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of between 340 and 400 C, compared with 310 C for lower grades. While that might result in a lower flux density (potentially by around 0.03 T), heat-resistant um can also have a higher maximum energy product, potentially an extra 25-30 kJ/m ³ . By comparison, magnets made from sintered aluminium-nickel-cobalt alloys (known as the Alnico family) tend to offer 0.6-1.4 T in flux density, have a coercivity of around 275 kA/m and an energy density of only 10-88 kJ/m ³ . A key advantage with such materials, however, is their high thermal resistivity, with Alnico 5 for example having a Curie temperature of 890 C. Ceramic (strontium ferrite) magnets can offer higher Curie temperatures than neodymium – up to 450 C – as can most commercially available permanent magnets, but tend to be the weakest in most other respects. The flux density of ceramic magnets varies between 0.2 and 0.78 T, and their energy product extends from 10 to 40 kJ/m ³ . They remain, however, among the most cost-effective permanent magnets, owing to the abundance of materials used to make them. That is in stark contrast to neodymium magnets, which use materials – rare earths – that are in short supply. High-temperature neodymium is even more troublesome than the more typical grades, as it requires adding elements such as terbium and dysprosium, which are rarer still. Currently, the overwhelming majority of minerals containing these elements, such as xenotime and euxenite, can only be obtained from mines in southern China. Although future potential sources exist in Western Australia, their rarity contributes in no small part to the expense of high- temperature neodymium. Also, neodymium can corrode more easily than other magnet materials, which forces many motor manufacturers to opt for samarium-cobalt permanent magnets – another rare earth alloy Electric motors | Focus The flux density of a magnet can determine the torque capability of the motor, and is typically measured in teslas, symbol T