Project

AMPHIBIAN: AnisoMetric Permanent HybrId magnets Based on Inexpensive And Non-critical materials

Goal: To develop up-scalable and cost-efficient methods for manufacturing improved rare earth free permanent magnets. We aim at producing permanent magnets with an enhanced performance with respect to already existing rare earth free magnets; and making them more enduring and sustainable. AMPHIBIAN is a Research and Innovation project funded by the European Commission. Grant agreement H2020-NMBP-2016-720853.
http://amphibianproject.eu/

Date: 1 January 2017 - 31 December 2019

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César De Julián Fernández
added 2 research items
The use of rare-earth-based permanent magnets is one of the critical points for the development of the current technology. On the one hand, industry of the rare-earths is highly polluting due to the negative environmental impact of their extraction and, on the other hand, the sector is potentially dependent on China. Therefore, investigation is required both in the development of rare-earth-free permanent magnets and in sintering processes that enable their greener fabrication with attractive magnetic properties at a more competitive price. This work presents the use of a cold sintering process (CSP) followed by a post-annealing at 1100 ºC as a new way to sinter composite permanent magnets based on strontium ferrite (SFO). Composites that incorporate a percentage ≤ 10% of an additional magnetic phase have been prepared and the morphological, structural and magnetic properties have been evaluated after each stage of the process. CSP induces a phase transformation of SFO in the composites, which is partially recovered by the post-thermal treatment improving the relative density to 92% and the magnetic response of the final magnets with a coercivity of up to 3.0 kOe. Control of the magnetic properties is possible through the composition and the grain size in the sintered magnets. These attractive results show the potential of the sintering approach as an alternative to develop modern rare-earth-free composite permanent 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.
César De Julián Fernández
added a research item
In the last few years, significant effort has again been devoted to ferrite-based permanent magnet research due to the so-called rare-earth crisis. In particular, a quest to enhance ferrites maximum energy product, BH max , is underway. Here, the influence of composition and sintering conditions on the microstructure and consequently magnetic properties of strontium ferrite-based hybrid composites was investigated. The powder mixtures consisted of hydrothermally synthesised Sr-ferrite with hexagonally shaped platelets with a diameter of 1 μ m and thickness up to 90 nm, and a soft magnetic phase in various ratios. Powders were sintered using a spark plasma sintering furnace. The crystal structure, composition and microstructure of the starting powders and hybrid magnets were examined. Their magnetic properties were evaluated by vibrating sample magnetometer, permeameter and by single-point-detection measurements.
César De Julián Fernández
added an update
Expected Conference on Permanent Magnets!
Including a session on Rare-Earth free Magnets and the contributions of the AMPHIBIAN team.
 
César De Julián Fernández
added 2 research items
Novel hybrid materials based on the soft nanocrystalline Fe73.9Si15.5Cu1Nb3B6.6 alloy are designed in this work with the purpose of reducing its particle size and developing different compositional, structural and magnetic properties depending on the high-energy ball milling time and route employed. Innovative processes combining mixer, vibratory and planetary motions at the same time are carried out under a dry and a wet route, obtaining reduced sub-micron particle size distributions after 2 h of milling. The established approach takes advantage of the nature of amorphous materials to promote the formation of different hybrid compounds and it is supported by the high virulence of the milling process performed at 2000 rpm. On the one hand, the results obtained by the dry route show a structural and magnetic evolution dominated by the presence of ferromagnetic α-Fe3Si nanocrystals. On the other hand, the wet approach that is supported by glycerol evidences the destruction of the α-Fe3Si with the milling time in favor of the non-magnetic β-FeSi2 phase, obtaining a new functionality for the Fe73.9Si15.5Cu1Nb3B6.6 alloy. Their properties as soft magnetic alloys are evaluated and compared depending on the followed route, opening up a new physicochemical method in the development of multiphase compounds with potential applications. With this work, a controlled formation of different hybrid compounds is achieved, revealing an effective mechanism to introduce functional materials with different properties embedded in an amorphous ferromagnetic matrix.
The exploitation of the exchange coupling between hard and soft magnetic materials has been proposed for enhancing the magnetic performances of rare-earth free permanent magnets, with the aim of extending their use to all applications where moderate energy product (35–100 kJ m ⁻³ ) is required. Strontium hexaferrite (SFO)/spinel ferrite composites seem particularly promising to achieve this target, although the conditions to maximize the effect while using techniques easily scalable to industrial production have not yet been identified. Within this framework, the optimization of the structural, chemical, and magnetic properties of the two moieties before the coupling procedure is crucial to enhance the energy product of the final composite. Here we report the syntheses of both nanometric SFO with high coercivity (ca. 525 kA m ⁻¹ ) and quasi-bulk saturation magnetization (68 Am ² kg ⁻¹ ) and a series of nanosized zinc-doped ferrite (Zn x Fe 3− x O 4 , 0.0 ⩽ x ⩽ 0.4) through cheap, easily scalable and eco-friendly approaches. The structural and chemical stability of the two magnetic phases as a function of temperature were investigated up to 1100 °C, with the aim of finding the best compromise between preservation of the nanometric scale and magnetic properties. A very high-magnetization (106 Am ² kg ⁻¹ ) ferrite was obtained by annealing Zn 0.3 Fe 2.7 O 4 nanopowder at the highest investigated temperature. A preliminary attempt at coupling the two phases, starting from a mixture of the nanopowders, was performed through a classic annealing process in the temperature range 500 °C–1100 °C. The adopted procedure allowed for obtaining an exchange coupled composite at 1100 °C where the two phases are intimately and homogeneously mixed, with micrometric (0.3–5 μ m) and nanometric (up to 50 nm) spinel ferrite particles. Despite these promising results, no enhancement of the energy product was found, highlighting the need for further experimental efforts to improve the coupling procedure.
César De Julián Fernández
added 5 research items
In order to obtain competitive strontium ferrite sintered magnets, SiO2 and CaO are added to avoid exaggerated grain growth. Besides favoring proper densification, these additives prevent the collapse of coercivity associated to grain growth. However, these additives may lead to slight decreases in density and the formation of paramagnetic α-Fe2O3 that hampers magnetization. Here, with the motivation of simplifying the production process, we present a study to maximize the magnetic performance of strontium ferrite ceramics using silica as the sole additive. A microscopic study offers insights into the grain growth mechanism activated by Silica. As a result, a compromise between relative density, coercivity and saturation magnetization is attained. It is found that sintering for 4 h up to 1200 °C with a SiO2 content of 1 wt% leads to the best compromise between coercivity, magnetization and density values. Competitive densities are reported in the absence of CaO, the usual co-additive. In addition, Confocal Raman Microscopy is employed for the first time to characterize the decomposition of strontium ferrite onto α-Fe2O3.
Composites of magnetically hard and soft phases are present in multiple and diverse applications, ranging from bulk permanent magnets in motors and generators to state-of-the-art recording media devices. The nature of the magnetic coupling between the hard and soft phases is of great technological relevance, as the macroscopic properties of the functional composite material ultimately depend on the atomic-scale interactions between phases. In this work, the hard/soft bilayer system SrFe12O19/Co has been studied based on photoemission electron microscopy combined with x-ray absorption and magnetic circular dichroism. Our experiments show that the magnetization of the hard magnetic oxide has a direction perpendicular to the layer plane, whereas the magnetization of the soft metallic overlayer remains in-plane. As a consequence, the magnetic domain patterns observed for the hard and soft phases are very different and completely uncorrelated to one another, indicating that no soft spins align with the hard phase by pure magnetodipolar arguments. The results are understood as the consequence of an absence of exchange-coupling between phases, in a scenario in which the shape anisotropy of the soft layer overcomes the Zeeman energy of the perpendicular magnetic field generated by the hard ferrite. Micromagnetic simulations of our system predict that low degrees of exchange-coupling effectively prevent substantial softening of the composite and lead to the alignment of soft and hard magnetic moments. A strategy thus emerges for the development of future hard-soft magnets, based on minimizing the degree of exchange-coupling while avoiding complete uncoupling.
We performed large-scale micromagnetic simulations on core–shell particle systems for S r F e 12 O 19 / F e and C o F e 2 O 4 / C o nanocomposites, where both magnetically hard and soft materials were considered as the core or shell materials of these particles. A detailed analysis of the influence of structural properties and exchange core–shell coupling within the same particle on the magnetization reversal of a nanocomposite was carried out, revealing different remagnetization scenarios in terms of the evolution of magnetization distributions in soft magnetic cores. The optimal nanocomposite compounds with respect to the energy product of material were predicted.
César De Julián Fernández
added an update
New works of AMPHIBIAN project pubblisehd in the " Journal of Physics D: Applied Physics "Special Issue on Ferrites for Permanent Magnet Applications"
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César De Julián Fernández
added 8 research items
The occurrence of ε-Fe2O3 in archaeological samples that have been subjected to high temperatures is gradually being detected by the use of micrometric structural characterization techniques. This work provides new information by revealing that the ε-Fe2O3 is formed as a response to temperature, the aggregation state and the position within the baked clay with respect to the nearest heat source. In addition, depending mainly on the atmospheric environment, the temperature reached by the combustion structure, the distance from the heating source and the particle aggregation, other iron oxide magnetic phases are produced. In the baked clay studied here, hematite is found over the whole range of samples studied but its magnetic contribution is negligible. Magnetite is observed at the sample surface, probably due to local atmospheric environment closest to the combustion source. Maghemite is found at all depths up to 6 cm below the sample surface. ε-Fe2O3 has a limited distribution, found within 2–3 cm of the sample surface. Furthermore, the viability of this compound as a palaeofield marker has been evaluated in both archaeological and synthetic samples. The results indicate that ε-Fe2O3 is able to register the direction of the magnetic field. Linear palaeointensity plots have been obtained in synthetic samples, although the value of the palaeofield could be, sometimes, overestimated.
Magnetic spinel ferrite MFe2O4 (M=Mn, Co, Ni, Zn) nanoparticles have been prepared via simple, green and scalable hydrothermal synthesis pathways utilizing sub- and supercritical conditions to attain specific product characteristics. The crystal-, magnetic- and micro-structures of the prepared crystallites have been elucidated through meticulous characterization employing several complementary techniques. Analysis of energy dispersive X-ray spectroscopy (EDS) and X-ray absorption near edge structure (XANES) data verifies the desired stoichiometries with divalent M and trivalent Fe ions. Robust structural characterization is carried out by simultaneous Rietveld refinement of a constrained structural model to powder X-ray diffraction (PXRD) and high-resolution neutron powder diffraction (NPD) data. The structural modeling reveals different affinities of the 3d transition metal ions for the specific crystallographic sites in the nanocrystallites, characterized by the spinel inversion degree, x, [M²⁺1-xFe³⁺x]tet[M²⁺xFe³⁺2-x]octO4, compared to the well-established bulk structures. The MnFe2O4 and CoFe2O4 nanocrystallites exhibit random disordered spinel structures (x=0.643(3) and 0.660(6)), while NiFe2O4 is a completely inverse spinel (x=1.00) and ZnFe2O4 is close to a normal spinel (x=0.166(10)). Furthermore, the size, size distribution and morphology of the nanoparticles have been assessed by peak profile analysis of the diffraction data, transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM). The differences in microstructure, spinel inversion and distinct magnetic nature of the M²⁺ ions directly alter the magnetic structures of the crystallites at the atomic-scale and consequently the macroscopic magnetic properties of the materials. The present study serves as an important structural benchmark for the rapidly expanding field of spinel ferrite nanoparticle research.
Magnetic materials are ubiquitous in electric devices and motors making them indispensable for modern-day society. The hexaferrites currently constitute the most widely used permanent magnets (PMs), accounting for 85% (by weight) of the PMs global sales. This work presents a complete bottom-up nanostructuring protocol for preparation of magnetically aligned, high-performance hexaferrite PMs with a record-high BHmax for dry-processed ferrites. The procedure includes the supercritical hydrothermal flow synthesis of anisotropic magnetic-single-domain strontium hexaferrite (SrFe12O19) nanocrystallites of various sizes, and their subsequent compaction into bulk magnets by spark plasma sintering (SPS). Interestingly, Rietveld modeling of neutron powder diffraction data reveals a significant difference between the magnetic structure of the thinnest nanoplatelets and the bulk compound, indicating the Sr-containing atomic layer to be the termination layer. Subsequently, high-density SrFe12O19 magnets (>95% of the theoretical density) are produced by SPS of the flow-synthesized nanoplatelets. Texture analysis by X-ray pole figure measurements demonstrates how the anisotropic shape of the nanoplatelets causes a self-induced alignment during SPS, without application of an external magnetic field. The self-induced texture is accompanied by crystallite growth along the magnetic easy-axis, i.e. the thickness of the platelets, resulting in high-performance PMs with square hysteresis curves and BHmax of 30 kJ/m³. The BHmax is further enhanced by annealing, reaching 36 kJ/m³ after 4 h at 850 °C, which exceeds the BHmax of the highest grade of dry processed commercial ferrites worldwide.
César De Julián Fernández
added 8 research items
Antiferromagnetic(AFM)|ferrimagnetic(FiM) core|shell (CS) nanoparticles (NPs) of formula Co0.3Fe0.7O|Co0.6Fe2.4O4 with mean diameter from 6 to 18 nm have been synthesized through a one-pot thermal decomposition process. The CS structure has been generated by topotaxial oxidation of the core region, leading to the formation of a highly monodisperse single inverted AFM|FiM CS system with variable AFM-core diameter and constant FiM-shell thickness (~2 nm). The sharp interface, the high structural matching between both phases and the good crystallinity of the AFM material have been structurally demonstrated and are corroborated by the robust exchange-coupling between AFM and FiM phases, which gives rise to one among the largest exchange bias (HE) values ever reported for CS NPs (8.6 kOe) and to a strongly enhanced coercive field (HC). In addition, the investigation of the magnetic properties as a function of the AFM-core size (dAFM), revealed a non-monotonous trend of both HC and HE, which display a maximum value for dAFM = 5 nm (19.3 and 8.6 kOe, respectively). These properties induce a huge improvement of the capability of storing energy of the material, a result which suggests that the combination of highly anisotropic AFM|FiM materials can be an efficient strategy towards the realization of novel Rare Earth-free permanent magnets.
We demonstrate that ion beam sputtering is a suitable deposition technique to obtain single-phase and highly (111)-oriented CoO and Co3O4 thin films. The control of substrate temperature and composition of the reactive atmosphere during deposition allows tailoring the phase structure and preferential growth of the films. Optimum substrate temperatures of 773 K and 603 K for CoO and Co3O4, respectively, have been determined and the presence of the single CoO and Co3O4 phases on the films has been confirmed by X-ray diffraction, X-ray photoelectron spectroscopy, X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopies. Transmittance differences in the visible region higher than 50% have been observed between low transmittance in Co3O4 films and high transmittance in CoO films. The semiconducting behavior of Co3O4 thin films was confirmed by temperature dependent resistance measurements. The Co3O4 thin films also show a dual optical band gap at energies of 1.47 and 2.14 eV associated to d-d and p-d transition, respectively. Additionally, at intermediate temperatures between the optimum ones to growth (111)-oriented single phase CoO and Co3O4 films, only one single phase, either CoO or Co3O4, is present in the films, in which the electrical and structural properties can be controlled by the oxygen partial pressure used during deposition.
Zinc substitution is often proposed as an efficient strategy to improve the performances of spinel ferrite nanoparticles, particularly related to their application as theranostic agents. In this work, a series of 8 nm spinel ferrite nanoparticles of formula CoxZnyFe3-(x+y)O4, is synthesized by thermal decomposition with the purpose of investigating the role of Zn ions in modifying the structural and magnetic properties. Contrary to most of the literature on this subject, where the sum of Co and Zn is kept constant (x+y=1), here the amount of Co is maintained at ca. x = 0.6, corresponding to the maximum of magnetic anisotropy of the Zn-undoped system, while the amount of Zn is progressively varied along the series from y = 0.05 to y = 0.4. This approach allows enlightening the effect of the Zn introduction on the magnetic and crystal structures and, particularly, on magnetic anisotropy, which is deeply investigated by several complementary techniques. A significant increase of the saturation magnetization, MS, upon the Zn content up to y = 0.4 is confirmed only at low temperature, while at room temperature this effect is partially nullified by the weakening of the magnetic exchange coupling constants, due to the increasing Zn substitution. Moreover, we demonstrate that the lattice modifications following the Zn introduction are responsible of a strong decrease of the particle magnetic anisotropy. Overall, these effects limit the use of Zn-substituted ferrites in biomedical applications like MRI and magnetic fluid hyperthermia, only to very low amount of Zn, as here confirmed by relaxometric and calorimetric measurements.
César De Julián Fernández
added 5 research items
We show that it is possible to tune the Néel temperature of nickel(II)-cobalt(II) oxide films by changing the Ni to Co ratio. We grow single crystalline micrometric triangular islands with tens of nanometers thickness on a Ru(0001) substrate using high temperature oxygen-assisted molecular beam epitaxy. Composition is controlled by adjusting the deposition rates of Co and Ni. The morphology, shape, crystal structure and composition are determined by low-energy electron microscopy and diffraction, and synchrotron-based x-ray absorption spectromicroscopy. The antiferromagnetic order is observed by x-ray magnetic linear dichroism. Antiferromagnetic domains up to micrometer width are observed.
Several M-type SrFe12O19 nanoparticle samples with different morphologies have been synthesized by different hydrothermal and sol-gel synthesis methods. Combined Rietveld refinements of neutron and X-ray powder diffraction data with a constrained structural model reveal a clear correlation between crystallite size and long-range magnetic order, which influences the macroscopic magnetic properties of the sample. The tailor-made powder samples were compacted into dense bulk magnets (>90% of the theoretical density) by spark plasma sintering (SPS). Powder diffraction as well as X-ray and neutron pole figure measurements and analyses have been carried out on the compacted specimens in order to characterize the nuclear (structural) and magnetic alignment of the crystallites within the dense magnets. The obtained results, combined with macroscopic magnetic measurements, reveal a direct influence of the nanoparticle morphology on the self-induced texture, crystallite growth during compaction and macroscopic magnetic performance. An increasing diameter-to-thickness aspect ratio of the platelet-like nanoparticles leads to increasing degree of crystallite alignment achieved by SPS. Consequently, magnetically aligned, highly dense magnets with excellent magnetic performance (30(3) kJ/m³) are obtained solely by nanostructuring means, without application of an external magnetic field before or during compaction. The demonstrated control over nanoparticle morphology and, in turn, crystal and magnetic texture is a key step on the way to designing nanostructured hexaferrite magnets with optimized performance.
César De Julián Fernández
added 7 research items
We show that it is possible to tune the Néel temperature of nickel(II)-cobalt(II) oxide films by changing the Ni to Co ratio. We grow single crystalline micrometric triangular islands with tens of nanometers thickness on a Ru(0001) substrate using high temperature oxygen-assisted molecular beam epitaxy. Composition is controlled by adjusting the deposition rates of Co and Ni. The morphology, shape, crystal structure and composition are determined by low-energy electron microscopy and diffraction, and synchrotron-based x-ray absorption spectromicroscopy. The antiferromagnetic order is observed by x-ray magnetic linear dichroism. Antiferromagnetic domains up to micrometer width are observed.
A combination of high energy ball milling, vacuum filtering and sedimentation processes has been demostrated as an useful approach to reduce, in a controlled way, the length of as-cast Fe73.5Si13.5Nb3Cu1B9 amorphous magnetic microwires (MWs) and annealed material at 550 ºC in nitrogen condidions. Homogeneous compositional microstructures with fairly narrow size distributions between 1300 µm and 11.7 µm are achieved exhibiting tunable response as a soft magnetic material and as a microwave absorber. From the magnetic perspective, the soft magnetic character is increased with smaller length of the MWs whereas the remanence has the opposite behavior mainly due to the structural defects and the loss of the shape anisotropy. From the microwave absorption perspective, a novel potential applicability is tested in these refine microstructures. According to this innovative line, coatings based on commercial paints with a filling percentage of 0.55% of MWs with different lengths are deposited on metallic sheets. Large attenuation values around -40 dB are obtained in narrow spectral windows located in the GHz range and their position can be varied by combining different optimized lengths of MW. As an example of this powerful mechanism for absorbing microwaves at specific frequencies, MW lengths of 2 mm and 50 µm are chosen, where a precise tailoring of the minimum reflection loss (RL) is obtained in a range between 8.85 GHz and 13.25 GHz. To confirm these experimental results an effective medium standard model proposed for electrical permittivity is used. Experimental and theoretical results are consistent and these novel composites are also proposed as a feasible candidate for designing frequency selective microwave absorbers on demand, with low filling percentages and high absorption intensity values.
Nanoparticles usually exhibit a specific structure and composition, which can influence the development of the microstructure during their sintering. Barium hexaferrite nanoplatelets have a specific, iron-rich structure defined by the termination at the surfaces with the S blocks of their SRS*R* hexaferrite structure (S and R represent a cubic (Fe6O8)²⁺ and a hexagonal (BaFe6O11)²⁻ structural block, respectively). The unsubstituted and Sc-substituted hexaferrite nanoplatelets were hydrothermally synthesized and fired at different temperatures. A combination of morpho-structural analyses (XRD, SEM, TEM, and aberration-corrected STEM) and magnetic measurements was used to reveal the evolution of the microstructure during sintering. During the initial stages of sintering the nanoplatelets thicken predominantly by the fusion of individual original nanoplatelets. Due to the Fe-rich surfaces of the nanoplatelets, the fusion growth results in an inhomogeneity that leads to the formation of planar defects in the grains and the precipitation of Fe2O3 as the secondary phase. In the Sc-substituted hexaferrite grains, superstructural compositional ordering was detected for the first time. The Sc substitution caused exaggerated grain growth in barium hexaferrite ceramics sintered at 1300 °C.
Sergey Erokhin
added a research item
We present a numerical algorithm for predicting the optimal conditions for the effective alignment of magnetic particles in dense powders during the compactization process using an externally applied field. This task is especially important for the permanent magnets development due to the fact that alignment of anisotropy axes of nanocomposite grains increases both remanence and coercivity of magnetic materials. In contrast to previously known methods where magnetic moment of each particle was assumed to be 'fixed' with respect to the particle itself, our approach takes into account the (field-dependent) deviation of this moment from the particle anisotropy axis that occurs even for magnetically 'hard' particles possessing a strong mechanical contact. We show, that this deviation leads to the existence of the optimal value of the applied field for which the particle orientation (or alignment) time is minimal. The influence of the external pressure and internal mechanical friction on the details of the compactization/orientation process is also studied.
César De Julián Fernández
added a research item
We report the results of an unpolarized small-angle neutron-scattering (SANS) study on Mn-Zn ferrite (MZFO) magnetic nanoparticles with the aim to elucidate the interplay between their particle size and the magnetization configuration. We study different samples of single-crystalline MZFO nanoparticles with average diameters ranging between 8 to 80 nm, and demonstrate that the smallest particles are homogeneously magnetized. However, with increasing nanoparticle size, we observe the transition from a uniform to a nonuniform magnetization state. Field-dependent results for the correlation function confirm that the internal spin disorder is suppressed with increasing field strength. The experimental SANS data are supported by the results of micromagnetic simulations, which confirm an increasing inhomogeneity of the magnetization profile of the nanoparticle with increasing size. The results presented demonstrate the unique ability of SANS to detect even very small deviations of the magnetization state from the homogeneous one.
César De Julián Fernández
added a research item
Platelets of strontium hexaferrite (SrFe12O19, SFO), up to several micrometers in width, and tens of nanometers thick have been synthesized by a hydrothermal method. They have been studied by a combination of structural and magnetic techniques, with emphasis on Mössbauer spectroscopy and X-ray absorption based-measurements including spectroscopy and microscopy on the iron-L edges and the oxygen-K edge, allowing us to establish the differences and similarities between our synthesized nanostructures and commercial powders. The Mössbauer spectra reveal a greater contribution of iron tetrahedral sites in platelets in comparison to pure bulk material. For reference, high-resolution absorption and dichroic spectra have also been measured both from the platelets and from pure bulk material. The O-K edge has been reproduced by density functional theory calculations. Out-of-plane domains were observed with 180° domain walls less than 20 nm width, in good agreement with micromagnetic simulations.
César De Julián Fernández
added an update
AMPHIBIAN partners will present several contributions in the Joint European Magnetic Symposia, JEMS, (https://jems2019.se/) that will take place the 26-30 August week in Uppsala.
Thursday Morning in the Materials for energy session
11:30 – 11:45 O117 – A Growing Improvement: Controlling the Nano-structuring of Ferrite Magnets
Frederik H. Gjørup,Matilde Saura-Múzquiz, Jakob V. Ahlburg ,Anna Z. Eikeland ,Mathias I. Mørch, Jennifer Hoelscher,Mogens Christensen
Friday morning in the Materials for energy session
10:45 – 11:00 O131 Ferrite magnets improved through size, shape and texture control
Matilde Saura-Múzquiz , Cecilia Granados-Miralles, Anna Eikeland, Frederik Gjørup, Mogens Christensen
11:45 – 12:00 O144 – Study of the high-coercivity of the Al and Cr doped Strontium hexaferrites
Durgamadhab Mishra,Marian Stingaciu ,Anna Zink Eikeland ,Riccardo Cabassi,Fulvio Bolzoni, Michele Petrecca, Blaž Belec, Franca Albertini, Claudio Sangregorio, Mogens Christensen, Stefano Deledda, César De Julián Fernández
 
Michele Petrecca
added a research item
The synthesis of highly compacted, nanostructured soft magnets is highly desirable due to their promising properties for the development of electronic devices working at frequency higher than 2 MHz. In this work we investigated the potentiality of High Pressure Field Assisted Sintering Technique (HP-FAST). To this aim, we first synthesized soft Mn-Zn ferrite magnetic nanoparticles (MNPs) through an easy-scalable, eco-friendly strategy based on aqueous co-precipitation in basic media, starting from transition metal chlorides. Powder X-ray diffraction (PXRD) and Transmission Electron Microscopy (TEM) analyses evidenced the formation of crystalline nanoparticles with the cubic spinel structure and average crystal size of 7.5 nm. Standard magnetometric measurements showed a saturation magnetization value of ca. 56 emu/g and no magnetic irreversibility at room temperature. The MNPs were then compacted applying an uniaxial pressure over a toroidal shaped die. In order to obtain a material with a density close to the bulk one, the as-prepared green toroids underwent either a classic sintering treatment, obtaining a microstructured system, or to High Pressure Field Assisted Sintering Technique (HP-FAST), which allowed for preserving the nanostructure. The relative permeability and core losses of the toroidal samples were evaluated in the frequency range 1-2 MHz using an in-house built setup. The comparison of the behavior of samples obtained by the two different sintering approaches showed the nanostructured samples had a much smaller relative magnetic permeability (ten times lower than the microstructured sample) and, consequently, higher core losses. However, when samples with similar μr were compared, a significant decrease of core losses at the larger frequencies was observed. This result suggests HP-FAST is a very promising approach to prepare high density nanostructured soft magnetic materials.
César De Julián Fernández
added an update
Today begins the XVI Ecers Conference (www. Ecers2019.org) in Turin (Italy) in which several contributions of the AMPHIBIAN’s teams will be presented:
Tuesday 18th, Jesús Gúzman Mínguez will present “Improving State-of-art Strontium ferrite magnets: strategies to optimize magnetic and microstructural properties”.
Tuesday 18th, Petra Jenus will present “ANISOTROPIC FERRITE-BASED HYBRID MAGNETS CONSOLIDATED BY SPARK PLASMA SINTERING: TOWARDS RARE-EARTH-FREE MAGNETS FOR ENERGY STORAGE”
 
César De Julián Fernández
added an update
AMPHIBIAN partners will contribute in the ICFPM Conference in Gijon (Spain) (https://icfpm2019.org) presenting several talks on the results on free Rare-Earth Magnets and developments obtained in the Framework of the AMPHIBIAN project. The main contributions are :
  • -"3D magnetometry in nanomaterials using XMCD-PEEM microscopy" by S. Ruiz-Gomez, A. Quesada, M. Foerster, L. Aballe, A. Mascaraque, J. de la Figuera, L. Perez
  • "Magnetic Anisotropy of Strontium Ferrite nanoparticles" by C. de Julian Fernandez, D. Mishra, R. Cabassi, F. Bolzoni, M. Petrecca M. Albino, M. Saura-Muzquiz, P. Jenus, A. Quesada, M. Christensen, T. Schliesch, B. Belec, C. Sangregorio and F. Albertini
  • "Controlled design of exchange coupled nano-heterostructures with high energy product" by B. Muzzi, M. Albino, M. Petrecca, C. Innocenti, B. Cortigiani, G. Bertoni, C. de Julian Fernandez, C. Sangregorio.
  • "Optimization of the magnetic and the structural properties of the FeSiCuNbB particles obtained by high energy ball milling processes" J. Lopez-Sanchez, E. Navarro, A. Serrano, A. del Campo, P. Marín.
  • "Exchange coupled core-shell nanoparticles as building blocks for permanent magnet" by B. Muzzi, A. Lopez-Ortega, M. Albino, M. Petrecca, C. Innocenti, G. Bertoni, C. de Julian Fernandez, C. Sangregorio.
Several contributions of other groups on this subject will be presented.
 
Blaž Belec
added a research item
In this investigation we analyze an unprecedented difference in the behavior of nanoparticles when compared to the corresponding bulk. We have found that a chemical substitution can have the opposite effect on the magnetic properties of nanoparticles compared to the bulk, as revealed for the first time in the case of Sc-substituted barium-hexaferrite nanoplatelets. Even though the Sc substitution is known to greatly decrease the saturation magnetization, M S , of the bulk barium hexaferrite, it showed the opposite effect for nanoplatelets. The M S values of the nanoplatelets (in average 50 nm wide and approximately 3 nm thick) increased to over 38 Am ² /kg, compared to ∼16 Am ² /kg for unsubstituted nanoplatelets of comparable average size. The Sc incorporation was investigated with a combination of atomic-resolution imaging and elemental mappings in a scanning-transmission electron microscope. As in the bulk, the Sc ³⁺ ions showed a clear preference for incorporation into an R block of the hexaferrite SRS ∗ R ∗ structure for the nanoplatelets (R and S represent a hexagonal (BaFe 6 O 11 ) ²⁻ and a cubic (Fe 6 O 8 ) ²⁺ structural block, respectively). A clear difference between the nano and the bulk observed for the first time was in the partial substitution of the Sc ³⁺ for the Ba ²⁺ in the nanoplatelets; however, this cannot explain the large increase in M S . Ab-initio calculations suggest that the opposite effect of the Sc substitution in the nanoplatelets to that in the bulk can be ascribed to specific, two-dimensional magnetic ordering in the platelets.
César De Julián Fernández
added 4 research items
An understanding of the adaptation of the crystal structure of materials confined at the nanoscale, the influences of their specific structures on the evolution of their morphologies and, finally, their functional properties is essential not only for expanding fundamental knowledge, but also for facilitating the designs of novel nanostructures for diverse technological and medical applications. Here we describe how the distinct structure of barium-hexaferrite nanoplatelets evolves in a stepwise manner in parallel with the development of their size and morphology during hydrothermal synthesis. The nanoplatelets are formed by reactions between Ba- and Fe-hydroxides in an aqueous suspension at temperatures below 80 oC. Scanning-transmission electron microscopy showed that the structure of the as-synthesized, discoid nanoplatelets (2.3 nm thick, 10 nm wide) terminates at the basal surfaces with Ba-containing planes. However, after subsequent washing of the nanoplatelets with water the top two atomic layers dissolve from the surfaces. The final structure can be represented by a SRS* sequence of the barium-hexaferrite SRS*R* unit cell, where S and R represent a hexagonal (BaFe6O11)2- and a cubic (Fe6O8)2+ structural block, respectively. Due to the stable SRS* structure, the thickness of the primary nanoplatelets remains unchanged up to approximately 150 oC, when some of the primary nanoplatelets start to grow exaggeratedly and their thicknesses increase discretely with the addition of the RS segments to their structure. The SRS* structure of the primary nanoplatelets is too thin for the complete development of magnetic ordering. However, the addition of just one RS segment (SRS*R*S structure) gives the nanoplatelets hard magnetic properties.
Atomic-resolution scanning-transmission electron microscopy showed that barium hexaferrite (BHF) nanoplatelets display a distinct structure, which represents a novel structural variation of hexaferrites stabilized on the nanoscale. The structure can be presented in terms of two alternating structural blocks stacked across the nanoplatelet: a hexagonal (BaFe6O11)2- R block and a cubic (Fe6O8)2+ spinel S block. The structure of the BHF nanoplatelets comprises only two, or rarely three, R blocks and always terminates at the basal surfaces with the full S blocks. The structure of a vast majority of the nanoplatelets can be described with a SR*S*RS stacking order, corresponding to a BaFe15O23 composition. The nanoplatelets display a large, uniaxial magnetic anisotropy with the easy axis perpendicular to the platelet, which is a crucial property enabling different novel applications based on aligning the nanoplatelets with applied magnetic fields. However, the HF nanoplatelets exhibit a modest saturation magnetization, Ms, of just over 30 emu/g. Given the cubic S block termination of the platelets, layers of maghemite, gama-Fe2O3, (M), with a cubic spinel structure, can be easily grown epitaxially on the surfaces of the platelets, forming a sandwiched M/BHF/M platelet structure. The exchange-coupled composite nanoplatelets exhibit a remarkably uniform structure, with an enhanced Ms of more than 50 emu/g while essentially maintaining the out-of-plane easy axis. The enhanced Ms could pave the way for their use in diverse platelet-based magnetic applications.
César De Julián Fernández
added a research item
The magnetic properties of SrFe 12 O 19 nanocrystallites produced by hydrothermal synthesis and consolidated by Spark Plasma Sintering (SPS) were optimized by varying the compaction parameters: sintering time, sintering temperature, uniaxial pressure or pre-compaction in a magnetic field. Highly textured compacts with a high degree of crystallite alignment were produced. Qualitative and quantitative textural information was obtained based on X-ray diffraction pole figure measurements. The optimum sintering conditions, relating the degree of alignment and bulk magnetic properties, were identified based on the resulting magnetic properties. It was found that one must strike a balance between the degree of crystallite alignment for high saturation magnetisation and coercivity (H c ) to gain the highest energy product (BH max ). It was found that the coercive field drops when the crystallite alignment increases. This was particularly pronounced in the case of magnetically pre-aligned powders prior to SPS, where H c and BH max decreased as the pellets became increasingly textured. The best BH max value of 29(4) kJ m ⁻³ was found for the sample sintered at 950 °C for 2 minutes with an applied pressure of 100 MPa for a powder pre-aligned in an applied field of 0.55 T. The results presented here show the potential of SPS consolidation of SrFe 12 O 19 with high relative densities and emphasize the effect of the degree of alignment on the decrease of coercive field and its influence on the magnetic performance.
Adrian Quesada
added a research item
Exchange-coupled hard-soft biphase magnets are technologically relevant systems in that they enable tailoring the magnetization reversal process. Here, exchange-spring behavior is observed in CoFe2O4/FeCo bilayers for soft thicknesses as thin as 2 nm, at least four times below the exchange length of the system. This result is in contrast with the accepted theory for spring magnets that states that the exchange length defines the critical thickness below which both magnetic phases should be rigidly coupled. In combination with micromagnetic calculations, this surprising observation is understood as a consequence of the dominance of domain-wall propagation in the soft phase during the reversal process, so far unaccounted for in theoretical descriptions. Our results emphasize the need to expand the existing spring theory from coherent rotation to domain-wall related processes in multidomain configurations in order to accurately design magnetic heterostructures with controllable reversal.
César De Julián Fernández
added a research item
Nanocrystallites of the permanent magnetic material SrFe12O19 were synthesised using a conventional sol-gel (CSG) and a modified sol-gel (MSG) synthesis route. In the MSG synthesis, crystallite growth takes place in a solid NaCl matrix, resulting in freestanding nanocrystallites, as opposed to the CSG synthesis, where the produced nanocrystals are strongly intergrown. The resulting nanocrystallites from both methods exhibit similar intrinsic magnetic properties, but significantly different morphology and degree of aggregation. The nanocrystallites were compacted into dense pellets using a Spark Plasma Sintering (SPS) press, this allows investigating the influence of crystallite morphology and the alignment of the nanocrystallites on the magnetic performance. A remarkable correlation was observed between the crystallites morphology and their ability to align in the compaction process. Consequently, a significant enhancement of the maximum energy product was obtained after SPS for the MSG prepared sample (22.0 kJ/m3), compared to CSG sample, which achieved an energy product of 11.6 kJ/m3.
César De Julián Fernández
added an update
Session: Y4 - Thin Film and Hybrid Nanostructures III
Talk: Y4-01: Studying The 3D Magnetization Of Ultrathin And Antiphase-Boundary Free Spinel Crystals.
Time and date: 1:30pm - 2:00pm: Fri, Jul 20
 
César De Julián Fernández
added an update
The talk will be on Tuesday, at 16.15 in a Session K5: MATERIALS FOR ENERGY APPLICATIONS II (which starts at 15.30) K5-03. Spark plasma sintering of ferrite-based hybrid magnets: towards rare-earth-free magnets for energy storage
 
César De Julián Fernández
added an update
César de Julian Fernandez, from IMEM-CNR, will speak in the Session FG "Magnetic Materials for Energy" on hybrid ferrite-based hard nanomaterials for magnets.
More details in
 
Adrian Quesada
added 2 research items
We have grown high quality magnetite microcrystals free from antiphase boundaries on Ru(0001) by reactive molecular beam epitaxy, conserving bulk magnetic properties below 20 nm thickness. Magnetization vector maps are obtained by X-ray spectromicroscopy and compared with micromagnetic simulations. The observed domain configurations are dictated purely by shape anisotropy, overcoming the possible influences of (magneto)crystalline anisotropy and defects, thus demonstrating the possibility of designing spin structures in ultrathin, magnetically soft magnetite at will.
We here present a simple model of a vibrating sample magnetometer (VSM). The system allows recording magnetization curves at room temperature with a resolution of the order of 0.01 emu and is appropriated for macroscopic samples. The setup can be mounted with different configurations depending on the requirements of the sample to be measured (mass, saturation magnetization, saturation field, etc.). We also include here examples of curves obtained with our setup and comparison curves measured with a standard commercial VSM that confirms the reliability of our device.
César De Julián Fernández
added a project goal
To develop up-scalable and cost-efficient methods for manufacturing improved rare earth free permanent magnets. We aim at producing permanent magnets with an enhanced performance with respect to already existing rare earth free magnets; and making them more enduring and sustainable. AMPHIBIAN is a Research and Innovation project funded by the European Commission. Grant agreement H2020-NMBP-2016-720853.