Low field controllable continuous spin switching in the thulium-ytterbium single crystal.

We report the spin reorientation transition (SRT) and the low field controllable continuous spin switching (SSW) of the Tm0.75Yb0.25FeO3 (TYFO) single crystal in this study. The SRT, characterized by the transition from Γ2(Fx, Cy, Gz)-Γ4(Gx, Ay, Fz), occurs within the temperature range of 20-27 K. Under an external magnetic field of 50 Oe, the SSW occurs along the c-axis at approximately 98 K due to the reversal of Tm3+ magnetic moment induced by the magnetic coupling change between Tm3+ and Fe3+, transitioning from a parallel to an antiparallel alignment. Notably, a continuous SSW is observed along the a-axis at low temperatures, which has not been previously reported in rare earth orthoferrites. This unique behavior can be easily manipulated by low magnetic fields within the temperature range of 2-20 K. Both the spin reorientation transition and spin switching phenomena in the TYFO single crystal arise from interactions between rare earth ions and iron ions and can be effectively regulated by applied low magnetic fields, making it a promising material for low-field spin devices.

Site-Selective Assembly of Centimeter-Scale Arrays of Precisely Oriented Magnetic Nanoellipsoids.

The precise organization and orientation of anisotropic nanoparticles (NPs) on substrates over a large area is key to the application of NP assemblies in functional optical, electronic, and magnetic devices, but achieving such high-precision NP assembly still remains challenging. Here, we demonstrate the site-selective assembly of magnetic nanoellipsoids into large-area precisely positioned, orientationally controlled arrays via a combination of chemical patterning and magnetic manipulation. Magnetic ellipsoidal NPs are selectively positioned on predetermined chemical patterns with high fidelity through electrostatic interactions and aligned uniformly in line with an applied magnetic field. The position, orientation, and interparticle spacing of the ellipsoids can be precisely tuned by controlling the chemical patterns and magnetic field. This approach is simple to implement and can generate centimeter-scale arrays in high yield (up to 99%). The arrays exhibit collective magnetic responses that are dependent on the orientation of the ellipsoids. This work offers a tool for the fabrication of precisely engineered arrays of anisotropic NPs for applications such as metasurface and artificial spin ice.

Tunable Noninteger Flux Quantum of Vortices in Superconducting Strips.

Flux quantization has been widely regarded as the hallmark of the macroscopic quantum state of superconductivity. However, practical design of superconductor devices exploiting finite size confinement effects may induce exotic phenomena, including nonquantized vortices. In our research, the magnetic flux of vortices has been studied in a series of superconducting strips as a function of the strip width and the penetration depth. In both circumstances, the observation using scanning Hall probe microscope (SHPM) displays a controlled evolution from singly quantized vortices to nonquantized ones. It is also found that the magnetic flux is immune to the flowing supercurrent. The simulations based on Ginzburg-Landau theory agree well with experimental results. The observed behavior of the vortex flux may open new perspectives for fundamental research and applications based on vortex matter, such as vortex-memory devices and magnetic field traps for ultracold atoms.

Innovative ochre processing and tool use in China 40,000 years ago.

Homo sapiens was present in northern Asia by around 40,000 years ago, having replaced archaic populations across Eurasia after episodes of earlier population expansions and interbreeding1-4. Cultural adaptations of the last Neanderthals, the Denisovans and the incoming populations of H. sapiens into Asia remain unknown1,5-7. Here we describe Xiamabei, a well-preserved, approximately 40,000-year-old archaeological site in northern China, which includes the earliest known ochre-processing feature in east Asia, a distinctive miniaturized lithic assemblage with bladelet-like tools bearing traces of hafting, and a bone tool. The cultural assembly of traits at Xiamabei is unique for Eastern Asia and does not correspond with those found at other archaeological site assemblages inhabited by archaic populations or those generally associated with the expansion of H. sapiens, such as the Initial Upper Palaeolithic8-10. The record of northern Asia supports a process of technological innovations and cultural diversification emerging in a period of hominin hybridization and admixture2,3,6,11.

Low field control of spin switching and continuous magnetic transition in an ErFeO3 single crystal.

The magnetic behavior of a rare-earth orthoferrite ErFeO3 single crystal can be controlled by low magnetic fields from a few to hundreds of Oe. Here we investigated a high-quality ErFeO3 single crystal in the temperature range of 5-120 K, with two types of spin switching in the field-cooled-cooling (FCC) and field-cooled-warming (FCW) processes below the temperature of the spin reorientation (SR) transition from Γ4 to Γ2 at 98-88 K. The magnitude of the applied magnetic fields can regulate two types of spin switching along the a-axis of the ErFeO3 single crystal but does not affect the type and temperature range of the SR transition. An interesting "multi-step" type-II spin switching is observed in FCW under low magnetic fields (H < 18 Oe) just below the SR transition temperature, which is associated with the interaction and the change of magnetic configurations from rare-earth and iron magnetic sublattices. When the magnetic field is lower than 15 Oe, the type-II spin switching in the FCW process gradually changes to a continuous magnetic transition along the a-axis of the ErFeO3 single crystal. As the magnetic field is reduced to less than 17 Oe, the type-I spin switching in the FCW process also transforms into a continuous magnetic transition. Understanding the magnetic reversal effects will help us explore the potential applications of these magnetic materials for future information devices.

Evolution of Superconducting Properties in Fe1.1Se0.8Te0.2 Films Before and After Structure Avalanche.

By preparing a series of high-quality Fe1.1Se0.8Te0.2 films on the CaF2 substrate via pulsed laser deposition, we reveal the evolution of the structure as well as the superconductivity with the film thickness. We have found that there exists a threshold thickness above which the critical temperature Tc reaches its optimal value of 23.18 K with large activation energy, promising for high-field technological applications. Most importantly, the thick films have been found in a metastable state due to the fragile balance between the increased strain energy and the large compressive stress. Once the balance is broken by an external perturbation, a unique structure avalanche happens with a large part of the film exfoliated from the substrate and curves out. The exfoliated part of the film remains a single phase, with its lattice parameter and Tc recovering the bulk values. Our results clearly demonstrate the close relation between the compressive stress of the film/substrate interface and the high critical temperature observed in FeSeTe films. Moreover, this also provides an efficient way to fabricate free-standing single-phase FeSeTe crystals in the phase-separation regime.

Emergence of exchange bias field in FeS superconductor with cobalt-doping.

Among all the iron-based superconductors, the 11 series has the simplest layered structure but exhibits rich physical phenomenon. In this work, we have synthesized Fe1-xCoxS single crystals with tetragonal structure and studied their structure and magnetic properties. Magnetic susceptibility measurements indicate that the cobalt doping would suppress superconductivity and even introduce weak ferromagnetism besides antiferromagnetism. Scanning electron microscopy study reveals that the Co-doped samples exhibit intrinsic phase separation. Moreover, magnetic force microscopy measurement shows no magnetic domain in Fe1-xCoxS, indicating that neither phase is pure ferromagnetic. The coexistence of ferromagnetism and antiferromagnetism leads to the relatively large exchange bias field. Since the exchange bias effect has been widely used in the field of information storage, spin-valves, and magnetic tunnel junctions, our study provides another option for further application.

Spin-orbit coupling in magnetoelectric Ba3(Zn1-xCox)2Fe24O41 hexaferrites.

The Z-type hexaferrites Ba3(Zn1-xCox)2Fe24O41 (x = 0.2, 0.4, 0.6, 0.8, defined as Z1-Z4) were synthesized by a sol-gel method. With increasing cobalt concentration, the origin of magnetoelectric (ME) coupling and the effects of crystal parameters, occupation of ions, and magnetocrystalline anisotropy (MCA) on ME current were studied systematically. The mechanism of magnetic phase transition, revealing the evolution of the magnetic order in the temperature range of 10-400 K, was discussed in detail. Our results suggest that the ferroelectricity of Z1-Z4 originates from both inverse Dzyaloshinskii Moriya (DM) interaction and p-d hybridization mechanism. In particular the ME coupling property is only dominated by p-d hybridization with spin-orbit coupling. This study provides an effective way to improve the ME coupling property of hexaferrites, which have potential applications in the design of new electronic devices.

Direct Observation of Nanoscale Light Confinement without Metal.

Manipulation of light below the diffraction limit forms the basis of nanophotonics. Metals can confine light at the subwavelength scale but suffer from high loss of energy. Recent reports have theoretically demonstrated the possibility of light confinement below the diffraction limit using transparent dielectric metamaterials. Here, nanoscale light confinement (<λ/20) in transparent dielectric materials is shown experimentally through a luminescent nanosystem with rationally designed dielectric claddings. Theoretically, green light with a wavelength of 540 nm has a transmission of 98.8% when passing through an ultrathin NaYF4 /NaGdF4 superlattice cladding (thickness: 6.9 nm). Unexpectedly, the complete confinement of green emission (540 nm) by such an ultrathin dielectric cladding is directly observed. FDTD calculations are used to confirm that the ultrathin dielectric cladding has negligible influence on the transmission of propagating light, but extraordinary confinement of evanescent waves. This will provide new opportunities for nanophotonics by completely averting the use of metals.

Tunable and switchable magnetic dipole patterns in nanostructured superconductors.

Design and manipulation of magnetic moment arrays have been at the focus of studying the interesting cooperative physical phenomena in various magnetic systems. However, long-range ordered magnetic moments are rather difficult to achieve due to the excited states arising from the relatively weak exchange interactions between the localized moments. Here, using a nanostructured superconductor, we investigate a perfectly ordered magnetic dipole pattern with the magnetic poles having the same distribution as the magnetic charges in an artificial spin ice. The magnetic states can simply be switched on/off by applying a current flowing through nanopatterned area. Moreover, by coupling magnetic dipoles with the pinned vortex lattice, we are able to erase the positive/negative poles, resulting in a magnetic dipole pattern of only one polarity, analogous to the recently predicted vortex ice. These switchable and tunable magnetic dipole patterns open pathways for the study of exotic ordering phenomena in magnetic systems.

Controlled Generation of Quantized Vortex-Antivortex Pairs in a Superconducting Condensate.

Quantized vortices, as topological defects, play an important role in both physics and technological applications of superconductors. Normally, the nucleation of vortices requires the presence of a high magnetic field or current density, which allow the vortices to enter from the sample boundaries. At the same time, the controllable generation of individual vortices inside a superconductor is still challenging. Here, we report the controllable creation of single quantum vortices and antivortices at any desirable position inside a superconductor. We exploit the local heating effect of a scanning tunneling microscope (STM) tip: superconductivity is locally suppressed by the tip and vortex-antivortex pairs are generated when supercurrent flows around the hot spot. The experimental results are well-explained by theoretical simulations within the Ginzburg-Landau approach.

Nanoscale assembly of superconducting vortices with scanning tunnelling microscope tip.

Vortices play a crucial role in determining the properties of superconductors as well as their applications. Therefore, characterization and manipulation of vortices, especially at the single-vortex level, is of great importance. Among many techniques to study single vortices, scanning tunnelling microscopy (STM) stands out as a powerful tool, due to its ability to detect the local electronic states and high spatial resolution. However, local control of superconductivity as well as the manipulation of individual vortices with the STM tip is still lacking. Here we report a new function of the STM, namely to control the local pinning in a superconductor through the heating effect. Such effect allows us to quench the superconducting state at nanoscale, and leads to the growth of vortex clusters whose size can be controlled by the bias voltage. We also demonstrate the use of an STM tip to assemble single-quantum vortices into desired nanoscale configurations.

Bound vortex dipoles generated at pinning centres by Meissner current.

One of the phenomena that make superconductors unique materials is the Meissner-Ochsenfeld effect. This effect results in a state in which an applied magnetic field is expelled from the bulk of the material because of the circulation near its surface of resistance-free currents, also known as Meissner currents. Notwithstanding the intense research on the Meissner state, local fields due to the interaction of Meissner currents with pinning centres have not received much attention. Here we report that the Meissner currents, when flowing through an area containing a pinning centre, generate in its vicinity two opposite sense current half-loops producing a bound vortex-antivortex pair, which eventually may transform into a fully developed vortex-antivortex pair ultimately separated in space. The generation of such vortex dipoles by Meissner currents is not restricted to superconductors; similar topological excitations may be present in other systems with Meissner-like phases.