  We propose a thermodynamic model of free energy expansion up to eighth order
of magnetisation to describe the complex magnetic phase transitions in
ferromagnetic superconductor UGe$_2$. The model successfully describes
transitions between ordered phases which take place without changing of
magnetic structure but only of magnetisation which is the case of UGe$_2$ where
two magnetic phases with same structure but with different magnitude occur with
decreasing of temperature. We consider the infuence of pressure on magnetic
transitions in the simplest form of including it only in pressure dependence of
Curie temperature for the transition between the paramagnetic and themagnetic
phase with the lower magnetic moment FM1. Our results show that for pressures
lower than some limiting pressure the transition betweenlow-and
high-magnetisation phase remains of crossover type. Above this limiting
pressure this transition changes to real thermodynamic transition of first
order.
