Magnetic structure,phase transitions and features of electromagnetic wave propagation in La2 Cu O4

The frequency-field diagram and the components of the tensor of the high-frequency magnetic susceptibility are obtained for antiferromagnetic structures realized in La2 Cu O4 in longitudinal magnetic field. The coefficient of the electromagnetic wave propagation through the sample is calculated, and the changes in the propagation spectrum due to the switch-over of the magnetic phases are analysed thoroughly. The polariton effects are shown to vary for different magnetic structures, and each of the possible scenarios of the phase switch-over is characterized by its specific changes in the propagation spectrum. The character of the changes is indicative of approaching the phase transition points. Thus, the spectrum of the electromagnetic wave propagation is helpful in restoring the phase diagram, finding the AFMR frequencies in different phases and restoring on their bases the constants of the spin Hamiltonian of La2 Cu O4