Nondestructive ultrafast steering of a magnetic vortex by terahertz pulses

Magnetic vortices, formed in confined soft ferromagnetic structures, are characterized by in-plane curling magnetization (chirality) and an out-of-plane core magnetization (polarity). Their thermal stability, negligible magnetostatic interactions, and diverse information storage potential make them attractive for high-density, nonvolatile memories and spintronic devices. However, reversing vortex polarity and/or chirality is challenging due to rotational symmetry and strong exchange interactions. Previous attempts using magnetic fields, spin-polarized currents, spin waves, or laser pulses have faced limitations in reproducibility and precise control of chirality switching. This research utilizes the magnetoelectric (ME) effect, facilitated by the inverse Dzyaloshinskii-Moriya (DM) mechanism, to achieve swift and controlled manipulation of magnetic vortex configurations by electric means. The ME effect allows an external THz electric field to couple with and drive the vortex via E-P, where P is the spin-driven emergent ferroelectric polarization. The ME coupling strength is determined by the spin-orbit interaction and is linearly related to the applied electric field. The study aims to demonstrate all-electric magnetization switching through ME coupling, offering a novel approach to overcome the limitations of existing methods.

Numerous methods have been explored to switch vortex core polarity, including alternating magnetic fields, in-plane magnetic field pulses, resonant microwave pulses, field-driven spin waves, spin-polarized currents, and photothermal-assisted femtosecond laser pulse excitation. However, achieving reliable and reproducible chirality switching has proven more difficult, often resulting in random emergence of clockwise and counterclockwise states. Structures with broken symmetry have been introduced to address chirality switching with magnetic fields. The simultaneous control of both vortex chirality and polarity remains a significant challenge, hindering the development of reliable magnetic vortex-based spintronics. This paper addresses this gap by proposing and demonstrating a novel approach based on magnetoelectric interactions.

The magnetization dynamics of a vortex were investigated using finite-difference micromagnetic simulations with GPU acceleration, based on the Landau-Lifshitz-Gilbert (LLG) equation. The simulations incorporated the isotropic Heisenberg exchange field, uniaxial magnetocrystalline anisotropy field, magnetostatic demagnetizing field, and the ME field. Material parameters were chosen for permalloy (Py) at room temperature: exchange constant A = 13 pJ/m, saturation magnetization Ms = 8 × 10⁵ A/m, and magnetocrystalline anisotropy Ku = 0. The Gilbert damping constant was set to α = 0.05. The ME coupling strength was set to ξm = 1 pC/m, which is realistic for Py films prepared on a Pt layer. An initially stable magnetic vortex with positive polarity and clockwise chirality was prepared in a Py square (width of 99 nm and thickness of 3 nm). The system was discretized by cubic cells with a size of (3 nm)³. The simulations involved applying time-asymmetric electric-field pulses with varying amplitudes and durations to the initial vortex state. A Maxwell solver was used to validate the electromagnetic field distribution in the samples for the THz fields used in the calculations, confirming the adequacy of the simulation approach. The stable magnetization configurations were characterized by the skyrmion number (s) and the chirality number (c).

The study successfully demonstrated ultrafast and highly reproducible switching between four vortex states (I-IV) by sequentially applying two types of time-asymmetric electric-field pulses. The time required for core polarization reversal was on the order of 15 ps, and for chirality reversal, approximately 325 ps. A phase diagram was generated showing the remanent magnetization configurations reached by applying a static Gaussian electric field. In addition to the four desired vortex states, four new packaged-skyrmion-like stable configurations (V-VIII) emerged under large and narrow Gaussian electric fields. Detailed analysis of the magnetization dynamics during vortex-core reversal revealed a packaged-skyrmion-mediated process, differing from the usual gyrotropic core excitation. The change in topological charge (Δs = ±1) occurred through the formation of an intermediate packaged-skyrmion with localized skyrmion number s = ±1. Reproducible chirality reversal was achieved using weaker and longer time-asymmetric pulses, involving the formation of a hedgehog skyrmion. Simultaneous control of both vortex polarity and chirality was also demonstrated by slightly shortening the pulse duration, leading to the emergence of vortex configuration II. The study showed efficient and reliable manipulation of four magnetic vortex states by combining different switching procedures. Finally, simulations on YIG disks with different material parameters confirmed the feasibility of reliable switching under weaker electric field pulses.

The findings demonstrate a novel and efficient method for controlling magnetic vortex states using time-asymmetric terahertz electric-field pulses. The packaged-skyrmion-mediated switching mechanism provides a faster and more reproducible alternative to existing methods. The ability to independently and simultaneously control both vortex chirality and polarity opens up exciting possibilities for high-density, nonvolatile magnetic memory devices. The use of electric fields offers advantages over magnetic fields in terms of energy efficiency, localized control, and scalability. The results provide deeper insights into the fundamentals of magnetoelectric effects and magnetization relaxation dynamics, highlighting the potential of THz sources for advanced spintronics applications. The success in achieving ultrafast and highly reproducible switching of both polarity and chirality significantly advances the prospects for developing reliable magnetic vortex-based spintronic devices.

This study successfully demonstrated a novel method for nondestructive ultrafast steering of a magnetic vortex using a sequence of time-asymmetric terahertz electric-field pulses. This approach offers a highly reproducible and energy-efficient way to control both the polarity and chirality of magnetic vortices, paving the way for advanced nonvolatile magnetic memory and spintronic devices. Future research could explore the optimization of pulse shapes and parameters for even faster and more efficient switching, investigate the scalability of the method to larger arrays of vortices, and explore its application in different materials systems.

The study primarily relied on micromagnetic simulations. Experimental validation is needed to confirm the findings and assess the practical limitations of the proposed method. The influence of imperfections and variations in material properties on the switching behavior warrants further investigation. The specific choice of materials and parameters used in the simulations might limit the generalizability of the results. Exploring the robustness of the method under different experimental conditions is essential before real-world applications can be considered.