The location of a tricritical point in the $\left(p,T\right)$ phase diagram of MnSi and the nature of the phase transition at both sides of this point are under discussion. While the tricritical point was traditionally placed at $p_{\text{tc}}=12\text{kbar}$ and $T_{\text{tc}}=12\text{K}$, with the first-order transition at $p>p_{\text{tc}}$, recent studies [Stishov et al., Phys. Rev. B 76, 052405 (2007)] suggested that it should be located at $p_{\text{tc}}=3.5\text{kbar}$ and $T_{\text{tc}}=25\text{K}$ with the transition being second order for $p>p_{\text{tc}}$. Based on a direct analysis of the magnetic phase transition under pressure, we demonstrate that the coordinates of the tricritical point are indeed $p_{\text{tc}}=3.5\left(3\right)\text{kbar}$ and $T_{\text{tc}}=24\text{K}$ but with the magnetic phase transition changing from second to first order as pressure increases. We also discuss the effect of spin fluctuations in the transport and magnetic properties of MnSi.

• Received 13 October 2008

DOI:https://doi.org/10.1103/PhysRevB.79.060401