  It is shown that the phase transition in low-T_c clean itinerant ferromagnets
is generically of first order, due to correlation effects that lead to a
nonanalytic term in the free energy. A tricritical point separates the line of
first order transitions from Heisenberg critical behavior at higher
temperatures. Sufficiently strong quenched disorder suppresses the first order
transition via the appearance of a critical endpoint. A semi-quantitative
discussion is given in terms of recent experiments on MnSi, and predictions for
other experiments are made.
