Why Is Iron Not Always Magnetic? It Is, in Patches That Cancel Out

Within iron and a few other metals, the magnetic contributions of neighbouring atoms line up with each other spontaneously, because doing so lowers the energy of the arrangement. That alignment does not extend through the whole piece, since a uniformly magnetised object carries a large amount of energy in the field it projects into the surrounding space. The material therefore divides into regions, each internally aligned and each pointing in a different direction, arranged so that the fields largely close on themselves within the material and project very little outside. An ordinary piece of iron is therefore fully magnetised everywhere internally and shows almost nothing externally, which is why it is attracted to a magnet and is not one.

The response happens in stages that can be measured separately:

• •Regions already pointing near the applied direction grow at the expense of others
• •The boundaries between regions move through the material to accomplish that
• •Movement is jerky rather than smooth, since boundaries snag on defects
• •Those jumps are audible as noise when amplified, which is how the process was confirmed
• •At higher fields, remaining regions rotate their direction bodily
• •Once everything points the same way the material is saturated and cannot give more

The difference between a permanent magnet and a piece of iron that forgets lies in how easily those internal boundaries move. In a soft material the boundaries move freely, so the regions return to a cancelling arrangement as soon as the applied field is removed, which makes such materials ideal for transformer cores and electromagnets where the magnetism must follow the current exactly. In a hard material the boundaries are pinned by deliberately introduced defects, impurities and crystal structure, so they cannot move back and the alignment persists, which is a permanent magnet. Making better permanent magnets is largely a matter of making boundary movement harder, which is why the materials involved are metallurgically elaborate.

The existence of these regions was proposed theoretically before anybody could see one, and the confirmation came in two elegant steps. Heinrich Barkhausen demonstrated in 1919 that magnetising a sample produces a series of tiny discrete jumps rather than a smooth change, by winding a coil around an iron rod, connecting it to an amplifier and a loudspeaker, and slowly bringing a magnet near, which produces a distinctive crackling as boundaries jump between pinning points. Francis Bitter developed a method in 1931 of sprinkling a fine suspension of magnetic particles onto a polished surface, where they collect along the boundaries and make the pattern of regions directly visible under a microscope. Both techniques remain in use for materials testing.

Several familiar observations follow directly. Dropping or hammering a magnet weakens it, because the shock supplies energy that lets boundaries move and the regions partially randomise again. Heating does the same more effectively, and above a characteristic temperature the spontaneous alignment collapses entirely and the material stops being magnetic at all, recovering on cooling. Cutting a magnet in half produces two magnets rather than a separated north and south, because each piece still contains aligned regions with their own ends. A magnet left near another in the wrong orientation weakens over time. And magnetic data storage works by setting the direction of very small regions and reading them back, which is the same physics at a scale of nanometres.

Iron divides into regions that are each fully magnetised internally and arranged so their fields cancel externally, which is why an ordinary piece is attracted to a magnet without being one. Applying a field grows the favourably aligned regions by moving the boundaries between them, jerkily rather than smoothly. Permanent magnets pin those boundaries with deliberate defects, and hammering or heating a magnet frees them again.