Electron energy-loss spectroscopy and ab initio electronic structure of the LaOFeP superconductor

The electronic band structures of the LaOFeP superconductor have been calculated theoretically by the first principles method and measured experimentally by electron energy loss spectroscopy. The calculations indicate that the Fe atom in LaOFeP crystal shows a weak magnetic moment and does not form a long-range magnetic ordering. Band structure, Fermi surfaces and fluorine-doping effects are also analyzed based on the data of the density functional theory. The fine structures of the EELS data have been carefully examined in both the low loss energy region and the core losses region (O K, Fe L2,3, and La M4,5). A slight bump edge at 44 eV shows notable orientation-dependence: it can be observed in the low loss EELS spectra with q parallel to c, but becomes almost invisible in the q vertical to c spectra. Annealing experiments indicate that low oxygen pressure favors the appearance of superconductivity in LaOFeP, this fact is also confirmed by the changes of Fe L2,3 and O K excitation edges in the experimental EELS data

Edge-Mediated Skyrmion Chain and Its Collective Dynamics in a Confined Geometry

The emergence of a topologically nontrivial vortex-like magnetic structure, the magnetic skyrmion, has launched new concepts for memory devices. There, extensive studies have theoretically demonstrated the ability to encode information bits by using a chain of skyrmions in one-dimensional nanostripes. Here, we report the first experimental observation of the skyrmion chain in FeGe nanostripes by using high resolution Lorentz transmission electron microscopy. Under an applied field normal to the nanostripes plane, we observe that the helical ground states with distorted edge spins would evolves into individual skyrmions, which assemble in the form of chain at low field and move collectively into the center of nanostripes at elevated field. Such skyrmion chain survives even as the width of nanostripe is much larger than the single skyrmion size. These discovery demonstrates new way of skyrmion formation through the edge effect, and might, in the long term, shed light on the applications.Comment: 7 pages, 3 figure

Deterministic generation of skyrmions and antiskyrmions by electric current

Magnetic skyrmions are nanoscale spin whirlpools that promise breakthroughs in future spintronic applications. Controlled generation of magnetic skyrmions by electric current is crucial for this purpose. While previous studies have demonstrated this operation, the topological charge of the generated skyrmions is determined by the direction of the external magnetic fields, thus is fixed. Here, we report the current-induced skyrmions creation in a chiral magnet FeGe nanostructure by using the \emph{in-situ} Lorentz transmission electron microscopy. We show that magnetic skyrmions or antiskyrmions can be both transferred from the magnetic helical ground state simply by controlling the direction of the current flow at zero magnetic field. The force analysis and symmetry consideration, backed up by micromagnetic simulations, well explain the experimental results, where magnetic skyrmions or antiskyrmions are created due to the edge instability of the helical state in the presence of spin transfer torque. The on-demand generation of skyrmions and control of their topology by electric current without the need of magnetic field will enable novel purely electric-controlled skyrmion devices.Comment: 5 pages and 4 figure

Electrical Probing of Field-Driven Cascading Quantized Transitions of Skyrmion Cluster States in MnSi Nanowires

Magnetic skyrmions are topologically stable whirlpool-like spin textures that offer great promise as information carriers for future ultra-dense memory and logic devices1-4. To enable such applications, particular attention has been focused on the skyrmions properties in highly confined geometry such as one dimensional nanowires5-8. Hitherto it is still experimentally unclear what happens when the width of the nanowire is comparable to that of a single skyrmion. Here we report the experimental demonstration of such scheme, where magnetic field-driven skyrmion cluster (SC) states with small numbers of skyrmions were demonstrated to exist on the cross-sections of ultra-narrow single-crystal MnSi nanowires (NWs) with diameters, comparable to the skyrmion lattice constant (18 nm). In contrast to the skyrmion lattice in bulk MnSi samples, the skyrmion clusters lead to anomalous magnetoresistance (MR) behavior measured under magnetic field parallel to the NW long axis, where quantized jumps in MR are observed and directly associated with the change of the skyrmion number in the cluster, which is supported by Monte Carlo simulations. These jumps show the key difference between the clustering and crystalline states of skyrmions, and lay a solid foundation to realize skyrmion-based memory devices that the number of skyrmions can be counted via conventional electrical measurements

Simultaneous Ni Doping at Atom Scale in Ceria and Assembling into Well-Defined Lotuslike Structure for Enhanced Catalytic Performance

Oxide materials with redox capability have attracted worldwide attentions in many applications. Introducing defects into crystal lattice is an effective method to modify and optimize redox capability of oxides as well as their catalytic performance. However, the relationship between intrinsic characteristics of defects and properties of oxides has been rarely reported. Herein, we report a facile strategy to introduce defects by doping a small amount of Ni atoms (∼1.8 at. %) into ceria lattice at atomic level through the effect of microstructure of crystal on the redox property of ceria. Amazingly, a small amount of single Ni atom-doped ceria has formed a homogeneous solid solution with uniform lotuslike morphology. It performs an outstanding catalytic performance of a reduced T50 of CO oxidation at 230 °C, which is 135 °C lower than that of pure CeO2 (365 °C). This is largely attributed to defects such as lattice distortion, crystal defects and elastic strain induced by Ni dopants. The DFT calculation has revealed that the electron density distribution of oxygen ions near Ni dopant, the reduced formation energy of oxygen vacancy originated from local chemical effect caused by local distortion after Ni doping. These differences have a great effect on increasing the concentration of oxygen vacancies and enhancing the migration of lattice oxygen from bulk to a surface which is closely related to optimized redox properties. As a result, oxygen storage capacity and the associated catalytic reactivity has been largely increased. We have clearly demonstrated the change of crystal lattice and the charge distribution effectively modify its chemical and physical properties at the atomic scale

Broadening microwave absorption via a multi-domain structure

Materials with a high saturation magnetization have gained increasing attention in the field of microwave absorption; therefore, the magnetization value depends on the magnetic configuration inside them. However, the broad-band absorption in the range of microwave frequency (2-18 GHz) is a great challenge. Herein, the three-dimensional (3D) Fe/C hollow microspheres are constructed by iron nanocrystals permeating inside carbon matrix with a saturation magnetization of 340 emu/g, which is 1.55 times as that of bulk Fe, unexpectedly. Electron tomography, electron holography, and Lorentz transmission electron microscopy imaging provide the powerful testimony about Fe/C interpenetration and multi-domain state constructed by vortex and stripe domains. Benefiting from the unique chemical and magnetic microstructures, the microwave minimum absorption is as strong as -55 dB and the bandwidth (<-10 dB) spans 12.5 GHz ranging from 5.5 to 18 GHz. Morphology and distribution of magnetic nano-domains can be facilely regulated by a controllable reduction sintering under H2/Ar gas and an optimized temperature over 450-850 C. The findings might shed new light on the synthesis strategies of the materials with the broad-band frequency and understanding the association between multi-domain coupling and microwave absorption performance.This work was supported by the Ministry of Science and Technology of China (973 Project Nos. 2013CB932901 and 2016YFE0105700) and the National Natural Science Foundation of China (Nos. 51672050 and 51172047) and NSAF-U1330118. The authors extend their appreciation to the International Scientific Partnership Program ISPP at King Saud University for funding this research work through ISPP# 0018.Scopu

In-plane Hall effect in rutile oxide films induced by the Lorentz force

The conventional Hall effect is linearly proportional to the field component or magnetization component perpendicular to a film. Despite the increasing theoretical proposals on the Hall effect to the in-plane field or magnetization in various special systems induced by the Berry curvature, such an unconventional Hall effect has only been experimentally reported in Weyl semimetals and in a heterodimensional superlattice. Here, we report an unambiguous experimental observation of the in-plane Hall effect (IPHE) in centrosymmetric rutile RuO2 and IrO2 single-crystal films under an in-plane magnetic field. The measured Hall resistivity is found to be proportional to the component of the applied in-plane magnetic field along a particular crystal axis and to be independent of the current direction or temperature. Both the experimental observations and theoretical calculations confirm that the IPHE in rutile oxide films is induced by the Lorentz force. Our findings can be generalized to ferromagnetic materials for the discovery of in-plane anomalous Hall effects and quantum anomalous Hall effects. In addition to significantly expanding knowledge of the Hall effect, this work opens the door to explore new members in the Hall effect family

Radially oriented mesoporous TiO2 microspheres with single-crystal–like anatase walls for high-efficiency optoelectronic devices

Highly crystalline mesoporous materials with oriented configurations are in demand for high-performance energy conversion devices. We report a simple evaporation-driven oriented assembly method to synthesize three-dimensional open mesoporous TiO2 microspheres with a diameter of ~800 nm, well-controlled radially oriented hexagonal mesochannels, and crystalline anatase walls. The mesoporous TiO2 spheres have a large accessible surface area (112 m2/g), a large pore volume (0.164 cm3/g), and highly single-crystal–like anatase walls with dominant (101) exposed facets, making them ideal for conducting mesoscopic photoanode films. Dye-sensitized solar cells (DSSCs) based on the mesoporous TiO2 microspheres and commercial dye N719 have a photoelectric conversion efficiency of up to 12.1%. This evaporation-driven approach can create opportunities for tailoring the orientation of inorganic building blocks in the assembly of various mesoporous materials.State Key Basic Research Program of China (2013CB934104 and 2012CB224805), the National Science Foundation (21210004), the Science and Technology Commission of Shanghai Municipality (08DZ2270500), the Shanghai Leading Academic Discipline Project (B108), King Abdulaziz City for Science and Technology (project no. 29-280), and Deanship of Scientific Research, King Saud University–The International Highly Cited Research Group Program (IHCRG#14-102). Y.L. also acknowledges the Interdisciplinary Outstanding Doctoral Research Funding of Fudan University (EZH2203302/001)

Excellent NiO–Ni Nanoplate Microwave Absorber via Pinning Effect of Antiferromagnetic–Ferromagnetic Interface

Materials with strong magnetic property that can provide excellent microwave absorption performance are highly desirable, especially if their dielectric and magnetic properties can be easily modulated, which make minimal thickness and ultrawide bandwidth become achievable. The magnetic properties of ferromagnetic (FM) and antiferromagnetic (AFM) composite materials are closely related to their ratio of composition, size, morphology, and structure. AFM–FM composites have become a popular alternative for microwave absorption; however, the controllable design and preparation need to be urgently optimized. Here, we have successfully prepared a series of platelike NiO–Ni composites and demonstrated the potential of such composites for microwave absorption. Strong magnetic coupling was found from NiO–Ni nanoparticles by electron holography, which makes NiO–Ni composites a highly efficient microwave absorber (strong reflection loss: −61.5 dB and broad bandwidth: 11.2 GHz, reflection loss < −10 dB). Our findings are helpful to develop a strong microwave absorber based on magnetic coupling