Magnetes permanentes nano-estruturados isentos de terras-raras

In this work we explore on the rare-earth free nanostructured permanent magnets, including thin films, nanoparticles and nanocomposites with the focus on Alnico magnets and hexaferrites. Here we investigate the effects of different heat treatment conditions on structural and magnetic properties of RFsputtered Alnico V thin films on Si substrates. We show an in-depth analysis of the various heat treated samples with high coercivity to unveil the origin of high coercivity in these thin films with a recently discovered Fe-Co rich Body- Centered Tetragonal (bct) phase. Exchange-spring magnets are also explored, namely barium hexaferrite (BaM) and strontium hexaferrite (SrM). We investigate on the possibility of coating BaM and SrM flake-like hexaferrite particles via a hydrothermal and coprecipitation method to prepare core-shelllike BaM/Fe3O4 and SrM/Fe3O4 nanocomposites, where the ferrite particles where prepared via a sol-gel auto-combustion method. We show how optimised hard to soft magnetic phase ratio and preparation conditions lead to a significant enhancement in their hard magnetic properties compared to commercial ferrite powders. Moreover, we employ the prepared highperformance exchange-coupled nanocomposite powder and investigate the mechanical and magnetic properties of warm compressed nanocomposite powder in an epoxy matrix. We show how the powder-to-resin ratio and preparation conditions lead to optimised mechanical properties, and enhancement in the maximum energy product of the composite magnet. Finally, micromagnetic simulations were employed to better understand and support the experimental results of the exchange coupling behaviour of the BaM/Fe3O4 hard-soft magnetic nanocomposites. We show how the thickness of BaM particles affect their coercivity and how the volume fraction of each magnetic phase, together with their interface area, affect the exchange coupling behaviour and maximum energy product of the nanocomposite magnets.Neste trabalho, exploramos magnetes permanentes nano-estruturados, livres de terras raras, incluíndo filmes finos, nanopartículas e nanocompósitos focando magnetes de Alnico e hexaferrites. Investigamos os efeitos de diferentes condições de tratamento térmico nas propriedades estruturais e magnéticas de filmes finos de Alnico V pulverizados por RF em substratos de Si. Fizemos uma análise mais aprofundada das várias amostras tratadas termicamente para desvendar a origem da alta coercividade nesses filmes finos com uma recentemente descoberta fase Tetragonal Centrada no Corpo (bct) rica em Fe-Co. Os magnetes exchange-spring também são explorados, e.g. hexaferrite de bário (BaM) e hexaferrite de estrôncio (SrM). Investigamos a possibilidade de revestir partículas de hexaferrite semelhantes a flocos de BaM e SrM por meio de métodos hidrotérmico e de coprecipitação para preparar nanocompósitos tipo núcleo-casca de BaM/Fe3O4 e SrM/Fe3O4, onde as partículas de ferrite foram preparadas por meio de método sol-gel de combustão. Mostramos como a relação de fases magnéticas macia e dura, mais as condições de preparação otimizadas, levam a um aprimoramento significativo das suas propriedades magnéticas duras em comparação com pós de ferrite comerciais. Além disso, usando o pó de nanocompósito de alto desempenho, investigamos as propriedades mecânicas e magnéticas do pó do nanocompósito comprimido a quente em uma matriz de epóxi. Mostramos como a combinação pó-resina e as condições de preparação levam à obtenção de propriedades mecânicas otimizadas e a um aprimoramento do produto de energia máxima do magnete composto. Finalmente, realizamos simulações micromagnéticas para melhor compreender e apoiar os resultados experimentais do comportamento de acoplamento de troca dos nanocompósitos magnéticos duros-macios de BaM/Fe3O4. Mostramos como a espessura das partículas BaM afetam a coercividade e como a fração de volume de cada fase magnética, assim como a área de interface entre elas, afetam o comportamento de acoplamento de troca bem como o produto energético máximo dos magnetes de nanocompósitos.Programa Doutoral em Ciência e Engenharia de Materiai

Hardening of cobalt ferrite nanoparticles by local crystal strain release: implications for rare earth free magnets

In this work, we demonstrate that the reduction of the local internal stress by a low-temperature solvent-mediated thermal treatment is an effective post-treatment tool for magnetic hardening of chemically synthesized nanoparticles. As a case study, we used nonstoichiometric cobalt ferrite particles of an average size of 32(8) nm synthesized by thermal decomposition, which were further subjected to solvent-mediated annealing at variable temperatures between 150 and 320 °C in an inert atmosphere. The postsynthesis treatment produces a 50% increase of the coercive field, without affecting neither the remanence ratio nor the spontaneous magnetization. As a consequence, the energy product and the magnetic energy storage capability, key features for applications as permanent magnets and magnetic hyperthermia, can be increased by ca. 70%. A deep structural, morphological, chemical, and magnetic characterization reveals that the mechanism governing the coercive field improvement is the reduction of the concomitant internal stresses induced by the low-temperature annealing postsynthesis treatment. Furthermore, we show that the medium where the mild annealing process occurs is essential to control the final properties of the nanoparticles because the classical annealing procedure (T > 350 °C) performed on a dried powder does not allow the release of the lattice stress, leading to the reduction of the initial coercive field. The strategy here proposed, therefore, constitutes a method to improve the magnetic properties of nanoparticles, which can be particularly appealing for those materials, as is the case of cobalt ferrite, currently investigated as building blocks for the development of rare-earth free permanent magnets.This work was supported by EU-H2020 AMPHIBIAN Project (Grant no. 720853). A.L.O. acknowledges support from the Universidad Pública de Navarra (Grant no. PJUPNA2020). Open access funding provided by Universidad Pública de Navarra

Magnetic Nanostructures

This thesis reports the results of studies conducted at LAboratory for Molecular Magnetism (LA.M.M.) of the University of Firenze concerning the synthesis and characterization of rare-earth free nanostructured materials for permanent magnet applications. Ferrite-based magnetic materials doped with transition metal ions are studied with particular attention to the correlation between their magnetic properties and nanostructures. Firstly, the magnetic behaviour of single-phase ferrites nanocrystals with enhanced anisotropy was analysed, in order to under stand the correlation between the final properties and particle size, shape, crystallinity, composition, etc. Then, hybrid bi magnetic core|shell nanoparticles were prepared focusing on the aftermath and required conditions of exchange-coupling establishment between the two moieties

FeCo Nanowire-Strontium Ferrite Powder Composites for Permanent Magnets with High-Energy Products

Due to the issues associated with rare-earth elements, there arises a strong need for magnets with properties between those of ferrites and rare-earth magnets that could substitute the latter in selected applications. Here, we produce a high remanent magnetization composite bonded magnet by mixing FeCo nanowire powders with hexaferrite particles. In the first step, metallic nanowires with diameters between 30 and 100 nm and length of at least 2 {\mu}m are fabricated by electrodeposition. The oriented as-synthesized nanowires show remanence ratios above 0.76 and coercivities above 199 kA/m and resist core oxidation up to 300 {\deg}C due to the existence of a > 8 nm thin oxide passivating shell. In the second step, a composite powder is fabricated by mixing the nanowires with hexaferrite particles. After the optimal nanowire diameter and composite composition are selected, a bonded magnet is produced. The resulting magnet presents a 20% increase in remanence and an enhancement of the energy product of 48% with respect to a pure hexaferrite (strontium ferrite) magnet. These results put nanowire-ferrite composites at the forefront as candidate materials for alternative magnets for substitution of rare earths in applications that operate with moderate magnet performance

Magnetic materials for magnetoelectric coupling: An unexpected journey

Magnetic materials for magnetoelectric coupling are reported. After an introduction of magnetoelectric effect and materials, an historical on the main developments in this field are presented. Then, the main concepts related to multiferroic and magnetoelectric materials are introduced, together with the description of the main types of magnetoelectric materials and structures. Finally, the magnetic materials used the development of magnetoelectric composites are presented and discussed, highlighting their main physico-chemical characteristics and processing methods. In this way, a complete account on concepts, materials and methods is presented in this strongly evolving research field, with strong application potential in the areas of sensors and actuators, among others.FCT- Fundação para a Ciência e Tecnologia- for financial support in the framework of the Strategic Funding UID/FIS/04650/2020 and under projects PTDC/BTM-MAT/28237/2017 and PTDC/EMD-EMD/28159/2017. P.M., A.C.L. and N.P. also support from FCT (for the contract under the Stimulus of Scientific Employment, Individual Support – 2017 Call (CEECIND/03975/2017, for the SFRH/BD/132624/2017 and for the SFRH/BD/131729/2017 grant, respectively). Finally, the authors acknowledge funding by the Spanish State Research Agency (AEI) and the European Regional Development Fund (ERFD) through the project PID2019-106099RB-C43/AEI/10.13039/501100011033 and from the Basque Government Industry and Education Department under the ELKARTEK, HAZITEK and PIBA (PIBA-2018-06) program

A detailed investigation of the onion structure of exchanged coupled magnetic Fe3-dO4@CoFe2O4@Fe3-dO4 nanoparticles

Nanoparticles that combine several magnetic phases offer wide perspectives for cutting edge applications because of the high modularity of their magnetic properties. Besides the addition of the magnetic characteristics intrinsic to each phase, the interface that results from core-shell and, further, from onion structures leads to synergistic properties such as magnetic exchange coupling. Such a phenomenon is of high interest to overcome the superparamagnetic limit of iron oxide nanoparticles which hampers potential applications such as data storage or sensors. In this manuscript, we report on the design of nanoparticles with an onion-like structure which has been scarcely reported yet. These nanoparticles consist of a Fe3-dO4 core covered by a first shell of CoFe2O4 and a second shell of Fe3-dO4, e.g., a Fe3-dO4@CoFe2O4@Fe3-dO4 onion-like structure. They were synthesized through a multistep seed-mediated growth approach which consists consists in performing three successive thermal decomposition of metal complexes in a high-boiling-point solvent (about 300 °C). Although TEM micrographs clearly show the growth of each shell from the iron oxide core, core sizes and shell thicknesses markedly differ from what is suggested by the size increasing. We investigated very precisely the structure of nanoparticles in performing high resolution (scanning) TEM imaging and geometrical phase analysis (GPA). The chemical composition and spatial distribution of atoms were studied by electron energy loss spectroscopy (EELS) mapping and spectroscopy. The chemical environment and oxidation state of cations were investigated by 57Fe Mössbauer spectrometry, soft X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD). The combination of these techniques allowed us to estimate the increase of Fe2+ content in the iron oxide core of the core@shell structure and the increase of the cobalt ferrite shell thickness in the core@shell@shell one, whereas the iron oxide shell appears to be much thinner than expected. Thus, the modification of the chemical composition as well as the size of the Fe3-dO4 core and the thickness of the cobalt ferrite shell have a high impact on the magnetic properties. Furthermore, the growth of the iron oxide shell also markedly modifies the magnetic properties of the core-shell nanoparticles, thus demonstrating the high potential of onion-like nanoparticles to accurately tune the magnetic properties of nanoparticles according to the desired applications. © 2021 American Chemical Society