Permanent magnets (PMs) have tremendous demand because of their applicability in various electronic devices. Rare earth (RE) elements are the major component in fabricating these PMs. Tremendous research interest has been generated over the last decade to design the RE-free PMs because of the ever-increasing cost of REs and shortage of their supply. Hexagonal ferrites show incredible potential as RE-free magnets owing to their high coercivity and moderate saturation magnetization. In this work, we focused on improving the magnetic properties of Sr0.8Ca0.2AlxCoxFe12−2xO19 hexagonal ferrites with respect to Co2+–Al3+ co-substitution synthesized by the sonochemical method. Improved saturation magnetization of 64.4 emu g−1, coercivity of 13.2 kOe, and a remnant ratio of 0.71 was achieved upon Co2+–Al3+ substitution. The prepared materials were also tested for microwave absorption characteristics, where a maximum reflection loss of −35.4 dB at 12.36 GHz was obtained. Results were discussed in light of the synthesis method, Co2+–Al3+ substitution, and electrical properties. Such improvement in the electromagnetic properties make these ferrites suitable for use as RE-free PMs and microwave absorbers.
Nanocrystalline Ce-substituted yttrium iron garnet (YIG) powders of different compositions, Y 3Àx Ce x Fe 5 O 12 (0 # x # 2.0), were synthesized by a combination of sol-gel auto-combustion and solid-state synthesis techniques. The as-obtained powder samples were sintered at 1150 C for 10 h. The garnet structure formation is confirmed by the X-ray diffraction pattern, which shows that the calculated lattice parameter increased for x ¼ 1.0 and shows a decreasing trend for x $ 1.0 with the addition of cerium ions. The lattice parameter increased from 12.38Å to 12.41Å for x # 1.0 whereas it decreased from 12.412Å to 12.405Å with the cerium composition for x > 1.0. The average particle size determined by high resolution transmission electron microscopy is in the range of 50 to 90 nm and found to increase with the substitution of cerium ions in YIG. The room temperature magnetic parameters such as saturation magnetization, coercivity and remanence magnetization are greatly affected by the substitution of cerium ions. The values of saturation magnetization decrease from 25.5 to 15 emu g À1 whereas coercivity increases from 1 to 28 Oe with the substitution of cerium ions. The pure YIG sample shows polycrystalline nature that changed towards a single-crystal structure leading to a preferred-(100) orientation with the Ce substitution. The change from a ring to a spotty pattern observed in SAED confirmed the crystalline phase transformation and is well supported by HRTEM and magnetic measurements. The behavior of magnetic and electrical properties is well supported by the poly-and single-crystalline nature of YIG and Ce-YIG, respectively. The crystal structure transformation in YIG brought about by Ce substitution could unveil enormous opportunities in the preparation of singlecrystal materials from their polycrystalline counterparts.
M-type barium hexaferrites (BaM) with the substitution of Ce–Dy ions were synthesized using the sol-gel auto-ignition method. The prepared materials were explored for their application as a permanent magnet and microwave absorbing material. The structural properties, phase evaluation, micro-strain, morphological analysis, magnetic behaviour, microwave absorbing properties and optical properties were studied by employing various techniques. The structural parameters and phase identification obtained by Rietveld refinement confirmed the formation of an M-type hexaferrite structure for pure BaM, whereas Ce–Dy substitution induced secondary phases of cubic CeO2 and ortho DyFeO3. Crystallite size obtained from Williamson–Hall plots increased from 27.1 nm to 30.8 nm with the introduction of Ce–Dy ions in BaM. The nanocrystalline nature of the prepared samples was confirmed using scanning and transmission electron microscopy techniques. Fourier transform infrared spectra of all the samples were recorded in the wavenumber range of 400–4000 cm−1 and also supported the x-ray diffraction findings by confirming the formation of samples with hexaferrite structures. Coercivity of the BaM hexaferrites improved from 4430 to 5721 Oe with the Ce–Dy substitution. A Ce–Dy substituted BaM hexaferrite sample of 3 mm thickness showed a maximum reflection loss of −16.3 dB around 16.7 GHz. Permittivity and permeability studies were carried out to understand the microwave absorption behaviour.
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