A REVIEW OF NEW MAGNETIC PHENOMENA 1165 



Consider a single electron. Because it is a spinning charge, it has a 

 magnetic moment which lies along its axis of spin. Because it has mass, it 

 has mechanical angular momentum. The ratio of these two quantities is 

 the magnetomechanical ratio, 7. If a steady magnetic field, Hq, is applied, 

 there will be a torque on the electron as a result of the interaction be- 

 tvv'een Ho and the magnetic moment of the electron. The electron will, 

 therefore, precess about the direction of Hq with a frequency which has 

 been shown to be given by coo = yHo. This phenomenon is the well-known 

 Larmor precession. ^^ We may say then, that the electron has a reso- 

 nance frequency coq. 



Now suppose a sample of ferrite to be placed in a magnetic field ^0. 

 By virtue of the contributions of its many spinning electrons, the sample 

 has a magnetic moment, M. If Hq is a strong field, M will be parallel to 

 i^o. However, there will be, as in the case of the single electron, a fre- 

 quency at which M will precess about the direction of Hq. This precession 

 frequency is proportional to an effective field. He, and to M/J, where / 

 is the vector sum of the individual angular momenta of the electrons. 

 He is a function of Hq and also of the demagnetizing fields within the 

 ferrite. If an alternating field of this frequency is supplied perpendicular 

 to ^0, then absorption will occur. The amplitude of the precessional 

 motion will become such that the energy supplied by the alternating field 

 is equal to the energy transformed into heat in the sample. At the resonant 

 frequency, fi is equal to 1 and /i" reaches a maximum. 



Although the above discussion postulates a strong external field, a 

 more detailed analysis leads to the conclusion that resonance may be 

 expected even with zero external field. This is attributed to the presence 

 of internal fields which result from such things as crystal anisotropy, 

 magnetostrictive strain, and internal demagnetizing fields in the ma- 

 terial. These demagnetizing fields are generally the most important factor 

 in determining the resonant frequency of a demagnetized ferrite. 



A number of investigators have studied ferromagnetic resonance in 

 ferrites. Some experimental methods and results are described in Refer- 

 ences 19 through 23. Much of the investigation has been carried out on 

 single crystals of ferrite. In these cases, the experimental results depend 

 upon the orientation of the crystal axis with respect to the external field. 

 In the case of polycrystalline samples, the resonance is still present, but 

 the resonance is not as sharp. 



Most of the experiments in which ferromagnetic resonance has been 

 studied have been with fixed frequency and varying magnetic field. Fig. 5 

 shows some typical results of such an experiment. From a practical ex- 

 perimental standpoint, this is preferable to varying frequency with a fixed 

 magnetic field. However, it is apparent that if one holds the magnetic 



