Papers by Vasiliki Tsakaloudi
Magnetochemistry
We review the basic phenomenology of magnetic losses from DC to 1 GHz in commercial and laborator... more We review the basic phenomenology of magnetic losses from DC to 1 GHz in commercial and laboratory-prepared soft ferrites considering recent concepts regarding their physical interpretation. This is based, on the one hand, on the identification of the contributions to the magnetization process provided by spin rotations and domain walls and, on the other hand, the concept of loss separation. It additionally contemplates a distinction between the involved microscopic dissipation mechanisms: spin damping and eddy currents. Selected experimental results on the broadband behavior of complex permeability and losses in Mn-Zn ferrites provide significant examples of their dependence on sintering methods, solute elements, and working temperature. We also highlight the peculiar frequency and temperature response of Ni-Zn ferrites, which can be heavily affected by magnetic aftereffects. The physical modeling of the losses brings to light the role of the magnetic anisotropy and the way its mag...
On the basis of todays demand for the design and development of new magnetic MnZn and NiZn ferrit... more On the basis of todays demand for the design and development of new magnetic MnZn and NiZn ferrite materials that satisfy the requirements of modern electronic and telecommunication applications, the present PhD thesis deals with the correlation between the several process parameters, the development of the polycrystalline microstructure and the electromagnetic behaviour of some MnZn and NiZn ferrite materials. The purpose of this research was the determination and understanding of the numerous phenomena that take place during the manufacturing of the above ferrite types and the combination of the mechanisms that are responsible for the required electromagnetic performance of the final materials with the chemical composition, the process parameters and the development of the polycrystalline microstructure during sintering. Through the completion of the above tasks, the design and manufacturing of high magnetic performing MnZn and NiZn ferrite materials that precisely meet the strict...
2020 IEEE 29th International Symposium on Industrial Electronics (ISIE), 2020
We analyze the physical mechanisms associated with addition of CoO to sintered Mn-Zn ferrites and... more We analyze the physical mechanisms associated with addition of CoO to sintered Mn-Zn ferrites and the ensuing stabilization versus temperature of their magnetic properties. We determine, in particular, the value and behavior of the magnetic anisotropy as a function of doping and temperature and we model in physical terms the evolution of the energy loss in the investigated frequency (DC–1 GHz) and temperature (-20 $\mathrm{C}-+130^{\circ}\mathrm{C})$ ranges. We show that magnetic aging by long exposure of the CoO-doped ferrites at 200 C is minimized by additional $\mathrm{T}\mathrm{i}\mathrm{O}_{2}$ doping. This is observed to restrain the increase of the extra-anisotropy induced by directional ordering of the $\mathrm{C}\mathrm{o}^{2+}$ cations.
Journal of Magnetism and Magnetic Materials, 2020
Abstract The soft magnetic properties of the sintered Mn-Zn ferrites can be stabilized versus cha... more Abstract The soft magnetic properties of the sintered Mn-Zn ferrites can be stabilized versus changing temperature by addition of CoO in suitable proportion. The material benefits in this case of anisotropy compensation brought about by the Co2+ cations. However, prolonged exposure of magnetic cores to high temperatures, as likely to occur in automotive applications, can be associated with local phenomena of induced anisotropy, resulting in magnetic viscosity and aging. We show in this paper that proper addition of TiO2 in optimally CoO-doped ferrites can restrain aging and the ensuing detrimental effects on loss and permeability. We find, in particular, that on passing from conventional doping with 1000 ppm to 5000 ppm of TiO2, the deterioration of the soft magnetic properties of the ferrite following an aging treatment of 100 h at 200 °C is in good part reduced below a few hundred kHz. It is concluded that this effect chiefly relates to a correspondingly reduced value of the magnetic anisotropy induced by directional ordering at 200 °C. This beneficial effect appears to descend from hindered diffusion of the Co2+ cations by the dissolved Ti4+ cations. The induced anisotropy, however, while leading to increased hysteresis and excess losses, brings about a decrease of the total energy loss at intermediate frequencies, by modifying the distribution of the resonance frequencies and the associated damping of the rotational processes. Little to negligible effects are observed beyond a few MHz.
Journal of Applied Physics, 2019
CoO-doping is known to stabilize the temperature dependence of initial permeability and magnetic ... more CoO-doping is known to stabilize the temperature dependence of initial permeability and magnetic losses in Mn-Zn ferrites, besides providing, with appropriate dopant contents, good soft magnetic response at and around room temperature. These effects, thought to derive from the mechanism of anisotropy compensation, are, however, poorly assessed from a quantitative viewpoint. In this work, we overcome such limitations by providing, besides extensive experimental investigation vs frequency (DC-1 GHz), CoO content (0 ≤ CoO ≤ 6000 ppm), and temperature (−20°C ≤ T ≤ 130°C) of permeability and losses of sintered Mn-Zn ferrites, a comprehensive theoretical framework. This relies on the separate identification of domain wall motion and moment rotations and on a generalized approach to magnetic loss decomposition. The average effective anisotropy constant ⟨K eff ⟩ is obtained and found to monotonically decrease with temperature, depending on the CoO content. The quasistatic energy loss W h is then predicted to pass through a deep minimum for CoO = 3000-4000 ppm at and below the room temperature, while becoming weakly dependent on CoO under increasing T. The rotational loss W rot (f) is calculated via the complex permeability, as obtained from the Landau-Lifshitz equation for distributed values of the local effective anisotropy field H k,eff (i.e., ferromagnetic resonance frequency). Finally, the excess loss W exc (f) is derived and found to comply with suitable analytical formulation. It is concluded that, by achieving, via the rotational permeability, value and behavior of the magnetic anisotropy constant, we can predict the ensuing properties of hysteresis, excess, and rotational losses.
AIP Advances, 2019
Improving soft magnetic properties of Mn-Zn ferrite by rare earth ions doping AIP Advances 8, 047... more Improving soft magnetic properties of Mn-Zn ferrite by rare earth ions doping AIP Advances 8, 047807 (2018);
AIP Advances, 2018
Paper published as part of the special topic on 23rd Soft Magnetic Materials Conference ARTICLES ... more Paper published as part of the special topic on 23rd Soft Magnetic Materials Conference ARTICLES YOU MAY BE INTERESTED IN Improving soft magnetic properties of Mn-Zn ferrite by rare earth ions doping AIP Advances 8, 047807 (2018);
Journal of Magnetism and Magnetic Materials, 2017
Mn-Zn ferrites prepared by different sintering schedules at 1325 °C, 1340 °C, and 1360 °C, have b... more Mn-Zn ferrites prepared by different sintering schedules at 1325 °C, 1340 °C, and 1360 °C, have been characterized from the structural, electrical, and magnetic viewpoint. Magnetic losses and complex permeability have been, in particular, measured and analyzed from quasi-static excitation up to 1 GHz. It is observed that lower sintering temperatures and shorter treatment times lead to more homogeneous grain structure and better soft magnetic response at all frequencies. It is shown, however, that, once the contribution by eddy currents is singled out, the energy losses tend to coincide beyond a few MHz in the differently treated samples. The interpretative approach consists in separating the contributions by the domain wall displacements and the magnetization rotations to complex permeability and losses as a function of frequency. This can be accomplished in a relatively simple way in the low induction region described by the Rayleigh law, where these quantities can be quantitatively related and the linear Landau-Lifshitz-Gilbert equation applies, account being taken of the distribution in amplitude and orientation of the local anisotropy fields.
Journal of Magnetism and Magnetic Materials, 2016
Abstract Current market trends of the switching power supplies industry require even lower energy... more Abstract Current market trends of the switching power supplies industry require even lower energy losses in power conversion systems with maintenance of satisfactory initial permeability levels. Typical operation conditions refer to a frequency of 100 kHz, an induction level of 200 mT and a steady state temperature of 100° C. In this work the development of a polycrystalline Mn–Zn ferrite material that exhibits initial relative magnetic permeability above 2500 and very low power losses at 100 kHz, 200 mmT and 100° C is presented. The Mn–Zn ferrite samples were prepared by the conventional solid state reaction method. Sintering was performed under controlled atmosphere conditions. The combinatorial role of TiO2 and CoO together with Zn content, as well as the effects of the process parameters on the magnetic performance of the Mn–Zn ferrite was evaluated. It is shown that the development of the adequate polycrystalline microstructure that is characterized by (a) high sintered density, (b) homogenous grain size that is free of morphological or chemical pinning defects and (c) high resistivity grain boundary structure, can be achieved by means of appropriate compositional and dopant adjustment, anisotropy control and specific resistivity optimization. The newly developed Mn–Zn ferrite is characterized by high sintered density of 4.91 g/cm3, initial magnetic permeability of 2512 (at 10 kHz, 0.1 mT, 25 °C), high saturation magnetic flux density of 560 mT (at 10 kHz, 1200 A/m, 25 °C) and very low power losses (Pv) of 224 mW/cm3 (at 100 kHz, 200 mT, 100 °C) combined with very low power losses of 470 mW/cm3 even at room temperature, establishing it as ideal for power applications.
Journal of the American Ceramic Society, 2008
IEEE Magnetics Letters, 2018
The magnetic properties of sintered Mn-Zn ferrites, Co 2+ enriched by addition of CoO up to 6000 ... more The magnetic properties of sintered Mn-Zn ferrites, Co 2+ enriched by addition of CoO up to 6000 ppm, were measured in ring samples for a broad range of peak polarization values (2-200 mT) and frequencies (dc-1 GHz). The results were analyzed by separating the contributions to the magnetization process of domain wall motion and magnetization rotation, and applying the concept of loss decomposition. By determining the value and behavior of the rotational permeability μ rot as a function of the CoO content, we obtain the average effective magnetic anisotropy <K> and its effect on the loss. We thus identify the hysteresis (quasi-static) W h , rotational W rot , and excess W exc loss components and their dependence on CoO. The quasi-static loss W h , the domain wall permeability μ dw , and <K> have minima, and μ rot has a maximum, for CoO in the range 3000-4000 ppm. The rotational loss by spin damping W rot,sd is calculated by use of the Landau-Lifshitz equation by assuming distributed anisotropy field amplitudes. W rot,sd covers the experimental loss behavior beyond about 1 MHz. W exc and W h , both directly generated by the moving domain walls, share the dissipative response of the material at lower frequencies and show similar trends versus CoO content. It is concluded that the modulation of the magnetic anisotropy of Mn-Zn ferrites through Co 2+ enrichment, leading to maximum magnetic softening for CoO in the range 3000-4000 ppm, can be assessed in terms of separate effects of domain wall motion and moment rotations and their related dissipative properties. Index Terms-Soft magnetic materials, Mn-Zn ferrites, magnetic losses, complex permeability, loss decomposition. I. INTRODUCTION The development of low-loss Mn-Zn ferrites, matching the working conditions nowadays posed by the high-speed semiconducting devices used in power electronics and trends toward miniaturization of components, requires understanding of the magnetization process upon a broad range of magnetizing frequencies. Guidelines for optimal preparation methods and composition adjustments [Kalarus 2012] would indeed benefit from a quantitative assessment of the role of structure and composition on magnetic loss and permeability. This is not a simple task because sintered ferrites are heterogeneous materials, where the magnetization process occurs by admixture of frequency-dependent rotations and domain wall (dw) processes, and two different energy dissipation channels, eddy currents and spin damping, operate. Besides the preparation methods and the ensuing structural properties of the sintered material, relevant effects on the magnetic properties are obtained, for example, by operating on the stoichiometry of Fe 2 O 3 , that is, on the concentration of the Fe 2+ ions, which interfere with anisotropy and conductivity [Pascard 1998, Li 2008]. Oxides segregating at the grain boundaries (e.g., CaO, Nb 2 O 5 , SiO 2) are routinely added, in order to increase the resistivity [Zaspalis 2002, Wang 2014], whereas oxides dissolving in the spinel lattice and releasing cations on the octahedral sites (e.g., TiO 2 , SnO 2 , CoO) can both hinder the hopping mechanism of conductivity and compensate the negative
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Papers by Vasiliki Tsakaloudi