Finding the Cause of Magnetism

To answer the foregoing questions, we will have to examine the basic building block of matter, the atom. It consists of a tightly packed nucleus made up of protons and neutrons, with varying numbers of electrons circling around it, much as the planets of our solar system orbit the sun. This movement of electrons actually results in a minute magnetic force within the atom. Most electrons are paired in such a way that their magnetic fields cancel each other. When all the electrons in an atom are paired, the net magnetic field is zero. Metals composed of such kind of atoms are nonmagnetic.

But if the atom has unpaired electrons, it has a net magnetic moment, as the scientists call it. The strength of this magnetic moment determines how the atoms line up in the solid metal. In most metals, the agitation of the atoms at ordinary temperatures is great enough to overcome the magnetic forces, and the atomic magnets are disarranged, in random directions. The net resultant of the magnetic fields of a large number of atoms averages out to zero.

However, magnetism can be induced in such metals when they are placed in another magnetic field. Chromium is such a metal. The force of the magnetic field causes the atoms to turn into a parallel alignment. But as soon as it is removed from the field, thermal agitation again prevails, and this destroys the alignment. The chromium loses its magnetism. Metals like this, which do not retain magnetism, are called paramagnetic.

By contrast, in some metals, including iron, cobalt, and nickel, the individual atoms have much stronger magnetic moments. They are so strong that when atoms are crystallizing out of a melt, one atom feels the influence of its neighbor, and clusters of atoms align themselves with their magnetic axes parallel. Each such group actually becomes a small magnet. However, these clusters are microscopic in size and they are randomly oriented in a fresh casting. Thus an ordinary iron nail, for example, is not a magnet.

But if a piece of iron is placed in a magnetic field, the groups that happen to be in line with the field tend to grow at the expense of neighboring groups, by pulling adjacent atoms into line with them. This action is enhanced if the metal is heated, or stressed as by drawing. The alignment formed in this way persists when the iron is removed from the field. Thus the metal has become a permanent magnet. Such metals, which can be permanently magnetized, are called ferromagnetic. The iron atoms in magnetite were so aligned, apparently by the earth’s magnetic field when the ore was crystallizing.

The larger the groups are that are aligned with the field, and the smaller the ones that are randomly oriented, the more powerful will be the resultant permanent magnet. Scientists have learned that by applying heat or stress on the metal while it is within a powerful magnetic field, the maximum number of atomic groupings can be permanently aligned. In this way, permanent magnets of great strength can be produced economically.

[Pictures on page 19]

In nonmagnetized metals, small atomic groups are arranged with their magnetic poles situated at random

When magnetized, atomic groups realign themselves so that they lie parallel to one another