Clusters as new materials

Over the past two decades methods have been developed to produce clusters with an exactly determined number of atoms. Due to their finite size these small particles have totally different structures and `materials properties' than their bulk crystalline counterparts. Even more, these properties sometimes change drastically whenever a single atom is added to or removed from the cluster. This opens the pathway for a whole new world of tailor made materials in the future. In this article we describe the present state of the knowledge of the properties of clusters of atoms which in their bulk form conventional metals or semiconductors. The questions addressed include the development of the electronic structure as a function of cluster size and for example what remains of the `metallic' properties of the bulk solid in these very small clusters. Technological advances are expected using clusters on a specific support material in the areas of catalysis, magnetic storage media or electronic materials, and even solids assembled totally from clusters. Examples from each of these fields will be discussed in the context of this article.     

Ferromagnetic behaviour in semiconductors: a new magnetism in search of spintronic materials

The future of the spintronic technology requires the development of magnetic semiconductor materials. Most research groups have focused on diluted magnetic semiconductors because of the promising theoretical predictions and initial results. In this work, the current experimental situation of ZnO based diluted magnetic semiconductors is presented. Recent results on unexpected ferromagnetic-like behaviour in different nanostructures are also revised, focusing on the magnetic properties of Au and ZnO nanoparticles capped with organic molecules. These experimental observations of magnetism in nanostructures without the typical magnetic atoms are discussed. The doubts around the intrinsic origin of ferromagnetism in diluted magnetic semiconductors along with the surprising magnetic properties in absence of the typical magnetic atoms of certain nanostructures should make us consider new approaches in the quest for room temperature magnetic semiconductors.     

Magnetic van der Waals materials: Synthesis,structure, magnetism,and their potential applications

As the family of magnetic materials is rapidly growing, two-dimensional (2D) van der Waals (vdW) magnets have attracted increasing attention as a platform to explore fundamental physical problems of magnetism and their potential applications. This paper reviews the recent progress on emergent vdW magnetic compounds and their potential applications in devices. First, we summarize the current vdW magnetic materials and their synthetic methods. Then, we focus on their structure and the modulation of magnetic properties by analyzing the representative vdW magnetic materials with different magnetic structures. In addition, we pay attention to the heterostructures of vdW magnetic materials, which are expected to produce revolutionary applications of magnetism-related devices. To motivate the researchers in this area, we finally provide the challenges and outlook on 2D vdW magnetism.     

Magnetic superlattices and multilayers

We briefly review the active areas of current research in magnetic superlattices, emphasizing later years. With recent widening use of advanced technologies, more emphasis has been made on quantitative atomic level chemical and structural characterization. Examples where the multilayer structure has been controlled, characterized and correlated with the physical properties are discussed. The physical properties are categorized according to the complexity of a structure needed to observe a particular effect. We outline a number of general important unsolved problems, which could considerably benefit from theoretical and experimental input. An extensive list of magnetic multilayer materials is provided, with references to recent publications.     

有机自组装低维圆偏振发光材料的研究进展

具有圆偏振发光(CPL)性质的材料由于在3D显示、光学存储以及光学防伪等领域的重要应用,近年来越来越受到研究人员的关注。超分子策略能够将不同类型的分子组装成具有独特功能的低维(零维、一维和二维等)结构,因而成为构筑CPL活性有机低维材料的最有效方法之一。本文从超分子自组装驱动力的角度综述了近几年自组装CPL活性有机低维材料的研究进展。首先,本文系统地总结了现阶段设计自组装CPL活性有机低维材料的策略,其次重点讨论了这类材料的性能及应用,最后探讨了这一领域未来的发展机遇和挑战。     

Recent progress on the prediction of two-dimensional materials using CALYPSO

In recent years, structure design and predictions based on global optimization approach as implemented in CALYPSO software have gained great success in accelerating the discovery of novel two-dimensional(2D) materials. Here we highlight some most recent research progress on the prediction of novel 2D structures, involving elements, metal-free and metal-containing compounds using CALYPSO package. Particular emphasis will be given to those 2D materials that exhibit unique electronic and magnetic properties with great potentials for applications in novel electronics, optoelectronics,magnetronics, spintronics, and photovoltaics. Finally, we also comment on the challenges and perspectives for future discovery of multi-functional 2D materials.     

反铁磁半金属 EuCd2Pn2（Pn = As,Sb）中磁交换诱导的外尔态

由于丰富的拓扑量子效应及巨大的潜在应用价值,拓扑材料逐渐成为凝聚态物理前沿的研究材料体系。其中,作为与石墨烯具有相似电子结构的材料,三维拓扑半金属吸引了越来越多的研究兴趣。目前已知的拓扑半金属大多为非磁性的,而磁性拓扑半金属数量有限,与非磁性拓扑半金属相比较,研究开展的还比较少。磁性与拓扑之间的相互作用能够导致非常规的物理性质,如反常霍尔效应甚至量子反常霍尔效应等。此外,在一些具有特殊磁结构的拓扑半金属中,施加外磁场能够调制其自旋结构,从而影响其拓扑能带结构。在该综述中,笔者将详细介绍利用外磁场在 EuCd2Pn2 (Pn = As, Sb) 反铁磁半金属材料中通过调制自旋结构从而改变晶体结构对称性来诱导拓扑相变。此外,笔者也将简单介绍包括 GdPtBi 和 MnBi2Te4 在内的几个相关材料。该综述中讨论的外磁场调控的磁交换诱导的拓扑相变不仅有望应用于拓扑器件,也有助于为理解磁性与拓扑态之间的紧密关联提供新的线索,对于设计新的磁性拓扑材料有启发意义。综述最后,笔者对发展磁性拓扑半金属做了一些简单展望。     

On the hydrodynamic theory of a magnetic liquid I. General description

Using Zubarev's method of nonequilibrium statistical operator, the generalized hydrodynamic equations are obtained for a model of magnetic liquid in an inhomogeneous external field. In this model the “liquid” subsystem is treated as a classical one and the “magnetic” subsystem is described by quantum mechanical methods. The properties of the transport equations are analysed in the case of a weak nonequilibrium. The equations for time correlation functions and collective mode spectrum are also found in the same manner. It is shown that the generalized hydrodynamic equations reduce to the well-known results in the limiting cases when the dynamic variables of one subsystem are formally neglected. As an illustration, a simple model of spin relaxation is considered, and the frequency matrix and the matrix of memory functions are calculated. A comparison with previous works is made.     

Functionalized Magnetite Nanoparticles—Synthesis,Properties, and Bio-Applications

Nanotechnology has generated tremendous hopes in recent years toward the design of advanced functional materials, especially in the bio-medical field. Nano-sized-materials such as magnetite nanoparticles display indeed fascinating physico-chemical properties that, if tuned properly, can be exploited to design new bio-diagnostic and therapeutic strategies as well as innovative biotechnology methodologies. Owing to their biocompatibility and excellent magnetic properties, magnetite nanocrystals have been the object of a tremendous amount of research in the last decade and numerous (bio)applications have been reported. Importantly, advances in the synthesis of magnetite nanoparticles enable excellent control over their size, shape, and composition. Despite these remarkable progresses, many issues remain to be overcome for these nanotechnology products to revolution the medical practice. The fine control and application of colloidal nanostructures such as magnetite nanoparticles in complex biological systems remains especially challenging. This article attempts to review the current status of magnetite nanoparticles preparation and use, with a special emphasis on bio-medical applications, but also to outline the promises and challenges associated to this emerging technology.     

Magnetism in low dimensionality

The collective creativity of those working in the field of surface magnetism has stimulated an impressive range of advances. Once wary, theorists are now eager to enter the field. The present article attempts to take a snapshot of where the field has been, with an eye to the more speculative issue of where it is going. Selective examples are used to highlight three general areas of interest: (i) characterization techniques, (ii) materials properties, and (iii) theoretical/simulational advances. Emerging directions are identified and discussed, including laterally confined nanomagnetism and spintronics.     

Data-driven curvilinear reconstructions of 3D MR images: application to cryptogenic extratemporal epilepsy

This paper presents a data-driven method for the reconstruction and visualisation of curvilinear slices from three-dimensional (3D) magnetic resonance (MR) scans of the head. Visualisation of curvilinear slices, rather than standard planar slices, produces symmetrical views of the cortex and allows small abnormalities to be detected by comparing the two hemispheres of the brain. In our method, the surface defined by the upper half of the brain is used as a reference shape for curvilinear reconstructions. The brain is first segmented from the 3D scan using a 3D region growing method associated to an unsupervised threshold selection technique. The upper half of the segmented brain is then extracted and fitted by a deformable surface model. This surface is finally interactively moved by the operator in the 3D scan, to visualise the desired curvilinear slice, which is projected on the screen as a two-dimensional image. We show an application of this visualisation technique to the localisation of cerebral epileptogenic lesions. The procedure has proven efficient and handy in clinical use.     

Survey of Plasmonic Nanoparticles: From Synthesis to Application

The unique properties of plasmonic nanostructures have fuelled research based on the tremendous amount of potential applications. Their tailor‐made assemblies in combination with the tunable size and morphology of the initial building blocks allow for the creation of materials with a desired optical response. In this respect, it is crucial to synthesize nanoparticles with a defined shape that are at the core of such developments. Moreover, the interaction of individual nanoparticles with an incident electromagnetic field cannot only be influenced by their structure, but in fact, also by their spatial arrangement to each other. To harvest such opportunities, a profound theoretical understanding of these interactions is required as well as concise strategies to create such ordered assemblies. A quantitative evaluation of their optical properties can only be conducted when discrete structures of high uniformity can be achieved. As a consequence, separation steps have to be applied in order to obtain materials of high purity and uniformity. This also allows for an easier structural characterization of the nanoparticles and their assembled superstructures. In this progress report, an overview about the current development in this field of research is provided.     

Order-disorder criticality, wetting, and morphological phase transitions in the irreversible growth of far-from-equilibrium magnetic films

An exhaustive numerical investigation of the growth of magnetic films in confined (d+1)-dimensional stripped geometries (d=1,2) is carried out by means of extensive Monte Carlo simulations. Films in contact with a thermal bath at temperature T, are grown by adding spins having two possible orientations and considering ferromagnetic (nearest-neighbor) interactions. At low temperatures, thin films of thickness L are constituted by a sequence of well-ordered domains of average length l_DL. These domains have opposite magnetization. So, the films exhibit “spontaneous magnetization reversal” during the growth process. Such reversal occurs within a short characteristic length l_R, such that l_Dl_RL. Furthermore, it is found that for d=1 the system is non-critical, while a continuous order-disorder phase transition at finite temperature takes place in the d=2 case. Using standard finite-size scaling procedures, the critical temperature and some relevant critical exponents are determined. Finally, the growth of magnetic films in (2+1) dimensions with competing short-range magnetic fields acting along the confinement walls is studied. Due to the antisymmetric condition considered, an interface between domains with spins having opposite orientation develops along the growing direction. Such an interface undergoes a localization–delocalization transition that is the precursor of a wetting transition in the thermodynamic limit. Furthermore, the growing interface also undergoes morphological transitions in the growth mode. A comparison between the well-studied equilibrium Ising model and the studied irreversible magnetic growth model is performed throughout. Although valuable analogies are encountered, it is found that the non-equilibrium nature of the latter introduces new and rich physical features of interest.     

Effect of the four-sheet Fermi surface on magnetoresistivity of MgB2

Recent experimental data of anisotropic magnetoresistivity measured in MgB₂ films have shown an intriguing behaviour: the angular dependence of magnetoresistivity changes dramatically with temperature and disorder. In order to explain such phenomenology, in this work, we extend our previous analyses on multiband transverse magnetoresistivity in magnesium diboride, by calculating its analytic expression, assuming a constant anisotropic Fermi surface mass tensor. The calculation is done for arbitrary orientation of the magnetic field with respect to the crystalline axes and for the current density either perpendicular or parallel to the magnetic field. This approach allows to extract quite univocally the values of the scattering times in the σ- and π-bands by fitting experimental data with a simple analytic expression. We also extend the analysis to the magnetoresistivity of polycrystalline samples, with an arbitrary angle between the current density and the magnetic field, taking into account the anisotropy of each randomly oriented grain. Thereby, we propose magnetoresistivity as a very powerful characterization tool to explore the effect of disorder by irradiation or selective doping as well as of phonon scattering in each one of the two types of bands, in single crystals and polycrystalline samples, which is a crucial issue in the study of magnesium diboride.     

Probing buried interfaces with non-linear optical spectroscopy

The importance of buried interfaces in our everyday lives and in current scientific research is highlighted, along with experimental difficulty associated with studying such systems. We present an overview of the application of second harmonic generation and sum-frequency spectroscopy to the study of buried interfaces. Several examples from the current literature are presented, ranging from chemical and biological, to electrical and magnetic interfaces. The importance of this work in the context of ongoing research in these areas is discussed. Finally, we provide a snapshot of the state of the art in non-linear optical spectroscopy by mentioning several new directions that are likely to have a large impact on future research into the physics and chemistry of buried interfaces.     

Hexagonal wurtzite MnO in ferromagnetic state: A magnetic topological spin-gapless Weyl semimetal

Magnetic topological materials have attracted increasingly attentions in recent years due to their exotic electronic behaviors emerging from the couplings of topological, magnetic, and crystalline symmetries. In this work, based on the first-principles calculations, we propose that hexagonal wurtzite MnO is a magnetic topological spin-gapless semi-half-metal with two pairs of type-I Weyl fermions near the Fermi level in ferromagnetic state, which is a promising candidate material in spintronic and piezoelectric applications. In the absence of spin-orbit coupling (SOC), it hosts one triple degeneracy point (TP) in the irreducible Brillouin zone. Owing to weak SOC, the TP splits into two type-I Weyl points that are very close to each other. The Fermi arc surface states connecting the projected Weyl points with opposite chirality are observed. Our results therefore provide a wonderful platform to study the interplay of magnetism and topology.     

Epitaxial self-organization: from surfaces to magnetic materials

Self-organization of magnetic materials is an emerging and active field. An overview of the use of self-organization for magnetic purposes is given, with a view to illustrate aspects that cannot be covered by lithography. A first set of issues concerns the quantitative study of low-dimensional magnetic phenomena (1D and 0D). Such effects also occur in microstructured and lithographically-patterned materials but cannot be studied in these because of the complexity of such materials. This includes magnetic ordering, magnetic anisotropy and superparamagnetism. A second set of issues concerns the possibility to directly use self-organization in devices. Two sets of examples are given: first, how superparamagnetism can be fought by fabricating thick self-organized structures, and second, what new or improved functionalities can be expected from self-organized magnetic systems, like the tailoring of magnetic anisotropy or controlled dispersion of properties. To cite this article: O. Fruchart, C. R. Physique 6 (2005).     

On the anomalous low-resistance state and exceptional Hall component in hard-magnetic Weyl nanoflakes

Magnetic topological materials, which combine magnetism and topology, are expected to host emerging topological states and exotic quantum phenomena. In this study, with the aid of greatly enhanced coercive fields in high-quality nanoflakes of the magnetic Weyl semimetal Co_3Sn_2S_2, we investigate anomalous electronic transport properties that are difficult to reveal in bulk Co_3Sn_2S_2 or other magnetic materials. When the magnetization is antiparallel to the applied magnetic field, the low longitudinal resistance state occurs, which is in sharp contrast to the high resistance state for the parallel case. Meanwhile, an exceptional Hall component that can be up to three times larger than conventional anomalous Hall resistivity is also observed for transverse transport. These anomalous transport behaviors can be further understood by considering nonlinear magnetic textures and the chiral magnetic field associated with Weyl fermions, extending the longitudinal and transverse transport physics and providing novel degrees of freedom in the spintronic applications of emerging topological magnets.     

The surface science of xerography

Over the past four decades xerography, the dry ink marking process developed by the photocopy industry, has grown from nothing into a $170 billion industry worldwide. This amazing commercial success is due to the fact that during this period, xerographic technology experienced constant and often-dramatic improvement created by sustained industrywide research and development. Indeed, the development of the xerographic copying and printing industry is one of the great applied surface science successes of all time. In this article we outline the story of the advances in xerographic technology during the past four decades, describe the profound dependence on these advances of the control of surface and interface properties of increasingly sophisticated multi-component materials systems, and indicate the potential impact on the industry of the continuing development of the surface and interface science of the multi-component materials packages used in xerographic technology.     

Perspectives of antiferromagnetic spintronics

Antiferromagnets are promising for future spintronic applications owing to their advantageous properties: They are magnetically ordered, but neighboring magnetic moments point in opposite directions, which results in zero net magnetization. This means antiferromagnets produce no stray fields and are insensitive to external magnetic field perturbations. Furthermore, they show intrinsic high frequency dynamics, exhibit considerable spin–orbit and magneto-transport effects. Over the past decade, it has been realized that antiferromagnets have more to offer than just being utilized as passive components in exchange bias applications. This development resulted in a paradigm shift, which opens the pathway to novel concepts using antiferromagnets for spin-based technologies and applications. This article gives a broad perspective on antiferromagnetic spintronics. In particular, the manipulation and detection of antiferromagnetic states by spintronics effects, as well as spin transport and dynamics in antiferromagnetic materials will be discussed. We will also outline current challenges and future research directions in this emerging field.