Highly spin-polarized materials and devices for spintronics∗

Abstract

The performance of spintronics depends on the spin polarization of the current. In this study half-metallic Co-based full-Heusler alloys and a spin filtering device (SFD) using a ferromagnetic barrier have been investigated as highly spin-polarized current sources. The multilayers were prepared by magnetron sputtering in an ultrahigh vacuum and microfabricated using photolithography and Ar ion etching. We investigated two systems of Co-based full-Heusler alloys, Co₂Cr_{1 ? x}Fe_xAl (CCFA(x)) and Co₂FeSi_{1 ? x}Al_x (CFSA(x)) and revealed the structure and magnetic and transport properties. We demonstrated giant tunnel magnetoresistance (TMR) of up to 220% at room temperature and 390% at 5 K for the magnetic tunnel junctions (MTJs) using Co₂FeSi_0.5Al_0.5 (CFSA(0.5)) Heusler alloy electrodes. The 390% TMR corresponds to 0.81 spin polarization for CFSA(0.5) at 5 K. We also investigated the crystalline structure and local structure around Co atoms by x-ray diffraction (XRD) and nuclear magnetic resonance (NMR) analyses, respectively, for CFSA films sputtered on a Cr-buffered MgO (001) substrate followed by post-annealing at various temperatures in an ultrahigh vacuum. The disordered structures in CFSA films were clarified by NMR measurements and the relationship between TMR and the disordered structure was discussed. We clarified that the TMR of the MTJs with CFSA(0.5) electrodes depends on the structure, and is significantly higher for L2₁ than B2 in the crystalline structure. The second part of this paper is devoted to a SFD using a ferromagnetic barrier. The Co ferrite is investigated as a ferromagnetic barrier because of its high Curie temperature and high resistivity. We demonstrate the strong spin filtering effect through an ultrathin insulating ferrimagnetic Co-ferrite barrier at a low temperature. The barrier was prepared by the surface plasma oxidization of a CoFe₂ film deposited on a MgO (001) single crystal substrate, wherein the spinel structure of CoFe₂O₄ (CFO) and an epitaxial relationship of MgO(001)[100]/CoFe₂ (001)]110]/CFO(001)[100] were induced. A SFD consisting of CoFe₂ /CFO/Ta on a MgO (001) substrate exhibits the inverse TMR of - 124% at 10 K when the configuration of the magnetizations of CFO and CoFe₂ changes from parallel to antiparallel. The inverse TMR suggests the negative spin polarization of CFO, which is consistent with the band structure of CFO obtained by first principle calculation. The - 124% TMR corresponds to the spin filtering efficiency of 77% by the CFO barrier.     

ZnO:Co diluted magnetic semiconductor or hybrid nanostructure for spintronics?

We have studied the influence of intrinsic and extrinsic defects in the magnetic and electrical transport properties of Co-doped ZnO thin films. X-ray absorption measurements show that Co substitute Zn in the ZnO structure and it is in the 2+ oxidation state. Magnetization (M) measurements show that doped samples are mainly paramagnetic. From M vs. H loops measured at 5 K we found that the values of the orbital L and spin S numbers are between 1 and 1.3 for L and S = 3/2, in agreement with the representative values for isolated Co 2+. The obtained negative values of the Curie–Weiss temperatures indicate the existence of antiferromagnetic interactions between transition metal atoms.     

Syntheses and characterization of 6,6′-diaryl-9,9′-dialkyl[3,3′]bicarbazoles as materials for electroluminescent devices

Electro-active materials containing 6,6′-diaryl-substituted [3,3′]bicarbazole core were synthesized by multistep synthetic rote and characterized. The derivatives were examined by differential scanning calorimetry and demonstrated formation of amorphous materials with rather high glass transition temperatures. The synthesized compounds have been tested as hole transporting layers in simple OLED devices with Alq₃ as the emitter/electron transporting layer. The green devices containing hole transporting films of diphenyl-9,9′-diethyl-[3,3′]bicarbazole exhibited the best overall performance (turn-on voltage: 3.5 V, maximum photometric efficiency: 4.2 cd/A, maximum brightness: ∼12,200 cd/m²).     

Eu–Co substituted Sr-hexaferrites for recording media and microwave devices

Journal of Materials Science: Materials in Electronics - Among ferrites materials, M-type hexaferrites are very important due to their excellent technological applications. In the present...     

Implicit yield function formulation for granular and rock-like materials

The constitutive modelling of granular, porous and quasi-brittle materials is based on yield (or damage) functions, which may exhibit features (for instance, lack of convexity, or branches where the values go to infinity, or ‘false elastic domains’) preventing the use of efficient return-mapping integration schemes. This problem is solved by proposing a general construction strategy to define an implicitly defined convex yield function starting from any convex yield surface. Based on this implicit definition of the yield function, a return-mapping integration scheme is implemented and tested for elastic–plastic (or -damaging) rate equations. The scheme is general and, although it introduces a numerical cost when compared to situations where the scheme is not needed, is demonstrated to perform correctly and accurately.     

Is surface chemical composition important for orthopaedic implant materials?

Ti-6Al-7Nb (NS) in its ‘standard’ implant form has been previously shown to be detrimental to fibroblast growth and colonisation on its surface. Specific aspects of the NS topography have been implicated, however, the contribution of its unique surface chemistry to the cell behaviour was unknown. By evaporating either gold or titanium on the surface of standard NS, two different model surface chemistries could be studied with the same characteristic standard NS topography. Two other ‘standard’ orthopaedic topographies, that of stainless steel (SS) and of ‘commercially pure’ titanium (TS) were also treated in a similar manner. All materials elicited behaviour similar to their uncoated counterparts. For coated SS and TS, cell proliferation was observed, cells were well spread and displayed mature focal adhesion sites, and associated cytoskeletal components. For coated NS, cell proliferation was compromised, cells remained rounded, filopodia attached and seemed to probe the surface, especially the β -phase particles, and both the focal adhesion sites and the microtubule network were disrupted by the presence of these particles. These results confirmed, that in the instance of NS, the topography was the primary cause for the observed stunted cell growth. For biomaterials studies, the standardisation of surface chemistry used here is a valuable tool in allowing vastly different materials and surface finishes to be compared solely on the basis of their topography.     

Bimodal microstructure – A feasible strategy for high-strength and ductile metallic materials

Introducing a bimodal grain-size distribution has been demonstrated an efficient strategy for fabricating high-strength and ductile metallic materials, where fine grains provide strength, while coarse grains enable strain hardening and hence decent ductility. Over the last decades, research activities in this area have grown enormously, including interesting results on fcc Cu, Ni and Al-Mg alloys as well as steel and Fe alloys via various thermo-mechanical processing approaches. However, investigations on bimodal Mg and other hcp metals are relatively few. A brief overview of the available approaches based on thermo-mechanical processing technology in producing bimodal microstructure for various metallic materials is given, along with a summary of unusual mechanical properties achievable by bimodality, where focus is placed on the microstructure-mechanical properties and relevant mechanisms. In addition, key factors that influencing bimodal strategies, such as compositions of starting materials and processing parameters, together with the challenges this research area facing, are identified and discussed briefly.     

Research progress on electrode materials and electrolytes for supercapacitors北大核心CSCD

Supercapacitors have great potential applications for electronic devices, and energy recyling and storage areas owing to their high power density, long cycle life, high safety and excellent performance at low temperatures. The electrode materials and electrolytes are two key factors that influence their performance. The electrode materials used in supercapacitors include carbon materials such as activated carbons, carbon nanotubes, graphene, carbon nanofibers and carbon nano-onions, metal oxides, conductive polymers and their composites. The electrolytes are aqueous electrolytes, organic electrolytes or ionic liquids. Here research progress on the electrode materials and liquid electrolytes for supercapacitors is summarized, their advantages and disadvantages are analyzed, and new electrode materials and electrolytes are suggested.     

Graphene-based materials and structures for energy harvesting with fluids – A review

Graphene and graphene-based systems have recently been recognized as promising platforms for energy harvesting, microelectronic components and energy storage owing to their excellent electrical and thermal conductivity, outstanding mechanical properties, good chemical stability, area adaptability, among other significant properties. Integration of energy harvesting systems relying on the graphene/graphene-based materials in contact with fluids has been emphasized in recent years, as well as their potential impact on electric energy generation for a wide range of applications (e.g. innovative medical devices, advanced electronic systems and highly-efficient transduction systems for renewable energy). This review summarizes, for the first time, major breakthroughs carried out in the scope of energy harvesting exploiting graphene-based material systems (comprising graphene films, graphene grids, graphene membranes, 3D graphene composites and tribological structures) in contact with ionic and non-ionic fluids. Several transduction mechanisms for energy harvesting have been thoroughly analyzed. Energy outputs, materials and structures, substrates, types of fluid, manufacture methodologies, and experimental test methodologies are systematically highlighted in this review. Finally, future research directions and innovative applications of these harvesters are proposed.     

Ion exchangers and adsorbents — Versatile aids for the materials processing industry

Ion exchangers and adsorbers are available nowadays in such a wide range of universally applicable or specialist products that they are used extensively in chemical industry. New applications are discovered each year. Hence their originally predominant use in water treatment has meanwhile been extended and diversified by numerous further applications.     

Investigation into refractories as crucible and mould materials for melting and casting γ-TiAl alloys

Abstract

An assessment has been made of the suitability of magnesia, calcia, alumina, and yttria for manufacturing melting crucibles and investment casting moulds for γ-TiAl alloys. Small refractory crucibles have been prepared by pressing or plasma spraying techniques and used to melt a small quantity of a Ti–48Al–2Nb–2Mn (at.-%) alloy in a carbon resistance furnace. The effects of the refractory type and melting time on the oxygen content, penetration, and microstructure have been established. The depth of penetration versus the holding time at 1550°C can be expressed by a power law. Based on these small­scale tests, it can be concluded that magnesia and silica containing alumina are unsuitable, whereas both pure calcia, yttria, and yttria­coated magnesia show promise for melting and casting γ-TiAl alloys.     

Why engineer porous materials?

A number of specific examples are briefly given for the use of pores in engineering materials: a porous ceramic to produce minimum thermal conduction; thin skeleton walls in silicon to produce photoluminescence; low dielectric constant materials. The desirable nature of the pores in fuel cell electrodes and sieves is described. Further examples are given in orthopaedics, prosthetic scaffolds and sound deadening and impact resistance materials. An attempt is made to describe the desirable pore size, whether open or closed, and the useful volume fraction. This short review does not deal with flexible foams.     

Direct Ink Writing of Highly Conductive MXene Frames for Tunable Electromagnetic Interference Shielding and Electromagnetic Wave?Induced Thermochromism

The highly integrated and miniaturized next-generation electronic products call for high-performance electromagnetic interfer?ence (EMI) shielding materials to ...     

Geopolymers and other alkali activated materials: why,how, and what?

This paper presents a review of alkali-activation technology, moving from the atomic scale and chemical reaction path modelling, towards macroscopic observables such as strength and durability of alkali-activated concretes. These properties and length scales are intrinsically interlinked, and so the chemistry of both low-calcium (‘geopolymer’) and high-calcium (blast furnace slag-derived) alkali-activated binders can be used as a starting point from which certain engineering properties may be discussed and explained. These types of materials differ in chemistry, binder properties, chemical structure and microstructure, and this leads to the specific material properties of each type of binder. The secondary binder products formed during alkali-activation (zeolites in low-Ca systems, mostly layered double hydroxides in alkali-activated slags) are of significant importance in determining the final properties of the materials, particularly in the context of durability. The production of highly durable concretes must remain the fundamental aim of research and development in the area of alkali-activation. However, to enable the term ‘highly durable’ to be defined in a satisfactory way, the underlying mechanisms of degradation—which are not always the same for alkali-activated binders as for Portland cement-based binders, and cannot always be tested in precisely the same ways—need to be further analysed and understood. The process of reviewing a topic such as this will inevitably raise just as many questions as answers, and it is the intention of this paper to present both, in appropriate context.     

Inorganic photovoltaics – Planar and nanostructured devices

Since its invention in the 1950s, semiconductor solar cell technology has evolved in great leaps and bounds. Solar power is now being considered as a serious leading contender for replacing fossil fuel based power generation. This article reviews the evolution and current state, and potential areas of near future research focus, of leading inorganic materials based solar cells, including bulk crystalline, amorphous thin-films, and nanomaterials based solar cells. Bulk crystalline silicon solar cells continue to dominate the solar power market, and continued efforts at device fabrication improvements, and device topology advancements are discussed. III–V compound semiconductor materials on c-Si for solar power generation are also reviewed. Developments in thin-film based solar cells are reviewed, with a focus on amorphous silicon, copper zinc tin sulfide, cadmium telluride, as well as nanostructured cadmium telluride. Recent developments in the use of nano-materials for solar power generation, including silicon and gallium arsenide nanowires, are also reviewed.     

Carbon–phenolic ablative materials for re-entry space vehicles: Manufacturing and properties

Thermal protection systems (TPS) are designed to protect re-entry space vehicles from the severe heating encountered during hypersonic flight through a planet’s or the earth’s atmosphere. A carbon–phenolic ablative TPS was developed, manufactured and tested with the aim of fulfilling the thermal and mechanical requirements corresponding to the actual loads experienced by a vehicle during a moon-earth re-entry. Experimental activities were carried out on two different composite systems (a resole resin coupled with a graphitic felt and a graphitic foam), and were aimed to the optimization of the manufacturing procedure and to the characterization of the mechanical behaviour and of the insulation performance of the fabricated composites.     

New chickens and new eggs——New catalytic materials for alternative fuels and chemical feedstock

The on-going depletion of the oil reserves leads to the needs for development of sustainable and renewable energies. Between fossil and solar eras,low carbon natural gas and clean coal become the most crucial intermediate alternatives as energy source and feedstock to produce chemicals.In meantime,lignocellulosic biomass will play the role as fuel or as raw material,especially for the future decades.This is of ultimate importance for the countries with increased demands for energy in a portable form and for synthetic materials,such as China. New catalytic materials are at the heart in the processes(existing and innovative)to converting natural gas or coal,the nonportable or non-pumpable energy carriers,to liquid form,and to upgrading less valuable to more value-added chemicals for a particular market requirement.In many instances,innovation of a new catalytic material makes a new process from dream to reality,and in the other cases,the need for a new process catalyzes the research for a new catalytic material. The process and material("chickens and eggs")interplay propels the recent advances in hydrogen fuel,GTL,CTL,CTC,SNG and BTL technologies.In this growing market,Siid-Chemie has not only strengthened its market presence,but also consolidated its position as the worldwide technology alytic materials in the new chemical processes,such as ShiftMax(R),ActiSorb(R),AmoMax(R),MegaMax(R),DME-1(R),FAMax(R),and MTPro-1(R).Siid-Chemie commits to provide the best performing tailored catalytic materials to the matching processes.