The role entropy plays on the interconversion of gigahertz sound waves and electromagnetic waves in ferromagnetic metals has been studied. In linearized form the heat flow equation leads to an entropy wave in the ferromagnet. For the simple configuration in which the microwave magnetic field and an applied, static magnetic field are parallel and in the plane of a ferromagnetic metal film, two pairs of terms arise which couple the entropy wave to the electromagnetic wave and the sound wave. The coupling to the electromagnetic wave is peculiar to ferromagnets and its primary effect is to modify the skin depth. The coupling to the sound wave controls whether the sound propagates under adiabatic or isothermal conditions. The frequency at which the transition from adiabatic to isothermal sound occurs can be low enough to affect the accuracy of experiments to measure the ferromagnetrsquo;s exchange stiffness constant by observing the spin wave/sound wave dispersion relation crossing.
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