* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Slide 1
Survey
Document related concepts
Ising model wikipedia , lookup
Superconducting magnet wikipedia , lookup
Relativistic quantum mechanics wikipedia , lookup
Magnetic stripe card wikipedia , lookup
Magnetometer wikipedia , lookup
Earth's magnetic field wikipedia , lookup
Magnetic monopole wikipedia , lookup
Magnetotactic bacteria wikipedia , lookup
Magnetotellurics wikipedia , lookup
Magnetohydrodynamics wikipedia , lookup
Neutron magnetic moment wikipedia , lookup
Magnetoreception wikipedia , lookup
Electromagnet wikipedia , lookup
Force between magnets wikipedia , lookup
History of geomagnetism wikipedia , lookup
Multiferroics wikipedia , lookup
Magnetochemistry wikipedia , lookup
Transcript
Spintronics in metals and semiconductors Tomas Jungwirth Institute of Physics ASCR Alexander Shick, Karel Výborný, Jan Zemen, Jan Masek, Vít Novák, Kamil Olejník, et al. University of Nottingham Bryan Gallagher, Tom Foxon, Richard Campion, Kevin Edmonds, Andrew Rushforth, Chris King et al. Hitachi Cambridge Jorg Wunderlich, Andrew Irvine, David Williams, Elisa de Ranieri, Sam Owen, et al. Outline 1. Tunneling anisotropic magnetoresistance in transition metals 2. Ferromagnetism in (Ga,Mn)As and related semiconductors 3. Spintronic transistors Spintronics: Spin-orbit & exchange interactions nucleus rest frame I Qv E electron rest frame Q 40 r spin-orbit interaction 3 H SO r 0 I r B 4 r 3 Thomas precession 1 B 0 0 v E 2 v E c g B e SB S vE 2 2 2mc DOS Coulomb repulsion & Pauli exclusion principle exchange interaction ferromagnetism AMR TMR ~ 1% MR effect Exchange int.: Spin-orbit int.: ~ 100% MR effect M ~ vg ( M vs. I ) magnetic anisotropy M (B ) TAMR TDOS (M ) Au Exchange int.: TDOS () TDOS () AFM-FM exchange bias TAMR in CoPt structures ab intio theory experiment Shick, et al, PRB '06, Park, et al, PRL '08 Park, et al, PRL '08 TAMR in TM structures Consider uncommon TM combinations Mn/W ~100% TAMR Shick, et al, unpublished spontaneous moment Consider both Mn-TM FMs & AFMs exchange-spring rotation of the AFM Scholl et al. PRL ‘04 Proposal for AFM-TAMR: first microelectronic device with active AFM component Shick, et al, unpublished Outline 1. Tunneling anisotropic magnetoresistance in transition metals 2. Ferromagnetism in (Ga,Mn)As and related semiconductors 3. Spintronic transistors TM-based semiconducting multiferroic spintronics sensors & memories transistors & logic Magnetic materials spintronic magneto-sensors, memories Ferroelectrics/piezoelectrics electro-mechanical transducors, large & persistent el. fields Semiconductors transistors, logic, sensitive to doping and electrical gating Ferromagnetic semiconductors Need true FSs not FM inclusions in SCs Ga Mn As GaAs - standard III-V semiconductor Group-II Mn - dilute magnetic moments & holes (Ga,Mn)As - ferromagnetic semiconductor Mn 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 due to p-d hybridization (Zener local-itinerant kinetic-exchange) Mn Mn-d-like local moments Mn As Strong spin-orbit coupling Ga Mn As Mn As-p-like holes H SO eS p 1 dV (r ) r Beff S L mc mc er dr V s p Beff Strong SO due to the As p-shell (L=1) character of the top of the valence band Beff Bex + Beff Note: TAMR discovered in (Ga,Mn)As Gold et al. PRL’04 (Ga,Mn)As synthesis high-T growth Low-T MBE to avoid precipitation High enough T to maintain 2D growth need to optimize T & stoichiometry for each Mn-doping Inevitable formation of interstitial Mn-donors compensating holes and moments need to anneal out optimal-T growth Polyscrystalline 20% shorter bonds Interstitial Mn out-diffusion limited by surface-oxide O GaMnAs-oxide x-ray photoemission GaMnAs MnI++ Olejnik et al, ‘08 10x shorther annealing with etch Optimizing annealing-T another key factor Rushforth et al, ‘08 Tc limit in (Ga,Mn)As remains open 180 160 140 120 Yu et al. ‘03 100 TC(K) Indiana & California (‘03): “ .. Ohno’s ‘98 Tc=110 K is the fundamental upper limit ..” 80 Nottingham & Prague (’08): Tc up to 185K so far 60 40 California (‘08): “…Tc =150-165 K independent of xMn>10% contradicting Zener kinetic exchange ...” Mack et al. ‘08 “Combinatorial” approach to growth with fixed growth and annealing T’s 20 0 0 1 2 3 4 5 6 7 Mntotal(%) ? 8 9 10 Other (III,Mn)V’s DMSs Kudrnovsky et al. PRB 07 Weak hybrid. Mean-field but low TcMF InSb Strong hybrid. Delocalized holes long-range coupl. d5 Impurity-band holes short-range coupl. Large TcMF but low stiffness GaP (Al,Ga,In)(As,P) good candidates, GaAs seems close to the optimal III-V host Other DMS candidates 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 Masek et al. PRL 07 no Mn concentration limit and self-compensation possibly both p-type and n-type ferromagnetic SC (Li / Zn stoichiometry) Towards spintronics in (Ga,Mn)As: FM & transport Dense-moment MS F<< d- Dilute-moment MS F~ d- Eu - chalcogenides Critical contribution to resistivity at Tc ~ magnetic susceptibility Broad peak near Tc disappeares with annealing (higher uniformity)??? Critical contribution at Tc to d/dT like TM FMs (Ga,Mn)As (Prague Nottingham) Fe Fisher & Langer ’68 Ni Novak et al., ‘08 d/dT ~ cv F ~ d- 2 (T ) ~ ( Ri , T ) ~ J pd [ Si S0 Si S0 ] 2 0 ~ uncor ~ (S ) (q ~ kF ~ 0) ~ (q ~ k F ~ 1 / d ) ~ cv Fisher&Langer ‘68 d/dT Tc Scattering off short range correlated spin-fluctuation Outline 1. Tunneling anisotropic magnetoresistance in transition metals 2. Ferromagnetism in (Ga,Mn)As and related semiconductors 3. Spintronic transistors Gating of the highly doped (Ga,Mn)As: p-n junction FET p-n junction depletion estimates -3 carrier density [ 10 cm ] 10 8 19 0V 3V 5V 10V 6 4 2 0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 GaMnAs layer thickness [nm] ~25% depletion feasible at low voltages Olejnik et al., ‘08 Increasing and decreasing AMR and Tc with depletion Vg = 0V Vg = 3V 24.5 -3 [10 cm] 19.4 24.0 19.2 19.0 23.5 18.8 23.0 18.6 20 22 24 26 28 30 32 34 22.5 T [K] -6 d/dT [10 AMR 100 0 0 -100 -100 -200 -300 -200 20 22 24 26 28 T [K] 30 32 34 Persistent variations of magnetic properties with ferroelectric gates Stolichnov et al., Nat. Mat.‘08 62K 65K depletion accumulation dR/dT 200 100 30 40 50 60 T (K) 70 80 90 100 Electro-mechanical gating with piezo-stressors exy = 0.1% exy = 0% Strain & SO Rushforth et al., ‘08 Electrically controlled magnetic anisotropies (Ga,Mn)As spintronic single-electron transistor Wunderlich et al. PRL ‘06 Huge, gatable, and hysteretic MR Single-electron transistor Two "gates": electric and magnetic AMR nature of the effect normal AMR Coulomb blockade AMR Single-electron charging energy controlled by Vg and M 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) Theory confirms chemical potential anisotropies in (Ga,Mn)As & predicts CBAMR in SO-coupled room-Tc metal FMs • CBAMR if change of |(M)| ~ e2/2C • In our (Ga,Mn)As ~ meV (~ 10 Kelvin) • In room-T ferromagnet change of |(M)|~100K • Room-T conventional SET (e2/2C >300K) possible 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 Device design Physics of SO & exchange Chemical potential CBAMR Materials TM FMs, MnAs, MnSb SET (III,Mn)V, I(II,Mn)V DMSs Tunneling DOS TAMR Tunneling device Resistor Mn-based TM FMs&AFMs Group velocity & lifetime AMR TM FMs END 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 DOS Giant Magneto-Resistance > P AP ~ 10% MR effect Tunneling Magneto-Resistance DOS DOS ~ 100% MR effect Spin Transfer Torque writing 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 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 Photogenerated ferromagnetism Electric-field controlled ferromagnetism in FET or piezo/FM hybrid Vgate ħw Ferro SC Ferro SC Magnetization Magnetization GaSb B (mT) Variable controlled strain using a Piezo stressor A.W. Rushforth, J. Zemen, K. Vyborny, et al. arXiv:0801.0886 Strain induced by piezo voltage +/- 150V: ~ 2 10-4 (at 50K) M. Overby, et al., arXiv:0801.4191 Easy axis rotation by 50 deg for Vpiezo = -150V +150V 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 Spintronics with spin-currents only Magnetic domain “race-track” memory 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 Spintronics in nominally non-magnetic materials Datta-Das transistor Spin Hall effect spin-dependent deflection transverse edge spin polarization _ __ FSO FSO I intrinsic skew scattering 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