Magnetic tricritical point and nematicity in FeSe under pressure

Magnetism induced by external pressure ($p$) was studied in a FeSe crystal sample by means of muon-spin rotation. The magnetic transition changes from second-order to first-order for pressures exceeding the critical value $p_{{\rm c}}\simeq2.4-2.5$GPa\mathrm{.}Themagneticorderingtemperature($T_{{\rm N}}$) and the value of the magnetic moment per Fe site ($m_{\mathrm{F}\mathrm{e}}$) increase continuously with increasing pressure, reaching $T_{\mathrm{N}}\simeq 50$~K and $m_{\mathrm{F}\mathrm{e}}\simeq 0.25$ ${\mu }_{\mathrm{B}}$ at $p\simeq 2.6$ GPa, respectively. No pronounced features at both $T_{\mathrm{N}}(p)$ and $m_{\mathrm{F}\mathrm{e}}(p)$ are detected at $p\simeq p_{\mathrm{c}}$, thus suggesting that the stripe-type magnetic order in FeSe remains unchanged above and below the critical pressure $p_{\mathrm{c}}$. A phenomenological model for the $(p,T)$ phase diagram of FeSe reveals that these observations are consistent with a scenario where the nematic transitions of FeSe at low and high pressures are driven by different mechanisms.