1. Spintronics and magnetic semiconductors Tomas Jungwirth University of Nottingham Bryan Gallagher, Tom Foxon, Richard Campion, et al. Hitachi Cambridge Jorg Wunderlich, David Williams, et al. Institute of Physics ASCR, Prague Sasha Shick, Jan Mašek, Vít Novák, et al. University of Texas Texas A&M Univ. Allan MacDonald, Qian Niu et al. Jairo Sinova, et al. NERC SWAN

2. 1. Current spintronics in HDD read-heads and memory chips 2. Basic physical principles of the operation of spintronic devices 3. Semiconductor spintronics research 4. Summary

3. Current spintronics applications First hard disc (1956) - classical electromagnet for read-out From PC hard drives ('90) to micro-discs - spintronic read-heads MB’s 10’s-100’s GB’s 1 bit: 1mm x 1mm 1 bit: 10 -3 mm x 10 -3 mm

4. Anisotropic magnetoresistance (AMR) read head 1992 - dawn of spintronics Appreciable sensitivity, simple design, scalable, cheap Giant magnetoresistance (GMR) read head - 1997 High sensitivity  and  are almost on and off states: “1” and “0” & magnetic  memory bit

5. MEMORY CHIPS DRAMhigh density, cheep. DRAM (capacitor) - high density, cheep x 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 lifetime, expensive charge Operation through electron charge manipulation

6. MRAM – universal memory fast, small, low-power, durable, and non-volatile 2006- First commercial 4Mb MRAM

7. RAM chip that actually won't forget  instant on-and-off computers Based on Tunneling Magneto-Resistance (similar to GMR but insulating spacer)

9. RAM chip that actually won't forget  instant on-and-off computers Based on Tunneling Magneto-Resistance (similar to GMR but insulating spacer)

10. 1. Current spintronics in HDD read-heads and memory chips 2. Basic physical principles of the operation of spintronic devices 3. Semiconductor spintronics research 4. Summary

11. 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

12. quantum mechanics & special relativity  particles/antiparticles & spin Dirac equation 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

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

14. 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) many-body

15. 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

16. Conventional ferromagnetic metals itinerant 4s: no exch.-split no SO localized 3d: exch. split SO coupled  ss sdsd sdsd Mott’s model of transport Ab initio Kubo (CPA) formula for AMR and AHE in FeNi alloys difficult to connect models and microscopics Banhart&Ebert EPL‘95 Khmelevskyi ‘PRB 03 Mott&Wills ‘36 AMR AHE

17. 1. Current spintronics in HDD read-heads and memory chips 2. Basic physical principles of the operation of spintronic devices 3. Semiconductor spintronics research 4. Summary

18. Mn Ga As Mn Ferromagnetic semiconductors GaAs - standard III-V semiconductor Group-II Mn - dilute magnetic moments & holes & holes (Ga,Mn)As - ferromagnetic semiconductor semiconductor More tricky than just hammering an iron nail in a silicon wafer

19. Mn-d-like local moments As-p-like holes Mn Ga As Mn - carriers with both strong SO coupling and exchange splitting, yet simple and exchange splitting, yet simple semiconductor-like bands semiconductor-like bands - Mn 3d 5 (S=5/2, L=0): no SO coupling just help to stabilize ferromagnetism Favorable systems for exploring physical origins of old spintronics effects and for finding new ones

20. FM without SO-couplingSO-coupling without FM FM & SO-coupling ~(k. s) 2 ~(k. s) 2 + M x. s x kyky kxkx kxkx kyky M kxkx kyky M Enhanced interband scattering near degeneracy ~M x. s x Hot spots for scattering of states moving  M  R(M  I)> R(M || I) AMR: a reflection of Fermi surface spin textures in transport

21. Family of new AMR effects: TAMR – anisotropic TDOS TAMR – discovered in GaMnAs  Au GaMnAs Au AlOx Au predicted and observed in metals [100] [010] [100] [010] [100] [010] Gould, et al., PRL'04, Brey et al. APL’04, Ruster et al.PRL’05, Giraud et al. APL’05, Saito et al. PRB’05, [ 010 ]  M [ 110 ] [ 100 ] [ 110 ] [ 010 ] Shick et al.PRB'06, Bolotin et al. PRL'06, Viret et al. EJP’06, Moser et al. 06, Grigorenko et al. ‘06 Resistance

22. TAMR spintronic diode classical spintronic TMR Au No need for exchange biased fixed magnet or spin coherent tunneling spintronic TAMR Au TMR

23. & electric & magnetic control of CB oscillations Coulomb blockade AMR spintronic transistor Wunderlich et al. PRL 06 SourceDrain Gate VGVG VDVD Q [ 010 ]  M [ 110 ] [ 100 ] [ 110 ] [ 010 ]  Anisotropic chemical potential

24. Generic effect in FMs with SO-coupling (predicted higher-T CBAMR for metals) Combines electrical transistor action with magnetic storage Switching between p-type and n-type transistor by M  programmable logic CBAMR SET

25. Dilute moment nature of ferromagnetic semiconductors Ga As Mn 10-100x smaller M s One Current induced switching replacing external field Tsoi et al. PRL 98, Mayers Sci 99 Key problems with increasing MRAM capacity (bit density): - Unintentional dipolar cross-links - External field addressing neighboring bits 10-100x weaker dipolar fields 10-100x smaller currents for switching Sinova et al., PRB 04, Yamanouchi et al. Nature 04

26. One Dipolar-field-free current induced switching nanostructures Micromagnetics (magnetic anisotropy) without dipolar fields (shape anisotropy) ~100 nm Domain wall Strain controlled magnetocrystalline (SO-induced) anisotropy Can be moved by ~100x smaller currents than in metals Humpfner et al. 06, Wunderlich et al. 06

27. 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 (Li / Zn stoichiometry) In (Ga,Mn)As T c ~ #Mn Ga (T c =170K for 6% MnGa) But the SC refuses to accept many group-II Mn on the group-III Ga sublattice Materials research of DMSs Masek et al. PRL 07

28. 1. Current spintronics in HDD read-heads and memory chips 2. Basic physical principles of the operation of spintronic devices 3. Semiconductor spintronics research 4. Summary

29. Information reading  Ferro Magnetization  Current Information reading & storage Tunneling magneto-resistance sensor and memory bit Information reading & storage & writing Current induced magnetization switching Information reading & storage & writing & processing : Spintronic single-electron transistor: magnetoresistance controlled by gate voltage Materials: Dilute moment ferromagnetic semiconductors Mn Ga As Mn Spintronics explores new avenues for:

31. (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

32. 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

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

34. Spintronics in non-magnetic semiconductors way around the problem of T c in ferromagnetic semiconductors & back to exploring spintronics fundamentals

35. 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

36. 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

37. 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