FERRITES IN MICROWAVK APIM.K ATIoNS 1343 



A ferromagnetic crystal consists entirely of regions called domains 

 which are completely magnetized along one of the directions of easy 

 magnetization. In general the direction of magnetization of these do- 

 mains is varied in an orderly manner as shown in Fig. 6 so that the 

 energy of the crystal as a whole is a minimum. In the region Ix^tween 

 adjacent domains there is a (usually) narrow wall in which the mag- 

 netization goes through a gradual change in direction from that of one 

 domain to that of the other. When an external field is applied the mag- 

 netization of the crystal is increased by the growth of some domains at 

 the expense of their neighbors. When the crystal is saturated substan- 

 tially all of the walls have disappeared and the material behaves as a 

 single large domain. 



There are currently two proposed mechanisms by which these domain 

 walls could cause a loss at low fields. Becker and Doring* have shown 

 that there can be associated with the motion of a domain wall either 

 relaxation or resonance frequencies. Galt^ has measured relaxations in a 

 single crystal of magnetite at 3,000 cps and in a single crystal of nickel 

 ferrite at approximately 2.5 mc and has presented a rather convincing 

 argument that these are due to domain wall motion. In general the 

 relaxation frequency would be expected to occur far below the micro- 

 wave frequencies, but resonances could conceivably occur at micro- 

 wave frequencies and could be quite broad. Until recently no other theory 

 had been advanced which would explain the losses so often observed at 

 low fields, and these were, therefore, attributed to a high frequency 

 domain wall resonance. 



There is a more satisfactory explanation which has recently been 

 stated in different ways by Rado* and by Smit and Polder.' Rado has 

 observed a resonance absorption in the microwave region with zero 

 applied field and has shown from temperature dependence that the fre- 

 quency of this resonance depends upon the saturation magnetization and 

 the crystalline anisotropy of the ferrite. Smit and Polder have presented 

 a model by which we can see how both of these quantities can enter to 

 produce a loss at low fields.' We consider an ellipsoidal crystallite as 

 shown in Fig. 7. The domain structure shown is one which could exist in 

 some crystallites in a polycrystallme ferrite. 



The magnetization in domains numbered 1 will respond to right cir- 

 cular polarization and the others to left circular. In other words a wave 

 rotating clockwise is positive circularly polarized in domains one while a 



4 Becker and Doring, Ferromagnetismus, Sringer, Berlin, 1939. 



6 J. K. Gait, Phys. Rev., 86, Feb 15, 1952 



• G. T. Rado, R. W. Wright, et al., Phys. Rev Nov^2. 



' D. Polder and J. Smit, Revs. Mod. Phys., 26, pp. 89-90, Jan. 1963. 



