Magnetic Resonance 



PART II— MAGNETIC RESONANCE OF 

 ELECTRONS 



By KARL K. DARROW 



(Manuscript received December 24, 1952) 



Magnetic resonance of electrons is the analogue of magnetic resonance of 

 nuclei, treated in the first part of this article. Though the analogy is close 

 and the fundamental laws are identical, the two topics are remarkably dif- 

 ferent in detail. Though electrons are the commonest of particles, they display 

 magnetic resonance only in somewhat exceptional cases. In many free atoms 

 and most solid and liquid substances, magnetic resonance is suppressed by 

 what is known as the ^^anti-parallel coupling^^ of electrons two by two. The 

 exceptional cases are those of certain free atoms, ferromagnetic substances, 

 and a restricted class of strongly paramagnetic substances; the resonance 

 has also been observed very lately for the conduction electrons in metals. In 

 the cases in which it does occur, resonance is likely to occur at a frequency 

 or frequencies very different from that which the elementary theory predicts. 

 This is sometimes because of the orbital motions of the electrons, oftener 

 mainly because of the electric and magnetic fields existing in solids, and the 

 deviations of the observed cases from the ideal case shed light upon these 

 fields. 



The subject of these pages is the magnetic resonance of electrons 

 — "electron resonance" for short. Electrons being everywhere, one might 

 expect it to be found in every substance; but for a fundamental reason 

 it is a rare phenomenon, and this magnifies its interest. Those who search 

 the literature for it under this its proper name will seldom find it, for 

 it is frequently called ''paramagnetic resonance" or, in appropriate 

 cases, "ferromagnetic resonance," These are lengthy names which tend 

 to veil the similarities between electron resonance and nuclear resonance, 

 which latter was the theme of Part I of this article (in the January issue 

 of this Journal) . I will introduce electron resonance by making use of 

 all these similarities. 



Magnetic resonance in general is due directly to the magnetism of 



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