Novel magnetic stimulation methodology for low-current implantable medical devices

Recent studies highlight the ability of inductive architectures to deliver therapeutic magnetic stimuli to target tissues and to be embedded into small-scale intracorporeal medical devices. However, to date, current micro-scale biomagnetic devices require very high electric current excitations (usually exceeding 1 A) to ensure the delivery of efficient magnetic flux densities. This is a critical problem as advanced implantable devices demand self-powering, stand-alone and long-term operation. This work provides, for the first time, a novel small-scale magnetic stimulation system that requires up to 50-fold lower electric current excitations than required by relevant biomagnetic technology recently proposed. Computational models were developed to analyse the magnetic stimuli distributions and densities delivered to cellular tissues during in vitro experiments, such that the feasibility of this novel stimulator can be firstly evaluated on cell culture tests. The results demonstrate that this new stimulative technology is able to deliver osteogenic stimuli (0.1-7 mT range) by current excitations in the 0.06-4.3 mA range. Moreover, it allows coil designs with heights lower than 1 mm without significant loss of magnetic stimuli capability. Finally, suitable core diameters and stimulator-stimulator distances allow to define heterogeneity or quasi-homogeneity stimuli distributions. These results support the design of high-sophisticated biomagnetic devices for a wide range of therapeutic applications.This work was funded by the Portuguese Foundation for Science and Technology (FCT), through the grant references SFRH/BPD/117475/2016, SFRH/BD/129340/2017 and IF/01089/2015, and by the European Structural and Investment Funds, through the project reference POCI-01-0145-FEDER-031132 and POCI-01-0145-FEDER-007679. It was also support by the TEMA - Centre for Mechanical Technology & Automation (UID/EMS/00481/2013-FCT and CENTRO-01-0145-FEDER-022083) and CICECO - Aveiro Institute of Materials (UID /CTM /50011/ 2013).in publicatio

Magnetovolume effects in Heusler Compounds via First-Principles Calculations

Heusler alloys are promising for several applications, including magnetic refrigeration, due to high magnetocaloric and magnetovolume effects. One way to optimize this potential is by increasing the magnetovolume effect. Using density functional theory with the Korringa-Kohn-Rostoker method, we calculate the effective exchange interaction energies and corresponding mean field Curie temperature as a function of the volume (hydrostatic pressure) in several L2(1)-type Co(2)YZ Heusler alloys. Different qualitative trends and signs of the pressure derivatives of the Curie temperature and moments are found among these compounds, discussed and compared with previous calculations and experiments

Localizacao electronica e magnetismo em amorfos metalicos

This thesis presents a theoretical and experimental study of the magnetic and transport properties of amorphous metallic alloys, including the following subjects: 1) Transport properties in the weak localization regime: effect of the addition of magnetic impurities on the magnetoresistance and electrical resistivity. We show that, contrary to usual belief, magnetic impurities do not simply destroy the effects of weak localization on magnetoresistance. As a matter of fact, these effects can even be enhanced, through the influence of the spin-splitting of the conduction band. This contribution, neglected in many previous studies, has a relevant role on the magnetoresistance of the studied series of amorphous alloys (Dy_xY_1_-_x)Ni. This results from the polarization of the conduction band due to the magnetic exchange interaction of conduction electrons with the moments of Dy. As a consequence, the spin splitting scales with the magnetic susceptibility of the samples, as temperature or composition are varied. We also studied in this series the influenece of the addition of magnetic impurities on the temperature dependence of the electrical resistivity, which, for x=0 (YNi), is dominated, at low temperatures, by electron-electron interaction effects, with a characteristic T temperature dependence. We could show, for the first time, the presence of an additional contribution varying as 1/T, and proportional to the concentration of magnetic impurities. A detailed re-analysis of data in the litterature could also reveal the presence of this contribution in other amorphous systems with magnetic impurities. 2) Magnetic properties in the weak localization regime: modification of spin fluctuations in systems with itinerant electron magnetism. We analyse the results of a study of the magnetic properties of the amorphous system Y_1_-_xNi. This data analysis reveals, for the first time and quantitatively, the presence of a contribution due to disorder in the magnetic susceptibility of magnetic itinerant systems, near the Stoner condition for ferromagnetism..Available from Fundacao para a Ciencia e a Tecnologia, Servico de Informacao e Documentacao, Av. D. Carlos I, 126, 1200 Lisboa / FCT - Fundação para o Ciência e a TecnologiaSIGLEPTPortuga

Superferromagnetism in mechanically alloyed fcc Fe23Cu77 with bimodal cluster size distribution

International audienceMagnetic measurements, x-ray diffraction and Mössbauer spectroscopy were used to characterize a nanostructured fcc Fe23Cu77 at.% alloy prepared by high-energy ball-milling, addressing in particular the effect of clustering on the nature of the interacting magnetic entities. The interpretation of magnetization measurements leads to the conclusion that grains, whose mean size is ~16 nm, contain two populations of magnetic Fe-rich nanoclusters with a bimodal size distribution. These two sets of clusters contain about 14 and 400 Fe atoms and have magnetic moments of 30 µB and 860 µB, respectively. The inter-cluster ferromagnetic interactions that lead to superferromagnetism with a Curie temperature TC~220 K can be described by a mean field determined by the smaller clusters only, which account for 90% of the magnetization

Dynamic off-centering of Cr3+^{3+} ions and short-range magneto-electric clusters in in CdCr2_2S4_4

The cubic spinel CdCr$_2$S$_4$ gained recently a vivid interest, given the relevance of relaxor-like dielectric behavior in its paramagnetic phase. By a singular combination of local probe techniques namely Pair Distribution Function and Perturbed Angular Correlation we firmly establish that the Cr ion plays the central key role on this exotic phenomenon, namely through a dynamic off-centering displacement of its coordination sphere. We further show that this off centering of the magnetic Cr-ion gives rise to a peculiar entanglement between the polar and magnetic degrees of freedom, stabilizing, in the paramagnetic phase, short range magnetic clusters, clearly seen in ultra-low field susceptibility measurements. Moreover, the Landau theory is here used to demonstrate that a linear coupling between the magnetic and polar order parameters is sufficient to justify the appearance of magnetic cluster in paramagnetic phase of this compound. These results open insights on the hotly debated magnetic and polar interaction, setting a step forward in the reinterpretation of the coupling of different physical degrees of freedom

Magnetic hyperfine field at Cr site in AgCrO2_2 given by perturbed angular correlations

This work presents an electric field gradient and magnetic hyperfine field study, in the AgCrO$_2$ multiferroic with triangular spin lattice. The temperature dependence of the electric field gradient (EFG) and magnetic hyperfine field (MHF) at Cr site was studied at ISOLDE via perturbed angular correlation measurements with the $^{111}$In probe, at room temperature and below the Neel temperature (T=21 K) down to presence of two distinct local environments. One axial symmetric EFG with a very low MHF, and a non-axially symmetric EFG with a much higher one. The temperature dependences of MHF magnitude and of the angle between the MHF and the principle component of the EFG are investigated

Effect of processing conditions on the properties of recycled cathode ray tube glass foams

Cathode ray tube glass waste was used to produce glass foams by a powder sintering route. The glass waste powder was mixed with small amounts (5 and 8 wt%) of coal fly ash, which acted as foaming agent, and the compacts of the mixed powders were heated at different sintering temperatures in the range 600-800 A degrees C for various dwell times (30-120 min). The effect of the different processing conditions on the microstructural characteristics (porosity, pore size and pore size distribution), mechanical resistance and thermal conductivity of the produced foams was investigated. The volume of pores tended to increase with sintering temperature and time, and glass foams (with a porosity higher than 50 %) were only achieved after sintering at 750 A degrees C. The average pore size increased with sintering temperature and dwell time, and pore growth was particularly accentuated at 800 A degrees C, where coalescence of the pores occurred, with a consequent decrease in compressive strength. Selected combinations of the sintering temperature, dwell time and foaming agent led to glass foams with a satisfactory microstructural homogeneity, which exhibited mechanical strength and thermal conductivity values similar to commercial foams used as thermal insulating materials