  The problem of quantum criticality in the context of itinerant ferro- or
metamagnetism has received considerable attention [S. A. Grigera et. al.,
Science 294, 329 (2001); C. Pfleiderer et. al., Nature, 414, 427 (2001)]. It
has been proposed that a new kind of quantum criticality is realised in
materials such as MnSi or Sr_3Ru_2O_7. We show based on a mean-field theory
that the low-temperature behaviour of the n-layer ruthenates
Sr_{n+1}Ru_nO_{3n+1} can be understood as a result of a Van Hove singularity
(VHS). We consider a single band whose Fermi energy, E_F, is close to the VHS
and deduce a complex phase diagram for the magnetism as a function of
temperature, magnetic field and E_F. The location of E_F with respect to the
VHS depends on the number of layers or can be tuned by pressure. We find that
the ferromagnetic quantum phase transition in this case is not of second but of
first order, with a metamagnetic quantum critical endpoint at high magnetic
field. Despite its simplicity this model describes well the properties of the
uniform magnetism in the single, double and triple layer ruthenates. We would
like to emphasise that the origin of this behaviour lies in the band structure.
