#3] Giant Magnetoresistance: Experimentally Driven ; Theoretically Modeled 1989; IT Applications into 1990’s First Commerical Hard-Disks with GMR Sensors (IBM) 1998 Effect parallel / antiparallel thin magnetic films separated by non- magnetic spacer Nobel Prize in Physics 2007: ‘‘Discovery of GMR’’ Basic physics involved Divice applications – computers hard disks Beyond the GMR Film from Juelich

1988: … simultaneously, but independent … Albert Fert “Does the electrical resistance depend on the magnetization alignment?” Peter Gr ü nberg

Magnetic Multilayers (Fe) with Nonmagnetic Spacers (Cr)

Epitaxial Growth of Multilayers (Idealized) Modern layer-by-layer fabrication techniques: Molecular Beam Epitaxial (MBE) and/or Pulsed Laser Deposition (PLD) Topical use in “interface superconductivity”: LaAlO 3 / SrTiO 3 -- complex oxides. 2DEG  2D-SC (over few nm) at T C = 0.2K

Two possible geometries film fabrication Small thicknessesSmall diameter

Original Magnetoresistance Measurements Gruenberg et al. Fert et al.

Density of States for Unpolarized and Polarized 3d Metal M = 0 M = (n↑ - n↓) ≠ 0 Paramagnet Ferromagnet

Two Ferromagnets with a Nonmagnetic Spacer in between SPACER SPACER Send current through device. Which has smallest resistance, AF or F?? AF aligned F aligned

Parallel Resistor Model with Current of Up/Down Electrons AF F ΔR ≈ 50% or less

Half-Metallic Ferromagnetic, e.g., Hauslers & Skutterudites Spin polarized conduction electrons at Fermi surface (E F ) – here 100% ↓- conduction electrons

Multilayer with Two Half-Metallic Ferromagnets Spacer Spacer AF F Spintronics: electronics based upon the spin degrees of freedom, i.e., electron transport controlled / manipulated by spins.

Spin-Dependent Scattering Theory of GRM Camley and Barnas PRL(1989) and Maekawa et al.JPSJ(1991) C & B: Boltzmann eq. approach with spin dependent coefficient for specular reflection, transmission and anisotropy diffuse scatterings (interface roughness) at the Fe/Cr boundary. N = D  / D . M et al.: Spin dependent random exchange potential at interfaces (F/NM) and performing a Born approximation: 1/τ = matrix elements of V(r) Boltzmann eq. to calculate differences between F and AF coupling between adjacent layers. (ρ ↑↓ - ρ ↑↑ ) / ρ ↑↑

Now place an insulator between the two magnetic metals New physics involve: Quantum mechanical tunneling of electrons. Magnetic fields dependence of tunneling processes. Oxide tunnel junction:

Theory of TMR “Old” Jullier PL (1975), Mathon & Umerski PRB (1999) Conductance ratios: R TRM = [  (0) -1 -  (H s ) -1 ] /  (H s ) -1  40% at Rm.T Electron tunneling from ferromagnet are spin polarized Spin polarized tunneling: P = [D  (E F ) – D  (E F )] / [ D  (E F ) + D  (E F )] via net difference of up/down density of states at E F. Julliere formula: R TMR = (2P L P R ) / [1 – P L P R ] at Left and Right electrodes Different if one has a nonmagnetic metallic interlayer between one of the ferromagnetic electrodes and the insulator. Due to quantum well states in the metallic interlayer that do not participate in the transport. Only in spin down channel causing an spin asymmetry of tunnel electrons.

IN RESERVE  to Juelich CARTOONS Film from Jülich at time of Noble Prize ?

Forschungszentrum Jülich in der Helmholtz-Gemeinschaft Magnetic interlayer exchange coupling (IEC) Consider two ferromagnetic layers separated by a thin spacer layer: Ferromagnet / Non-Ferromagnet / Ferromagnet The ferromagnetic layers interact across the spacer and align … … parallel … “ferromagnetic coupling” … antiparallel … “antiferromagnetic coupling” … at 90º… “biquadratic or 90º-coupling”

Forschungszentrum Jülich in der Helmholtz-Gemeinschaft Scanning electron microscopy with spin analysis (SEMPA) [2]: Oscillatory interlayer exchange coupling [1] S.S.P. Parkin, Phys. Rev. Lett. 67, 3958 (1991) [2] D.T. Pierce et al., Phys. Rev. B 49, (1994) 3) Domain picture of Fe layer grown on Cr wedge 1) Domain picture of Fe single crystal (whisker) with two domains 2) Wedge-shaped Cr spacer Cr spacer thickness D (ML) only occurs for thin spacers with a thickness of a few nm is observed for many metallic spacer layers (see [1] for a “periodic table of interlayer coupling”) oscillates as a function of the spacer thickness D

Forschungszentrum Jülich in der Helmholtz-Gemeinschaft Origin of oscillatory interlayer exchange coupling Spin-dependent interface reflection gives rise to spin-dependent quantum-well states (QWS). They only form for parallel alignment of the FM layers, but not for antiparallel alignment! Parallel alignment:Antiparallel alignment:

Forschungszentrum Jülich in der Helmholtz-Gemeinschaft Quantum Well States The energy related to k  is quantized. Energy levels shift when the spacer thickness D is varied. A new level crosses E F when D is changed by (Similar to an electron in a box, where E decreases with increasing D) after M. Stiles

Forschungszentrum Jülich in der Helmholtz-Gemeinschaft Spin-dependent reflectivity arises from the “potential landscape” seen by the electrons due to the layered structure. Example Co / Cu / Co: Similar band structure for majority electrons and shifted band structure for minority electrons: What is the origin of spin-dependent reflectivity? P. Lang et al., Phys. Rev. B 53, 9092 (1996)

Forschungszentrum Jülich in der Helmholtz-Gemeinschaft The electrical resistance depends on the relative magnetic alignment of the ferromagnetic layers 19% for 80% for RT Giant magnetoresistance (GMR) Ferromagnet Metal Ferromagnet Electrical resistance: R P ) R AP GMR is much larger than the anisotropic magnetoresistance (AMR)

Forschungszentrum Jülich in der Helmholtz-Gemeinschaft First observations of GMR [1] G. Binasch, P. Grünberg et al., Phys. Rev B 39, 4828 (1989) [2] M.N. Baibich, A. Fert et al., Phys. Rev. Lett. 61, 2472 (1988) P. Grünberg, FZJ [1]A. Fert, Paris-Sud [2] GMR AMR Both experiments employ antiferromagnetic interlayer coupling to achieve the antiparallel alignment

Forschungszentrum Jülich in der Helmholtz-Gemeinschaft GMR of a spin-valve B. Dieny, J. Magn. Magn. Mater. 136, 335 (1994) 6 nm Ni 80 Fe 20 4 nm Ni 80 Fe 20 7 nm FeMn 2.2 nm Cu The steep slope at zero field makes spin-valves sensitive field sensors. CIP-geometry Spin-valves make use of the exchange bias effect at the AFM/FM interface Ferromagnet Anti- Ferromagnet

Forschungszentrum Jülich in der Helmholtz-Gemeinschaft Microscopic picture of GMR: Spin-dependent scattering 1) Spin-dependent scattering: r min  r maj 2) Mott’s two current model: independent current channels for spin-up and spin-down (no spin-flip scattering)

Forschungszentrum Jülich in der Helmholtz-Gemeinschaft Microscopic picture of GMR: Scattering spin asymmetry The origin of the spin-dependent scattering lies in the spin-split band structure and density of states of 3d transition metals: minority resistance r min  majority resistance r maj For Co/Cu: r min > r maj

Forschungszentrum Jülich in der Helmholtz-Gemeinschaft Application of GMR: Magnetic field sensor [1] K.M.H. Lenssen et al., J. Appl. Phys. 85, 5531 (1999) Example of a real layer structure [1]: NAF = natural antiferromagnet, SAF = synthetic antiferromagnet

Forschungszentrum Jülich in der Helmholtz-Gemeinschaft Application of GMR: Read heads in hard disk drives IBM-HGST Microdrive: - 1 inch diameter rpm Gbit/in 2 (Bit size:180 x 30 nm 2) - 8 Gbyte in 2006 (340 Mbyte in 1999) Disk rotation fixed Ferromagnet Current Anti-Ferromagnet free Ferromagnet

Forschungszentrum Jülich in der Helmholtz-Gemeinschaft Advantages of GMR-based read heads compared to AMR or inductive read heads: 1) Stronger MR signal  Better signal-to-noise  Smaller bits can be read 2) GMR is an interface effect (AMR is a bulk effect):  Thinner MR elements  Less demagnetization  Less wide MR elements  Higher sensitivity Application of GMR in hard-disks GMR AMR

Forschungszentrum Jülich in der Helmholtz-Gemeinschaft What’s beyond GMR?  Current-induced magnetization switching by spin-transfer torque Apply Newton’s third law “Actio = Reactio”: The electric current flow controls the magnetization state Negative current  parallel alignment Positive current  Antiparallel alignment J.C. Slonczewski, J. Magn. Magn. Mater. 159, L1 (1996); L. Berger, Phys. Rev B 54, 9353 (1996)

Forschungszentrum Jülich in der Helmholtz-Gemeinschaft Advanced switching concept for spintronic devices Spintronic devices employ the electron spin for data storage and processing: Advantages of current-induced switching over field-induced switching: nano-scale addressability and favorable scalability MRAM Bit “Spin-Transistor” Gate Bit

Very New “Spin Caloritronics” by adding ΔT E.G., Spin Seebeck effect - Sinova and Uchida et al. NM (2010) ΔTΔT Voltage across Pt bar is due to Inverse Spin Hall Effect, transmitted along the F slab by long-lived, long-range F spin waves.

What is the (Inverse) Spin Hall effect: J C =D ISHE (J S x  ) F pumps polarized spins into Pt bar. Spin current J S carrying a magnetic moment  flows downward. As a result of the spin-orbit coulping (large in Pt) asymmetry electron scattering occurs deflecting the  electrons in the same direction, i.e., to the right. Thereby a charge current J C flows to the left generating a voltage + to -

Forschungszentrum Jülich in der Helmholtz-Gemeinschaft FILM and THE END STOP

Forschungszentrum Jülich in der Helmholtz-Gemeinschaft What’s beyond GMR? - Resistance changes up to 80% at RT - Widely employed in HDD read-heads and sensors Giant magnetoresistance (GMR): The magnetization state controls to the electric current flow Parallel alignment  low resistive Antiparallel alignment  high resistive G. Binasch et al., Phys. Rev. B 39, 4828 (1989); N.M. Baibich et al., Phys. Lett. 61, (1988) R P = low I P = high R AP = high I AP = low

Forschungszentrum Jülich in der Helmholtz-Gemeinschaft Current-induced magnetization switching The polarity of the electric current flow controls the magnetization state. High current densities: >10 7 A/cm 2 or several mA per (100 nm) 2 Electron flux 100 nm contact diameter  parallel  antiparallel Negative current  parallel alignment Positive current  Antiparallel alignment J.C. Slonczewski, J. Magn. Magn. Mater. 159, L1 (1996); L. Berger, Phys. Rev B 54, 9353 (1996) E.B. Myers et al., Science 285, 867 (1999); J.A. Katine et al., Phys. Rev. Lett. 84, 3149 (2000)

Forschungszentrum Jülich in der Helmholtz-Gemeinschaft Nanopillar devices fabricated by e-beam lithography Wafer 70 nm H. Dassow, D.E. B ü rgler et al., Appl. Phys. Lett. 89, (2006) 20 nm fixed FM 2 nm free FM 6 nm spacer bottom electrode top electrode

Forschungszentrum Jülich in der Helmholtz-Gemeinschaft Pioneering work by the Cornell group Columnar structure (“nanopillar”) and measurement via GMR effect: Sputtered, polycrystalline system: 2.5 nm Co: thin, “free” FM layer 6.0 nm Cu: spacer 10.0 nm Co: thick, “fixed” FM layer  = 130 nm Au low resistive, parallel state for negative current high resistive, antiparallel state for positive current E.B. Myers et al., Science 285, 867 (1999); J.A. Katine et al., Phys. Rev. Lett. 84, 3149 (2000)

Forschungszentrum Jülich in der Helmholtz-Gemeinschaft Physical picture A second FM layer with tilted magnetization polarizes the incident current. One layer (M free ) is easier to switch than the other (M fixed ): M free rotates towards M fixed  parallel alignment M free rotates away from M fixed  antiparallel alignment Note importance of reflected current and asymmetry of FM layers M free electron flux M fixed  M free electron flux M fixed  X. Waintal et al., Phys. Rev. B 62, (2000)

Forschungszentrum Jülich in der Helmholtz-Gemeinschaft For H extern > H c there is only one stable alignment and no switching  spin-transfer torque can excite oscillatory motions of M free with frequencies of several GHz.  GHz voltage signal due to GMR Current-driven magnetization dynamics  nano-scale, solid-state, on-chip microwave oscillator operating at RT dc current M free constant H extern M fixed

Forschungszentrum Jülich in der Helmholtz-Gemeinschaft Current-induced microwave spectra Quality factor f/  f of up to 90 Microwave power per line is estimated to be of the order of 1 nW 2 nm Fe / 6 nm Ag / 10 nm Fe / 0.9 nm Cr / 14 nm Fe at 50 K I bias = 8 mA B || easy axis

Forschungszentrum Jülich in der Helmholtz-Gemeinschaft 1986Discovery of magnetic interlayer exchange coupling (Grünberg) 1988 Discovery of GMR (Grünberg, Fert) 1995 Realization of TMR at room temperature (Miyazaki, Moodera) 1996Prediction of spin-transfer effects (Slonczewski, Berger) 1998 First commercial harddisks with GMR sensors (IBM) 2000Experimental observation of spin-transfer effects (Cornell) 2001Commercial harddisks with AFC media (IBM, now HGST) 2004Giant TMR across epitaxial MgO barriers 2006MRAM based on TMR (Freescale) 2007Demo: MRAM based on giant TMR and spin-transfer (Hitachi) … there is more to come: e.g. quantum information technology … short transfer times from basic research to applications in mass markets GMR and its impact on information technology

Forschungszentrum Jülich in der Helmholtz-Gemeinschaft The goal for the future

Forschungszentrum Jülich in der Helmholtz-Gemeinschaft STOP

Forschungszentrum Jülich in der Helmholtz-Gemeinschaft Once upon a time, … Once upon a time, in the early 1980’s … Peter Gr ü nberg N S S N ? “What happens if I bring two ferromagnets close –I mean really close– together?”

Forschungszentrum Jülich in der Helmholtz-Gemeinschaft Thanks to … … Albert Fert and Peter Gr ü nberg … … for opening the door to spintronics and its applications!

Forschungszentrum Jülich in der Helmholtz-Gemeinschaft Typical hysteresis loops for different types of interlayer coupling FM coupling or decoupled AF coupling90° couplingDominant 90° plus AF coupling

Forschungszentrum Jülich in der Helmholtz-Gemeinschaft Experiment: Anisotropy (“The normal compass”)

Forschungszentrum Jülich in der Helmholtz-Gemeinschaft Experiment: Interlayer coupling (“The crazy compass”)

Forschungszentrum Jülich in der Helmholtz-Gemeinschaft Application of GMR: Read heads in hard disk drives An animation explaining the application of GMR in readheads of hard disk drives can be found at:

Forschungszentrum Jülich in der Helmholtz-Gemeinschaft Giant Magnetoresistance Effect Physical Picture, Applications, and Future Daniel E. Bürgler Institut für Festkörperforschung, Elektronische Eigenschaften (IFF–9) CNI – Center of Nanoelectronic Systems for Information Technology Forschungszentrum Jülich GmbH, Germany University of Leoben, Leoben, November 14, 2007

Forschungszentrum Jülich in der Helmholtz-Gemeinschaft

Fundamental Physics of Nano. & Info. Technology – Dec. 2008

Difference between F and AF Configurations ΔR ≈ 50% … Best to date 10%