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Spintronics Tomas Jungwirth ([email protected]) Institute of Physics ASCR, Prague University of Nottingham 1. Current spintronics in HDD read-heads and MRAMs 2. Basic physical principles of spintronics 3. Spintronics research – overview 4. Ferromagnetic semiconductors 5. Single-electron spintronic transistor 6. Summary Hard disk drive First hard disc (1956) - classical electromagnet for read-out 1 bit: 1mm x 1mm MB’s From PC hard drives ('90) to micro-discs - spintronic read-heads 1 bit: 10-3mm x 10-3mm 10’s-100’s GB’s Dawn of spintronics Magnetoresistive read element Inductive read/write element Anisotropic magnetoresistance (AMR) – 1850’s 1990’s Giant magnetoresistance (GMR) – 1988 1997 MRAM – universal memory fast, small, low-power, durable, and non-volatile 2006- First commercial 4Mb MRAM Based on Tunneling Magneto-Resistance (similar to GMR but insulating spacer) RAM chip that actually won't forget instant on-and-off computers 1. Current spintronics in HDD read-heads and MRAMs 2. Basic physical principles of spintronics 3. Spintronics research – overview 4. Ferromagnetic semiconductors 5. Single-electron spintronic transistor 6. Summary Spin-orbit coupling from classical E&M and postulated electron spin nucleus rest frame electron rest frame I Qv 1 B 0 0 v E 2 v E c H SO E Q 40 r 3 r 0 I r B 4 r 3 g B e SB S vE 2 2 2mc Lorentz transformation Thomas precession e… it’s all about spin and charge of electron communicating SO coupling from relativistic QM quantum mechanics & special relativity Dirac equation E=p2/2m E2/c2=p2+m2c2 E ih d/dt Spin (E=mc2 for p=0) p -ih d/dr Anisotropic Magneto-Resistance & HSO (2nd order in v/c around the non-relativistic limit) ~ 1% MR effect Current sensitive to magnetization direction Ferromagnetism = Pauli exclusion principle & Coulomb repulsion etotal wf antisymmetric e- = orbital wf antisymmetric * spin wf symmetric (aligned) DOS e- DOS • Robust (can be as strong as bonding in solids) • Strong coupling to magnetic field (weak fields = anisotropy fields needed only to reorient macroscopic moment) DOS Giant Magneto-Resistance > P AP ~ 10% MR effect Tunneling Magneto-Resistance DOS DOS ~ 100% MR effect 1. Current spintronics in HDD read-heads and MRAMs 2. Basic physical principles of spintronics 3. Spintronics research – overview 4. Ferromagnetic semiconductors 5. Single-electron spintronic transistor 6. Summary GMR ~ 1% MR effect ~ 10% MR effect < AMR FM & SO-coupling (M ) FM only ( ) + larger MR + linear sensing, low-noise - low MR, low-resistance - TAMR AlOx Au TDOS low-resistance, non-linear, spin-coherence, exchange biasing or interlayer coupling, higher noise TMR Au ~ 100% MR effect TDOS TDOS (M ) Combining “+” and eliminating “-” of AMR and TMR(GMR) + very large MR, high resistance, bistable memory - non-linear, spin-coherence, exchange biasing, higher noise Spin Transfer Torque writing Semiconducting multiferroic structures Ferromagnetic/magnetostrictive magneto-sensors, transducors, memory, storage piezo/FM hybrids FM semiconductors Semicondicting/gatable Ferroelectric/piezoelectric electro-sensors, transducors, memory FeFET transistors, processors Systems integrating all three basic elements of current microelectronics Photogenerated ferromagnetism Electric-field controlled ferromagnetism in FET or piezo/FM hybrid Vgate ħw Ferro SC Ferro SC Magnetization Magnetization GaSb B (mT) Fast Precessional switching via gatevoltage (I) (II) Beff M=(M,0,0) (III) M=(0,M,0) M Beff Beff 0 VG = V0, t < 0 VG 0 VG = VC, t = 0 x 0 x VG = V0, t > Δt90° VC Δt90° (a) time V0 M=(0,0,M) (I) z (II) z Beff VG VG = V0, t < 0 (III) Beff M=(0,0,-M) Beff VG = VC, t = 0 VG = V0, t > Δt180° VC Δt180° (b) V0 time Nonvolatile programmable logic Variant p- or n-type FET-like transistor in one single nano-sized CBAMR device 1 0 V DD ON OFF ON VB ON OFF VB ON OFF 10 Vout 10 ON OFF 1 0 VA 1 0 1 0 0 1 VA ON OFF OFF ON OFF “OR” A 0 1 0 1 B 0 0 1 1 Vout 0 1 1 1 Nonvolatile programmable logic Variant p- or n-type FET-like transistor in one single nano-sized CBAMR device 1 0 V DD ON OFF 1 0 VA VB ON OFF Vout VB VA “OR” A 0 1 0 1 B 0 0 1 1 Vout 0 1 1 1 Nonvolatile programmable logic Variant p- or n-type FET-like transistor in one single nano-sized CBAMR device 1 0 V DD ON OFF 1 0 VA VB ON OFF Vout VB VA “NAND” A 0 1 0 1 B 0 0 1 1 Vout 1 1 1 0 Spintronics with spin-currents only Magnetic domain “race-track” memory Spintronics in nominally non-magnetic materials Datta-Das transistor Spin Hall effect spin-dependent deflection transverse edge spin polarization skew scattering intrinsic _ __ side jump FSO FSO I 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 supercondicting magnet, 100 A SHE edge spin accumulation can be extracted and moved further into the circuit 1. Current spintronics in HDD read-heads and MRAMs 2. Basic physical principles of spintronics 3. Spintronics research – overview 4. Ferromagnetic semiconductors 5. Single-electron spintronic transistor 6. Summary Dilute moment ferromagnetic semiconductors More tricky than just hammering an iron nail in a silicon wafer Ga Mn As Mn GaAs - standard III-V semiconductor Group-II Mn - dilute magnetic moments & holes (Ga,Mn)As - ferromagnetic semiconductor GaAs: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 Ga As-p-like holes FM coupling between Mn local moments mediated by SC valence band holes Mn Mn-d-like local moments Mn As Dilute moment nature of ferromagnetic semiconductors Key problems with increasing MRAM capacity (bit density): - Unintentional dipolar cross-links - External field addressing neighboring bits One 10-100x weaker dipolar fields 10-100x smaller Ms Ga As Mn 10-100x smaller currents for switching Mn Strong spin-orbit coupling Ga As-p-like holes Mn As Mn Mn-d-like local moments H SO eS p 1 dV (r ) r Beff S L mc mc er dr V s Beff Strong SO due to the As p-shell (L=1) character of the top of the valence band Beff Bex + Beff p Ga As • (Ga,Mn)As ferromagnetic semiconductor • dilute moment system e.g., low currents needed for writing • Mn-Mn coupling mediated by spin-polarized & spin-orbit coupled delocalized holes spintronics • tunability of magneto-electronics properties by same means as in conventional semiconductors – doping, gating (normal, piezo). • but maximum Curie temperature so far below 200 K Mn Mn coupling strength / Fermi energy Magnetism in systems with coupled dilute moments and delocalized band electrons band-electron density / local-moment density (Ga,Mn)As Hole transport and ferromagnetism at relatively large dopings conducting p-type GaAs: - shallow acc. (C, Be) ~ 1018 cm-3 - Mn ~1020 cm-3 Non-equilibrium growth - technological difficulties 1. Current spintronics in HDD read-heads and MRAMs 2. Basic physical principles of spintronics 3. Spintronics research – overview 4. Ferromagnetic semiconductors 5. Single-electron spintronic transistor 6. Summary (Ga,Mn)As spintronic single-electron transistor Spintronic transistor - magnetoresistance controlled by gate voltage Bptp B90 Huge hysteretic low-field MR Sign & magnitude tunable by small gate valtages I B0 Strong dependence on field angle hints to AMR origin Anisotropic magnetoresistive effect AMR in the resistor AMR in the transistor Single electron transistor Narrow channel SET dots due to disorder potential fluctuations (similar to non-magnetic narrow-channel GaAs or Si SETs) Coulom blockade oscillations low Vsd blocked due to SE charging CB oscillation shifts by magnetication rotations magnetization angle At fixed Vg peak valley or valley peak MR comparable to CB negative or positive MR(Vg) Single Electron Transistor Source Q VD Drain • Vg = 0 Q Q2 U dQ VD ( Q ) &VD Q / C U 2C 0 ' Gate VG e2 k BT 2C ' Coulomb blockade • Vg 0 ( Q Q0 )2 U & Q0 CGVG 2C QQind0 = (n+1/2)e Q=ne - discrete Q0=CgVg - continuous QQ0ind = ne eE2/2C C n-1 n n+1 n+2 Q0=-ne blocked Q0=-(n+1/2)e open Coulomb blockade AMR QQind0 = (n+1/2)e QQ0ind = ne eE2/2C C n-1 n n+1 n+2 [110] F [100] [110] Q( M ) U dQ'VD ( Q' ) e 0 Q ( Q Q0 ) ( M ) C U & Q0 CG [ VG VM ( M )] & VM 2C e CG 2 electric & magnetic control of Coulomb blockade oscillations [010] M [010] SO-coupling (M) 1. Current spintronics in HDD read-heads and MRAMs 2. Basic physical principles of spintronics 3. Spintronics research – overview 4. Ferromagnetic semiconductors 5. Single-electron spintronic transistor 6. Summary Magnetization Spintronics explores new avenues for: • 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 transistor: magnetoresistance controlled by gate voltage Ga As • New materials Ferromagnetic semiconductors, Multiferroics Non-magnetic SO-coupled systems Mn Mn