The frequencies absorbed in magnetic resonance are in the megahertz (million cycles per second) and gigahertz (billion cycles per second) ranges. The absorption frequencies for any particular substance are directly proportional to the strength of the magnetic field. This characteristic is unique to magnetic resonance.
There are two distinct subcategories of magnetic resonance. One of these, for which the absorbing particles are electrons, is known by either of two interchangeable names: electron spin resonance (ESR) or electron paramagnetic resonance (EPR). The other category, in which the absorbing particles are atomic nuclei, is called nuclear magnetic resonance (NMR).2
The resonances are actually the absorptions of particular frequencies and are found in EPR and NMR arises from some of the most fundamental properties of matter. A general theory of EPR and NMR must be derived from quantum mechanics, but a classical analogy provides some insight. In the familiar model, every atom has a massive nucleus containing N positively charged protons and a number of uncharged neutrons. ...