Magnets are found in everyday items and technology such as phones, computers and cars. In ambient temperatures, magnets create their own magnetic field but temperature extremes can affect the way a magnet behaves.
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How Does a Magnet Work When Heated?
To understand how temperature might affect a magnet, you need to look at the atomic structure of the elements it is made of. Magnets are made of atoms and, in normal conditions, these atoms align between poles and foster magnetism. There is a delicate balance between temperature and magnetic domains – that is the atom’s inclination to ‘spin’ in a certain direction.
Temperature can either strengthen or weaken a magnet’s attractive forces. Cooling or exposing the magnet to low temperatures will enhance and strengthen the magnetic properties, while heating will weaken them.
As you heat a magnet, you supply it with more thermal energy; this allows the individual charged particles to move around at an increasingly faster and more sporadic rate. In between the weakening of overall magnetism and the availability of extra thermal energy, the spin of individual electrons within the atom – which behaves like mini-magnets – are more likely to be in high energy states.
So, heating a magnet disrupts the domain walls and it becomes easy for the magnetic domains, which are ordinarily lined up, to rotate and become misaligned. They are now less aligned and point in the opposite direction to their neighbors, causing a decrease in the magnetic field and loss of magnetism.
As you heat a magnet further, the individual spins within the domains become even more likely to point in opposite directions to their neighbors, decreasing their average alignment seen by their neighbors, decreasing the effect which favors their initial lining up.
At a well-defined temperature – known as the Curie temperature – the entire tendency of atoms to align into domains collapses and the material stops being a magnet. Named after Pierre Curie, the French physicist, the Curie Temperature is the temperature at which the atoms are too frantic to preserve their aligned spins, so no magnetic domain can exist. Even if the magnet is then cooled, once it has become demagnetized, it will not become magnetized again.
If a magnet is exposed to high temperatures, the delicate balance between temperature and the domains in a magnet are destabilized. At around 80 °C, a magnet will lose its magnetic force and it will become demagnetized permanently if exposed to this temperature for a period, or if heated above its Curie temperature. Heat the magnet even more, and it will melt, and eventually vaporize.
The ease with which a magnet becomes demagnetized decreases with increased temperature. Different materials react differently under heat, so what the magnet is made of is important; different magnetic materials have different Curie temperatures, the average being between 600 to 800 °C. Magnets consisting of Alnico – an iron alloy containing aluminum, nickel and cobalt – has the best strength resistance, then SmCo (Samarian cobalt) and NdFeB (neodymium-iron-boron), followed by ceramics. NdFeB magnets have the highest resistance to demagnetization but the largest change with temperature.
- Neodymium magnets lose some of their performance for every degree rise in temperature. Up to 150 °C neodymium magnets are considered to have the best magnetic performance of all permanent magnetic materials.
- Samarium cobalt magnets are not as strong as neodymium magnets at room temperature but have a better resistance to demagnetization than neodymium magnets.
- Alnico magnets are second only to neodymium magnets in terms of magnetic strength but are significantly more susceptible to demagnetization by external magnetic fields and physical shock, although not by elevated temperature.
The shape of a magnet can also affect its maximum useable temperature as the length of the magnetized axis increases, and resistance to demagnetization also increases. Small, thin magnets are generally more susceptible than magnets greater in volume to rising temperatures.
References and Further Reading
Temperature effects on permanent magnets
How does temperature affect magnetism?
Ask the Van; heating magnets
What happens when a magnet is heated?
How does heat affect magnets?