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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: 1mm x 1mm MByte From PC hard drives ('90) to micro-discs - spintronic read-heads GByte 1 bit: 10-3mm x 10-3mm 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 4Mb 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 4Mb 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=p2/2m E ih d/dt p -ih d/dr ... E2/c2=p2+m2c2 (E=mc2 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) & 2nd-order in v /c around non-relativistic limit) Ingredients: - potential V(r) E - motion of an electron e- Produces an electric field 1 E V (r ) e In the rest frame of an electron the electric field generates and effective magnetic field - gives an effective interaction with the electron’s Beff magnetic moment 1 ( V ) p 2m 2 c 2 • Current sensitive to magnetization direction V Beff s p H SO s Beff e- Spintronics Ferromagnetism Coulomb repulsion & Pauli exclusion principle Spin-orbit coupling Dirac eq. in external field V(r) & 2nd-order in v /c around non-relativistic limit V s p Beff H SO s Beff Beff 1 ( V ) p 2m 2 c 2 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 ~Mx . sx kx FM without SO-coupling ~(k . ~(k . s)2 + Mx . s x s)2 SO-coupling without FM FM & SO-coupling AMR M Ferromagnetism: sensitivity to magnetic field scattering SO-coupling: anisotropies in Ohmic transport characteristics; ~1-10% MR sensor M 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 QQind0 = (n+1/2)e Q VD Drain QQ0ind = ne Gate VG eE2/2C C n-1 Q( M ) U dQ'VD ( Q' ) e 0 n n+1 n+2 Q ( Q Q0 ) ( M ) C U & Q0 CG [ VG VM ( M )] & VM 2C e CG [110] F 2 [100] [110] electric & magnetic control of Coulomb blockade oscillations [010] M [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 GaAs - 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 (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 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 GaAs - Hole mediated Mn-Mn FM coupling 5 d-electrons with L=0 S=5/2 local moment moderately shallow acceptor (110 meV) hole Mn–hole spin-spin interaction Ga Mn As 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. InSb, InAs, GaAs d5 Strong hybrid. GaP 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 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 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 side jump majority _ __ FSO FSO I _ minority FSO __ FSO non-magnetic V I V=0 Spin Hall effect in non-magnetic conductors: spin-dependent deflection transverse edge spin polarization Spin Hall effect detected optically in GaAs-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, 100A 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 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