Towards ferrite based rare-earth free permanent magnets: from model systems to new technological applications

Tesis Doctoral inédita leída en la Universidad Autónoma de Madrid, Facultad de Ciencias, Departamento de Física la Materia Condensada y Nanotecnología. Fecha de lectura: 19-12-2017Esta tesis tiene embargado el acceso al texto completo hasta el 19-06-2019Permanent magnets are essential in many applications of very relevant technological areas (transport, communications technology, energy) and are present in virtually all smart devices. However, they are not without controversy and have generated in recent years serious economic and political problems, as well as having important repercussions on the environment. In 2012 the global alarm was raised due to the monopoly derived from the strategic geographical situation of the so-called rare earths, fundamental constituent elements of these materials. Advances in nanoscience and nanotechnology are key in the search for alternatives to permanent magnets based on rare earths. In this sense, and in a general way, this thesis work combines fundamental studies in nanomagnetism with energetically efficient technological processes in order to be able to develop permanent magnets free of competitive rare earth of last generation, as well as to implement new technological applications. To this end, the objectives set out in the present study have included: 1) The study of rare earth-free magnetic systems exploiting anisotropy, shape and microstructure in both model systems (epitaxial layers and manganese nanowires) and in isotropic ferrite powders. 2) The search for general relations to improve / enhance the magnetic properties of rare earthfree materials based on nanostructured ferrites by effective, reproducible and scalable methods. 3) Understanding and controlling the microstructural effects on the magnetic properties of treated and refined ferrite isotropic powders. 4) The development of new methodologies to enhance the properties of permanent magnets based on isotropic ferrite powder, and the prototyping of new applications making use of the permanent magnets free of developed rare earths. From the scientific point of view, it is necessary to highlight the microscopic determination of the magnetization reversal processes in model systems as well as the experimental demonstration of the generic effects induced by engineered microstructure (tensions and grain size) on the magnetic properties in processed isotropic powders. From the technological point of view, to review the development of an efficient, reproducible and scalable methodology to produce isotropic powder with improved magnetic properties (coercivity and / or (BH)max product) as well as the design and prototyping of new technological applications.

Study of Phases Evolution in High-coercive MnAl Powders Obtained Through Short Milling Time of Gas-atomized Particles

Gas-atomized Mn54Al46 particles constituted nominally of only ε- and γ2-phases, i.e. no content of the ferromagnetic L10-type τ-phase, have been used to study the evolution of phases during short time of high-energy milling and subsequent annealing. Milling for 3 min is sufficient to begin formation of the τ-MnAl phase. A large coercivity of 4.9 kOe has been obtained in milled powder after annealing at 355 °C for 10 min. The large increase in coercivity, by comparison with the lower value of 1.8 kOe obtained for the starting material after the same annealing conditions, is attributed to the combined formation of the τ-MnAl and β-Mn phases and the creation of a very fine microstructure with grain sizes on the order of 20 nm. Correlation between morphology, microstructure and magnetic properties of the rapidly milled MnAl powders constitutes a technological advance to prepare highly coercive MnAl powders.United States Department of Energy AR0000188Ministerio de Economía y Competitividad MAT2014-56955-R, PCIN-2015-126, MAT2013- 45165-P, PEJ-2014Comunidad Autónoma de Madrid S2013/MIT-28

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

Synthesis and characterisation of CoFe2O4 + CaCu3Ti4O12composite of ferrite

Composite of cobalt ferrite (CoFe2O4) and CCTO are fabricated by chemical route method and its dielectric and structural properties are investigated. The composite powder fired at and above 8000c and have perovskite structure. The composite then formed pellet and sintered at 9000C. It possess of both magnetic as well as dielectric property simultaneously. With X-ray diffraction, the phase transformation is confirmed. By the help of SEM characterization, the surface morphology of composite is confirmed. With EDAX images the percentage of elements in composite is also confirmed.Its dielectric properties have been studied in the temperature range 300-700 K. It is found that dielectric constant (ɛ) increases with increase in temperature.The dielectric loss increases with increase in temperature, irrespective of CCTO transition temperature, that ensures the semiconducting nature or thermally activated mechanism of conduction in the composites

Exchange-Coupling Behavior in SrFe12O19/La0.7Sr0.3MnO3 Nanocomposites

Magnetically hard-soft (100-x) SrFe12O19–x wt % La0.7Sr0.3MnO3 nanocomposites were synthesized via a one-pot auto-combustion technique using nitrate salts followed by heat treatment in air at 950 °C. X-ray diffraction (XRD), transmission electron microscopy (TEM), and vibrating sample magnetometry (VSM) were used to characterize the structural and magnetic properties of the samples. XRD spectra revealed the formation of a mixture of ferrite and magnetite phases without any trace of secondary phases in the composite. Microstructural images show the proximity grain growth of both phases. The room temperature hysteresis loops of the samples showed the presence of exchange-coupling between the hard and soft phases of the composite. Although saturation magnetization reduced by 41%, the squareness ratio and coercivity of the nanocomposite improved significantly up to 6.6% and 81.7%, respectively, at x = 40 wt % soft phase content in the nanocomposite. The enhancement in squareness ratio and coercivity could be attributed to the effective exchange-coupling interaction, while the reduction in saturation magnetization could be explained on the basis of atomic intermixing between phases in the system. Overall, these composite particles exhibited magnetically single-phase behavior. The adopted synthesis method is low cost and rapid and results in pure crystalline nanocomposite powder. This simple method is a promising way to tailor and enhance the magnetic properties of oxide-based hard-soft magnetic nanocomposites

Approaching Ferrite-Based Exchange-Coupled Nanocomposites as Permanent Magnets

During the past decade, CoFe2O4 (hard)/Co-Fe alloy (soft) magnetic nanocomposites have been routinely prepared by partial reduction of CoFe2O4 nanoparticles. Monoxide (i.e., FeO or CoO) has often been detected as a byproduct of the reduction, although it remains unclear whether the formation of this phase occurs during the reduction itself or at a later stage. Here, a novel reaction cell was designed to monitor the reduction in situ using synchrotron powder X-ray diffraction (PXRD). Sequential Rietveld refinements of the in situ data yielded time-resolved information on the sample composition and confirmed that the monoxide is generated as an intermediate phase. The macroscopic magnetic properties of samples at different reduction stages were measured by means of vibrating sample magnetometry (VSM), revealing a magnetic softening with increasing soft phase content, which was too pronounced to be exclusively explained by the introduction of soft material in the system. The elemental compositions of the constituent phases were obtained from joint Rietveld refinements of ex situ high-resolution PXRD and neutron powder diffraction (NPD) data. It was found that the alloy has a tendency to emerge in a Co-rich form, inducing a Co deficiency on the remaining spinel phase, which can explain the early softening of the magnetic material

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

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

Cation distribution of cobalt ferrite electrosynthesized nanoparticles: A methodological comparison

Final publication at http://doi.org/10.1016/j.jallcom.2017.12.342, © 2017 Elsevier B.V.The present work seeks to analyse the structural and magnetic properties of cobalt ferrite nanoparticles obtained by electrochemical synthesis by high-resolution transmission electronic microscopy (HRTEM), X-ray absorption spectroscopy (XAS), Mössbauer spectroscopy (MS), neutron diffraction (ND) and SQUID magnetometer. The cationic distribution is analyzed by different techniques. The inversion degree determined by the most accurate measurements was 0.73(1), and the formula for the nanoparticles therefore was (↑Co 0.27 Fe 0.73 )[↓Co 0.73 Fe 1.27 ]O 4 . The magnetic moment found from DC and Mössbauer spectroscopy measurements was 3.8(3) μB, and the coercivity was 7870 Oe at 100 K.This work is supported by the MINECO/FEDER Project MAT2015-67557-C2-2-

Cation distribution of cobalt ferrite electrosynthesized nanoparticles. A methodological comparison

The present work seeks to analyse the structural and magnetic properties of cobalt ferrite nanoparticles obtained by electrochemical synthesis by high-resolution transmission electronic microscopy (HRTEM), X-ray absorption spectroscopy (XAS), Mössbauer spectroscopy (MS), neutron diffraction (ND) and SQUID magnetometer. The cationic distribution is analyzed by different techniques. The inversion degree determined by the most accurate measurements was 0.73(1), and the formula for the nanoparticles therefore was (↑Co0.27Fe0.73)[↓Co0.73Fe1.27]O4. The magnetic moment found from DC and Mössbauer spectroscopy measurements was 3.8(3) μB, and the coercivity was 7870 Oe at 100 K.This work is supported by the MINECO/FEDER Project MAT2015- 67557-C2-2-P. The authors are grateful to the Institut Laue Langevin and the Spanish CRG D1B for the neutron beam-time allocated (experiment codes 5-31-2259 and CRG-1940; https://doi.org/10. 5291/ILL-DATA.5-31-2259) and to the SpLine CRG beamline staff at ESRF for assistance during XAS experiments