A Review of New Magnetic Phenomena

Up until a few years ago, the classical concept of magnetic materials, characterized by hysteresis and eddy current effects, was adequate for the communications engineer. Recent new and important developments, however, have made it necessary for him to have a broader knowledge of magnetic phenomena than is given by the old picture. Because of the low resistivities of existing magnetic materials, their properties at high frequencies have been dominated by eddy currents which in many cases have completely masked other magnetic effects. In contrast, the newly developed ferrites have resistivities from 10 to 10 times greater, and eddy currents are usually negligible. The ferrites are, therefore, useful at far higher frequencies than previously available materials. Furthermore they have revealed new and important magnetic phenomena. It is the purpose of this paper to present the modern picture of magnetism from the standpoint of the engineer. It describes the new phenomena and relates the experimentally observed behavior of magnetic materials at high frequency to the present physical theory of magnetism. The new phenomena include dimensional and domain wall motion effects in the low megacycle region and ferromagnetic resonance and the recently observed Faraday effect in the microwave region. 6 12