  The itinerant quasi-ferromagnetic metal MnSi has been studied by detailed
thermal expansion measurements under pressures and magnetic fields. A sudden
decrease of the volume at the critical pressure Pc ~1.6 GPa has been observed
and is in good agreement with the pressure variation of the volume fraction of
the spiral magnetic ordering. This confirms that the magnetic order disappears
by a first order phase transition. The energy change estimated by the volume
discontinuity on crossing Pc is of similar order as the Zeeman energy of the
transition from the spiral ground state to a polarized paramagnetic one under
magnetic field. In contrast to the strong pressure dependence of the transition
temperature, the characteristic fields are weakly pressure dependent,
indicating that the strength of the ferromagnetic and the Dzyaloshinskii-Moriya
interactions do not change drastically around Pc. The evaluated results of the
thermal expansion coefficient and the magnetostriction are analyzed
thermodynamically. The Sommerfeld coefficient of the linear temperature term of
the specific heat is enhanced just below Pc. The magnetic field-temperature
phase diagrams in the ordered and paramagnetic phases are also compared.
Comparison is made with other heavy fermion compounds with first order phase
transition at 0 K.
