Search

[EN] 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 Am2 kg−1) and a series of nanosized zinc-doped ferrite (ZnxFe3−xO4, 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 Am2 kg−1) ferrite was obtained by annealing Zn0.3Fe2.7O4 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 μm) 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., We acknowledge Dr S Ciattini and Dr L Chelazzi of the Centro Servizi di Cristallografia Strutturale at the University of Florence for help in x-ray powder diffraction data collection, Dr L Capozzoli and Dr A Lavacchi of Ce.M.E.—Centro di Microscopie Elettroniche Laura Bonzi of CNR for SEM and STEM images, Professor S Caporali and Dr S Martinuzzi of the University of Florence for SEM images, and the European Commission for funds through project H2020 No. 720853 (AMPHIBIAN).

[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.

Recent studies on magnetic nanoparticles (MNPs) used for Magnetic Fluid Hyperthermia treatments have shown that Brownian rotation is suppressed when they are confined within a cell. To investigate this effect we conducted a systematic study of the Specific Absorption Rate (SAR) of colloidal suspensions of MNPs in water and gels at different agarose concentration. SAR measurements were conducted by varying the frequency (f = 110–990 kHz) and amplitude (up to 17 kA/m) of the applied alternating magnetic field (AMF). MNP samples with different diameter (d = 10, 14, and 18 nm) were used. Our results show that Neel relaxation dominates SAR with negligible contribution from Brownian motion for smaller MNPs (d = 10 nm). For the largest MNPs (d = 18 nm) we observed a more significant SAR decrease in gel suspensions as compared to those in solution. In particular, when applying AMFs as the ones used in a clinical setting (16.2 kA/m at f = 110 kHz), we measured SAR value of 67 W/g in solution and 25 W/g in gel. This experimental finding demonstrates that investigation of MNPs properties should be conducted in media with viscosity similar to the one found in mammalian tissues.

[EN] We present here an investigation aimed at exploring the role of the microstructure on the magnetic properties of nanostructured cobalt ferrite. Bulk, almost fully dense, nanograined ferrites have been obtained starting from nanopowders prepared by a simple, inexpensive, water-based, modified Pechini method. This synthesis yielded largely aggregated, pure single-phase cobalt ferrite nanoparticles of ca. 35 nm average size, which have been then densified by high-pressure field-assisted sintering. Different sintering conditions (pressure up to 650 MPa and temperature up to 800 °C) and procedures have been used on both as-prepared and milled nanopowders in order to obtain materials with a spectrum of complex microstructures. In all cases, the sintering process did not produce any change in the phase composition. At the same time, using a high uniaxial pressure in combination with relatively low sintering temperatures and times, allowed for obtaining a high degree of densification while preserving the nanometric size of the crystallites. Moreover, we observed that in the densified materials the best magnetic properties are not necessarily associated with a more uniform microstructure, but rather arise from a delicate balance between moderate aggregation, grain size and high density., European Commission for funds through project H2020 No. 720853 (Amphibian).

30 pags., 22 figs., 2 tabs. Permanent magnets based on hard hexaferrite represent the largest family of magnets being used today by volume. They generate moderate remanence induction, but present crucial advantages in terms of availability, cost, resistance to demagnetization and corrosion and absence of eddy current losses. As a consequence, ferrites are the most logical candidate for substitution of rare-earths in selected applications that do not demand the best performing magnets. If the remanence of ferrite-based magnets was to be improved, even mildly, the door to a larger scale substitution could be opened. In this framework, we review here current strategies to improve the properties of hexaferrites for permanent magnet applications. We first discuss the potential of exploring the nanoscale. Second, progress related to controllably doping hexaferrites is revised. Third, results achieved by fabricating hard-soft magnetic composites using ferrites as the hard phase are presented. Finally, future prospects and new potential end applications for ferrite magnets are discussed. This work was supported by the Slovenian Research Agency through research programs Nos. P2-0089 and P2-0412 and project J2-8166, by the Spanish Ministry of Science and Innovation through projects RTI2018-095303-BC51 and RTI2018-095303-A-C52, by the European Commission through Project H2020 No. 720853 (Amphibian) and by the Regional Government of Madrid through project S2018-NMT-4321.