Multisublattice antiferromagnets with complex crystallographic structures can give rise to nontrivial spin ordering due to competing exchange couplings within and between different subsystems of magnetic ions. In nickel orthoborate ${\mathrm{Ni}}_3{\left({\mathrm{BO}}_3\right)}_2$ there are two distinct subsystems of the ${\mathrm{Ni}}^{2+}$ ions $(S=1)$ that are antiferromagnetically ordered below $T_N=46\mathrm{K}$, forming complex spin ordering and multiple branches of magnons. We report a comprehensive experimental study of single-crystal spin dynamics using polarized Raman scattering and infrared absorption spectroscopy. A rich multibranch spin-wave spectrum covering a broad energy range was observed via one-magnon scattering. Additionally, we detected the formation of a complex-shape two-magnon band in the range of $60–200{\mathrm{cm}}^{-1}$. These findings were supported by calculations using linear spin-wave theory, which allowed us to describe the shape of the two-magnon band and propose an exchange structure for the antiferromagnetic nickel orthoborate ${\mathrm{Ni}}_3{\left({\mathrm{BO}}_3\right)}_2$ compatible with experimental observations.

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• Magnons
• Complex oxides
• Infrared spectroscopy
• Linear spin wave theory
• Raman spectroscopy