Magnetic phase diagrams of UPd2Si2: searching for Lifshitz point

The ternary compounds with composition UT$_{2}$M$_{2}$, where T is a d-electron transition metal, and M stands for Si or Ge atom, crystallize mostly with tetragonal unit cells of the ThCr$_{2}$Si$_{2}$- (space group I4/mmm) or CaBe$_{2}$Ge$_{2}$-type (space group P4/nmm). Competition between crystallization of the two polymorphs may result in a structural instability that strongly influences the bulk physical characteristics [1]. Some UT$_{2}$M$_{2}$ compounds exhibit complex magnetic properties. Specifically, the magnetic phase diagram of UPd$_{2}$Si$_{2}$, constructed for $H \parallel c$ (easy magnetic direction), comprises as many as three ordered phases, namely, simple antiferromagnetic (AFM), ferrimagnetic (FiM) and incommensurate longitudinal spin wave (ICLSW) states. Most remarkably, in high magnetic fields, paramagnetic (P), FiM and ICLSW phases, meet each other at a point [2,3] that has been suggested to be a bicritical Lifshtz point (LP), whose “critical behavior is strikingly different from any other” [3,4]. It is worthwhile noting, that to date the existence of magnetic LP has been unambiguously confirmed only for MnP [5].
However, the magnetic phase diagrams of UPd$_{2}$Si$_{2}$ reported in the literature, show some notable differences [2,3], presumably related to the structural instability of the compound. In order to clarify the issue, we synthesized several single crystals of UPd$_{2}$Si$_{2}$ using the Czochralski pulling technique in a tetra-arc furnace. The chemical composition and the crystal structure of the particular crystals were checked by means of energy-dispersive x-ray spectroscopy and x-ray diffraction (Laue backscattering and four-circle diffractometers). Their magnetic, electrical transport and thermal properties were characterized in wide ranges of temperature (2–300 K), and magnetic fields (up to 14 T).
We constructed the magnetic phase diagrams of the grown crystals and identified three main types of behavior, two of them being consistent with those reported before in the literature [1,2]. In zero magnetic field, the crystals differ in terms of number of phase transitions and values of critical temperatures. However, their properties vary mostly in the low-field regime, while in high magnetic fields, the P, FiM and ICLSW phase boundaries tend to merge for each crystal type, as expected for the appearance of LP. Further experiments are currently underway to characterize in details the critical behavior near the LP anomaly.

This work was supported by the National Science Centre of Poland grant no. 2018/31/D/ST3/03295.

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