Investigation of the phase diagram of UNi2Si2
It is now well
established that the strong anisotropy in the magnetic properties of the
intermetallic compounds UT2Si2, where T stand for a
transition metal, is responsible for their rich magnetic phase diagram. However, within that series of compounds
(URu2Si2, UPd2Si2, URh2Si2
just to name a few), UNi2Si2 is one that shows an
unusual sequence of magnetically ordered states which cannot be accounted for
by the axial next‑nearest‑neighbor Ising model (ANNNI). For that reason, UNi2Si2certainly
continues to attract much attention. Thus, in order to better understand its
unusual magnetic properties, we have investigated, by means of sound velocity
measurements, the influence of pressure on its magnetic phase diagram. First of all, our measurements, in the
magnetically ordered states, indicate that the temperature dependence of the
elastic properties along the c-axis is dominated by magnetoelastic
effects. Moreover, the analysis of the
temperature dependence for the incommensurate longitudinal spin-density state
reveals a critical exponent b = 0.38 which is consistent with
other type of measurements. From all
these measurements, we derive the magnetic phase diagram of UNi2Si2
at different pressures. Our observations clearly indicate that the
commensurate longitudinal spin‑density‑wave with a ferromagnetic
component (<12>‑phase below 50 K) is the only phase that sees its
stability increases under pressure and magnetic field. Finally, our
investigation also reveals that the coordinates of the triple-point (Tp, Hp)
decrease with pressure at a rate of dTp/dP = -.1 K/kbar and dHp/dP = 0.1
T/kbar, respectively.