  We consider an unconventional superconductor in a low frequency harmonic
magnetic field. In the Meissner regime at low T a nonlinear magnetic response
arises from quasiparticle excitations near minima in the energy gap. Various
physical quantities then acquire higher harmonics of the frequency of the
applied field. We discuss how examination of the field and angular dependence
of these harmonics allows determination of the structure of the energy gap. We
show how to distinguish nodes from small finite minima ("quasinodes"). Gaps
with nodal lines give rise to universal power law field dependences for the
nonlinear magnetic moment and the nonlinear torque. They both have separable
temporal and angular dependences. In contrast, when there are quasinodes these
quantities have more complicated and nonseparable field, temporal, and angular
dependences. We illustrate this on the example of an s+id gap. We discuss how
to perform measurements so as to maximize the nonlinear signal and how to
investigate in detail the properties of the superconducting minima, thus
determining the gap function symmetry.
