Intrinsic and extrinsic dopings in epitaxial films MnBi2Te4.

The intrinsic antiferromagnetic topological insulator MnBi2Te4and members of its family have been the subject of theoretical and experimental research, which has revealed the presence of a variety of defects and disorders that are crucial in determining the topological and magnetic properties. This also brings about challenges in realizing the quantum states like the quantum anomalous Hall and the axion insulator states. Here, utilizing cryogenic magnetoelectric transport and magnetic measurements, we systematically investigate the effects arising from intrinsic doping by antisite defects and extrinsic doping by Sb in MnBi2Te4epitaxial films grown by molecular beam epitaxy. We demonstrate that the nonequilibrium condition in epitaxy allows a wide growth window for optimizing the crystalline quality and defect engineering. While the intrinsic antisite defects caused by the intermixing between Bi and Mn can be utilized to tune the Fermi level position as evidenced by a p-to-n conductivity transition, the extrinsic Sb-doping not only compensates for this doping effect but also modifies the magnetism and topology of the film, during which a topological phase transition is developed. Conflicting reports from the theoretical calculations and experimental measurements in bulk crystals versus epitaxial films are addressed, which highlights the intimate correlation between the magnetism and topology as well as the balance between the Fermi-level positioning and defect control. The present study provides an experimental support for the epitaxial growth of the intrinsic topological insulator and underlines that the topology, magnetism, and defect engineering should be revisited for enabling a steady and reliable film production.

Unusual spin-orbit torque switching in perpendicular synthetic antiferromagnets with strong interlayer exchange coupling.

Synthetic antiferromagnet (SAF) is an outstanding system for controlling magnetic coupling via tuning the layer thickness and material composition. Here, we control the interlayer exchange coupling (IEC) in a perpendicularly magnetized SAF Pt/Co/Ir/CoFeB/MgO multilayer, which is tuned by varying the nonmagnetic layer Ir thickness and the magnetic layer Co thickness. And we study the spin-orbit torque (SOT) driven magnetization switching of the SAF. In the SAF with a weak IEC, the SOT-driven switching behavior is similar to that of a single ferromagnet system, which is dominated by the external magnetic field. In contrast, in the SAF with an ultra-strong IEC, the saturation magnetic field is large than 50 kOe, and the SOT-driven switching behavior is decided by the effective magnetic field. The effective field is correlated to the external magnetic field, the IEC field, magnetic moments of CoFeB and Co, and magnetic anisotropy. These results may advance the understanding of SOT switching of perpendicular SAFs and promote the applications of SAFs with low stray fields and lower power in spintronic devices.

Comprehensive Study of the Current-Induced Spin-Orbit Torque Perpendicular Effective Field in Asymmetric Multilayers.

The spin-orbit torques (SOTs) in the heavy metal (HM)/ferromagnetic metal (FM) structure hold promise for next-generation low-power and high-density spintronic memory and logic applications. For the SOT switching of a perpendicular magnetization, an external magnetic field is inevitable for breaking the mirror symmetry, which is not practical for high-density nanoelectronics applications. In this work, we study the current-induced field-free SOT switching and SOT perpendicular effective field (Hzeff) in a variety of laterally asymmetric multilayers, where the asymmetry is introduced by growing the FM layer in a wedge shape. We show that the design of structural asymmetry by wedging the FM layer is a universal scheme for realizing field-free SOT switching. Moreover, by comparing the FM layer thickness dependence of (Hzeff) in different samples, we show that the efficiency (ß =Hzeff/J, J is the current density) is sensitive to the HM/FM interface and the FM layer thickness. The sign of ß for thin FM thicknesses is related to the spin Hall angle (θSH) of the HM layer attached to the FM layer. ß changes its sign with the thickness of the FM layer increasing, which may be caused by the thickness dependence of the work function of FM. These results show the possibility of engineering the deterministic field-free switching by combining the symmetry breaking and the materials design of the HM/FM interface.

ICG-ER: a new probe for photoimaging and photothermal therapy for breast cancer.

Breast cancer is common cancer type with high mortality. There are still inperfections in the traditional diagnosis and treatment methods for cancer. Photoacoustic imaging combines the advantages of high specificity and deep tissue penetration and is especially suitable for early cancer detection and treatment monitoring. With its specificity and noninvasiveness; photothermal therapy has become one of the best representative treatment methods. Indocyanine green (ICG) is a near-infrared imaging reagent approved by the FDA for clinical application, with a potential application for photothermal therapy. ICG has low targeting specificity. Through the combination of EB and ICG, the timeliness of ICG circulation in vivo is improved, and the tumor targeting of ICG-E is improved by using RGD. ICG-ER, an integrated optical probe for diagnosis and treatment, was constructed, and high uptake of ICG-ER by 4T1 cells was observed by flow cytometry and confocal laser scanning microscopy (CLSM). ICG-ER photoacoustic signal intensity is concentration-dependent. In vivo photoacoustic imaging showed that the ICG-ER concentration time in the tumor site was long and reached a peak at 42 hours. Under laser irradiation, the temperature of the tumor site in mice that were injected with ICG-ER reached 56°C. After photothermal treatment, the tumor tissue in the mice showed obvious necrosis and no tumor recurrence, proving that ICG-ER has a good photothermal effect. Based on the above results, ICG-ER can be used in breast cancer optical imaging and photothermal therapy, which is expected to provide new ideas for breast cancer clinical diagnosis and treatment.

Néel-Type Elliptical Skyrmions in a Laterally Asymmetric Magnetic Multilayer.

Magnetic skyrmions, topological-chiral spin textures, have potential applications in next-generation high-density and energy-efficient spintronic devices for information storage and logic technologies. Tailoring the detailed spin textures of skyrmions is of pivotal importance for tuning skyrmion dynamics, which is one of the key factors for the design of skyrmionic devices. Here, the direct observation of parallel aligned elliptical magnetic skyrmions in Pt/Co/Ta multilayers with an oblique-angle deposited Co layer is reported. Domain wall velocity and spin-orbit-torque-induced out-of-plane effective field analysis demonstrate that the formation of unusual elliptical skyrmions is correlated to the anisotropic effective perpendicular magnetic anisotropy energy density (Keff u ) and Dzyaloshinskii-Moriya interaction (DMI) in the film plane. Structural analysis and first-principles calculations further show that the anisotropic Keff u and DMI originate from the interfacial anisotropic strain introduced by the oblique-angle deposition. The work provides a method to tune the spin textures of skyrmions in magnetic multilayers and, thereby, a new degree of freedom for the design of skyrmionic devices.

A novel NIR-II probe for improved tumor-targeting NIR-II imaging.

In this work, we report a novel probe IR-RGD, which possesses a bright emission tail in the NIR-II region along with high quantum yield. Further, IR-RGD has been successfully used for tumor-targeting NIR-II fluorescence imaging with great potential for clinical translation.