Semiconductor spintronics Tomáš 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, Kamil Olejník Jan Kučera, Karel Výborný, Jan Zemen, et al. University of Texas Texas A&M Univ. Allan MacDonald, Qian Niu et al. Jairo Sinova, et al. NERC SWAN

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

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

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

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

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

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

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

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: mm x mm

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

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

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

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

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

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

Dilute moment nature of ferromagnetic semiconductors Ga As Mn x 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 x weaker dipolar fields x smaller currents for switching Sinova et al., PRB 04, Yamanouchi et al. Nature 04

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

Mn-d-like local moments As-p-like holes Mn Ga As Mn EFEF DOS Energy spin  spin  GaAs:Mn – extrinsic p-type semiconductor with 5 d-electron local moment on the Mn impurity valence band As-p-like holes As-p-like holes localized on Mn acceptors << 1% Mn onset of ferromagnetism near MIT Jungwirth et al. RMP ‘06 ~1% Mn >2% Mn

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 see J. Zemen 12:05, T2

& 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

Combines electrical transistor action with magnetic storage Switching between p-type and n-type transistor by M  programmable logic CBAMR SET

Spintronics in non-magnetic semiconductors way around the problem of low Curie T in ferromagnetic semiconductors & back to exploring spintronics fundamentals

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

intrinsic skew scattering 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 VV B eff p s Spin-orbit coupling

n n p SHE microchip, 100  A superconducting 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 Wunderlich et al. PRL 05

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

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: & non-magnetic – spin Hall effect

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

(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

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

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