Radiation pressure and the linear momentum of the electromagnetic field in magnetic media

Abstract: We examine the force of the electromagnetic radiation on linear, isotropic, homogeneous media specified in terms of their permittivity epsilon and permeability mu. A formula is proposed for the electromagnetic Lorentz force on the magnetization M, which is treated here as an Amperian current loop. Using the proposed formula, we demonstrate conservation of momentum in several cases that are amenable to rigorous analysis based on the classical Maxwell equations, the Lorentz law of force, and the constitutive relations. Our analysis yields precise expressions for the density of the electromagnetic and mechanical momenta inside the media that are specified by their (epsilon, mu) parameters. An interesting consequence of this analysis is the identification of an "intrinsic" mechanical momentum density, ExM/c^2, analogous to the electromagnetic (or Abraham) momentum density, ExH/c^2. (Here E and H are the magnitudes of the electric and magnetic fields, respectively, and c is the speed of light in vacuum.) This intrinsic mechanical momentum, associated with the magnetization M in the presence of an electric field E, is apparently the same "hidden" momentum that was predicted by W. Shockley and R.P. James nearly four decades ago.