Brief introduction to spintronics devices

1. Brief introduction to spintronics devices Zhihao Zang 07.20.2019

2. Content • Introduction and future of spintronics • Spin injection and detection: some spintronics devices • Spin valve • Spin FET • Spin LED • Spin relaxation mechanism • Perspective • Perovskite LED • Chiral 2D perovskite • FM/2D semiconductors

3. Introduction and future of spintronics NPG Asia Mater. 3, 65–73 (2011) Traditional: ON/OFF Spintronics: spin up/spin down

4. Introduction and future of spintronics Gate-tuning of effective magnetic field Three basic problems in spintronics: • Injection • Relaxation and transportation • Detection Drain-detection Source-injection Appl. Phys. Lett. 56, 665 (1990)

5. Content • Introduction and future of spintronics • Spin injection and detection: some spintronics devices • Spin valve • Spin FET • Spin LED • Spin relaxation mechanism • Perspective • Perovskite LED • Chiral 2D perovskite • FM/2D semiconductors

6. Spin valve Without spin flip process, the polarization of injection current: Conductivity mismatch: FM to semiconductors With spin flip process, the polarization of injection current: The degree of current polarization is too low due to large relative conductivities. With introducing a spin selective barrier, it can be written as: As the ferromagnet is a metal and the contact resistance is large, . NPG Asia Mater. 3, 65–73 (2011)

7. Spin valve Non-local spin valve Magnetic resistance is dependent on the relative magnetization configurations of two FM electrodes. Non-local measurement can get higher Signal-to-noise, owing to the absence of net charge flow between injector and detector. Local spin valve Nature Nano. 9, 794–807 (2014)

8. Spin valve electrochemical potentials spin flip length The polarized current injected from Co1 can induce an unbalanced distribution of spin-up and spin-down electrons in Al strip, and this unbalance can diffuse to Co2, which can be detected. Nature, 416, 713–716 (2002)

9. Spin valve With a out-plane magnetic field , the spin direction of electrons in Co2 will precess around an axis parallel to , called Larmor precession. • Hanle effect However, in an (infinite) diffusive conductor the diffusion time t from Co1 to Co2 has a broad distribution: . The signal we get: . At large , the magnetization direction of the Co electrodes is tilted out of the substrate plane with an angle . When we include this effect we calculate: . Nature, 416, 713–716 (2002)

10. Spin FET Rashba field - provided by the intrinsic electric field caused by structural asymmetry of 2DEG channel, which can be controlled by Vg. An external-magnetic field, Ba = 0.5 T, was applied to fix the magnetization orientations of the FM electrodes in a chosen direction, thereby determining the axis of spin injection and detection. Due to slightly different coercivities between two FM because of different aspect ratio, magnetization alignment of FM electrodes will change between parallel and antiparallel configurations. Ni81Fe19/InAs/InP 10.1126/science.1173667

11. Spin FET Shubnikov-de Haas (SdH) oscillations: The theory of Datta and Das describes the transport of ballistic carriers in a narrow 2DEG channel: T/2=1.24V T/2=1.53V 10.1126/science.1173667

12. Spin LED In GaAs, the conduction band (s-character) is two-fold spin degenerate, whereas the valence band (p-character) is four-fold degenerate (heavy- and light-hole spin). The limit of circular polarization: ~ 50%. Some strain and confinement can make valence bands separated. Circular-polarized selection rules: . Phys. Rev. B 62, 8180

13. Spin LED Denoting Bloch states according to , the wave function can be expressed as the combination of spatial components: To assess the degree of spin polarization of a system (e.g. GaAs): The irreducible representations of point group - Td. The dipole operator corresponding to optical transition is proportional to (spherical harmonic). We have the relative intensity for transition: =3 NPG Asia Mater. 3, 65–73 (2011)

14. Content • Introduction and future of spintronics • Spin injection and detection: some spintronics devices • Spin valve • Spin FET • Spin LED • Spin relaxation mechanism • Perspective • Perovskite LED • Chiral 2D perovskite • FM/2D semiconductors

15. Spin relaxation mechanism e.g. GaAs: inversion symmetry broken. Generally, causing a effective magnetic field D’yakonov–Perel’ mechanism: spin splitting of the conduction band due to the inversion symmetry breaking of a crystal. Two regimes for the spin relaxation can be considered: : interval between two scattering events : spin-relaxation time Phys. Rev. Lett. 89, 236601

16. Spin relaxation mechanism Net polarization curve is consistent with SQUID curve We can rewrite circular polarization as: ratio of the electron populations between the upper and lower Zeeman levels If, the system reaches a quasi-equilibrium state defined by Zeeman splitting r increases above 150 K . TheZeeman polarization decreases monotonously. If the Zeeman polarization is vanishing, Appl. Phys. Lett. 85, 3492 (2004)

17. Content • Introduction and future of spintronics • Spin injection and detection: some spintronics devices • Spin valve • Spin FET • Spin LED • Spin relaxation mechanism • Perspective • Perovskite LED • Chiral 2D perovskite • FM/2D semiconductors

18. Perovskite LED Circularly emission from triplet states: T2 and T3. With a magnetic field, quenching happens to PL (optical Hanle effect): Nature Communications 10, 129 (2019)

19. Perovskite LED P(EL)~0.8% Nature Communications 10, 129 (2019)

20. Chiral 2D perovskite Chiral 2D perovskite: chirality transfer from chiral ligands to perovskite. R-RDCP and S-RDCP show different CD signals while rac-RDCP shows no signal, which means that chirality has been transferred to 2D perovskite. CD signals exhibit dispersive features centered around the absorption energy(~380nm), “Cotton effect” . Nature Photonics 12, 528–533 (2018)

21. Chiral 2D perovskite Different absorption rates lead to different spin-polarized light Emission. Thus, the chiral perovskite can emit spin-polarized light without an external magnetic field. At non-zero magnetic field, the Zeeman effect breaks time-reversal symmetry in the system and gives rise to energy splitting of the spin subbands. A competition between Zeeman splitting and chirality transfer. Nature Photonics 12, 528–533 (2018)

22. Chiral 2D perovskite Steady state equations: Electron-hole picture for calculating DP Assuming that there is only small difference between the bright exciton emission rate: If Δ1and Δ2are small enough then we can simplify the equation above: Hence, Nature Photonics 12, 528–533 (2018)

23. FM/2D semiconductors Based on current conditions, we could demonstrate a device: AC-driven spin-LED with chiral perovskite. Emitting spin-polarized light with/without magnetic field Gr Gr h-BN h-BN Chiral 2D perovskite 2D semiconductors Gr FM h-BN h-BN Si/SiO2 Si/SiO2 Emitting spin-polarized light with magnetic field

24. FM/2D semiconductors few-layer tunneling barrier(M) Gr Gr few-layer tunneling barrier(M) 2D semiconductors With two spin tunneling barrier, we can get a device with higher degree of polarization. Si/SiO2

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