A team of Russian scientists has produced hexaferrite particles with the highest coercivity reported to date. Lev A. Trusov from Lomonosov Moscow State University explained, “The strongest industrial magnets are made of alloys of rate-earth elements – NdFeB and SmCo compounds – which create very strong magnetic fields, but are also very difficult to demagnetize. Hard magnetic ferrites based on iron oxide demonstrate more moderate magnetic properties but have some useful advantages.”
The advantages are low cost, plentiful supply, biocompatibility, stability at the nanoscale, and high frequency radiation absorption in the 1-220 GHz range. But only one ferrite material till date has shown coercivity over 20 KOe: so-called epsilon-Fe203. The usage in the industrial applications, however, has been hampered by difficult mass production, which requires a complex process of particle formation in a mesoporous amorphous silica matrix and subsequent removal of the silica.
“In contrast, our hexaferrites can be obtained by a very simple method, which is readily scalable can be efficiently integrated into modern ferrite technology,” says Evgeny A. Gorbachev, first author of the study.
The team along with the colleagues from Moscow Institute of Physics and technology and Prokhorov General Physics Institute of the Russian Academy of Sciences, came up with a simple means of producing particles of hexaferrite Sr1-x/12Cax/12Fe12-xAlxO19, which show high coercivity values up to 40 KOe. The method depends on a highly porous precursor, which is made using the well-known citrate-nitrate auto-combustion method, in which citric acid acts as a fuel and the nitrate ion as an oxidizer. When aqueous nitrates and citric acid solutions are heated, the viscous melt self-ignites producing a low-density amorphous mixture of metal oxides. Annealing this highly porous powder at 1200°C forms Al-substituted hexaferrite particles less than a micron in diameter.
“The highly porous nature of the precursor prevents intensive particles growth and sintering during high temperature annealing,” explains Trusov. “So our hexaferrite materials have particle dimensions below the critical size of a single magnetic domain, which results in very hard magnetic properties.”
Moreover, the inclusion of aluminum in the material boosts coercivity and can be used to fine-tune the properties. Since the hexaferrite is produced in the form of a typical oxide powder, it can be easily transformed into coatings, composites, or even paints.
“We can imagine highly stable magnetic memory media, if the particle size is decreased,” points out Gorbachev, “and the microwave absorption [properties] may find application in new generations of wireless communication and radar technology.”
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