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Aharonov-Bohm oscillations have been observed in a lattice formed by a two dimensional rhombus tiling. This observation is in good agreement with a recent theoretical calculation of the energy spectrum of this so-called T3 lattice. We have investigated the low temperature magnetotransport of the T3 lattice realized in the GaAlAs/GaAs system. Using an additional electrostatic gate, we have studied the influence of the channel number on the oscillations amplitude. Finally, the role of the disorder on the strength of the localization is theoretically discussed., Comment: 6 pages, 11 EPS figures

We report measurements of the low temperature magnetic response of a line of 16 GaAs/GaAlAs connected mesoscopic rings whose total length is much larger than l(straight phi). Using an on-chip micro-SQUID technology, we have measured a periodic response, with period h/e, corresponding to persistent currents in the rings of a typical amplitude of 0.40+/-0.08 nA per ring. Direct comparison with measurements on the same rings but isolated is presented.

Shot noise refers to the fluctuations in electrical current through a device arising from the discrete nature of the charge-carrying particles. Recent experiments have exploited the fact that the shot noise is proportional to the charge of the carriers to establish fractional quantization of quasiparticles in the fractional quantum Hall effect1,2. By a similar argument, it is expected that when a superconducting reservoir emits Cooper pairs, (which have a charge twice that of an electron) into a short normal-metal wire, the shot noise should be double that obtained for a normal-metal reservoir3,4. Although the charge of Cooper pairs has been well established by flux quantization and tunnel experiments5, doubling of their shot noise has not yet been observed. Here we report a shot-noise experiment using a short diffusive normal-metal superconductor contact, in which we confirm the predicted noise behaviour for double charges. The measurements, taken over a large range of bias current, establish that phase coherence is not required to observe the effect.

Various single particle measuring techniques are briefly reviewed and the basic concepts of a new micro-SQUID technique are discussed. It allows measurements of the magnetization reversal of single nanometer-sized particles at low temperature. The influence of the measuring technique on the system of interest is discussed., 3 pages, 3 figures, conference proceedings of MMM 1999, San Jose, 15-18 Nov., session number BE-01

Recent progress in experiment on quantum tunnelling of the magnetic moment in mesoscopic systems will be reviewed. The emphasis will be made on measurements of individual nanoparticles. These nanomagnets allow one to test the border between classical and quantum behaviour. Using the micro-SQUID magnetometer, waiting time, switching field and telegraph noise measurements show unambiguously that the magnetisation reversal of small enough single crystalline nanoparticles is described by a model of thermal activation over a single-energy barrier. Results on insulating BaFeO nanoparticles show strong deviations from this model below 0.4 K which agree with the theory of macroscopic quantum tunnelling in the low dissipation regime., 6 pages, 2 figures, conference proceedings of LT22-Helsinki

International audience; The first three-dimensional switching field measurements of an individual nanometer-sized magnetic particle are presented. The angular dependence of the switching field can be described by the model of uniform rotation of magnetization. However, the anisotropy function proposed by Stoner and Wohlfarth appears to be simplistic. In order to reproduce the measured switching fields, one has to take into account higher order anisotropy terms. These terms are very important to understand dynamical magnetization reversal properties in the classical and the quantum regimes.

In order to investigate the effect of electronic phase coherence on screening we have measured the flux dependent polarizability of isolated mesoscopic rings at 350 MHz. At low temperature (below 100 mK) both non-dissipative and dissipative parts of the polarizability exhibit flux oscillations with a period of half a flux quantum in a ring. The sign and amplitude of the effect are in good agreement with recent theoretical predictions. The observed positive magneto-polarizability corresponds to an enhancement of screening when time reversal symmetry is broken. The effect of electronic density and temperature are also measured., Comment: 4 pages, revtex, 4 figures, to appear in Phys. Rev. Lett

We have detected oscillations of the charge around a potential hill (antidot) in a two-dimensional electron gas as a function of a perpendicular magnetic field B. The field confines electrons around the antidot in closed orbits, the areas of which are quantised through the Aharonov-Bohm effect. Increasing B reduces each state's area, pushing electrons closer to the centre, until enough charge builds up for an electron to tunnel out. This is a new form of the Coulomb blockade seen in electrostatically confined dots. We have also studied h/2e oscillations and found evidence for coupling of opposite spin states of the lowest Landau level., 3 pages, 3 Postscript figures, submitted to the proceedings of EP2DS-13

In a recent paper [B.A. Piot, et al., Phys. Rev. (B) 72 (2005) 245325], we have shown that the lifting of the electron spin degeneracy in the integer quantum Hall effect at high filling factors should be interpreted as a magnetic-field-induced Stoner transition. Here, we show that within this framework, the transition to the polarized state with increasing magnetic field allows a determination of the Landau level shape.

We have detected oscillations of the charge around a potential hill (antidot) in a two-dimensional electron gas as a function of a large magnetic field B. The field confines electrons around the antidot in closed orbits, the areas of which are quantised through the Aharonov-Bohm effect. Increasing B reduces each state's area, pushing electrons closer to the centre, until enough charge builds up for an electron to tunnel out. This is a new form of the Coulomb blockade seen in electrostatically confined dots. Addition and excitation spectra in DC bias confirm the Coulomb blockade of tunnelling., Comment: 4 pages, 4 Postscript figures

We present an experimental study of the transport properties of a ferromagnetic metallic wire (Co) in metallic contact with a superconductor (Al). As the temperature is decreased below the Al superconducting transition, the Co resistance exhibits a significant dependence on both temperature and voltage. The differential resistance data show that the decay length for the proximity effect is much larger than we would simply expect from the exchange field of the ferromagnet., 4 pages, 6 included epsf figures, published version with small changes

We have measured the electromagnetic response of micron-size isolated mesoscopic GaAs/GaAlAs square dots down to temperature T=16mK, by coupling them to an electromagnetic micro-resonator. Both dissipative and non dissipative responses exhibit a large magnetic field dependent quantum correction, with a characteristic flux scale which corresponds to a flux quantum in a dot. The real (dissipative) magnetoconductance changes sign as a function of frequency for low enough density of electrons. The signal observed at frequency below the mean level spacing corresponds to a negative magnetoconductance, which is opposite to the weak localization seen in connected systems, and becomes positive at higher frequency. We propose an interpretation of this phenomenon in relation to fundamental properties of energy level spacing statistics in the dots., 4 pages, 4 eps figures

Presented are switching field measurements of individual ferrimagnetic and insulating BaFeCoTiO nanoparticles containing about ${10}^{5}{\ensuremath{\mu}}_{B}$ at very low temperatures (0.1--6 K). For temperatures higher than 0.4 K, the quantitative agreement with the N\'eel-Brown theory of thermal activated magnetization reversal allowed us to identify unambiguously the magnetization reversal of uniform rotation. Below 0.4 K, strong deviations from this model are evidenced which are quantitatively in agreement with the predictions of the theory of macroscopic quantum tunneling of magnetization in the low dissipation regime.

International audience; Presented are the first magnetization measurements of individual ferromagnetic nanoparticles (15–30 nm) at very low temperatures (0.1–6 K). The angular dependence of the hysteresis loop evidenced the single domain character of the particles. Waiting time, switching field, and telegraph noise measurements showed for the first time that the magnetization reversal of a well prepared ferromagnetic nanoparticle can be described by thermal activation over a single-energy barrier as originally proposed by Néel and Brown. The “activation volume” estimated by these measurements was close to the particle volume.

We observe an inversion of the low temperature dependence for the conductivity of doped GaAs by application of a magnetic field. This inversion happens when Wct = 1, as predicted by Houghton (PRB25, 2196, 1982) for the correction to conductivity due to screened Coulomb repulsion in the diffusive regime. This correction follows the oscillating behavior of the transport elastic time entering the Shubnikov-de Haas regime. For Wct > 1, we observe that the Hartree part of the interaction correction is suppressed. Moreover, the total correction seems strongly reduced although its dependence stays logarithmic., 4 pages, 4 figures

International audience; GdFe (1000 Å)/TbFe (e)/GdFe (500 Å) sandwiches, where the thickness e of the TbFe film can be as small as 2 Å, constitutes a Domain Wall Junction (DWJ). In this device, a 180° domain wall, nucleated in the thicker GdFe layer, crosses an artificial energy barrier (the TbFe layer), by thermal activation (above 1 K). DWJs constitute a model system for the study of nucleation, pinning and depinning of a domain wall, in both thermal activation and quantum regimes. In the present case, quantum tunnelling of magnetisation is suggested below 0.7 K.

Low temperature magnetization measurements of individual ferromagnetic particles and wires are presented (0.1 < T(K) < 6). The detector was a Nb micro-bridge-DC-SQUID, fabricated using electron-beam lithography. The angular dependence of the switching field could be explained approximatively by simple classical micromagnetic concepts (uniform rotation, curling…). However, dynamical measurements evidenced nucleation and propagation of domain walls, except for the smallest particles of about 20 nm. The variation of the mean switching field distribution (as a function of temperature and field sweeping rate) and of the probabilities of switching (as a function of temperature and the applied field) allowed to study in details the dynamics of magnetization reversal of individual particles. For sub-micron particles, we found that above a crossover temperature of 1K, the mean switching field and the switching probability follow a thermally activated model. For temperatures below IK, the dynamics of magnetization reversal becomes temperature independent which is interpreted in terms of deviations from the Néel-Brown model of magnetization reversal due to surface roughness and oxidazation. Although this crossovei temperature is much too large to be interpreted with current models of quantum tunneling, such an effect cannot be excluded. Measurements performed on ferromagnetic nanoparticles of good quality (single crystalline and with a diameter smaller than 25 nm), allowed us to show for the first time that the magnetization reversal can be described by thermal activation over the anisotropy energy barrier, as originally proposed by Néel. The observation of telegraph noise strengthens these results. Our measurements open the door to the observation of macroscopic quantum tunneling oí the magnetization in an individual particle containing 103-105 spins.

We present the first magnetization measurements of individual ferromagnetic particles and wires at very low temperatures. The angular dependence of the magnetization reversal could be explained by simple classical micromagnetic concepts: uniform rotation and curling. Probabilities of switching and switching field distributions showed for the first time that the magnetization reversal of a well defined nanoparticle can be described by thermal activation over a single-energy barrier as originally proposed by Neel and Brown. Furthermore, the influence of sample defects on the magnetization reversal is discussed.

The mechanisms of magnetization reversal in small magnetic particles have been much discussed in the last decades and prompted intense research activities, motivated in particular by applications in magnetic recording technology [1]. However, experiments were performed, in general, on large assemblies of particles, and the dispersion of morphologies, compositions, orientations, and separations of the magnetic entities limited the interpretation of the results. Furthermore, interactions between particles were difficult to take into account. Single particle studies were possible only in few cases [2]. Recently, insights into the magnetic properties of individual and isolated particles were obtained with the help of near field magnetic force microscopy [3], electron Lorentz microscopy or holography [4], and micro-SQUID (superconducting quantum interference device) magnetometry [5]. It is now possible to make a clear link between experiments performed on nanometer-sized single objects (particles, wires, etc.) and the numerical calculations based on the Brown micromagnetic equations [6]. We report the first study of isolated nanoscale wires with diameters smaller than 100 nm, for which singledomain states could be expected. The cylindrical geometry, with its large shape anisotropy, is well suited for comparison with theory. We obtained unique insight into the process of magnetization reversal by measuring histograms of the switching field as a function of the orientation of the wires in the applied field, their diameter, and the temperature. Furthermore, we measured the probability of switching as a function of the applied field and the temperature. Our studies reveal that the magnetization reversal proceeds by a distortion of the magnetization followed by a nucleation and a propagation process. The observed behavior illustrates the fundamental importance of the study of single, isolated magnetic particles in comparing models and experiments. We developed planar microbridge dc SQUID [7], made of Nb (thickness 20 nm), which were shown to be able to detect 10 4 mB [8]. The SQUID is made of a thin (20 nm) Nb layer in order to prevent flux trapping. The experimental setup allows measurements of hysteresis loops in magnetic fields of up to 0.5 T and temperatures below 6 K, with a time resolution of 100 ms. Ni wires were produced by electrochemically filling the pores of commercially available nanoporous tracketched polycarbonate membranes of thicknesses of 6 to 10 mm [9]. The pore size was chosen in the range of 30 to 100 nm [10]. In order to place one wire on the SQUID detector, we dissolved the membrane in chloroform and put a drop on a chip of some hundreds of SQUID’s. Magnetization measurements were performed on SQUID’s with a single isolated wire. Scanning electron microscopy (SEM) (Fig. 1) was used to determine the position and morphology of the wire. The surface roughness was around 5 nm, corresponding to our SEM resolution.

We present magnetization measurements of individual amorphous Co particles (300 nm\ifmmode\times\else\texttimes\fi{}200 nm\ifmmode\times\else\texttimes\fi{}30 nm) patterned by electron-beam lithography. The hysteresis loops show mainly two magnetization jumps corresponding to domain-wall nucleation and annihilation. The angular dependence of these magnetization jumps is measured and the temperature dependence (0.1--6 K) of the nucleation and the annihilation processes is investigated by two independent methods: switching field and switching time measurements. Finally we compare the results obtained on one particle with those of an array of about 2\ifmmode\times\else\texttimes\fi{}${10}^{7}$ particles. We show experimentally that the dynamical properties measured on individual particles are connected with an unusual 1/T relaxation of the array of particles. \textcopyright{} 1996 The American Physical Society.

We identify the different contributions to quantum interference in a mesoscopic metallic loop in contact with two superconducting electrodes. At low temperature, a flux-modulated Josephson coupling is observed with strong damping over the thermal length L_{T}. At higher temperature, the magnetoresistance exhibits large h/2e-periodic oscillations with 1/T power law decay. This flux-sensitive contribution arises from coherence of low-energy quasiparticles states over the phase-breaking length L_{\phi}. Mesoscopic fluctuations contribute as a small h/e oscillation, resolved only in the purely normal state., Comment: 4 pages, 3 figures

We present an experimental study of the diffusive transport in a normal metal near a superconducting interface, showing the re-entrance of the metallic conductance at very low temperature. This new mesoscopic regime comes in when the thermal coherence length of the electron pairs exceeds the sample size. This re-entrance is suppressed by a bias voltage given by the Thouless energy and can be strongly enhanced by an Aharonov Bohm flux. Experimental results are well described by the linearized quasiclassical theory., Comment: improved version submitted to Phys. Rev. lett., 4 pages, 5 included epsf figures

We describe general considerations important for the choice of the material and the sensors for persistent current measurements. We show the interest of using a combination of a microscopic SQUID device on top of a loop made of a two-dimensional electron gas. We present our results obtained and present the developments which we undertake to increase the performances, in the aim to explore a wider parameter range. We mention other applications of micro-SQUID sensors.

In this paper we present a general picture of the cyclic change in resonant structure seen in conductance across quantum Hall transitions in quantum antidots. The cycle begins with resonant dips below the ${\mathit{ne}}^{2}$/h plateau, which evolve into resonant peaks above it. A sinusoidal oscillation, of shorter period in magnetic field, then appears, which evolves into resonant dips below the (n+1)${\mathit{e}}^{2}$/h plateau. The cycle repeats for successive transitions. We provide an explanation for this behavior in terms of the edge-state model and show by numerical calculation that this picture accounts in detail for the line shapes and dependencies observed experimentally.

Mesoscopic metallic rings present a spectacular thermodynamic property : they carry a persistent non dissipative current when they are threaded by a magnetic flux [1][2][3]. The existence of such a persistent current is a consequence of the coherence of the electronic wave functions along the ring, but unlike a superconductor, a mesoscopic ring presents a finite Ohmic conductivity when it is connected to a voltage source. The average value of this conductivity is given by the Drude formula which reads, in the zero frequency limit, σ0 = ne 2 T e /m, n is the volume concentration of electrons and T e the elastic scattering time. Inelastic processes do not appear in this formula, however the sample is implicitly coupled to a thermodynamic reservoir, realized for instance by the macroscopic measuring leads, where the dissipation takes place. Thus the mechanism of dissipation is not intrinsic to the sample.

International audience; The magnetoconductance of insulating amorphous alloys is investigated at very low temperature. In bulk 3D samples we observe a crossover at T=100mK from a high temperature Mott hopping regime to an activated regime. The latter regime extends up to higher temperature in thin films, and the I-V curves indicate the existence of single electron charging effects. The magnetic field reinforces the electronic localization in these insulators with strong spin-orbit scattering.

We have processed very small high electron mobility transistor (HEMT) on the two dimensional electron gas of a GaAs/GaInAs heterostructure. The contacts (down to 2 μm wide) connect channels of different lengths and widths. We measure the saturation I(V) curves and obtain the source drain voltage at saturation. Experiments demonstrate a universal linear dependence of the saturation voltage drop with the length of the channel, whatever its width. Our experimental result is understood with basic equations of the 2D channel, taking into account the contact resistivity. The universal law Usat(L) gives a nice way to measure easily and precisely the resistivity of contacts in ultrasmall devices when their width cannot be precisely known, like in narrow two dimensional electron gas.

The microwave power density absorbed in the normal state by continuously irradiated superconducting Nb and NbN nanostrips is extracted from their electrical transport properties. The procedure is based on the reduction of the retrapping current (i.e., the minimum applied current required to sustain a dissipative region inside a superconducting microbridge) that results from the microwave dissipation. The power effectively absorbed by the nanostrips varies linearly with the input power level and falls in the μWμm–3 range. At a given microwave frequency, the relation between the input power and the absorbed power does not depend on the coolant temperature. In addition, the upper limit of the detection range is given by the heat removal capabilities through the substrate. When the absorbed microwave power exceeds this temperature-dependent heat removal threshold, normal hotspot domains are stabilized down to zero bias current. The determination of the microwave power absorbed by superconducting samples in the normal state may be of practical interest in the field of incident power detection.

We have observed reproducible conductance fluctuations at low temperature in a small GaAs:Si wire driven across the Anderson transition by the application of a gate voltage. We analyse quantitatively the log-normal conductance statistics in terms of truncated quantum fluctuations. Quantum fluctuations due to small changes of the electron energy (controlled by the gate voltage) cannot develop fully due to identified geometrical fluctuations of the resistor network describing the hopping through the sample. The evolution of the fluctuations versus electron energy and magnetic field shows that the fluctuations are non-ergodic, except in the critical insulating region of the Anderson transition, where the localization length is larger than the distance between Si impurities. The mean magnetoconductance is in good accordance with simulations based on the Forward-Directed-Paths analysis, i.e. it saturates to $ {\rm ln} (\sigma (H>1)/\sigma (0))\simeq 1, $ as $ \sigma (0) $ decreases over orders of magnitude in the strongly localized regime., Comment: Email contact: sanquer@amoco.saclay.cea.fr

We have detected the presence of persistent currents of about 4 nA in a mesoscopic single loop etched in a GaAs-GaAlAs heterojunction. For this purpose a dedicated device with an integrated DC-SQUID has been developed.

We report on a study of the spatio-temporal variation of magnetic induction in a superconducting niobium sample during a slow sweep of external magnetic field. A sizable fraction of the increase in the local vortex population occurs in abrupt jumps. We compare the size distribution of these avalanches with the predictions of self-organized-criticality models for vortex dynamics.

Electrical transport measurements were performed on NbN multicontact microstrips in order to investigate the non-equilibrium dissipative states in highly disordered granular superconducting nanostructures. By applying a dc voltage-bias, a peculiar discontinuous normal phase propagation was observed in a current interval well below the switching point. These stabilized resistive states are explained within the framework of a localized normal hotspot maintained by Joule heating. The thermal healing length of our NbN microstrips is estimated by taking into account both kinds of heat transfer (i.e. by conduction within the microstrip and by surface heat transfer through the substrate). It appears that the estimated healing length properly matches the length associated with a normal region across the whole width and whose resistance is given by the difference of resistance between two nearest intermediate dissipative states as observed in the voltage-biased curves.

We have realized reliable DC-SQUIDs in aluminium with two Dayem bridges in only one lithographic operation. These bridges appear to behave like classic Josephson junctions even at low temperature far from Tc. They are very durable and reproducible upon temperature cycling. They exhibit a considerable hysteresis and critical currents as low as 10 μA. An electronic setup has been developed in order to measure the critical current of these hysteretic SQUIDs with a typical noise of 8 10−6 φ0/Hz1/2. This non-destructive and damage free technique allows one to perform magnetization measurements on any material.

The Landau Zener model has recently been used to measure very small tunnel splittings in molecular clusters of Fe8, which at low temperature behaves like a nanomagnet with a spin ground state of S = 10. The observed oscillations of the tunnel splittings as a function of the magnetic field applied along the hard anisotropy axis are due to topological quantum interference of two tunnel paths of opposite windings. Transitions between quantum numbers M = -S and (S - n), with n even or odd, revealed a parity effect which is analogous to the suppression of tunnelling predicted for half integer spins. This observation is the first direct evidence of the topological part of the quantum spin phase (Berry or Haldane phase) in a magnetic system. We show here that the quantum interference can also be measured by ac susceptibility measurements in the thermal activated regime., Comment: 3 pages, 2 figures, conference proceedings of LT22 (Helsinki, Finland, August 4-11, 1999

We show in this article the first full three dimensional measurements of switching fields of a nanometer-scale magnetic particle. The field direction dependence of the switching fields can be successfully described by the uniform rotation model. Nevertheless, we could see that the Stoner–Wohlfarth form for the anisotropy function, which is often assumed together with the uniform rotation model, is an oversimplification. In order to reproduce the measured fields, one has to take into account higher order anisotropy terms. The complete form of the anisotropy function seems to be accessible only by magnetic measurements on each individual nanoparticle.

Electrical transport measurements were performed on superconducting NbN multicontact microstrips in order to investigate non-equilibrium dissipative states in type-II nanostructures. Current-biased and voltage-biased approaches point out the ability to stabilize a normal phase that grows discontinuously, in opposition to the usual self-spreading hotspot propagation. Results obtained by applying a dc voltage to two segments in series support this idea of a peculiar hotspot expansion. This last voltage-biasing configuration, which involves five contacts, could be used in the future to distinguish more easily between hotspot discontinuous growth and phase-slippage mechanisms.

In this report we present a procedure to fabricate high transmissive superconducting (S) contacts by diffusing indium into GaAs/AlxGa1−xAs based high-mobility two-dimensional electron gas (2DEG). We use a thermostable trilayer resist for indium lift-off. By doing electrical measurements at low temperature and transmission electronic microscopy observations, we show that the current through the S/2DEG interface is carried by a few number of localized indium filaments providing a good contact with the 2DEG. The low dimensional nature of those filaments is characterized by phase slip centers in the I(V) curve of a shunted S/2DEG/S junction.

The Hall resistance of small Hall sensors and its nonlinear dependence on magnetic field B were investigated at the temperature of 4.2 K. The sensors were four-terminal crosses fabricated by etching AlGaAs/InGaAs/GaAs delta-doped heterostructures. While large sensors exhibit good linearity of the Hall voltage on magnetic field, in sensors smaller than 5 mu m we detected pronounced nonlinearities for magnetic fields between 0 and 2 T. We attribute the latter to ballistic corrections to the classical Hall effect, and we model the Hall and the bend resistances at low temperature using Monte Carlo simulation. We also carried out temperature studies of the Hall sensors. We show that the nonlinearities persist up to room temperature. The effect of nonlinearities on the performance of Hall sensors is discussed. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3103303]

In the last ten years the progresses of microtechnology has opened the field of meso-scopic systems, where quantum information is not lost at low temperature during the travel of electrons through the sample. Beside the weak localization (WL) phenomena, that has become the appropriate tool to determine the scale of inelastic processes in macroscopic systems, mesoscopic samples exhibit a very unusual phenomena: the complex interference pattern resulting from the diffusive motion of electrons results in a very complicated but reproducible magnetoconductance curve (MC curve), which is sensitive to any microscopic disorder configuration change. The MC curve contains so a very large amount of informations but it obeys to universal statistical rules, as predicted by Universal Conductance Fluctuations (UCF) theory.

The existence of persistent currents in a mesoscopic ring in presence of disorder was suggested by Biittiker, Imry and Landauer [1]. However a complete theoretical approach, easy in the case of one-dimensional (ID) system [2], is very difficult to extend to a real three-dimensional (3D) system with disorder and at finite temperature [3]. The direct observation of such currents would give very important informations for the understanding of such mesoscopic systems.

Magnetic imaging at the micrometer scale with high sensitivity is a challenge difficult to be met. Magnetic force microscopy has a very high spatial resolution but is limited in magnetic resolution. Hall probe microscopy is very powerful but sensor fabrication at the one micron scale is difficult and effects due to discreteness of charge appear in the form of significant 1/f noise. SQUID microscopy is very powerful, having high magnetic resolution, but spatial resolution is usually of the order of 10 μm. The difficulties lay mostly in an efficient way to couple flux to the sensor. The only way to improve spatial resolution is to place the probe close to the very edge of the support, thus maximising coupling and spatial resolution. If there has been found a way to bring close the tip, there must be also found a reliable a way to maintain distance during scanning. We want to present recent improvements on scanning microsquid microscopy: Namely the improved fabrication of microSQUID tips using silicon micro machining and the precise positioning of the micrometer diameter microSQUID loop by electron beam lithography. The microSQUID is a microbridge DC SQUID, with two opposite microbridges. The constrictions are patterned by high-resolution e-beam lithography and have a width of 20 nm and a length of about 100 nm. The distance control during scanning is obtained by integrating the microSQUID sensor with a piezoelectric tuning fork acting as a force sensor allowing to control height and even topographic imaging. The detector is placed in a custom built near field microscope and the sample temperature can be varied between 0.1 Kelvin and 10 K. The microscope is used to study magnetic flux structures in unconventional superconductors and will be used to observe thermal domains in superconducting detectors in the voltage state.

Nanofabrication at the “Groupement Scientifique CNET-CNRS” in Bagneux has been performed using either a JEOL 5D2U e-beam pattern generator, or a modified Philips 515 scanning electron microscope. In this paper we present some of the results we obtained on the transport and optical properties of 1D an 0D structures in III–V semiconductors.

We aim to explore the nanostructuring potential of a highly focused pencil of ions. We show that focused ion beam technology (FIB) is capable of overcoming some basic limitations of current nanofabrication techniques and allowing innovative patterning schemes for nanoscience. In this work, we first detail the very high resolution FIB instrument developed specifically to meet nanofabrication requirements. Then we introduce and illustrate some new patterning schemes for next-generation FIB processing. These patterning schemes are: 1. nanoengraving of membranes as a template for nanopores and nanomask fabrication; 2. local defect injection for magnetic thin film direct patterning; 3. function of graphite substrates to prepare 2-D organized arrays of clusters; and 5. selective epitaxy of III-V semiconductors on FIB patterned surfaces. Finally, we show that FIB patterning allows "bottom-up" or "organization" processes.

We report on the first observation of the re-entrance of the metallic resistance in a mesoscopic proximity superconductor. This occurs at low temperature when the coherence length of electron pairs with a thermal energy is larger than the sample size. This regime breaks down when the voltage is increased above the Thouless energy. We find a qualitative agreement between experimental results and the quasiclassical theory including the Usadel equations.

Presented are magnetization measurements of single submicronic particles (500–50 nm) at very low temperature (0.1–6 K). We are studying the mean switching field and the switching probability as a function of the field sweeping rate and temperature. For temperatures down to 1 K we observe an increase of the mean switching field. Below, the mean switching field becomes temperature independent. The results are compared with models of thermally activated switching and macroscopic quantum tunneling.

We performed measurements of the magnetic response of a single mesoscopic ring made of a GaAlAs/GaAs heterojunction (2 DEG). We detected a magnetic signal, with a period h/e, which is an evidence, of persistent currents. The amplitude and the sign of this signal seem to vary in time despite a stable amplitude and phase of the Aharonov-Bohm conductance oscillations. We discuss the improvements necessary to determine the nature of these variations.

We present the first magnetization measurements of single submicron (Co, CoZr, Ni) particles. As a detector we use a microbridge-dc-SQUID deposited onto or next to the sample. Sample and detector are patterned by electron-beam lithography. The dynamics and the temperature dependence of the magnetization reversal are studied.

The low-energy cluster beam deposition (LECBD) technique is used to deposit gold nanoclusters preformed in the gas phase on functionalized graphite substrates (highly oriented pyrolitic graphite (HOPG)), to prepare 2D- organized arrays of cluster assembled dots. Functionalized HOPG substrates are obtained using the focused ion beam (FIB) nanoengraving technique to pattern 2D-organized arrays of defects (nanoholes, nanobumps) which act as traps for the diffusing clusters. Depending on the deposition conditions (nature, size and fluence of the deposited clusters) and the functionalized substrates (nature and size of the FIB-induced defects, geometry of the 2D array of defects and temperature during deposition) high-quality quantum-dot arrays can be obtained with well controlled and reproducible morphologies. Kinetic Monte Carlo simulations of the cluster deposition experiments on functionalized substrates allow us to obtain quite good fits of the experimental images performed by tapping mode atomic force microscopy (TMAFM), leading to systematic investigations of the best conditions to realize high-quality quantum dots systems. This combined top-down-bottom-up approach (LECBD-FIB) seems a promising method for preparing high-integration-density devices (∼Tbit cm −2 ) well suited for future applications to data storage, nanoelectronics, nano-optics, nanomagnetic systems.

We study the temperature dependence of the magnetization switching fields of single nanoparticles with an improved micro-superconducting quantum interference device (SQUID) technique, that allows us to perform measurements independently of the critical superconducting temperature of the SQUID. A simplified Neel–Brown model that describes the magnetization reversal by thermal activation over a single anisotropy barrier is compared to measurements on a 3 nm cobalt nanoparticle embedded in a niobium matrix.