Collective Effects Kang L. Wang Raytheon Distinguished Professor of Physical Electronics Device research Laboratory Center on Functional Engineered NanoArchitectonics -- FENA (www.fena.org) Western Institute of Nanoelectronics – WIN (www.win-nano.org) California NanoSystems Institute – CNSI (www.cnsi.ucla.edu) University of California - Los Angeles E-mail: wang@ee.ucla.edu)

Outline Introduction Collective effects and state variables Interaction in the space and the Order Parameter Collective effects and state variables Variability issues of spintronics versus nanoelectronics Examples: Spin wave bus MQCA SPIN FET Molecules and atoms Summary A collection of atoms may be MEMS devices.

Charge State Variable (RT) Conventional Electronics employs indept electron entity and Coulomb interaction r u-nm As the size of the devices goes down, the Coulomb (electrostatic) Capacitance energy arises. Leading to the increase of the energy per one electron and thus to high variability as quantum fluctuations become important Making order parameters such as phase transition, magnetism, etc. Order Parameter The solution: To switch to interactions other than Coulomb

Corrections for Coulomb Energy Whatever the new interaction will be it is going to the some part of the ELECTRODYNAMIC interaction: ElectroDynamic Interaction = Coulomb + Corrections Dynamic: of relativistic origin including spins, magnetic, multi-ferroics Many-body or Quantum Effective interactions in many-electron collective variables Static: Multi-pole, short ranged ~ 1/rn, n>2 Ferroelectric Big Molecules (collective variables) Single electron level E> KT Too weak to work with

Many-electron collective variables for information processing Examples of the order parameters and collective variables These we can call a first level collective variables, they are actually fields in space Molecules Excitations of these can be called a second level collective variables Collective variable representing the state of many-electron system (e.g., position)

Excitations of the order parameters as the second level collective variables Domain walls in ferromagnets Topological excitations of the order parameters: for example ferromagnetic vortices off on Goldstone excitations of the order parameter: for example spin waves: MTJ memory unit can be view as a domain-wall trap off (no wall) oxide layer Free layer Is it possible to use Ferroelectric or even MultiFerroic , Domain walls, Topological excitations, Goldstones? Are they advantageous in any way ? Fixed layer MEMS is the second level of state variables for charge. on (1 wall) oxide layer Free layer Fixed layer

Variability: Electronics vs Spintronics The Same Principle for elemental Electrics and Spintronics circuit units (FET and spin-FET)

Variability Issues Electronics Spintronics Total range spin vector = 2S+1 The total range spin vector is 2S+1 Thermal fluctuations give Gaussians:

Quantum fluctuations from the quantization of the state variable Spin Fluctuation Na: number of atoms constituting the gate This is like to shot noise versus Johson’s noise.

Variability Charge Spin or a linear length of 77 nm or a linear size of 1.6 nm High enough energy Collective particles Quantum fluctuations of charge For To= 300K we will have the limitation on the size: or a linear length of 77 nm or a linear size of 1.6 nm. Quantum fluctuations of the Total Spin Room Temperature. Quantum fluctuations of the projection of the Spin Ovchinnikov and Wang, APL 2008

Spintronics for low power – Spin as a state variable For Single Spin Bmin155 Tesla – Not practical! (DE=2 mB B = 1.157×10-4 eV at 1 T) For N Spins Datta, APL 90, 093503(2007) Single electron or collective variables should be used to satisfy thermal stability and power dissipation requirements Depending on Gilbert damping factor. 11 11

Summary Comparison of Electronic, Spin and Optical State Computing Independent electrons Lower bound (Impractical Limit) Mechanism Energy Size Electronic 3kBT 1 nm Practical limit ~3-5 nm Spin 70kBT 7 nm Practical limit >20 nm Optical 3600kBT 20 nm Practical limit >90 nm Victor Zhirnov

Summary Comparison of Electronic, Spin and Optical State Computing Correlated electrons Electronic Spin Optical 3kBT 3600kBT 1 nm 20 nm 2 nm Mechanism Energy Size Lower bound (Impractical Limit) Practical limit~20~70 nm Practical limit ~2~7 nm Practical limit >90 nm

Spin Logic Devices Spin Waves Magnetic Cellular Automata 3-terminal Phase modulation/Amplification/ Superposition Magnetic Cellular Automata I1 I2 I3 output 3-terminal Spin FET Sugahara- Tanaka Parallel Anti- RAP Spin Valves/Spin Torque

Spin Wave Bus -- Spin-Based Logic Device and transfer of information (Phasetronics) Three terminal device (three MOS with a common ferromagnetic film) Two inputs – One output The input is provided by a Source -Drain current pulse - ISD The output is the inductive voltage between two nearest source ( or drain) contacts - VSS This structure is a modification of the previous one : (i) The ferromagnetic film is placed on the top; (ii) It is a three terminal device. All technological parameters are the same as reported by Covington et all. (slide 5)

Experiment – Spin wave Propagation Signal/Pulse Generator circulator Oscilloscope 50 GHz 100 nm NiFe The general idea of using inductively coupled circuits for information processing is to transmit information between the circuits using a magnetic flux without transmitting current via wires. According to the Faraday’s law, the change of the magnetic flux through the surface generates an inductive voltage proportional to the rate of the magnetic flux change. The change of magnetization in the magnetic material alters the outgoing magnetic flux and results in the inductive voltage signal. In this case, the change of magnetization is through the propagation of spin waves. The figure shows the inductive voltage measured for the device – simulated and measured. Time-resolved inductive voltage measurement technique used for measuring spin wave. The dashed line depicts the voltage pulse applied to the excitation line. The pulse characteristics are as follows: pulse amplitude 24.5V; rising time 1.2ns; and pulse length 20ns. The solid line depicts the inductive voltage signal detected by the detection line. One can see the inductive voltage oscillation at the detection line caused by the inductive coupling via the spin waves. The output voltage signal has maximum pulse amplitude 26mV, and the period of oscillation is 9ns. Time resolved inductive voltage measured

Experimental Data – SW transport in CoFe film Prominent modulation by weak (10  50 Gauss) magnetic field M. Bao, J-Y Lee, A Khitun, K. L Wang, D. W. Lee and S. Wang, 3-D mapping of spin wave propagation in CoFe thin film, (2007).

General Concept and Some Results Experimental data on amplitude and phase modulation for the structure with 100nm CoFe film in the frequency range (0.5  6 GHz) and magnetic field range (0  350G) Prominent power (8dB/20G) and phase modulation ( 60Deg/10G) in the specific frequency regions “AND”, “OR”, “NOT” gates Maj

Prototype Three-Terminal Device Logic state - spin wave phase Spin wave interferometer Phase control by the direction of current in the excitation loop Only two phases 0 and  detection Input 1 Input 2 In-Phase Out-of-Phase In Phase: Amplification Out of Phase: Cancellation A. Khitun, M. Bao, Y. Wu, J-Y Kim, A. Hong, A. Jacob, K. Galatsis, and K. L. Wang, Logic Devices with Spin Wave Buses – an Approach to Scalable Magneto-Electric Circuitry, Proceeding of MRS, (in press), 2008

Mitigating eddy current losses in nanoscale devices Continuous metallic Insulating film Eddy current losses severely damp spin waves in a metallic film. 0.1 T CoFe; 100 nm Ferrite (Fe3O4) 2, 4, or 8 mm Eddy current loss can be reduced by laminations. Fig. (1) Fig. (2) Jim Allen – UCSB

Prototype Device by Kostylev et al: Logic state - spin wave amplitude Spin wave interferometer Phase modulation by magnetic field Gradual phase shift control up to 2.5 I, A / YIG (Yittrium iron Garnet) was used. L is 8 mm between the transducer and detector. Backward volume magnetostatic waves (BVMSW) and MSSW (magneto static spin waves) Kostylev, M.P., et al., Spin-wave logical gates. APL, 2005. 87(15): p. 153501-1-3.

Follow-up work by T. Schneider et al. The same device structure as for the prototype (Kostylev et al.) Logic state - spin wave amplitude Phase modulation by magnetic field (Input current 1200mA XNOR, NAND logic gates demonstrated YIG (Yittrium iron Garnet) was used. Backward volume magnetostatic waves (BVMSW) and MSSW (magneto static spin waves) T. Schneider, A.A. Serga, B. Leven, B. Hillebrands, R.L. Stamps and M.P. Kostylev, Realization of spin-wave logic gates, APL, 92, 0022505, 2008

Propagation distance: 2 Group velocity: ~105 m/s or 107cm/s Speed of Operation Internal delay time = propagation distance/group velocity Propagation distance: ~ (submicron) Group velocity: gr= d/dk (~ 107 cm/s ) Delay time ~ 10-100 ps Experimental Data: 100nm CoFe film, RT Propagation distance: 2 Group velocity: ~105 m/s or 107cm/s Current device: 1 ns Ultimate limit: <10 ps The fundamental limit for device operation speed – limited spin wave group velocity. operation speed by the scaling down the signal propagation distance (submicron)

Numerical modeling: Multifunctional MagnetoElectric Cell m - the unit magnetization vector Ms - the saturation magnetization - the gyro-magnetic ratio  - the phenomenological Gilbert coefficient Landau-Lifshitz-Gilbert formalism A - the exchange constant K - the uniaxial anisotropy constant e - the unit vector along with the uniaxial direction Hpulse - the pulse field M V Sang-Koog Kim, Sung-Chul Shin, and Kwangsoo No Seoul National University, IEEE TRANSACTIONS ON MAGNETICS, VOL. 40, NO. 4, JULY 2004

Spin Wave Modulation by Electric Field R. Ramesh (Berkeley) Modulation via the exchange bias coupling in FM/MF structure K. Wang (UCLA) S. Wang (Stanford) Work Integrated by Ajey P. Jacob (Intel)

Magnetic Nanofabric: Spin Wave multibit processor Piezoelectric Ferromagnetic Film Silicon Substrate Modulator f n ACPS Line (input 1 ,f 2 3 ,…f ) ME Cell (output Silicon Oxide Here we have a set of converters, modulators, and magneto-electric cells arranged in a sequence. Two antenna transmission lines on the edge are aimed to excite and detect spin wave signals on different frequencies. The pair of modulator and magneto-electric cell is adjusted to modulate/amplify spin wave signals on specific frequency. At the bottom, we have an equivalent circuit. Equivalent circuit

Magnetic Nanofabrics:- Spin Wave device’s building blocks- A. Khitun, M. Bao and K. L. Wang (UCLA) (1) (2) The set of figures on the left are the basic elements that are developed for building magnetic circuits on the right. so, (1) we have a converter to translate input voltage pulse into spin wave signal and vice versa. (2) Spin wave guides to transmit spin wave signal through the circuit. Depending on the design, we could make a splitter or a combiner of the spin wave. (3) Modulator to provide pi-phase shift to the propagating spin wave. (4) Magneto-electric cell – multifunctional element to serve as a memory and an amplifier for spin wave signal (5) this is the simple circuit of a one-bit logic comprising of input converter, modulator, magneto electric cell and output converter. (6) Three input majority gate that has a converters, combiner and the magnetoelectric cell. (7) Multifunctional logic gate built on the base of the majority gate by introducing two modulators. Depending on the control voltages applied to the modulators, the gate operates as AND, OR, NAND, and NOR logic gates. 3/27/20173/27/20173/27/20173/27/20173/27/2017

SW Logic Efficiency Estimates   Physical Parameter Estimated Range Energy per bit Spin wave energy 1kT – 100kT (Hext ~ 100Oe, VSW: 0.1um2 - 0.01um2) Energy to excite spin wave a) External magnetic field (e.g. coil) b) Internal excitation (e.g. spin torque) Energy to create a magnetic field a) 102kT – 104kT (M/M ~0.01, ~107 rad/s/Oe, Z~50Ohm,  ~ 10-12s) h: 1um – 10nm Ref.1 Number of functions without restoration (amplification) Spin wave coherence length /wavelength 100-1000 (L ~ 50um@RT) : 100nm-10nm Signal restoration energy Electromagnetic coupling 102kT-105kT  - magnetoelectric coupling range from 10 to 1000 mV/(cm Oe) Ref.2 Signal propagation speed Spin wave group velocity 106 cm/s - 107cm/s (function of film thickness) Time delay Propagation length/Spin wave velocity 0.05ns-1ns d range from 1um to 100nm Scaling factor and Defect Tolerance Spin wavelength  10nm - 100nm (insensitive to defects with size << ) Operation frequency Spin wave frequency  1GHz - 200GHz (NiFe, CoFe) Ref.3,4 (depends on the material structure) 1) Khitun A., Nikonov D.E., Bao M., Galatsis K., and Wang K.L., Feasibility study of logic circuits with spin wave bus. Nanotechnology 18, p. 465202, 2007. 2) Eerenstein, W., N.D. Mathur, and J.F. Scott, Multiferroic and magnetoelectric materials. Nature, 2006. 442(17): p. 759-65. 3) Covington, M., T.M. Crawford, and G.J. Parker, Time-resolved measurement of propagating spin waves in ferromagnetic thin films. Physical Review Letters, 002. 89(23): p. 237202-1-4. 4) Vasiliev S.V., Kruglyak V.V.,Sokolovskii M.L., and Kuchko A.N., Spin wave interferometer employing a local nonuniformity of the effective magnetic field, JOURNAL OF APPLIED PHYSICS 101, p. 113919 (2007).

Spin Wave Logic Devices Experimentally demonstrated devices: M.P. Kostylev, A.A. Serga, T. Schneider, B. Leven, B. Hillebrands, Spin-wave logical gates. APL, 87(15): p. 153501-1-3, 2005. T. Schneider, A.A. Serga, B. Leven, B. Hillebrands, R.L. Stamps and M.P. Kostylev, Realization of spin-wave logic gates, APL, 92, 0022505, 2008 A. Khitun, M. Bao, Y. Wu, J-Y Kim, A. Hong, A. Jacob, K. Galatsis, and K. L. Wang, Logic Devices with Spin Wave Buses – an Approach to Scalable Magneto-Electric Circuitry, Proceeding of MRS, (in press), 2008 Ferromagnetic resonance controlled by electric field: A.A. Semenov, S.F. Karmanenko, V.E. Demidov, B.A. Kalinikos, S. Grinivasan, A.N. Slavin, J.V. Mantese, Ferrite-ferroelectric layered structures for electrically and magnetically tunable microwave resonators. APL 88, 033503, 2006. Spin wave modulation using multiferroics: None

Magnetic Logic - Cellular Automata NAND gates form the building blocks for circuits inside your computer

Logic Gates using MQCA = Current state-of-the-art: the majority logic gate. Imre et al, Science 311, 205 (2006)

Instability of bits 0° is unstable Energy (normalized) vs. θ

Vertical Lines The Problem – stray fields cause vertical bits to flip first The Solution – Add stabilizing bits to left and right

The B-gate (NAND function) 00 10 01 11 D. Carlton, UCB

these gates can be linked together to do logic... = D. Carlton, UCB

Nano magnet Switching speed Direct observation of spin transfer switching by x-ray microscopy. d) 8.6 ns e) 9.0 ns f) 9.6 ns g) 12.0 ns h) 12.2 ns i) 13.2 ns a) 0 ns b) 0.15 ns c) 0.6 ns a b c d e f i h g 20 nm CoPt free layer 5 nm Cu as a tunneling layer Fe as Fixed layer 20 nm CoPt free layer: 5 nm Cu as a tunneling layer and the Fe as Fixed layer. The circularly polarized x-ray beam from an undulator on beam line 11.0.2 at the Advanced Light Source (ALS) is incident 30 from the surface normal. It is focused to 30 nm by a zone plate. The transmission of the x rays through the whole pillar is monitored by a fast avalanche detector as a function of the position x; y while the pillar is scanned in steps of 10 nm across the x-ray focus. Tuning the photon energy to the characteristic Co L3 resonance provides magnetic contrast through the x-ray magnetic circular dichroism (XMCD) effect [19]. Y. Acremann et al., PRL 96, 217202/1-4 (2006) Joachim Stöhr – SLAC with Yves Acremann

Field Effect in DMS Confirmed Transistor with Memory Spin FET Field Effect in DMS Confirmed Ge MnGe Al2O3 Al Schematic Spin gain FET structure with a MnGe/SiGe quantum well. Transistor with Memory Bohr magneton: e*hbar/2m μB = 9.274 009 49(80) × 10-24 J·T-1 JingJing Chen and KL Wang et al., App. Phys. Letts. 90, 012501 2007

Molecular Building Blocks Physical Molecular Change Molecular Motion Rotational Conformation Phase Change Put mems materials to this. MEMORY applications LOGIC applications

Molecular Rotation - metallacarboranes Metal carborane molecules “electronic switching” Atomic Scale: 90 rotation Cu(II)   Cu(I) Rotor Stator Tetrahedral Square planar “ON” “OFF” LUMO HOMO EF Metal Ec Ev P+ Si 5.2eV 4.6eV 4.1eV Cu Carborane has 90 degree switching ability. (Phen), Switching voltage may be ~ 1V. As part of the FENA center, we are in the process of taking these molecules to the solid state Similarly, these nickel carborane molecules Ni (iii) to (IV) are based on a redox process, which rotates the ligands by 180 degrees changing the tunneling current and alterning the conductivity through the structure I-V characteristics Negative differential resistance due to tunneling through molecular rotor Hysteresis due to rotation

Acknowledgments V Zhirnov and R Cavin A Jacob, J Allen, A Khitun, I Ovchinnikov, M Bao H Ohno, Tanaka, and K Ando All the FENA, WIN & CNSI participants All students, postdoctoral fellows, Faculty and visitors as well as collaborators around the world Support: DARPA, SRC, NSF, Marco, NERC, ARO, AFOSR, ONR, and many industrial companies