Magnetic topological semimetals are increasingly fueling interests in exotic electronic-thermal physics including thermoelectrics and spintronics. To control the transports of topological carriers in such materials becomes a central issue. However, the topological bands in real materials are normally intricate, leaving obstacles to understand the transports in a physically clear way. Parallel to the renowned effective two-band model in magnetic field scale for semiconductors, here, an effective Weyl-band model in temperature scale was developed with pure Weyl state and a few meaningful parameters for topological semimetals. Based on the model, a universal scaling was established and subsequently verified by reported experimental transports. The essential sign regularity of anomalous Hall and Nernst transports was revealed with connection to chiralities of Weyl nodes and carrier types. Upon a double-Weyl model, a concept of Berry-curvature ferrimagnetic structure, as an analogy to the real-space magnetic structure, was further proposed and well described the emerging sign reversal of Nernst thermoelectric transports in temperature scale. Our study offers a convenient tool for scaling the Weyl-fermion-related transport physics, and promotes the modulations and applications of magnetic topological materials in future topological quantum devices.

In 1966, Pierre-Gilles de Gennes proposed a non-volatile mechanism for switching superconductivity on and off in a magnetic device. This involved a superconductor (S) sandwiched between ferromagnetic (F) insulators in which the net magnetic exchange field could be controlled through the magnetisation-orientation of the F layers. Because superconducting switches are attractive for a range of applications, extensive studies have been carried out on $F/S/F$ structures. Although these have demonstrated a sensitivity of the superconducting critical temperature ($T_{c}$) to parallel (P) and antiparallel (AP) magnetisation-orientations of the F layers, corresponding shifts in $T_c$ (i.e., ${\Delta}T_c = T_{c,AP} - T_{c,P}$) are lower than predicted with ${\Delta}T_c$ only a small fraction of $T_{c,AP}$, precluding the development of applications. Here, we report $EuS/Au/Nb/EuS$ structures where EuS is an insulating ferromagnet, Nb is a superconductor and Au is a heavy metal. For P magnetisations, the superconducting state in this structure is quenched down to the lowest measured temperature of 20 mK meaning that ${\Delta}T_c/T_{c,AP}$ is practically 1. The key to this so-called absolute switching effect is a sizable spin-mixing conductance at the $EuS/Au$ interface which ensures a robust magnetic proximity effect, unlocking the potential of $F/S/F$ switches for low power electronics.

AbstractPhase transition is a fundamental phenomenon in condensed matter physics, in which states of matter transform to each other with various critical behaviors under different conditions. The magnetic martensitic transformation features significant multi‐caloric effects that benefit the solid‐state cooling or heat pumping. Meanwhile, the electronic topological transition (ETT) driven by pressure has been rarely reported in martensitic systems. Here, the modulation effects of hydrostatic pressure on phase transitions in a magnetic martensitic alloy are reported. Owing to the huge volume expansion during the transition, the martensitic transition temperature is driven from 339 to 273 K by pressure within 1 GPa, resulting in highly tunable giant baro‐ and magneto‐caloric effects (BCE and MCE) in a wide working temperature range. Interestingly, an ETT is further induced by pressure in the martensite phase, with a sudden drop of the measured saturation magnetization around 0.6 GPa. First‐principles calculations reveal a sharp change in the density of states (DOS) due to the orbit shift around the Fermi level at the same pressure and reproduce the experimental observation of magnetization. Besides, the ETT is accompanied by remarkable changes in the lattice parameters and the unit‐cell orthorhombicity. The study provides insight into pressure‐modulated exotic phase‐transition phenomena in magnetic martensitic systems.

During the phase transitions, diverse states evolve with multiplex phenomena arising from the critical competition. In this study, a displacive martensitic transformation with a lattice shear distortion was unexpectedly observed at the reconstructive phase boundary that usually connects multiple phases without crystallographic relation, in a Ni–Co–Mn–V all-d-metal Heusler alloy system. Experiments and theoretical calculations suggest that the parent phase becomes increasingly unstable when approaching the phase boundary. The lattice-distorted transformation with moderate first-order nature survives due to the critical phase competition from the structural frustration, in which the comparable energy and the diminished formation preference of different phases emerge. In this critical state, the phase selection including the martensitic transformation can be tuned by external fields such as rapid cooling, annealing, and magnetic field. Our research reveals a novel manner to destabilize the parent phase, through which one could attain new functional materials based on the phase transitions.

TiNi-based shape memory alloys have been extensively investigated due to their significant applications, but a comprehensive understanding of the evolution of electronic structure and electrical transport in a system with martensitic transformations (MT) is still lacking. In this work, we focused on the electronic transport behavior of three phases in Ni$_{50-x}$Fe$_{x}$Ti$_{50}$ across the MT. A phase diagram of Ni$_{50-x}$Fe$_{x}$Ti$_{50}$ was established based on x-ray diffraction, calorimetric, magnetic, and electrical measurements. To reveal the driving force of MT, phonon softening was revealed using first-principles calculations. Notably, the transverse and longitudinal transport behavior changed significantly across the phase transition, which can be attributed to the reconstruction of electronic structures. This work promotes the understanding of phase transitions and demonstrates the sensitivity of electron transport to phase transition.

All-d-metal full-Heusler alloys possess superior mechanical properties and high spin polarization, which would play an important role in spintronic applications. Despite this, their electrical and thermal transport properties have not been comprehensively investigated till now. In this work, we present an analysis on the evolution of structural, magnetic and transport properties of Cr- and Ti-substituted Fe2CrV all-d-metal Heusler alloys by combining theoretical calculations and experiments. Both series of alloys crystallize in Hg2CuTi-type structure. With increasing Ti doping, the calculated total magnetic moments of Fe50Cr25V25-xTix decrease linearly. The experimental saturation magnetization is highly consistent with theoretical calculations and Slater-Pauling rule when x < 4, indicating the highly ordered atomic occupation. The magnetization and Curie temperature can be significantly tuned by altering spin polarizations and exchange interactions. The introduction of the foreign atom, Ti, results in a linear increase in residual resistivity, while electron-phonon scattering keeps relatively constant. The maximum values for electrical and thermal transport properties are observed in the stoichiometric Fe2CrV composition.

Nodal chain semimetals protected by nonsymmorphic symmetries are distinct from Dirac and Weyl semimetals, featuring unconventional topological surface states and resulting in anomalous magnetotransport properties. Here, we reveal that ferromagnetic FeIn2S4 is a suitable nodal chain candidate in theory. Centrosymmetric FeIn2S4 with nonsymmorphic symmetries shows half metallicity and clean band crossings with hourglass-type dispersion tracing out nodal lines. Owing to glide mirror symmetries, the nontrivial nodal loops form a nodal chain, which is associated with the perpendicular glide mirror planes. These nodal chains are robust against spin-orbital interaction, giving rise to the coexistence of drumhead-type surface states and closed surface Fermi arcs. Moreover, the nodal loops protected by nonsymmorphic symmetry contribute to large anomalous Hall conductivity and the anomalous Nernst conductivity. Our results provide a platform to explore the intriguing topological state and transverse transport properties in a magnetic system.

The kagome lattice system has been identified as a fertile ground for the emergence of a number of new quantum states,including superconductivity,quantum spin liquids,and topological electronic states.This has attracted significant interest within the field of condensed matter physics.Here,we present the observation of an anomalous Hall effect in an iron-based kagome antiferromagnet LuFe6Sn6,which implies a non-zero Berry curvature in this compound.By means of extensive magnetic measurements,a high Neel temperature,TN=552 K,and a spin reorientation behavior were identified and a simple temperature-field phase diagram was constructed.Furthermore,this compound was found to exhibit a large Sommerfeld coefficient of γ=87 mJ·mol-1·K-2,suggesting the presence of a strong electronic correlation effect.Our research indicates that LuFe6Sn6 is an intriguing compound that may exhibit magnetism,strong correlation,and topological states.

The modulation of topological electronic state by an external magnetic field is highly desired for condensed-matter physics. Schemes to achieve this have been proposed theoretically, but few can be realized experimentally. Here, combining transverse transport, theoretical calculations, and scanning tunneling microscopy/spectroscopy (STM/S) investigations, we provide an observation that the topological electronic state, accompanied by an emergent magneto-transport phenomenon, was modulated by applying magnetic field through induced non-collinear magnetism in the magnetic Weyl semimetal EuB6. A giant unconventional anomalous Hall effect (UAHE) is found during the magnetization re-orientation from easy axes to hard ones in magnetic field, with a UAHE peak around the low field of 5 kOe. Under the reasonable spin-canting effect, the folding of the topological anti-crossing bands occurs, generating a strong Berry curvature that accounts for the observed UAHE. Field-dependent STM/S reveals a highly synchronous evolution of electronic density of states, with a dI/dV peak around the same field of 5 kOe, which provides evidence to the folded bands and excited UAHE by external magnetic fields. This finding elucidates the connection between the real-space non-collinear magnetism and the k-space topological electronic state and establishes a novel manner to engineer the magneto-transport behaviors of correlated electrons for future topological spintronics.

Creating materials that do not exist in nature can lead to breakthroughs in science and technology. Magnetic skyrmions are topological excitations that have attracted great attention recently for their potential applications in low power, ultrahigh density memory. A major challenge has been to find materials that meet the dual requirement of small skyrmions stable at room temperature. Here we meet both these goals by developing epitaxial FeGe films with excess Fe using atomic layer molecular beam epitaxy (MBE) far from thermal equilibrium. Our atomic layer design permits the incorporation of 20% excess Fe while maintaining a non-centrosymmetric crystal structure supported by theoretical calculations and necessary for stabilizing skyrmions. We show that the Curie temperature is well above room temperature, and that the skyrmions have sizes down to 15 nm as imaged by Lorentz transmission electron microscopy (LTEM) and magnetic force microscopy (MFM). The presence of skyrmions coincides with a topological Hall effect-like resistivity. These atomically tailored materials hold promise for future ultrahigh density magnetic memory applications.

We present a study on the magnetocaloric properties of a CaBaCo4O7 polycrystalline cobaltite along with research on the nature of magnetic phase transition. The magnetization as a function of temperature identifies the ferrimagnetic to paramagnetic transition at a Curie temperature of 60 K. Moreover, a Griffiths-like phase is confirmed in a temperature range above T C. The compound undergoes a crossover from the first to second-order ferrimagnetic transformation, as evidenced by the Arrott plots, scaling of the universal entropy curve, and field-dependent magnetic entropy change. The maximum of entropy change is 3 J/kg⋅K for ΔH = 7 T at T C, and a broadening of the entropy peak with increasing magnetic field indicates a field-induced transition above T C. The analysis of the magnetic entropy change using the Landau theory reveals the second-order phase transition and indicates that the magnetocaloric properties of CaBaCo4O7 are dominated by the magnetoelastic coupling and electron interaction. The corresponding values of refrigerant capacity and relative cooling power are estimated to be 33 J/kg and 42 J/kg, respectively.

Nanowire AlGaN III‐nitride LEDs are claimed as potential high‐efficiency solid‐state photon sources spanning to the short‐wavelength deep ultraviolet (UV). Nanowire LEDs (NWLEDs) emitting in the UV are compared with a transparent n‐AlGaN top electrode formed by coalescing the top region of nanowire–ensemble LEDs with commonly employed opaque conformal metallic electrodes used for nanowire‐based devices. The use of a transparent contact results in an increase in the wall plug efficiency of >25×, exceeding the expected increase due to enhanced photon‐extraction efficiency. Increased nanowire connectivity reduces the short‐circuit pathways, enabling higher device yields of relatively large‐area (>1 mm2) UV nanowire–ensemble LEDs. Despite these large relative improvements, the absolute output efficiency remains miniscule (<1 m%). Electroluminescence microscopy demonstrates that <0.1% of nanowires within the ensemble contribute to emission. The single‐nanowire efficiency is estimated and points toward improvement of the homogeneity of the injection current as a crucial step for realizing commercially viable UV NWLEDs.

Kagome materials have been studied intensively in condensed matter physics. With rich properties, various Kagome materials emerge during this process. Here, we grew single crystals of Y 0.5 Fe 3 Sn 3 and confirmed an YCo 6 Ge 6 -type Kagome-lattice structure by detailed crystal structure characterizations. This compound bears an antiferromagnetic ordering at T N = 551 K, and shows a weak ferromagnetism at low temperatures, where an anomalous Hall effect was observed, suggesting the non-zero Berry curvature. With the unstable antiferromagnetic ground state, our systematic investigations make Y 0.5 Fe 3 Sn 3 a potential Kagome compound for Kagome or topological physics.

Magnetic skyrmions are promising for next-generation information storage and processing owing to their potential advantages in data storage density, robustness, and energy efficiency. The magnetic multilayers consisting of Pt, Co, and a third metal element $X$ provide an ideal platform to study the skyrmions due to their highly tunable magnetic properties. Here, we report the observation of room-temperature bubble-like N\'eel skyrmions in epitaxial Pt/Co/Cu multilayers in samples with multidomain states in zero field. The magneto-optic Kerr effect (MOKE) and superconducting quantum interference device (SQUID) magnetometry are applied to investigate the shapes of the hysteresis loops, the magnetic anisotropy, and the saturation magnetization. By tuning the Co thickness and the number of periods, we achieve perpendicular and in-plane magnetized states and multidomain states that are identified by a wasp-waisted hysteresis loop. Skyrmions are directly imaged by magnetic force microscopy (MFM) and Lorentz transmission electron microscopy (LTEM). The development of room-temperature skyrmions in Pt/Co/Cu multilayers may lead to advances in skyrmion-related research and applications.

In recent years, more and more magnetic topological materials, especially magnetic Weyl semimetals, have been discovered, providing a platform for studying the electronic transport behavior. The strong Berry curvature of magnetic topological materials can significantly enhance the conventional transverse transport behaviors, and can also make the transport phenomena that have been overlooked or unobserved appear gradually. In this review, the semi-classical equation is used to understand the anomalous transport behaviors in magnetic topological materials. The intrinsic anomalous Hall conductivity is obtained by integrating the Berry curvature of the occupied states, which is determined by the electronic band structure. The topological electronic state can be modulated by magnetic field and doping, and the anomalous Hall conductivity was changed with the evolution of the Berry curvature. A linear positive magnetoresistance behavior associated with the Berry curvature and magnetic field is introduced, which establishes the relation between the Berry curvature and the longitudinal transport. Due to the presence of tilted Weyl cone, the conductivity terms related to the first power of magnetic field are observed in magnetic Weyl systems. These behaviors under the interaction of topology and magnetic provide a new understanding and insight for the electric transport behaviors. At last, this review also provides a viewpoint on the field of magnetic topological physics.

In-plane transport properties, including anisotropic magnetoresistance (AMR) and planar Hall effect (PHE), are of great interest in electrical transport and spintronic applications. Unconventional transport behavior emerging from the topological physics has been intensively studied and very much desired. In this study, a large AMR of −18% at a very low magnetic field of 0.2 T is observed in a soft magnetic Weyl semimetal EuB6 based on the characteristics of both high magnetization and large magnetoresistance. Furthermore, the intrinsic antisymmetric AMR and PHE are unambiguously observed and interpreted as the modification in conductivity owing to the Berry curvature in a tilted Weyl system instead of the out-of-plane magnetic field component. Our study provides a strategy for low-magnetic-field applications of large AMR and enriches the transport physics of spintronic devices.

Journal Article Insights into the Origin of Skyrmion Pinning in [Pt/Co/Cu] Magnetic Multilayers Get access Binbin Wang, Binbin Wang Center for Electron Microscopy and Analysis, Ohio State University, Columbus, OH, USADept. of Material Science and Engineering, Ohio State University, Columbus, OH, USA Search for other works by this author on: Oxford Academic Google Scholar Núria Bagués, Núria Bagués Center for Electron Microscopy and Analysis, Ohio State University, Columbus, OH, USA Search for other works by this author on: Oxford Academic Google Scholar Shekhar Das, Shekhar Das Dept. of Physics, The Ohio State University, Columbus, OH, USA Search for other works by this author on: Oxford Academic Google Scholar Shuyu Cheng, Shuyu Cheng Dept. of Physics, The Ohio State University, Columbus, OH, USA Search for other works by this author on: Oxford Academic Google Scholar Camelia Selcu, Camelia Selcu Dept. of Physics, The Ohio State University, Columbus, OH, USA Search for other works by this author on: Oxford Academic Google Scholar Denis V Pelekhov, Denis V Pelekhov Dept. of Physics, The Ohio State University, Columbus, OH, USA Search for other works by this author on: Oxford Academic Google Scholar P Chris Hammel, P Chris Hammel Dept. of Physics, The Ohio State University, Columbus, OH, USA Search for other works by this author on: Oxford Academic Google Scholar Mohit Randeria, Mohit Randeria Dept. of Physics, The Ohio State University, Columbus, OH, USA Search for other works by this author on: Oxford Academic Google Scholar Roland K Kawakami, Roland K Kawakami Dept. of Physics, The Ohio State University, Columbus, OH, USA Search for other works by this author on: Oxford Academic Google Scholar David W McComb David W McComb Center for Electron Microscopy and Analysis, Ohio State University, Columbus, OH, USADept. of Material Science and Engineering, Ohio State University, Columbus, OH, USA Corresponding author: mccomb.29@osu.edu Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 28, Issue S1, 1 August 2022, Pages 2328–2330, https://doi.org/10.1017/S1431927622008947 Published: 01 August 2022

Magnetic materials with kagome crystal structure exhibit rich physics, such as frustrated magnetism, skyrmion formation, topological flat bands, and Dirac/Weyl points. Until recently, most studies on kagome magnets have been performed on bulk crystals or polycrystalline films. Here, we report the atomic layer molecular beam epitaxy synthesis of high-quality thin films of topological kagome magnet Fe3Sn2. The structural and magnetic characterization of Fe3Sn2 on epitaxial Pt(111) identifies highly ordered films with c-plane orientation and an in-plane magnetic easy axis. Studies on the local magnetic structure by anomalous Nernst effect imaging reveal in-plane oriented micrometer size domains. Superlattice structures consisting of Fe3Sn2 and Fe3Sn are also synthesized by atomic layer molecular beam epitaxy, demonstrating the ability to modulate the sample structure at the atomic level. The realization of high-quality films by atomic layer molecular beam epitaxy opens the door to explore the rich physics of this system and investigate novel spintronic phenomena by interfacing Fe3Sn2 with other materials.

We introduce a phase imaging mechanism for scanning transmission electron microscopy that exploits the complementary intensity changes of transmitted disks at different scattering angles. For scanning transmission electron microscopy, this method provides a straightforward, dose-efficient, and noise-robust phase imaging, from atomic resolution to intermediate length scales, as a function of scattering angles and probe defocus. At atomic resolution, we demonstrate that the phase imaging using the method can detect both light and heavy atomic columns. Furthermore, we experimentally apply the method to the imaging of nanoscale magnetic phases in FeGe samples. Compared with conventional methods, phase retrieval using the new method has higher effective spatial resolution and robustness to non-phase background contrast. Our method complements traditional phase imaging modalities in electron microscopy and has the potential to be extended to other scanning transmission techniques and to characterize many emerging material systems.