Laboratoire de Photonique et de Nanostructures R. Giraud Aussois, March 2007 Nano Team 1922 March 2007 GDR Physique Quantique Msoscopique Universal conductance fluctuations, and Domain-wall dephasing in ferromagnetic GaMnAs nanowires: Laboratoire de Photonique et de Nanostructures CNRS/LPN Route de Nozay, F Marcoussis, France R. Giraud, L. Vila, L. Thevenard, A. Lematre, F. Pierre, J. Dufouleur, D. Mailly, B. Barbara, G. Faini a large phase coherence length for a quantum spintronics in nanomagnets Laboratoire de Photonique et de Nanostructures R. Giraud Aussois, March 2007 Nano Team Outline The MesoSpin project at LPN Phase-coherent spin transport in nanostructures: Spin-orbit coupling vs. 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 ? Laboratoire de Photonique et de Nanostructures R. Giraud Aussois, March 2007 Nano Team i) Spin-SET, Spin Qu-bit Phase-coherent manipulation of a localized or a delocalized spin All-electrical control of the two spin-states of a single electron spin localized in a vertical quantum dot The MesoSpin project contributes to make a new link between spintronics and mesoscopic physics It is based on all-semiconducting III-V materials, and uses an epitaxial ferromagnet GaMnAs, eventually integrated onto a conventional semiconducting heterostructure ii) Coherent spin transport Phase-coherent control of a spin current in a ferromagnet or a 2DEG L = 1 m W = 150 nm ~ 100 nm W ~ 30 nm ~ 100 nm Electrical spectroscopy of TAMR in a spin Zener-Esaki diode R. Giraud et al., APL 05 Laboratoire de Photonique et de Nanostructures R. Giraud Aussois, March 2007 Nano Team Spin-Orbit coupling Coherent manipulation of a spin-polarized current Exchange interaction vs. L. Vila et al., PRL 07 Gate-controlled spin precession in a 2DEG Spin precession through a domain wall (non adiabatic spin transport) possibly tuned by an electric and/or a magnetic field The Datta/Das spin FET S. Datta, B. Das, APL 90 Dephasing of a coherent spin current by DW J. Nitta, et al., PRL 97 From J. Nitta the micron scalethe nano scale Bychkov-Rashba coupling in III-V materials DW l spin flip < l spin-orbit (-rotation) l exchange (-rotation) = 2m * L 22 = 2M S L2L2 D Laboratoire de Photonique et de Nanostructures R. Giraud Aussois, March 2007 Nano Team Quantum corrections to the conductance as a probe of L Saturation of L due to magnetic impurities in a metal F. Pierre et al., PRB 68, (2003) Mesoscopic transport & Magnetism (1) V I + time-reversed paths Closed orbits... P. Mohanty, et al., PRL 97 H. Pothier, et al., PRL 97 Exchange field to freeze single-spin fluctuations 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 T = 30 mK Classical magneto-conductance dominates over Quantum corrections Laboratoire de Photonique et de Nanostructures R. Giraud Aussois, March 2007 Nano Team How to preserve phase coherence in a ferromagnet ? reduce spin-flip scattering Conditions for quantum interferences in ferromagnets Anisotropy field to freeze low-energy spin waves fluctuations Epitaxial ferromagnet to avoid low-energy spin fluctuations at grain boundaries Reduced role of the spin-orbit interaction Internal fields large L inel Mesoscopic transport & Magnetism (2) Exchange interaction to minimize energy exchanges within a spin bath 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 B exchange At best: if spin-induced decoherence is limited perpendicular anisotropy B int = B ext Laboratoire de Photonique et de Nanostructures R. Giraud Aussois, March 2007 Nano Team Substitutional Mn = Acceptor Bound magnetic polaron Impurity band / Mobility edge Delocalized hole-induced ferromagnetism Mn : 3d 5 4s 2 Ga Mn Mn 2+ (S=5/2) + h (J.-C. Girard, LPN) GaMnAs : an epitaxial ferromagnet Tuning of the magnetic properties Hydrogen-control of the hole density p p TCTC (L. Thevenard, A. Lematre, LPN) Domain Imaging by Kerr microscopy (coll.: N. Vernier, J. Ferr, LPS) 135 m x 176 m GaMnAs/InGaAs: Demagnetized 70 K Strain-induced magnetic anisotropy SQUID magnetometry (coll.: R.-M. Galra, Institut Nel) Laboratoire de Photonique et de Nanostructures R. Giraud Aussois, March 2007 Nano Team Tailored by the strains induced by the substrate on the epilayer GaMnAs/GaAs : In-plane anisotropyGaMnAs/InGaAs : Out-of-plane anisotropy GaMnAs MBE growth engineering of the anisotropy Large positive AMR H > H A Single-domain magnetization state H < H A Coherent rotation vs. Domain walls Single-domain magnetization state What a magnet ! Large Anomalous Hall Effect, No AMR at any field... Laboratoire de Photonique et de Nanostructures R. Giraud Aussois, March 2007 Nano Team Quantum Transport in GaMnAs nanowires Nanowire: L=220nm, W=50nm L W Quantum corrections to G L. Vila, R. Giraud, L. Thevenard, A. Lematre, F. Pierre, J. Dufouleur, D. Mailly, B. Barbara and G. Faini Phys. Rev. Lett. 98, (2007) Classical conductance and Quantum corrections Laboratoire de Photonique et de Nanostructures R. Giraud Aussois, March 2007 Nano Team L = 1 m W = 150 nm Metallic behaviour! g ~ 26 p ~ cm -3 D ~ 1 cm 2 /s ~ 2 m.cm Perpendicular anisotropy : low temperature & bias behaviour Measurements at eV < k B T ~ Much larger than measured with 3d-metal nanowires! large L Laboratoire de Photonique et de Nanostructures R. Giraud Aussois, March 2007 Nano Team Three traces at base temperature First run Second run, 2 days later (T raised to 4K..?) 24h after the green trace (with T=30mK) Perpendicular anisotropy : Universal Conductance Fluctuations Each trace is a 12h-measurement Universal Conductance Fluctuations as a probe of L Laboratoire de Photonique et de Nanostructures R. Giraud Aussois, March 2007 Nano Team Perpendicular anisotropy : UCF temperature and bias behaviour Temp. dependence Bias Current dependence Equivalence of the Bias and Temperature dependence of UCF Laboratoire de Photonique et de Nanostructures R. Giraud Aussois, March 2007 Nano Team Perpendicular anisotropy : check of the UCF nature Redistribution of the microscopic structural disorder configuration New reproducible magneto-fingerprint Single-domain magnetization state Laboratoire de Photonique et de Nanostructures R. Giraud Aussois, March 2007 Nano Team Universal Conductance Fluctuations in epitaxial ferromagnetic GaMnAs nanowires: A large effective phase coherence length Dephasing by a spin disorder (domain walls) Dominant mechanisms ? Outline The MesoSpin project at LPN Phase-coherent spin transport in nanostructures: Spin-orbit coupling vs. Exchange interaction Phase-coherent spin transport in ferromagnets: Strong decoherence mechanisms Strategy for mesoscopic transport in nano-magnets Laboratoire de Photonique et de Nanostructures R. Giraud Aussois, March 2007 Nano Team Perp. anisotropy : Length dependence of the UCF amplitude only if L < L (T) L LL Direct evidence of large L L > L regime Laboratoire de Photonique et de Nanostructures R. Giraud Aussois, March 2007 Nano Team L=220nm, W=50nm Temperature dependence of the UCF amplitude & Decoherence Laboratoire de Photonique et de Nanostructures R. Giraud Aussois, March 2007 Nano Team Large effective phase coherence lengths extracted from the classical analysis L ~ mK L ~ L T ~ 1/T 1/2 Universal scaling law L=220nm, W=50nm Temperature dependence of the UCF amplitude & Decoherence Unusual decoherence mechanism ? Deviations from standard classical diffusive transport (Breakdown of Fermi liquid theory + Multi-band hole transport) Or simply (a similar result was obtained in Regensburg: K. Wagner et al., PRL 97, (2006)) D is an intrinsic limitation to larger values of L in ferromagnetic DMS Decoherence ? Laboratoire de Photonique et de Nanostructures R. Giraud Aussois, March 2007 Nano Team Universal Conductance Fluctuations as a probe of L Effective phase coherence length extracted from UCF within the framework of the semi-classical approximation Quasi-1D regime (L T > W)Quasi-2D regime (L T < W) G rms (e 2 /h) = (L /L) 3/2 G rms (e 2 /h) = L T /L*(L /L) 1/2 Aperiodic conductance fluctuations Analysis of the UCF amplitude Extraction of L Similar values are obtained because of the same temperature dependence of L and L T No dimensional crossover observed with temperature for nanowire widths W < 150 nm V I Laboratoire de Photonique et de Nanostructures R. Giraud Aussois, March 2007 Nano Team Direct evidence of a large L : Non local effects for L~L L L The wave function feels the microscopic configuration of the voltage probes L > L Only the nanowire contribution is probed R. Giraud, L. Vila, A. Lematre and G. Faini to appear in Appl. Surf. Science (2007) Laboratoire de Photonique et de Nanostructures R. Giraud Aussois, March 2007 Nano Team Outline The MesoSpin project at LPN Phase-coherent spin transport in nanostructures: Spin-orbit coupling vs. 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 ? Laboratoire de Photonique et de Nanostructures R. Giraud Aussois, March 2007 Nano Team Modification of the correlation fields Another insight of the partial failure of the semi-classical approximation... Structural vs. Spin disorder EFEF + Strain-induced inter-subbands mixing Planar vs. Perpendicular anisotropy Laboratoire de Photonique et de Nanostructures R. Giraud Aussois, March 2007 Nano Team B Z < B A : Onset of coherent scattering by Domain Walls (no decoherence) Planar anisotropy : Dephasing by spin disorder (domain walls) First evidence of dephasing of free carriers by DWs L. Vila, R. Giraud, L. Thevenard, A. Lematre, F. Pierre, J. Dufouleur, D. Mailly, B. Barbara and G. Faini Phys. Rev. Lett. 98, (2007) AMR saturation field H A Pinned Domain Walls Planar anisotropy Uniform Single Domain Perpendicular anisotropy Laboratoire de Photonique et de Nanostructures R. Giraud Aussois, March 2007 Nano Team Dephasing by a spin disorder (domain walls) -1- Dephasing of free carriers by DWs L. Vila et al., Phys. Rev. Lett. 98, (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 Modified AB phase Negligible influence in ferromagnetic DMS ! Perpendicular applied field B Z = H Z + H D + 4 M = 0 ! for a thin film (ba) Narrow nanowires stray fields but the magnetization is small (M S DW ) should be sensitive to the internal DW structure Spin dephasing Spin manipulation of a coherent spin current based on exchange coupling On-going measurements on a single pinned DW to identify the dominant mechanism Laboratoire de Photonique et de Nanostructures R. Giraud Aussois, March 2007 Nano Team 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 Laboratoire de Photonique et de Nanostructures R. Giraud Aussois, March 2007 Nano Team Dephasing by spin disorder (domain walls) -4- Dephasing of free carriers by DWs L. Vila, R. Giraud, L. Thevenard, A. Lematre, F. Pierre, J. Dufouleur, D. Mailly, B. Barbara and G. Faini Phys. Rev. Lett. 98, (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) 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 Control and Stability of the magnetic configuration (anisotropy, pinning) Magnet manipulation : requires a small applied magnetic field Well-defined, tunable length scales in a metallic regime (e.g., DW, L sf ) Laboratoire de Photonique et de Nanostructures R. Giraud Aussois, March 2007 Nano Team Summary & Conclusions Large quantum interference effects have been unambiguously observed in epitaxial GaMnAs nanowires Comparison between planar & perpendicular anisotropy: quantum interferences are dominated by structural disorder and 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 current gives 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) Laboratoire de Photonique et de Nanostructures R. Giraud Aussois, March 2007 Nano Team Spin-Orbit coupling Coherent manipulation of a spin-polarized current Exchange interaction vs. L. Vila et al., PRL 07 Gate-controlled spin precession in a 2DEG Spin precession through a domain wall (non adiabatic spin transport) possibly tuned by an electric and/or a magnetic field The Datta/Das spin FET S. Datta, B. Das, APL 90 Dephasing of a coherent spin current by DW J. Nitta, et al., PRL 97 From J. Nitta the micron scalethe nano scale Bychkov-Rashba coupling in III-V materials DW l spin flip l spin-orbit (-rotation) l exchange (-rotation) = 2m * L 22 = 2M S L2L2 D