The annealing of 1 MeV implantations of boron in silicon

Buried layers of boron in silicon have been made by 1 MeV implantations up to a dose of 1013 cm−2. The annealing of the implantation damage has been studied with Van der Pauw and Hall measurements. It is concluded that lattice damage reduces the mobility only for annealing temperatures below 600°C. The average mobilities measured after annealing at temperatures above 600°C correspond accurately to the values calculated from the most recent literature data, based on scattering by the lattice and by the active impurities. Complete activation was obtained after 60 min annealing at 700°C

Narrow channel Si-MOSFETs for electron transport studies

We have fabricated narrow channel Si-MOSFETs for electron transport studies at low temperature. The fabrication process combines optical lithography for large structures and high resolution e-beam lithography for narrow gates. The smallest working devices have a 0.14 μm wide gate. This paper reports the fabrication process and gives some examples of the quantum transport phenomena observed in these devices

Drug-Loaded IRONSperm clusters: modeling, wireless actuation, and ultrasound imaging

Individual biohybrid microrobots have the potential to perform biomedical in vivo tasks such as remote-controlled drug and cell delivery and minimally invasive surgery. This work demonstrates the formation of biohybrid sperm-templated clusters under the influence of an external magnetic field and essential functionalities for wireless actuation and drug delivery. Ferromagnetic nanoparticles are electrostatically assembled around dead sperm cells, and the resulting nanoparticle-coated cells are magnetically assembled into three-dimensional biohybrid clusters. The aim of this clustering is threefold: First, to enable rolling locomotion on a nearby solid boundary using a rotating magnetic field; second, to allow for noninvasive localization; third, to load the cells inside the cluster with drugs for targeted therapy. A magneto-hydrodynamic model captures the rotational response of the clusters in a viscous fluid, and predicts an upper bound for their step-out frequency, which is independent of their volume or aspect ratio. Below the step-out frequency, the rolling velocity of the clusters increases nonlinearly with their perimeter and actuation frequency. During rolling locomotion, the clusters are localized using ultrasound images at a relatively large distance, which makes these biohybrid clusters promising for deep-tissue applications. Finally, we show that the estimated drug load scales with the number of cells in the cluster and can be retained for more than 10 h. The aggregation of microrobots enables them to collectively roll in a predictable way in response to an external rotating magnetic field, and enhances ultrasound detectability and drug loading capacity compared to the individual microrobots. The favorable features of biohybrid microrobot clusters place emphasis on the importance of the investigation and development of collective microrobots and their potential for in vivo applications

Binary data corruption due to a Brownian agent

We introduce a model of binary data corruption induced by a Brownian agent (active random walker) on a d-dimensional lattice. A continuum formulation allows the exact calculation of several quantities related to the density of corrupted bits \rho; for example the mean of \rho, and the density-density correlation function. Excellent agreement is found with the results from numerical simulations. We also calculate the probability distribution of \rho in d=1, which is found to be log-normal, indicating that the system is governed by extreme fluctuations.Comment: 39 pages, 10 figures, RevTe

Macrospin approximation and quantum effects in models for magnetization reversal

The thermal activation of magnetization reversal in magnetic nanoparticles is controlled by the anisotropy-energy barrier. Using perturbation theory, exact diagonalization and stability analysis of the ferromagnetic spin-s Heisenberg model with coupling or single-site anisotropy, we study the effects of quantum fluctuations on the height of the energy barrier. Opposed to the classical case, there is no critical anisotropy strength discriminating between reversal via coherent rotation and via nucleation/domain-wall propagation. Quantum fluctuations are seen to lower the barrier depending on the anisotropy strength, dimensionality and system size and shape. In the weak-anisotropy limit, a macrospin model is shown to emerge as the effective low-energy theory where the microscopic spins are tightly aligned due to the ferromagnetic exchange. The calculation provides explicit expressions for the anisotropy parameter of the effective macrospin. We find a reduction of the anisotropy-energy barrier as compared to the classical high spin-s limit.Comment: 10 pages, 11 figure

Ferromagnetic Liquid Thin Films Under Applied Field

Theoretical calculations, computer simulations and experiments indicate the possible existence of a ferromagnetic liquid state, although definitive experimental evidence is lacking. Should such a state exist, demagnetization effects would force a nontrivial magnetization texture. Since liquid droplets are deformable, the droplet shape is coupled with the magnetization texture. In a thin-film geometry in zero applied field, the droplet has a circular shape and a rotating magnetization texture with a point vortex at the center. We calculate the elongation and magnetization texture of such ferromagnetic thin film liquid droplet confined between two parallel plates under a weak applied magnetic field. The vortex stretches into a domain wall and exchange forces break the reflection symmetry. This behavior contrasts qualitatively and quantitatively with the elongation of paramagnetic thin films.Comment: 10 pages, 4 figures, Submitted to Phys. Rev.

Dépendance entre l'épaisseur des couches magnétiques minces et le champ coercitif de déplacement des parois

Experimental measurements of the thickness dependence of the wall motion coercive force in thin magnetic films are not in agreement with the so-called 4/3 power law of Néel. It will be shown that this 4/3 power law of Néel is only valid for Bloch walls in thick films under the assumption that the magnitude of the thickness variations of the film is independent of the thickness. Using the same assumption one finds that the wall motion coercive force of Néel walls is independent of the thickness of the film.La relation expérimentale entre le champ coercitif et l'épaisseur des couches magnétiques minces n'est pas toujours en accord avec la théorie proposée par Néel. La discussion suivante montre que cette théorie n'est valable que pour les parois de Bloch et pour une fluctuation d'épaisseur indépendante de l'épaisseur elle-même. En présence de parois de Néel, on démontre que le champ coercitif ne dépend pas de l'épaisseur de la couche