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Electronic, magnetic and optical materials

When ferromagnetic iron is heated above its Curie temperature (about 770 °C), what happens to its magnetism?

  1. AIt becomes diamagnetic
  2. BIt becomes antiferromagnetic
  3. CIt becomes paramagnetic
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Answer: C. It becomes paramagnetic

In a ferromagnet, the magnetic moments of neighboring atoms are aligned in the same direction by the exchange interaction, giving a spontaneous magnetization. As temperature rises, thermal motion disturbs the alignment, and above the Curie temperature the spontaneous magnetization disappears, leaving paramagnetism, a slight magnetization only when an external field is applied. The Curie temperature is about 770 °C for iron, about 358 °C for nickel and about 1120 °C for cobalt. Antiferromagnetism is an ordered state in which neighboring moments align antiparallel, and the temperature at which that order disappears is called the Néel temperature. Diamagnetism is a weak property of all substances and does not appear as a result of ferromagnetism vanishing.

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