Introduction

Synopsis

1. Isolated ions (2 lectures)
   
   

   Magnetic properties become particularly simple if we are able to ignore the interactions between ions. In this case we are able to treat the ions as effectively ``isolated'' and can discuss diamagnetism and paramagnetism. For the latter phenomenon we revise the derivation of the Langevin and Brillouin functions outlined in the part A course. The ions can interact with the crystal field and this can be probed experimentally using magnetic resonance (in particular ESR and NMR).

2. Interactions (2 lectures)
   
   

   Now we turn on the interactions! I will discuss what sort of interactions there might be, including dipolar interactions and the different types of exchange interaction. The interactions lead to various types of ordered magnetic structures which can be measured using neutron diffraction. I will then discuss the Weiss model of ferromagnetism, antiferromagnetism and ferrimagnetism and also consider the magnetism of metals.

3. Symmetry breaking (3 lectures)
   
   

   The concept of broken symmetry is at the heart of condensed matter physics. The first three lectures aim to demonstrate how the existence of the solid phase, ferromagnetism and superconductivity, are all the result of breaking symmetry. The consequences of breaking symmetry are that systems show some kind of rigidity (in the case of ferromagnetism this is permanent magnetism), low temperature elementary excitations (in the case of ferromagnetism these are spin waves, also known as magnons), and defects (in the case of ferromagnetism these are domain walls). These concepts are explored in the context of low-dimensional magnetism.