Magnetic phase transitions in the triangular-lattice spin-1 dimer compound K2Ni2(SeO3)3

In our study, we conduct magnetization and heat capacity measurements to investigate field-induced magnetic phase transitions within the newly synthesized compound ${\mathrm{K}}_{2}{\mathrm{Ni}}_{2}{({\mathrm{SeO}}_{3})}_{3}$, a spin-1 dimer system arranged on a triangular lattice. From our first-principles simulations, we determine that the spin system in ${\mathrm{K}}_{2}{\mathrm{Ni}}_{2}{({\mathrm{SeO}}_{3})}_{3}$ can be represented as a two-dimensional triangular-lattice spin-1 dimer model, including an intra-dimer exchange of ${J}_{1}=0.32\phantom{\rule{0.16em}{0ex}}\mathrm{meV}$, an inter-dimer exchange of ${J}_{2}=0.79\phantom{\rule{0.16em}{0ex}}\mathrm{meV}$, and an easy-axis anisotropy of $D=0.14\phantom{\rule{0.16em}{0ex}}\mathrm{meV}$. The presence of easy-axis magnetic anisotropy explains the distinct magnetic phase diagrams observed under $c$-axis directional and in-plane magnetic fields. Notably, our investigation unveils a two-step phase transition with the magnetic field aligned with the $c$ direction. Our findings yield valuable insights into the magnetic phase transitions inherent to geometrically frustrated magnetic systems featuring dimer structures.