Making hard magnets the easy way

Ceramic-like iron oxide ferrites are widely used as permanent magnets, magnetic storage media, and microwave absorbers. But these materials have relatively modest magnetic properties, including low coercivity, which measures the ability of a ferromagnetic material to resist demagnetization. Now a team of Russian scientists has produced hexaferrite particles with the highest coercivity reported to date.

“The strongest industrial magnets are made of alloys of rare-earth elements – NdFeB and SmCo compounds – which create very strong magnetic fields, but are also very difficult to demagnetize,” explains Lev A. Trusov from Lomonosov Moscow State University. “Hard magnetic ferrites based on iron oxide demonstrate more moderate magnetic properties but have some useful advantages.”

These advantages include 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 to date has shown coercivity over 20 kOe: so-called epsilon-Fe₂O₃. Its use in 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.

Along with colleagues from Moscow Institute of Physics and Technology and Prokhorov General Physics Institute of the Russian Academy of Sciences, the team devised a simple means of producing particles of the hexaferrite Sr_1-x/12Ca_x/12Fe_12-xAl_xO₁₉, which show high coercivity values up to 40 kOe. The process relies 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 solutions of aqueous metal nitrates and citric acid 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.”

Gorbachev et al., Materials Today (2019), https://doi.org/10.1016/j.mattod.2019.05.020