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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 .

Submillimeter dynamic properties (dielectric permittivity e′(ν,T), dynamic conductivity σ′(ν,T)) at frequencies ν=3–20 cm−1 as well as static ones (magnetization, AC magnetic susceptibility, resistance and magnetoresistance) of the La1−xSrxMnO3 (x=0.1, 0.175, 0.2 and 0.3) ferromagnets have been studied at temperatures between 5 K and 400 K. A sharp increase of σ′(ν,T) was observed near the Curie temperature at a metal–semiconductor transition in La0.825Sr0.175MnO3. σ′(ν,T) was described by the Drude model and was equal or slightly greater than its static value. In La0.9Sr0.1MnO3 σ′(ν,T) was considerably greater than the static value, because not only the Drude-like charge carriers contribute to the dynamic conductivity σ′(ν,T) but also the crystal lattice. A significant anisotropy of e′(T) was observed for different crystallographic directions. A jump of e′(T) as well as anomalies in the AC magnetic susceptibility were found at T=100–120 K in La0.9Sr0.1MnO3 which indicate the existence of an additional phase transition (polaron ordering) besides the magnetic transition at 160 K.

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.

Antiferromagnetic resonance (AFMR) have been studied in La1−xSrxMnO3 single crystals (0 25 cm −1 at T=3– 300 K . Two AFMR modes, quasiferromagnetic (F) and quasiantiferromagnetic (AF) were observed and studied below TN. A strong decrease of the resonance frequency of the F-mode was revealed with increasing x, while the frequency of the AF-mode is only slightly decreased (20%). The observed behaviour of the AFMR modes corresponds to a canted magnetic structure and its evolution with increasing x.

We report on detailed susceptibility, magnetization and magnetoresistance studies in La1−xSrxMnO3 for x⩽0.2. We arrive at a phase diagram which shows a ferromagnetic and insulating ground state for 0.1⩽x⩽0.15, followed by a canted antiferromagnetic phase at higher temperatures. The ferromagnetic transition is strongly coupled to a structural transition from a Jahn–Teller distorted phase to a pseudocubic orthorhombic phase, and is accompanied by the appearance of large positive magnetoresistance effects. We interpret the rich phase diagram close to x= 1 8 in terms of orbital ordering, due to an interplay of superexchange interactions and Jahn–Teller distortions. In addition, we report on the temperature dependence of phonon spectra and on optical conductivities. The temperature dependence of bending and stretching modes clearly indicate the structural and magnetic-phase transitions. The optical conductivities are explained in terms of small polaron absorption.

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.

Dielectric and magnetic properties of Gd2CuO4 single crystal were investigated by means of FIR reflectivity and submillimeter transmissivity measurements at temperatures 5 – 300 K. Besides the four known Eu optical phonon modes we observed unexpected phonon lines at 104, 136 and 577 cm−1, which could be caused by structural deformations in the T' phase. The observed behaviour of Cu AFMR mode confirms the existence of the weak ferromagnetic moment and testifies to the strong influence of the Cu-Gd exchange on the magnetic properties of Gd2CuO4.