In our discussion of the electronic properties of bulk and nanoscale materials, we have so far concentrated on the charge carried by the electrons. However, the spin degree of freedom is also tremendously important, from the standpoints of fundamental science and technological impact. The collective response of electronic spins (in concert with their orbital motion) gives rise to the magnetic properties of materials. Magnetism's technological impact cannot be overstated, from the first primitive compasses thousands of years ago to advanced magnetic data storage and magnetic imaging techniques today.

In this chapter we introduce magnetism and magnetic materials, beginning with bulk systems. After discussing how to characterize magnetic response down to very small scales, we look at the effects of surfaces and interfaces, including confinement down to the nanoscale. After reviewing the history and evolution of magnetic data storage and magneto electronic devices, we consider nanoscale magnetism and its potential impact.

Definitions and units

One unfortunate and unavoidable challenge of discussing magnetism in materials is the unwieldy nature of magnetic units. This is largely the result of the historical development of our understanding of electricity, magnetism, and their relationship.

Let's begin by introducing the different types of vector field relevant to magnetism. The truly fundamental field is B, the magnetic induction or magnetic flux density (though, truth be told, most physicists would call this the magnetic field, as was done in the previous chapter). This is the field that shows up in the Lorentz force law and determines the force on a moving charge. It is also the field that determines magnetic resonance frequencies, and is related to the vector potential by ∇ × A = B. The SI unit of B is the Tesla. The Earth's magnetic field is about 3 × 10− 5 T over most of the planet.