The rare earths have a unique place among the elements. Although very much alike chemically and in most phy ical properties they each have very different and striking magnetic properties. The reason, of course, lies in their 4f electrons which determine the magnetic properties but have little effect on other chemical and physical behaviour. Although they are not rare, some indeed are among the more common heavy elements in the earth's crust, the difficulty of separation has meant that their intricate magnetic properties have only recently been unravelled. Now, however, the general pattern of...