SPINTRONICS Tom Jungwirth Fyzikln stav AVR University of

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 1 bit: 1 mm x 1 mm MByte From PC hard drives ('90) to micro-discs - spintronic read-heads GByte 1 bit: 10 -3 mm x 10 -3 mm

HARD DISKS

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

Anisotropic magnetoresistance (AMR) read head 1992 - dawn of spintronics Appreciable sensitivity, simple design, scalable, cheap

Giant magnetoresistance (GMR) read head 1997 High sensitivity

. MEMORY CHIPS DRAM (capacitor) - high density, cheep x slow, high power, volatile . SRAM (transistors) - low power, fast x low density, expensive, volatile . Flash (floating gate) - non-volatile x slow, limited life, expensive Operation through electron charge manipulation

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

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

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 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 eq. solid-state physics and microelectronics

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

Non-relativistic (except for the spin) 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) Ingredients: - potential V(r) E - motion of an electron e- 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 • Current sensitive to magnetization direction V Beff s p

e- Spintronics Ferromagnetism Coulomb repulsion & Pauli exclusion principle Spin-orbit coupling Dirac eq. in external field V(r) & 2 nd-order in v /c around non-relativistic limit V s p Beff Fermi surfaces ky ~Mx. sx FM without SO-coupling kx ~(k. s)2 SO-coupling without FM ~(k. s)2 + Mx. sx FM & SO-coupling

Fermi surfaces ky kx FM without SO-coupling ~(k. FM & SO-coupling without FM AMR M kx k SO-coupling: anisotropies in Ohmic transport characteristics; ~1 -10% MR sensor scattering y Ferromagnetism: sensitivity to magnetic field M kx ky ~Mx. sx ~(k. s)2 + Mx. sx s)2 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 EF large spin-up DOS available at EF

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

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

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

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

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

Spintronic transistor based on CBAMR Source Q VD Drain Q 0 Gate VG e 2/2 C [110] [010] M F [100] [110] electric & magnetic control of Coulomb blockade oscillations [010] SO-coupling (M)

CBAMR SET • 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 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 Beff Difficulties with injecting spin polarized currents from metal ferromagnets to semiconductors, with spincoherence, etc. not yet realized s p

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

Ga (Ga, Mn)As (and other III-Mn-V) ferromagnetic semiconductor • compatible with conventional III-V semiconductors (Ga. As) • 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 Mn As Mn

(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

No apparent physical barriers for achieving room Tc 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. In. Sb, In. As, Ga. As d 5 Strong hybrid. Ga. P Impurity-band holes short-range coupl.

And look into related semiconductor host families like e. g. I-II-V’s 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 no Mn concentration limit possibly both p-type and n-type ferromagnetic SC

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

Anomalous Hall effect in ferromagnetic conductors: spin-dependent deflection & more spin-ups transverse voltage skew scattering intrinsic _ __ FSO minority V side jump majority 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 mikročip, 100 A SHE detected elecrically in metals Cu supravodivý magnet, 100 A 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)

Anisotropic magneto-resistance sensor Electromagnet • Information reading Ferr o 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 Ga As Dilute moment ferromagnetic semiconductors Mn • Spintronics fundamentals AMR, anomalous and spin Hall effects Mn