Controlling magnetic states at femtosecond timescales is crucial for future information technology. While all-optical magnetization reversal has been achieved in ferrimagnetic and ferromagnetic alloys and multilayers using ultrafast laser pulses, it remains elusive in elementary ferromagnets. This study aims to address this challenge by investigating the possibility of achieving all-optical magnetization reversal in bulk nickel using a single laser pulse. The successful control of magnetization direction in an elementary ferromagnet with a single pulse would represent a significant advancement in spintronics, leading to more efficient and faster information storage and processing devices. Existing research has demonstrated all-optical switching in materials like GdFeCo, primarily due to laser-induced heating, and in Co/Pt multilayers, where laser helicity plays a key role. However, these methods often require multiple pulses or high energy consumption, hindering practical applications. Therefore, understanding the underlying mechanism and identifying optimal pulse parameters for single-pulse switching in an elementary ferromagnet is crucial for developing next-generation spintronic devices.

Numerous studies have explored the ultrafast manipulation of magnetic materials using laser pulses, starting with the observation of optically driven ultrafast demagnetization in nickel. The three-temperature model, while providing a good semi-phenomenological description of demagnetization in bulk Ni and generalized to ferrimagnetic GdFeCo, doesn't fully capture the complexity of the process. Other models, like those incorporating stochastic magnetization dynamics using Landau-Lifshitz-Gilbert and Landau-Lifshitz-Bloch equations, have also been employed. Microscopic explanations for laser-induced demagnetization include the inverse Faraday effect (IFE), light-induced magnetic torques, and the superdiffusive spin transport model, each contributing to different aspects of the ultrafast dynamics. Time-dependent density functional theory (TDFT) simulations, while providing valuable insights into ultrafast demagnetization, have so far been largely limited to these processes, rather than full magnetization reversal. Previous simulations based on realistic electronic structure descriptions primarily focused on ultrafast demagnetization processes. This study, using a newly developed time-dependent tight-binding framework, advances these efforts by exploring the possibility of all-optical magnetization reversal with a single laser pulse.

The research employs a time-dependent tight-binding framework, parameterized from density functional theory (DFT) calculations, to simulate the magnetization dynamics of bulk nickel interacting with laser pulses. The Hamiltonian includes kinetic energy, spin-orbit coupling (SOC), electron-electron exchange interactions, and the effect of laser-induced excitations. The time-dependent Schrödinger equation is solved to propagate the ground state eigenvectors up to a few picoseconds, allowing for the investigation of the effects of linearly and circularly polarized pulses with varying widths and intensities. The simulations monitor the time-dependent magnetization components (longitudinal and transverse) and the absorbed energy, along with the redistribution of electronic populations among different orbitals. The pulse shape is defined using a vector potential, with parameters adjusted to vary pulse width (τ), intensity (E₀), and polarization (linear or circular). The absorbed laser fluence is approximated by the energy change in the material divided by its cross-section. The simulations focus on circularly polarized pulses in the yz-plane, given the superior results observed in the study. The calculations utilize the TITAN code, which has been shown to be accurate and stable through tests involving variations in k-points, error tolerances, and sensitivity to laser parameter changes. Bulk face-centered cubic Nickel with a theoretical lattice constant of 3.46 Å is utilized in the simulations. Computational details such as k-point grid, temperature, and time propagation step size are explicitly provided. The study’s methodology allows for the investigation of the complex dynamics of the magnetization, including its three-dimensional trajectory and the contribution of individual orbitals to the overall magnetization behavior.

The study successfully demonstrates all-optical magnetization reversal in bulk nickel using a single laser pulse. A phase diagram mapping the laser pulse parameter space reveals a region where switching is highly probable, indicating a critical minimum pulse width (around 60 fs) and a fluence window for switching to occur. The simulations reveal that ultrafast light-induced torques, arising from the spin-orbit interaction, are the primary mechanism for magnetization reversal. These torques are most effective when the laser pulse is circularly polarized in a plane containing the initial magnetization orientation. The magnetization dynamics are found to be three-dimensional, with initial rotation in the yz-plane (containing the initial magnetization and laser polarization), followed by a rotation out of this plane. The study also reveals rich intra-atomic orbital-dependent dynamics. The magnetization vectors carried by sp and d orbitals display a transient non-collinear state, featuring a ferromagnetic coupling followed by an antiferromagnetic coupling. The magnetization switching involves a transfer of spin angular momentum from the d orbitals to the sp orbitals, accompanied by changes in orbital occupation. Three distinct dynamic regimes are identified during the laser pulse interaction, corresponding to different dominant processes such as intra-orbital spin-flips, inter-orbital optical transitions, and the resulting coupling between sp and d orbitals. The key observation is that the complete reversal of the magnetization is achieved through the laser-driven torque with specific laser parameters for the intensity and the pulse widths.

The findings address the research question by demonstrating the feasibility of all-optical magnetization reversal in an elementary ferromagnet using a single laser pulse. The identification of ultrafast light-induced torques as the central mechanism clarifies a key aspect of ultrafast magnetization dynamics. The detailed analysis of orbital-dependent dynamics provides a deeper understanding of the microscopic processes involved. The mapping of the laser pulse parameter space provides practical guidelines for future experiments and device design. This work has significant implications for the field of spintronics by providing a pathway for more energy-efficient and faster all-optical magnetization switching in elementary ferromagnets. This opens the door to implementing all-optical addressed spintronic storage and memory devices. The fact that the switching can occur even while the pulse is still active suggests a potential for even greater efficiency. The identified interplay between different orbital contributions to the magnetization further enriches our understanding of ultrafast spin dynamics and contributes to the development of more efficient, faster and low-power all-optical spintronic devices.

This study successfully demonstrates all-optical magnetization reversal in bulk nickel using a single sub-picosecond laser pulse. The identified mechanism involves ultrafast light-induced torques, which are most effective with circularly polarized light. The detailed mapping of the laser parameter space and the analysis of orbital-dependent dynamics provide valuable insights for future research and technological applications. Future studies could focus on incorporating dissipation mechanisms into the simulations to further refine the model and explore the potential for stabilizing the reversed magnetization state. Investigating other elementary ferromagnets and exploring the use of different laser wavelengths could also provide further insights and expand the scope of all-optical magnetization control.

The current model does not include dissipation mechanisms that would become relevant at longer timescales beyond the few picoseconds simulated. This might affect the long-term stability of the switched magnetization state. The absorbed laser fluence is approximated in the model and may be subject to further refinement. While the study demonstrates the possibility of single-pulse switching, the long-term stability of the magnetization reversal needs further investigation to assess the practicality of the method for technological applications.