Zi-An Li

Improved hydrogen storage properties of LiBH4 destabilized by carbon

The hydrogen storage properties of LiBH4 ball milled with various ratios of carbon nanotubes (Cnano) were investigated. The LiBH4∕Cnano mixtures showed superior dehydrogenation, hydrogen desorption starting at 250°C, and the majority of hydrogen being released below 600°C. The rehydrogenation results revealed that the Li2C2, formed during the dehydrogenation, could be reversed to LiH, in which the hydrogen capacity corresponds to 1∕4 of the original hydrogen content of LiBH4, and C at 10MPa hydrogen pressure and 400°C.

Experimental observation of chiral magnetic bobbers in B20-type FeGe

Chiral magnetic skyrmions1,2 are nanoscale vortex-like spin textures that form in the presence of an applied magnetic field in ferromagnets that support the Dzyaloshinskii–Moriya interaction (DMI) because of strong spin–orbit coupling and broken inversion symmetry of the crystal3,4. In sharp contrast to other systems5,6 that allow for the formation of a variety of two-dimensional (2D) skyrmions, in chiral magnets the presence of the DMI commonly prevents the stability and coexistence of topological excitations of different types7. Recently, a new type of localized particle-like object—the chiral bobber (ChB)—was predicted theoretically in such materials8. However, its existence has not yet been verified experimentally. Here, we report the direct observation of ChBs in thin films of B20-type FeGe by means of quantitative off-axis electron holography (EH). We identify the part of the temperature–magnetic field phase diagram in which ChBs exist and distinguish two mechanisms for their nucleation. Furthermore, we show that ChBs are able to coexist with skyrmions over a wide range of parameters, which suggests their possible practical applications in novel magnetic solid-state memory devices, in which a stream of binary data bits can be encoded by a sequence of skyrmions and bobbers.Electron holography enables direct experimental verification of the existence of chiral bobbers in thin films of chiral magnets.The use of chiral skyrmions, which are nanoscale vortex-like spin textures, as movable data bit carriers forms the basis of a recently proposed concept for magnetic solid-state memory. In this concept, skyrmions are considered to be unique localized spin textures, which are used to encode data through the quantization of different distances between identical skyrmions on a guiding nanostripe. However, the conservation of distances between highly mobile and interacting skyrmions is difficult to implement in practice. Here, we report the direct observation of another type of theoretically-predicted localized magnetic state, which is referred to as a chiral bobber (ChB), using quantitative off-axis electron holography. We show that ChBs can coexist together with skyrmions. Our results suggest a novel approach for data encoding, whereby a stream of binary data representing a sequence of ones and zeros can be encoded via a sequence of skyrmions and bobbers. The need to maintain defined distances between data bit carriers is then not required. The proposed concept of data encoding promises to expedite the realization of a new generation of magnetic solid-state memory.

Observation of nanoscale magnetic fields using twisted electron beams

Electron waves give an unprecedented enhancement to the field of microscopy by providing higher resolving power compared to their optical counterpart. Further information about a specimen, such as electric and magnetic features, can be revealed in electron microscopy because electrons possess both a magnetic moment and charge. In-plane magnetic structures in materials can be studied experimentally using the effect of the Lorentz force. On the other hand, full mapping of the magnetic field has hitherto remained challenging. Here we measure a nanoscale out-of-plane magnetic field by interfering a highly twisted electron vortex beam with a reference wave. We implement a recently developed holographic technique to manipulate the electron wavefunction, which gives free electrons an additional unbounded quantized magnetic moment along their propagation direction. Our finding demonstrates that full reconstruction of all three components of nanoscale magnetic fields is possible without tilting the specimen.Beyond high resolving power, electron microscopy can be used to study both the electronic and magnetic properties of a sample. Here, Grillo et al. combine electron vortex beams with holographic detection to measure out-of-plane nanoscale magnetic fields.

An in-plane magnetic chiral dichroism approach for measurement of intrinsic magnetic signals using transmitted electrons.

Electron energy-loss magnetic chiral dichroism is a powerful technique that allows the local magnetic properties of materials to be measured quantitatively with close-to-atomic spatial resolution and element specificity in the transmission electron microscope. Until now, the technique has been restricted to measurements of the magnetic circular dichroism signal in the electron beam direction. However, the intrinsic magnetization directions of thin samples are often oriented in the specimen plane, especially when they are examined in magnetic-field-free conditions in the transmission electron microscope. Here, we introduce an approach that allows in-plane magnetic signals to be measured using electron magnetic chiral dichroism by selecting a specific diffraction geometry. We compare experimental results recorded from a cobalt nanoplate with simulations to demonstrate that an electron magnetic chiral dichroism signal originating from in-plane magnetization can be detected successfully.

Mapping the magnetization fine structure of a lattice of Bloch-type skyrmions in an FeGe thin film

Bloch-type magnetic skyrmions are nanoscale vortex-like spin objects that form densely packed lattice arrangements in B20-type chiral magnets in the presence of a magnetic field. Here, we use off-axis electron holography, in combination with an iterative model-based reconstruction algorithm, to study the geometries of the projected in-plane magnetization distributions of individual skyrmions in an FeGe thin film as a function of applied magnetic field. We compare our results with micromagnetic simulations and find a departure from magnetic chirality in the transition regions between adjacent skyrmions when they are in lattice arrangements.

Lorentz microscopy and off-axis electron holography of magnetic skyrmions in FeGe

Magnetic skyrmions are vortex-like spin structures that are of great interest scientifically and for applications in low-power magnetic memories. The nanometer size and complex spin structure require high-resolution and quantitative experimental methods to study the physical properties of skyrmions. Here, we illustrate how Lorentz TEM and off-axis electron holography can be used to study the spin textures of magnetic skyrmions in the noncentrosymmetric B20-type helimagnet FeGe as a function of temperature and applied magnetic field. By reversing the magnetic field inside the microscope, the switching mechanism of the skyrmion lattice at 240 K is followed, showing a transition of the skyrmion lattice to the helical structure before the anti-skyrmion lattice is formed.

Defect effects on spatiotemporal evolution of photoinduced martensitic transition in MnNiSn

Martensitic transition and reverse transition in ferromagnetic shape memory alloy MnNiSn contain a variety of structural dynamic features accompanied directly by atomic motions and micro-domain alterations. To investigate the effects of crystalline defects on the dynamical structural phase transitions, we use ultrafast transmission electron microscopy (UTEM) to directly image the rapid structural phase transition in MnNiSn initiated by femtosecond laser pulses. Via high spatiotemporal resolution images, we reveal the pinning effect by the grain boundary on the phonon-driven martensitic transition after fs-laser pulse excitations, and the structural oscillation is also observed as driven by coherent acoustic phonons that start at the sites of the grain boundary and propagate with the speed of sound. These results elucidate the roles of crystallographic defects in the dynamical processes of martensitic transition and highlight the unprecedented capability of UTEM for direct imaging lattice motions with nanometer spatial and picosecond temporal resolutions.Martensitic transition and reverse transition in ferromagnetic shape memory alloy MnNiSn contain a variety of structural dynamic features accompanied directly by atomic motions and micro-domain alterations. To investigate the effects of crystalline defects on the dynamical structural phase transitions, we use ultrafast transmission electron microscopy (UTEM) to directly image the rapid structural phase transition in MnNiSn initiated by femtosecond laser pulses. Via high spatiotemporal resolution images, we reveal the pinning effect by the grain boundary on the phonon-driven martensitic transition after fs-laser pulse excitations, and the structural oscillation is also observed as driven by coherent acoustic phonons that start at the sites of the grain boundary and propagate with the speed of sound. These results elucidate the roles of crystallographic defects in the dynamical processes of martensitic transition and highlight the unprecedented capability of UTEM for direct imaging lattice motions with nano...