Viewpoint: A magnetic future - hexagonal ferrites

Materials World magazine
,
8 May 2013

Their high frequency properties are making
hexagonal ferrites a smart option for stealth
technology and electronics. Dr Robert Pullar of the
Department of Materials and Ceramic Engineering
and CICECO at the University of Aveiro, Portugal,
looks at the changing face of this materials.

Hexagonal ferrite ceramics were first
discovered and characterised by scientists
at Philips Laboratories. The development
of hexagonal ferrites started in the
1950s, when scientists studied and tried
to reproduce the structure of magnetoplumbite, a
natural magnetic mineral containing lead and iron
oxides. In the synthetic hexagonal ferrites, lead is
replaced by barium or strontium. Today, hexagonal
ferrites are the major global magnetic material by
volume, with over 400,000 tonnes of hexagonal
ferrites produced in 2012.

Hexagonal ferrites, also known as hexaferrites,
are magnetic iron oxides with a hexagonal structure.
They are formed by iron, oxygen and at least one
other metallic element, usually barium or strontium
and often contain other metals such as cobalt as well.
Hexagonal ferrites are the most common magnetic
material and are worth almost US$4bln globally. More
than 300,000 tonnes of the most common hexagonal
ferrite, BaFe12O19, also known as barium M ferrite, are
produced every year. This equates to 50g for every
person on Earth.

The widespread use of ferrite magnets is due to
their low cost compared to other magnets, such as
metallic alloys, the best of which are based on the
expensive rare earth metal neodymium. Although
inferior to rare earth metal alloys as permanent
magnets, with the rising costs and scarcity of
rare earth elements, hexaferrites are becoming an
increasingly attractive alternative. They also possess
unique high-frequency properties in the microwave/
GHz range, giving the material applications in
electronics, wireless communications, stealth and
radar absorbing material technology.

Hexagonal ferrites are currently employed in many sectors. As well as permanent
magnets such as fridge magnets (both the magnet that keeps the door shut, and
the ones 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 five terabytes, so large that one
tape could store the equivalent of eight million books.

Although these current applications are very important, the search is on to
improve the properties of these materials and to find more innovative applications.
There has been increasing interest in hexaferrite nanofibres and fibre alignment
effects on magnetic properties. Nanotechnology and composites are also growing
areas of research.

More recently, it has been discovered that some hexaferrites are single-phase
room-temperature multiferroics, which extends the capabilities of these materials
tremendously, and the last year has seen a sudden growth of interest in hexagonal
ferrites because of this. Multiferroics are materials that can be both ferromagnetic
and ferroelectric. Moreover, the magnetic properties can affect the electric ones and
vice versa, in a process known as coupling. There are very few single-phase materials
that show multiferroic behaviour at room temperature – they usually need cooling
to cryogenic temperatures. However, in 2010 the hexaferrite Sr3Co2Fe24O41 was found
to be a room-temperature multiferroic, and several other hexagonal ferrites have
also been recently reported as room temperature multiferroics.

Last year, there was a record number of papers published on hexaferrites. Their
use in multiferroic applications has immense potential in various technologies, such
as highly sensitive magnetic field sensors (used in biomedicine), a new generation
of smart stealth technology (used in military/defence), improved data storage
solutions for IT and computing, and field-responsive smart filters and switches for
wireless communications. Furthermore, hexaferrites are already a mass commercial
product and so can be cheaply produced in great quantities, and because no special
processing is required to make them, their production for multiferroic applications
is feasible. With this in mind, hexaferrites will become an increasingly important
ceramic material.