Semiconductor spintronics Tom Jungwirth Institute of Physics ASCR

Semiconductor spintronics Tomáš Jungwirth Institute of Physics ASCR, Prague Sasha Shick, Jan Mašek, Vít Novák, Kamil Olejník Jan Kučera, Karel Výborný, Jan Zemen, et al. University of Nottingham Bryan Gallagher, Tom Foxon, Richard Campion, et al. University of Texas A&M Univ. Hitachi Cambridge Allan Mac. Donald, Qian Niu et al. Jairo Sinova, et al. Jorg Wunderlich, David Williams, 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 E=p 2/2 m 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 Dirac equation solid-state physics and microelectronics

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

many-body e- Pauli exclusion principle & Coulomb repulsion Ferromagnetism total wf antisymmetric FERO = orbital wf antisymmetric * spin wf symmetric (aligned) MAG • Robust (can be as strong as bonding in solids) • Strong coupling to magnetic field (weak fields = anisotropy fields needed only to reorient macroscopic moment) NET

relativistic single-particle Spin-orbit coupling (Dirac eq. in external field V(r) & 2 nd-order in v /c around non-relativistic limit) e- V Beff • Current sensitive to magnetization direction s p

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

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

. 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-volatile x slow, limited lifetime, expensive Operation through electron charge manipulation

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

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) RAM chip that actually won't forget instant on-and-off computers

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

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

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

Dipolar-field-free current induced switching nanostructures see J. Zemen 12: 05, T 2 Micromagnetics (magnetic anisotropy) without dipolar fields (shape anisotropy) One Domain wall ~100 nm Can be moved by ~100 x smaller currents than in metals Humpfner et al. 06, Wunderlich et al. 06 Strain controlled magnetocrystalline (SO-induced) anisotropy

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

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

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 Ordinary magnetoresistance: response in normal metals to external magnetic field via classical Lorentz force B Extraordinary magnetoresistance: response to internal spin polarization in ferromagnets often via quantum-relativistic spin-orbit coupling anisotropic magnetoresistance _ _ _ FL +++++++ V I M __ FSO I e. g. ordinary (quantum) Hall effect V and anomalous Hall effect Known for more than 100 years

Spin-orbit coupling V s p Beff skew scattering intrinsic Anomalous Hall effect in ferromagnetic conductors: spin-dependent deflection & more spin-ups transverse voltage majority __ FSO minority V I _ FSO __ FSO non-magnetic I V=0 Spin Hall effect in non-magnetic conductors: spin-dependent deflection transverse edge spin polarization

Spin Hall effect detected optically in Ga. As-based structures Same magnetization achieved by external field generated by a superconducting magnet with 106 x larger dimensions & 106 x larger currents p n n SHE microchip, 100 A superconducting magnet, 100 A Wunderlich et al. PRL 05 SHE detected elecrically in metals Cu SHE edge spin accumulation can be extracted and moved further into the circuit

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

Magnetization Spintronics explores new avenues for: Ferr o • Information reading 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 Ga • Materials: Dilute moment ferromagnetic semiconductors Mn As Mn & non-magnetic – spin Hall effect

Materials research of DMSs In (Ga, Mn)As Tc ~ #Mn. Ga (Tc=170 K for 6% Mn. Ga) But the SC refuses to accept many group-II Mn on the group-III Ga sublattice III = I + II Ga = Li + Zn Ga. As and Li. Zn. As are twin SC (Ga, Mn)As and Li(Zn, Mn)As should be twin ferromagnetic SC But Mn isovalent in Li(Zn, Mn)As Masek et al. PRL 07 no Mn concentration limit possibly both p-type and n-type ferromagnetic SC (Li / Zn stoichiometry)

(Ga, Mn)As material Ga Mn As Mn - Mn local moments too dilute (near-neghbors cople AF) - Holes do not polarize in pure Ga. As - Hole mediated Mn-Mn FM coupling 5 d-electrons with L=0 S=5/2 local moment moderately shallow acceptor (110 me. V) hole

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

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