  The experimental technique of a.c. susceptibility can be used as a probe of
magnetic dynamics in a wide variety of systems. Its use is restricted to the
low-frequency regime and thus is sensitive to relatively slow processes. Rather
than measuring the dynamics of single spins, a.c. susceptibility can be used to
probe the dynamics of collective objects, such as domain walls in ferromagnets
or vortex matter in superconductors. In some frustrated systems, such as spin
glasses, the complex interactions lead to substantial spectral weight of
fluctuations in the low-frequency regime, and thus a.c. susceptibility can play
a unique role. We review the theory underlying the technique and magnetic
dynamics more generally and give applications of a.c. susceptibility to a wide
variety of experimental situations.
