Ultrafast Optically Induced Ferromagnetic State in an Elemental Antiferromagnet

Ultrafast spin transfer and its impact on the electronic structure

Optically induced intersite spin transfer (OISTR) promises manipulation of spin systems within the ultimate time limit of laser excitation. Following its prediction, signatures of ultrafast spin transfer between oppositely aligned spin sublattices have been observed in magnetic alloys and multilayers. However, it is known neither from theory nor from experiment whether the band structure immediately follows the ultrafast change in spin polarization or whether the exchange split bands remain rigid. We show that ultrafast spin transfer occurs even in ferromagnetic gadolinium metal. Charge transfer between localized surface and extended valence-band states leads to a decrease of the surface spin polarization. This synchronously alters the exchange splitting of the bulk valence bands during laser excitation. Moreover, the onset of demagnetization can be tuned by over 200 fs by changing the temperature-dependent spin mixing. Our results show a promising route to ultrafast control of the magnetization, widening the impact and applicability of OISTR.

Unconventional magnetism mediated by spin-phonon-photon coupling

Magnetic order typically emerges due to the short-range exchange interaction between the constituent electronic spins. Recent discoveries have found a crucial role for spin-phonon coupling in various phenomena from optical ultrafast magnetization switching to dynamical control of the magnetic state. Here, we demonstrate theoretically the emergence of a biquadratic long-range interaction between spins mediated by their coupling to phonons hybridized with vacuum photons into polaritons. The resulting ordered state enabled by the exchange of virtual polaritons between spins is reminiscent of superconductivity mediated by the exchange of virtual phonons. The biquadratic nature of the spin-spin interaction promotes ordering without favoring ferro- or antiferromagnetism. It further makes the phase transition to magnetic order a first-order transition, unlike in conventional magnets. Consequently, a large magnetization develops abruptly on lowering the temperature which could enable magnetic memories admitting ultralow-power thermally-assisted writing while maintaining a high data stability. The role of photons in the phenomenon further enables an in-situ static control over the magnetism. These unique features make our predicted spin-spin interaction and magnetism highly unconventional paving the way for novel scientific and technological opportunities.

Ultrafast Chiral Precession of Spin and Orbital Angular Momentum Induced by Circularly Polarized Laser Pulse in Elementary Ferromagnets

Despite spin (SAM) and orbital (OAM) angular momentum dynamics being well-studied in demagnetization processes, their components receive less focus. Here, we utilize real-time time-dependent density functional theory (rt-TDDFT) to unveil significant x and y components of SAM and OAM induced by circularly left (σ+) and right (σ-) polarized laser pulses in ferromagnetic Fe, Co, and Ni. Our results show that the magnitude of the OAM is an order of magnitude larger than that of the SAM, highlighting a stronger optical response from the orbital degrees of freedom of electrons. Intriguingly, σ+ and σ- pulses induce chirality in the precession of SAM and OAM, respectively, with clear associations with laser frequency and duration. Finally, we demonstrate the time scale of the OAM and SAM precession occurs even earlier than that of the demagnetization process and the OISTR effect. Our results provide detailed insight into the dynamics of SAM and OAM during and shortly after a polarized laser pulse.

Light-induced ultrafast spin transport in multilayer metallic films originates from sp-d spin exchange coupling

Ultrafast interaction between the femtosecond laser pulse and the magnetic metal provides an efficient way to manipulate the magnetic states of matter. Numerous experimental advancements have been made on multilayer metallic films in the last two decades. However, the underlying physics remains unclear. Here, relying on an efficient ab initio spin dynamics simulation algorithm, we revealed the physics that can unify the progress in different experiments. We found that light-induced ultrafast spin transport in multilayer metallic films originates from the sp-d spin-exchange interaction, which can induce an ultrafast, large, and pure spin current from ferromagnetic metal to nonmagnetic metal without charge carrier transport. The resulting trends of spin demagnetization and spin flow are consistent with most experiments. It can explain a variety of ultrafast light-spin manipulation experiments with different systems and different pump-probe technologies, covering a wide range of work in this field.

Optically controlling the competition between spin flips and intersite spin transfer in a Heusler half-metal on sub-100-fs time scales

The direct manipulation of spins via light may provide a path toward ultrafast energy-efficient devices. However, distinguishing the microscopic processes that can occur during ultrafast laser excitation in magnetic alloys is challenging. Here, we study the Heusler compound Co2MnGa, a material that exhibits very strong light-induced spin transfers across the entire M-edge. By combining the element specificity of extreme ultraviolet high-harmonic probes with time-dependent density functional theory, we disentangle the competition between three ultrafast light-induced processes that occur in Co2MnGa: same-site Co-Co spin transfer, intersite Co-Mn spin transfer, and ultrafast spin flips mediated by spin-orbit coupling. By measuring the dynamic magnetic asymmetry across the entire M-edges of the two magnetic sublattices involved, we uncover the relative dominance of these processes at different probe energy regions and times during the laser pulse. Our combined approach enables a comprehensive microscopic interpretation of laser-induced magnetization dynamics on time scales shorter than 100 femtoseconds.

Ultrafast Laser Pulse Induced Transient Ferrimagnetic State and Spin Relaxation Dynamics in Two-Dimensional Antiferromagnets

We employ real-time time-dependent density functional theory (rt-TDDFT) and ab initio nonadiabatic molecular dynamics (NAMD) to systematically investigate the ultrafast laser pulses induced spin transfer and relaxation dynamics of two-dimensional (2D) antiferromagnetic-ferromagnetic (AFM/FM) MnPS3/MnSe2 van der Waals heterostructures. We demonstrate that laser pulses can induce a ferrimagnetic (FiM) state in the AFM MnPS3 layer within tens of femtoseconds and maintain it for subpicosecond time scale before reverting to the AFM state. We identify the mechanism in which the asymmetric optical intersite spin transfer (OISTR) effect occurring within the sublattices of the AFM and FM layers drives the interlayer spin-selective charge transfer, leading to the transition from AFM to FiM state. Furthermore, the unequal electron-phonon coupling of spin-up and spin-down channels of AFM spin sublattice causes an inequivalent spin relaxation, in turn extending the time scale of the FiM state. These findings are essential for designing novel optical-driven ultrafast 2D magnetic switches.

Inverse Design of Light Manipulating Structural Phase Transition in Solids

This Perspective focuses on recent advances in understanding ultrafast processes involved in photoinduced structural phase transitions and proposes a strategy for precise manipulation of such transitions. It has been demonstrated that photoexcited carriers occupying empty antibonding or bonding states generate atomic driving forces that lead to either stretching or shortening of associated bonds, which in turn induce collective and coherent motions of atoms and yield structural transitions. For instance, phase transitions in IrTe₂ and VO₂, and nonthermal melting in Si, can be explained by the occupation of specific local bonding or antibonding states during laser excitation. These cases reveal the electronic-orbital-selective nature of laser-induced structural transitions. Based on this understanding, we propose an inverse design protocol for achieving or preventing a target structural transition by controlling the related electron occupations with orbital-selective photoexcitation. Overall, this Perspective provides a comprehensive overview of recent advancements in dynamical structural control in solid materials.

Ultrafast Laser Control of Antiferromagnetic-Ferrimagnetic Switching in Two-Dimensional Ferromagnetic Semiconductor Heterostructures

Realizing ultrafast control of magnetization switching is of crucial importance for information processing and recording technology. Here, we explore the laser-induced spin electron excitation and relaxation dynamics processes of CrCl₃/CrBr₃ heterostructures with antiparallel (AP) and parallel (P) systems. Although an ultrafast demagnetization of CrCl₃ and CrBr₃ layers occurs in both AP and P systems, the overall magnetic order of the heterostructure remains unchanged due to the laser-induced equivalent interlayer spin electron excitation. More crucially, the interlayer magnetic order switches from antiferromagnetic (AFM) to ferrimagnetic (FiM) in the AP system once the laser pulse disappears. The microscopic mechanism underpinning this magnetization switching is dominated by the asymmetrical interlayer charge transfer combined with a spin-flip, which breaks the interlayer AFM symmetry and ultimately results in an inequivalent shift in the moment between two FM layers. Our study opens up a new idea for ultrafast laser control of magnetization switching in two-dimensional opto-spintronic devices.

All-optical control of spin in a 2D van der Waals magnet

Two-dimensional (2D) van der Waals magnets provide new opportunities for control of magnetism at the nanometre scale via mechanisms such as strain, voltage and the photovoltaic effect. Ultrafast laser pulses promise the fastest and most energy efficient means of manipulating electron spin and can be utilized for information storage. However, little is known about how laser pulses influence the spins in 2D magnets. Here we demonstrate laser-induced magnetic domain formation and all-optical switching in the recently discovered 2D van der Waals ferromagnet CrI₃. While the magnetism of bare CrI₃ layers can be manipulated with single laser pulses through thermal demagnetization processes, all-optical switching is achieved in nanostructures that combine ultrathin CrI₃ with a monolayer of WSe₂. The out-of-plane magnetization is switched with multiple femtosecond pulses of either circular or linear polarization, while single pulses result in less reproducible and partial switching. Our results imply that spin-dependent interfacial charge transfer between the WSe₂ and CrI₃ is the underpinning mechanism for the switching, paving the way towards ultrafast optical control of 2D van der Waals magnets for future photomagnetic recording and device technology.

The use of light in driving the magnetization of materials has great technological potential, as well as allowing for insights into the fast dynamics of magnetic systems. Here, the authors combine CrI₃, a van der Waals magnet, with WSe₂, and demonstrate all optical switching of the resulting heterostructure.

Light-Controlled Ultrafast Magnetic State Transition in Antiferromagnetic-Ferromagnetic van der Waals Heterostructures

Manipulating spin in antiferromagnetic (AFM) materials has great potential in AFM opto-spintronics. Laser pulses can induce a transient ferromagnetic (FM) state in AFM metallic systems but have never been proven in two-dimensional (2D) AFM semiconductors and related van der Waals (vdW) heterostructures. Herein, using 2D vdW heterostructures of FM MnS₂ and AFM MXenes as prototypes, we investigated optically induced interlayer spin transfer dynamics based on real-time time-dependent density functional theory. We observed that laser pulses induce significant spin injection and interfacial atom-mediated spin transfer from MnS₂ to Cr₂CCl₂. In particular, we first demonstrated the transient FM state in semiconducting AFM-FM heterostructures during photoexcited processes. The proximity magnetism breaks the magnetic symmetry of Cr₂CCl₂ in heterostructures. Our results provide a microscopic understanding of optically controlled interlayer spin dynamics in 2D magnetic heterostructures and open a new way to manipulate magnetic order in 2D materials for ultrafast opto-spintronics.

Two-dimensional magnetic atomic crystals

Two-dimensional (2D) magnetic crystals show many fascinating physical properties and have potential device applications in many fields. In this paper, the preparation, physical properties and device applications of 2D magnetic atomic crystals are reviewed. First, three preparation methods are presented, including chemical vapor deposition (CVD) molecular beam epitaxy (MBE) and single-crystal exfoliation. Second, physical properties of 2D magnetic atomic crystals, including ferromagnetism, antiferromagnetism, magnetic regulation and anomalous Hall effect are presented. Third, the application of 2D magnetic atomic crystals in heterojunctions reluctance and other aspects are briefly introduced. Finally, the future development direction and possible challenges of 2D magnetic atomic crystals are briefly addressed.

Tuning magnetism at the two-dimensional limit: a theoretical perspective

The discovery of two-dimensional (2D) magnetic materials provides an ideal testbed for manipulating the magnetic properties at the atomically thin and 2D limit. This review gives recent progress in the emergent 2D magnets and heterostructures, focusing on the theory side. We summarize different theoretical models, ranging from the atomic to micrometer-scale, used to describe magnetic orders. Then, the current strategies for tuning magnetism in 2D materials are further discussed, such as electric field, magnetic field, strain, optics, chemical functionalization, and spin-orbit engineering. Finally, we conclude with the future challenges and opportunities for 2D magnetism.

Resonant Soft X-ray Reflectivity in the Study of Magnetic Properties of Low-Dimensional Systems

In this review, the technique of resonant soft X-ray reflectivity in the study of magnetic low-dimensional systems is discussed. This technique is particularly appealing in the study of magnetization at buried interfaces and to discriminate single elemental contributions to magnetism, even when this is ascribed to few atoms. The major fields of application are described, including magnetic proximity effects, thin films of transition metals and related oxides, and exchange-bias systems. The fundamental theoretical background leading to dichroism effects in reflectivity is also briefly outlined.