Magnetic performance of SrFe12O19–Zn0.2Fe2.8O4 hybrid magnets prepared by spark plasma sintering

[EN] 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, BHmax, 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 μm 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.This work is supported by the European Commission through Project H2020 No. 720853 (AMPHIBIAN) and Slovenian Research Agency is acknowledged for funding the research program Ceramics and complementary materials for advanced engineering and biomedical applications (P2-0087), CEMM, JSI is acknowledged for the use of EM.Peer reviewe

Magnetic performance of SrFe12O19-Zn0.2Fe2.8O4 hybrid magnets prepared by Spark Plasma Sintering

[EN] 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, BHmax, 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 μm 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.This work is supported by the European Commission through Project H2020 No. 720853 (AMPHIBIAN) and Slovenian Research Agency is acknowledged for funding the research program Ceramics and complementary materials for advanced engineering and biomedical applications (P2-0087), CEMM, JSI is acknowledged for the use of EM.Peer reviewe

Strongly Exchange Coupled Core|Shell Nanoparticles with High Magnetic Anisotropy: A Strategy toward Rare-Earth-Free Permanent Magnets

Antiferromagnetic­(AFM)|ferrimagnetic­(FiM) core|shell (CS) nanoparticles (NPs) of formula Co_0.3Fe_0.7O|Co_0.6Fe_2.4O₄ 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 (<i>H</i> _E) values ever reported for CS NPs (8.6 kOe) and to a strongly enhanced coercive field (<i>H</i> _C). In addition, the investigation of the magnetic properties as a function of the AFM-core size (<i>d</i> _AFM), revealed a nonmonotonous trend of both <i>H</i> _C and <i>H</i> _E, which display a maximum value for <i>d</i> _AFM = 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 toward the realization of novel rare-earth-free permanent magnets

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 μm 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 °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.We would like to thank Dr. Vić tor Fuertes for his advice on the processing of the bonded magnets. This work is supported by the Spanish Ministerio de Economía y Competitividad y Ministerio de Ciencia e Innovación (Project Nos. MAT2017- 86450-C4-1-R, MAT2015-64110-C2-1-P, MAT2015-64110- C2-2-P, MAT2017-87072-C4-2-P, RTI2018-095303-A-C52, and FIS2017-82415-R) and by the European Commission through Project H2020 (No. 720853; AMPHIBIAN). C.G.-M. acknowledges financial support from MICINN through the “Juan de la Cierva” Program (FJC2018-035532-I). A.Q. acknowledges financial support from MICINN through the “Ramón y Cajal” Program (RYC-2017-23320). The work also is funded by the Regional Government of Madrid (Project S2018/ NMT-4321; NANOMAGCOST)

Exploring the Effect of Co Doping in Fine Maghemite Nanoparticles

We present a study of the structural, magnetic, and magneto-optical properties of a series of Co-substituted ferrite nanoparticles (NPs) prepared by thermal decomposition of metallo-organic precursors in high boiling solvents. The structural characterization, carried out by using several techniques (transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), and magnetic circular dichroism measurements), showed all the samples are high crystalline, 5–6 nm spherical NPs with the cubic spinel structure typical of ferrites. The evolution of lattice parameters with cobalt content suggests that the material is Co-substituted maghemite, also confirmed by XAS and magneto optical (MO) characterizations. The investigation of the magnetic and magneto-optical properties displays peculiar trends with the cobalt content, the main features being the large increase of the saturation magnetization and the anomalous dependence of magnetic anisotropy which reaches its maximum values for intermediate compositions. The large tuneability of this material makes it possible to implement the performances of devices used in biomedical and sensing applications