SPINTRONICS Tomáš Jungwirth Fyzikální ústav AVČR University of Nottingham

1. Current spintronics in HDD read-heads and memory chips 2. Physical principles of operation of current spintronic devices 3. Research at the frontiers of spintronics 4. Summary

Current spintronics applications First hard disc (1956) - classical electronics for read-out From PC hard drives ('90) to micro-discs - spintronic read-heads MByte GByte 1 bit: 1mm x 1mm 1 bit: mm x mm

HARD DISKS

HARD DISK DRIVE READ HEADS horse-shoe read/write heads spintronic read heads

Anisotropic magnetoresistance (AMR) read head dawn of spintronics dawn of spintronics Appreciable sensitivity, simple design, scalable, cheap

Giant magnetoresistance (GMR) read head High sensitivity

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

MRAM – universal memory fast, small, non-volatile RAM chip that won't forget ↓ instant on-and-off computers Tunneling magneto-resistance effect (TMR) First commercial 4Mb MRAM

MRAM – universal memory fast, small, non-volatile RAM chip that won't forget ↓ instant on-and-off computers Tunneling magneto-resistance effect (TMR) First commercial 4Mb MRAM

1. Current spintronics in HDD read-heads and memory chips 2. Physical principles of current spintronic devices operation 3. Research at the frontiers of 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 eq. 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) Non-relativistic (except for the spin) many-body

Ingredients: - potential V(r) - motion of an electron Produces an electric field In the rest frame of an electron the electric field generates and effective magnetic field - gives an effective interaction with the electron’s magnetic moment E 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

Spin-orbit coupling Dirac eq. in external field  V(r) & 2nd-order in v /c around non-relativistic limit e-e-e-e- VV B eff p s Spintronics Ferromagnetism Coulomb repulsion & Pauli exclusion principle ~(k. s) 2 kyky kxkx ~M x. s x Fermi surfaces FM without SO-couplingSO-coupling without FMFM & SO-coupling ~(k. s) 2 + M x. s x

FM without SO-couplingSO-coupling without FM FM & SO-coupling ~(k. s) 2 + M x. s x kyky kxkx kxkx kxkx kyky kyky M M scattering ~M x. s x Fermi surfaces AMR Ferromagnetism: sensitivity to magnetic field SO-coupling: anisotropies in Ohmic transport characteristics; ~1-10% MR sensor hot spots for scattering of states moving  M  R(M  I)> R(M || I)

Diode classical spin-valve TMR Based on ferromagnetism only; ~100% MR sensor or memory no (few) spin-up DOS available at E F large spin-up DOS available at E F

1. Current spintronics in HDD read-heads and memory chips 2. Physical principles of current spintronic devices operation 3. Research at the frontiers of spintronics 4. Summary

Removing external magnetic fields (down-scaling problem)

EXTERNAL MAGNETIC FIELD problems with integration - extra wires, addressing neighboring bits

Current (instead of magnetic field) induced switching Angular momentum conservation  spin-torque

current magnetic field local, reliable, but fairly large currents needed Myers et al., Science '99; PRL '02 Likely the future of MRAMs

Spintronics in the footsteps of classical electronics from resistors and diodes to transistors

TAMR Au TMR - TAMR sensor/memory elemets no need for exchange biasing or spin coherent tunneling AMR based diode FM AFM Simpler design without exchange-biasing the fixed magnet contact

Single-electron transistor Two "gates": electric and magnetic Spintronic transistor based on AMR type of effect Huge, gatable, and hysteretic MR

& electric & magnetic control of Coulomb blockade oscillations Q0Q0 Q0Q0 e 2 /2C  [ 010 ]  M [ 110 ] [ 100 ] [ 110 ] [ 010 ] SO-coupling   (M) Spintronic transistor based on CBAMR SourceDrain Gate VGVG VDVD Q

Generic effect in FMs with SO-coupling Combines electrical transistor action with magnetic storage Switching between p-type and n-type transistor by M  programmable logic CBAMR SET In principle feasible but difficult to realize at room temperature

Spintronics in the footsteps of classical electronics from metals to semiconductors

Spin FET – spin injection from ferromagnet & SO coupling in semiconductor VV B eff p s Difficulties with injecting spin polarized currents from metal ferromagnets to semiconductors, with spin- coherence, etc.  not yet realized

Ferromagnetic semiconductors – all semiconductor spintronics GaAs - standard semiconductor Mn - dilute magnetic element (Ga,Mn)As - ferromagnetic semiconductor semiconductor Mn Ga As Mn More tricky than just hammering an iron nail in a silicon wafer

(Ga,Mn)As (and other III-Mn-V) ferromagnetic semiconductor Mn Ga As Mn compatible with conventional III-V semiconductors (GaAs) dilute moment system  e.g., low currents needed for writing Mn-Mn coupling mediated by spin-polarized delocalized holes  spintronics tunability of magnetic properties as in the more conventional semiconductor electronic properties. strong spin-orbit coupling  magnetic and magnetotransport anisotropies Mn-doping (group II for III substitution) limited to ~10% p-type doping only maximum Curie temperature below 200 K

(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

No apparent physical barriers for achieving room T c in III-Mn-V or related functional dilute moment ferromagnetic semiconductors Need to combine detailed understanding of physics and technology Weak hybrid. Delocalized holes long-range coupl. InSb, InAs, GaAs d5d5 Strong hybrid. Impurity-band holes short-range coupl. GaP

And look into related semiconductor host families like e.g. I-II-V’s 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

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

1. Current spintronics in HDD read-heads and memory chips 2. Physical principles of current spintronic devices operation 3. Research at the frontiers of spintronics 4. Summary

Downscaling approach about to expire currently ~ 30 nm feature size interatomic distance in ~20 years Spintronics: from straighforward downscaling to more "intelligent" device concepts: simpler more efficient realization for a given functionality (AMR sensor) multifunctional (integrated reading, writing, and processing) new materials (ferromagnetic semiconductors) fundamental understanding of quantum-relativistic electron transport (extraordinary MR)

Information reading Electromagnet Anisotropic magneto-resistance sensor  Ferro Magnetization  Current Information reading & storage Tunneling magneto-resistance sensor and memory bit Information reading & storage & writing Current induced magnetization rotation

Information reading & storage & writing & processing : Spintronic single-electron transistor: magnetoresistance controlled by gate voltage New materials Dilute moment ferromagnetic semiconductors Mn Ga As Mn Spintronics fundamentals AMR, anomalous and spin Hall effects