Torque exerted on anisotropic magnetic medium by electromagnetic wave

Abstract

The possibility of exerting a torque with a nonzero time average value as a result of interaction of a plane electro-magnetic wave with anisotropic magnetic medium is discussed. The torque per unit volume produced by the action of the magnetic field on the magnetization of the medium is T = M \times H and it arises from the fact that the vector of magnetization M is in general not parallel to the magnetic field intensity H . Three cases of magnetic media are distinguished: 1) medium with a natural or static anisotropy, 2) gyrotropic medium, 3) medium with shape anisotropy. In the first case, the lossless medium having a permeability \mu x, \mu y in the x and y directions perpendicular to the direction of electromagnetic plane wave propagation, the maximum of the torque per unit area corresponds to the thickness of the section \Delta z = \lambda_{0}/2(nx - ny) , where nx,ny are the refraction constants of the medium, and the sign of torque is opposite in the neighboring sections. In the case of a gyrotropic medium characterized by Polder's permeability tensor, it can be shown that the average value of torque exerted on a lossless medium by a plane wave propagating in the direction of dc magnetic field is zero. The nonzero average value of torque exists in the case of gyrotropic medium with losses as in the third case when a plane linearly polarized wave propagates along a ferrite cylinder with elliptical cross section. The results of not yet finished experimental investigations, which had to confirm the possibility of utilization of the torque for the construction of a feed-through wattmeter for the measurement of microwave power in the X band, are shown at the end of the paper.