3.1 Introduction to Magneto-optical Materials and Concepts
As implied by its name, magneto-optics embraces all activities that concern the interaction of light with a magnetic material. A vast range of substances can be accessed for such investigations, and insulating materials such as YIG (yttrium iron garnet) are popular for isolator applications. It is metallic materials, however, that are discussed in this tutorial, so that the basics of the all-important Kerr effect can be exposed in a manner that exploits popular ferromagnetic materials that yield effects that are of practical use. In this spirit, the discussion will always refer to metallic conductors, even though, of course, all magneto-optic materials can display the full range of magneto-optic properties to a greater or lesser degree. Of the five ferromagnetic elements, cobalt, iron, and nickel are by far the most important. In bulk form, each material contains what are called domains. Domains are magnetised subregions that, although quite often chaotically related to each other, can be brought into alignment by the application of a modest applied magnetic field. In this way, a magnetic material and its magnetic state can be organised to have an electromagnetic impact simply by reflecting p-polarised light from it. In fact, the magnetisation of the material produces a small rotation of the plane of polarisation of this type of incident light. Upon reflection from a magnetised material, an incident plane p-polarised light beam can become elliptically polarised, with the principal axis moving a few degrees away from the axis of the incoming light. Such a rotation is known as the Kerr effect, named after the Rev. John Kerr, who announced his discovery in 1888. Kerr could not have imagined just how significant this discovery was; its impact on data storage technologies is still holding out fantastic prospects. As discussed below, magnetic materials can be magnetised in a number of ways. A popular orientation for the magnetisation is to be perpendicular to the optical reflection surface. It should also be possible to observe such a magneto-optic effect using only a nanomagnet that is a member of an array of nanomagnets. The aim is to deposit a massive number of isolated magnetised units in the multiterabit range over a surface so that each unit can be interrogated by bouncing a laser beam off of it. The âbitsâ are digital data that can be read optically through the Kerr effect. This is not the only use for Kerr effect; it is genetically useful as a tool for monitoring all types of magnetic states. The data storage aspect is amazing, however, when it is realised that the print collection in the U.S. Library of Congress is about 10 terabytes. The fact that 1 terabyte represents about 50,000 trees turned into printed paper makes the development of magneto-optic storage even more imperative and exciting.
Ferromagnetic elements such as iron and cobalt exhibit the maximum magnetisation that they can attain. This is technically known as the saturation magnetisation and is very high. Such elements also have a high Curie point, the temperature at which the magnetisation is destroyed; e.g., for cobalt this temperature is 1120Â° C. For nickel, this value drops to 360Â° C. It is against the background of these attractive properties of ferromagnetic metals that the rest of the chapter develops a working discussion of the Kerr effect.