A REVIEW OF NEW MAGNETIC PHENOMENA 



1167 



of propagation. Upon emerging from the sample, the wave passes into C, 

 a circular to rectangular waveguide transition which may be rotated 

 for maximum transmission of energy down section C. The angle of dis- 

 placement of C with respect to A is a measure of the rotation of the plane 

 of polarization of the wave in its passage through the ferrite. The rota- 

 tion per centimeter of material depends upon the longitudinal field in 

 the sample, increasing with this field and reaching a constant value when 

 the ferrite is saturated. 



Hogan has given a discussion of the theory of this ferromagnetic 

 effect. The incident linearly polarized wave may be described as a com- 

 bination of two oppositely rotating circularly polarized waves. The 

 real part, ^t , of the permeability of the ferrite varies with magnetic 

 field in a different way for the two circular polarizations, as shown in 

 Fig. 7. The velocity of propagation of the two polarizations is therefore 

 different, and in passing through the ferrite they will fall out of phase by 

 an amount proportional to sample length. Upon emerging they will com- 

 bine to form a linearly polarized wave whose plane is rotated with re- 

 spect to the incident wave. Reference to Fig. 7 shows that the most 

 useful region for obtaining this effect lies below the field required to pro- 

 duce ferromagnetic resonance (i.e., at frequencies above the ferromag- 

 netic resonance frequency) in the region where the two curves are prac- 

 tically parallel. In this region the device is relatively insensitive to small 

 field changes and is somewhat ''broadband" with respect to frequency. 



For the practical application of the Faraday rotation, it is desirable 

 that the attenuation per degree of rotation be small. Attenuation varies 

 widely for different kinds of ferrites and is a function of frequency. For 



SECTION C 



MAY BE ROTATED 



FOR MAXIMUM 



TRANSMISSION 



SOLENOID FOR PRODUCING 

 LONGITUDINAL FIELD 



Fig. 6 — Apparatus for demonstrating the microwave Faraday effect. Energy 

 is supplied to section A. Rotation of plane of polarization occurs in section B and 

 is controlled by controlling the longitudinal field. Section C is rotated for maxi- 

 mum transmission. The angular displacement of C with respect to A is a measure 

 of the rotation. 



