Roman M. Dubrovin

Two-magnon modes are terahertz-frequency magnetic excitations in antiferromagnets, governed by exchange interactions, involving magnons from the entire Brillouin zone and dominated by zone-edge magnons. The ability to couple to light promotes two-magnon modes as contenders for ultrafast optical manipulation of the magnetic state, beyond conventional zone-center magnonics. While magnon–magnon interactions are known to critically shape the two-magnon line in spontaneous Raman scattering spectra, their role in coherent time-domain excitations remains unexplored. We report a detailed experimental and theoretical study of the influence of magnon–magnon interactions on coherent two-magnon modes in a cubic antiferromagnet excited via impulsive stimulated Raman scattering. We reveal the nontrivial evolution of coherent magnetic dynamics in the time domain and the corresponding spectrum and compare it with the spontaneous Raman scattering spectrum. By extending the spin-correlation based theory for two-magnon modes, we derive a unified description of their spectra in Raman scattering and impulsive stimulated Raman scattering and highlight the role of magnon–magnon interactions.

Harnessing nonlinear interactions between phonon modes in condensed matter presents a powerful avenue for tailoring material properties for innovative applications in future information devices. Here we report a study of nonlinear coupling between infrared (IR) active and Raman-active phonon modes in rare-earth orthoferrites ErFe O 3 , TmFe O 3 , and TbFe O 3 . To reveal the nonlinear phonon interactions, we resonantly excite IR-active phonons with an ultrashort midinfrared pulse, with its central wavelength tunable in the 3–18 µm (16.5–100 THz) range, and trace the phonon modes in the time domain after the excitation. We identify two main coupling mechanisms: quadratic linear and trilinear, which stand for a nonlinear coupling of the Raman-active phonon with either one or two distinct IR-active modes, respectively. The origin of driving each Raman-active phonon mode is identified and analyzed with polarization-dependent pump-probe spectroscopy. Our results show a strong dependence of the trilinear coupling mechanism on the crystallographic orientation of the sample, thus revealing that only specific phonon modes can exhibit this coupling.

The idea to find a magnet that responds to an electric field as efficiently as to its magnetic counterpart has long intrigued people's minds and recently became a cornerstone for future energy efficient and nano-scalable technologies for magnetic writing and information processing. In contrast to electric currents, a control by electric fields promises much lower dissipations and in contrast to magnetic fields, electric fields are easier to apply to a nanoscale bit. Recently, the idea to find materials and mechanisms facilitating a strong and simultaneously fast response of spins to electric field has fueled an intense research interest to electromagnons in non-collinear antiferromagnets. Here we show that THz spin resonance at the frequency 0.165 THz in collinear antiferromagnet Cr$_{2}$O$_{3}$, which does not host any electromagnons, can be excited by both THz magnetic and electric fields. The mechanisms result in comparable effects on spin dynamics, when excited by freely propagating electromagnetic wave, but have different dependencies on the orientation of the applied THz electric field and the antiferromagnetic Néel vector. Hence this discovery opens up new chapters in the research areas targeting to reveal novel principles for the fastest and energy efficient information processing - ultrafast magnetism, antiferromagnetic spintronics, and THz magnonics.

Rare-earth orthoferrites are a promising platform for antiferromagnetic spintronics with a rich variety of terahertz spin and lattice dynamics phenomena. For instance, it has been experimentally demonstrated that the light-driven optical phonons can coherently manipulate macroscopic magnetic states via nonlinear magnetophononic effects. Here using TbFeO$_{3}$ as an example, we reveal the origin of the mode mixing between the LO and TO phonons, which is important for understanding of nonlinear phononics. We performed a comprehensive study of the lattice dynamics of the TbFeO$_{3}$ single crystal by polarized infrared and Raman scattering spectroscopic techniques, and experimentally obtained and carefully analyzed the spectra of anisotropic complex dielectric functions in the far-infrared spectral range. This allowed us to reliably identify the symmetries and parameters of most infrared- and Raman-active phonons. Next, the experimental studies were supplemented by the lattice dynamics calculations which allowed us to propose the normal mode assignments. We reveal that the relation between LO and TO polar phonons is complex and does not strictly follow the ``LO-TO rule'' due to the strong mode mixing. We further analyze how displacements of different ions contribute to phonon modes and reveal that magnetic Fe ions are not involved in Raman-active phonons, thus shedding light on a lack of spin phonon coupling for such phonons. The obtained results establish a solid basis for further in-depth experimental research in the field of nonlinear phononics and magnetophononics in rare-earth orthoferrites.