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The Curie temperature is a point at which materials lose their ferromagnetism, the ability to align their atoms even in the absence of a magnetic field. They become paramagnetic, requiring an external magnetic field to magnetize and remain in that state. Magnetic materials may change properties at a variety of temperatures, but they are often quite high. When the temperature drops again, the material recovers its magnetism, illustrating that the phase change is reversible. The Curie temperature phenomenon is named for noted researcher Pierre Curie.
When materials are in a ferromagnetic state, exposure to a magnetic field can align their atoms and create an attraction. If the field is taken away, the magnetism remains, as the material has a form of memory. This can be used to make permanent magnets and demonstrate a variety of interesting physical phenomena. Paramagnetic materials, however, require the maintenance of an external magnetic field to remain magnetized.
At the Curie temperature, the heat agitates the atoms inside the material so much that they cannot align, and it loses its magnetism. This can have important implications. In geology, for example, high heat can occur in lightning strikes and volcanic eruptions, and is capable of causing the properties of minerals in the Earth’s crust to change. Observers looking at magnetic minerals need to consider their history and what may have influenced them.
This can also be important for magnetic materials used in high heat environments. If the temperature gets too high, they can lose their properties, and may fail to function as expected. A simple example is sometimes used for demonstrations in science classes, where a lecturer shows how heating above the Curie temperature causes a paperclip or similar small metal object to lose its attraction to a magnet. In a situation where the magnetic attraction might be relied upon for a process or function, hitting the Curie temperature would be undesirable.
Data sheets on the properties of ferromagnetic and ferrimagnetic materials discuss their Curie temperatures. This information is provided for reference, so people know how the materials should behave in response to temperature changes. For companies preparing products, it can be especially important to have technical specifications about the materials used, so they can plan design and function accordingly. If a material would lose its magnetic attraction in normal use, the company might need to choose a different material to use in production.