The atomic basis of magnetism

Abstract

In the large-scale world we commonly associate magnetic fields with electrical currents. Our experience in the large-scale world is of little value, however, in providing a theoretical description of the origin of the magnetism of remanence-carrying materials. Although we can use our experience of classical physics to equate an orbiting electron to a current loop and determine the resultant magnetic dipole moment, this turns out to be of little help, as the observed magnetism of solids is due almost exclusively to the magnetic dipole moment associated with the ‘spin’ of the electron. The spin of the electron has no analogue in classical physics and is a feature of the quantum mechanical description of the inhabitants of the submicroscopic world of atoms and electrons. The electron has angular momentum quantized in units of h/2π (= ћ) where h,Planck’s constant, equals 6.63 × 10−34 joule s, and a spin quantum number s = 1/2. The component of spin angular momentum in a specified direction is sћ and the associated magnetic moment (elm)sћ where e and m are the electronic charge and mass. The electron can therefore be regarded simply as a microscopic magnet with magnetic moment (eћ)/(2m),this being the fundamental unit of magnetic moment, the Bohr magneton, s(= 9.27 × 10−24A m2).