1. SPINTRONICS Tomáš Jungwirth Fyzikální ústav AVČR University of Nottingham

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 From PC hard drives ('90) to micro-discs - spintronic read-heads MByte GByte 1 bit: 1mm x 1mm 1 bit: 10 -3 mm x 10 -3 mm

4. HARD DISKS

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

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

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

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

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

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

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=p 2 /2m E  ih d/dt p  -ih d/dr... E 2 /c 2 =p 2 +m 2 c 2 (E=mc 2 for p=0) high-energy physics solid-state physics and microelectronics

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

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

17. Ingredients: - potential V(r) - motion of an electron Produces an electric field 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 E e-e-e-e- Relativistic "single-particle" VV B eff p s Spin-orbit coupling (Dirac eq. in external field  V(r) & 2nd-order in v /c around non-relativistic limit ) Current sensitive to magnetization Current sensitive to magnetization direction direction

18. Spin-orbit coupling Dirac eq. in external field  V(r) & 2nd-order in v /c around non-relativistic limit e-e-e-e- VV B eff p s Spintronics Ferromagnetism Coulomb repulsion & Pauli exclusion principle ~(k. s) 2 kyky kxkx ~M x. s x Fermi surfaces FM without SO-couplingSO-coupling without FMFM & SO-coupling ~(k. s) 2 + M x. s x

19. FM without SO-couplingSO-coupling without FM FM & SO-coupling ~(k. s) 2 + M x. s x kyky kxkx kxkx kxkx kyky kyky M M scattering ~M x. s x Fermi surfaces AMR Ferromagnetism: sensitivity to magnetic field SO-coupling: anisotropies in Ohmic transport characteristics; ~1-10% MR sensor 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 E F large spin-up DOS available at E F

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

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

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

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

29. & electric & magnetic control of Coulomb blockade oscillations Q0Q0 Q0Q0 e 2 /2C  [ 010 ]  M [ 110 ] [ 100 ] [ 110 ] [ 010 ] SO-coupling   (M) Spintronic transistor based on CBAMR SourceDrain Gate VGVG VDVD Q

30. 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 CBAMR SET In principle feasible but difficult to realize at room temperature

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

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

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

34. (Ga,Mn)As (and other III-Mn-V) ferromagnetic semiconductor Mn Ga As Mn 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 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 Mn Ga As Mn

36. Ga As Mn Mn–hole spin-spin interaction hybridization Hybridization  like-spin level repulsion  J pd S Mn  s hole interaction Mn-d As-p

37. H eff = J pd || -x Mn As Ga h eff = J pd || x Hole Fermi surfaces Ferromagnetic Mn-Mn coupling mediated by holes

38. No apparent physical barriers for achieving room T c in III-Mn-V or related functional dilute moment ferromagnetic semiconductors Need to combine detailed understanding of physics and technology Weak hybrid. Delocalized holes long-range coupl. InSb, InAs, GaAs d5d5 Strong hybrid. Impurity-band holes short-range coupl. 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 T c in ferromagnetic semiconductors & back to exploring spintronics fundamentals

41. Spintronics relies on extraordinary magnetoresistance B V I _ + + + + + + + + + + + + + _ _ _ _ _ FLFL 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 e.g. ordinary (quantum) Hall effect I _ F SO _ _ V and anomalous Hall effect anisotropic magnetoresistance M Known for more than 100 years but still controversial

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

43. n n p SHE mikročip, 100  A supravodivý magnet, 100 A Spin Hall effect detected optically in GaAs-based structures Same magnetization achieved by external field generated by a superconducting magnet with 10 6 x larger dimensions & 10 6 x larger currents Cu SHE detected elecrically in metals SHE edge spin accumulation can be extracted and moved further into the circuit

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

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