  This thesis uses a quantity that is defined and justified by information
theory -- mutual information -- to examine models of condensed matter systems.
More precisely, it studies models which are made up out of ferromagnetically
interacting spins. Quantum information theory often focuses on the ground state
of such systems; we will however be interested in what happens at finite
temperature.
  Using mutual information, which can be seen as a generalization of
entanglement entropy to the finite-temperature case, we can study the different
phases occurring in these models, and in particular the phase transitions
between those. We examine broadly two different classes of models: classical
spins on two-dimensional lattices, and fully-connected models of
quantum-mechanical spin-1/2 particles.
  (Abstract abridged.)
