• electron paramagnetic resonance;
  • paramagnets;
  • rare-earth aluminum borates;
  • spin–phonon interaction


We have presented new results on the EPR spectra and the ground state of the Cr3+ ion replacing Y3+ in YAl3(BO3)4 and Eu3+ in EuAl3(BO3)4 single crystals. The EPR studies performed at frequencies of about 9.4 and 37 GHz in a temperature range of 300–4.2 K have revealed the peculiarities and differences of EPR spectra of Cr3+ ion as well as determined the spin-Hamiltonian parameters (g-factors, D, and E fine-structure parameters and their signs) in both crystals. The value and sign of the D parameter in both crystals are determined by both the nearest ligands around the Cr3+ ion and the spin–orbit interaction. The interaction between Cr3+ ion and rare-earth ion weakly influences on the ground-state splitting, while the changes of g-factors are mainly caused by the Cr3+ and Eu3+ ions interactions. The EPR spectra and contributions determining the value of spin-Hamiltonian parameters were analyzed on the basis of the superposition model of the crystal field.