EPR-determined anisotropy of the $g$-factor and magnetostriction of a Cu$_2$MnBO$_5$ single crystal with a ludwigite structure

Electron paramagnetic resonance (EPR) and magnetostriction of the Cu$_2$MnBO$_5$ single crystal have been studied. The EPR spectrum consists of a single Lorentzian line due to the exchange-coupled system of spins of Cu$^{2+}$ and Mn$^{3+}$ ions. It has been established experimentally that the $g$-factor in the paramagnetic region is strongly anisotropic and anomalously small, which is not typical of the exchange-coupled system of spins of Cu$^{2+}$ and Mn$^{3+}$ ions. At a temperature of $150$ K, the $g$-factors along the crystallographic $a$, $b$, and $c$ axes are $2.04$, $1.96$, and $1.87$, respectively. Such small effective $g$-factor values can be due to the effect of the anisotropic Dzyaloshinskii–Moriya exchange interaction between the spins of Cu$^{2+}$ and Mn$^{3+}$ ions directed along the $a$ axis. The presence of two Cu$^{2+}$ and Mn$^{3+}$ Jahn-Teller ions occupying four nonequivalent positions in the crystal is responsible for the absence of the inversion center. It is found that the behavior of the magnetostriction of Cu$_2$MnBO$_5$ is not typical of transition-metal crystals but is closer to the behavior of crystals containing rare-earth ions.