SPINTRONICS

1. SPINTRONICS Tomáš Jungwirth University of Nottingham Fyzikální ústav AVČR

2. 1. Current spintronics in HDD read-heads and memory chips 2.Physical principles of operation of current spintronic devices 3. Research at the frontiers of spintronics 4. Summary

3. Current spintronics applications First hard disc(1956) - classical electronics for read-out 1 bit: 1mm x 1mm MByte From PC hard drives ('90) to micro-discs - spintronic read-heads 1 bit: 10-3mm x 10-3mm GByte

4. HARD DISKS

5. HARD DISK DRIVE READ HEADS spintronic read heads horse-shoe read/write heads

6. Anisotropic magnetoresistance (AMR) read head 1992 - dawn of spintronics Appreciable sensitivity, simple design, scalable, cheap

7. Giant magnetoresistance (GMR) read head 1997 High sensitivity

8. MEMORY CHIPS .DRAM(capacitor) - high density, cheep x slow, high power, volatile .SRAM(transistors) - low power, fast x low density, expensive, volatile .Flash (floating gate) - non-volatilex slow, limited life, expensive Operation through electron chargemanipulation

9. MRAM – universal memory fast, small, non-volatile First commercial 4Mb MRAM Tunneling magneto-resistance effect (TMR) RAM chip that won't forget ↓ instant on-and-off computers

11. MRAM – universal memory fast, small, non-volatile First commercial 4Mb MRAM Tunneling magneto-resistance effect (TMR) RAM chip that won't forget ↓ instant on-and-off computers

12. 1. Current spintronics in HDD read-heads and memory chips 2.Physical principles of current spintronic devices operation 3. Research at the frontiers of spintronics 4. Summary

13. Electron has a charge (electronics) and spin (spintronics) Electrons do not actually “spin”, they produce a magnetic moment that is equivalent to an electron spinning clockwise or anti-clockwise

14. quantum mechanics & special relativity  particles/antiparticles & spin Dirac eq. E=p2/2m E ih d/dt p -ih d/dr . . . E2/c2=p2+m2c2 (E=mc2 for p=0) high-energy physics solid-state physics and microelectronics

15. e- Resistor classical spintronic external manipulation of charge & spin internal communication between charge & spin

16. total wf antisymmetric = * spin wf symmetric (aligned) orbital wf antisymmetric e- FERO MAG NET Non-relativistic (except for the spin) many-body Pauli exclusion principle & Coulomb repulsionFerromagnetism • Robust(can be as strong as bonding in solids) • Strong coupling to magnetic field • (weak fields = anisotropy fields needed • only to reorient macroscopic moment)

17. p s V e- Beff Relativistic "single-particle" Spin-orbit coupling (Dirac eq. in external field V(r) & 2nd-order in v /c around non-relativistic limit) Produces an electric field Ingredients: - potential V(r) - motion of an electron E In the rest frame of an electron the electric field generates and effective magnetic field - gives an effective interaction with the electron’s magnetic moment • Current sensitive to magnetization • direction

18. ky kx p s V e- Beff Spintronics Ferromagnetism Coulomb repulsion & Pauli exclusion principle Spin-orbit coupling Dirac eq. in external field V(r) & 2nd-order in v /c around non-relativistic limit Fermi surfaces ~(k . s)2 + Mx . sx ~(k . s)2 ~Mx . sx FM without SO-coupling SO-coupling without FM FM & SO-coupling

19. ky kx Fermi surfaces ~(k . s)2 + Mx . sx ~(k . s)2 ~Mx . sx FM & SO-coupling FM without SO-coupling SO-coupling without FM AMR M Ferromagnetism: sensitivity to magnetic field SO-coupling: anisotropies in Ohmic transport characteristics; ~1-10% MR sensor scattering ky kx M ky kx hot spots for scattering of states moving  M  R(M  I)> R(M || I)

20. Diode classical spin-valve TMR Based on ferromagnetism only; ~100% MR sensor or memory no (few) spin-up DOS available at EF large spin-up DOS available at EF

21. 1. Current spintronics in HDD read-heads and memory chips 2.Physical principles of current spintronic devices operation 3. Research at the frontiers of spintronics 4. Summary

22. Removing external magnetic fields (down-scaling problem)

23. EXTERNAL MAGNETIC FIELD problems with integration - extra wires, addressing neighboring bits

24. Current (instead of magnetic field) induced switching Angular momentum conservation  spin-torque

25. magnetic field current Myers et al., Science '99; PRL '02 local, reliable, but fairly large currents needed Likely the future of MRAMs

26. Spintronics in the footsteps of classical electronics from resistors and diodes to transistors

27. AMR based diode - TAMR sensor/memory elemets TAMR TMR no need for exchange biasing or spin coherent tunneling Au FM AFM Simpler design without exchange-biasing the fixed magnet contact

28. Spintronic transistor based on AMR type of effect Huge, gatable, and hysteretic MR Single-electron transistor Two "gates": electric and magnetic

29. M [010] [110] F Q VD [100] Source Drain [110] [010] Gate VG Q0 Q0 e2/2C Spintronic transistor based on CBAMR magnetic electric & SO-coupling  (M) control of Coulomb blockade oscillations

30. CBAMR SET • Generic effect in FMs with SO-coupling • Combines electrical transistor action • with magnetic storage • Switching between p-type and n-type transistor • by M  programmable logic In principle feasible but difficult to realize at room temperature

31. Spintronics in the footsteps of classical electronics from metals to semiconductors

32. p s V Beff Spin FET – spin injection from ferromagnet & SO coupling in semiconductor Difficulties with injecting spin polarized currents from metal ferromagnets to semiconductors, with spin-coherence, etc.  not yet realized

33. Ga Mn As Mn Ferromagnetic semiconductors – all semiconductor spintronics More tricky than just hammering an iron nail in a silicon wafer GaAs - standard semiconductor Mn - dilute magnetic element (Ga,Mn)As - ferromagnetic semiconductor

34. Ga Mn As Mn (Ga,Mn)As (and other III-Mn-V) ferromagnetic semiconductor • compatible with conventional III-V semiconductors (GaAs) • dilute moment system  e.g., low currents needed for writing • Mn-Mn coupling mediated by spin-polarized delocalized holes  spintronics • tunability of magnetic properties as in the more conventional semiconductor electronic properties. • strong spin-orbit coupling  magnetic and magnetotransport anisotropies • Mn-doping (group II for III substitution) limited to ~10% • p-type doping only • maximum Curie temperature below 200 K

35. Ga Mn As Mn (Ga,Mn)As material 5 d-electrons with L=0 S=5/2 local moment moderately shallow acceptor (110 meV)  hole -Mn local moments too dilute (near-neghbors cople AF) - Holes do not polarize in pure GaAs - Hole mediated Mn-Mn FM coupling

36. Ga Mn As Mn Mn–hole spin-spin interaction As-p Mn-d hybridization Hybridization  like-spin level repulsion  Jpd SMn shole interaction

37. Mn As Ga Ferromagnetic Mn-Mn coupling mediated by holes heff = Jpd <SMn> || x Hole Fermi surfaces Heff = Jpd <shole> || -x

38. No apparent physical barriers for achieving room Tc in III-Mn-V or related functional dilute moment ferromagnetic semiconductors Need to combine detailed understanding of physics and technology Impurity-band holes short-range coupl. Delocalized holes long-range coupl. Weak hybrid. Strong hybrid. InSb, InAs, GaAs d5 GaP

39. And look into related semiconductor host families like e.g. I-II-V’s III = I + II  Ga = Li + Zn • GaAs and LiZnAs are twin SC • (Ga,Mn)As and Li(Zn,Mn)As • should be twin ferromagnetic SC • But Mn isovalent in Li(Zn,Mn)As • no Mn concentration limit • possibly both p-type and n-type ferromagnetic SC

40. Spintronics in non-magnetic semiconductors way around the problem of Tc in ferromagnetic semiconductors & back to exploring spintronics fundamentals

41. V Spintronics relies on extraordinary magnetoresistance Ordinary magnetoresistance: response in normal metals to external magnetic field via classical Lorentz force Extraordinary magnetoresistance: response to internal spin polarization in ferromagnets often via quantum-relativistic spin-orbit coupling B anisotropic magnetoresistance _ _ _ _ _ _ _ _ _ _ _ FL + + + + + + + + + + + + + I V _ _ FSO M _ I e.g. ordinary (quantum) Hall effect and anomalous Hall effect Known for more than 100 years but still controversial

42. _ _ _ FSO _ non-magnetic FSO I majority _ _ _ FSO _ FSO I minority V V=0 Anomalous Hall effect in ferromagnetic conductors: spin-dependent deflection & more spin-ups  transverse voltage skew scattering side jump intrinsic Spin Hall effect in non-magnetic conductors: spin-dependent deflection  transverse edge spin polarization

43. n p n Cu Spin Hall effect detected optically in GaAs-based structures Same magnetization achieved by external field generated by a superconducting magnet with 106 x larger dimensions & 106 x larger currents SHE mikročip, 100A supravodivý magnet, 100 A SHE edge spin accumulation can be extracted and moved further into the circuit SHE detected elecrically in metals

44. 1. Current spintronics in HDD read-heads and memory chips 2.Physical principles of current spintronic devices operation 3. Research at the frontiers of spintronics 4. Summary

45. Downscaling approach about to expire currently ~ 30 nm feature size interatomic distance in ~20 years • Spintronics: from straighforward downscaling to • more "intelligent" device concepts: • simpler more efficient realization for a given functionality (AMR sensor) • multifunctional (integrated reading, writing, and processing) • new materials (ferromagnetic semiconductors) • fundamental understanding of quantum-relativistic electron transport (extraordinary MR)

46. Ferro Magnetization  Current Anisotropic magneto-resistance sensor Electromagnet • Information reading  • Information reading & storage Tunneling magneto-resistance sensor and memory bit • Information reading & storage & writing Current induced magnetization rotation

47. Ga Mn As Mn • Information reading & storage & writing & processing Spintronic single-electron transistor: magnetoresistance controlled by gate voltage • New materials Dilute moment ferromagnetic semiconductors • Spintronics fundamentals AMR, anomalous and spin Hall effects