What physical property distinguishes paramagnetic materials from ferromagnetic materials?

Paramagnetic materials are characterized by their weak magnetism, which means they exhibit a magnetic response in the presence of an external magnetic field, but this response is relatively weak and only occurs while the field is applied. This weak magnetism is due to the presence of unpaired electrons in the material's atomic structure, which align in the direction of the magnetic field. However, once the external magnetic field is removed, the magnetic properties of paramagnetic materials disappear quickly.

In contrast, ferromagnetic materials exhibit permanent magnetism. This means that they not only respond to external magnetic fields like paramagnetic materials but can also retain a significant level of magnetism even after the external field is removed. The unpaired electrons in ferromagnetic materials can become aligned in a way that results in a lasting magnetic field, hence their ability to be permanent magnets.

Therefore, the defining characteristic that distinguishes paramagnetic materials from ferromagnetic materials is the weakness of the magnetic response in paramagnetic materials, which is accurately captured by the concept of weak magnetism.