Bulk and surface switching in Mn-Fe-based Prussian Blue Analogues

Many Prussian Blue Analogues are known to show a thermally induced phase transition close to room temperature and a reversible, photo-induced phase transition at low temperatures. This work reports on magnetic measurements, X-ray photoemission and Raman spectroscopy on a particular class of these molecular heterobimetallic systems, specifically on Rb0.81Mn[Fe(CN)6]0.95_1.24H2O, Rb0.97Mn[Fe(CN)6]0.98_1.03H2O and Rb0.70Cu0.22Mn0.78[Fe(CN)6]0.86_2.05H2O, to investigate these transition phenomena both in the bulk of the material and at the sample surface. Results indicate a high degree of charge transfer in the bulk, while a substantially reduced conversion is found at the sample surface, even in case of a near perfect (Rb:Mn:Fe=1:1:1) stoichiometry. Thus, the intrinsic incompleteness of the charge transfer transition in these materials is found to be primarily due to surface reconstruction. Substitution of a large fraction of charge transfer active Mn ions by charge transfer inactive Cu ions leads to a proportional conversion reduction with respect to the maximum conversion that is still stoichiometrically possible and shows the charge transfer capability of metal centers to be quite robust upon inclusion of a neighboring impurity. Additionally, a 532 nm photo-induced metastable state, reminiscent of the high temperature Fe(III)Mn(II) ground state, is found at temperatures 50-100 K. The efficiency of photo-excitation to the metastable state is found to be maximized around 90 K. The photo-induced state is observed to relax to the low temperature Fe(II)Mn(III) ground state at a temperature of approximately 123 K.Comment: 12 pages, 8 figure

Bifunctional CoFe2O4/ZnO Core/Shell Nanoparticles for Magnetic Fluid Hyperthermia with Controlled Optical Response

Conjugation of optical and magnetic responses in a unique system at the nanoscale emerges as a powerful tool for several applications. Here, we fabricated bifunctional CoFe2O4-core/ZnO-shell nanoparticles with simultaneous photoluminescence in the visible range and ac magnetic losses suitable for hyperthermia. The structural characterization confirms that the system is formed by a ≈7 nm CoFe2O4 core encapsulated in a ≈1.5-nm-thick semiconducting ZnO shell. As expected from its high anisotropy, the magnetic losses in an ac magnetic field are dominated by the Brown relaxation mechanism. The ac magnetic response of the core/shell system can be accurately predicted by the linear response theory and differs from that one of bare CoFe2O4 nanoparticles as a consequence of changes in the viscous relaxation process due to the effect of the magnetostatic interactions. Concerning the optical properties, by comparing core/shell CoFe2O4/ZnO and single-phase ZnO nanoparticles, we found that the former exhibits a broader optical absorption and photoluminescence, both shifted to the visible range, indicating that the optical properties are closely associated with the shell-morphology of ZnO. Being focused on bifunctional nanoparticles with an optical response in the visible range and a tunable hyperthermia output, our results can help to address current open questions on magnetic fluid hyperthermia.Fil: Lavorato, Gabriel Carlos. Centro Brasileiro de Pesquisas Físicas; Brasil. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; Argentina. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; ArgentinaFil: Lima, Enio Junior. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; ArgentinaFil: Vasquez Mansilla, Marcelo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; Argentina. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; ArgentinaFil: Troiani, Horacio Esteban. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; ArgentinaFil: Zysler, Roberto Daniel. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; ArgentinaFil: Winkler, Elin Lilian. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; Argentin

The meaning of maxima and minima in first order reversal curves: Determining the interaction between species in a sample

First-order reversal curves (FORCs) are a characterization technique for magnetic materials used in a wide range of research fields. Since their first application in the Earth Sciences two decades ago, their importance in science has been continuously growing and new experimental techniques have been subsequently designed based on the original idea of FORCs. Nonetheless, very recent experimental works on very well designed and simple magnetic structures demonstrate that even for the most simple cases the interpretation of FORC data lacks understanding. In this work, we address this problem analytically, explaining the meaning of maxima, minima and noisy tails and set a strategy to extract the interaction field between magnetic structures. The origin of this interaction field is often the magnetostatic energy, however, we propose that this strategy could be applied for estimating exchange interactions too

Optimum performances for non-linear finite elements model of 8/6 switched reluctance motor based on intelligent routing algorithms

This paper presents torque ripple reduction with speed control of 8/6 Switched Reluctance Motor (SRM) by the determination of the optimal parameters of the turn on, turn off angles Theta_(on), Theta_(off), and the supply voltage using Particle Swarm Optimization (PSO) algorithm and steady state Genetic Algorithm (ssGA). With SRM model, there is difficulty in the control relapsed into highly non-linear static characteristics. For this, the Finite Elements Method (FEM) has been used because it is a powerful tool to get a model closer to reality. The mechanism used in this kind of machine control consists of a speed controller in order to determine current reference which must be produced to get the desired speed, hence, hysteresis controller is used to compare current reference with current measured up to achieve switching signals needed in the inverter. Depending on this control, the intelligent routing algorithms get the fitness equation from torque ripple and speed response so as to give the optimal parameters for better results. Obtained results from the proposed strategy based on metaheuristic methods are compared with the basic case without considering the adjustment of specific parameters. Optimized results found clearly confirmed the ability and the efficiency of the proposed strategy based on metaheuristic methods in improving the performances of the SRM control considering different torque loads

Passiv damping on spacecraft sandwich panels

For reusable and expendable launch vehicles as well as for other spacecraft structural vibration loads are safety critical design drivers impacting mass and lifetime. Here, the improvement of reliability and safety, the reduction of mass, the extension of service life, as well as the reduction of cost for manufacturing are desired. Spacecraft structural design in general is a compromise between lightweight design and robustness with regard to dynamic loads. The structural stresses and strains due to displacements caused by dynamic loads can be reduced by mechanical damping based on passive or active measures. Passive damping systems can be relatively simple and yet are capable of suppressing a wide range of mechanical vibrations. Concepts are low priced in development, manufacturing and application as well as maintenancefree. Compared to active damping measures passive elements do not require electronics, control algorithms, power, actuators, sensors as well as complex maintenance. Moreover, a reliable application of active dampers for higher temperatures and short response times (e. g. re-entry environment) is questionable. The physical effect of passive dampers is based on the dissipation of load induced energy. Recent activities performed by OHB have shown the function of a passive friction-damping device for a vertical tail model of the German X-vehicle PHÖNIX but also for general sandwich structures. The present paper shows brand new results from a corresponding ESA-funded activity where passive damping elements are placed between the face sheets of large spacecraft relevant composite sandwich panels to demonstrate dynamic load reduction in vibration experiments on a shaker. Several passive damping measures are investigated and compared

On the Absence of Photospheric Net Currents in Vector Magnetograms of Sunspots Obtained From Hinode (SOT/SP)

Various theoretical and observational results have been reported regarding the presence/absence of net electric currents in the sunspots. The limited spatial resolution of the earlier observations perhaps obscured the conclusions. We have analyzed 12 sunspots observed from Hinode (SOT/SP) to clarify the issue. The azimuthal and radial components of magnetic fields and currents have been derived. The azimuthal component of the magnetic field of sunspots is found to vary in sign with azimuth. The radial component of the field also varies in magnitude with azimuth. While the latter pattern is a confirmation of the interlocking combed structure of penumbral filaments, the former pattern shows that the penumbra is made up of a "curly interlocking combed" magnetic field. The azimuthally averaged azimuthal component is seen to decline much faster than 1/$\varpi$ in the penumbra, after an initial increase in the umbra, for all the spots studied. This confirms the confinement of magnetic fields and absence of a net current for sunspots as postulated by \cite{parker96}. The existence of a global twist for a sunspot even in the absence of a net current is consistent with a fibril-bundle structure of the sunspot magnetic fields.Comment: 15 pages, 4 figures, 1 table; accepted for publication in the ApJ Letter

Flux observer algorithms for direct torque control of brushless doubly-fed reluctance machines

Direct Torque Control (DTC) has been extensively researched and applied to most AC machines during the last two decades. Its first application to the Brushless Doubly-Fed Reluctance Machine (BDFRM), a promising cost-effective candidate for drive and generator systems with limited variable speed ranges (such as large pumps or wind turbines), has only been reported a few years ago. However, the original DTC scheme has experienced flux estimation problems and compromised performance under the maximum torque per inverter ampere (MTPIA) conditions. This deficiency at low current and torque levels may be overcome and much higher accuracy achieved by alternative estimation approaches discussed in this paper using Kalman Filter (KF) and/or Sliding Mode Observer (SMO). Computer simulations accounting for real-time constraints (e.g. measurement noise, transducer DC offset etc.) have produced realistic results similar to those one would expect from an experimental setup

Properties of ferromagnetic Josephson junctions for memory applications

In this work we give a characterization of the RF effect of memory switching on Nb-Al/AlOx-(Nb)-Pd$_{0.99}$Fe$_{0.01}$-Nb Josephson junctions as a function of magnetic field pulse amplitude and duration, alongside with an electrodynamical characterization of such junctions, in comparison with standard Nb-Al/AlOx-Nb tunnel junctions. The use of microwaves to tune the switching parameters of magnetic Josephson junctions is a step in the development of novel addressing schemes aimed at improving the performances of superconducting memories.Comment: IEEE Trans. Appl. Supercond. Special Issue ISEC201