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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