It happens that in iron, and a few other substances, the spins interact with each other in such a way that their magnetic moments tend to all line up in the same direction, as shown in FIGURE 29.53. Materials that behave in this fashion are called ferromagnetic, with the prefix ferro meaning “iron-like.”

FIGURE 29.53 In a ferromagnetic material, the spin magnetic dipole moments are aligned.

In ferromagnetic materials, the individual spin magnetic moments add together to create a macroscopic magnetic dipole moment. The material has a north and a south magnetic pole, generates a magnetic field, and aligns parallel to an external magnetic field. In other words, it is a magnet!

Although iron is a magnetic material, a typical piece of iron is not a strong permanent magnet. You need not worry that a steel nail, which is mostly iron and is easily lifted with a magnet, will leap from your hands and pin itself against the hammer because of its own magnetism. It turns out, as shown in FIGURE 29.54 , that a piece of iron is divided into small regions, typically less than $100\mu \text{m}$ in size, called magnetic domains. The magnetic dipole moments of all the iron atoms within each domain are perfectly aligned, so each individual domain, like Figure 29.53, is a strong magnet.

FIGURE 29.54 Magnetic domains in a ferromagnetic material. The net magnetic dipole is nearly zero.

However, the various magnetic domains that form a larger solid, such as you might hold in your hand, are randomly arranged. Their magnetic dipoles largely cancel, much like the cancellation that occurs on the atomic scale for nonferromagnetic substances, so the solid as a whole has only a small magnetic dipole moment. That is why the nail is not a strong permanent magnet.