Magnetic phase transitions in the antiferromagnet CuB2O4 optically detected via Frenkel excitons

The copper metaborate CuB₂O₄ with a unique noncentrosymmetric crystal structure and the two structurally nonequivalent $4b$ and $8d$ magnetic subsystems of Cu²⁺ ions are characterized by several commensurate and incommensurate magnetic phases below the N\'eel temperature of TN=20 K. Transitions between them can be induced by varying the sample temperature or the applied magnetic field. High-resolution optical spectroscopy of the Frenkel exciton states is used to probe the magnetic phase transitions. The absorption spectra are measured in the temperature range from 4.3 K up to TN and in magnetic fields up to 2 T. The exciton transitions are observed in the range of $1.405--1.407$ eV and are related to the lowest-energy electronic transition in the $4b$ magnetic subsystem of Cu²⁺ ions, thus allowing us to monitor the behavior of this particular subsystem. The exciton lines show abrupt changes of intensity and energy splitting, which can be unambiguously attributed to the magnetic phase transitions in the $4b$ subsystem. The magnetic phase diagram reconstructed from the spectroscopic experiments on the $4b$ subsystem is compared with those from magnetic measurements, which reflect the magnetic behavior of both the $4b$ and $8d$ subsystems. The suggested spectroscopic approach enables separate monitoring of the temperature and magnetic field behavior of different magnetic subsystems in structurally and magnetically complex compounds.