SPIN-DEPENDENT TRANSPORT IN NANOSTRUCTURES

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

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

3. MOTIVATIONS BASIC RESEARCH • 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

4. APPLICATIONS • - DATA STORAGE • BIOMEDICINE AND BIOTECHNOLOGIES

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

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

7. 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 In ferromagnetic metals SPIN-POLARIZED ELECTRONIC BANDS + SPIN-DEPENDENT SCATTERING PROCESSES S-L (spin-orbit)

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

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

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

11. Late in the 80’s, french and german research teams led byA. FERTandP. 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.

12. GIANT MAGNETORESISTANCE (1988) The Nobel Prize in Physics 2007"forthediscovery of GiantMagnetoresistance"

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

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

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

16. CIP I CPP I COUPLED MULTILAYERS SPIN VALVES GRANULAR ALLOYS SPINTRONICS

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

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

19. 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

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

21. Metallic multilayers CIP CPPMagnetic tunnel junctions Insulating barriers Semiconducting barriers STRUCTURES different length scales

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

23. ADVANTAGES OF PEROVSKITE OXIDES: optimal properties for magnetic tunnel electrodes…. 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 METALICITYSUPERCONDUCTIVITY OPTICS

24. A B:Mn O PEROVSKITES ABO3 90s: Manganites R. Ushibara, Phys. Rev. B 51, 14103 (1995)

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

26. HC t (nm) SUBSTRATE-INDUCED EFFECTS ON MAGNETIC AND TRANSPORT PROPERTIES La0.6Sr0.4MnO3 films grown by sputtering Anisotropy fields Coercive fields KA1 = KV + 1/t * KS LS-STO LS-MGO Kv[erg/cm3] -3.6x105 +2.6x105 Ks [erg/cm2] +0.89 +0.66 SrTiO3 full symbols, MgO: empty L.B. Steren, M. Sirena, and J. Guimpel, J. Appl. Phys., Vol. 87, No. 8, (2000)

27. TRANSPORT PROPERTIES OF LSMO FILMS Grown on MgO SrTiO3 L.B. Steren et al, J.Magn. Magn. Mater. 211, 28 (2000). J. Guimpel et al, Thin Solid Films 373, 102 (2000).

28. METAL-INSULATOR TRANSITION INDUCED BY ANNEALING TREATMENTS UNDER OXYGEN ATMOSPHERE Bulk La0.96Sr0.04MnO3 is a canted antiferromagnetic insulator La0.96Sr0.04MnO3 thin films show: FERROMAGNETISM 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%. M. Sirena et al, J. Appl. Phys. 105, 33902 (2009).

29. = 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

30. Growth of metallic multilayers La0.7Sr0.3MnO3/LaNiO3 in order to look for giant magnetoresistance in oxide-based multilayers LaSrMnO multilayers La0.7Sr0.3MnO3/LaNiO3 M. Granada et al, Appl. Phys. Lett. 91, 072110 (2007) Roughness N LSMO=2.33Å N LNO =1.53Å C =0.05Å J.C. Rojas Sanchez et al, Appl. Surf. Science, (07)

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

32. Ni- O La Mn - O IN OXIDES: FM/AFM oxide-based multilayers Phys. Rev. 64, 94429 (2001); Phys. Rev. B 60, 485 (1999) YBaCuO/LSMO N. Haberkorn et al, APL 84, 3927 (2004) dead layers, Phys. Rev. B 69. 134428 (2004) hole transfer from the high Tc to the manganite THEORY S. Dong et al, Phys. Rev. Lett 2009 => weak FM/FM origin of EB FUNDAMENTAL ROLE OF INTERFACES A/B o A/substrate • Lattice mistmatch => deffects, strain • Interdifussion • - Roughness

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

34. BILAYERS Normalized XAS (left) and absolute XAS (right) to directly compare lineshape, but also see relative concentrations of elements. PROBE OF THE MAGNETISM AND OXIDATION STATE OF Mn AND Ni separately  MAGNETIC Ni2+ AT INTERFACE!

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

36. BULK INTERFACE Temperature dependence of manganites magnetization at interfaces!

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

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

39. SUMMARY OF THE TALK 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)

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

41. 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 2004. • Oxide spintronics, IEEE TRANS. ELECTRON. DEVICES, VOL. X, NO. XX, AUGUST 2006 ; M. Bibes and A. Barthelemy Laura Steren steren@tandar.cnea.gov.ar

42. TEAM CAB- CAC 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) In collaboration with: V.H. Etgens, M. Marangolo, M. Eddrief (INSP,France) G. Leyva (CAC, Bs.As.) H. Pastoriza (CAB) G. Faini (LPN, France)

43. AT THE FRONTIERS OF CONDENSED MATTER V Current Trends and Novel Materials • 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.