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 pc≃2.4-2.5 GPa. The magnetic ordering temperature (TN) and the value of the magnetic moment per Fe site (mFe) increase continuously with increasing pressure, reaching TN≃50 K and mFe≃0.25μB at p≃2.6 GPa, respectively. No pronounced features at both TN(p) and mFe(p) are detected at p≃pc, thus suggesting that the stripe-type magnetic order in FeSe remains unchanged above and below the critical pressure pc. 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.
Bibliographical noteFunding Information:
This work was performed at the Swiss Muon Source (), Paul Scherrer Institute (PSI, Switzerland). R.M.F. is supported by the U. S. Department of Energy, Office of Science, Basic Energy Sciences, under Award No. DE-SC0012336. The work of G.S. is supported by the Swiss National Science Foundation, Grants No. 200021_149486 and No. 200021_175935. The work of X.D., F.Z., and Z.Z. was supported by the “National Key Research and Development Program of China (Grant No. 2016YFA0300301),” the “Strategic Priority Research Program (B)” of the Chinese Academy of Sciences (Grant No. XDB07020100), and the Natural Science Foundation of China (Grant No. 11574370).