Discovery of Deactivation Phenomenon in NiCo2 S4 /NiS2 Electromagnetic Wave Absorbent and Its Reactivation Mechanism.

Over the past century, extensive research has been carried out on various types of microwave absorption (MA) materials, primarily emphasizing mechanism, performance, and even toward smart device. However, the deactivation, a crucial concern for practical applications, has long been long-neglected. In this work, an in-depth exploration of the deactivation mechanism reveals a significant competition between metal and oxygen, leading to the replacement of the S-M (M = Ni and Co) bond by a new S─O bond on the surface of absorber. This substitution initiates a series of collapse effect that introduces additional defective sites and diminishes the potential for charge transport. Subsequently, passive and active anti-deactivation strategies are developed to target the deactivation. The passive strategy involved intentionally creating electron-deficient structures at the initial Ni and Co sites in the crystal through the Fe doping engineering, with the objective of preventing the generation of S─O bonds. Furthermore, the active anti-deactivation strategy allows for the precise control of absorber deactivation and reactivation by employing accelerated thermodynamic and kinetic methods, enabling a reversible transformation of S-M through competitive reactions with S─O bonds. Finally, a fast deactivation and reactivation method is first proposed promising to stimulate further innovations and breakthroughs in practical applications.

Advancements in Microwave Absorption Motivated by Interdisciplinary Research.

Microwave absorption materials (MAMs) are originally developed for military purposes, but have since evolved into versatile materials with promising applications in modern technologies, including household use. Despite significant progress in bench-side research over the past decade, MAMs remain limited in their scope and have yet to be widely adopted. This review explores the history of MAMs from first-generation coatings to second-generation functional absorbers, identifies bottlenecks hindering their maturation. It also presents potential solutions such as exploring broader spatial scales, advanced characterization, introducing liquid media, utilizing novel toolbox (machine learning, ML), and proximity of lab to end-user. Additionally, it meticulously presents compelling applications of MAMs in medicine, mechanics, energy, optics, and sensing, which go beyond absorption efficiency, along with their current development status and prospects. This interdisciplinary research direction differs from previous research which primarily focused on meeting traditional requirements (i.e., thin, lightweight, wide, and strong), and can be defined as the next generation of smart absorbers. Ultimately, the effective utilization of ubiquitous electromagnetic (EM) waves, aided by third-generation MAMs, should be better aligned with future expectations.

Ultrasonic Field Induces Better Crystallinity and Abundant Defects at Grain Boundaries to Develop CuS Electromagnetic Wave Absorber.

Ultrasonic field (USF) is widely used to regulate the intrinsic properties of materials that are not applied in electromagnetic wave (EMW) absorption. One reason is that the lack of a response mechanism for the materials to USF hinders the expansion of their EMW absorption performance. Therefore, to address this issue, a series of CuS nanoparticles with diverse anions are constructed in the presence or absence of USF. The ultrasonic-induced cavitation effect can significantly promote CuS crystallization and lead to the accumulation of S defects at the grain boundaries (GBs). Furthermore, the S defects at the GBs are easily oriented and arranged, allowing the polarization relaxation retention to be maintained at 10 wt%. Consequently, the CuS with a nitrate precursor under USF shows an optimum effective absorption bandwidth (EAB) of 10.24 GHz at a thickness of 3.5 mm, which is 228.6% more than that without the USF. CuS with a chloride precursor also achieves an EAB of 3.92 GHz, even at a considerably low filler ratio. Thus, this study demonstrates the response mechanism of diverse anions to the USF for the first time and provides a novel technique to optimize the EMW absorption performance of semiconductors.

Regulating conduction and polarization losses by adjusting bonded N in N-doped Cu/CuO/C composites.

Conduction and polarization losses are the main forms of dielectric loss, and regulating these mechanisms is key to obtaining favorable electromagnetic wave absorption performance. In this study, the conversion of graphite N and pyridine N in Cu-based metal-organic framework (MOF)-derived composites was adopted to modulate conduction and polarization losses by tuning the pyrolysis temperature and Cu salt concentration. The results show that increasing the pyrolysis temperature facilitates the conversion of pyridine N to graphite N, which is beneficial for conduction loss. Moreover, increasing the Cu concentration promotes the transformation of pyridine N to graphite N as well as, and then promotes the reverse conversion of graphite N to pyridine N, which is conducive to defect-induced polarization. The unique layered Cu/CuO/C composite obtained at 700 °C with a moderate Cu content exhibited the optimal performance with an effective absorption bandwidth of 5.5 GHz (11.6 âˆ¼ 17.1 GHz) at an ultra-thin thickness of 1.56 mm. This is owed to its favorable impedance matching, significant conduction loss, and polarization loss (defect-induced polarization and interfacial polarization). This study provides a novel strategy for regulating conduction and polarization losses.

Strain-relaxation induced transverse resistivity anomaly in epitaxial films through lithography engineering.

The exotic transverse resistivity in magnetic materials has received intense research because of possible new emergent physics. Here, we report the strain-relaxation induced transverse resistivity anomaly in Mn2CoAl epitaxial films through lithography engineering. The anomalous Hall resistivityρxyAHdecreases from 0.48 to 0.17µΩ cm at 10 K when the widths of the Hall bar decreases from 40 to 1 µm, and the temperature dependence ofρxyAHreverses for the 1.4 µm deep-etched samples. Importantly, Hall resistivity anomalies appear in the 1µm-wide and 1.4µm-wide deep-etched Hall bar samples, which can be well explained by the two-channel transport mechanism. We believe that these observations can be attributed to the strain relaxation effect when the Hall bar width is narrowed to around 1 µm. Our work shows that the induced strain relaxation can possibly lead to the alternation of the materials' electronic structure, and the size effect should be considered when the sample size is reduced to about 1 µm.

Roles of Joule heating and spin-orbit torques in the direct current induced magnetization reversal.

Current-induced magnetization reversal via spin-orbit torques (SOTs) has been intensively studied in heavy-metal/ferromagnetic-metal/oxide heterostructures due to its promising application in low-energy consumption logic and memory devices. Here, we systematically study the function of Joule heating and SOTs in the current-induced magnetization reversal using Pt/Co/SmOx and Pt/Co/AlOx structures with different perpendicular magnetic anisotropies (PMAs). The SOT-induced effective fields, anisotropy field, switching field and switching current density (Jc) are characterized using electric transport measurements based on the anomalous Hall effect and polar magneto-optical Kerr effect (MOKE). The results show that the current-generated Joule heating plays an assisted role in the reversal process by reducing switching field and enhancing SOT efficiency. The out-of-plane component of the damping-like-SOT effective field is responsible for the magnetization reversal. The obtained Jc for Pt/Co/SmOx and Pt/Co/AlOx structures with similar spin Hall angles and different PMAs remains roughly constant, revealing that the coherent switching model cannot fully explain the current-induced magnetization reversal. In contrast, by observing the domain wall nucleation and expansion using MOKE and comparing the damping-like-SOT effective field and switching field, we conclude that the current-induced magnetization reversal is dominated by the depinning model and Jc also immensely relies on the depinning field.

Current-Induced Domain Wall Motion and Tilting in Perpendicularly Magnetized Racetracks.

The influence of C insertion on Dzyaloshinskii-Moriya interaction (DMI) as well as current-induced domain wall (DW) motion (CIDWM) and tilting in Pt/Co/Ta racetracks is investigated via a magneto-optical Kerr microscope. The similar DMI strength for Pt/Co/Ta and Pt/Co/C/Ta samples reveals that DMI mainly comes from the Pt/Co interface. Fast DW velocity around tens of m/s with current density around several MA/cm2 is observed in Pt/Co/Ta. However, it needs double times larger current density to reach the same magnitude in Pt/Co/C/Ta, indicating DW velocity is related to the spin-orbit torque efficiency and pinning potential barrier. Moreover, in CIDWM, DW velocity is around 103 times larger than that in field-induced DW motion (FIDWM) with current-generated effective field keeping the same magnitude as applied magnetic field, revealing that the current-generated Joule heating has an influence on DW motion. Interestingly, current-induced DW tilting phenomenon is observed, while this phenomenon is absent in FIDWM, demonstrating that the current-generated Oersted field may also play an essential role in DW tilting. These findings could provide some designing prospects to drive DW motion in SOT-based racetrack memories.

Magnetization switching through domain wall motion in Pt/Co/Cr racetracks with the assistance of the accompanying Joule heating effect.

Heavy metal/ferromagnetic layers with perpendicular magnetic anisotropy (PMA) have potential applications for high-density information storage in racetrack memories and nonvolatile magnetic random access memories. In these devices, deterministic magnetization switching has been achieved via electric current induced spin orbital torques (SOTs) with the assistance of a current directional external in-plane bias field, which causes technological obstacles for the real application of SOT based spintronic devices. Here, we report that reversible field-free magnetization switching could be achieved via current-driven domain wall motion (DWM) in Pt/Co/Cr micro-sized racetracks with PMA owing to the preformation of the homochiral Néel-type domain wall, in which an in-plane inherent Dzyaloshinskii-Moriya interaction field was generated acting as the external in-plane bias field to break the symmetry. A full magnetization switching can be realized in this device based on the enhanced SOTs from a dedicated design of Pt/Co/Cr structures with Pt and Cr showing opposite signs of spin Hall angles. Therefore, the generated spin currents are expected to work in concert to improve the SOTs. We also demonstrated that the simultaneously accompanying Joule heating effect also plays a key role in the field-free magnetization switching process, including the propagation field as well as the domain wall motion velocity.