Dmytro Afanasiev

Understanding spin dynamics at ever shorter time and smaller length scales is one of the major challenges in fundamental and applied magnetism, leading in particular to the discovery of giant magnetoresistance and All-Optical Switching (AOS) of magnetization and motivating the development of large-scale facilities such as x-ray free-electron lasers. Here, we propose a conceptually distinct approach to explore ultrafast laser-induced spin dynamics at the nanoscale by combining tabletop Magnetic Force Microscopy (MFM) with femtosecond laser excitation. By writing magnetic domains not with a single but with a pair of mutually delayed ultrashort laser pulses, we observe the spin dynamics by analyzing the final static nanotextured domain pattern as a function of pump-to-pump delay. We show that using MFM, we are able to deduce not only the average reversed magnetization but also the switched areas of nanoscale domains with picosecond temporal resolution. The capabilities of the technique are further demonstrated by applying it to study ultrafast helicity-dependent AOS of magnetization in ferromagnetic Pt/Co/Pt, where magnetization can be reversed with a pair of 100-fs and 3-ps laser pulses. Tracking the laser-induced area of the nanotextured domains as a function of the time separation between the pump pulses reveals critical slowing down of the spin dynamics near the Curie temperature of Co, thereby increasing the efficiency of dual-pulse AOS.

Controlling magnetic textures at ever smaller length scales and timescales is of fundamental and technological interest. External stimuli capable of acting at the nanoscale pose a challenge, motivating alternative approaches that exploit the intrinsic inhomogeneity of magnetic textures. Here we use a Pt/Co/Pt ferromagnetic thin film to investigate magnetization reversal with circularly polarized picosecond laser pulses. Magnetic force microscopy reveals stochastic nucleation of complex nanotextured domains from an initial monodomain state. Subsequent illumination of these domains with laser pulses induces deterministic and homogeneous magnetization switching. We find that the domain growth depends on the complexity of the texture, revealing a helicity- and texture-dependent mechanism that contrasts with temperature-gradient-driven domain expansion. We complement our observations with a stochastic model in which domain nucleation is governed by light helicity and the local magnetic environment. These results provide an insight into the mechanism of multipulse helicity-dependent all-optical switching.

Femtosecond optical pulses have opened a new frontier in ultrafast dynamics, enabling direct access to fundamental interactions in quantum materials. In antiferromagnets (AFMs), where the fundamental quantum mechanical exchange interaction governs spin dynamics, this access is especially compelling, enabling the excitation of magnons - collective spin-wave modes - that naturally reach terahertz (THz) frequencies and supersonic velocities. Femtosecond optical pulses provided a route to coherently excite such magnons across the entire Brillouin zone. Controlling their spectral properties - such as the magnon gap and dispersion - represents the next monumental step, enabling dynamic tuning of group velocities, coherence, and interaction pathways. Yet, achieving this remains a challenge, requiring ultrafast and long-lasting manipulation of the underlying exchange interaction. Here, we show that in DyFeO3 - an insulating AFM with strongly coupled electronic and magnetic degrees of freedom - resonant above-bandgap optical excitation leads to a dramatic renormalization of the THz magnon spectrum, including a near-total collapse of the magnon gap. Our analysis reveals this transformation to be consistent with a transient reduction of the exchange interaction by nearly 90% in the near-surface nanoscale region. These findings establish a pathway for light-driven, nanoscale control of AFM spin dynamics, opening opportunities for reconfigurable, high-speed magnonic and spintronic applications.

Resonant pumping of the electronic f-f transitions in the orbital multiplet of dysprosium ions (Dy^{3+}) in a complex perovskite DyFeO_{3} is shown to impulsively launch THz lattice dynamics corresponding to the B_{2g} phonon mode, which is dominanted by the motion of Dy^{3+} ions. The findings, supported by symmetry analysis and density-functional theory calculations, not only provide a novel route for highly selective excitation of the rare-earth crystal lattices but also establish important relationships between the symmetry of the electronic and lattice excitations in complex oxides.

Traditionally, magnetic solids are divided into two main classes—ferromagnets and antiferromagnets with parallel and antiparallel spin orders, respectively. Although normally the antiferromagnets have zero magnetization, in some of them an additional antisymmetric spin–spin interaction arises owing to a strong spin–orbit coupling and results in canting of the spins, thereby producing net magnetization. The canted antiferromagnets combine antiferromagnetic order with phenomena typical of ferromagnets and hold great potential for spintronics and magnonics^1–5. In this way, they can be identified as closely related to the recently proposed new class of magnetic materials called altermagnets^6–9. Altermagnets are predicted to have strong magneto-optical effects, terahertz-frequency spin dynamics and degeneracy lifting for chiral spin waves¹⁰ (that is, all of the effects present in the canted antiferromagnets^11,12). Here, by utilizing these unique phenomena, we demonstrate a new functionality of canted spin order for magnonics and show that it facilitates mechanisms converting a magnon at the centre of the Brillouin zone into propagating magnons using nonlinear magnon–magnon interactions activated by an ultrafast laser pulse. Our experimental findings supported by theoretical analysis show that the mechanism is enabled by the spin canting.

Controlled generation of coherent spin waves with highest possible frequencies and shortest possible wavelengths is a cornerstone of spintronics and magnonics. Here, using Heisenberg antiferromagnet RbMnF3, we demonstrate that laser-induced THz spin dynamics corresponding to pairs of mutually coherent counter-propagating spin waves with the wavevectors up to the edge of the Brillouin zone cannot be understood in terms of magnetization and antiferromagnetic (Néel) vectors, conventionally used to describe spin waves. Instead, we propose to model such spin dynamics using the spin correlation function. We derive a quantum-mechanical equation of motion for the latter and emphasize that unlike the magnetization and antiferromagnetic vectors the spin correlations in antiferromagnets do not exhibit inertia.