  Optical magneto-spectroscopy methods (Raman scattering, far-infrared
transmission, and photoluminescence) have been applied to investigate the
properties of the NiPS3 semiconducting antiferromagnet. The fundamental magnon
gap excitation in this van der Waals material has been found to be split into
two components, in support of the biaxial character of the NiPS3
antiferromagnet. Photoluminescence measurements in the near-infrared spectral
range show that the intriguing 1.475 eV-excitation unique to the NiPS3
antiferromagnetic phase splits upon the application of the in-plane magnetic
field. The observed splitting patterns are correlated with properties of magnon
excitations and reproduced with the simple model proposed. Possible routes
toward a firm identification of the spin-entangled 1.475 eV-optical excitation
in NiPS3, which can hardly be recognized as a coherent Zhang-Rice exciton, are
discussed.
