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.

We report on the low-frequency optical excitations in the multiferroic ground state of polycrystalline ${\mathrm{FeCr}}_2{\mathrm{S}}_4$ in the frequency range 0.3–3 THz and their changes upon applying external magnetic fields up to 7 T. In the ground state below the orbital-ordering temperature $T_{\mathrm{OO}}=9$ K we observe the appearance of several new modes. By applying the external magnetic field parallel and perpendicular to the propagation direction of the THz radiation, we can identify the strongest absorptions to be of predominantly electric-dipole origin. We discuss these modes as the low-energy electronic excitations of the ${\mathrm{Fe}}^{2+}$ ions ($3d^6, S=2$) in an tetrahedral ${\mathrm{S}}^{2-}$ environment. The eigenfrequencies and relative intensities of these absorption lines are satisfactorily reproduced by our calculation assuming an effective exchange field of $12.8{\mathrm{cm}}^{-1}$ at the ${\mathrm{Fe}}^{2+}$-ions sites. The direction of the exchange field is found to be slightly tilted out of the $ab$ plane. With our approach we can also describe previously reported results from Mössbauer studies and the order of magnitude of the electric polarization induced by orbital and noncollinear spin ordering.

In the last decade, Fe₂Mo₃O₈ was recognized for a giant magnetoelectric effect, the origin of which is still not clear. In the present paper, we contribute to the microscopic theory of the magnetoelectric coupling in this compound. Using crystal field theory and the molecular field approximation, we calculated the low-lying energy spectrum for iron ions and their interaction with electric and magnetic fields. Classical ionic contribution to the electric polarization related to the ionic shifts is also estimated. It is found that the electronic and ionic contributions to the electric polarization are comparable and these mechanisms support each other at $T<T_N$. The suggested electronic mechanism provides insight into the nature of huge jumps in polarization upon phase transitions from paramagnetic (PM) to antiferromagnetic (AFM) and then to ferrimagnetic (FRM) states under an applied external magnetic field as well as the large differential magnetoelectric coefficient.

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.

The lack of both time-reversal and spatial inversion symmetry in polar magnets is a prerequisite for the occurrence of optical magnetoelectric effects such as nonreciprocal directional dichroism, i.e., a difference in refractive index and absorption for two counter-propagating electromagnetic waves, which has the potential for the realization of optical diodes. In particular, antiferromagnetic materials with magnetic excitations in the THz range such as ${\mathrm{Fe}}_2{\mathrm{Mo}}_3{\mathrm{O}}_8$ are promising candidates for next-generation spintronic applications. In a combined experimental and theoretical effort we investigated the THz excitations of the polar honeycomb antiferromagnet ${\mathrm{Fe}}_2{\mathrm{Mo}}_3{\mathrm{O}}_8$ in external magnetic fields and their nonreciprocal directional dichroism, together with the temperature dependence of the electronic transitions in the mid- and near-infrared frequency range. Using an advanced single-ion approach for the Fe ions, we are able to describe optical excitations from the THz to the near-infrared frequency range quantitatively and model the observed nonreciprocal directional dichroism in the THz regime successfully.

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.