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The copper metaborate CuB₂O₄ with a unique noncentrosymmetric crystal structure and the two structurally nonequivalent 4b and 8d magnetic subsystems of Cu^2+ ions are characterized by several commensurate and incommensurate magnetic phases below the N\'eel temperature of T₍=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 T₍ 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 Cu^2+ 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.
Kudlacik et al. (Mon,) studied this question.
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