THE MICROWAVE GYRATOR 7 



will luiNO its plane of polarization rotated a total of 180° in passing in 

 both directions through the material. Thus, the Faraday rotation in 

 optics affords an anti-reciprocal relation quite analogous to the anti- 

 reciprocal property of the gyrator postulated by Tellegen. 



Lord Raylcigh^ described a one-way transmission system in optics 

 which makes use of the Faraday rotation. Lord Rayleigh's "one-way" 

 system consisted of two polarizing Nicol prisms (oriented so that their 

 planes of acceptance made an angle of 45° with each other), with the 

 material causing the Faraday rotation placed between them. Thus, 

 light which was passed by the first crystal and whose plane of polariza- 

 tion was rotated 45° would be passed by the second crystal also. But, 

 in the reverse direction, the rotation would be in such a sense that light 

 which was admitted to the system by the second crystal would not be 

 passed by the first. 



Although Rajdeigh's one-way transmission system can be actually 

 realized, it is experimentally difficult since most substances show ex- 

 tremely small Faraday rotations. In fact, large rotations for transparent 

 substances in the optical region are of the order of one degree per cm 

 path length for an applied magnetic field of 1000 oersteds. To realize 

 a rotation of 45° would require maintaining a field of 1000 oersteds over 

 a distance of approximately one-half meter. The Faraday effect in 

 ferromagnetic substances, however, is unique in that it shows rotations 

 many orders of magnitude greater than the rotations exhibited by any 

 other substances. For instance, K6nig^° reports rotations of 382,000°/cm 

 by passing light through thin layers of magnetized iron. These data, of 

 necessity, however, were taken on extremely thin sections and the total 

 rotation obtained for any specimen did not exceed 10°. In order to 

 obtain appreciable rotations in a device of practical size, it is necessary 

 to obtain a material which shows a rotation at least intermediate be- 

 tween those reported for iron and other ordinary materials. In addition, 

 in order to make effective use of these rotations, the material must be 

 transparent to the radiation which is being used. 



THEORY OF THE FERROMAGXETIC FARADAY EFFECT 



Polder^^ has shown in his analysis of the ferromagnetic resonance 

 phenomenon, that a plane electromagnetic wave at microwave fre- 

 quencies should show appreciable Faraday rotation when propagated 

 through a ferromagnetic material which is magnetized in a direction 

 parallel to the direction of propagation of the wave. Polder has neglected 

 both magnetic and dielectric losses in his analysis and although for the 

 ferrites which are of greatest interest, this approximation is quite 



