Magnetic Materials and Properties

Abstract

The above reference to lodestone is interesting in that it is over 2000 years old. The property of the magnetic field to attract or generate a force is universally known and is used in practical devices, probably more than most of us realize. How many applications of the permanent magnet do you recall? Did you know, for example, that many electric motors use permanent magnets, or that when you push a button on the keyboard of a computer you are most likely pushing down a small magnet which then activates a Hall-element switch? It is therefore quite useful to identify the properties of the permanent magnet since sooner or later you will encounter it in design. Thus follows the study of magnetic properties of materials. Many materials exhibit magnetic properties, some quite surprising. The permanent magnet is only one of them. Iron, nickel, or chromium oxides on audio tapes, video tapes, or computer disks store information in the form of magnetic field variations. Solid nickel contracts when placed in a magnetic field, while strong magnetic fields cause atoms to tilt about their spin axes, a phenomenon that leads directly to magnetic resonance imaging (MRI). Naturally occurring materials, such as magnetite (Fe₃O₄) are found in bacteria and in brains of many animals which use this material as a biological compass for navigation in the geomagnetic field. Magnetite and hematite (Fe₂O₃) are the basis of the lodestone (a naturally occurring, magnetic stone).

Now sing my muse, fir ’tis a weighty cause. Explain the Magnet, why it strongly draws, And brings rough iron to its fond embrace. This men admire; for they have often seen Small rings of iron, six, or eight, or ten,Compose a subtile chain, no tye between; But, held by this, they seem to hang in air, One to another sticks and wantons there; So great the Loadstone’s force, so strong to bear!...

—Titus Lucretius Carus (94–50 BCE), De Rerum Natura (On the nature of things), T. Creech, Translation, London 1714.