There is associated with each atom in paramagnetic and ferromagnetic materials a net magnetic moment. Explain why ferromagnetic materials can be permanently magnetized whereas paramagnetic ones cannot.

Paramagnetic materials have unpaired electrons in their atomic structure, causing them to have a net magnetic moment. In the absence of an external magnetic field, the individual magnetic moments of the atoms in a paramagnetic material are randomly oriented, causing the net magnetization to be zero. Ferromagnetic materials, on the other hand, also have unpaired electrons, but due to a special kind of interaction called exchange coupling, the individual magnetic moments of atoms in these materials tend to align parallel to each other, forming domains. In the absence of an external magnetic field, the magnetization of these domains may still cancel each other out, causing the material to have little or no net magnetization.

When an external magnetic field is applied to a paramagnetic material, the magnetic moments of its atoms begin to align with the magnetic field, causing a net magnetization in the material. However, this alignment is relatively weak and disappears when the external magnetic field is removed. Therefore, paramagnetic materials cannot be permanently magnetized. Similarly, when a ferromagnetic material is subjected to an external magnetic field, the magnetic moments in the material align with the field, causing a strong net magnetization. However, unlike paramagnetic materials, the magnetic domains in ferromagnetic materials have a tendency to remain aligned even when the external magnetic field is removed.

The tendency of ferromagnetic materials to keep their magnetic alignment is called hysteresis. When an external magnetic field is strong enough, the alignment of the magnetic domains in a ferromagnetic material can become "locked" in place, resulting in a permanent magnetization. This is what makes ferromagnetic materials suitable for creating permanent magnets, whereas paramagnetic materials cannot hold their magnetization once the external magnetic field is removed.

In summary, the difference between ferromagnetic and paramagnetic materials lies in the interaction and alignment of their atomic magnetic moments. Ferromagnetic materials have stronger alignment due to exchange coupling, allowing them to maintain their magnetization even without an external magnetic field. This makes them suitable for permanent magnetization, while paramagnetic materials lose their magnetization once the external field is removed.