MERGING NANOPHOTONICS AND SPINTRONICS
Bradlee Beauchamp – ECE 695 Dec. 1st, 2017
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OUTLINE • Importance of merging nanophotonics and
spintronics • Spintronics Overview and Motivation • Plasmonics and spintronics
• Plasmonic all-optical switching • Heat-assisted magnetic recording
• Metamaterials and spintronics • Photonic spin hall effect • Plasmonic spin hall effect
• Conclusion
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SYNERGISM OF SPINTRONICS AND NANOPHOTONICS • Speed of nanophotonics, non-volatility and
efficiency of spintronics
Spintronics
https://engineering.purdue.edu/~shalaev/teaching.php
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SPINTRONICS • Exploit the spin of an electron to store, carry,
and read information • Benefits
• Non-volatile • Low-power • Scalable
• Relevant topics for discussion:
• Spin valve/magnetic tunnel junction, giant magnetoresistance/tunneling magnetoresistance, spin-transfer torque, spin-orbit torque
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NANOMAGNETS IN SPINTRONICS • Giant magnetoresistance (GMR)
• GMR nobel prize 2007, nobel prize winners • Resistance change for parallel vs antiparallel
magnetizations in iron separated by non-magnetic spacer (copper)
• Basis of spin valve
https://www.nobelprize.org/nobel_prizes/physics/laureates/2007/
• Tunneling magnetoresistance (TMR) has a fixed, free, and tunneling layer
• Basis of magnetic tunnel junction (MTJ)
ferromagnet (free layer)
ferromagnet (reference layer)
nonmagnetic tunneling barrier
S. Ikeda et al.
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CURRENT-INDUCED SWITCHING LIMITED BY PRECESSION
• Spin-transfer torque • Spin-polarized current can impart
angular momentum to free layer, allowing precessional rotation of magnetization
• Limited to nanosecond (GHz) speed
• Spin-orbit torque • Recent efforts of having spin
current adjacent to FM, causing magnetization switching
• Room temp. SOT switching from topological insulator demonstrated last month
• Still depends on precessional switching
J. Magn. Magn. Mater., D.C. Ralph & M.D. Stiles, 2007
Nat. Commun., Yi Wang et al., 2017 6
SPINTRONICS AND PLASMONICS An opportunity for THz light-controlled nanoscale data storage
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OPTICAL INTERACTION WITH MAGNETISM Motivation • fundamental limit for
deterministic magnetic field switching is 2 ps (Nature, Tudosa et al., 2004)
• Sub-picosecond optical interaction with magnetism, demagnetization via 60fs laser first demonstrated 1996 (Phys. Rev. Lett., Beaurepaire et al.)
• can be integrated with mature spintronics devices such as the MTJ (where spin-orbit torque used to switch) and used at THz speeds (J. App. Phys., Walowski, 2016)
Rev. Mod. Phys., Kirilyuk, Kimel, Rasing, 2010
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ALL – OPTICAL MAGNETIZATION SWITCHING • Switching via 40 fs circularly polarized
laser pulses demonstrated in by CD Stanciu et al. (2007)
• Actual magnetization switching is slower but is still sub-picosecond
• Vahaplar et al. (2012) have described this as a combination of laser absorption bringing material to non-equibrilium state with no net magnetization and light helicity providing an opto-magnetic field and linear switching
• Inverse Faraday Effect (IFE) • Circularly polarized light induces a static
magnetization in a crystal • Circularly polarized light causes angular
momentum-flux
Phys. Rev. Lett.,CD Stanciu et al. 2007
To increase IFE: Either use material with high V or increase intensity
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STANCIU ET AL. SINGLE PULSE SWITCHING
Switching limited to spot size of laser! (~10 um) nanophotonics needed for miniaturization – Tbit/in^2 data storage desirable
Phys. Rev. Lett.,CD Stanciu et al. 2007
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PLASMONICS FOR MINIATURIZATION AND FIELD-ENHANCEMENT
International Conference on Magnetism, Belotelov et al., 2010
Enhancement of Inverse Faraday Effect
Magnetoplasmonics: plasmonic enhancement and miniaturization of magneto-optical phenonema • enhancements shown by Belotelov: Faraday effect, Inverse transverse magneto-
optical Kerr effect (TMOKE), longitudinal and polar MOKE
paramagnetic metal dielectric
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PLASMONICS FOR MINIATURIZATION AND FIELD-ENHANCEMENT
Nano Letters, Liu et al., 2015
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SURFACE-PLASMON OPTO-MAGNETIC FIELD ENHANCEMENT – EFFORTS HERE AT PURDUE
• Dutta et al. simulations show enhancement of opto-magnetic field via surface plasmons is ~10 times that of direct photo-magnetic coupling due to localized surface plasmon resonance (LSPR)
MgO/TiN/BIG/Si3N4 MgO/BIG/Si3N4
Opt. Mat. Express, Dutta et al., 2017 13
HEAT-ASSITED MAGNETIC RECORDING (HAMR) • Plasmonic resonance used to
assist with magnetic recording for high-coercivity magnetic materials needed for larger areal densities (e.g. FePt)
• Help push off “trilemma” of magnetic storage (storage, SNR, writability)
Nanophotonics (review), Zhou et al., 2014
http://nptel.ac.in/courses/115103038/40
Indian Institutes of Technology 14
SPINTRONICS AND METAMATERIALS
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PHOTONIC SPIN HALL EFFECT • Manipulation of
strong spin-orbit interaction of light
• Phase gradient introduced by metasurface enables transverse motion of opposite helicities in light
• Axial symmetry removed, strong spin-orbit coupling
Science, Yin et al., 2013 16
PLASMONIC SPIN HALL EFFECT • Shown in silver/air
grating which can act as a hyperbolic metasurface (HMS)
• PSHE: lack of inversion symmetry, supports E field components perpendicular to SPP direction, dispersion frequency-dependent
• Enables dispersion-dependent PSHE where SPP propagation and helicity are linked Nature, High et al., 2015
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SUMMARY • Merging spintronics and nanophotonics
enables >THz speeds • Plasmonics needed for coupling light to
magnetic structures via inverse faraday effect to ensure scalability
• Meta-surfaces provide possible on-chip manipulation of light to couple with plasmonic structures
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THANK YOU
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