Pressure induced crossover of the magnetic transition from second to first order near the quantum critical point in MnSi

The magnetic ordering transition temperature in MnSi decreases with increasing pressure and tends towards absolute zero near 15 kbar. As the critical pressure is approached, we observe a crossover from a conventional transition of second order to one of weak first order. The crossover point is found to be near a minimum in the inverse susceptibility as a function of temperature. A qualitative description of this behaviour is given in terms of a temperature dependent renormalisation of the coefficients in a sixth order Landau expansion for the free energy.