IV Phenomena and Devices

1. IV Phenomena and Devices Direct detection of spin injection All-Metal structures and Domain Wall Velocity The magnetophotovoltaic effect in Schottky junctions MgO barrier magnetic tunnel junctions Planned work MANSE Midterm Review

2. Staff, Publications • Plamen Stamenov Postdoc from November 2007 • Huseyin KurthPostdoc • Tomohiko NiizekiPostdoc • Gen Feng Postdoc • Ciaran FowleyPostgrad • Cathy BoothmanPostgrad • Kaan Oguz Postgrad MANSE Midterm Review

3. Publications; — On the direct magnetic detection of spin injection and adiabatic depolarisation in aluminium, P. Stamenov and JMD Coey, Journal of Magnetism and Magnetic Materials, 320, 403-406 (2008) —Influence of annealing on the bias voltage dependence of tunnelling magnetoresistance in MgO double-barrier magnetic tunnel junctions with CoFeB electrodes, G Feng, S van Dijken and J.M.D. Coey, Applied Physics Letters 89 162501 (2006) —Effect of barrier sputtering parameters on Co80 Fe10 B10 – MgO magnetic tunnel junctions G. Feng, S. van Dijken and J. M. D. Coey, J. Magnetism Magnetic Materials 316 E984-986 (2007) —Noise in MgO barrier magnetic tunnel junctions with CoFeB electrodes; influence of annealing temperature, J. Scola, H Polovy, C. Fermon, M. Pannetier-Lecoeur, G. Feng, K. Fahy and J. M. D. Coey, Applied Physics Letters 90 252501 (2007) —High inverted tunneling magnetoresistance in MgO –based magnetic tunnel juctions, J. F. Feng, Gen Feng, J. M. D. Coey, X.F. Han, and W.S. Zhan. Applied Physics Letters 91 102505 (2007) — Room-temperature magnetoresistance in CoFeB/STO/CoFeB magnetic tunnel junctions, K. Oguz and J. M. D. Coey, Journal of Magnetism and Magnetic Materials, (2008) — Magnetic annealing of CoFeB/MgO based single and double tunnel junctions: tunnel magnetoresistance, bias dependence and output voltage, G. Feng, S. van Dijken, J.M.D. Coey, T. Loo and D.J. Smith. Journal of Applied Physics, 105 (2009) in press MANSE Midterm Review

4. — An approach to fabricate pure metallic Ni-Ni and metallic oxide Ni-NiO-Ni nanocontacts by a repeatable microfabrication method, H X Wei, T. X. Wang, H. Wang, X. F. Han, M. A. Bari and J. M. D. Coey, International Journal of Nanotechnology 4 21-31 (2007) — Magnetoresistance in NiOx nanoconstrictions controlled by magnetic fields and currents, O.Cespedes, M. Viret, JMD Coey, Journal of Applied Physics, 103, 083901 (2008) — Size-dependent scaling of perpendicular exchange bias in magnetic nanostructures, G Malinowski, M. Albrecht, I. L. Guhr, J. M. D. Coey and S. van Dijken, Phys. Rev. B 75 012423 (2007) — Reply to Comment on ‘Size-dependent scaling of perpendicular exchange bias in magnetic nanostructures’ , G. Malinowski, M. Albrecht, I.L. Guhr, J.M.D. Coey. S. van Dijken, Physical Review B 77 017402 (2008) — Magnetic dead layers in sputtered Co40Fe40B20 films, K. Oguz, P. Jivrajka, M.Venkatesan, G. Feng, J.M.D. Coey, Journal of Applied Physics, 103, 07B526 (2008) — Point contact Andreev reflection by nanoindentation of polymethyl methacrylate, E. Clifford and J. M. D. Coey, Applied Physics Letters 89, 092506 (2006) MANSE Midterm Review

5. Introduction Junctions & Devices Metallic Structures (Non GMR) Metal-Semiconductor Contacts GMR and TMR Junctions Direct Spin Injection Detection Magnetic Field Effects Theory & Experiment Large Area Junctions Electronic & Magnetic Response Anomalous Magnetoresistance Structures (In-plane Anisotropy) Sensors (Linear Response) Spontaneous Hall Effect Structures & Perpendicular Anisotropy & Domain Wall Velocity Low Barrier Height Junctions for Spin Injection Field & Current Driven Switching Small Area Junctions Oscillatory & High Frequency Response MANSE Midterm Review

6. Spin Injection – Spin-Self-Diffusion Hence the spin diffusion length is much greater than the mean free path. MANSE Midterm Review

7. Direct Measurement of Injected Polarisation • Using a commercial second-order gradiometer system • The magnetic background of the injectors is a major concern MANSE Midterm Review

8. Magnetisation ProfileTheory & Experiment • The injected magnetisation is small even for 100 % efficiency MANSE Midterm Review

9. Why should it work? / Why should it not? • Long spin diffusion length – 3 nm (300 K) 300 μm (20 K) ~ 3 mm (2 K) • High polarisation of ferromagnetic injectors – 40 % • Signal magnitude – Zeeman shift of electrochemical potential is 0.1 meV/T, Spin injection shift 1 eVm/V (10-2 V/m achievable → ~ 10 meV) • Spatial discrimination – fully decorrelated at 1 cm • Short timescales (10 – 100 ns) – audio frequency modulation is possible • Complications arising from injector stability and superconducting transitions (Al, In) are avoidable • Small signals moments of ~ 10-9 Am2 • Small injection efficiencies ~ 5 % • Large background – 10 times the signal • Background drifts – up to 100 %/min • High power dissipation levels – 10 mW/cm • Parasitic inductive pickup – angular errors of 0.3 mm/10 cm – antisymmetric with respect to current • Signal and noise spatial frequency spectrum overlap • Unexpected effects, symmetric with respect to current • … • 1985M. Johnson and R. H. Silsbee – electrical detection of the “Hanle Effect” • 1993M. Johnson – spin accumulation in Au • … MANSE Midterm Review

10. Various Aspects of the Observed Effects Symmetries of the Effects Temperature Dependence Current Dependence Field Dependence MANSE Midterm Review

11. AC rod sample - Fe @ 1.8 K, 20 mT cross-induction • Only cross-induction from the injection electrodes is observable MANSE Midterm Review

12. Spin InjectionConclusions • No spin injection or adiabatic electronic heating down to δM of the order of 1 A/m, current densities of 108 A/m2 and fields up to 0.5 T • Non-trivial current, field and temperature dependencies for most observed effects • Further work on custom-designed gradiometers MANSE Midterm Review

13. AHE Sample, Setup and Specs • Spontaneous (Anomalous) Hall • [Pt1/Co0.5]3Pt2 • [Pt1/Co0.5]3IrMn10Pt2 • Size (0.1 – 0.3) x (10 - 500 μm) • DC – 50 MHz broadband • AC – 1-10 MHz LIA • Bmax = 200 mT, 1.2 T, 14 T • dB/dT = 200 T/s, 0.5 T/s, 13 mT/s • 2 K < T < 350 K • Imax < 100 μA • VDW < 1000 m/s MANSE Midterm Review

14. CoFe/Pt Domain Topology 2 μm, 5 μm 1 μm, 2 μm 500 nm, 1 μ m 400 nm, 500 nm 200 nm, 200 nm MANSE Midterm Review

15. M,UAHE vs μoH without Exchange BiasT = 300 K • Symmetric with ± B • Reversal through multiple domain states, NOT a single nucleation-propagation event • DC Bias offsets • Asymmetric with ± B • Reversal may be through a single nucleation-propagation event • Advantageous to come back from the EB direction • Opposite EB directions on the two crosses MANSE Midterm Review

16. Example without Exchange Bias • Independent reversal at the two crosses • Field-sweep-rate determined time delay • Large number of events • Small induction effects MANSE Midterm Review

17. Example with Exchange Bias • Correlated reversal on the two crosses • Field-sweep-rate independent time delay • Small number of evens • Negligible induction effects MANSE Midterm Review

18. Schottky BarriersCurrent Components • Thermionic emission over the barrier • Tunneling through the barrier • Recombination in the space-charge region • Recombination in the neutral region After: Rhoderick, E.H. & Williams, R.H. (1988). Metal-Semiconductor Contacts. Oxford: Clarendon. MANSE Midterm Review

19. Schottky BarriersSimple Models The Schottky Model The Bethe Model The Sze Model MANSE Midterm Review

20. Schottky BarriersEffective Circuit • The far from simple effective circuit of the real diode makes the analysis of all possible magnetic field effects difficult • The extraction of spin polarisation information is, by necessity, model dependent MANSE Midterm Review

21. Schottky BarriersMagnetic Field Effects Thermionic-emission: metal semiconductor Drift-diffusion: Ambipolar diffusion: Recombination: MANSE Midterm Review

22. Schottky BarriersDerivative Spectroscopy CoFe/Si<111> MANSE Midterm Review

23. Schottky BarriersDerivative Spectroscopy Cu/Si <111> MANSE Midterm Review

24. Schottky BarriersPhoto-illumination • Illumination eliminates the need for external biasing • The contribution of the series magneto-resistance of the diode base is strongly diminished MANSE Midterm Review

25. Schottky BarrierMagneto-Photo-Voltaic Effect ? • The photo-voltage does saturate as a function of the illumination light intensity at sufficiently low temperatures • The photo-voltage does become a good measure of the barrier height and can be used to extract spin polarisation MANSE Midterm Review

26. Schottky BarriersMagneto-Photo-Voltaic Effect 100 K 200 K MANSE Midterm Review

27. Schottky BarriersPhotovoltaic Measurements • The Schottky barrier height should be sufficiently different from the band-gap of the semiconductor, to avail for experimental separation of the internal photoemission • The metal layer should be sufficiently transparent at the frequencies of interest, but sufficiently thick to preserve bulk behaviour • The temperature dependence of the Schottky barrier height should be sufficiently weak • The Schottky barrier height should be determined by the difference of the work functions of the two materials and not by interface pinning MANSE Midterm Review

28. R3D – GdCo2 MANSE Midterm Review

29. MTJ Optimisation MANSE Midterm Review

30. Tunnel Junction Fabrication 1 ) Bottom contact patterning: UV lithography + 45o Ar+ Ion Milling 4) Lift off: Ar+ Ion Milling (5o) + Hot Ultrasonic for 5-6 hours in remover 2) Pillar patterning: E-Beam lithography + Ar+ Ion Milling (85o + 5o) 5) Top Contact deposition: UV lithography + Sputtering (Ta5/Cu100nm) + lift off 3) Sputtering SiO2 Deposition (100 nm) MTJ stack Ebeam Resist SiO2 Cu contact MANSE Midterm Review

31. Tunnel JunctionsDerivative Spectroscopy Annealed As Deposited MANSE Midterm Review

32. Tunnel BarriersMagneto-conductance Delta barrier Realistic adiabatic barriers MANSE Midterm Review

33. Tunnel JunctionsMicromagnetic Effects Conventional magnetisation reversal process in exchange biased junction Small angle deviations of the electrodes MANSE Midterm Review

34. Tunnel JunctionsThe High Field Limit Is there any detail in the high field limit, when the magnetisations of junction electrodes are aligned parallel to each other and to the applied field? MANSE Midterm Review

35. Tunnel BarriersHigh Field TAMR? As deposited Annealed • Detail appears in the derivative spectra only after the constant derivative background has been subtracted • The symmetry of the effect is high and one base function should be sufficient to describe it • Unannealed junctions show at least three times lower amplitudes MANSE Midterm Review

36. Tunnel BarriersTAMR Base Function? MANSE Midterm Review

37. Tunnel BarriersTAMR Fit? 10 K 2 K • The fit is a set of four Lorentzians of width 0.35 eV and approximately equivalent spacing of 0.25 eV, corresponding to the anisotropy of both spin-up and spin-down bands near the Fermi surface. MANSE Midterm Review

38. Tunnel BarriersNo Effect in the Plane • One privileged direction only – after crystallization • The angle between the electron propagation direction and the magnetisation remains constant • Micromagnetic effects tend to dominate the low field transport • Directional anisotropy is obvious – exchange bias MANSE Midterm Review

39. Current Driven SwitchingTMR Junctions HEB , Hext, Hd Mfree x MANSE Midterm Review

40. Tunnel JunctionsConclusions • Well characterized tunnel junctions with high TMR, good patterning and well-behaved micromagnetically • There is high field anisotropy of the tunnelling magnetoresistance • Origin is the anisotropy of the electronic structure • The fundamental reason is spin-orbit coupling MANSE Midterm Review

41. Sensors (Linear Response)GMR Junctions • Reversal behaviour typical of exchange-biased spin-valves • It is possible to engineer structures where the SAF looses magnetic integrity at small external fields, therefore resulting in negative GMR ratios MANSE Midterm Review

42. Sensors (Linear Response)TMR Junctions HEB Mfree Hext, Hd MANSE Midterm Review

43. Oscillatory &High Frequency Response Bias T Device MW Source Idc MANSE Midterm Review

44. Conclusions • Direct detection of spin injection will require materials with long spin diffusion lengths > 10 μm and optimized gradiometer assemblies • Technology of fabricating and nanoscale pattering of MgO barrier magnetic tunnel junctions has been mastered. Installation of CMP in Spring 2009 will improve yield • Optimized low-barrier height Schottky contacts still deserve a detailed investigation as spin-injectors • Working thin film stacks and devices based on charge transfer ferromagnetism has yet to be demonstrated MANSE Midterm Review

45. Future work • Noise setup • Stripline setup • High resolution planar and volume GQUID gradiometers • Electric field gated spin electroinic devices MANSE Midterm Review

46. Outline MANSE Midterm Review