Spintronic phenomena and components for memory, logic and RF applications Giant MagnetoResistance Benefit in magnetic recording technology Tunnel Magnetoresistance Spin-transfer Magnetic Random Access Memories (MRAM) Hybrid CMOS/magnetic components for non-volatile and reprogrammable logic Radio Frequency oscillators based on spin-transfer Conclusion IRFU SPhNucléaire 21/01/2011 B.Dieny Acknowledgements R.C. Sousa C.Papusoi J.Hérault M.Souza L.Nistor E.Gapihan S.Bandiera G.Prenat U.Ebels D.Houssamedine B.Rodmacq O. Redon M.C.Cyrille Work partly supported by the projects B.Delaet SPINSWITCH (MRTN-CT-2006-035327) CILOMAG (ANR 2007) J.-P. Nozières L. Prejbeanu HYMAGINE (ERC2010) V.Javerliac K.Mc Kay IRFU SPhNucléaire 21/01/2011 B.Dieny Spin electronics Spin electronics or spintronics : electrons = electrical charge + spin ↑ or ↓ Purpose of spin-electronics : Use spin as a new degree of freedom ⇒ New phenomena ⇒ new components (MRAM, Logic gates, RF components) Classical image of spin : Spin up Spin down All electrons have a spin: wave function described with « up » and « down » components. In non-magnetic material : « up » and « down » spin populations are equal In magnetic material : net spin polarization parallel to magnetization (~50% in Co). IRFU SPhNucléaire 21/01/2011 B.Dieny Birth of spin electronics : Giant magnetoresistance discovery (1988) Fe/Cr multilayers H=-Hsat H=0 H=+Hsat Fe Cr Fe Cr Fe ~ 80% FeCr Cr Fe Antiferromagnetically coupled multilayers R − R GMR = AP P RP Magnetic field (kOe) A.Fert et al, PRL (1988); P.Grunberg et al, patent (1988)+PRB (1989) Nobel Prize 2007 IRFU SPhNucléaire 21/01/2011 B.Dieny Spin dependent transport in magnetic metals Current carried in parallel by “spin up” and by “spin down” electrons Schematic electronic Scattering of electrons determined by DOS at EF : structure of magnetic metals σ 2 E Fermi Golden rule : P ∝ < iW f > D f (EF ) s↓(E) s↑(E) D↓(E) D (E) Different density of states at Fermi energy for ↑ spin-up and spin-down electrons Different mean free paths and different resistivities for spin-up and spin-down electrons Example: λ↑Co = 10nm;λ↓Co = 1nm ρ↑Co ρ↓Co ρCo = =16.4μΩ.cm ρ↑Co =18μΩ.cm ρ↓Co =180μΩ.cm ρ↑Co + ρ↓Co IRFU SPhNucléaire 21/01/2011 B.Dieny Simple model of Giant Magnetoresistance Parallel config Antiparallel config Fe Cr Fe Fe Cr Fe 2 Δρ ⎛ ρ − ρ ⎞ α −1 2 ⎜ ↑ ↓ ⎛ ⎞ = ⎟ = ⎝ ⎠ ρap ⎝ ρ↑ + ρ↓ ⎠ α +1 Equivalent resistances : ρ ρ ρ ρ ρ α = ↑ ρ ρ ρ ρ ρ ↓ Key role of 2ρ ρ ρ + ρ scattering contrast α ρ = ↑ ↓ ( ↑ ↓ ) P ρAP = ()ρ↑ + ρ↓ 2 IRFU SPhNucléaire 21/01/2011 B.Dieny Low field GMR: Spin-valves (a) emu) 1 Antiferromagnetic -3 FeMn 90Å pinning layer 0 M (10 -1 NiFe 40Å Ferromagnetic pinned layer Cu 22Å Non magnetic spacer 4 (b) Soft ferromagnetic layer NiFe 70Å 3 or 2 R/R (%) R/R Ta 50Å Buffer layer Δ 1 0 -200 0 200 400 600 800 substrate H(Oe) 4 (c) 3 2 •Ultrasensitive magnetic field sensors (MR heads) R/R (%) •Spin engineering of magnetic multilayers Δ 1 B.Dieny et al, Phys.Rev.B.(1991)+patent US5206590 (1991). 0 -40 -20 02040 H(Oe) IRFU SPhNucléaire 21/01/2011 B.Dieny Benefit of GMR in magnetic recording GMR spin-valve heads from 1998 to 2004 IRFU SPhNucléaire 21/01/2011 B.Dieny Dramatic increase inareal storage density over thepast 50 years IRFU SPhNucléaire 21/01/2011 B.Dieny 108 increase Magnetic tunnel junctions – Tunnel magnetoresistance Structure of a magnetic tunnel junction NiFe CoFe Al(Zr)Ox Storage layer: CoFe 4 Magnetic Tunnel junctionnm CoFe Al O barrier 1.5nm or 2 3 IrMn Reference layer :CoFe 3nm NiFe IrMn 7nm Acts as a couple polarizer/analyzer with the spin of the electrons. •First observation of TMR at low T in MTJ: Julliere (1975) (Fe/Ge/Co) •TMR at 300K : Moodera et al, PRL (1995); Myazaki et al, JMMM(1995). ΔR/R~50% in AlOx based junctions IRFU SPhNucléaire 21/01/2011 B.Dieny Julliere model of TMR Jullière,Phys. Lett. Isolant A54 225 (1975) Ferro 1 F1 F2 EF Isolant eV EF Ferro 2 0 σ 2 Fermi Golden rule: proba of tunneling P ∝ < iW f > D f (EF ) Nb of electrons candidate for tunneling ∝ Di (EF ) σ σ σ ⇒ tunneling current in each spin channel J ∝ D1 (EF )× D2 (EF ) Parallel configuration Antiparallel configuration parallel ↑ ↑ ↓ ↓ antiparallel ↑ ↓ ↓ ↑ J ∝ D1 D2 + D1 D2 J ∝ D1 D2 + D1 D2 D ↑ ( E ) − D ↓ ( E ) ΔR 2 P P ΔR 2 P P P = F F TMR = = 1 2 TMR = = 1 2 D ↑ ( E ) + D ↓ ( E ) F F RP 1 − P1 P2 R AP 1 + P1 P2 P~50% in Fe, Co IRFU SPhNucléaire 21/01/2011 B.Dieny ΔR/R~40 - 70% with alumina barriers Spin polarization of 3d metals E Metals s↑(E) s↓(E) Parkin et al D↑(E) D↓(E) EF (Ristoiu et al.) Half metals NiMnSb Half metals are E 100% spin polarized ! Heusler alloys LaSrMnO3 Fe O 300K EF δ 3 4 Intensité (u.a.) Intensité D (E) D (E) CrO2 ↑ Δ ↓ spin up … spin down Photoemission intensity Δ- Exchange splitting -1012 E-E (eV) δ- half metallic gap F IRFU SPhNucléaire 21/01/2011 B.Dieny Giant TMR of MgO tunnel barriers S.S.P.Parkin et al, Nature Mat. (2004), nmat1256. S.Yuasa et al, Nature Mat. (2004), nmat 1257. Very well textured MgO barriers grown by sputtering or MBE on bcc CoFe or Fe magnetic electrodes, or on amorphous CoFeB electrodes followed by annealing to recrystallize the electrode. Yuasa et al, APL89, 042505(2006) IRFU SPhNucléaire 21/01/2011 B.Dieny Tunneling through crystalline MgO barriers (cont’d) Co|MgO|Co and CoFe|MgO|CoFe were predicted to show extremely high TMR for well ordered interfaces (W.Butler, Phys.Rev.B.(2000)). New mechanism of spin-filtering during tunneling through MgO according to symmetry of wave functions. Spin up-up down- up-down GP/GAP alignment down or down=up Fe|MgO|Fe 2.55 X109 7.08 X107 2.41 X107 54.3 Co|MgO|Co 8.62 X108 7.51 X107 3.60 X106 147.2 FeCo|MgO|FeCo 1.19 X109 2.55 X106 1.74 X106 353.5 The conductances above were calculated by integrating over the entire Fermi surface. They assumed 8 layers of MgO. W.Butler, Alabama Univ IRFU SPhNucléaire 21/01/2011 B.Dieny Magnetic tunnel junctions based on MgO tunnel barriers •As-deposited, CoFeB amorphous, MgO polycristalline •Upon annealing, recrystallization of CoFeB from the MgO interfaces and improvement in MgO crystallinity with (100) bcc texture J. Hayakawa et al. Jap. J. Appl. Physics 2005 °C °C Also, Yuasa et al. Applied Physics Letters, 2005 IRFU SPhNucléaire 21/01/2011 B.Dieny IRFU SPhNucléaire 21/01/2011 B.Dieny Magnetic Tunnel Junctions (MTJ): a reliable path for CMOS/magnetic integration MgO based MTJ 160 120 • Resistance of MTJ compatible with 80 TMR(%) resistance of passing FET (few kΩ) 40 0 -300 -200 -100 0 100 200 300 • MTJ can be deposited in magnetic H(Oe) back end process • No CMOS contamination • MTJ used as variable resistance controlled by field or current/voltage (Spin-transfer) •Commercial CMOS/MTJ products available from EVERSPIN since 2006 (4Mbit MRAM) Above CMOS technology Implemented in Airbus flight controller Cross-section of Freescale (Everspin) 4Mbit MRAM based on field switching IRFU SPhNucléaire 21/01/2011 B.Dieny Spin-transfer Predicted by Slonczewski (JMMM.159, L1(1996)) and Berger (Phys.Rev.B54, 9359 (1996)) Giant or Tunnel magnetoresistance: Acting on electrical current via the magnetization orientation Spin transfer is the reciprocal effect: Acting on the magnetization via the spin polarized current CoCu ~1nm Co M.D.Stiles et al, Phys.Rev.B.66, 014407 (2002) Polarizing layer Free layer Conduction electron flow Reorientation of the direction of polarization of current via incoherent precession/relaxation of the electron spin around the local exchange field ⇒ Torque on the free layer magnetization IRFU SPhNucléaire 21/01/2011 B.Dieny Magnetization dynamics: Effective field + spin-torque Slonczewski (JMMM.159, L1(1996)) and Berger (Phys.Rev.B54, 9359 (1996)), Stiles, Levy, Fert, Barnas, Vedyaev Polarizer M dM dM = −γM × ()H + bI.M + γaI.M × (M × M ) + αM × dt eff p p dt Spin-torque term: Effective field term damping Gilbert (conserves energy) (or antidamping) term Damping term ( Modified LLG) Non conservative a and b are coefficients proportional to the spin polarization of the current Precession from Damping torque (intrinsic Gilbert damping α) effective field Heff Current-induced torque M (Spin-torque) Effective field term is relatively weak in metallic pillars (<10% of spin-torque term) but more important in MTJ (~30% of spin-torque term) IRFU SPhNucléaire 21/01/2011 B.Dieny Energy dissipation and energy pumping due to spin transfer torque Without spin torque (standard LLG) : <0 Dissipation, leading to relaxation towards effective field Z.Li and S.Zhang, Phys.Rev.B68, 024404 (2003) With spin torque term : Proportional to current Large damping dE/dt can be either >0 or <0 With large damping: standard dynamical behavior, With low damping: New dynamical effects such as spin current induced steady excitations. Low damping The magnetization pumps energy from the spin- current. IRFU SPhNucléaire 21/01/2011 B.Dieny Magnetization switching induced by a polarized current Katine et al, Phys.Rev.Lett.84, 3149 (2000) on Co/Cu/Co sandwiches (Jc ~2-4.107A/cm²) Field scan Current scan I = -0.4 mA 9,1 H = -4 Oe 9,1 I+ )) 9 )) 9 CoFeCu2 Cu 8,9 8,9 CoFeCu1 R(ohms R(ohms R(ohms R(ohms 8,8 I- 8,8 8,7 8,7 -400 -200 0 200 400 -8 -4 0 4 8 H(Oe) I(mA) P-AP 7 jc =1.9.10 A/cm² AP-P 7 jc =1.2.10 A/cm² By spin transfer, a spin-polarized current can be used to manipulate the magnetization of magnetic nanostructures instead of by magnetic field.