Abnormal expansion during the ferro- to para-magnetic transition in pure iron

The strain change near the Curie temperature was investigated during continuous heating of pure iron using two different types of dilatometers with and without a radio frequency (RF) field. Contrary to the well-known fact that the ferro- to para-magnetic transition accompanies volumetric contraction, abnormal expansion was observed near the Curie temperature only when the quench dilatometer employing the RF field for heating was used. The abnormal expansion was caused primarily by the change in electron-phonon interaction under the RF field.