Which class of materials exhibits spontaneous alignment of magnetic moments in the absence of an external magnetic field?

• A. Ferromagnetic
• B. Diamagnetic
• C. Paramagnetic
• D. Nonmagnetic

Answer: (A)

Spontaneous magnetization happens when the interactions between neighboring electron spins favor them pointing in the same direction. In ferromagnetic materials, these exchange interactions are strong enough that, below the Curie temperature, large regions form where all the moments align parallel to each other. This intrinsic alignment occurs even without any external magnetic field, although the sample may appear unmagnetized overall if different regions (domains) point in different directions. This is different from paramagnetic materials, where moments only align with an external field and quickly lose that alignment when the field is removed, and from diamagnetic materials, where induced moments oppose the applied field. Nonmagnetic materials have little or no intrinsic magnetic moments. So the class that shows spontaneous alignment of magnetic moments without an external field is ferromagnetic.