Specific Heat of the Skyrmion Lattice Phase and Field-Induced Tricritical Point in MnSi

We report high-precision measurements of the temperature and magnetic field dependence of the specific heat, $C(T,H)$, across the magnetic phase diagram of MnSi. Clear anomalies establish the Skyrmion lattice unambiguously as a thermodynamic phase. The evolution of the specific heat anomalies, the field dependence of the entropy released at the phase transitions, and the temperature versus field dependence of crossover lines provide striking evidence of a tricritical point at ${$\mu${}}_{0}{H}_{\mathrm{TCP}}^{\mathrm{int}}=340\text{ }\text{ }\mathrm{mT}$ and ${T}_{\mathrm{TCP}}=28.5\text{ }\text{ }\mathrm{K}$. The existence of this tricritical point represents strong support of a helimagnetic Brazovskii transition, i.e., a fluctuation-induced first-order transition at $H=0$.