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19–22 March 2007 “ GDR Physique Quantique Mésoscopique ”

19–22 March 2007 “ GDR Physique Quantique Mésoscopique ”. Universal conductance fluctuations, and Domain-wall dephasing in ferromagnetic GaMnAs nanowires:. a large phase coherence length for a quantum spintronics in nanomagnets. R. Giraud , L. Vila, L. Thevenard,

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19–22 March 2007 “ GDR Physique Quantique Mésoscopique ”

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  1. 19–22 March 2007 “ GDR Physique Quantique Mésoscopique ” Universal conductance fluctuations, and Domain-wall dephasing in ferromagnetic GaMnAs nanowires: a large phase coherence length for a quantum spintronics in nanomagnets R. Giraud, L. Vila, L. Thevenard, A. Lemaître, F. Pierre, J. Dufouleur, D. Mailly, B. Barbara, G. Faini Laboratoire de Photonique et de Nanostructures – CNRS/LPN Route de Nozay, F – 91460 Marcoussis, France

  2. Outline • The MesoSpin project at LPN • Phase-coherent spin transport in nanostructures: • Spin-orbit couplingvs.Exchange interaction • Phase-coherent spin transport in ferromagnets: • Strong decoherence mechanisms • Strategy for mesoscopic transport in nano-magnets • Universal Conductance Fluctuations in epitaxial ferromagnetic GaMnAs nanowires: • A large effective phase coherence length • Dephasing by a spin disorder (domain walls) • Dominant mechanisms?

  3. i) Spin-SET , Spin Qu-bit ii) Coherent spin transport Electrical spectroscopy of TAMR in a spin Zener-Esaki diode R. Giraud et al., APL 05’ Ø~ 100 nm L = 1 µm Phase-coherent control of a spin current in a ferromagnet or a 2DEG W = 150 nm W ~ 30 nm Phase-coherent manipulation of a localized or a delocalized spin The MesoSpin project contributes to make a new link between spintronicsandmesoscopic physics It is based on all-semiconducting III-V materials, and uses an epitaxial ferromagnet GaMnAs, eventually integrated onto a conventional semiconducting heterostructure All-electrical control of the two spin-states of a single electron spin localized in a vertical quantum dot Ø~ 100 nm

  4. Exchange interaction Spin-Orbit coupling 2πγMS 2m*  = L2  = L D ħ2 the micron scale the nano scale Coherent manipulation of a spin-polarized current « Spin precession through a domain wall Gate-controlled spin precession in a 2DEG (non adiabatic spin transport) The Datta/Das spin FET possibly tuned by an electric and/or a magnetic field S. Datta, B. Das, APL 90’ vs. From J. Nitta <  DW  l spin flip l spin-orbit(π-rotation) l exchange(π-rotation) Bychkov-Rashba coupling  in III-V materials Dephasing of a coherent spin current by DW J. Nitta, et al., PRL 97’ L. Vila et al., PRL 07’

  5. Closed orbits... I V + time-reversed paths …but only ultra-short L were reported to date, probably due to a short inelastic mean free path Jaroszynski et al. 95’, Aprili et al. 97’, Lee 04’, Saitoh et al. 05’ L < 30 nm @ T = 30 mK Classical magneto-conductance dominates over Quantum corrections Mesoscopic transport & Magnetism (1) Quantum corrections to the conductance as a probe of L Saturation of L due to magnetic impurities in a metal P. Mohanty, et al., PRL 97’ H. Pothier, et al., PRL 97’ F. Pierre et al., PRB 68, 085413 (2003) Exchange ‘field’to freeze single-spin fluctuations

  6. Mesoscopic transport & Magnetism (2) Conditions for quantum interferences in ferromagnets How to preserve phase coherence in a ferromagnet ? → reduce spin-flip scattering… large Linel Exchange interactionto minimize energy exchanges within a spin bath Anisotropy ‘field’to freeze low-energy spin waves fluctuations Epitaxial ferromagnetto avoid low-energy spin fluctuations at grain boundaries Reduced role of the spin-orbit interaction Internal fields… perpendicular anisotropy  Bint = Bext At best: if spin-induced decoherence is limited A ferromagnetic epilayer with a strong uniaxial magnetic anisotropy is a good candidate and should be equivalent to a non magnetic, low S-O coupling system, but with an internal Bexchange

  7. Domain Imaging by Kerr microscopy (coll.: N. Vernier, J. Ferré, LPS) SQUID magnetometry (coll.: R.-M. Galéra, Institut Néel) GaMnAs/InGaAs: Demagnetized state @ 70 K 135 µm x 176 µm GaMnAs : an epitaxial ferromagnet Mn : …3d54s2 Ga → Mn  Mn2+ (S=5/2) + h Substitutional Mn = Acceptor Bound magnetic polaron Impurity band / Mobility edge Delocalized hole-induced ferromagnetism (J.-C. Girard, LPN) Tuning of the magnetic properties (L. Thevenard, A. Lemaître, LPN) p • Hydrogen-control • of the hole density p TC  • Strain-induced magnetic anisotropy

  8. What a magnet ! ‘Large’ positive AMR Large Anomalous Hall Effect, No AMR H > HA Single-domain magnetization state Single-domain magnetization state H < HA Coherent rotation vs. Domain walls at any field... GaMnAs MBE – growth engineering of the anisotropy Tailored by the strains induced by the substrate on the epilayer GaMnAs/GaAs:In-planeanisotropy GaMnAs/InGaAs:Out-of-planeanisotropy

  9. L W Quantum corrections to G Classical conductance and Quantum corrections Quantum Transport in GaMnAs nanowires Nanowire:L=220nm, W=50nm L. Vila, R. Giraud, L. Thevenard, A. Lemaître, F. Pierre, J. Dufouleur, D. Mailly, B. Barbara and G. Faini Phys. Rev. Lett. 98, 027204 (2007)

  10. L = 1 µm W = 150 nm Much larger than measured with 3d-metal nanowires! large Lφ Measurements at eV < kBT Perpendicular anisotropy : low temperature & bias behaviour ρ ~ 2 mΩ.cm Metallic behaviour! g ~ 26 p ~ 5.1020 cm-3 D ~ 1 cm2/s Ω  Ω ~

  11. Perpendicular anisotropy : Universal Conductance Fluctuations Universal Conductance Fluctuations as a probe of L Three traces at base temperature… Each trace is a 12h-measurement First run 24h after the green trace (with T=30mK) Second run, 2 days later (T raised to 4K..?)

  12. Perpendicular anisotropy : UCF temperature and bias behaviour Equivalence of the Bias and Temperature dependence of UCF Bias  Current dependence Temp. dependence

  13. Perpendicular anisotropy : check of the UCF nature Redistribution of the microscopic structural disorder configuration New reproducible magneto-fingerprint Single-domain magnetization state

  14. Outline • The MesoSpin project at LPN • Phase-coherent spin transport in nanostructures: • Spin-orbit couplingvs.Exchange interaction • Phase-coherent spin transport in ferromagnets: • Strong decoherence mechanisms • Strategy for mesoscopic transport in nano-magnets • Universal Conductance Fluctuations in epitaxial ferromagnetic GaMnAs nanowires: • A large effective phase coherence length • Dephasing by a spin disorder (domain walls) • Dominant mechanisms?

  15. LF L Perp. anisotropy : Length dependence of the UCF amplitude only if L < L(T) Direct evidence of large L  L > L regime

  16. Temperature dependence of the UCF amplitude& Decoherence L=220nm, W=50nm

  17. Temperature dependence of the UCF amplitude& Decoherence Large effective phase coherence lengths extracted from the ½classical analysis L=220nm, W=50nm (a similar result was obtained in Regensburg: K. Wagner et al., PRL 97, 056803 (2006)) L~ 100 nm @ 100mK L~ LT ~ 1/T1/2 D is an intrinsic limitation to larger values of Lφ in ferromagnetic DMS Decoherence ? • Unusual decoherence mechanism ? Or ‘simply’… • Deviations from standard ½ classical diffusive transport (Breakdown of Fermi liquid theory + Multi-band hole transport) Universal scaling law

  18. I V Universal Conductance Fluctuations as a probe of L Effective phase coherence length extracted from UCF within the framework of the semi-classical approximation… → Aperiodic conductance fluctuations Analysis of the UCF amplitude Extraction of L Quasi-1D regime(LT > W) Quasi-2D regime (LT < W) Grms(e2/h) = (L/L)3/2 Grms(e2/h) = LT/L*(L/L)1/2 • Similar values are obtained because of the same temperature dependence of L and LT • No dimensional crossover observed with temperature for nanowire widths W < 150 nm

  19. Direct evidence of a large L : ‘Non local’ effects for L~L R. Giraud, L. Vila, A. Lemaître and G. Faini to appear in Appl. Surf. Science (2007)  L  L The wave function feels the microscopic configuration of the voltage probes L > L Only the nanowire contribution is probed

  20. Outline • The MesoSpin project at LPN • Phase-coherent spin transport in nanostructures: • Spin-orbit couplingvs.Exchange interaction • Phase-coherent spin transport in ferromagnets: • Strong decoherence mechanisms • Strategy for mesoscopic transport in nano-magnets • Universal Conductance Fluctuations in epitaxial ferromagnetic GaMnAs nanowires: • A large effective phase coherence length • Dephasing by a spin disorder (domain walls) • Dominant mechanisms?

  21. Structuralvs. Spin disorder Planar vs. Perpendicular anisotropy Modification of the correlation fields → Another insight of the partial failure of the semi-classical approximation... + Strain-induced inter-subbands mixing EF

  22. Perpendicular anisotropy Planar anisotropy Pinned Domain Walls Uniform Single Domain Planar anisotropy :Dephasing by spin disorder (domain walls) AMR saturation field HA BZ < BA : Onset of coherent scattering by Domain Walls (no decoherence)  First evidence of dephasing of free carriers by DWs L. Vila, R. Giraud, L. Thevenard, A. Lemaître, F. Pierre, J. Dufouleur, D. Mailly, B. Barbara and G. Faini Phys. Rev. Lett. 98, 027204 (2007)

  23. Modified AB phase Perpendicular applied field DW BZ = HZ + HD + 4M = 0 ! for a thin film (b«a) … Narrow nanowires  stray fields … … but the magnetization is small (MS<100mT) Negligible influence in ferromagnetic DMS ! DW Dephasing by a spin disorder (domain walls) -1-  Dephasing of free carriers by DWs L. Vila et al., Phys. Rev. Lett. 98, 027204 (2007) Possible mechanisms for the additional dephasing in a ferromagnet • Internal exchange field due to the perpendicular magnetization component (modification of the Aharonov-Bohm flux) → much too small in a DMS ... • Dipolar stray field of a domain wall → even smaller

  24. Spin dephasing On-going measurements on a single pinned DW to identify the dominant mechanism Dephasing by spin disorder (domain walls) -2-  Dephasing of free carriers by DWs L. Vila et al., Phys. Rev. Lett. 98, 027204 (2007) Possible mechanisms for the additional dephasing in a ferromagnet • Berry phase in presence of a domain wall → but low density of DW (no multiplication at small fields) Yet, possible contribution in a non adiabatic spin transport regime ? • Spin precession inside a domain wall (non adiabaticregime; Lsf > DW) → should be sensitive to the internal DW structure Spin manipulation of a coherent spin current based on exchange coupling

  25. Dephasing by spin disorder (domain walls) -3- Possible mechanisms for the additional dephasing in a ferromagnet • Berry phase in presence of a domain wall → but low density of DW (no multiplication at small fields) Yet, possible contribution in a non adiabatic spin transport regime ? Spin dephasing Spin manipulation of a coherent spin current based on exchange coupling ... but possibly also affected by a perturbative spin-orbit coupling (anisotropy) Bi-axial anisotropy Geometrical contribution interfering trajectories on the Bloch sphere

  26. Dephasing by spin disorder (domain walls) -4-  Dephasing of free carriers by DWs L. Vila, R. Giraud, L. Thevenard, A. Lemaître, F. Pierre, J. Dufouleur, D. Mailly, B. Barbara and G. Faini Phys. Rev. Lett. 98, 027204 (2007) Advantage over non magnetic semiconductors for phase-coherent spin manipulation • Fast evolution of the phase on a short length scale fixed by the domain wall width (~ 20 nm) • Well-defined, tunablelength scales in a metallic regime (e.g., DW , Lsf) • Combination of different accumulated phases : e.g., phase shift in an AB interferometer • Controled phase manipulation under domain wall distortion (static E or B fields) Some possible drawbacks/difficulties of spin manipulation in a ferromagnetic nanostructure • Magnet manipulation : requires a small applied magnetic field • Control and Stability of the magnetic configuration (anisotropy, pinning)

  27. Large quantum interferenceeffects have been unambiguously observed in epitaxial GaMnAs nanowires Comparison between planar & perpendicular anisotropy: quantum interferences are dominated by structural disorderand an Aharonov-Bohm phase at large magnetic fields (single-domain regime) an additional phase is observed at small fields in presence of a spin disorder (dephasing by domain walls, due to either a Berry phase and/or non adiabatic spin transport -precession- through a magnetic domain wall) This magnetic control of a phase coherent spin currentgives a new way to manipulate the spin of free carriers, on a rather short lengthscale, using the exchange interaction and pinned domain walls. (alternative to the use of spin-orbit couplings) Summary & Conclusions

  28. Exchange interaction Spin-Orbit coupling 2πγMS 2m*  = L2  = L D ħ2 the micron scale the nano scale Coherent manipulation of a spin-polarized current « Spin precession through a domain wall Gate-controlled spin precession in a 2DEG (non adiabatic spin transport) The Datta/Das spin FET possibly tuned by an electric and/or a magnetic field S. Datta, B. Das, APL 90’ vs. From J. Nitta «  DW  l spin flip l spin-orbit(π-rotation) l exchange(π-rotation) Bychkov-Rashba coupling  in III-V materials Dephasing of a coherent spin current by DW J. Nitta, et al., PRL 97’ L. Vila et al., PRL 07’

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