Some samples of the iron ore magnetite(Fe3O4) have a strong natural permanent magnetic field (1). These lodestones have been used since ancient times to make compasses — the name is from old English meaning ‘leading stone’. Until the development of steel alloys lodestones were the strongest known permanent magnets (PMs) (2).
High-power electric motors, such as those used in electric cars, need a field of about 1T. Motors that use permanent magnets (PMs) are smaller, lighter and more efficient than those that use electric current to produce a magnetic field. Practical motors containing PMs first became possible with the development of alnico alloys (3). Ferrite, used for ceramic PMs, is made by heating a mixture of iron oxide (Fe2O3) and strontium carbonate (SrCO3) or barium carbonate (BaCO3) to over 1000°C.
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