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 K2Ni2(SeO3)3, a spin-1 dimer system arranged on a triangular lattice. The Ni-Ni dimers exhibit a ferromagnetic intra-dimer interaction, effectively behaving as an ensemble with a total spin of S=2. In contrast, antiferromagnetic interactions manifest between these dimers on the triangular lattice. The trigonal distortion of the NiO6 octahedra introduces easy-axis magnetic anisotropy, accounting for the distinct magnetic phase diagrams observed when applying c-axis directional and in-plnae magnetic fields. Notably, our investigation unveils a two-step phase transition with the magnetic field aligned with the c direction. We propose that the system at the first transition is from a paramagnetic state to an up-up-down state, characterized by the Z3 lattice-symmetry breaking. Subsequently, a Berezinskii-Kosterlitz-Thouless transition, involving the breaking of the c-axis spin-rotation symmetry, leads to the formation of the ``Y state'' at low temperatures. These findings yield valuable insights into the magnetic phase transitions inherent to geometrically frustrated magnetic systems featuring dimer structures.