Competition of magnetocrystalline anisotropy of uranium layers and zigzag chains in UNi0.34Ge2 single crystals

Structural and thermodynamic properties of single-crystalline ${\mathrm{UNi}}_{1\ensuremath{-}x}{\mathrm{Ge}}_{2}$ with $x=0.66$ have been investigated by measuring magnetization, specific heat, and thermal expansion over a wide range of temperatures and magnetic fields. The measurements revealed the emergence of a long-range antiferromagnetic ordering of uranium magnetic moments below the N\'eel temperature ${T}_{\mathrm{N}}=45.5(1)\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ and the existence of two easy axes in the studied compound, namely, $b$ and $c$, which correspond to the plane of the uranium zigzag chains. Magnetic field applied along these two crystallographic directions induces in the system a first-order metamagnetic phase transition (from antiferromagnetism to field-polarized paramagnetism), and the width of the magnetic hysteresis associated with that transition reaches as much as about 40 kOe at the lowest temperatures. A magnetic phase diagram developed from the experimental data showed that the metastable region associated with that magnetic hysteresis forms a funnel that narrows toward the N\'eel point in a zero magnetic field. The four-layer Ising model has successfully predicted the collinear antiferromagnetic structure in ${\mathrm{UNi}}_{0.34}{\mathrm{Ge}}_{2}$ (known from earlier reports), its magnetic phase diagram, and temperature and field variations of its magnetization. Moreover, it suggests that the first-order phase transition extends down to a zero magnetic field, although it is barely detectable in the experiments performed in low magnetic fields. According to this model, the second-order phase transition occurs in the compound only in a zero field.