Freezing and thawing of artificial ice by thermal switching of geometric frustration in magnetic flux lattices.

The problem of an ensemble of repulsive particles on a potential-energy landscape is common to many physical systems and has been studied in multiple artificial playgrounds. However, the latter usually involve fixed energy landscapes, thereby impeding in situ investigations of the particles' collective response to controlled changes in the landscape geometry. Here, we experimentally realize a system in which the geometry of the potential-energy landscape can be switched using temperature as the control knob. This realization is based on a high-temperature superconductor in which we engineer a nanoscale spatial modulation of the superconducting condensate. Depending on the temperature, the flux quanta induced by an applied magnetic field see either a geometrically frustrated energy landscape that favours an ice-like flux ordering, or an unfrustrated landscape that yields a periodic flux distribution. This effect is reflected in a dramatic change in the superconductor's magneto-transport. The thermal switching of the energy landscape geometry opens new opportunities for the study of ordering and reorganization in repulsive particle manifolds.

Stochastic current-induced magnetization switching in a single semiconducting ferromagnetic layer.

We show experimental evidence of magnetization switching in a single (Ga,Mn)(As,P) semiconducting ferromagnetic layer, attributed to a strong reduction of the magnetization and the anisotropy due to current injection. The nucleation of magnetization reversal is found to occur even in the absence of a magnetic field and to be both anisotropic and stochastic. Our findings highlight a new mechanism of magnetization manipulation based on spin accumulation in a semiconductor material.

Structural and electrical characterization of ultra-thin SrTiO3 tunnel barriers grown over YBa2Cu3O7 electrodes for the development of high Tc Josephson junctions.

The transport properties of ultra-thin SrTiO(3) (STO) layers grown over YBa(2)Cu(3)O(7) electrodes were studied by conductive atomic force microscopy at the nano-scale. A very good control of the barrier thickness was achieved during the deposition process. A phenomenological approach was used to obtain critical parameters regarding the structural and electrical properties of the system. The STO layers present an energy barrier of 0.9 eV and an attenuation length of 0.23 nm, indicating very good insulating properties for the development of high-quality Josephson junctions.

Quantum coherence engineering in the integer quantum Hall regime.

We present an experiment where the quantum coherence in the edge states of the integer quantum Hall regime is tuned with a decoupling gate. The coherence length is determined by measuring the visibility of quantum interferences in a Mach-Zehnder interferometer as a function of temperature, in the quantum Hall regime at a filling factor 2. The temperature dependence of the coherence length can be varied by a factor of 2. The strengthening of the phase coherence at finite temperature is shown to arise from a reduction of the coupling between copropagating edge states. This opens the way for a strong improvement of the phase coherence of quantum Hall systems. The decoupling gate also allows us to investigate how interedge state coupling influences the quantum interferences' dependence on the injection bias. We find that the finite bias visibility can be decomposed into two contributions: a Gaussian envelope which is surprisingly insensitive to the coupling, and a beating component which, on the contrary, is strongly affected by the coupling.

Spin drift velocity, polarization, and current-driven domain-wall motion in (Ga,Mn)(As,P).

Current-driven domain-wall motion is studied in (Ga,Mn)(As,P) ferromagnetic semiconducting tracks with perpendicular anisotropy. A linear steady state flow regime is observed over a large temperature range of the ferromagnetic phase (0.1T(c)

Determination of the spin torque non-adiabaticity in perpendicularly magnetized nanowires.

Novel nanofabrication methods and the discovery of an efficient manipulation of local magnetization based on spin polarized currents has generated a tremendous interest in the field of spintronics. The search for materials allowing for fast domain wall dynamics requires fundamental research into the effects involved (Oersted fields, adiabatic and non-adiabatic spin torque, Joule heating) and possibilities for a quantitative comparison. Theoretical descriptions reveal a material and geometry dependence of the non-adiabaticity factor ß, which governs the domain wall velocity. Here, we present two independent approaches for determining ß: (i) measuring the dependence of the dwell times for which a domain wall stays in a metastable pinning state on the injected current and (ii) the current-field equivalence approach. The comparison of the deduced ß values highlights the problems of using one-dimensional models to describe two-dimensional dynamics and allows us to ascertain the reliability, robustness and limits of the approaches used.

Current induced domain wall motion in GaMnAs close to the Curie temperature.

Domain wall dynamics produced by spin transfer torques is investigated in (Ga, Mn)As ferromagnetic semiconducting tracks with perpendicular anisotropy, close to the Curie temperature. The domain wall velocities are found to follow a linear flow regime which only slightly varies with temperature. Using the Döring inequality, boundaries of the spin polarization of the current are deduced. A comparison with the predictions of the mean field k·p theory leads to an estimation of the carrier density whose value is compatible with results published in the literature. The spin polarization of the current and the magnetization of the magnetic atoms present similar temperature variations. This leads to a weak temperature dependence of the spin drift velocity and thus of the domain wall velocity. A combined study of field- and current-driven motion and deformation of magnetic domains reveals a motion of domain walls in the steady state regime without transition to the precessional regime. The ratio between the non-adiabatic torque ß and the Gilbert damping factor α is shown to remain close to unity.

Imprinting nanoporous alumina patterns into the magneto-transport of oxide superconductors.

We used oxygen ion irradiation to transfer the nanoscale pattern of a porous alumina mask into high-T(C) superconducting thin films. This causes a nanoscale spatial modulation of superconductivity and strongly affects the magneto-transport below T(C), which shows a series of periodic oscillations reminiscent of the Little-Parks effect in superconducting wire networks. This irradiation technique could be extended to other oxide materials in order to induce ordered nanoscale phase segregation.

Tuning decoherence with a voltage probe.

We present an experiment where we tune the decoherence in a quantum interferometer using one of the simplest objects available in the physics of quantum conductors: an Ohmic contact. For that purpose, we designed an electronic Mach-Zehnder interferometer which has one of its two arms connected to an Ohmic contact through a quantum point contact. At low temperature, we observe quantum interference patterns with a visibility up to 57%. Increasing the connection between one arm of the interferometer to the floating Ohmic contact, the voltage probe, reduces quantum interference as it probes the electron trajectory. This unique experimental realization of a voltage probe works as a trivial which-path detector whose efficiency can be simply tuned by a gate voltage.

Exploration of the ultimate patterning potential achievable with focused ion beams.

Decisive advances in the field of nanosciences and nanotechnologies are intimately related to the development of new instruments and of related writing schemes and methodologies. Therefore we have recently proposed the exploitation of the nano-structuring potential of a highly focused ion beam (FIB) as a tool, to overcome intrinsic limitations of current nano-fabrication techniques and to allow innovative patterning schemes that are urgently needed in many nanoscience challenges. In this work, we will first detail a very high-resolution FIB instrument we have developed specifically to meet these nano-fabrication requirements. Then we will introduce and illustrate an advanced FIB processing scheme that is the fabrication of artificial nanopores.

Nonadiabatic spin transfer torque in high anisotropy magnetic nanowires with narrow domain walls.

Current induced domain wall (DW) depinning of a narrow DW in out-of-plane magnetized (Pt/Co)_{3}/Pt multilayer elements is studied by magnetotransport. We find that for conventional measurements Joule heating effects conceal the real spin torque efficiency and so we use a measurement scheme at a constant sample temperature to unambiguously extract the spin torque contribution. From the variation of the depinning magnetic field with the current pulse amplitude we directly deduce the large nonadiabaticity factor in this material and we find that its amplitude is consistent with a momentum transfer mechanism.

Quantitative determination of the nonlinear pinning potential for a magnetic domain wall.

Using microwave currents, we excite resonances of geometrically confined pinned domain walls, detecting the resonance by the rectification of the microwave current. By applying magnetic fields, the resonance frequency of the domain wall oscillator can be tuned over a wide range. Increasing the power leads to a redshift due to the nonlinearity of the system. From this frequency shift, we directly deduce the quantitative shape of the potential, so that a complete characterization of the pinning potential is obtained.

Noise dephasing in edge states of the integer quantum Hall regime.

An electronic Mach-Zehnder interferometer is used in the integer quantum Hall regime at a filling factor 2 to study the dephasing of the interferences. This is found to be induced by the electrical noise existing in the edge states capacitively coupled to each other. Electrical shot noise created in one channel leads to phase randomization in the other, which destroys the interference pattern. These findings are extended to the dephasing induced by thermal noise instead of shot noise: it explains the underlying mechanism responsible for the finite temperature coherence time tau_{phi}(T) of the edge states at filling factor 2, measured in a recent experiment. Finally, we present here a theory of the dephasing based on Gaussian noise, which is found to be in excellent agreement with our experimental results.

Coupling efficiency for phase locking of a spin transfer nano-oscillator to a microwave current.

The phase locking behavior of spin transfer nano-oscillators (STNOs) to an external microwave signal is experimentally studied as a function of the STNO intrinsic parameters. We extract the coupling strength from our data using the derived phase dynamics of a forced STNO. The predicted trends on the coupling strength for phase locking as a function of intrinsic features of the oscillators, i.e., power, linewidth, agility in current, are central to optimize the emitted power in arrays of mutually coupled STNOs.

Direct measurement of the coherence length of edge states in the integer quantum Hall regime.

We have determined the finite temperature coherence length of edge states in the integer quantum Hall effect regime. This was realized by measuring the visibility of electronic Mach-Zehnder interferometers of different sizes, at filling factor 2. The visibility shows an exponential decay with the temperature. The characteristic temperature scale is found inversely proportional to the length of the interferometer arm, allowing one to define a coherence length l_(phi). The variations of l_(phi) with magnetic field are the same for all samples, with a maximum located at the upper end of the quantum Hall plateau. Our results provide the first accurate determination of l_(phi) in the quantum Hall regime.

Ion beam nanopatterning in graphite: characterization of single extended defects.

The morphology and the electronic structure of a single focused ion-beam-induced artificial extended defect is probed by several methods including micro-Raman spectroscopy, atomic force and scanning tunneling microscopies and Monte Carlo and/or semi-analytical simulation within standard codes. The efficiency of the artificial defect for deposited metallic cluster pinning is also investigated. We show a correlation between the ion dose, morphology, electronic structure and cluster trapping efficiency. At room temperature, cluster pinning is efficient when the displacement per atom is one or more. Well-ordered patterned cluster networks are considered for potential applications.

Detection of current-induced resonance of geometrically confined domain walls.

Magnetic domain walls are found to exhibit quasiparticle behavior when subjected to geometrical variations. Because of the spin torque effect such a quasiparticle in a potential well is excited by an ac current leading to a dip in the depinning field at resonance for current densities as low as 2 x 10(10) A/m2. Independently the resonance frequencies of transverse walls and vortex walls are determined from the dc voltage that develops due to a rectifying effect of the resonant domain wall oscillation. The dependence on the injected current density reveals a strongly nonharmonic oscillation.

Universal conductance fluctuations in epitaxial GaMnAs ferromagnets: dephasing by structural and spin disorder.

Mesoscopic transport measurements reveal a large effective phase coherence length in epitaxial GaMnAs ferromagnets, contrary to usual 3d-metal ferromagnets. Universal conductance fluctuations of single nanowires are compared for epilayers with a tailored anisotropy. At large magnetic fields, quantum interferences are due to structural disorder only, and an unusual behavior related to hole-induced ferromagnetism is evidenced, for both quantum interferences and decoherence. At small magnetic fields, phase coherence is shown to persist down to zero field, even in presence of magnons, and an additional spin disorder contribution to quantum interferences is observed under domain walls nucleation.

Temperature dependence of the spin torque effect in current-induced domain wall motion.

We present an experimental study of domain wall motion induced by current pulses as well as by conventional magnetic fields at temperatures between 2 and 300 K in a 110 nm wide and 34 nm thick Ni80Fe20 ring. We observe that, in contrast with field-induced domain wall motion, which is a thermally activated process, the critical current density for current-induced domain wall motion increases with increasing temperature, which implies a reduction of the spin torque efficiency. The effect of Joule heating due to the current pulses is measured and taken into account to obtain critical fields and current densities at constant sample temperatures. This allows for a comparison of our results with theory.

Direct observation of domain-wall configurations transformed by spin currents.

Direct observations of current-induced domain-wall propagation by spin-polarized scanning electron microscopy are reported. Current pulses move head-to-head as well as tail-to-tail walls in submicrometer Fe20Ni80 wires in the direction of the electron flow, and a decay of the wall velocity with the number of injected current pulses is observed. High-resolution images of the domain walls reveal that the wall spin structure is transformed from a vortex to a transverse configuration with subsequent pulse injections. The change in spin structure is directly correlated with the decay of the velocity.