-From ScienceDaily-

Magnets that can readily switch their polarity are widely used in the computer industry for data storage, but they present an engineering challenge: A magnet’s polarity must be easily switched when writing data to memory, but be difficult to switch when storing or reading it.

These conflicting requirements are typically met by heating and softening the magnet for saving data, then cooling and hardening the magnet for storage and reading.

(Credit: University of Chicago)

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