Optically addressable high-spin states (S ≥ 1) of defects in semiconductors are the basis for the development of solid-state quantum technologies. Recently, one such defect has been found in hexagonal boron nitride (hBN) and identified as a negatively charged boron vacancy (${\mathrm{V}}_{\mathrm{B}}^{-}$). To explore and utilize the properties of this defect, one needs to design a robust way for its creation in an hBN crystal. We investigate the possibility of creating ${\mathrm{V}}_{\mathrm{B}}^{-}$ centers in an hBN single crystal by means of irradiation with a high-energy (E = 2 MeV) electron flux. Optical excitation of the irradiated sample induces fluorescence in the near-infrared range together with the electron spin resonance (ESR) spectrum of the triplet centers with a zero-field splitting value of D = 3.6 GHz, manifesting an optically induced population inversion of the ground state spin sublevels. These observations are the signatures of the ${\mathrm{V}}_{\mathrm{B}}^{-}$ centers and demonstrate that electron irradiation can be reliably used to create these centers in hBN. Exploration of the ${\mathrm{V}}_{\mathrm{B}}^{-}$ spin resonance line shape allowed us to establish the source of the line broadening, which occurs due to the slight deviation in orientation of the two-dimensional B-N atomic plains being exactly parallel relative to each other. The results of the analysis of the broadening mechanism can be used for the crystalline quality control of the 2D materials, using the ${\mathrm{V}}_{\mathrm{B}}^{-}$ spin embedded in the hBN as a probe.

${\mathrm{SrEr}}_2{\mathrm{O}}_4$ is a geometrically frustrated magnet which demonstrates rather unusual properties at low temperatures including a coexistence of long- and short-range magnetic order, characterized by two different propagation vectors. In the present work, the effects of crystal fields (CFs) in this compound containing four magnetically inequivalent erbium sublattices are investigated experimentally and theoretically. We combine the measurements of the CF levels of the ${\mathrm{Er}}^{3+}$ ions made on a powder sample of ${\mathrm{SrEr}}_2{\mathrm{O}}_4$ using neutron spectroscopy with site-selective optical and electron paramagnetic resonance measurements performed on single-crystal samples of the lightly Er-doped nonmagnetic analog, ${\mathrm{SrY}}_2{\mathrm{O}}_4$. Two sets of CF parameters corresponding to the ${\mathrm{Er}}^{3+}$ ions at the crystallographically inequivalent lattice sites are derived which fit all the available experimental data well, including the magnetization and dc susceptibility data for both lightly doped and concentrated samples.