Spinelectronics: From Basic Phenomena to Applications [email protected] Spintronics tutorial Santander 20/06/2017 John Slonczewski We will miss you, John [email protected] Spintronics tutorial Richmond 03/06/2019 Spin-electronics Electronics Magnetism Electron charge Electron Spin Main goal : Discover new phenomena taking advantage of the electrons’ spin and try to use them in devices having new functionalities or improved peformances (higher sensitivity, lower power consumption…etc). Started in 1988 with the discovery of Giant Magnetoresistance [email protected] Spintronics tutorial Richmond 03/06/2019 Spintronics/nanomagnetism broadening spectrum of interest 1988 Spin injection/detection 1995 1989 Giant 2000 Tunnel 2006 /manipulation: Magnetoresistance 2004 2010 -semiconductors (Si, GaAs) (GMR) Magnetoresistance -Graphene and other 2D materials (TMR) -Topological insulators 1996 Spin transfer 2000 Control of torque (STT) 2004 magnetic properties by electric field/strain : Spintronic -Interfacial anisotropy phenomena Current induced motion 2004 -Multiferroïcs of magnetic textures: 2008 2013 2003-2007 domain walls, skyrmions Antiferro spintronics TeraHertz emission Spin orbit torque (SOT) 1996-2010 Dzyaloshinskii-Moriya interaction (DMI) Spincaloritronics Interconversion Interplay Heat/Spin/Charge spin charge current 2008 2010 2008-2010 2008 [email protected] Spintronics tutorial Richmond 03/06/2019 Spintronics/nanomagnetism broadening spectrum of interest Magnetic recording : MRAM -Heat Assisted Standalone -Microwave assisted MR Embedded Sensors : -MR heads -3D position sensors Non-volatile logic -Biotechnology Normally-Off/Instant-On electronics Spintronics Spin-waves based Energy : applications Thermoelectricity with logic spincaloritronics effects Race track STT or SOT based Shift registers RF devices -oscillators, filters and demodulators -data communication Memristor applications -neuromorphic computing Neuromorphic architecture -power harvesting [email protected] Spintronics tutorial Richmond 03/06/2019 Spinelectronics: From Basic Phenomena to Applications OUTLINE • Part 1 : Basic phenomena in spintronics: - Giant Magnetoresistance - Tunnel magnetoresistance (TMR) - Spin-Transfer Torque (STT) - Spin-orbit Torques (SOT) • Part 2 : Spintronics main applications - Magnetic Recording (Hard disk drives Read-heads) - MRAMs - Magnetic field sensors - RF applications [email protected] Spintronics tutorial Richmond 03/06/2019 Spinelectronics: From Basic Phenomena to Applications OUTLINE • Part 1 : Basic phenomena in spintronics: - Giant Magnetoresistance - Tunnel magnetoresistance (TMR) - Spin-Transfer Torque (STT) - Spin-orbit Torques (SOT) • Part 2 : Spintronics main applications - Magnetic Recording (Hard disk drives Read-heads) - MRAMs - Magnetic field sensors - RF applications [email protected] Spintronics tutorial Richmond 03/06/2019 Giant magnetoresistance (1988) Fe/Cr multilayers Baibich, M. et al, Phys.Rev.Lett., 61 (1988) 2472. Two geometries of measurement: I ~ 80% V I Current-in-plane I V Magnetic field (kG) I I Current-perpendicular-to-plane R − R GMR = AP P R Antiferromagnetically coupled multilayers P [email protected] Spintronics tutorial Richmond 03/06/2019 NOBEL PRIZE IN PHYSICS IN 2007 Albert Fert Peter Grünberg Albert Fert & Peter Grünberg received the Nobel Prize from His Majesty King Carl XVI Gustaf of Sweden at the Stockholm Concert Hall, 10 December 2007. [email protected] Spintronics tutorial Richmond 03/06/2019 -9- Other GMR systems B.Dieny et al , Phys. Rev.B.(Condensed-Matter) 43 (1991)1297 [email protected] Spintronics tutorial Richmond 03/06/2019 Two current model (Mott 1930) for transport in magnetic metals As long as spin-flip is negligible, current can be considered as carried in parallel by two categories of electrons: spin ↑ and spin ↓ (parallel and antiparallel to quantization axis) − 1 ρ ρ ρ = 1 + 1 = ↑ ↓ ρ ρ ρ + ρ ↑ ↓ ↑ ↓ Sources of spin flip: magnons and spin-orbit scattering Negligible spin-flip often crude approximation (spin diffusion length in NiFe~4.5nm, 30% spin memory loss at Co/Cu interfaces) Mott N.F. and H.H.Wills, Proc.Roy.Soc.A156, 368 (1936). [email protected] Spintronics tutorial Richmond 03/06/2019 Spin dependent transport in magnetic transition metals (1) Band structure of 3d transition metals In transition metals, partially filled bands which participate to conduction are s and d bands Non-magnetic Cu : Magnetic Ni : E E s (E) s (E) s↓(E) s↑ (E) ↓ ↑ D (E) D↑ (E) D↓ (E) D↑ (E) ↓ EF EF ≠ D↑(EF) = D↓(EF) D↑(EF) D↓(EF) Most of transport properties are determined by DOS at Fermi energy Spin-dependent density of state at Fermi energy Fert, A., Campbell, I.A., J.Phys.F6, 849 (1976). [email protected] Spintronics tutorial Richmond 03/06/2019 Spin dependent transport in magnetic metals (2) m*(d) >> m*(s) J mostly carried by s electrons in transition metals Scattering of electrons determined by DOS at EF : E σ s↓(E) s↑(E) Fermi Golden rule : ∝ < >2 P iW f D f (EF ) D↓(E) D (E) s↑ s↑ ↑ d↑ s↓ s↓ d↓ Most efficient scattering channel Spin-dependent scattering rates in magnetic transition metals Example: λ↑ = 10 nm;λ↓ = 1nm Co Co Fert, A., Campbell, I.A., J.Phys.F6, 849 (1976). [email protected] Spintronics tutorial Richmond 03/06/2019 Simple model of Giant Magnetoresistance Parallel config Antiparallel config Baibich, M. et al, Phys.Rev.Lett., 61 (1988) 2472. Fe Cr Fe Fe Cr Fe 2 ∆ρ ρ − ρ α −1 2 = ↑ ↓ = ρ ρ + ρ α + ap ↑ ↓ 1 Equivalent resistances : ρ ρ ρ ρ ρ α = ↑ ρ ρ ρ ρ ρ ↓ Key role of 2ρ ρ (ρ + ρ ) ρ = ↑ ↓ ρ = ↑ ↓ scattering contrast α P ()ρ + ρ AP ↑ ↓ 2 [email protected] Spintronics tutorial Richmond 03/06/2019 Two configurations of GMR measurement 1) Current in-plane (CIP) I V I • Straightforward to measureCIP at wafer level, no need for patterning • Measured in 4 point probe geometry CIP-GMR described by Boltzman formalism (R. E. Camley and J. Barna ś, Phys. Rev. Lett. 63 , 664 (1989) • Important characteristic lengths : elastic spin-dependent mean free paths λ λ e.g. = 7 ; = 1 [email protected] Spintronics tutorial Richmond 03/06/2019 2) Current Perpendicular to Plane GMR (CPP-GMR) I V I I Much more difficult to measure but richer physics, Either on macroscopic samples (0.1mm diameter) with superconducting leads (R~ ρ . thickness / area ~ 10 −5Ω) or on patterned microscopic pillars of area < µm² (R~ a few Ohms) Pratt,W.P.Jr, et al, Phys.Rev.Lett. 66 (1991) 3060 Eid, K.,Pratt Jr., W.P., and Bass, J. Journ.Appl.Phys.93, 3445 (2003). Michigan State Univ [email protected] Spintronics tutorial Richmond 03/06/2019 Current Perpendicular to Plane GMR Measurement limited at 4K [email protected] Spintronics tutorial Richmond 03/06/2019 Serial resistance model for CPP-GMR Without spin-filp, serial resistance network can be used for CPP transport CPP transport through F/NM/F sandwich described by: (a) Parallel magnetic configuration : ρ ↑ ↑ ρ t ↑ ρ ↑ F tF AR F / NM NM NM AR F / NM F tF ρ ↓ ↓ ρ t ↓ ρ ↓ F tF AR F / NM NM NM AR F / NM F tF (b) Antiparallel magnetic configuration : ρ ↑ ↑ ρ t ↓ ρ ↓ F tF AR F / NM NM NM AR F / NM F tF ρ ↓ ↓ ρ t ↑ ρ ↑ F tF AR F / NM NM NM AR F / NM F tF [email protected] Spintronics tutorial Richmond 03/06/2019 Spin accumulation – spin relaxation in CPP geometry F1 F2 J J↑ ()1+ β e flow 2 e flow J ()1− β 2 J↓ lSF lSF Valet and Fert theory of CPP-GMR (Phys.Rev.B48, 7099(1993)) z In F1: Different scattering rates for spin ↑ and spin ↓ electrons ⇒ different spin ↑ and spin ↓ currents. Larger scattering rates for spin ↓ : J ↑ >>J ↓ far from the interface. In F2: Larger scattering rates for spin ↑ : J↓ >> J↑ far from the interface. Majority of incoming spin ↑electrons, majority of outgoing spin ↓ electrons Building up of a spin ↑accumulation around the interface balanced in steady state by spin-relaxation [email protected] Spintronics tutorial Richmond 03/06/2019 Starting point : Valet and Fert theory of CPP-GMR (Phys.Rev.B48, 7099(1993)) Spin-relaxation at F ↑/F ↓ interface: µσ : spin-dependent chemical potential In homogeneous material, µ=ε -eφ F J J↑ ()1+ β r 2 Spin-dependent current driven by ∇µ ()− β J ∂µ 1 1 σ 2 J l l Jσ = ↓ SF SF eρσ ∂z Generalization of Ohm law z Spin relaxation : β ρ * F e F lSF J ∂Jσ µσ − µ−σ eρσ = ∂ 2 ∆µ z 2lSF lSF =spin-diffusion length (~5nm in NiFe, ~20nm in Co)) lSF lSF [email protected] Spintronics tutorial Richmond 03/06/2019 z Interfacial boundary conditions ↑ ↓ ↑ ↓ ↑ ↓ ↑ ↓ µ ( ) ( )− µ ( ) ( ) = ( ) ( ) ( ) (Ohm law at interfaces) i+1 zi+1 i zi+1 ri+1 J i zi+1 ↑ ↓ ↑ ↓ ( ) ( ) = ( ) ( ) (if no interfacial spin-flip is considered) J i+1 zi+1 J i zi+1 Note: Interfacial spin memory loss can be introduced by : ↑(↓) ( ) = δ ↑(↓) ( ) Ji+1 zi+1 Ji zi+1 30% memory loss as at Co/Cu interface yields δ=0.7 [email protected] Spintronics tutorial Richmond 03/06/2019 Input microscopic transport parameters to describe macroscopic CPP properties : Within each layer : ρ = ρ [ − + β ] -The measured resistivity ρ. ↑()↓ 2 * 1 ( ) β ρ ρ -The scattering asymmetry . ρ = ↑ ↓ = ρ * ()− β 2 measured 1 ρ↑ + ρ↓ -The spin diffusion length l sf . At each interface : -The measured interfacial area*resistance product rmeasured -The interfacial scattering asymmetry γ. = [ − + γ ] r↑()↓ 2r * 1 ( ) r r r = ↑ ↓ = r *()1−γ 2 measured + r↑ r↓ [email protected] Spintronics tutorial Richmond 03/06/2019 Examples of bulk parameters β Material Measured lSF resistivity Bulk 4K/300K scattering asymmetry Cu 0.5-0.7 µΩ .cm 0 500nm 3-5 0 50-200nm Au 2µΩ .cm 0 35nm 8 0 25nm Ni 80 Fe 20 10-15