Magnetic tricritical point and nematicity in FeSe under pressure

Abstract

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 ${p}_{\mathrm{c}}\ensuremath{\simeq}2.4--2.5$ GPa. The magnetic ordering temperature (${T}_{\mathrm{N}}$) and the value of the magnetic moment per Fe site (${m}_{\mathrm{Fe}}$) increase continuously with increasing pressure, reaching ${T}_{\mathrm{N}}\ensuremath{\simeq}50$ K and ${m}_{\mathrm{Fe}}\ensuremath{\simeq}0.25{\ensuremath{\mu}}_{\mathrm{B}}$ at $p\ensuremath{\simeq}2.6$ GPa, respectively. No pronounced features at both ${T}_{\mathrm{N}}(p)$ and ${m}_{\mathrm{Fe}}(p)$ are detected at $p\ensuremath{\simeq}{p}_{\mathrm{c}}$, 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.