Magnon Coherent States and Condensates

It is known that spin waves that have quanta called magnons can be excited in magnetic materials by means of several processes. Most spin-wave phenomena involve a large number of magnons and can be well described by the semi-classical Landau-Lifshitz equation. Perhaps this is one reason for the coherent magnon states, defined in analogy to the coherent photon states that are widely used in quantum optics, to be not well known by the magnetism community. In this paper, we review the concept of coherent magnon states and show that they are the quantum states generated in a linear microwave driving process. We also address the recently observed Bose-Einstein condensation of magnons in magnetic films under strong microwave driving. Using a microscopic theoretical model that relies on the cooperative mechanisms made possible by magnon interactions, we explain quantitatively the spontaneous generation of quantum coherence and magnetic dynamic order observed above a critical microwave power level.