Magnetic Resonance: Part II - Magnetic Resonance of Electrons

PART II--MAGNETIC RESONANCE OF ELECTRONS (Manuscript received December 24, 1952) By KARL K. DARROW 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 different 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.