SPIN-DEPENDENT TRANSPORT IN NANOSTRUCTURES Laura B. Steren Condensed Matter Dept. Centro Atómico Constituyentes San Martin, Argentina

OUTLINE - MOTIVATIONS - INTRODUCTION TO SPIN-POLARIZED TRANSPORT PHENOMENA AND MAGNETORESISTIVE EFFECTS IN MAGNETIC NANOSTRUCTURES - SPINTRONICS GIANT MAGNETORESISTANCE EFFECT MAGNETIC TUNNEL JUNCTIONS - OXIDE-BASED SPINTRONICS

BASIC RESEARCH MOTIVATIONS -CONFINEMENT AND SIZE EFFECTS ON THE TRANSPORT PROPERTIES OF MAGNETIC NANOSTRUCTURES (ELECTRONIC BANDS, TRANSPORT MECHANISMS, …) - STRONG CORRELATION BETWEEN MAGNETISM AND TRANSPORT (MAGNETIC ORDER, ANISOTROPIES,….) -EFFECT OF INTERFACES AND SURFACES IN SPINTRONICS Ex. GMR TUNNEL JUNCTIONS/ SPIN FILTERS

APPLICATIONS - DATA STORAGE -BIOMEDICINE AND BIOTECHNOLOGIES

ELECTRONIC TRANSPORT & SIZE EFFECTS N ~ 30nm noble metals (Au, Ag, Cu) FM ~ 8nm transition metals (Fe,Co,Ni) The mean free path depends on electronic bands, impurities, carriers scattering processes, etc CHARACTERISTIC LENGTH IN ELECTRONIC TRANSPORT: MEAN FREE PATH OF THE CARRIERS BETWEEN COLLISIONS BALLISTIC REGIME (QM) DIFFUSIVE REGIME (SEMIC) SIZE Compare with?

d The surfaces and interfaces affect the solution of the Boltzmann equation when d/  1 SEMICLASSICAL APPROACH applied to thin films ROUGHNESS, COMPOSITION,…. FUCHS-SONDHEIMER (1952) Electric transport in thin films REFLECTION DIFUSSION

ELECTRICAL TRANSPORT IN MAGNETIC SYSTEMS KNOWN FOR MANY YEARS…. Interaction between electric carriers and localized moments for example: magnetic impurities in metals. Other cases: Kondo, ferromagnets S-L (spin-orbit) In ferromagnetic metals SPIN-DEPENDENT SCATTERING PROCESSES SPIN-POLARIZED ELECTRONIC BANDS +

MAGNETORESISTANCE in metals Lorentz force acts on the electron trayectory (MR). A.D. Kent et al J. Phys. Cond. Mat. 13, R461 (2001) Application of a H

ANISOTROPIC MAGNETORESISTANCE IN FM Coupling between electronic current and the magnetization of the FM (SPIN–ORBIT) DEPENDENCE ON THE ANGLE BETWEEN ELECTRIC CURRENT AND MAGNETIZATION Application of H ….

HALL EFFECT IN FERROMAGNETIC MATERIALS Anomalous:  M, Rs prop ρ origin: SO Ordinary: Bands/ Scattering ()

Late in the 80’s, french and german research teams led by A. FERT and P. GRUNBERG, respectively, discovered the giant magnetoresistant effect in Fe/Cr multilayers following different motivations WORKING WITH STRUCTURES OF NANOMETRIC SIZE NOT ONLY SERVE TO MINIATURIZE DEVICES BUT ALSO TO FINELY TUNE THE TRANSPORT PROPERTIES OF MATERIALS.

The Nobel Prize in Physics 2007 "for the discovery of Giant Magnetoresistance" GIANT MAGNETORESISTANCE (1988)

MAGNETIZATION LOOP T=294K MAGNETORESISTANCE T=294K MAGNETORESISTANCE T=10K CORRELATION BETWEEN MAGNETISM AND MAGNETO- TRANSPORT!! GIANT MAGNETORESISTANCE GMR = (Rap-Rp)/ Rp

PARALLEL/ANTIPARALLEL CONFIGURATION TWO CURRENT MODEL  ,  

J. Barnas, A. Fuss, R.E. Camley, P. Grunberg, W. Zinn, Phys. Rev. B 42, 1990 SEMICLASSICAL APPROACH: BOLTZMANN EQUATION SPIN -DEPENDENT BOUNDARY CONDITIONS TO SOLVE THE BOLTZMANN EQUATION AND CALCULATE THE ELECTRICAL CURRENT SPIN-DEPENDENT DENSITY FUNCTION G

I CIP I CPP COUPLED MULTILAYERS SPIN VALVES GRANULAR ALLOYS

A few years later, MAGNETIC TUNNEL JUNCTIONS TMR = (Rap-Rp)/ Rp

AMR TMR CoFe/AlO/Co Tunneling through the insulating barrier without spin-flip !

Experimental challenge: FABRICATION OF THE JUNCTIONS 1- SAMPLES GROWTH 2- NANOSTRUCTURATION -SURFACE ROUGHNESS OF THE FM ELECTRODES -FM-I INTERFACE QUALITY -QUALITY OF THE TUNNEL BARRIER OPTICAL LITHOGRAPHY + ION ETCHING M. Sirena, CAB Bariloche

Oxides for Spintronics A. Barthelemy, M. Bibes, IEEE Trans. Electron Devices vol. X, August 2006 Au/NiFeO/LSMO SPIN FILTER The height of the barrier is spin-dependent due to the magnetic character of the spacer M FM-M FM-I

Metallic multilayers CIP CPP Magnetic tunnel junctions Insulating barriers Semiconducting barriers different length scales STRUCTURES

GMR IN METALLIC SYSTEMS ’88 TM/NM MAGNETIC JUNCTIONS / SPIN FILTERS ‘92 TM/BINARY ALLOYS DOMAIN-WALL IN WIRES ‘06 TM OXIDES IN THE BEGINNING THE TYPICAL MATERIALS USED IN SPINTRONICS DEVICES

The oxides composition could be changed and so as the substrate-induced strains in order to design nanomaterials with specific physical properties!! FERROELECTRICITY FERROMAGNETISM/ AF METALICITY SUPERCONDUCTIVITY OPTICS ADVANTAGES OF PEROVSKITE OXIDES: optimal properties for magnetic tunnel electrodes….

PEROVSKITES ABO 3 90s: Manganites R. Ushibara, Phys. Rev. B 51, (1995) A B:Mn O

Highly textured when grown on insulator single- crystalline substrates like SrTiO3 and MgO GROWTH OF OXIDE FILMS BY SPUTTERING OR PLD

SUBSTRATE-INDUCED EFFECTS ON MAGNETIC AND TRANSPORT PROPERTIES La 0.6 Sr 0.4 MnO 3 films grown by sputtering SrTiO 3 full symbols, MgO: empty Anisotropy fields Coercive fields t (nm) HCHC L.B. Steren, M. Sirena, and J. Guimpel, J. Appl. Phys., Vol. 87, No. 8, (2000) K A1 = K V + 1/t * K S LS-STO LS-MGO Kv[erg/cm 3 ] -3.6x x10 5 Ks [erg/cm 2 ]

TRANSPORT PROPERTIES OF LSMO FILMS Grown on MgO SrTiO 3 L.B. Steren et al, J.Magn. Magn. Mater. 211, 28 (2000). J. Guimpel et al, Thin Solid Films 373, 102 (2000).

METAL-INSULATOR TRANSITION INDUCED BY ANNEALING TREATMENTS UNDER OXYGEN ATMOSPHERE Bulk La 0.96 Sr 0.04 MnO 3 is a canted antiferromagnetic insulator La 0.96 Sr 0.04 MnO 3 thin films show: FERROMAGNETISM M. Sirena et al, J. Appl. Phys. 105, (2009). All films display a ferromagnetic transition and their Tc and Mr increase with increasing oxygen content. The magnetic saturation Ms is almost the same for all the samples with a variation of less than 10%.

= Our results indicate that the oxygenation dynamic of LaSr0.04MnO manganites depend on strain fields and defects induced by the substrates. = The faster oxygenation dynamics observed in LSMO-STO films could be attributed to a high oxygen diffusion rate in strained films. = On the other hand, oxygen vacancies in LSMO-MGO films are probably removed from dislocations

M. Granada et al, Appl. Phys. Lett. 91, (2007) multilayers La0.7Sr0.3MnO3/LaNiO3 LaSrMnO J.C. Rojas Sanchez et al, Appl. Surf. Science, (07) Growth of metallic multilayers La0.7Sr0.3MnO3/LaNiO3 in order to look for giant magnetoresistance in oxide- based multilayers Roughness  N LSMO =2.33Å  N LNO =1.53Å  C =0.05Å

BROADENING OF THE HYSTERESIS AND SHIFT OF THE FC LOOPS EXISTENCE OF EXCHANGE BIAS AT LSMO/LNO INTERFACES EFFECTS OBSERVED AT FM/AFM INTERFACES: PINNING OF THE FM PHASE WHEN THE SAMPLE IS COOLED UNDER A MAGNETIC FIELD BELOW T N (AFM layer) First report: Co/CoO

Ni- O La Mn - O FUNDAMENTAL ROLE OF INTERFACES A/B o A/substrate -Lattice mistmatch => deffects, strain - Interdifussion - Roughness FM/AFM oxide-based multilayers Phys. Rev. 64, (2001); Phys. Rev. B 60, 485 (1999) YBaCuO/LSMO N. Haberkorn et al, APL 84, 3927 (2004) dead layers, Phys. Rev. B (2004) hole transfer from the high Tc to the manganite S. Dong et al, Phys. Rev. Lett 2009 => weak FM/FM origin of EB THEORY IN OXIDES:

CHEMICAL COMPOSITION AND MAGNETIC STATE AT THE INTERFACES? X-ray absorption spectroscopy experiments! Depth probe ~ 5nm These samples were d Esigned in order to test both LNO/LSMO and LSMO/LNO interfaces Brittany B. Nelson-Cheeseman, University of California, Berkeley Depart. Materials Science and Engineering, USA

PROBE OF THE MAGNETISM AND OXIDATION STATE OF Mn AND Ni separately  MAGNETIC Ni 2+ AT INTERFACE! BILAYERS Normalized XAS (left) and absolute XAS (right) to directly compare lineshape, but also see relative concentrations of elements.

From the temperature dependence of  the height of the barrier is deduced MAGNETIC TUNNEL JUNCTIONS LSMO/CaMnO/LSMO Bariloche, Argentina I-V curves

Temperature dependence of manganites magnetization at interfaces! BULK INTERFACE

APPLICATION TO DATA STORAGE Writing Reading Storage 2001, Hitachi AFC media GMR reading heads TMR W/R heads.

ROAD MAP FOR DATA STORAGE Hitachi 2007 Miniaturization and new technologies: spintronics CPP-GMR heads? TMR heads (1992)  recording AFC media GMR heads (1988) AMR heads Thin films heads

SPIN-DEPENDENT TRANSPORT PHENOMENA Spin valves / Giant magnetoresistance GMR Tunnel junctions/ Spin filters MULTIFUNCTIONAL MATERIALS CHEMISTRY/PHYSICS Oxides magnetism/electricity/multiferroics Metal/semiconductor DEVICES PHYSICS AND ENGINEERING (Top-down) SUMMARY OF THE TALK

THERE IS A LOT OF THINGS TO DO!! ► DEVELOPMENT OF NEW MATERIALS ► STUDY OF QUANTUM AND LOW DIMENSIONAL PHENOMENA ► NEW APPLICATIONS

BIBLIOGRAPHY MAGNETOELECTRONICS, Ed. by M. Johnson Elsevier 2004 NANOMAGNETISM AND SPINTRONICS, T. SHINJO, Elsevier 2009 SPINTRONICS: Fundamentals and applications; Reviews of modern physics, vol. 76, april Oxide spintronics, IEEE TRANS. ELECTRON. DEVICES, VOL. X, NO. XX, AUGUST 2006 ; M. Bibes and A. Barthelemy Laura Steren

EXPERIMENTAL RESEARCH Gabriela Alejandro (R) Julian Milano (R) Martin Sirena (R) Mara Granada (R) Marina Tortarolo (PD) Juan. C. Rojas Sanchez (GS) Federico Fernandez Baldis (GS) TEAM CAB- CAC In collaboration with: V.H. Etgens, M. Marangolo, M. Eddrief (INSP,France) G. Leyva (CAC, Bs.As.) H. Pastoriza (CAB) G. Faini (LPN, France)

December 6-10, 2010 Buenos Aires, Argentina Multifunctional materials. Mesoscopic and nanoscopic devices. Magnetic oxides and related topics. Dilute magnetic semiconductors and semiconducting heterostructures. Intermetallic compounds. General field theory applications, experimental and computational techniques in condensed matter. AT THE FRONTIERS OF CONDENSED MATTER V Current Trends and Novel Materials

IDEALLY HOWEVER MOST OF THE TIMES, NANOSTRUCTURED MATERIALS AND HETEROSTRUCTURES REVEAL NEW PHENOMENA/PROPERTIES, GIVING PLACE TO NEW STUDIES AND/OR MOTIVATIONS STRUCTURES THAT COMBINE ELEMENTS AND/ OR SAMPLES DESIGN WITH SPECIFIC GEOMETRIES FOR THE STUDY OF PARTICULAR PROPERTIES OR APPLICATIONS + MATERIAL ENGINEERING

ELECTRONIC STRUCTURE TRANSPORT EQUATIONS To discuss transport two calculations are necessary:

SEMICLASSICAL APPROACH TO TRANSPORT PROPERTIES The Boltzmann equation says that at any point and for any value of k, the net rate of change of fk(r) is zero: Assuming that the number of carriers in the neighbourhood of r at time t is equal to the number of them in the neighbourhood of r-tv k at time 0 : STEADY STATE SCATTERING MAGNETIC and ELECTRIC FIELD TEMPERATURE GRADIENT

RELAXATION TIME AND MEAN FREE PATH To solve the Boltzmann equation, a relaxation time, , is introduced. This characteristic time is related to the process of relaxation of the system to equilibrium once the external fields are turned off. People usually introduces a characteristic length, instead of using  : MEAN FREE PATH The mean free path depends on impurities, carriers scattering processes, etc

HALL EFFECT IN MnAs THIN FILMS SIGN CHANGE OF THE HALL EFFECT AROUND 40K THERE ARE HOLES AND ELECTRONS FERMI SURFACE CALCULATION C. Helman, A.M. Llois

Transverse wall vortex MFM IMAGE SIMULATIONS wire of Ni 81 Fe 19 width: 300 nm, thickness: 10nm M. Hayashi et al, Phys. Rev. Lett. 97, (2006) DOMAIN WALL DISPLACEMENT BY CURRENT INJECTION DOMAIN WALL AT ARTIFICIAL DEFFECTS

LaAlO 3 /SrTiO 3 INTERFACE PROPERTIES Conducting interfaces between insulating oxides Ohtomoand Hwang Nature 427, 423 (2004) INTERFACE PROPERTIES DEPENDENCE ON OXYGEN PRESSURE Mapping the spatial distribution of charge carriers in LaAlO 3 /SrTiO 3 heterostructures M. Basletic et al; Nature Materials VOL 7 AUGUST 2008 High Mobility in LaAlO3/SrTiO3 Heterostructures: Origin, Dimensionality, and Perspectives G. Herranz et al; PRL 98, (2007) GAS 2DEG Metal!! AlLaO 3 SrTiO 3

Ionic relaxation contribution to the electronic reconstruction at the n-type LaAlO3/SrTiO3 interface; R. Pentcheva and W. Pickett. Phys. Rev. B 78, (2008). and references therein DFT calculations of the perovskite electronic band structures at interfaces Besides the influence of oxygen defects, there are intrinsic effects that can explained the observed behaviour: AlLaO3SrTiO3 (LaO)+(SrO)0 (AlO2)-(TiO2)0 n-type p-type (electron doped) (hole doped) (LaO/TiO2) AlO2/SrO Local charge neutrality disrupted!

ADVANCED MAGNETIC MATERIALS / NEW DEVICES? MOLECULAR MAGNETS MULTIFERROICS MAGNETOSTRICTIVES MATERIALS (SHAPE MEMORY) MAGNETO-OPTICS MATERIALS LOGIC CIRCUITS WITH MAGNETIC WIRES HARD-MAGNETIC FILMS