Magnetic Domains & Hysteresis
We know magnets aren’t attracted to everything; if we put a magnet on a wooden wall, it will fall right down. Generally, magnets are attracted to objects that are made of the metals iron, nickel, or cobalt. These materials are called ferromagnetic materials. The reason magnets stick to these metals is because of a special characteristic about the atoms inside these metals. In most other materials that are not magnetic, the magnetic moments of the atoms inside are all oriented in random directions that cancel each other out. In ferromagnetic materials, the atoms form structures called domains. A domain is a region inside of a material where groups of magnetic moments naturally align in the same direction.
There can be numerous domains within an object. When there is no external magnetic field present, the domains are also oriented randomly so that there is no net magnetic field. However, when an external magnetic field is present, the domains will rotate and align with the external magnetic field. When all or most of the domains are aligned in the same direction, the whole object becomes magnetized in that direction and becomes a magnet.
The process of using a magnetic field to magnetize another object is called induction. Once a magnet has been induced, it produces its own magnetic field as long as its domains are aligned.
Once we have induced a magnet, we can observe an interesting effect when the external magnetic field is removed. Depending on the material, the domains will stay lined up together in the same direction even after the external field is gone. The domains do not instantly return to normal. This tendency to stay aligned is called hysteresis. Hysteresis is what allows us to make permanent magnets. To make permanent magnets, we take our material, create whatever shape we want, and then place the material inside of a very strong magnetic field. The domains inside the material align with the magnetic field, and when we remove the field, the domains stay aligned, and we now have a new magnet. While these are magnets are not truly “permanent”, some magnets’ domains will not return to their original state for much longer than a single lifetime.