Structural and magnetic properties of FIB-irradiated microstructures on a Pt/Co multilayer

We present a study of the structural and magnetic effects induced by focused 30 keV Ga⁺ ion beam irradiation on a Pt(2.8 nm)/[Pt(0.6 nm)/Co(0.3 nm)]₆/Pt(6.5 nm) ferromagnetic multilayer, using transmission electron microscopy, optical and magneto-optical microscopy. The work is of relevance to high-density data-storage applications, where media planarity is crucial for device reliability.     

Magnetic interactions and Preisach distributions of nanostructured barium hexaferrite

Barium hexaferrite (phase M) samples with different nanostructures were studied. Sample M1 is composed of nanocrystals of BaFe₁₂O₁₉ produced after milling the elemental oxides (Fe₂O₃ and BaCO₃) and heating in air atmosphere. Two more samples were produced by milling the same oxides and a 20% excess of -Fe. The resulting powder (composed of phase M and a 20% hematite) was heat-treated in different conditions, resulting in samples MF1 (with a partially sintered structure) and MF8 (with almost completely sintered structure).

In order to have an insight into the interactions in each sample, Preisach distributions were obtained using first-order reversion curves (FORCs) measurements. The Preisach distribution corresponding to M1 is a Gaussian-shaped one, with a maximum around 4.1 kOe. The distribution of MF1 has a narrow and high peak at 5.3 kOe, a number of overlapping small peaks down to 2.6 kOe and a distinct and low-intensity peak at 2 kOe. MF8 has a Preisach distribution with a succession of equally spaced distinct peaks from 4.2 to 1.5 kOe.

The found Preisach distributions suggest that the interactions occur among nanocrystals inside conglomerates with different number of particles.     

Probing buried interfaces with non-linear optical spectroscopy

The importance of buried interfaces in our everyday lives and in current scientific research is highlighted, along with experimental difficulty associated with studying such systems. We present an overview of the application of second harmonic generation and sum-frequency spectroscopy to the study of buried interfaces. Several examples from the current literature are presented, ranging from chemical and biological, to electrical and magnetic interfaces. The importance of this work in the context of ongoing research in these areas is discussed. Finally, we provide a snapshot of the state of the art in non-linear optical spectroscopy by mentioning several new directions that are likely to have a large impact on future research into the physics and chemistry of buried interfaces.     

Periodic structures and scaling Laws in a magnetic colloid

Summary Phase separation was found in an eicosane-based ferrofluid in zero applied field. Upon applying a magnetic field, the condensed phase breaks up to form a lattice of periodic columns. The periodicity λ scales with the layer thicknessL. Two scaling regimes were found. When thickness is small, where no branching is observed, λ ∞L ^1/2. In the large-thickness regime, a tree type of structure develops at the tail of each column, and the columns are separated by side bands while the scaling relation crosses over to λ ∞L ^2/3. Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4–8, 1994. this work is in collaboration with Hao Wang, Yun Zhu, C. Boyd, A. Cebers and R. E. Rosensweig.     

A general 3-D nonlinear magnetostrictive constitutive model for soft ferromagnetic materials

In this paper, a new general nonlinear magnetostrictive constitutive model is proposed for soft ferromagnetic materials, and it can predict magnetostrictive strain and magnetization curves under various pre-stresses. From the viewpoint of magnetic domain, it is based on the important physical fact that a nonlinear part of the elastic strain produced by magnetic domain wall motion under a pre-stress is responsible for the change of the maximum magnetostrictive strain in accordance with the pre-stress. Then the reduction of magnetostrictive strain from the maximum is caused by the domain rotation. Meanwhile, the magnetization under various pre-stresses in this model is introduced by magnetostrictive effect under the same pre-stress. A simplified 3-D model is put forward by means of linearizing the nonlinear function, i.e. the nonlinear part of the elastic strain produced by domain wall motion, and by using the quartic of magnetization to describe domain rotation. Besides, for the convenience of engineering applications, two-dimensional (plate or film) and one-dimensional (rod) models are also given for isotropic materials and their application ranges are discussed too. In comparison with the experimental data of Kuruzar and Jiles, it is found that this model can predict magnetostrictive strain and magnetization curves under various pre-stresses. The numerical simulation further illustrates that the new model can effectively describe the effects of the pre-stress or residual stress on the magnetization and magnetostrictive strain curves. Additionally, this model can be degenerated to the existing magnetostrictive constitutive model for giant magnetostrictive materials (GMM), i.e. a special soft ferromagnetic material.     

In vivo determination of the time and location of mucoadhesive drug delivery systems disintegration in the gastrointestinal tract

OBJECTIVE: The objective of this study was to use magnetic resonance imaging (MRI) to detect the time when and the location at which orally delivered mucoadhesive drugs are released. MATERIALS AND METHODS: Drug delivery systems comprising tablets or capsules containing a mucoadhesive polymer were designed to deliver the polymer to the intestine in dry powder form. Dry Gd-DTPA [diethylenetriaminepentaacetic acid gadolinium(III) dihydrogen salt hydrate] powder was added to the mucoadhesive polymer, resulting in a susceptibility artifact that allows tracking of the application forms before their disintegration and that gives a strong positive signal on disintegration. Experiments were performed with rats using T(1)-weighted spin-echo imaging on a standard 1.5-T MRI system. RESULTS: The susceptibility artifact produced by the dry Gd-DTPA powder in tablets or capsules was clearly visible within the stomach of the rats and could be followed during movement towards the intestine. Upon disintegration, a strong positive signal was unambiguously observed. The time between ingestion and observation of a positive signal was significantly different for different application forms. Quantification of the remaining mucoadhesive polymer in the intestine 3 h after observed release showed significant differences in mucoadhesive effectiveness. CONCLUSION: MRI allows detection of the exact time of release of the mucoadhesive polymer in vivo, which is a prerequisite for a reliable quantitative comparison between different application forms.     

Magnetic moment collapse induced by high-pressure in semi-borides TM2B (TM = Fe,Co). A first-principles study

The high-pressure effects are investigated on the structure, magnetic phase transition, and anisotropic elastic properties of the 3d transition-metal semi-borides TM₂B (TM?=?Fe, Co) by using the generalized gradient approximation (GGA) within the framework of density functional theory (DFT). At equilibrium spin polarization, calculations show that the Fe₂B and Co₂B compounds are ferromagnetic (FM). In the applied pressure range from 0 to 90?GPa, the magnetic moment of Fe₂B and Co₂B slowly decreases and then abruptly drops to zero at 85?GPa, indicating a state transition from the ferromagnetic to the nonmagnetic (NM) state (a first-order quantum phase transition). The collapse of the magnetic moment is accompanied by an abrupt change in the lattice parameters and elastic constants. In addition to this phenomenon, the density of states (DOS), and anisotropic elastic properties are presented at 0?GPa and at the critical transition pressure. Furthermore, I have plotted the three-dimensional (3D) surfaces and planar contours for the Young and bulk moduli of the compounds at several crystallographic planes, ((100) and (001)) to reveal their elastic anisotropy. On the basis of anisotropic elastic properties, I have predicted the easy and hard axes of magnetization for the TM₂B compounds.     

Modelling of DC Electric Fields Induced by RF Sheath in Front of ICRF Antenna

Reducing the ICRH (ion cyclotron range frequency) antenna-plasma interaction is one of the key points for reaching very long tokamak discharges. One problem which limits such discharges, is the appearance of hot spots on the surface of the antenna: Radio Frequency (RF) sheaths modify the properties of the edge plasma by rectifying the RF potential along open magnetic field lines and can induce hot spots. This paper investigates the corrections to sheath potentials introduced by the interactions between adjacent flux tubes. Our theoretical study started from an oscillating double Langmuir probe model, in which a transverse influx of current was included. This model was confronted with 1D PIC simulations along a magnetic field line, and demonstrated that current exchanges can decrease mean potentials. A 2D electrostatic fluid code was then developed, which couples adjacent flux tubes in a poloidal cross section with collisional conductivity or polarization currents. It showed that transverse currents are able to smooth structures smaller than a characteristic size in the sheath potential maps (results for Tore Supra). These computed rectified potentials can be used to obtain the DC electric fields in front of the antenna. And then, it gives an estimate of the particle drift and the energy flux on the antenna structure, which can explain hot spots.