Optical isolators, devices that only allow unidirectional light propagation, constitute an essential building block for photonic integrated circuits. For near-infrared communications wavelengths, most current isolator designs rely on the incorporation of magneto-optical (MO) materials to break time-reversal symmetry, such as iron garnets or magnetically substituted semiconductors. MO garnets form the backbone of traditional bulk isolators, but suffer from large lattice and thermal mismatch with common semiconductor substrates, which has significantly impeded their integration into on-chip optical isolators. Materials innovations over the past few years have overcome these barriers and enabled monolithic deposition of MO oxide thin films on silicon using techniques such as pulsed laser deposition and magnetron sputtering. On-chip optical isolator devices with polarization diversity in the telecommunication band have been demonstrated based on these materials. This article reviews the latest technological breakthroughs in MO oxide material growth as well as device design and integration strategies toward practical implementation of on-chip optical isolation.

This article overviews the current status of magnetocaloric materials for room-temperature refrigeration. We discuss the underlying mechanism of the magnetocaloric effect and illustrate differences between first- and second-order type materials starting with gadolinium as a reference system. Beyond the key functional properties of magnetocaloric materials, the adiabatic temperature, and entropy change, we briefly address the criticality of the most promising materials in terms of their supply risk. Looking at practical applications, suitable geometries and processing routes for magnetocaloric heat exchangers for device implementation are introduced.

Conventional electron microscopy during the last three decades has experienced tremendous developments, especially in equipment design and engineering, to become one of the most widely recognized and powerful tools for key research areas in materials science and nanotechnology. In this article, we discuss scanning ultrafast electron microscopy (S-UEM) as a new methodology for four-dimensional electron imaging of material surfaces. We also illustrate a few unique applications. By monitoring secondary electrons emitted from surfaces of photoactive materials, photo- and electron-impact-induced electrons and holes near surfaces, interfaces, and heterojunctions can be imaged with adequate spatial and temporal resolution. Charge separation, transport, and anisotropic motions as well as their dependence on carrier energies can be resolved. S-UEM is poised to directly image and visualize relevant interfacial dynamics in real space and time for emerging optoelectronic devices and help push their performance.

Dynamic random-access memory (DRAM) is the main memory in most current computers. The excellent scalability of DRAM has significantly contributed to the development of modern computers. However, DRAM technology now faces critical challenges associated with further scaling toward the ∼10-nm technology node. This scaling will likely end soon because of the inherent limitations of charge-based memory. Much effort has been dedicated to delaying this. Novel cell architectures have been designed to reduce the cell area, and new materials and process technologies have been extensively investigated, especially for dielectrics and electrodes related to charge storage. In this article, the current issues, recent progress in and the future of DRAM materials, and fabrication technologies are discussed.

Piezoelectric ceramics generate strain through the intrinsic piezoelectric effect, the motion of ferroelectric domain walls, or through field-induced phase transitions. The enhanced piezoelectric properties observed in morphotropic solid solutions arise from several distinct, but interrelated, mechanisms associated with the near degeneration of the energy surface from cubic to spherical symmetry. The phenomenological theory of ferroelectricity is used to explain the thermodynamic origins of strain generation mechanisms in these solid solutions. The displacement of ferroelectric domain walls is an extrinsic contribution to the piezoelectric response that can be controlled by modifying the host material with small concentrations of dopants. The concept of “hardening” is introduced; hardening can be useful in applications where piezoelectric energy conversion and low energy loss are more important than large strain. The operative mechanisms of strain generation and energy conversion in technologically important lead-based and lead-free piezoelectric materials are summarized.

Heat-assisted magnetic recording (HAMR) is the next-generation technology that is required to deliver areal densities in excess of 2 terabit/in2 for high-capacity, low-cost hard drives.The recording process relies on spatially and temporally localized heating of the media surface to lower its coercivity during the magnetic writing process. This scheme drives substantial changes to the recording head write element architecture, combining the conventional electromagnet structure with integrated optical light delivery layers, focusing optics, and plasmonic nanostructures to generate subwavelength optical spots. Power losses associated with the strong optical fields required for heating the media can cause local temperatures in excess of 400°C at the recording head surface. Coupled with high pressures, an oxidative/corrosive air-bearing environment, and a sub-3 nm head-media spacing, this introduces new challenges for the functional materials in recording heads required to balance performance and long-term reliability demands. Here, we briefly review specific challenges associated with HAMR heads, highlighting the major requirements, failure modes, and needed innovations for the near-field transducer and optical-waveguide subsystems.

The expansion of programmatically accessible materials data has cultivated opportunities for data-driven approaches. Workflows such as the Automatic Flow Framework for Materials Discovery not only manage the generation, storage, and dissemination of materials data, but also leverage the information for thermodynamic formability modeling, such as the prediction of phase diagrams and properties of disordered materials. In combination with standardized parameter sets, the wealth of data is ideal for training machine-learning algorithms, which have already been employed for property prediction, descriptor development, design rule discovery, and the identification of candidate functional materials. These methods promise to revolutionize the path to synthesis, and ultimately transform the practice of traditional materials discovery to one of rational and autonomous materials design.

Magnetocaloric heat pumps (MHPs) use the solid-state magnetocaloric effect (MCE) to move heat from cold to hot using an intermediate heat-transfer fluid. Work input is required to drive the MCE via a change in a magnetic field. Work input is also required to drive the heat-transfer fluid flow. Thus design of a MHP involves the coupling of materials, magnetics, heat transfer, and fluid flow. We discuss design principles and operational devices that have brought this technology toward technical feasibility, and the approaches to overcome remaining hurdles to commercial feasibility.

Historically, batteries with lithium metal anodes have been a hazard, as the lithium becomes rough and eventually finely divided during cycling. The promise of higher energy density, however, continues to drive the search for novel approaches to manage this light and reactive material. Significant improvement has been achieved by designing new liquid-electrolyte compositions and interface barriers to stabilize the lithium in traditional batteries, but it is clear that solid-state batteries ensure a higher level of safety and perhaps higher energy density and lifetimes. The materials challenge then is to fabricate a cost-effective solid electrolyte that effectively maintains lithium as a dense uniform metal layer. This article describes the ideal cycling behavior of lithium and progress toward this goal of a solid electrolyte using glassy, ceramic, polymer, and composite electrolytes, as well as the challenges that continue to arise toward long-term, high-rate, and efficient cycling of lithium metal.

Dealloying has evolved from a problematic corrosion process to a versatile tool for scalable fabrication of nanostructured metals. While the original, and majority of, work in the area has focused on electrochemical dealloying, a powerful variation of dealloying—liquid metal dealloying—has recently gained popularity. This process relies on a melt to carry out selective dissolution, replacing the traditional electrolyte solution. While electrolytes and molten metals are both suitable dealloying media, they can lead to very different morphologies. In this article, we compare and contrast what is known about the microscale physics and chemistry controlling microstructural evolution in electrochemical and liquid metal dealloying. We conclude that the core phenomenology of porosity evolution—a competition between dissolution and interface diffusion—is similar in both dealloying processes, but that the relative magnitudes of these two processes control interfacial pattern formation.

Third-generation semiconductors, such as ZnO and GaN, exhibit strong piezoelectric polarization due to the lack of inversion symmetry. The piezotronic effect observed in these semiconductors was proposed for tuning carrier transport in electronic devices by utilizing the induced piezoelectric potential as a virtual gate. This novel concept allows effective interactions between micro-/nanoelectronic devices and external mechanical stimuli. Piezotronics provide a promising approach for designing future electronic devices beyond Moore’s Law with potential for developing smart sensors, environment monitoring systems, human–machine interaction elements, and other transducers. In this article, we review recent progress in piezotronics using one-dimensional materials, heterojunctions, and large-scale arrays. We provide guidance for future piezotronic devices based on these materials.

In the quest for dynamic multimodal probing of a material’s structure and functionality, it is critical to be able to quantify the chemical state on the atomic-/nanoscale using element-specific electronic and structurally sensitive tools such as electron energy-loss spectroscopy (EELS). Ultrafast EELS, with combined energy, time, and spatial resolution in a transmission electron microscope, has recently enabled transformative studies of photoexcited nanostructure evolution and mapping of evanescent electromagnetic fields. This article aims to describe state-of-the-art experimental techniques in this emerging field and its major uses and future applications.

Ferroelectrics are promising for nonvolatile memories. However, the difficulty of fabricating ferroelectric layers and integrating them into complementary metal oxide semiconductor (CMOS) devices has hindered rapid scaling. Hafnium oxide is a standard material available in CMOS processes. Ferroelectricity in Si-doped hafnia was first reported in 2011, and this has revived interest in using ferroelectric memories for various applications. Ferroelectric hafnia with matured atomic layer deposition techniques is compatible with three-dimensional capacitors and can solve the scaling limitations in 1-transistor-1-capacitor (1T-1C) ferroelectric random-access memories (FeRAMs). For ferroelectric field-effect-transistors (FeFETs), the low permittivity and high coercive field Ec of hafnia ferroelectrics are beneficial. The much higher Ec of ferroelectric hafnia, however, makes high endurance a challenge. This article summarizes the current status of ferroelectricity in hafnia and explains how major issues of 1T-1C FeRAMs and FeFETs can be solved using this material system.

Research in thermoelectric (TE) quantum structures was greatly propelled by the prediction in the early 1990s of a significant boost in TE efficiency by quantum size effects. Recently, research interest has shifted from quantum size effects in conventional semiconductors toward new types of quantum materials (e.g., topological insulators [TIs], Weyl and Dirac semimetals) characterized by their nontrivial electronic topology. Bi2Te3, Sb2Te3, and Bi2Se3, established TE materials, are also TIs exhibiting a bulk bandgap and highly conductive and robust gapless surface states. The signature of the nontrivial electronic band structure on TE transport properties can be best verified in transport experiments using nanowires and thin films. However, even in nanograined bulk, the typical peculiarities in the transport properties of TIs can be seen. Finally, the remarkable transport properties of Dirac and Weyl semimetals are discussed.

Optical isolators and circulators are important elements in many photonic systems. These nonreciprocal devices are typically made of bulk optical components and are difficult to integrate with other elements of photonic integrated circuits. This article discusses the best performance for waveguide isolators and circulators achieved with heterogeneous bonding. By virtue of the bonding technology, the devices can make use of a large magneto-optical effect provided by a high-quality single-crystalline garnet grown in a separate process on a lattice-matched substrate. In a silicon-on-insulator waveguide, the low refractive index of the buried oxide layer contributes to the large penetration of the optical field into a magneto-optical garnet used as an upper-cladding layer. This enhances the magneto-optical phase shift and contributes greatly to reducing the device footprint and the optical loss. Several versions of silicon waveguide optical isolators and circulators, both based on the magneto-optical phase shift, are demonstrated with an optical isolation ratio of ≥30 dB in a wavelength band of 1550 nm. Furthermore, the isolation wavelength can be effectively tuned over several tens of nanometers.

Highly sensitive magnetic field sensors using magnetoelectric (ME) bulk and thin-film composites consisting of magnetostrictive and piezoelectric phases are discussed. Examples include PZT (Pb(ZrxTi1–x)O3) fibers and AlN as the piezoelectric component and amorphous magnetostrictive material, respectively, or their multilayers. Additionally, self-organized ME composites are discussed. These ME sensors offer a passive (consuming little to no power) nature, high sensitivities, large effect enhancements at mechanical resonances, and large linear dynamic ranges. At mechanical resonance, limits of detection in the fT/Hz1/2 range can be achieved. Below the mechanical resonance frequency, the sensitivity can be enhanced through frequency conversion using alternating current magnetic or electric fields or by using magnetic field-induced changes of the elastic properties, the delta-E effect, where E represents Young’s modulus. Noise floors of about 1–100 pT/Hz1/2 at a frequency of f = 1 Hz can be obtained depending on the sensor size and the operational mode. For applications in unshielded environments, approaches to suppress acoustic and vibrational cross-sensitivities are presented.

Data are a crucial raw material of this century. The amount of data that have been created in materials science thus far and that continues to be created every day is immense. Without a proper infrastructure that allows for collecting and sharing data, the envisioned success of big data-driven materials science will be hampered. For the field of computational materials science, the NOMAD (Novel Materials Discovery) Center of Excellence (CoE) has changed the scientific culture toward comprehensive and findable, accessible, interoperable, and reusable (FAIR) data, opening new avenues for mining materials science big data. Novel data-analytics concepts and tools turn data into knowledge and help in the prediction of new materials and in the identification of new properties of already known materials.

This article discusses recent studies of on-chip integration of a plasmonic isolator on a Si substrate and a hybrid isolator on an InP substrate. The key characteristics of the plasmonic isolator are reviewed and future prospects are discussed. A method to enhance the magneto-optical figure of merit (FOM) and reduce the propagation loss of a surface plasmon in order to realize the proposed design of the plasmonic isolator is described. One hundred percent enhancement of the FOM and 20× reduction of propagation loss in the optimized ferromagnetic plasmonic structure are experimentally demonstrated.

Nanoporous metals made by dealloying are macroscopic network architectures that can contain ∼1015 nanoscale struts or ligaments per sample. Their mechanical performance is critical to their applications as functional or lightweight high-strength materials. Testing nanoporous metals at the macroscopic scale offers opportunities for unraveling the properties of nanoscale solids in general. The central questions in this area include whether the macroscopic strength and elastic modulus of nanoporous metals can be correlated with the properties of nanoscale ligaments by the classical Gibson–Ashby equations, whether the dealloying-made network structure differs from the conventional foam metals, how network connectivity influences mechanical response, and how ligament size and surface properties affect the elastic and plastic response of nanoscale solids and that of nanoporous metals, particularly the tension–compression asymmetry in strength. This article reviews the fundamental observations of the mechanical response of nanoporous metals with a focus on gold and the emerging understanding of the aforementioned issues.

Ultrafast measurement technology provides essential contributions to our understanding of the properties and functions of solids and nanostructures. Atomic-scale vistas with ever-growing spatial and temporal resolution are offered by methods based on short pulses of x-rays and electrons. Time-resolved electron diffraction and microscopy are among the most powerful approaches to investigate nonequilibrium structural dynamics. In this article, we discuss recent advances in ultrafast electron imaging enabled by significant improvements in the coherence of pulsed electron beams. Specifically, we review the development and first application of ultrafast low-energy electron diffraction for the study of structural dynamics at surfaces, and discuss novel opportunities for ultrafast transmission electron microscopy facilitated by laser-triggered field-emission sources. These and further developments will render coherent electron beams an essential component in future ultrafast nanoscale imaging.