Domain wall propagation and pinning induced by current pulses in cylindrical modulated nanowires.

The future developments in 3D magnetic nanotechnology require the control of domain wall dynamics by means of current pulses. While this has been extensively studied in 2D magnetic strips (planar nanowires), few reports on this exist in cylindrical geometry, where Bloch point domain walls are expected to have intriguing properties. Here, we report an investigation on cylindrical magnetic Ni nanowires with geometrical notches. An experimental work based on synchrotron X-ray magnetic circular dichroism (XMCD) combined with photoemission electron microscopy (PEEM) indicates that large current densities induce domain wall nucleation, while smaller currents move domain walls preferably antiparallel to the current direction. In the region where no pinning centers are present, we found a domain wall velocity of about 1 km s-1. Thermal modelling indicates that large current densities temporarily raise the temperature in the nanowire above the Curie temperature, leading to nucleation of domain walls during the system cooling. Micromagnetic modelling with a spin-torque effect shows that for intermediate current densities, Bloch point domain walls with chirality parallel to the Oersted field propagate antiparallel to the current direction. In other cases, domain walls can be bounced from the notches and/or get pinned outside their positions. We thus found that current is not only responsible for domain wall propagation, but also is a source of pinning due to the Oersted field action.

Micromagnetism of permalloy antidot arrays prepared from alumina templates.

Magnetic hysteresis processes of hexagonal arrays of permalloy antidots have been studied by means of micromagnetic simulations as a function of geometrical parameters. The ideal system shows a maximum of the coercive field as a function of the antidot diameter. The simulated magnetic behavior has been compared with experimental values for antidot arrays of permalloy prepared from alumina templates with thicknesses between 2 and 60 nm, showing a monotonic increase of the coercive field as a function of the antidot diameter. We show that the introduction into simulations of the combination of variable antidot diameters from bottom to top due to the fabrication process and, more importantly, large geometrical domains, which break the sample symmetry, solves the discrepancy between the simulations and the experiment.

Nanomagnets with high shape anisotropy and strong crystalline anisotropy: perspectives on magnetic force microscopy.

We report on a new approach for magnetic imaging, highly sensitive even in the presence of external, strong magnetic fields. Based on FIB-assisted fabricated high-aspect-ratio rare-earth nanomagnets, we produce groundbreaking magnetic force tips with hard magnetic character where we combine a high aspect ratio (shape anisotropy) together with strong crystalline anisotropy (rare-earth-based alloys). Rare-earth hard nanomagnets are then FIB-integrated to silicon microcantilevers as highly sharpened tips for high-field magnetic imaging applications. Force resolution and domain reversing and recovery capabilities are at least one order of magnitude better than for conventional magnetic tips. This work opens new, pioneering research fields on the surface magnetization process of nanostructures based either on relatively hard magnetic materials-used in magnetic storage media-or on materials like superparamagnetic particles, ferro/antiferromagnetic structures or paramagnetic materials.

Focused-ion-beam-assisted magnet fabrication and manipulation for magnetic field detection applications.

A focused-ion-beam-assisted technique intended for ultrasmall, hard-magnet fabrication has been developed. By means of ion-beam-induced milling and deposition, reduced-size NdFeB magnets were extracted from a macroscopic quarry and bonded to the surface of a thin-film bulk acoustic resonator (FBAR). Electrical characterization of the FBAR before and after bonding of the magnet was carried out, thus observing both a downshifting of the resonance frequency and a reduction of the quality factor of the resonator. The magnetic behavior of the nanomagnet has been confirmed by means of magnetometry measurements based on atomic force microscopy.

Magnetostriction measuring device based on an optical fiber sensor with an annular photodiode.

A new simple and sensitive dilatometer to measure magnetostriction of ribbons has been developed, based on an optical fiber sensor using an annular photodiode. The optical fiber is used bidirectionally, both for emission and detection of light, simplifying the access to the ribbon under test. The working principle is based on the measurement by reflection of the longitudinal displacement of the ribbon end. For a Vitroperm amorphous ribbon of 100 mm length, 21 microm thickness, and 8.3 mm width, a displacement of 2.571 microm with a maximum uncertainty of 8 nm has been obtained.

In vivo bone response to porous calcium phosphate cement.

We conducted an in vivo experiment to evaluate the resorption rate of a calcium phosphate cement (CPC) with macropores larger than 100 microm, using the CPC called Biocement D (Merck Biomaterial, Darmstadt, Germany), which after setting only shows pores smaller than 1 microm. The gas bubble method used during the setting process created macroporosity. Preset nonporous and porous cement implants were inserted into the trabecular bone of the tibial metaphysis of goats. The size of the preset implants was 6 mm and the diameter of the drill hole was 6.3 mm, leaving a gap of 0.3 mm between implant surface and drill wall. After 2 and 10 weeks, the animals were euthanized and cement implants with surrounding bone were retrieved for histologic evaluation. Light microscopy at 2 weeks revealed that the nonporous implants were surrounded by connective tissue. On the cement surface, we observed a monolayer of multinucleated cells. Ten weeks after implantation, the nonporous implants were still surrounded by connective tissue. However, a thin layer of bone now covered the implant surface. No sign of cement resorption was observed. In contrast, the porous cement evoked a completely different bone response. At 2 weeks, bone formation had already occurred inside the implant porosity. Bone formation even appeared to occur as a result of osteoinduction. Also, at their outer surface, the porous implants were completely surrounded by bone. At 2 weeks, about 31% of the initial cement was resorbed. After 10 weeks, 81% of the initial phosphate cement was resorbed and new bone was deposited. On the basis of these observations, we conclude that the creation of macropores can significantly improve the resorption rate of CPC. This increased degradation is associated with almost complete bone replacement.

Biphasic materials for bone grafting and hyperthermia treatment of cancer.

Three biphasic materials have been synthesized from a magnetic glass-ceramic (Si-Ca-Fe) and a bioactive sol-gel glass (Si-P-Ca). The ratios of glass-ceramic:sol-gel glass used in this work were 1:1, 2:1, and 5:1. These materials show bioactive and magnetic properties and can be used as thermoseeds for hyperthermia treatment of bone tumors. The sol-gel glass content affects the textural properties of the glass-ceramic, giving rise to porosity, which plays a fundamental role in the formation of an apatite-like layer on the surface. On the other hand, as the sol-gel glass content increases, the magnetic properties change due to the diffusion of Fe ions to the glassy phases of the biphasic materials. The biphasic nature of these materials allows the changing of both properties, depending on the requirements of the patient.

In vitro release of gentamicin from OHAp/PEMA/PMMA samples.

The influence of hydroxyapatite (OHAp) and gentamicin sulphate (GEN) contents on the release kinetics of GEN, in samples composed of OHAp, poly(methyl methacrylate) (PMMA) and poly(ethyl methacrylate) (PEMA) has been studied. For this purpose, samples with 30 and 40% of OHAp and 5 and 9% of GEN were prepared. The in vitro release study was carried out soaking the samples in simulated body fluid (SBF) at 37 degrees C for 70 days. The release profiles showed a faster release during the first 10 h, diminishing progressively until the end of the study. It was noticed that the percentage of released GEN increased with the OHAp content. For samples with 40% of OHAp, GEN release is nearly independent of the initial amount of such drug (in the range 5-9%), whereas for samples with 30% of OHAp, the release process is favoured by higher contents of GEN, which would favour a higher SBF uptake. GEN release is related to SBF uptake, which is in turn related, on the one hand, to the OHAp content (increase of the porosity and the hydrophilic character of the samples) and on the other hand, to content of GEN.

A new method to produce macropores in calcium phosphate cements.

A new way to create macropores in calcium phosphate cements has been developed. The method consists in adding NaHCO3 to the starting cement powder (Biocement D) and using two different liquids: first a basic liquid to form the paste and later an acid liquid to obtain CO2 bubbles. Mercury intrusion measurements showed a dramatic increase both in macropores with an average size of 100 m and in the total porosity (even higher than 50% with respect to the Biocement D). This method does not change in any significant way the final reaction products of the starting material after being soaked 3 days in Ringer solution. Only, due to the increase of the porosity. the compressive strength of the porous cement decreases significantly.

A novel bioactive and magnetic biphasic material.

A novel biphasic material has been synthetised from a sol-gel-derived glass (Si-Ca-P) and a glass-ceramic obtained from a melt-derived glass (Si-Ca-Fe). Both components of such a biphasic mixture are bioactive, but with different kinetics for the growth of an apatite-like layer on the surface of these materials, needing only one day for the sol-gel-derived glass and one month for the glass-ceramic. The glass-ceramic shows magnetic properties. The biphasic material, obtained from a mixture 1:1 of these components, is bioactive, and its surface is coated after 15 days of soaking in SBF. The biphasic material also exhibited magnetic behaviour, useful for hyperthermia.

Gentamicin release from hydroxyapatite/poly(ethyl methacrylate)/poly(methyl methacrylate)composites.

In this work the release kinetics of gentamicin sulfate (GEN) in samples composed by hydroxyapatite, poly(methyl methacrylate), and poly(ethyl methacrylate) has been studied. The release study was performed by soaking three samples in simulated body fluid at 37 degrees C; the medium was periodically replaced during 70 days. The concentration of GEN was determined by the o-phtaldialdehyde method. The release profile shows three stages: the first stage, occurring during the first 10 h, corresponds to a fast release (nearly 30% of the drug is released in this period). The second stage is slower and includes from the first 10 h to 16 days, releasing 60% of the total amount of GEN. The final stage is the slowest and it takes from 16 to 70 days (10% of GEN is released). The fraction of released GEN versus square root of time can be fitted to a third order polynomial, corresponding with the model proposed by Cobby et al. (J Pharm Sci 1974;63:725-732). The characterization of the samples after the release study shows that a carbonate hydroxyapatite layer has grown on the whole surface of the composites.