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Resonance modes that are due to magnetic excitations in the exchange-coupled subsystems of rare-earth ions (R = Nd3+, Sm3+, and Gd3+) and Fe3+ ions have been detected in submillimeter transmission spectra (0.1–0.6 THz) of RFe3(BO3)4 iron borate-multiferroic single crystals. The strong interaction between spin oscillations of the Fe and R subsystems has been revealed, which determines the behavior of the modes depending on the anisotropy of the exchange splitting of the ground doublet of the R ion. It has been shown that the intensities of coupled modes (contributions to the magnetic permeability) depend strongly on the difference between the g factors of Fe and R ions. This dependence makes it possible to determine the sign of the latter g factor. In particular, a noticeable intensity of exchange Nd modes in NdFe3(BO3)4 is due to an increase in their contribution at g ⊥, ‖ Nd < 0, while in GdFe3(BO3)4 with g Gd ≈ g Fe ≈ 2, the Fe and Gd contributions compensate each other and the exchange (Gd) mode is not observed. In spite of the weak interaction of Sm ions with the magnetic field, SmFe3(BO3)4 exhibits resonance modes, which are attributed to the excitation of Sm ions through the Fe subsystem.

The colossal (more than threefold) decrease in the dielectric constant ɛ in the easy-plane SmFe3(BO3)4 ferroborate in a magnetic field of ∼5 kOe applied in the basal ab plane of the crystal has been found. A close relation of this effect to anomalies in the field dependence of the electric polarization has been established. It has been shown that this magnetodielectric effect is due to the contribution to ɛ from the electric susceptibility, which is related to the rotation of spins in the ab plane, arises in the region of the antiferromagnetic ordering T < TN = 33 K, and is suppressed by the magnetic field. A theoretical model describing the main features of the behavior of ɛ and electric polarization in the magnetic field has been proposed, taking into account the additional anisotropy in the basal plane induced by the magnetoelastic stresses.

The magnetic, magnetoelectric, and magnetoelastic properties of DyFe3(BO3)4 single crystals and the Nd0.75Dy0.25Fe3(BO3)4 substituted compound with the competitive Nd-Fe and Dy-Fe exchange interactions have been investigated in magnetic fields up to 210 kOe. It has been shown that the antiferromagnetic state in Nd0.75Dy0.25Fe3(BO3)4 below the Neel temperature is easy-plane and a spontaneous spin-reorientation transition to a uniaxial state occurs near 25 K. Anomalies are observed in various physical characteristics in a spin-flop transition induced by the magnetic field along the trigonal axis. The H-T phase diagrams are constructed; they are in good agreement with a simple model taking into account the anisotropy of the exchange splitting and g factors of the ground doublets of the Dy3+ and Nd3+ ions. The features of the magnetoelectric effect in various magnetic phases have been studied. Spontaneous electric polarization has been revealed in the region of the existence of an easy-plane antiferromagnetic state in the substituted compound. A qualitative theoretical justification of the possibility of the appearance of the spontaneous spin-reorientation transition in the Nd0.75Dy0.25Fe3(BO3)4 substituted compound is given.

The magnetic, magnetoelectric, and magnetoelastic properties of a PrFe 3 (BO 3 ) 4 single crystal and the phase transitions induced in this crystal by the magnetic field are studied both experimentally and theoretically. Unlike the previously investigated ferroborates, this material is characterized by a singlet ground state of the rare-earth ion. It is found that, below T N = 32 K, the magnetic structure of the crystal in the absence of the magnetic field is uniaxial ( l || c ), while, in a strong magnetic field H || c ( H cr ~ 43 kOe at T = 4.2 K), a Fe 3+ spin reorientation to the basal plane takes place. The reorientation is accompanied by anomalies in magnetization, magnetostric- tion, and electric polarization. The threshold field values determined in the temperature interval 2-32 K are used to plot an H - T phase diagram. The contribution of the Pr 3+ ion ground state to the parameters under study is revealed, and the influence of the praseodymium ion on the magnetic and magnetoelectric properties of praseodymium ferroborate is analyzed.

For the DyMnO3 multiferroic with a modulated magnetic structure, switching of its spontaneous electric polarization (P ‖ c axis) near the ferroelectric transition (T < TFE ∼ 20 K) is revealed by measuring the dielectric hysteresis loops. It is found that the coercive field strongly increases as the temperature decreases (up to 2.6 kV/mm at 17.6 K). The values obtained for the spontaneous polarization are found to agree well with the data obtained from pyroelectric measurements. In addition, anomalies are observed in the temperature dependences of the spontaneous polarization Pc, dielectric constant ɛc, and magnetic susceptibility xb at T ∼ 6 K; these anomalies are attributed to the antiferromagnetic ordering of the Dy3+ ions.

Complex experimental and theoretical investigations of the magnetic, magnetoelectric, and magnetoelastic properties of neodymium iron borate NdFe3(BO3)4 along various crystallographic directions have been carried out in strong pulsed magnetic fields up to 230 kOe in a temperature range of 4.2–50 K. It has been found that neodymium iron borate, as well as gadolinium iron borate, is a multiferroic. It has a much larger (above 300 μC/m2) electric polarization controlled by the magnetic field and giant quadratic magnetoelectric effect. The exchange field between the rare-earth and iron subsystems (∼50 kOe) has been determined for the first time from experimental data. The theoretical analysis based on the magnetic symmetry and quantum properties of the Nd ion in the crystal provides an explanation of the unusual behavior of the magnetoelectric and magnetoelastic properties of neodymium iron borate in strong magnetic fields and correlation observed between them.