Bernard Doudin

Bio: Bernard Doudin is an academic researcher from University of Strasbourg. The author has contributed to research in topics: Magnetoresistance & Magnetization. The author has an hindex of 36, co-authored 178 publications receiving 4969 citations. Previous affiliations of Bernard Doudin include École Normale Supérieure & University of Lausanne.

Giant magnetoresistance of nanowires of multilayers

Abstract: A new technique is required which enables tailoring of the morphology of a metallic nanostructured material down to the 10 nm length scale. Using nanoporous nuclear track etched membranes as templates for electrodeposition, an assembly of wires with diameters as low as 30 nm could be obtained. Alternating the electrodeposition of two metals resulted in multilayers grown perpendicular to the wire axis. Layer thicknesses as low as 2 nm could be reached. Application is demonstrated by making wires 6 μm long, 80 nm in diameter, having a succession of either Co and Cu layers or of (Ni,Fe) and Cu layers. Wires containing layers of 5–10 nm in thickness exhibited a giant magnetoresistance. The current was naturally perpendicular to the layers. At ambient temperature, a magnetoresistance of 14% for Co/Cu and of 10% for (Fe,Ni)/Cu was observed.

Nucleation of Magnetization Reversal in Individual Nanosized Nickel Wires.

Abstract: 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.

Conductivity in organic semiconductors hybridized with the vacuum field

Abstract: The surface plasmon modes of periodic hole arrays in Ag and Al films enhance by one order of magnitude the conductivity and the carrier mobility of organic semiconducting films deposited on these structures.

Magnetoelectronics with magnetoelectrics

Abstract: Magnetoelectric films are proposed as key components for spintronic applications. The net magnetic moment created by an electric field in a magnetoelectric thin film influences the magnetization state of a neighbouring ferromagnetic layer through exchange coupling. Pure electrical control of magnetic configurations of giant magnetoresistance spin valves and tunnelling magnetoresistance elements is therefore achievable. Estimates based on documented magnetoelectric tensor values show that exchange fields reaching 100 mT can be obtained. We propose a mechanism alternative to current-induced magnetization switching, providing access to a wide range of device impedance values and opening the possibility of simple logic functions.

Random and exchange anisotropy in consolidated nanostructured Fe and Ni: Role of grain size and trace oxides on the magnetic properties

Abstract: Results of magnetization measurements on nanocrystalline Fe and Ni produced by inert-gas condensation are presented. The grain size, which is about 10 to 20 nm in the as-prepared state, is increased by annealing the samples incrementally from 100 \ifmmode^\circ\else\textdegree\fi{}C to 1000 \ifmmode^\circ\else\textdegree\fi{}C. The coercive field shows a pronounced variation with grain size, with a maximum at around 30 nm and a steep decrease for smaller grain sizes. The coercivity is discussed on the basis of the random-anisotropy model that predicts that the effective anisotropy constant is reduced by averaging over magnetically coupled grains. This behavior is observed as long as the grain size is smaller than the effective bulk domain-wall width. The model also accounts for the approach to saturation in nanostructured Fe yielding values for the ferromagnetic correlation length and the anisotropy constant of the grains. The latter is about four times higher than the bulk value of Fe. Hysteresis measurements at 5 K after field cooling show a shift and broadening of the hysteresis loops for both Fe and Ni, which is attributed to an exchange coupling between the ferromagnetic grains and antiferromagnetic or ferrimagnetic (oxide) interfacial phases. The hysteresis shift decreases and finally vanishes with increasing grain size. This is indicative of a restructuring of the oxides, which is confirmed by the coercive field of the Fe samples showing a step at about 120 K caused by a phase transition of ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}.$ The step vanishes again with further increasing grain size. The saturation magnetization of the Ni samples increases with increasing annealing temperature, a fact that is attributed to the evolution of the oxides also.

Crystalline Ropes of Metallic Carbon Nanotubes

Abstract: The major part of this chapter has already appeared in [1], but because of the length restrictions (in Science), the discussion on why we think this form is given in only brief detail. This chapter goes into more depth to try to answer the questions of why the fullerenes form themselves. This is another example of the very special behavior of carbon. From a chemist’s standpoint, it is carbon’s ability to form multiple bonds that allows it to make these low dimensional forms rather than to produce tetrahedral forms. Carbon can readily accomplish this and it is in the mathematics and physics of the way this universe was put together, that carbon is given this property. One of the consequences of this property is that, if left to its own devices as carbon condenses from the vapor and if the temperature range is just right, above 1000°C, but lower than 1400°C, there is an efficient self-assembly process whose endpoint is C60.

Multiferroics: progress and prospects in thin films.

Abstract: Multiferroic materials, which show simultaneous ferroelectric and magnetic ordering, exhibit unusual physical properties — and in turn promise new device applications — as a result of the coupling between their dual order parameters. We review recent progress in the growth, characterization and understanding of thin-film multiferroics. The availability of high-quality thin-film multiferroics makes it easier to tailor their properties through epitaxial strain, atomic-level engineering of chemistry and interfacial coupling, and is a prerequisite for their incorporation into practical devices. We discuss novel device paradigms based on magnetoelectric coupling, and outline the key scientific challenges in the field.

Ferromagnetic materials

Abstract: In this chapter, we will restrict our attention to the ferrites and a few other closely related materials. The great interest in ferrites stems from their unique combination of a spontaneous magnetization and a high electrical resistivity. The observed magnetization results from the difference in the magnetizations of two non-equivalent sub-lattices of the magnetic ions in the crystal structure. Materials of this type should strictly be designated as “ferrimagnetic” and in some respects are more closely related to anti-ferromagnetic substances than they are to ferromagnetics in which the magnetization results from the parallel alignment of all the magnetic moments present. We shall not adhere to this special nomenclature except to emphasize effects, which are due to the existence of the sub-lattices.