SEM photograph of hexagonal ferrite crystals. 30000 X magnification, scale bar = 1 microns (0.0001 cm). Credit: Robert C. Pullar.
Hexagonal ferrite ceramics have well known abilities and properties, and are used in everything from fridge magnets to high-tech applications. Now, scientists have discovered room-temperature multiferroic hexaferrites, a development that extends the capabilities of these materials tremendously.
Hexagonal ferrites, also known as hexaferrites, are magnetic iron oxides with a hexagonal structure. They are formed by iron, oxygen and one or more other elements, which could be barium, strontium, cobalt, or a combination of these.
The development of hexagonal ferrites started in the 1950s, when scientists studied and tried to reproduce the structure of magnetoplumbite, a natural magnetic mineral. In the synthetic hexagonal ferrites, lead is replaced by barium or strontium, the simplest example being BaFe12O19.
Hexagonal ferrites are the most common magnetic materials used today, covering about 90% of the market – and it’s a big market: in 2012, it was worth almost $4 billion. Over 300,000 tons of the most common hexagonal ferrite, BaFe12O19, are produced every year, which corresponds to 50 grams for every person on earth.
The wide spread use of ferrite magnets is due to their lower cost compared to other magnets, such as metallic alloys, the best of which are based on the expensive rare earth metal neodymium.
Hexagonal ferrites are currently employed in many different sectors: permanent magnets, such as fridge magnets (both the magnet which keeps the door shut, and the one you stick on the outside), electric motors (there are about 100 different ferrite-based motors in every car) and loudspeakers.
Another important use is data storage. Many computer hard disks and tapes are made of hexagonal ferrites, some having very impressive storage capacity. In 2011, for example, Fujifilm produced a barium hexaferrite-based tape with a memory of 5 terabytes, so large that one tape could store the equivalent of eight million books.
Other uses include microwave devices, wireless communications and stealth and RAM (radar absorbing material) technology.
Although these current applications are very important, the search is on to improve the properties of these materials and to find innovative applications. Recently there has been increasing interest in hexaferrite nanofibers, and fiber alignment effects on magnetic properties. Nanotechnology and composites are also growing areas of research.
SrCoZnFe16O27 (W hexaferrite) crystal unit cell. Green = strontium; red = iron, cobalt or zinc; brown & pink = oxygen atoms. Credit: Robert C. Pullar.
In the last year, there has been a sudden growth of interest in hexagonal ferrites because of their room temperature multiferroic properties. Multiferroics are materials that can be both ferromagnetic and ferroelectric; moreover, the magnetic properties can affect the electric ones, and vice versa, a process known as coupling. There are very few single-phase materials that show multiferroic behavior at room temperature. Instead, they usually need cooling to cryogenic temperatures to exhibit these properties. However, the hexaferrite Sr3Co2Fe24O41 was found in 2010 to be a room-temperature multiferroic, and several other hexagonal ferrites have also been recently reported as room-temperature multiferroics.
Last year there were a record number of papers published on hexaferrites. The use of hexaferrites in multiferroic applications has immense potential, and could be used for various technologies, such as highly sensitive magnetic field sensors (used in biomedicine), a new generation of smart stealth technology (used in military and defense), improved data storage solutions for IT and computing, and field-responsive smart filters and switches for wireless communications. Furthermore, hexaferrites are already major global commercial products, cheaply produced in great quantities, and no special processing is required to make them. Therefore, their production for multiferroic applications is definitely feasible.
For more information on these materials, see a new in-depth review in Progress in Materials Science, which continues to looks at the past, present and future of these important ceramic materials.
(Robert C. Pullar is a member of the Department of Materials and Ceramic Engineering, University of Aveiro, Aveiro 3810-193, Portugal)