Synthesis and optical properties of nickel-doped copper metaborate crystals

This work presents information on the growth and spectroscopic study of single crystals of copper metaborate doped with nickel Cu$_{1-x}$Ni$_x$B$_2$O$_4$ ($x$ = 0.05, 0.1). In the absorption spectra of both crystals, satellites related to Cu centers distorted by impurity Ni atoms were observed near the lines of zero phonon transitions. Polarization studies in the isotropic $ab$-plane of the tetragonal crystal Cu$_{1-x}$Ni$_x$B$_2$O$_4$ show the presence of linear magnetic dichroism in the magnetically ordered state, which was previously observed both in manganese-doped and undoped copper metaborates CuB$_2$O$_4$. The temperature of magnetic phase transitions into the collinear antiferromagnetic and into helicoidal structures, $T_{\mathrm{N}}$ = 19.1 K and $T^*$ = 8.6 K, respectively, were determined from the temperature dependence of the dichroic signal.