Lorentz Transmission Electron Microscopy Research Articles

Electron Ptychography for Investigating Magnetic Textures in Micro- and Nano- Scale Magnets via Lorentz Transmission Electron Microscopy.

Electron Ptychography for Investigating Magnetic Textures in Micro- and Nano- Scale Magnets via Lorentz Transmission Electron Microscopy.

Light-Induced Metastable Hidden Skyrmion Phase in the Mott Insulator Cu2 OSeO3.

The discovery of a novel long-lived metastable skyrmion phase in the multiferroic insulator Cu2 OSeO3 visualized with Lorentz transmission electron microscopy for magnetic fields below the equilibrium skyrmion pocket is reported. This phase can be accessed by exciting the sample non-adiabatically with near-infrared femtosecond laser pulses and cannot be reached by any conventional field-cooling protocol, referred as a hidden phase. From the strong wavelength dependence of the photocreation process and via spin-dynamics simulations, the magnetoelastic effect is identified as the most likely photocreation mechanism. This effect results in a transient modification of the magnetic free energy landscape extending the equilibrium skyrmion pocket to lower magnetic fields. The evolution of the photoinduced phase is monitored for over 15 min and no decay is found. Because such a time is much longer than the duration of any transient effect induced by a laser pulse in a material, it is assumed that the newly discovered skyrmion state is stable for practical purposes, thus breaking ground for a novel approach to control magnetic state on demand at ultrafast timescales and drastically reducing heat dissipation relevant for next-generation spintronicdevices.

CP2 skyrmions and skyrmion crystals in realistic quantum magnets

Magnetic skyrmions are nanoscale topological textures that have been recently observed in different families of quantum magnets. These objects are called CP1 skyrmions because they are built from dipoles—the target manifold is the 1D complex projective space, CP1 ≅ S2. Here we report the emergence of magnetic CP2 skyrmions in a realistic spin-1 model, which includes both dipole and quadrupole moments. Unlike CP1 skyrmions, CP2 skyrmions can also arise as metastable textures of quantum paramagnets, opening a new road to discover emergent topological solitons in non-magnetic materials. The quantum phase diagram of the spin-1 model also includes magnetic field-induced CP2 skyrmion crystals that can be detected with regular momentum- (diffraction) and real-space (Lorentz transmission electron microscopy) experimental techniques.

Grain boundary construction and properties enhancement for hot deformed (Ce,La,Y)-Fe-B magnet by a two-step diffusion process

The rare earth-iron-boron magnets based on high abundance rare earths (REs) show potential for cost-effective permanent magnets but their hard magnetic properties have to be greatly improved. The grain boundary diffusion process (GBDP) is known as an effective way to improve the coercivity of Nd-Fe-B magnets, however, the conventional diffusion method faces a challenge for Ce-based magnets since there is no enough continuous GB layer as the diffusion channel. Here, a two-step (Nd-Cu doping followed by Nd-Cu diffusion) GBDP was introduced for hot deformed (Ce,La,Y)-Fe-B magnet, and the excellent magnetic properties of μ0Hc=0.63 T, μ0Mr=0.68 T, and (BH)max=72.4 kJ/m3 were achieved. The Nd-Cu doping helps the formation of RE-rich GB layer, and then it acts as the diffusion channel for increasing the efficiency of the subsequent Nd-Cu diffusion and results in the increased volume fraction of continuously distributed GB phase, whose paramagnetism was verified by 57Fe Mössbauer spectrometry. Those paramagnetic GB phases help to form the discontinuous domain walls, as observed by Lorentz transmission electron microscopy, and break the magnetic exchange coupling of RE2Fe14B grains. It thus contributes to the coercivity enhancement of the hot deformed magnet with two-step diffusion, which is further proved by micromagnetic simulation. This study proposes a potential technique to prepare anisotropic hot deformed (Ce,La,Y)-Fe-B magnet with high cost-performance.

Néel Skyrmion Bubbles in La0.7Sr0.3Mn1–xRuxO3 Multilayers

Ferromagnetic La0.7Sr0.3Mn1-xRuxO3 epitaxial multilayers with controlled variation of the Ru/Mn content were synthesized to engineer canted magnetic anisotropy and variable exchange interactions, and to explore the possibility of generating a Dzyaloshinskii-Moriya interaction. The ultimate aim of the multilayer design is to provide the conditions for the formation of domains with nontrivial magnetic topology in an oxide thin film system. Employing magnetic force microscopy and Lorentz transmission electron microscopy in varying perpendicular magnetic fields, magnetic stripe domains separated by Néel-type domain walls as well as Néel skyrmions smaller than 100 nm in diameter were observed. These findings are consistent with micromagnetic modeling, taking into account a sizable Dzyaloshinskii-Moriya interaction arising from the inversion symmetry breaking and possibly from strain effects in the multilayer system.

Creation of transverse domain walls in permalloy nanowires using Lorentz transmission electron microscopy: progress, opportunities, and challenges

Creation of transverse domain walls in permalloy nanowires using Lorentz transmission electron microscopy: progress, opportunities, and challenges

Inducing skyrmion flop transitions in Co8Zn8Mn4 at room temperature

Magnetic skyrmions are topologically-protected spin textures that manifest in certain noncentrosymmetric ferromagnets under the right conditions of temperature and field. In thin-film skyrmion hosts, demagnetization effects combined with geometric confinement can result in two distinct types of spin textures: those with their axis of symmetry in the plane of the film (IP) and those with their axis pointing out-of-plane (OOP). Here we present Lorentz transmission electron microscopy evidence in conjunction with numerical modeling showing a flop transition between IP and OOP skyrmions in ${\mathrm{Co}}_{8}{\mathrm{Zn}}_{8}{\mathrm{Mn}}_{4}$ at room temperature. We show that this skyrmion flop transition is controllable via the angle of the external field relative to the film normal and we illustrate how this transition depends on thickness. Finally, we propose a skyrmion-writing device that utilizes the details of this transition.

Manipulation of topological spin textures in centrosymmetric rare-earth magnets

Topologically protected magnetic skyrmions are expected to be used in the next-generation spintronic devices. Realizing their nucleation and manipulation at room temperature is fundamental for future practical applications. Here, using in situ Lorentz transmission electron microscopy and micromagnetic simulation, we demonstrate that magnetic biskyrmions can spontaneously exist at room temperature in Nd1–xTbxCo5 (x = 0.3, 0.5) alloys. The spontaneous biskyrmions are controllably obtained over a wide temperature range across room temperature by switching atomic chemical environment. Furthermore, the density of biskyrmions can be tuned by a small magnetic field. High-density biskyrmions are stimulated to form in the thinner region at room temperature by introducing an in-plane magnetic field component. These results provide valuable insights into the manipulation of topological states, which is of great significance to their practical applications.

Discovery of Topological Magnetic Textures near Room Temperature in Quantum Magnet TbMn6 Sn6.

The study of topology in quantum materials has fundamentally advanced the understanding in condensed matter physics and potential applications in next-generation quantum information technology. Recently, the discovery of a topological Chern phase in the spin-orbit-coupled Kagome lattice TbMn6 Sn6 has attracted considerable interest. Whereas these phenomena highlight the contribution of momentum space Berry curvature and Chern gap on the electronic transport properties, less is known about the intrinsic real space magnetic texture, which is crucial for understanding the electronic properties and further exploring the unique quantum behavior. Here, the stabilization of topological magnetic skyrmions in TbMn6 Sn6 using Lorentz transmission electron microscopy near room temperature, where the spins experience full spin reorientation transition between the a- and c-axes, is directly observed. An effective spin Hamiltonian based on the Ginzburg-Landau theory is constructed and micromagnetic simulation is performed to clarify the critical role of Ruderman-Kittel-Kasuya-Yosida interaction on the stabilization of skyrmion lattice. These results not only uncover nontrivial spin topological texture in TbMn6 Sn6 , but also provide a solid basis to study its interplay with electronic topology.

Development and applications ofultrafast transmission electron microscopy.

We present a review on the development and applications of the ultrafast transmission electron microscopy (UTEM) at RIKEN. We introduce the UTEM system for the pump-probe TEM observation in a wide temporal range. By combining the UTEM and pixelated detector, we further develop five-dimansional scanning TEM (5D STEM) which provides the ultrafast nanoscale movie of physical quantities in nano-materials, such as crystal lattice information and electromagnetic field, by convergent-beam electron diffraction (CBED) and differential phase contrast (DPC) imaging technique. We show our recent results on the nanosecond-to-microsecond magnetic skyrmion dynamics observed by Lorentz TEM (LTEM) and photo-induced acoustic wave generation in picosecond regime by bright-field TEM and electron diffraction measurements by UTEM. We also show the demonstartion of the 5D STEM on the quantitative time (t)-dependent strain mapping by CBED with an accuracy of 4 ps and 8 nm, and the ultrafast demagnetization under zero magnetic field observed by DPC with 10 ns and 400 nm resolution.

Transformation from Magnetic Soliton to Skyrmion in a Monoaxial Chiral Magnet.

Topological spin textures are of great interest for both fundamental physics and applications in spintronics. The Dzyaloshinskii-Moriya interaction underpins the formation of single-twisted magnetic solitons or multi-twisted magnetic skyrmions in magnetic materials with different crystallographic symmetries. However, topological transitions between these two kinds of topological objects have not been verified experimentally. Here, the direct observation of transformations from a chiral soliton lattice (CSL) to magnetic skyrmions in a nanostripe of the monoaxial chiral magnet CrNb3 S6 using Lorentz transmission electron microscopy is reported. In the presence of an external magnetic field, helical spin structures first transform into CSLs and then evolve into isolated elongated magnetic skyrmions. The detailed spin textures of the elongated magnetic skyrmions are resolved using off-axis electron holography and are shown to comprise two merons, which enclose their ends and have unit total topological charge. Magnetic dipolar interactions are shown to play a key role in the magnetic soliton-skyrmion transformation, which depends sensitively on nanostripe width. The findings here, which are consistent with micromagnetic simulations, enrich the family of topological magnetic states and their transitions and promise to further stimulate the exploration of their emergent electromagnetic properties.

Coexistence of Merons with Skyrmions in the Centrosymmetric Van Der Waals Ferromagnet Fe5- x GeTe2.

Fe5- x GeTe2 is a centrosymmetric, layered van der Waals (vdW) ferromagnet that displays Curie temperatures Tc (270-330 K) that are within the useful range for spintronic applications. However, little is known about the interplay between its topological spin textures (e.g., merons, skyrmions) with technologically relevant transport properties such as the topological Hall effect (THE) or topological thermal transport. Here, via high-resolution Lorentz transmission electron microscopy, it is shown that merons and anti-meron pairs coexist with Néel skyrmions in Fe5- x GeTe2 over a wide range of temperatures and probe their effects on thermal and electrical transport. A THE is detected, even at room T, that senses merons at higher T's, as well as their coexistence with skyrmions as T is lowered, indicating an on-demand thermally driven formation of either type of spin texture. Remarkably, an unconventional THE is also observed in absence of Lorentz force, and it is attributed to the interaction between charge carriers and magnetic field-induced chiral spin textures. These results expose Fe5-x GeTe2 as a promising candidate for the development of applications in skyrmionics/meronics due to the interplay between distinct but coexisting topological magnetic textures and unconventional transport of charge/heat carriers.

Driving a magnetic texture by magnon currents

Thermally induced spin dynamics in solids have sparked broad interest in both fundamental physics and spintronic applications. As theoretically proposed, thermally excited magnons created by temperature gradients can be used to manipulate spin textures such as topological magnetic solitons. However, so far, the effectiveness of such thermomagnonic torques remained a problem in practice. Here, the dynamics of magnetic vortex cores in thin ferromagnetic platelets driven by thermomagnonic torques are investigated using high-resolution Lorentz transmission electron microscopy. Large deflections of the magnetic vortex core transverse to the direction of the temperature gradient are observed. A generalized Thiele equation model is used to identify the magnitude of the contribution of the involved torques. Our results pave the way for the manipulation of magnetic domains on the nanoscale by thermomagnonic currents and provide insights into the interplay of temperature and spin.

Visualizing the Effect of Oxidation on Magnetic Domain Behavior of Nanoscale Fe3GeTe2 for Applications in Spintronics

Magnetic van der Waals (vdW) materials offer an opportunity to design heterostructures that will lead to exotic functionalities that arise from interfacial interaction. In addition to coupling to different vdW materials, the naturally oxidized surface layer of a vdW material also forms a heterostructure with its bulk film, giving rise to intriguing phenomena. Here, we directly observe the impact of oxidation on the magnetic domains, namely, magnetic stripe domain and skyrmions, in a nanoscale Fe3GeTe2 flake using cryo Lorentz transmission electron microscopy. After the Fe3GeTe2 is exposed to ambient conditions, partial oxidation leads to an increase in the density of skyrmions even under zero magnetic field. Complete oxidation leads to a loss of the magnetic domain structure. We observe a gradual change in Fe3GeTe2 from single crystal to amorphous as the oxidation increases. The oxidized Fe3GeTe2 primarily consists of iron oxide, which could be antiferromagnetic in nature. We hypothesize that the interfacial interaction between these surface antiferromagnetic oxides and the bulk ferromagnetic Fe3GeTe2, as well as the effect of interfacial roughness, leads to the increase in Néel skyrmion creation. This work opens a path to harness controlled oxidation as a build block to create dense skyrmion lattices without the need for an external magnetic field, leading to potential future applications in spintronic devices.

Topological spin texture in the pseudogap phase of a high-Tc superconductor.

An outstanding challenge in condensed-matter-physics research over the past three decades has been to understand the pseudogap (PG) phenomenon of the high-transition-temperature (high-Tc) copper oxides. A variety of experiments have indicated a symmetry-broken state below the characteristic temperature T* (refs. 1-8). Among them, although the optical study5 indicated the mesoscopic domains to be small, all these experiments lack nanometre-scale spatial resolution, and the microscopic order parameter has so far remained elusive. Here we report, to our knowledge, the first direct observation of topological spin texture in an underdoped cuprate, YBa2Cu3O6.5, in the PG state, using Lorentz transmission electron microscopy (LTEM). The spin texture features vortex-like magnetization density in the CuO2 sheets, with a relatively large length scale of about 100 nm. We identify the phase-diagram region in which the topological spin texture exists and demonstrate the ortho-II oxygen order and suitable sample thickness to be crucial for its observation by our technique. We also discuss an intriguing interplay observed among the topological spin texture, PG state, charge order and superconductivity.

Triple-meron crystal in high-spin Kitaev magnets

Skyrmions hold great promise in future spintronics applications since they are robust against local deformations. The meron, due to its topological equivalence to a half skyrmion, has been widely found to appear in pairs. Motivated by recent progresses in high-spin Kitaev magnets, here we investigate numerically a classical Kitaev-Γ model with a single-ion anisotropy. An exotic spin texture consisting of three merons is discovered. Such a state features a peculiar property with an odd number of merons in one magnetic unit cell. Therefore, these merons cannot be dissociated from skyrmions as reported in the literature and their origin is briefly discussed. Moreover, we find that these three merons contribute a finite topological number and thus it can induce the topological Hall effect (THE). Experimentally this spin texture can be observed by the Lorentz transmission electron microscopy and the THE can be used to identify the finite topological number. Our work demonstrates that high-spin Kitaev magnets can host robust unconventional spin textures and thus they offer a versatile platform for exploring exotic spin textures as well as their applications in spintronics.

Out-of-plane and in-plane magnetic phases of a FeGe slab detected by dynamic cantilever magnetometry

Using dynamic cantilever magnetometry (DCM), the magnetic phases in H -out-of-plane and H -in-plane configurations of a FeGe slab were investigated. We recorded magnetic field-dependent frequency shifts with static temperature (f -H measurements) and temperature-dependent frequency shifts with static magnetic field (f- T measurements). Based on these data, we constructed the H – T phase diagrams in both configurations. Skyrmion phase (SkX) can be recognized in H -out-of-plane configuration. While no signature of SkX can be recognized in H -in-plane configuration. Meanwhile, the critical magnetic field for the transition from conical phase to field-polarized phase in H -out-of-plane configuration is higher than in H -in-plane configuration. We attribute these two differences to signal-to-noise ratio and demagnetizing field in two configurations, respectively. This work demonstrated the ability of DCM to probe magnetizations of thick or tilted magnetic films, which are difficult to measure using Lorentz transmission electron microscopy or vibrating sample microscopy.

Dzyaloshinskii-Moriya-Interaction-Like Behavior in Confined Permalloy Nanostructures via Coupled Magnetic Vortices.

Dzyaloshinskii-Moriya interaction (DMI), one of antisymmetric exchanges, originates from the combination of low structural symmetry and large spin-orbit coupling and favors magnetization rotations with fixed chirality. Herein, this work reports a DMI-like behavior in permalloy via coupled vortices in confined structures. Under the in-plane magnetic fields, continuous reversals of different coupled vortices are directly observed by in situ Lorentz transmission electron microscopy, and reproduced by complementary micromagnetic simulations. The statistical results show that coupled vortices with opposite chirality appear more frequently with the frequency up to about 60%. Such an asymmetric phenomenon mainly arises from a DMI-like behavior, associated with the increased total energy difference between different ground-state coupled vortices. Moreover, in the reversal process, the junction between disks accelerates the annihilation of vortices moving toward it and is also the starting point of vortex nucleation. These results provide an effective method to generate a DMI-like behavior in magnetic systems with symmetry breaking surface and benefit the future development of vortex-based spintronic devices.

Особенности магнитных текстур в многослойных периодических структурах Co/Pt

The results of systematic experimental studies of the magnetic state and crystal structure of multilayer films based on a ferromagnet/heavy metal pair (Co/Pt) by optical magnetometry, magnetic force microscopy, Lorentz and analytical transmission electron microscopy are presented. It is shown that with an increase in the number of Co/Pt periods in the films, an increase in the average size of crystal grains is observed, which leads to an increase in the dispersion of perpendicular anisotropy and, as a consequence, to a decrease in the size of magnetic domains and magnetization reversal fields. In addition, in films with n≥6 periods, the domain wall becomes hybrid; has an intermediate structure between the walls of the Neel and Bloch types.

Significant Zero Thermal Expansion Via Enhanced Magnetoelastic Coupling in Kagome Magnets.

Zero-thermal-expansion (ZTE) alloys, as dimensionally stable materials, are urgently required in many fields, particularly in highly advanced modern industries. In this study, high-performance ZTE with a negligible coefficient of thermal expansion av as small as 2.4ppm K-1 in a broad temperature range of 85-245 K is discovered in Hf0.85 Ta0.15 Fe2 C0.01 magnet. It is demonstrated that the addition of trace interstitial atom C into Ta-substituted Hf0.85 Ta0.15 Fe2 exhibits significant capability to tune the normal positive thermal expansioninto high-performance ZTE via enhanced magnetoelastic coupling in stabilized ferromagnetic structure. Moreover, direct observation of the magnetic transition between ferromagnetic and triangular antiferromagnetic states via Lorentz transmission electron microscopy, along with corresponding theoretical calculations, further uncovers the manipulation mechanism of ZTE and negative thermal expansion. A convenient and effective method to optimize thermal expansion and achieve ZTE with interstitial C addition may result in broadened applications based on the strong correlation between the magnetic properties and crystal structure.