Abstract] |
Monday, March 2, 2015 10:48AM - 11:00AM |
A30.00013: Electric-field-induced modification in Curie temperature of Co monolayer on Pt(111) Kohji Nakamura, Mikito Oba, Toru Akiyama, Tomonori Ito, Michael Weinert Magnetism induced by an external electric field ($E$-field) has received much attention as a potential approach for controlling magnetism at the nano-scale with the promise of ultra-low energy power consumption. Here, the $E$-field-induced modification of the Curie temperature for a prototypical transition-metal thin layer of a Co monolayer on Pt(111) is investigated by first-principles calculations by using the full-potential linearized augmented plane wave method that treats spin-spiral structures in an $E$-field. An applied $E$-field modifies the magnon (spin-spiral formation) energies by a few meV, which leads to a modification of the exchange pair interaction parameters within the classical Heisenberg model. With inclusion of the spin-orbit coupling (SOC), the magnetocrystalline anisotropy and the Dzyaloshinskii-Morita interaction are obtained by the second variation SOC method. An $E$-field-induced modification of the Curie temperature is demonstrated by Monte Carlo simulations, in which a change in the exchange interaction is found to play a key role. [Preview Abstract] |
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