5:30 pm

Registration and reception (with wine) and posters
(Almaden lobby and outside auditorium

7:00-8:00

Dinner in Almaden Cafeteria

8:00-8:10

Welcome by Stuart Parkin

8:10-8:55

Sadamichi Maekawa, University of Tohoku
Perspective on spin momentum transfer

8:55-9:40

Shoucheng Zhang, Stanford University
Perspective on Spin Hall Effect

8:15-8:45

Breakfast: Coffee and bagels at Almaden

8:45-9:00

Opening remarks

9:00- 9:45 am

Stuart Barnes, University of Miami
Spin angular momentum transfer, modified Gilbert relaxation and the Berry phase emf in the dynamics of magnetic domains

9:45- 10:30

Gen Tatara, Tokyo Metropolitan University
Current-driven domain wall dynamics based on collective coordinates

10:30- 10:45

BREAK

10:45-11:30

Shufeng Zhang, University of Missouri
Spin transfer torques: theory and simulation

11:30-12:15

Jun’ichi Ieda, University of Tohoku
Spin Accumulation and Resistance Due To Domain Walls in Magnetic Nano-Wires

12:15-1:15pm

Lunch at Almaden

1:15-2:00

Mathias Kläui, Universitaet Konstanz
Interactions between domain walls and spin polarized currents

2:00- 2:45

Yves Acremann, Stanford University
Magnetization Dynamics in a Spin Valve Imaged with X-rays

2:45-3:15

BREAK

3:15-4:00

Jonathan Sun, IBM T.J. Watson Research Center
Spin-torque-induced magnetic reversal in magnetic tunnel junctions with MgO barriers

4:00- 4:45

Bill Gallagher, IBM T.J. Watson Research Center
Spin-Current MRAM in Context

4:45-5:30

Stu Wolf, University of Virginia
Spin Momentum Transfer-Is it the future of MRAM?

6:30

Dinner/ party at Stuart’s house

8:15-9:00

Breakfast: Coffee and bagels at Almaden

9:00-9:45

Kristen Buchanan, Argonne National Laboratory
Magnetic vortex dynamics in confined geometries

9:45-10:30

Sergei Demokritov, University of Muenster
Spin waves in confined structures: quantization, localization and tunneling

10:30-11:00

BREAK

11:00-11:45

Paul Crowell, University of Minnesota
Spin Injection and Accumulation in Ferromagnet-Semiconductor Devices

11:45-12:30

Yuichiro Kato, University of California, Santa Barbara
Imaging the spin Hall effect and current-induced polarization in semiconductors and heterostructures

12:30-1:30pm

Lunch at Almaden

1:30-2:15

Gerrit Bauer, TU Delft
Synchronization of magnetization motion in multilayers by spin currents

2:15-3:00

Yoshichika Otani, University of Tokyo
Spin-transfer induced magnetization reversal in lateral devices

3:00-3:15

BREAK

3:15-4:00

Mike Coey, Trinity College, Dublin
Magnetic Detection of Current-Induced Electron Spin-Polarization in Aluminum

4:00-4:45

Ching-Yao Fong, University of California, Davis
Design Half Metals with Simple Structures

5:00

Closing of meeting

6pm

Dinner for invited speakers

Perspective on spin momentum transfer

Sadamichi Maekawa
Institute for Materials Research, Tohoku University, Sendai 980-8577 Japan

Spin current in magnetic nanostructures provides a variety of quantum phenomena and their applications to spin-electronics devices [1,2].  Here, I present the brief review of the basic concepts in spin current and hope to discuss the perspective on the physics. Special emphasis will be placed on the spin angular momentum conservation between spin current and various magnetic structures such as magnetic domain walls and spin accumulation.
[1] Spin Dependent Transport in Magnetic Nanostructures, eds. S. Maekawa and T. Shinjo (Taylor and Francis, 2002).

[2] Concepts in Spin Electronics, ed. S. Maekawa (Oxford University Press, 2006)

Shoucheng Zhang, Stanford University

I shall review the theoretical and experimental status of the intrinsic spin Hall effect. A recent theory predicts that dissipationless spin currents can be induced purely by an electric field in conventional semiconductors. The dissipationless spin current is derived from a novel topological structure in momentum space, is independent of the sample disorder and leads to the intrinsic spin Hall effect. In hole doped semiconductors, with or without inversion symmetry breaking, there are no vertex corrections due to impurities scattering. I shall analyze a recent experiment on the spin Hall effect in the hole doped system, and show that it is consistent with the intrinsic nature of the effect. I shall also show that the spin Hall effect can be quantized in semiconductors with appropriate strain gradients, but in the absence of any external magnetic fields or the associated time reversal symmetry breaking. Certain band insulator with a non-vanishing topological invariant can also lead to the quantum spin Hall effect without time reversal symmetry breaking. Stability of the edge states are discussed as well.

Spin angular momentum transfer, modified Gilbert relaxation and the Berry phase emf in the dynamics of magnetic domains

S. E. Barnes

Physics Department, University of Miami, Coral Gables, FL 33124, USA

The transfer of angular momentum between a spin polarised current and a magnetic domain has potentially wide practical applications. The Landau-Lifhshitz-Gilbert (LLG) equations are often used to describe the dynamics of such domains. In order to be consistent with the second law of thermodynamics, the Gilbert relaxation term must be modified to involve a 'particle derivative'.  Faraday’s law e = −df/dt equates the induced electromotive-force (emf) to the derivative of the flux. A Berry phase g is accumulated by a spin 1/2 particle e.g., an electron (or even a neutron), during its adiabatic motion in the effective magnetic field it sees, e.g., in a ferromagnet. For a closed path this defines a flux F_s. As was pointed out by O. Stern in the context of mesoscopic rings, this can result in a motive-force −dF_s/dt. This arises from the coupling to fields due to the spin and not the charge and should therefore be called a spinmotive-force (smf). O. Stern, followed by others, suggested that such an smf arises when the external fields are time dependent, i.e., when the system is pumped by an external energy source.  Here it is shown that such a smf (and an associated emf) is commonplace in the spin angular transfer process, directly reflecting its non-conservative nature, even when the applied fields are stationary. A moving domain wall generates such an smf which results in current flow even in the absence of any external emf/smf.  Due to the coupling with the environment, it is notoriously difficult to put a macroscopic system in a state other than that which corresponds to a solution of the classical problem defined by an appropriate Lagrangian.  Here, including dissipation effects, this classical problem reduces to LLG equations modified as described above. The implicit 'measurement' made by the environment has experimental consequences for the angular dependence of the torque transfer process for both valves and partially or wholly pinned domain walls.

Current-driven domain wall dynamics based on collective coordinates

Gen Tatara

Tokyo Metropolitan University

Threshold current of domain wall motion under spin-polarized electric current in ferromagnets is theoretically studied. The wall is  described by use of equations of motion in terms of two collective  coordinates, wall position and spin polarization out-of easy-plane.   Effects of non-adiabaticity and spin relaxation, both represented by  so-called a beta-term in Landau-Lifshits equation, are taken account  in addition to extrinsic pinning.  It is demonstrated that there are four different regime where  threshold current is characterized by different dependence on  extrinsic pinning, shape-anistoropy and beta. The results are  compared with experimental results.

Spin transfer torques: theory and simulation

Shufeng Zhang

University of Missouri

We present our recent theoretical and computational efforts in understanding the spin transfer torque. Various spin torque models in ferromagnets will be reviewed and compared. Three current-driven effects will be discussed: 1) thermal effect as a subject in statistical physics, 2) domain wall dynamics as a challenging problem in computational micromagnetics, and 3) driven chaos and synchronization as an emerging topic in non-linear physics.

Spin Accumulation and Resistance Due To Domain Walls in Magnetic Nano-Wires

Jun’ichi Ieda

Institute for Materials Research, Tohoku University, 980-8577 Japan;
CREST, Japan Science and Technology Agency, 332-0012, Japan

Recent technological advances in material fabrication have led to growing interest in spin-polarized transport in ferromagnet materials [1]. Here we examine the coupling between the spin current and a domain wall (DW). While conservation of the spin angular momentum leads to the spin transfer torque driving DW [2], direct observation shows that the spin transfer is about 10% efficiency [3]. This suggests that the rest of the spins dissipate elsewhere, which is not yet resolved. On the other hand, transport measurements on ferromagnetic wires with electric currents traversing 'pinned' DW have revealed that the DW acts as a source of extra resistance [4]. Motivated by these experiments, we study the spin accumulation around DW under a finite current. Due to the asymmetry of the spin-dependent conductivities the spin accumulation gives rise to local shift of the spin-dependent electrochemical potentials that amounts to be detected as extra resistance. We solve the diffusion equation for the spin accumulation around the DW of width w and obtain the excess resistance. This decreases as a function of the ratio x = w/lF where lF is the spin diffusion length, and in the abrupt wall limit (x -> 0) reproduces the well-known result for interfacial resistance [5]. This work has been done in collaboration with S. Takahashi, M. Ichimura, H. Imamura and S. Maekawa.

[1] Concepts in Spin Electronics, ed. by S. Maekawa (Oxford Univ. Press, 2006).

[2] S. E. Barnes and S. Maekawa, Phys. Rev. Lett. 95, 107204 (2005).

[3] A. Yamaguchi et al., Phys. Rev. Lett. 92, 077205 (2004).

[4] U. Ebels et al., Phys. Rev. Lett. 84, 983 (2000).

[5] T. Valet and A. Fert, Phys. Rev. B48, 7099 (1993).

Interactions between domain walls and spin polarized currents

Mathias Kläui, Universitaet Konstanz

A promising novel approach for switching magnetic nanostructures is current-induced domain wall propagation (CIDP) where due to a spin torque effect, electrons transfer angular momentum to a head-to-head domain wall and thereby push it in the direction of the electron flow without any externally applied fields. We use magnetoresistance measurements, spin polarized scanning electron microscopy and photoemission electron microscopy to directly observe domain wall propagation in-situ in ferromagnetic nanostructures induced by current pulses [1]. In addition to wall propagation, we observe a range of wall transformations including the theoretically predicted nucleation and annihilation of vortices [2] and find that the wall velocity is directly correlated with the wall spin structure. Studying different geometries we conclude that the fastest and most reproducible wall propagation is found in wires, where no transformation occurs.  The observed wall velocities and critical current densities, where wall motion sets in at room temperature, do not agree well with theoretical 0K calculations [2]. We have therefore measured the critical current densities as a function of the sample temperature. We find that the spin torque effect becomes more efficient at low temperatures, which can be attributed to thermally activated spin waves and which could account for some of the observed discrepancies between the 300K experiment and the 0K simulations.

 [1] M. Klaui et al., PRL 94, 106601 (2005), PRL 95, 26601 (2005); [2] A. Thiaville et al., EPL 69, 990 (2005); G. Tatara et al., APL 86, 252509 (2005);

Magnetization Dynamics in a Spin Valve Imaged with X-rays

Y. Acremann¹, J.P. Strachan², V. Chembrolu², S.D. Andrews³, T. Tyliszczak⁴, J.A. Katine⁵, M.J. Carey⁵, B.M. Clemens³, H.C. Siegmann¹, J. Stöhr¹

¹ Stanford Synchrotron Radiation Laboratory, Stanford, California 94309, USA

² Department of Applied Physics, Stanford University, Stanford, California 94305, USA

³ Department of Materials Science and Engineering, Stanford University, Stanford, California 94305, USA

⁴ Advanced Light Source, Berkeley, California 94720, USA

⁵ Hitachi Global Storage Technologies San Jose Research Center, San Jose, California 95120, USA

Presented by

Yves Acremann

Advanced x-ray imaging techniques can image magnetic structures much below 1µm at presently 200 ps time resolution. Magnetization dynamics are excited in a CPP-spin valve by a pulsed spin polarized electron current. The Oerstedt field of the associated charge current dominates the dynamics, and magnetization switching takes place in less than 0.5 ns. As opposed to the Néel or Stoner-Wohlfarth model, the switching does not occur by uniform rotation of the magnetization, but rather by the motion of a magnetic vortex across the sample. We discuss the use of spin currents to construct a spin amplifier with ferromagnetic metals, similar to the one proposed by Dimtri E. Nikonov and George I. Bourianoff (IEEE Trans. on Nanotechnology 4, 206, 2005) for the still rather elusive magnetic semiconductors.

Spin-torque-induced magnetic reversal in magnetic tunnel junctions with MgO barriers

Jonathan Sun, IBM T.J. Watson Research Center

            Spin-torque induced magnetic reversal has been unambiguously demonstrated in magnetic tunnel junctions with MgO barriers. For quasi-static measurements, the reversal is dominated by events determined by spin-current amplified thermal activation, resulting in a measured average switching current below that of the zero-temperature dynamic threshold. Such sub-threshold switching current generally shows stronger and non-linear magnetic field dependence, following a shape determined by the magnetic field dependence of the thermal barrier height. Time-resolved measurements are required for adequately assessing the dynamic switching threshold current for fast (nano-second-level) deterministic switching, and for revealing the magnetic field dependence of the threshold current. The later would give direct experimental verification of the role a large easy-plane demagnetization field plays as it determines the value of the dynamic switching current threshold.

Spin-Current MRAM in Context

W.J. Gallagher

IBM Thomas J. Watson Research Center, Yorktown Heights, NY 10598

glgr@us.ibm.com

In recent years there has been an explosion of research on memory alternatives to the dominant three chip memory families:  SRAM, DRAM, and FLASH.   Two factors driving this are (1) scaling limitations faced by each of these technologies and (2) the desirability for system simplification that one 'universal' memory could bring, especially in an era of very high integration approaching a billion transistors per chip.  Among the most promising new-memory candidates for near-term commercialization are field-switched MRAM, which has some potential as a universal memory, and Phase-Change Memory, which has potential as a high-density nonvolatile memory that is scalable beyond the limits of some FLASH technologies.   This talk will look at spin-MRAM in the context of these developments.   Field-switched MRAM is being developed primarily in the 90nm to 180nm nodes, and appears extendable to perhaps the 45 node.  Spin-MRAM appears attractive at and beyond this node.  The reasons for this will be discussed as well the device attributes required for spin MRAM to be competitive in the overall memory context. 

Spin Momentum Transfer-Is it the future of MRAM?

Stuart Wolf

University of Virginia

In this brief talk I will discuss spin momentum transfer as providing the pathway for continual scaling of MRAM beyond the 32 nm node.  This will be based on novel new materials for the barriers, electrodes and perhaps even the overall structure of the memory cell.  The potential use of oxides, magnetic semiconductors, multierroics and a hybrid memory cell structure will be some of things discussed.

Magnetic vortex dynamics in confined geometries

Kristen S. Buchanan, ¹ Pierre E. Roy, ²  Marcos Grimsditch,¹ Frank Y. Fradin, ¹ Konstantin Yu. Guslienko, ¹ Sam D. Bader, ¹ and Valentyn Novosad¹

1. Materials Science Division and Center for Nanoscale Materials, Argonne National Laboratory, , IL 60439
2. Department of Engineering Science, Uppsala University, Uppsala, Sweden

The magnetic vortex state is the ground state for a wide variety of magnetically-soft structures with dimensions on the order of a micrometer or smaller.  A magnetic vortex in a ferromagnet with restricted geometry possesses a characteristic dynamic mode called the translational mode that corresponds to spiral-like motion of the vortex core around its equilibrium position.  Elliptical nanodots can take on a single vortex or vortex pair magnetization state, providing a convenient model system for investigating the effects of geometric confinement and dynamic vortex interactions.  Experimentally we have measured the eigenmodes of lithographically-defined Permalloy ellipses (2 x 1 mm² and 3 by 1.5 mm², both of 40-nm thick) using a microwave reflection technique.  The single vortex magnetization state exhibits a single, low-frequency excitation (77 MHz for the larger dots, 117 MHz for the smaller for small applied fields). The frequency of this mode changes very little for a field applied along the ellipse long axis, however, it more than doubles in frequency for a field along the short axis, which can be understood in terms of the shape of the vortex energy profile extracted from micromagnetic simulations.  A much richer excitation spectrum was measured for vortex pairs confined in the same elliptical dots [1]. By comparing with micromagnetic simulations, the detected resonances were assigned to the translational modes of vortex pairs with parallel or antiparallel core polarizations. Although the vortex core polarizations play a negligible role in the static interaction between two vortices, their effect dominates the dynamics.  

[1]  K. S. Buchanan, P. E. Roy, M. Grimsditch, F. Y. Fradin, K. Yu. Guslienko, S. D. Bader, and V. Novosad, Nature Physics 1, 172-176 (2005).

Spin waves in confined structures: quantization, localization and tunneling

S.O. Demokritov

University of Muenster, Germany.

The non-local nature of magnetic-dipole interaction and lateral confinement are responsible for a series of sophisticated physical effects determining spin wave properties and dynamics of magnetic nanostructures.   I will pay a special attention to the recently observed spin wave quantization and the spin wave well effect in patterned magnetic films and consider in detail magnetic dynamics of non-ellipsoidal elements which are mostly important for applications. Spin wave tunneling via potential barrier created by an inhomogeneous magnetic field and via vacuum will be also addressed.

Spin Injection and Accumulation in Ferromagnet-Semiconductor Devices

Paul Crowell

University of Minnesota

I will discuss recent experiments on ferromagnet-semiconductor heterostructures.^1,2  In the first, we image the accumulation of spin-polarized electrons near both the source and drain of a lateral Fe/GaAs/Fe device.  Both Fe contacts are epitaxial Schottky tunnel barriers.  The spin accumulation near the source is due to spin injection through the barrier.  The accumulation near the drain is due to the spin-dependent reflectivity of the barrier under forward bias.   For transport through the drain contact, we can also invert the imaging experiment so that we inject spin-polarized electrons optically and detect them electrically.  All of these experiments are conducted in a geometry sensitive to spin precession and can be analyzed in detail using a variation of the drift-diffusion model applied previously to spin transport experiments in all-metallic structures.    The success of this approach leads one to ask about the possibility of an all-electronic measurement.   I will discuss the status of this effort, focusing on measurements of the spin accumulation at the drain due to the forward-bias current.  This work was supported by the DARPA SpinS Program, ONR, the NSF MRSEC program under DMR 02-12032, the NSF NNIN program, and the LANL LDRD program.

In collaboration with X. Lou, M. Furis, C. Adelmann, S.A. Crooker, and C.J. Palmstrøm

1.  S.A. Crooker et al., Science 309, 2191 (2005).

2.  X. Lou et al., cond-mat/0602096.

Imaging the spin Hall effect and current-induced polarization in semiconductors and heterostructures

Yuichiro Kato
Center for Spintronics and Quantum Computation
University of California
Santa Barbara, CA 93106

Spin-orbit coupling in semiconductors relates the spin of an electron to its momentum, and provides a pathway for electrically initializing and manipulating electron spins in zero magnetic field for applications in spintronics and spin-based quantum information processing. This coupling can be regulated with strain in bulk semiconductors and quantum confinement in semiconductor heterostructures.

Using Faraday and Kerr rotation spectroscopies with temporal and spatial resolution, current-induced spin polarization [1] and the spin Hall effect [2] have been observed in bulk semiconductors. More recently, we have investigated the spin Hall effect and current-induced spin polarization in a two-dimensional electron gas confined in (110) AlGaAs quantum wells using Kerr rotation microscopy [3]. In contrast to previous measurements, the spin Hall profile shows complex structure and the current-induced spin polarization is out-of-plane. The experiments map the strong dependence of the current-induced spin polarization to the crystal axis along which the electric field is applied, reflecting the anisotropy of the spin-orbit interaction. These results reveal opportunities for tuning a spin source using quantum confinement, strain and device engineering in non-magnetic materials. This work was supported by ARO, DARPA, NSF and ONR.

1. Y. K. Kato, R. C. Myers, A. C. Gossard, D. D. Awschalom, Phys. Rev. Lett. 93, 176601 (2004).
2. Y. K. Kato, R. C. Myers, A. C. Gossard, D. D. Awschalom, Science 306, 1910 (2004).
3. V. Sih, R. C. Myers, Y. K. Kato, W. H. Lau, A. C. Gossard and D. D. Awschalom, Nature Physics 1, 31 (2005).

Synchronization of magnetization motion in multilayers by spin currents

Gerrit Bauer, TU Delft

The combination of the spin currents emitted by moving ferromagnets and the spin transfer torque that excites magnetization motion leads to a cross-talk of magnetic films in hybrid metallic multilayers. This dynamical coupling through the normal metals can lead to a synchronized magnetization motion that is favored by a smaller global damping and observed in ferromagnetic resonance. Similar phenomena are expected in granular ferromagnets.

*This work has been carried out in collaboration with Yaroslav Tserkovnyak, Arne Brataas, Bret Heinrich, and Babak Hosseinkhani.

Spin-transfer induced magnetization reversal in lateral devices

Yoshichika Otani and Takashi Kimura

ISSP University of Tokyo
& FRS-The Institute of Physical and Chemical Research

The spin torque responsible for the current induced magnetization switching is known to be proportional to the spin current density. Therefore not the charge current causing the Joule heat but the spin current is essential to realize efficient magnetization reversal due to the spin torque. Thus non-local spin injection technique is employed to realize such a reversal.  Here we discuss the magnetization reversal due to non-local spin injection into a nano-scale ferromagnetic particle in a lateral ferromagnetic/nonmagnetic hybrid device.

Magnetic Detection of Current-Induced Electron Spin-Polarization in Aluminum

P. Stamenov and J. M. D. Coey
School of Physics and CRANN, Trinity College, Dublin 2, Ireland

Design Half Metals with Simple Structures*

C. Y. Fong, University of California, Davis

          Transition metal pnictides (group V), a carbide (group IV), their related superlattices with the zincblende structure, and several carbon quantum wires doped with transition metal elements have been designed to exhibit half metallic properties. First-principles VASP code based on density functional theory (DFT) and the generalized gradient approximation (GGA) has been used. MnAs [1] serves as an example for the pnictides to explain the features of a half metal, the value of the magnetic moment/unit-cell and the interactions causing the half metallicity. MnC [2], a carbide, shows different half metallic properties from the pnictides. The physical implication of this difference will be presented. We will discuss half metallic superlattices made of pnictides [3] and the related ballistic properties [4]. Finally, we will present results on doped carbon quantum wires exhibiting robust half metallic properties, using the Cr case as a prototype [5]. 

[1]        J. E. Pask, L. H. Yang, C. Y. Fong, W. E. Pickett, and S. Dag, Phys. Rev. B 67, 104417 (2003).

[2]        M. C. Qian, C. Y. Fong, and L. H. Yang, Phys. Rev. B 70, 052404 (2004).

[3]        C. Y. Fong, M. C. Qian, J. E. Pask, L. H. Yang, and S. Dag, Appl. Phys. Lett. 84, 239 (2004).

[4]        M. C. Qian, C. Y. Fong, W. E. Pickett, J. E. Pask, L. H. Yang, and S. Dag, Phys. Rev. B 71, 012414 (2005).

[5]        S. Dag, S. Tongay, T. Yildirim, E. Durgun, R. T. Senger, C. Y. Fong, and S. Ciraci, Phys. Rev. B 72, 155444 (2005).

* This work is supported by National Science Foundation.