Current-induced Dynamics Research Articles

Development of ns × nm Magnetic Imaging Technique for Current-induced Dynamics based on Ultrafast Transmission Electron Microscopy

Development of ns × nm Magnetic Imaging Technique for Current-induced Dynamics based on Ultrafast Transmission Electron Microscopy

Decoupling the influences of chiral damping and Dzyaloshinskii-Moriya interaction in chiral magnetic domain walls

We revisit the nature and impact of both the chiral damping (CD) and Dzyaloshinskii-Moriya interaction (DMI) in uniaxial chiral ferromagnetic nanowires with broken inversion symmetry. We propose that CD, akin to its chiral energy counterpart (DMI), can be described in terms of the Lifshitz invariants permissible by the underlying symmetry of the system. This representation offers a clearer foundation for integrating CD into the dynamics of various chiral magnetic textures. We theoretically investigate the current-induced motion of chiral domain walls (DWs), driven by both spin-transfer torque and the spin Hall effect in the presence of CD. We demonstrate that it is possible to unambiguously separate the influence of CD from that of DMI by analyzing the current-induced dynamics. In particular, below the Walker breakdown (WB), the DMI does not affect the DW velocity, whereas increases in the strength of CD result in a decrease in the DW velocity. Moreover, for the spin-orbit torque driven motion, while the DMI enhances both the WB current density and the maximum attainable velocity below the WB, the CD only enhances the WB without affecting the maximum attainable velocity below the WB. Our findings open up intriguing opportunities for exploitation in exotic magnetic textures. Published by the American Physical Society 2024

Tunability of Microwave Frequency Using Spin Torque Nano Oscillator by the Generated Oersted Field with Tunable Free Layer

The current-induced magnetization precession dynamics provoked by the spin transfer torque (STT) in a spin valve device i.e. tri-layer device (commonly spin torque nano oscillator (STNO)) is investigated numerically by solving the governing Landau–Lifshitz–Gilbert–Slonczewski (LLGS) equation. In this study, we have devised an STNO device made of EuO-based ferromagnetic alloy in free and fixed magnetic layers. The copper acts as a nonmagnetic spacer. Here, we have introduced the current induced Oesterd field (CIOF), which is generated when a spin-polarized current passes through the STNO device. In the device, we have tuned the free layer angle [Formula: see text] from [Formula: see text] to [Formula: see text] as an increment of [Formula: see text]. For every individual [Formula: see text] ranging from [Formula: see text] to [Formula: see text], the generated Oersted field’s strength can be altered by increasing the STNO device’s diameter. Henceforth, it is apparent that the frequency tunability is achieved in the device for all the values of [Formula: see text]. The frequency and power of the device depend entirely on the material’s saturation magnetization, which inherently reflects the current density and coherence of spin-polarized DC. From the results, it is apparent that for a particular [Formula: see text], the frequency keeps increasing with the eventual decrease in power when we increase the strength of the Oersted field from 10[Formula: see text]kA/m to 50[Formula: see text]kA/m. By doing so, the maximum frequency can be tuned up to 212[Formula: see text]GHz for [Formula: see text] with [Formula: see text] as 50[Formula: see text]kA/m. The high frequency emitted by the device acts as a linchpin ingredient, as well as a launch pad element in much of scientific and technological point of view. It paves way for a new route in the areas such as high capacity, high precision, high density as well as the sensing applications.

Tunable and inhomogeneous current-induced THz-oscillation dynamics in the ferrimagnetic spin-chain

Ferrimagnets perform versatile properties, attributed to their antiferromagnetic sublattice coupling and finite net magnetization. Despite extensive research, the inhomogeneous dynamics in ferrimagnets, including domain walls and magnons, remain not fully understood. Therefore, we adopted a multi-spin model by considering the effect of the spin torques and explored the localized phase-dependent and inhomogeneous THz-oscillation dynamics in a ferrimagnetic spin-chain. Our results demonstrate that the exchange oscillation mode, induced by spin transfer torque, exhibits three typical phases, and the oscillation frequency is dominated by a joint effective field derived in the spin-chain. We also found that the localized spin configurations can be used to tune the bandwidth and sensitivity of the frequency response. Furthermore, we propose an anti-parallel exchange length to reveal the inhomogeneity in the ferrimagnetic spin-chain, which could serve as a valuable tool for characterizing the spin dynamics of these systems. Our findings offer understandings beyond uniform spin-dynamics in ferrimagnets.

Variation of magnetic properties with current in ferrimagnetic semiconductor Mn3Si2Te6

Orbital currents play a fundamental role in a wide range of transport phenomena. Recently, the discovery of a novel chiral orbital current state in the ferrimagnetic nodal-line semiconductor Mn3Si2Te6 has attracted significant interest, supported by anomalous I–V characteristics and time-dependent bistable switching. However, the direct experimental verifications, combining electrical transport and magnetic measurement, that detect the variation of the magnetic properties vs the current are still rare. Here, we investigate the transport properties of Mn3Si2Te6 and track the current-induced dynamics of the magnetic moment. Reflective magnetic circular dichroism reveals that significant alterations in Mn3Si2Te6 magnetoresistance in response to an electric field are necessarily coupled with a magnetic phase transition, establishing a rare correlation. Our findings indicate the predominance of magnetic chiral orbital currents in the colossal angular magnetoresistance effect, offering a unique platform for advanced studies in orbital magnetism.

Homochiral antiferromagnetic merons, antimerons and bimerons realized in synthetic antiferromagnets

The ever-growing demand for device miniaturization and energy efficiency in data storage and computing technology has prompted a shift towards antiferromagnetic topological spin textures as information carriers. This shift is primarily owing to their negligible stray fields, leading to higher possible device density and potentially ultrafast dynamics. We realize in this work such chiral in-plane topological antiferromagnetic spin textures namely merons, antimerons, and bimerons in synthetic antiferromagnets by concurrently engineering the effective perpendicular magnetic anisotropy, the interlayer exchange coupling, and the magnetic compensation ratio. We demonstrate multimodal vector imaging of the three-dimensional Néel order parameter, revealing the topology of those spin textures and a globally well-defined chirality, which is a crucial requirement for controlled current-induced dynamics. Our analysis reveals that the interplay between interlayer exchange and interlayer magnetic dipolar interactions plays a key role to significantly reduce the critical strength of the Dzyaloshinskii-Moriya interaction required to stabilize topological spin textures, such as antiferromagnetic merons, in synthetic antiferromagnets, making them a promising platform for next-generation spintronics applications.

Spin-torque-driven antiferromagnetic resonance.

The intrinsic fast dynamics make antiferromagnetic spintronics a promising avenue for faster data processing. Ultrafast antiferromagnetic resonance-generated spin current provides valuable access to antiferromagnetic spin dynamics. However, the inverse effect, spin-torque-driven antiferromagnetic resonance (ST-AFMR), which is attractive for practical utilization of fast devices but seriously impeded by difficulties in controlling and detecting Néel vectors, remains elusive. We observe ST-AFMR in Y3Fe5O12/α-Fe2O3/Pt at room temperature. The Néel vector oscillates and contributes to voltage signal owing to antiferromagnetic negative spin Hall magnetoresistance-induced spin rectification effect, which has the opposite sign to ferromagnets. The Néel vector in antiferromagnetic α-Fe2O3 is strongly coupled to the magnetization in Y3Fe5O12 buffer, resulting in the convenient control of Néel vectors. ST-AFMR experiment is bolstered by micromagnetic simulations, where both the Néel vector and the canted moment of α-Fe2O3 are in elliptic resonance. These findings shed light on the spin current-induced dynamics in antiferromagnets and represent a step toward electrically controlled antiferromagnetic terahertz emitters.

Direct Observation of Magnetic Skyrmions and Their Current-Induced Dynamics in Epitaxial Single-Crystal Oxide Films.

Magnetic skyrmions are scarcely investigated for single-crystal quality films, for which skyrmions may have a remarkable performance. Even in the limited studies in this aspect, the skyrmions are usually probed by the topological Hall effect, missing important information on dynamic properties. Here, we present a comprehensive investigation on the generation/manipulation of magnetic skyrmions in La0.67Ba0.33MnO3 single-crystal films. Using the technique of magnetic force microscopy, the current-driven skyrmion dynamics are directly observed. Unlike isolated skyrmions produced by magnetic field alone, closely packed skyrmions can be generated by electric pulses in a magnetic background, with a high density (∼60/μm2) and a small size (dozens of nanometers). The threshold current moving skyrmions is ∼2.3 × 104 A/cm2, 2-3 orders of magnitude lower than that required by metallic multilayers or van der Waals ferromagnetic heterostructures. Our work demonstrates the great potential of single-crystal oxide films in developing skyrmion-based devices.

On the current-induced domain-wall dynamics

On the current-induced domain-wall dynamics

Observation of long-range orbital transport and giant orbital torque

Modern spintronics relies on the generation of spin currents through spin-orbit coupling. The spin-current generation has been believed to be triggered by current-induced orbital dynamics, which governs the angular momentum transfer from the lattice to the electrons in solids. The fundamental role of the orbital response in the angular momentum dynamics suggests the importance of the orbital counterpart of spin currents: orbital currents. However, evidence for its existence has been elusive. Here, we demonstrate the generation of giant orbital currents and uncover fundamental features of the orbital response. We experimentally and theoretically show that orbital currents propagate over longer distances than spin currents by more than an order of magnitude in a ferromagnet and nonmagnets. Furthermore, we find that the orbital current enables electric manipulation of magnetization with efficiencies significantly higher than the spin counterpart. These findings open the door to orbitronics that exploits orbital transport and spin-orbital coupled dynamics in solid-state devices.

Order of magnitude reduction in Joule heating of single molecular junctions between graphene electrodes.

Maintaining stability of single-molecular junctions (SMJs) in the presence of current flow is a prerequisite for their potential device applications. However, theoretical understanding of nonequilibrium heat transport in current-carrying SMJs is a challenging problem due to the different kinds of nonlinear interactions involved, including electron-vibration and anharmonic vibrational coupling. Here, we overcome this challenge by accelerating Langevin-type current-induced molecular dynamics using machine-learning potential derived from density functional theory. We show that SMJs with graphene electrodes generate an order of magnitude less heating than those with gold electrodes. This is rooted in the better phonon spectral overlap of graphene with molecular vibrations, rendering harmonic phonon heat transport being dominant. In contrast, in a spectrally mismatched junction with gold electrodes, anharmonic coupling becomes important to transport heat away from the molecule to surrounding electrodes. Our work paves the way for studying current-induced heat transport and energy redistribution in realistic SMJs.

Toggle-like current-induced Bloch point dynamics of 3D skyrmion strings in a room temperature nanowire

Research into practical applications of magnetic skyrmions, nanoscale solitons with interesting topological and transport properties, has traditionally focused on two dimensional (2D) thin-film systems. However, the recent observation of novel three dimensional (3D) skyrmion-like structures, such as hopfions, skyrmion strings (SkS), skyrmion bundles, and skyrmion braids, motivates the investigation of new designs, aiming to exploit the third spatial dimension for more compact and higher performance spintronic devices in 3D or curvilinear geometries. A crucial requirement of such device schemes is the control of the 3D magnetic structures via charge or spin currents, which has yet to be experimentally observed. In this work, we utilise real-space imaging to investigate the dynamics of a 3D SkS within a nanowire of Co8Zn9Mn3 at room temperature. Utilising single current pulses, we demonstrate current-induced nucleation of a single SkS, and a toggle-like positional switching of an individual Bloch point at the end of a SkS. The observations highlight the possibility to locally manipulate 3D topological spin textures, opening up a range of design concepts for future 3D spintronic devices.

Dynamic transformation between a skyrmion string and a bimeron string in a layered frustrated system

Frustrated topological spin textures have unique properties that may enable novel spintronic applications, such as helicity-based information storage and computing. Here, we report the statics and current-induced dynamics of two-dimensional (2D) pancake skyrmions in a stack of weakly coupled frustrated magnetic monolayers, which form a three-dimensional (3D) skyrmion string. The Bloch-type skyrmion string is energetically more stable than its N\'eel-type counterpart. It can be driven into translational motion by the dampinglike spin-orbit torque and shows the damping-dependent skyrmion Hall effect. Most notably, the skyrmion string can be transformed to a dynamically stable bimeron string by the dampinglike spin-orbit torque. The current-induced bimeron string rotates stably with respect to its center, which can spontaneously transform back to a skyrmion string when the current is switched off. Our results reveal unusual physical properties of 3D frustrated spin textures, and may open up different possibilities for spintronic applications based on skyrmion and bimeron strings.

Configurable pixelated skyrmions on nanoscale magnetic grids

Topological spin textures can serve as non-volatile information carriers. Here we study the current-induced dynamics of an isolated magnetic skyrmion on a nanoscale square-grid pinning pattern formed by orthogonal defect lines with reduced magnetic anisotropy. The skyrmion on the square grid can be pixelated with a quantized size of the grid. We demonstrate that the position, size, and shape of skyrmion on the square grid are electrically configurable. The skyrmion center is quantized to be on the grid and the skyrmion may show a hopping motion instead of a continuous motion. We find that the skyrmion Hall effect can be perfectly prohibited due to the pinning effect of the grid. The pixelated skyrmion can be harnessed to build future programmable racetrack memory, multistate memory, and logic computing device. Our results will be a basis for digital information storage and computation based on pixelated topological spin textures on artificial pinning patterns.

Dynamics of ferrimagnetic skyrmionium driven by spin-orbit torque

Magnetic skyrmionium is a skyrmionlike spin texture with nanoscale size and high mobility. It is a topologically trivial but dynamically stable structure, which can be used as a nonvolatile information carrier for next-generation spintronic storage and computing devices. Here we study the dynamics of a skyrmionium driven by the spin torque in a ferrimagnetic nanotrack. It is found that the direction of motion is jointly determined by the internal configuration of a skyrmionium and the spin polarization vector. Besides, the deformation of a skyrmionium induced by the intrinsic skyrmion Hall effect depends on both the magnitude of the driving force and the net angular momentum. The ferrimagnetic skyrmionium is most robust at the angular momentum compensation point, whose dynamics is quite similar to the skyrmionium in antiferromagnet. The skyrmion Hall effect is perfectly prohibited, where it is possible to observe the position of the skyrmionium by measuring the magnetization. Furthermore, the current-induced dynamics of a ferrimagnetic skyrmionium is compared with that of a ferromagnetic and antiferromagnetic skyrmionium. We also make a comparison between the motion of a ferrimagnetic skyrmionium and a skyrmion. Our results will open a new field of ferrimagnetic skyrmioniums for future development of ferrimagnetic spintronics devices.

Orbital angular momentum and current-induced motion of a topologically textured domain wall in a ferromagnetic nanotube

We theoretically study the current-induced dynamics of a domain wall in a ferromagnetic nanotube by developing a theory for the orbital angular momentum of a domain wall and the current-induced torque on it. Specifically, a domain wall with nontrivial magnetization winding along the circumference is shown to possess finite orbital angular momentum, which is proportional to the product of its Skyrmion charge and position, and the current is shown to exert a torque changing the orbital angular momentum of the domain wall and thereby drives it. The current-induced torque is interpreted as the transfer of orbital angular momentum from electrons to the domain wall, which occurs due to the emergent magnetic field associated with the Skyrmion charge. Our results reveal a hitherto unrecognized utility of the orbital degree of freedom of magnetic solitons.

Current-induced dynamics and tunable spectra of a magnetic chiral bobber

Chiral bobber (ChB) is an exotic three-dimensional topological texture in chiral magnets. It is composed of a Bloch-point singularity and a skyrmion tube; the incommensurate characteristic length scales of these two components would thus lead to novel dynamics of a ChB. Here, we report the current-induced motion and the accompanying Hall effect of a ChB. Surprisingly, we find that the Hall angle of a ChB can be tuned in a wide range from \ensuremath{-}90 \ifmmode^\circ\else\textdegree\fi{} to positive angles. A further study of spin-wave excitations identified high-frequency resonant modes, which are related to the velocity of ChB and lattice constants. The excited frequency can be two orders of magnitude higher than that of skyrmions. The broad ranges of Hall angles and frequency spectra could boost the development of spintronic devices using topological textures.

Logic Gates Based on Synthetic Antiferromagnetic Bilayer Skyrmions

Technologies based on magnetic skyrmions, such as computational devices that can operate at high speed or with low energy consumption, have been proposed by many researchers. Recently, synthetic antiferromagnetic (SAF) structures have been proposed to increase the stability and mobility of skyrmions by reducing or eliminating the skyrmion Hall effect. Here, we numerically study the current-induced dynamics of skyrmions on SAF bilayer structures. We demonstrate the effective control and manipulation of SAF skyrmions, including directional displacement and alignment. Furthermore, we design SAF-skyrmion-based logic gates, such as the and, or, xor, and not gates. Our design provides guidance for future development of spintronic computing devices that use topological nanoscale spin textures as information carriers.

Dynamics of a pipeline buried in loosely deposited seabed to nonlinear wave [formula omitted] current

Submarine pipeline is a type of important infrastructure in petroleum industry, used for transporting crude oil or natural gas. Understanding of the dynamics characteristics under hydrodynamic loading is crucial for engineers when assessing the stability of offshore pipelines in their designed service period. In this study, taking the integrated numerical model FSSI-CAS 2D as the tool, the nonlinear ocean wave & current-induced dynamics of a shallowly buried submarine steel pipeline and its surrounding loosely deposited seabed soil is numerically investigated. The excellent soil model Pastor-Zienkiewicz-Mark III (PZIII) is adopted to describe the complicated mechanical behaviour of loose seabed soil under cyclic loading. Computational results indicate that the shallowly buried submarine pipeline eventually significantly float up, and extrude the seabed soil over the pipeline driven by the gradually increased buoyancy on the pipeline caused by the accumulation of pore pressure around the pipeline, making the seabed over the pipeline hunch significantly. As a result, considerable deformation occurs in the seabed soil surrounding the pipeline. Two effective stress-based criterion are proposed to judge the occurrence of soil liquefaction. Adopting the two criterion. it is found that the surrounding seabed soil around the pipeline does not become liquefied; only stiffness softening has occurred in it. However, the soil in the upper seabed with shallow depth away from the pipeline becomes liquefied with a liquefaction depth 1.2–1.5 m. Comparative analysis indicates that the pipeline transporting natural gas floats upward with a much greater displacement than that if crude oil is transported. The computational results show that the integrated mode FSSI-CAS 2D has successfully and subtly captured a series of nonlinear physical phenomena of the intensive interaction between pipeline and its surrounding seabed soil. Finally, it is indicated that the integrated model FSSI-CAS 2D has an advantage to investigate the complicated interaction between fluid-structure-seabed foundation.

Current-induced dynamics of skyrmion tubes in synthetic antiferromagnetic multilayers

Topological spin textures can be found in both two-dimensional and three-dimensional nanostructures, which are of great importance to advanced spintronic applications. Here we report the current-induced skyrmion tube dynamics in three-dimensional synthetic antiferromagnetic (SyAF) bilayer and multilayer nanostructures. It is found that the SyAF skyrmion tube made of thinner sublayer skyrmions is more stable during its motion, which ensures that a higher speed of the skyrmion tube can be reached effectively at larger driving current. In the SyAF multilayer with a given total thickness, the current-induced deformation of the SyAF skyrmion tube decreases with an increasing number of interfaces; namely, the rigidity of the SyAF skyrmion tube with a given thickness increases with the number of ferromagnetic (FM) layers. For the SyAF multilayer with an even number of FM layers, the skyrmion Hall effect can be eliminated when the thicknesses of all FM layers are identical. Larger damping parameter leads to smaller deformation and slower speed of the SyAF skyrmion tube. Larger fieldlike torque leads to larger deformation and a higher speed of the SyAF skyrmion tube. Our results are useful for understanding the dynamic behaviors of three-dimensional topological spin textures and may provide guidelines for building SyAF spintronic devices.