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

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

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. 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

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

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

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

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 - 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

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

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

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

& 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

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

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

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

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

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

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:

(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

Spintronics in non-magnetic semiconductors way around the problem of T c 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 but still controversial

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

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