Kirill Vasin

The generation of monochromatic electromagnetic radiation in the terahertz (THz) frequency range has remained a challenging task for many decades. Here, the emission of monochromatic sub‐THz radiation by optical phonons in the dielectric material KY(MoO₄)₂ is demonstrated. The layered crystal structure of KY(MoO₄)₂ causes infrared‐active shear lattice vibrations to have energies below 3.7 meV, corresponding to frequencies lower than 900 GHz where solid‐state‐based monochromatic radiation sources are rare. Directly excited by a 5 ps long broadband THz pulse, infrared‐active optical vibrations in KY(MoO₄)₂ re‐emit narrowband sub‐THz radiation as a time‐varying dipole for tens of picoseconds, which is exceptionally long for oscillators with frequencies below 1 THz. Such a long coherent emission allows for the detection of more than 50 periods of radiation with frequencies of 568 and 860 GHz. The remarkably long decay time together with the chemical stability of the employed material suggests a variety of possible applications in THz technology.

The effective Hamiltonians of the fine structure of Fe²⁺ (⁵D) terms are derived using the methods of operator perturbation theory taking into account covalent bonds between iron and oxygen ions. The energy operators of coupling of magnetic and electric dipole moments with the electric field are obtained for states of multiplets Fe²⁺ (⁵E) and Fe²⁺ (⁵T₂) with minimal sets of parameters. The energy spectra of low-lying states, the magnetic anisotropy parameters, and the distribution of quadrupole and induced electric dipole moments over all positions in the unit cell are calculated. The results of calculations are compared with available experimental data.

Magnetization reversal in ferro- and ferrimagnets is a well-known archetype of non-equilibrium processes, where the volume fractions of the oppositely magnetized domains vary and perfectly compensate each other at the coercive magnetic field. Here, we report on a fundamentally new pathway for magnetization reversal that is mediated by an antiferromagnetic state. Consequently, an atomic-scale compensation of the magnetization is realized at the coercive field, instead of the mesoscopic or macroscopic domain cancellation in canonical reversal processes. We demonstrate this unusual magnetization reversal on the Zn-doped polar magnet Fe₂Mo₃O₈. Hidden behind the conventional ferrimagnetic hysteresis loop, the surprising emergence of the antiferromagnetic phase at the coercive fields is disclosed by a sharp peak in the field-dependence of the electric polarization. In addition, at the magnetization reversal our THz spectroscopy studies reveal the reappearance of the magnon mode that is only present in the pristine antiferromagnetic state. According to our microscopic calculations, this unusual process is governed by the dominant intralayer coupling, strong easy-axis anisotropy and spin fluctuations, which result in a complex interplay between the ferrimagnetic and antiferromagnetic phases. Such antiferro-state-mediated reversal processes offer novel concepts for magnetization control, and may also emerge for other ferroic orders.

The parameters of even and odd crystal fields acting on iron ions in tetrahedral and octahedral positions of the Fe2Mo3O8 crystal are calculated. The obtained energy level schemes of the lowest multiplets are discussed in the context of the available experimental data. By comparing the calculated intensities of magnetic and electric dipole transitions with experimental data, the parameters of the effective Hamiltonian of the interaction of 3d electrons with an electric field are refined. It is found that the main absorption lines at T < TN in the region of terahertz frequencies are due to excitations of iron ions and are not associated with collective oscillations of magnetic moments. The splitting of absorption lines upon application of an external magnetic field is a consequence of the difference in the orientations of the magnetic sublattices relative to the crystallographic axes.

Within the framework of the quantum mechanical approach, the available experimental data are analyzed to identify the electronic structure of the multiferroic FeCr ₂ O ₄ . The relative values of the key contributions to the parameters of even and odd crystal fields acting on the 3 d electrons of the Fe ²⁺ ion are determined. Data on local lattice distortions are systematized. The parameter of the electron-deformation interaction of the ground term Fe ²⁺ ( ⁵ E ) is determined considering lattice distortions, and the parameters of binding of the spins of Fe ²⁺ and Cr ³⁺ to the electric field are estimated. The calculation results are compared with the available experimental data on the magnetic and structural characteristics of FeCr ₂ O ₄ , the critical temperature of the transition to an orbitally ordered state, optical conductivity data, the Mössbauer effect study, and measurements of spontaneous electric polarization.

Analytical expressions are obtained for the interaction between orbitally doubly degenerate states of ions through the strain field in weak-anisotropy cubic crystals. The arrangement of...