The practical difficulty in distinguishing the impact of magnetic circular dichroism and the inverse Faraday effect fuels intense debates over which mechanism predominantly drives the process of helicity dependent all-optical switching of magnetization in ferromagnets. Here, we quantitatively measure the efficiency of the switching process in a Pt/Co/Pt multilayered stack using visible- to near-infrared optical pulses. We find that the switching efficiency increases by a factor of 8.6 upon increasing the pumping wavelength from 0.5 $ \mu $m to 1.1 $ \mu $m, becoming 100 % efficient at even longer wavelengths up to 2.0 $ \mu $m. Our experimental results can be successfully explained by the phenomenon of magnetic circular dichroism, making a significant step towards resolving the long-standing controversy over the origin of the all-optical process of magnetization reversal in ferromagnets.

Recent experimental findings have revealed exceptionally diverse laser-induced spin dynamics in ferrimagnetic Gd/FeCo multilayers, tunable by magnetic field (H) and temperature (T); however, a comprehensive theoretical understanding of these phenomena remains lacking. Here, we introduce a two-step workflow that first establishes the complete static H-T phase diagram of the system, thereby identifying all distinct magnetic phases. With this solid foundation, we then employ the Landau-Lifshitz-Bloch equation to simulate the full spectrum of spin dynamics, capturing both the transverse and longitudinal responses. This integrated approach not only reproduces helicity-independent all-optical switching at low laser fluences but also reveals the critical role of the exchange relaxation mechanism at higher fluences. Our theoretical predictions show excellent agreement with experimental observations, underscoring the effectiveness of our method in unifying the static and dynamic magnetic behavior of ferrimagnetic systems.

Ultrafast heating of FeRh by a femtosecond laser pulse launches a magneto-structural phase transition from an antiferromagnetic to a ferromagnetic state. Aiming to reveal the ultrafast kinetics of this transition, we studied magnetization dynamics with the help of the magneto-optical Kerr effect in a broad range of temperatures (from 4 K to 400 K) and magnetic fields (up to 25 T). Three different types of ultrafast magnetization dynamics were observed and, using a numerically calculated H-T phase diagram, the differences were explained by different initial states of FeRh corresponding to a (i) collinear antiferromagnetic, (ii) canted antiferromagnetic and (iii) ferromagnetic alignment of spins. We argue that ultrafast heating of FeRh in the canted antiferromagnetic phase launches practically the fastest possible emergence of magnetization in this material. The magnetization emerges on a time scale of 2 ps, which corresponds to the earlier reported time-scale of the structural changes during the phase transition.

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.

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.

Abstract Understanding spin-lattice interactions in antiferromagnets is a critical element of the fields of antiferromagnetic spintronics and magnonics. Recently, coherent nonlinear phonon dynamics mediated by a magnon state were discovered in an antiferromagnet. Here, we suggest that a strongly coupled two-magnon-one phonon state in this prototypical system opens a novel pathway to coherently control magnon-phonon dynamics. Utilizing intense narrow-band terahertz (THz) pulses and tunable magnetic fields up to μ 0 H ext = 7 T, we experimentally realize the conditions of magnon-phonon Fermi resonance in antiferromagnetic CoF2. These conditions imply that both the spin and the lattice anharmonicities harvest energy from the transfer between the subsystems if the magnon eigenfrequency f m is half the frequency of the phonon 2f m = f ph. Performing THz pump-infrared probe spectroscopy in conjunction with simulations, we explore the coupled magnon-phonon dynamics in the vicinity of the Fermi-resonance and reveal the corresponding fingerprints of nonlinear interaction facilitating energy exchange between these subsystems.

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 magnonics1–5. In this way, they can be identified as closely related to the recently proposed new class of magnetic materials called altermagnets6–9. 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 antiferromagnets11,12). 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.

The interaction of light with spins in a Heisenberg antiferromagnet with a negligibly low magnetic anisotropy as in RbMnF3 has been theoretically analyzed. In particular, an optical pump–probe experiment, where coherent terahertz magnons are excited by short pump laser pulses in the antiferromagnet and are then detected by a probe pulse delayed with respect to the pump pulse, has been simulated. It has been shown that the giant increase in the intensity of excitation of terahertz magnons under the influence of the inverse Cotton–Mouton effect occurs near parametric resonance, i.e., in the parametric instability region.

Finding methods for the most efficient and fastest detection and control of magnetic domains in antiferromagnets is presently among the main challenges of magnetic research at large. We analyse the problem of optical read-out and control of the antiferromagnetic Néel vector using symmetry analysis and the principles of equilibrium thermodynamics. Following the pioneering approach of Dzyaloshinksii, we divide all antiferromagnets in three classes. It is shown that, using the magneto-optical Faraday effect or other effects which scale linearly with the antiferromagnetic Néel vector, it is possible to distinguish antiferromagnetic domains with mutually opposite Néel vectors in two of the three classes. Symmetry properties of one of these two classes are similar to those of altermagnets. The analysis also reveals multiple mechanisms to directly excite spins with light for practically every type of antiferromagnet.

Ultrafast excitation of matter can violate Curie's principle that the symmetry of the cause must be found in the symmetry of the effect. For instance, heating alone cannot result in a deterministic reversal of magnetization. However, if the heating is ultrafast, it facilitates toggle switching of magnetization between stable bit-states without any magnetic field. Here we show that the regime of ultrafast toggle switching can be also realized via a mechanism without relying on heat. Ultrafast laser excitation of iron-garnet with linearly polarized light modifies magnetic anisotropy and thus causes toggling magnetization between two stable bit states. This new regime of 'cold' toggle switching can be observed in ferrimagnets without a compensation point and over an exceptionally broad temperature range. The control of magnetic anisotropy required for the toggle switching exhibits reduced dissipation compared to laser-induced-heating mechanism, however the dissipation and the switching-time are shown to be competing parameters.

The Barnett effect, discovered more than a century ago, describes how an inertial body with otherwise zero net magnetic moment acquires spontaneous magnetization when mechanically spinning. Breakthrough experiments have recently shown that an ultrashort laser pulse destroys the magnetization of an ordered ferromagnet within hundreds of femtoseconds, with the spins losing angular momentum to circularly-polarized optical phonons as part of the ultrafast Einstein-de Haas effect. However, the prospect of using such high-frequency vibrations of the lattice to reciprocally switch magnetization in a nearby magnetic medium has not yet been experimentally explored. Here we show that the spontaneous magnetization temporarily gained via the ultrafast Barnett effect, through the resonant excitation of circularly-polarized optical phonons in paramagnetic substrates, can be used to permanently reverse the magnetic state of the substrate-mounted heterostructure. With the handedness of the phonons steering the direction of magnetic switching, the ultrafast Barnett effect offers a selective and potentially universal method for exercising ultrafast non-local control over magnetic order.

It is found that subtle changes in the external magnetic field and temperature result in dramatic changes in the ultrafast response of spins to a femtosecond laser excitation in a ferrimagnetic Gd/FeCo multilayer. A total of six distinct types of spin dynamics were observed and explained by considering the spin-flop transition to the noncollinear phase and the concept of a tricritical point in the $H$-$T$ phase diagram. A particularly interesting type of dynamics is the exchange-driven reversal. These exchange-driven dynamics provide new insights into the tricritical point, which is shown to separate two thermodynamically distinct noncollinear phases with the transition-metal magnetization pointing on adjacent sides of the anisotropy plane.

Correlated materials display macroscopic properties arising from short-range electronic interactions. Dynamics of the lattice, electrons and spins faster than the time required to reach thermodynamical equilibrium can be photo-induced, manipulated and detected. Despite the remarkable progress in understanding the equilibrium physics of correlated materials, this approach has succeeded in disclosing a plethora of novel phenomena. After the illumination, the relaxation toward equilibrium via non-equilibrium states takes place on the femtosecond and picosecond timescales. During this process, correlated materials display macroscopic properties not-observable in the thermodynamic phase diagram. This research field discloses even the potential to implement the photo-induced manipulations of the macroscopic electronic and magnetic properties of correlated materials in applications, such as information technology.

Exploiting the ability of optical second harmonic generation (SHG) to visualize antiferromagnetic domains in Cr2O3, here we explore the potential of SHG to probe the THz-induced ultrafast spin dynamics in this antiferromagnet. Our experiments clearly show that the observed THz-induced SHG transients do not reflect the dynamics of the antiferromagnetic N & eacute;el vector. Moreover, depending on the helicity of light at the fundamental frequency, the transients may have different form, corresponding either to linear or quadratic dependence of the SHG intensity on the THz electric field.

Silicon nanoparticles with Mie resonances are among the most prospective building blocks for state-of-the-art all-dielectric metasurfaces. Here we focus on linear and nonlinear optical responses of silicon nanoparticles printed by laser-induced transfer from silicon-on-insulator wafer. Second-harmonic generation and broadband multiphoton-absorption-induced luminescence are studied as a function of pump wavelength. The key role of magnetic quadrupole Mie resonances in the nonlinear optical response from silicon nanoparticles is revealed. We also show the influence of Si nanoparticle shape and structure modification, realized by additional femtosecond laser irradiation, on their linear and nonlinear optical properties.

We investigate the ultrafast demagnetization in Co2MnSi1-xAlx quaternary Heusler compounds induced by terahertz (THz) and infrared (IR) pulses. Adjusting the alloy's composition allows us to tailor the spin polarization at the Fermi energy from 97% & PLUSMN; 3% (Co2MnSi) due to its minority spin gap around 0.6 eV, down to 63% & PLUSMN; 3% (Co2MnAl) without a spin gap. Here we experimentally compare the cases of ultrafast demagnetization, when the material is excited with the help of laser pulses with photon energies below and above the minority spin gap, respectively. More particularly, the pump-photon energies were tuned from 1.02 eV (IR) down to 4.1 meV (THz). We found that the ultrafast demagnetization time decreases upon substitution of Si by Al and thus destroys the minority spin gap. Moreover, a decrease of the pump-photon energies in the near-infrared spectral range results in a slight increase of the demagnetization time. Nevertheless, further decrease of the photon energy by a factor of 250 hardly changes the characteristic time of ultrafast demagnetization. Both THz and IR pulses cause very similar ultrafast magnetization dynamics in our Co2MnSi1-xAlx Heusler compounds.

Current-inducing switching of magnetization is crucial for future magnetic data processing technologies, but switching it with speed and energy efficiency remains challenging. Using femtosecond optical pulses, instead of conventional charge currents, is found to make spintronics not only ultrafast but also counterintuitive.