Licong Peng

Preparation and structure analysis of titanium oxide nanotubes

Well crystallized nanoscale tubular materials have been synthesized via the reaction of TiO2 crystals of either anatase or rutile phase and NaOH aqueous solution. The atomic structure of the synthesized tubular material is imaged by high-resolution transmission electron microscopy (HRTEM), and the composition of individual tubular structures is determined using selected area energy dispersive X-ray spectroscopy (EDX). Our results show that the tubular materials are well crystallized tubes with an average diameter of about 9 nm and little dispersion, and are composed of mainly titanium and oxygen. The atomic ratio of O/Ti is found, however, to vary from tube to tube. Detailed electron and x-ray diffraction studies show that the structure of our titanium oxide nanotubes do not agree with those made of TiO2 crystals with either anatase or rutile phase. HRTEM observations revealed that the titanium oxide nanotubes usually have multiple shells, in analogy with multiwalled carbon nanotubes, but the shell spacin...

A Centrosymmetric Hexagonal Magnet with Superstable Biskyrmion Magnetic Nanodomains in a Wide Temperature Range of 100–340 K

Superstable biskyrmion magnetic nanodomains are experimentally observed for the first time in a hexagonal MnNiGa, a common and easily produced centrosymmetric material. The biskyrmion states in MnNiGa thin plates, as determined by the combination of in situ Lorentz transmission electron microscopy images, magnetoresistivity, and topological Hall effect measurements, are surprisingly stable over a broad temperature range of 100-340 K.

Real-Space Observation of Nonvolatile Zero-Field Biskyrmion Lattice Generation in MnNiGa Magnet

Magnetic skyrmions, particular those without the support of external magnetic fields over a wide temperature region, are promising as alternative spintronic units to overcome the fundamental size limitation of conventional magnetic bits. In this study, we use in situ Lorentz microscope to directly demonstrate the generation and sustainability of robust biskyrmion lattice at zero magnetic field over a wide temperature range of 16-338 K in MnNiGa alloy. This procedure includes a simple field-cooling manipulation from 360 K (higher than Curie temperature TC ∼ 350 K), where topological transition easily occurs by adapting the short-range magnetic clusters under a certain magnetic field. The biskyrmion phase is favored upon cooling below TC. Once they are generated, the robust high-density biskyrmions persist even after removing the external magnetic field due to the topological protection and the increased energy barrier.

Accurate measurement of phase shift in electron holography

A method is proposed for the accurate measurement of phase shift in electron holography. The method is based on the use of moire fringes resulting from the subtraction of a null electron hologram by a real object hologram recorded under slightly different experimental conditions. This method does not require any optical or digital reconstruction of the electron hologram, and is shown to be highly sensitive to the phase shift of the electron wave passing through an object. Using experimental results obtained from a single particle of silicon, we demonstrate that the sensitivity of this method to phase shift may easily be amplified by more than 11 times compared with the conventional method using an ordinary electron hologram.

VOID-LIKE DEFECTS IN ANNEALED CZOCHRALSKI SILICON

Void-like defects of octahedron structure having {111} facets were observed in annealed Czochralski silicon. The amorphous coverage of SiOx and SiCx on the inner surface of the defects was identified using transmission electron microscopy and electron energy-loss spectroscopy. It is suggested that these defects are a kind of amorphous precipitate origin. A mechanism for the generation of these defects and the previously reported solid amorphous precipitates is proposed.

Realization of zero-field skyrmions with high-density via electromagnetic manipulation in Pt/Co/Ta multilayers

Taking advantage of the electron-current ability to generate, stabilize, and manipulate skyrmions prompts the application of skyrmion multilayers in room-temperature spintronic devices. In this study, the robust high-density skyrmions are electromagnetically generated from Pt/Co/Ta multilayers using Lorentz transmission electron microscopy. The skyrmion density is tunable and can be significantly enhanced. Remarkably, these generated skyrmions after optimized manipulation sustain at zero field with both the in-plane current and perpendicular magnetic field being switched off. The skyrmion generation and manipulation method demonstrated in this study opens up an alternative way to engineer skyrmion-based devices. The results also provide key data for further theoretical study to discover the nature of the interaction between the electric current and different spin configurations.

Zero-field skyrmions generated via premartensitic transition in Ni50Mn35.2In14.8 alloy

Magnetic phase transition contributes to magnetocaloric effects and magnetoelastic coupling, producing significant multifunctions in Ni-based Heusler alloys. In this paper, the peculiar intermediate premartensitic phase during the transition from parent phase to martensite is identified in Ga-free Ni50Mn35.2In14.8 alloy via Lorentz transmission electron microscopy combined with in situ magnetizing and cooling manipulation. The simultaneous coexistence of three skyrmion configurations at zero field in martensite is directly observed with correlation to the appearance of the intermediate magnetic phase and martensite twinning confinement. The evolution of magnetic domains demonstrates a mechanism to generate skyrmions with magnetization orientation adjusted via magnetic phase transition, which illustrates the associated physical properties.

The indispensable role of the transversal spin fluctuations mechanism in laser-induced demagnetization of Co/Pt multilayers with nanoscale magnetic domains

The switching of magnetic domains induced by an ultrashort laser pulse has been demonstrated in nanostructured ferromagnetic films. This leads to the dawn of a new era in breaking the ultimate physical limit for the speed of magnetic switching and manipulation, which is relevant to current and future information storage. However, our understanding of the interactions between light and spins in magnetic heterostructures with nanoscale domain structures is still lacking. Here, both time-resolved magneto-optical Kerr effect experiments and atomistic simulations are carried out to investigate the dominant mechanism of laser-induced ultrafast demagnetization in [Co/Pt]20 multilayers with nanoscale magnetic domains. It is found that the ultrafast demagnetization time remains constant with various magnetic configurations, indicating that the domain structures play a minor role in laser-induced ultrafast demagnetization. In addition, both in experiment and atomistic simulations, we find a dependence of ultrafast demagnetization time τ M on the laser fluence, which is in contrast to the observations of spin transport within magnetic domains. The remarkable agreement between experiment and atomistic simulations indicates that the local dissipation of spin angular momentum is the dominant demagnetization mechanism in this system. More interestingly, we made a comparison between the atomistic spin dynamic simulation and the longitudinal spin flip model, highlighting that the transversal spin fluctuations mechanism is responsible for the ultrafast demagnetization in the case of inhomogeneous magnetic structures. This is a significant advance in clarifying the microscopic mechanism underlying the process of ultrafast demagnetization in inhomogeneous magnetic structures.

Spontaneous nanometric magnetic bubbles with various topologies in spin-reoriented La1−xSrxMnO3

Topological zero-field nanometric domains and their capability to be manipulated by external fields show potential applications in spintronics. Here, the spontaneous magnetic bubbles (≈100 nm in diameter) are observed at zero field in a ferromagnetic manganite La1−xSrxMnO3 (0.15 < x < 0.2) by using Lorentz transmission electron microscopy. The spin reorientation as a function of temperature drives the magnetic domain transition from traditional 180° in-plane domains to helical stripes and bubbles, resulting in rich magnetic configurations with various topologies. It directly demonstrates that the dynamic motion of Bloch lines in bubbles introduces the topologic transition under the application of magnetic fields.Topological zero-field nanometric domains and their capability to be manipulated by external fields show potential applications in spintronics. Here, the spontaneous magnetic bubbles (≈100 nm in diameter) are observed at zero field in a ferromagnetic manganite La1−xSrxMnO3 (0.15 < x < 0.2) by using Lorentz transmission electron microscopy. The spin reorientation as a function of temperature drives the magnetic domain transition from traditional 180° in-plane domains to helical stripes and bubbles, resulting in rich magnetic configurations with various topologies. It directly demonstrates that the dynamic motion of Bloch lines in bubbles introduces the topologic transition under the application of magnetic fields.