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Spintronics and magnetic semiconductors

NERC SWAN. Spintronics and magnetic semiconductors. Tom as Jungwirth. Universit y of Nottingham Bryan Gallagher, Tom Foxon, Richard Campion, et al. Institute of Physics ASCR, Prague Sasha Shick , Jan Ma šek, Vít Novák, et al.

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Spintronics and magnetic semiconductors

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

  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 1 bit: 1mm x 1mm MB’s From PC hard drives ('90) to micro-discs - spintronic read-heads 1 bit: 10-3mm x 10-3mm 10’s-100’s GB’s

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

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

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

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

  9. 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

  10. 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

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

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

  13. total wf antisymmetric = * spin wf symmetric (aligned) orbital wf antisymmetric e- FERO MAG NET 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)

  14. 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) • Current sensitive to magnetization • direction

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

  16. 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

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

  18. Ga Mn As Mn As-p-like holes Mn-d-like local moments • carriers with both strong SO coupling • and exchange splitting, yet simple • semiconductor-like bands • - Mn 3d5 (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

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

  20. AlOx Au GaMnAs [100] [100] M [010] [110] [010] F [100] [100] [110] Au [010] [010] [010] Family of new AMR effects: TAMR – anisotropic TDOS predicted and observed in metals TAMR– discovered in GaMnAs  Au Shick et al.PRB'06, Bolotin et al. PRL'06, Viret et al. EJP’06, Moser et al. 06, Grigorenko et al. ‘06 Resistance 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,

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

  22. Q VD Source Drain Gate VG M [010] [110] F [100]  [110] [010] Coulomb blockade AMR spintronic transistor Anisotropic chemical potential magnetic electric & control of CB oscillations Wunderlich et al. PRL 06

  23. CBAMR SET • 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

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

  25. ~100 nm Dipolar-field-free current induced switching nanostructures Micromagnetics (magnetic anisotropy) without dipolar fields (shape anisotropy) One Domain wall Can be moved by ~100x smaller currents than in metals Humpfner et al. 06, Wunderlich et al. 06 Strain controlled magnetocrystalline (SO-induced) anisotropy

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

  27. 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

  28. Ferro Ga Magnetization Mn As  Mn Current Spintronics explores new avenues for:  • Information reading • 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

  29. 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

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

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

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

  33. 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

  34. _ _ _ 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

  35. 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

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