396 THE BELL SYSTEM TECHNICAL JOURNAL, MARCH 1953 



(c) The value of g may depend upon the orientation of the applied 

 magnetic field with respect to the crystal. 



(d) The frequency of the resonance-peak or peaks may not be pro- 

 portional to H. In fact, it may deviate so far from being proportional to 

 H that extrapolation to ^ = will indicate that even in the absence of 

 ar applied magnetic field there would be a separation of the levels. Thus 

 the asymmetric electric field within a strongly paramagnetic crystal may 

 by itself produce the effect, which hitherto we have been ascribing en- 

 tirely to the applied magnetic field. This is called "zero-field splitting." 



One sees only too well that the interior of a strongly paramagnetic 

 salt is no place to look for the ideal case, and that resonance in such a 

 salt is a theme for deep study and not for facile interpretation. As a 

 matter of fact, electron resonance in paramagnetic salts is valued for 

 its contribution to our knowledge of the electric fields in these crystals ; 

 which is to say, that it is a part of solid-state physics, the details of 

 which lie beyond the scope of this article. 



HYPERFINE STRUCTURE OF ELECTRON RESOXAXri: 



One of the most beautiful phenomena in this province of physics 

 — and, I venture to say, not in this province only but in the whole of 

 physics — is the "hyperfine structure" or "hyperfine splitting" of the 

 electronic resonance. Here we see the spin and the magnetic moment of 

 the nucleus collaborating with those of the electron to produce an ex- 

 quisite and lucid joint effect. It is still the electronic resonance, and must 

 never be confused with the nuclear resonance; but the single resonance- 

 peak of the ideal case is split into a group of peaks, the number of which 

 is determined by the spin of the nucleus. 



Fig. 3 relates to neodymium — not however to the metal, but to 

 neodymium atoms in a salt of neodymium, diluted with a salt of another 

 metal so that the neodymium atoms may not influence one another 

 through undue proximity. Neodymium is an element with two "odd" 

 isotopes — that is to say, isotopes of odd mass-number — and several 

 "even" isotopes. The even isotopes have non-magnetic nuclei, and so 

 do not perturb the electron resonance. Each of the two odd isotopes has 

 a nucleus of spin 7/2 and non-zero magnetic moment. Such a nucleus 

 will have eight permitted orientations in the big magnetic field. It will 

 produce a local magnetic field in the region of the resonating electrons, 

 and the strength of this field will depend on the orientation. The reso- 

 nance-frequency depends on the big field compounded with the local 

 field (we met with instances of this rule in the study of nuclear reso- 



