ABSTRACT
In this chapter, we discuss the extremely important notion of exchange interaction between electrons, which plays a major role in the operation of many spin-based devices (both classical – see Chapter 15 – and quantum mechanical – see Chapter 16). We start by describing the formulation of the Exclusion Principle enunciated by Pauli in 1924. We then use this principle to calculate the energy eigenvalues of the Helium atom based on perturbation theory. This leads to the concept of exchange energy which was extended by Heisenberg to a system of spins to propose a model for the origin of ferromagnetism. The concept of exchange coupling and Heisenberg Hamiltonian are crucial in understanding the operation of many solid state spin-based devices described in this book and elsewhere.
