ABSTRACT
In 1930, Bloch introduced the concept of a magnon in order to account for the reduction of spontaneous magnetization in a ferromagnet as a function of temperature T. At T = 0 K, all atomic spins point along the same direction so that the total energy is at a minimum. As the temperature increases, the spins begin to deviate randomly at an increasing rate from the common direction, thereby reducing the spontaneous magnetization. At su ciently low temperatures, the low-lying energy states of spin systems have only a few misaligned spins and can be treated as a gas of quasiparticles called magnons or quantized spin waves. Because of exchange interactions, the disturbance arising from spin misalignments propagates through the spin system, as a collective motion of the atomic spins, as waves with discrete energies. us, spin waves can be viewed as the analog for magnetically ordered systems of lattice waves in solid systems. Based on this spin wave theory, Bloch derived the T 3/2 law for the
decrease of spontaneous magnetization. e quantitative theory of quantized spin waves was developed further by Holstein and Primako (1940) and Dyson (1956). For a comprehensive treatment of spin waves, the reader is referred to a book by Kittel (2004).
