Nikolai E. Khokhlov

Antiferromagnets are emerging as a transformative platform for next-generation spintronics and information storage technologies, promising improved speed and energy efficiency. However, unlike in ferromagnets---where magnetic domain stability is governed by long-range magnetostatic fields---the physical principles defining the minimum writable domain size in antiferromagnets remain poorly understood. Here we identify the Larkin length---the scale at which disorder-induced pinning of domain walls and elastic restoring forces balance---as a key parameter governing the ultimate resolution for stable antiferromagnetic domain writing. We observe that the Larkin length decreases sharply near the Neel transition, enabling deterministic electrical writing of micrometer-scale domains in the archetypal antiferromagnet Cr ₂ O ₃ . Exploiting this mechanism, we demonstrate Heat-Assisted Antiferromagnetic Recording using picosecond laser heating, as a direct analog of Heat-Assisted Magnetic Recording on ferromagnetic media. These findings establish a framework for understanding domain stability in antiferromagnets and provide practical pathways for deterministic electrical and optical domain writing towards extreme-density ferroic devices.

Ultrafast pump-probe imaging reveals that the efficiency of optical excitation of coherent spin waves in epitaxial iron garnet films can be effectively controlled by an external electric field at room temperature. Although a femtosecond laser pulse alone does not excite any pronounced coherent spin oscillations, an electrical gating with the field of 0.5 MV/m dramatically changes the outcome in a laser-induced launching of spin waves. The effect, demonstrated under room-temperature conditions, is estimated to be orders of magnitude larger than in magnetic van der Waals semiconductors observed at 10 K. This electrical gating of laser-induced spin dynamics enriches optomagnonics with a new tool and thus opens up a new avenue in fundamental and applied magnonics research.

The optical pump-probe technique is a common tool for the investigation of ultrafast spin dynamics, which usually utilizes single-diode detection averaging the dynamics over the pumped area. Using an ultrafast imaging technique, we show experimentally that a femtosecond laser pulse with a gradient distribution of intensity efficiently excites strongly inhomogeneous spin dynamics on spatial scales much smaller than the pump spot size. The mechanism responsible for the inhomogeneous distribution is based on temperature gradients and corresponds to a sign change of the torque derivative in different areas of the pump. We argue that the observed phenomenon is general for the systems with competitive magnetic anisotropies. Overlooking this effect in the majority of pump-probe experiments can result in a dramatic underestimate of the lifetime and amplitude of the laser-induced spin dynamics.

We present a concept for selective excitation of magnetostatic surface waves with a quasi-discrete spectrum using spatially patterned femtosecond laser pulses inducing either an ultrafast change of magnetic anisotropy or an inverse Faraday effect. We micromagnetically simulate the excitation of the waves with a periodically patterned uni- or bipolar laser impact. Such excitation yields multiple wavepackets propagating with different group velocities, whose dispersion corresponds to the set of quasi-discrete points. In addition, we show that the frequency of the spectral peaks can be controlled by the polarity of the periodic impact and its spatial period. The presented consideration of multiple spatially periodic magnetostatic surface wave sources as a whole enables implementation of a comprehensive toolkit of spatiotemporal optical methods for tunable excitation and control of spin-wave parameters.

A pair of circularly polarized laser pulses of opposite helicities are shown to control the route of spin reorientation phase transition in the rare-earth antiferromagnetic orthoferrite (Sm0.55Tb0.45)FeO3. The route can be efficiently controlled by the delay between the pulses and the sample temperature. Simulations employing previously published models of laser-induced spin dynamics in orthoferrites failed to reproduce the experimental results. We suggest that the failure is due to neglected temperature dependence of the antiferromagnetic resonance damping in the material. Taking into account the experimentally deduced temperature dependence of the damping, we obtained good agreement between the simulations and the experiment.

We analytically discuss and micromagnetically prove the ways to tune the spatial asymmetry of the initial phase, amplitude, and wavevectors of magnetostatic waves driven by ultrafast laser excitation. We consider that the optical pulse heats a thin ferromagnetic metallic film and abruptly decreases the saturation magnetization and the parameter of uniaxial anisotropy. The two corresponding terms of laser-induced torque have different azimuthal symmetries, with the 4-fold symmetry of the demagnetization-related term, and the isotropic distribution of the anisotropy-related term. As a result, the initial phase and amplitude of excited magnetostatic waves have a non-trivial azimuthal distribution tunable with the angle between the external magnetic field and anisotropy axis, and the laser spot diameter. Moreover, the variation of these parameters tunes the distribution of wavevectors, resulting in additional asymmetry between the spectral components of the waves propagating in different directions.

Nonreciprocity, i.e. inequivalence in amplitudes and frequencies of spin waves propagating in opposite directions, is a key property underlying functionality in prospective magnonic devices. Here we demonstrate experimentally and theoretically a simple approach to induce frequency nonreciprocity in a magnetostatically coupled ferromagnetic bilayer structure with a nonmagnetic spacer by its geometrical asymmetry. Using Brillouin light scattering, we show the formation of two collective spin wave modes in Fe₈₁Ga₁₉/Cu/Fe₈₁Ga₁₉ structure with different thicknesses of ferromagnetic layers. Experimental reconstruction and theoretical modeling of the dispersions of acoustic and optical collective spin wave modes reveal that both possess nonreciprocity reaching several percent at the wavenumber of 22 × 10⁴ rad cm⁻¹. The analysis demonstrates that the shift of the amplitudes of counter-propagating coupled modes towards either of the layers is responsible for the nonreciprocity because of the pronounced dependence of spin wave frequency on the layers’ thickness. The proposed approach enables the design of multilayered ferromagnetic structures with a given spin wave dispersion for magnonic logic gates.

Low-energy magnonic logic circuits are an actively developing field of modern magnetism. The potential benefits of magnonics for data processing are vitally dependent on units based on nonreciprocal propagation of spin waves, in analogy to semiconductor diodes and transistors in electronics. In this paper, we suggest the approach to realize nonreciprocal propagation of spin waves in a ferromagnetic metallic waveguide by exciting them with femtosecond laser pulse. Using micromagnetic modeling, we show that the combination of an external magnetic field and the position of the excitation laser spot across the waveguide leads to unidirectional propagation of the excited spin-wave packet. The results are meaningful for the design of hybrid magnonic-photonic circuits in future generations of data-processing devices.