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Magnetic gyrotropy effects near antiferromagnetic resonance (AFMR) determined by weak ferromagnetism (YFeO 3 ) and magnetoelectric effect (Cr 2 O 3 ) have been studied using quasi-optical submillimeter backward-wave-oscillator technique in the frequency range 3.2–13 cm −1 . A noticeable rotation of an optical indicatrix and increase of an ellipticity have been revealed near AFMR in both antiferromagnets, which present a manifestation of a Faraday rotation in YFeO 3 and gyrotropic birefringence in Cr 2 O 3 . Theoretical calculations and computer simulations made it possible to describe the observed spectra, taking into account the resonance behaviour of magnetic, magnetoelectric and electric susceptibilities, and to determine corresponding parameters, in particular, an AFMR mode contribution to the static magnetic and magnetoelectric susceptibilities.

Dynamic properties and magnetic excitations in PrBa 2 Cu 3 O 6 were investigated by means of a quasi-optical submillimeter backward-wave-oscillator technique and a Fourier spectrometer at the wavelengths v = 4 – 33 and 50–650 cm −1 , respectively. A weak and wide absorption line was observed in the submillimeter transmission spectra near 15 cm −1 at T , which was identified as a rare-earth mode originating from electronic transitions between the low-energy crystal-field states of Pr 3+ . Temperature dependence of the resonance frequency, linewidth and mode contribution to static magnetic permeability were obtained. Dynamic magnetic susceptibility χ(ω, q, T) and resonance frequencies ω(q, T) of the magnetic excitations of the Pr-subsystem were calculated and analyzed above and below the Neel temperature (T N = 10K).

We have revealed a strong influence of non-magnetic impurity Al 3+ ions (magnetic vacancies) on spin excitations in GdFe 0.9 Al 0.1 O 3 and HoFe 0.925 Al 0.075 O 3 at frequencies ν = 2–33 cm −1 . A significant growth of the frequencies of both AFMR modes ν 1 and ν 2 at low temperatures was found in GdFe 0.9 Al 0.1 O 3 as against the pure GdFeO 3 . The anomalies observed in the behaviour of AFMR modes for HoFe 0.925 Al 0.075 O 3 at T M = 60 K make evidence for a new first-order spin reorientational phase transition from Γ 4 ( G x F z ) phase to antiferromagnetic phase Γ 1 (G y ) induced by the magnetic vacancies.