Magnetic materials

Tuesday 7-15-96

The relevant section in the textbook is 18.3

Main concepts:

• The phenomenon of electron spin is primarily responsible for magnetism in most materials. In some sense, electron spin is similar to the rotation of the Earth around its own axis, but in truth it's a quantum mechanical effect which is difficult to make a comparison to.

• Electron spins can be thought of as being up or down. In many materials there are as many up spins as down, so the magnetic effects cancel out and the material is non-magnetic. Aluminum is a good example of such a material.

• In other materials, the spins can be aligned. In ferromagnetic materials, such as iron, the spins of atoms in large neighborhoods will align. These neighborhoods are known as domains. When a piece of un-magnetized iron is placed in a magnetic field, domains aligned with the field will grow at the expense of domains which are aligned opposite to the field, making the iron act like a magnet. In hard iron, the domains will not shift back to the starting point when the field is taken away, and a permanent magnet will be the result. In soft iron, the domains return to being randomly aligned when the field is removed.

• Hard iron is used in permanent magnets, while soft iron is perfect for use in an electromagnet. By turning on a current to set up a magnetic field, the soft iron makes the field much stronger by becoming magnetized itself. Being able to turn this field off by switching off the current can often be desirable

• magnetic effects are sensitive to temperature. It is much easier to keep permanent magnets magnetized at low temperatures, because at higher temperatures the atoms tend to move around much more, throwing the spins out of alignment. Above a critical temperature known as the Curie temperature, ferromagnets lose their magnetic properties.