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A tendency to form multiferroic states with a spatially modulated antiferromagnetic structure and electric polarization in pure orthorhombic manganites RMnO 3 upon decreasing of the rare-earth ionic radii has been realized in single crystals of substituted compounds Eu 1x- Y x MnO 3 (0.2 ≤ x ≤ 0.5) and Gd 1-y Y y MnO 3 (0 < y ≤ 0.2). While the parent pure and slightly substituted (x ≤ 0,1) compounds exhibited a spontaneous transition from the incommensurate (IC) to the canted antiferromagnetic (CAF) state at T CA < T N = 45-50 K, the transitions from IC to commensurate ferroelectric (C/FE) phase were observed at T lock < T N for x ≥ 0.3 and y ≥ 0.05 accompanied by dielectric anomalies. Various phase transitions were observed in magnetic fields up to 250 kOe along the a-, b-, c-axes by magnetization, magnetostriction and electric polarization measurements. The latter indicate the existence of a spontaneous electric polarization below T lock .

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.

Antiferromagnetic resonance (AFMR) in Co 1− c Mn c F 2 ( c ∼ 10 -4 ) in the external magnetic field is investigated. It was established that coherent and incoherent rearrangements (CR and ICR) of spectrum in the region of AFMR and impurity lines crossing are possible in the same sample depending on field orientation. Topology of the phase diagram of spin excitation (SE) spectrum in variables c - H is obtained.