Putting the Electron s Spin to Work Dan Ralph Kavli Institute at Cornell Cornell University

Transcription

1 Putting the Electron s Spin to Work Dan Ralph Kavli Institute at Cornell Cornell University Yongtao Cui, Ted Gudmundsen, Colin Heikes, Wan Li, Alex Mellnik, Takahiro Moriyama, Joshua Parks, Sufei Shi, Eugenia Tam, Chen Wang, Lin Xue, David Muller, Paul McEuen, Hector Abruña, Bob Buhrman

2 Projects in Ralph Group Single-Molecule Electronics (with Abruña, Chan) Molecular Interfacing with Graphene (with Park, McEuen, Abruña) Torque Nano Opto-Electronics (with McEuen, Gaeta) done in KIC Photon Facility Manipulating Nanomagnets with Spin Torque (with Muller, Buhrman) Spin Manipulation Using Topological Insulators (with Muller, Kim)

3 Spin-Based Devices An electron can maintain its spin direction over 100 s of nm in a metal like Cu. In nanoscale devices is possible to generate spin currents and control their dynamics in ways that are impossible in larger devices: new opportunities for science and applications I will highlight two projects today: Controlling dynamics in nanoscale magnetic tunnel junctions using spin transfer torque Learning to measure and manipulate spin in single molecules

4 Thin magnetic layers act like filters for spins Magnetic Layer" unpolarized incident electrons (e.g. sp-like states in Cu)

5 Putting Spin Filters in Series: Giant Magnetoresistance discovered 1988 Parallel layers: low resistance Antiparallel layers: high resistance Disk drive read heads

6 Spin Transfer -- An Efficient Way to Apply Torques to Magnets (proposed by Slonczewski, Berger 1996) A magnetic layer can absorb the transverse component of incoming spin angular momentum, and therefore receives a torque. Cartoon Picture, for a Perfect Spin Filter Spin-polarized Current! Torque z" x" Magnetic Layer

7 Spin-Transfer-Driven Magnetic Switching 70 nm" view from above CoFeB MgO (~1 nm) CoFeB 130 nm" Single-shot time resolved measurements of magnetic dynamics Huai et al., APL 84, 3118 (2004). Fuchs et al., APL 85, 1205 (2004). Y-T. Cui et al., Phys. Rev. Lett. 104, (2010)

8 Low-Resistance Tunnel Junctions for Spin-Torque Studies view from above 40 nm Pt Tunnel Barrier 100 nm CoFeB CoFeB Ta Cu

9 STEM EELS Spectra: Composition Maps Reveal Nanoscale Variations in Chemistry ADF STEM (with Muller, Buhrman) BF STEM B Mn Composite RGB Map (O, B, Fe) Ta CoFeB MgO CoFeB 2 nm Fe O IrMn nm 22 nm Red+Green=Yellow 5 nm Co B EELS Spectrum Imaging Reveals: Nanoscale inclusions of B and other materials in the bottom electrode Areas of Fe diffusion into the MgO Mn diffusion into the MgO/CoFeB interfaces all in an excellent junction ~180% TMR

10 Spin-Transfer-Driven Magnetic Precession Sampling oscilloscope measurement H = 625 G V pulse = 240 mv Dephasing time 50 nsec, hundreds of oscillations can be observed ~1 nsec required to establish a steady precession state (~ 4 oscillation periods) I. N. Krivorotov et al., Science 307, 228 (2005).

11 New Experiments to be Enabled by the Tools in the Kavli Institute at Cornell Photon Facility Probing the limits of magnetic dynamics with ultrashort pulses Ultrafast Ti:Sapph laser V Delay Pulse shaping Photoswitch THz detector Injected current pulse Parabolic mirror Emitted THz waveform Magnetic AFM device device Testing predictions that strong spin torque can be generated by heat flow Heat and Spin

12 Studying Spin in Single Molecules Using Mechanically-Controllable Break Junctions The electrode spacing can be varied by bending the substrate. Junctions exhibit excellent stability.

13 Cobalt bis(terpy-sh) SH free metal ion octahedral field N N N Co N N N SH Spin state for Co 1+ is S = 1 For a purely octahedral bonding environment, the 3 levels in the ground state S = 1 multiplet are degenerate. If the molecule is stretched axially, this degeneracy is broken due to spin-orbit coupling. J. J. Parks et al., Science 328, 1370 (2010).

14 Mechanical Manipulation of Spin States in Single Molecules Splitting of the Kondo peak illustrates mechanical control of the spin states (only possible for spins with S = 1 or greater) The temperature dependence exhibits for the first time underscreened Kondo scaling predicted for high-spin systems The breaking of spin degeneracy is confirmed by magnetic field studies. J. J. Parks et al., Science 328, 1370 (2010).

15 Kavli Instrumentation Project: Optical Manipulation and Read-Out of Electron and Spin Dynamics High NA(0.7) objecfve lens Focal Plane Piezo Tube xy scanner: 10 microns at 1K AIocube z- posifoner Sample Holder Initial Projects: Nanoscale electronic domain structure in ruthenates (with Davis, Kim) Optical spin control in single molecules (with Gaeta) CollimaFng OpFcs

16 Conclusion! Advances in understanding how to generate and control spins on the nanoscale is enabling a revolution in the science of magnetic devices. New instrumentation for atomic scale characterization and dynamical measurements is the key to progress in this field.