Long-lived spin waves in a metallic antiferromagnet.

Collective spin excitations in magnetically ordered crystals, called magnons or spin waves, can serve as carriers in novel spintronic devices with ultralow energy consumption. The generation of well-detectable spin flows requires long lifetimes of high-frequency magnons. In general, the lifetime of spin waves in a metal is substantially reduced due to a strong coupling of magnons to the Stoner continuum. This makes metals unattractive for use as components for magnonic devices. Here, we present the metallic antiferromagnet CeCo2P2, which exhibits long-living magnons even in the terahertz (THz) regime. For CeCo2P2, our first-principle calculations predict a suppression of low-energy spin-flip Stoner excitations, which is verified by resonant inelastic X-ray scattering measurements. By comparison to the isostructural compound LaCo2P2, we show how small structural changes can dramatically alter the electronic structure around the Fermi level leading to the classical picture of the strongly damped magnons intrinsic to metallic systems. Our results not only demonstrate that long-lived magnons in the THz regime can exist in bulk metallic systems, but they also open a path for an efficient search for metallic magnetic systems in which undamped THz magnons can be excited.

Skyrmion Echo in a System of Interacting Skyrmions.

We consider helical rotation of skyrmions confined in the potentials formed by nanodisks. Based on numerical and analytical calculations we propose the skyrmion echo phenomenon. The physical mechanism of the skyrmion echo formation is also proposed. Because of the distortion of the lattice, impurities, or pinning effect, confined skyrmions experience slightly different local fields, which leads to dephasing of the initial signal. The interaction between skyrmions also can contribute to the dephasing process. However, switching the magnetization direction in the nanodiscs (e.g., by spin transfer torque) also switches the helical rotation of the skyrmions from clockwise to anticlockwise (or vice versa), and this restores the initial signal (which is the essence of skyrmion echo).

Origin of the quasi-quantized Hall effect in ZrTe5.

The quantum Hall effect (QHE) is traditionally considered to be a purely two-dimensional (2D) phenomenon. Recently, however, a three-dimensional (3D) version of the QHE was reported in the Dirac semimetal ZrTe5. It was proposed to arise from a magnetic-field-driven Fermi surface instability, transforming the original 3D electron system into a stack of 2D sheets. Here, we report thermodynamic, spectroscopic, thermoelectric and charge transport measurements on such ZrTe5 samples. The measured properties: magnetization, ultrasound propagation, scanning tunneling spectroscopy, and Raman spectroscopy, show no signatures of a Fermi surface instability, consistent with in-field single crystal X-ray diffraction. Instead, a direct comparison of the experimental data with linear response calculations based on an effective 3D Dirac Hamiltonian suggests that the quasi-quantization of the observed Hall response emerges from the interplay of the intrinsic properties of the ZrTe5 electronic structure and its Dirac-type semi-metallic character.

Plasmonic Skyrmion Lattice Based on the Magnetoelectric Effect.

The physical mechanism of the plasmonic skyrmion lattice formation in a magnetic layer deposited on a metallic substrate is studied theoretically. The optical lattice is the essence of the standing interference pattern of the surface plasmon polaritons created through coherent or incoherent laser sources. The nodal points of the interference pattern play the role of lattice sites where skyrmions are confined. The confinement appears as a result of the magnetoelectric effect and the electric field associated with the plasmon waves. The proposed model is applicable to yttrium iron garnet and single-phase multiferroics and combines plasmonics and skyrmionics.

Magnetic Weyl semimetal phase in a Kagomé crystal.

Weyl semimetals are crystalline solids that host emergent relativistic Weyl fermions and have characteristic surface Fermi-arcs in their electronic structure. Weyl semimetals with broken time reversal symmetry are difficult to identify unambiguously. In this work, using angle-resolved photoemission spectroscopy, we visualized the electronic structure of the ferromagnetic crystal Co3Sn2S2 and discovered its characteristic surface Fermi-arcs and linear bulk band dispersions across the Weyl points. These results establish Co3Sn2S2 as a magnetic Weyl semimetal that may serve as a platform for realizing phenomena such as chiral magnetic effects, unusually large anomalous Hall effect and quantum anomalous Hall effect.

Giant intrinsic spin Hall effect in W3Ta and other A15 superconductors.

The spin Hall effect (SHE) is the conversion of charge current to spin current, and nonmagnetic metals with large SHEs are extremely sought after for spintronic applications, but their rarity has stifled widespread use. Here, we predict and explain the large intrinsic SHE in ß-W and the A15 family of superconductors: W3Ta, Ta3Sb, and Cr3Ir having spin Hall conductivities (SHCs) of -2250, -1400, and 1210 ℏ e ( S / cm ) , respectively. Combining concepts from topological physics with the dependence of the SHE on the spin Berry curvature (SBC) of the electronic bands, we propose a simple strategy to rapidly search for materials with large intrinsic SHEs based on the following ideas: High symmetry combined with heavy atoms gives rise to multiple Dirac-like crossings in the electronic structure; without sufficient symmetry protection, these crossings gap due to spin-orbit coupling; and gapped crossings create large SBC.

Anomalous Hall effect in Weyl semimetal half-Heusler compounds RPtBi (R = Gd and Nd).

Topological materials ranging from topological insulators to Weyl and Dirac semimetals form one of the most exciting current fields in condensed-matter research. Many half-Heusler compounds, RPtBi (R = rare earth), have been theoretically predicted to be topological semimetals. Among various topological attributes envisaged in RPtBi, topological surface states, chiral anomaly, and planar Hall effect have been observed experimentally. Here, we report an unusual intrinsic anomalous Hall effect (AHE) in the antiferromagnetic Heusler Weyl semimetal compounds GdPtBi and NdPtBi that is observed over a wide temperature range. In particular, GdPtBi exhibits an anomalous Hall conductivity of up to 60 Ω-1⋅cm-1 and an anomalous Hall angle as large as 23%. Muon spin-resonance (µSR) studies of GdPtBi indicate a sharp antiferromagnetic transition (TN) at 9 K without any noticeable magnetic correlations above TN Our studies indicate that Weyl points in these half-Heuslers are induced by a magnetic field via exchange splitting of the electronic bands at or near the Fermi energy, which is the source of the chiral anomaly and the AHE.

Signature of type-II Weyl semimetal phase in MoTe2.

Topological Weyl semimetal (TWS), a new state of quantum matter, has sparked enormous research interest recently. Possessing unique Weyl fermions in the bulk and Fermi arcs on the surface, TWSs offer a rare platform for realizing many exotic physical phenomena. TWSs can be classified into type-I that respect Lorentz symmetry and type-II that do not. Here, we directly visualize the electronic structure of MoTe2, a recently proposed type-II TWS. Using angle-resolved photoemission spectroscopy (ARPES), we unravel the unique surface Fermi arcs, in good agreement with our ab initio calculations that have nontrivial topological nature. Our work not only leads to new understandings of the unusual properties discovered in this family of compounds, but also allows for the further exploration of exotic properties and practical applications of type-II TWSs, as well as the interplay between superconductivity (MoTe2 was discovered to be superconducting recently) and their topological order.

THz-Driven Ultrafast Spin-Lattice Scattering in Amorphous Metallic Ferromagnets.

We use single-cycle THz fields and the femtosecond magneto-optical Kerr effect to, respectively, excite and probe the magnetization dynamics in two thin-film ferromagnets with different lattice structures: crystalline Fe and amorphous CoFeB. We observe Landau-Lifshitz-torque magnetization dynamics of comparable magnitude in both systems, but only the amorphous sample shows ultrafast demagnetization caused by the spin-lattice depolarization of the THz-induced ultrafast spin current. Quantitative modeling shows that such spin-lattice scattering events occur on similar time scales than the conventional spin conserving electronic scattering (∼30 fs). This is significantly faster than optical laser-induced demagnetization. THz conductivity measurements point towards the influence of lattice disorder in amorphous CoFeB as the driving force for enhanced spin-lattice scattering.

Correlation-Driven Insulator-Metal Transition in Near-Ideal Vanadium Dioxide Films.

We use polarization- and temperature-dependent x-ray absorption spectroscopy, in combination with photoelectron microscopy, x-ray diffraction, and electronic transport measurements, to study the driving force behind the insulator-metal transition in VO_{2}. We show that both the collapse of the insulating gap and the concomitant change in crystal symmetry in homogeneously strained single-crystalline VO_{2} films are preceded by the purely electronic softening of Coulomb correlations within V-V singlet dimers. This process starts 7 K (±0.3 K) below the transition temperature, as conventionally defined by electronic transport and x-ray diffraction measurements, and sets the energy scale for driving the near-room-temperature insulator-metal transition in this technologically promising material.

Structural and electronic properties of epitaxial multilayer h-BN on Ni(111) for spintronics applications.

Hexagonal boron nitride (h-BN) is a promising material for implementation in spintronics due to a large band gap, low spin-orbit coupling, and a small lattice mismatch to graphene and to close-packed surfaces of fcc-Ni(111) and hcp-Co(0001). Epitaxial deposition of h-BN on ferromagnetic metals is aimed at small interface scattering of charge and spin carriers. We report on the controlled growth of h-BN/Ni(111) by means of molecular beam epitaxy (MBE). Structural and electronic properties of this system are investigated using cross-section transmission electron microscopy (TEM) and electron spectroscopies which confirm good agreement with the properties of bulk h-BN. The latter are also corroborated by density functional theory (DFT) calculations, revealing that the first h-BN layer at the interface to Ni is metallic. Our investigations demonstrate that MBE is a promising, versatile alternative to both the exfoliation approach and chemical vapour deposition of h-BN.

Focused-electron-beam-induced-deposited cobalt nanopillars for nanomagnetic logic.

Nanomagnetic logic (NML) intends to alleviate problems of continued miniaturization of CMOS-based electronics, such as energy dissipation through heat, through advantages such as low power operation and non-volatile magnetic elements. In line with recent breakthroughs in NML with perpendicularly magnetized elements formed from thin films, we have fabricated NML inverter chains from Co nanopillars by focused electron beam induced deposition (FEBID) that exhibit shape-induced perpendicular magnetization. The flexibility of FEBID allows optimization of NML structures. Simulations reveal that the choice of nanopillar dimensions is critical to obtain the correct antiferromagnetically coupled configuration. Experiments carrying the array through a clocking cycle using the Oersted field from an integrated Cu wire show that the array responds to the clocking cycle.

Field Effect and Strongly Localized Carriers in the Metal-Insulator Transition Material VO(2).

The intrinsic field effect, the change in surface conductance with an applied transverse electric field, of prototypal strongly correlated VO(2) has remained elusive. Here we report its measurement enabled by epitaxial VO(2) and atomic layer deposited high-κ dielectrics. Oxygen migration, joule heating, and the linked field-induced phase transition are precluded. The field effect can be understood in terms of field-induced carriers with densities up to ∼5×10(13) cm(-2) which are trongly localized, as shown by their low, thermally activated mobility (∼1×10(-3) cm(2)/V s at 300 K). These carriers show behavior consistent with that of Holstein polarons and strongly impact the (opto)electronics of VO(2).

Massive Dirac Fermion Observed in Lanthanide-Doped Topological Insulator Thin Films.

The breaking of time reversal symmetry (TRS) in three-dimensional (3D) topological insulators (TIs), and thus the opening of a 'Dirac-mass gap' in the linearly dispersed Dirac surface state, is a prerequisite for unlocking exotic physical states. Introducing ferromagnetic long-range order by transition metal doping has been shown to break TRS. Here, we present the study of lanthanide (Ln) doped Bi2Te3, where the magnetic doping with high-moment lanthanides promises large energy gaps. Using molecular beam epitaxy, single-crystalline, rhombohedral thin films with Ln concentrations of up to ~35%, substituting on Bi sites, were achieved for Dy, Gd, and Ho doping. Angle-resolved photoemission spectroscopy shows the characteristic Dirac cone for Gd and Ho doping. In contrast, for Dy doping above a critical doping concentration, a gap opening is observed via the decreased spectral intensity at the Dirac point, indicating a topological quantum phase transition persisting up to room-temperature.

Study of Dy-doped Bi2Te3: thin film growth and magnetic properties.

Breaking the time-reversal symmetry (TRS) in topological insulators (TIs) through ferromagnetic doping is an essential prerequisite for unlocking novel physical phenomena and exploring potential device applications. Here, we report the successful growth of high-quality (Dy(x)Bi(1-x))2Te3 thin films with Dy concentrations up to x = 0.355 by molecular beam epitaxy. Bulk-sensitive magnetisation studies using superconducting quantum interference device magnetometry find paramagnetic behaviour down to 2 K for the entire doping series. The effective magnetic moment, µeff, is strongly doping concentration-dependent and reduces from ∼12.6 µ(B) Dy(-1) for x = 0.023 to ∼4.3 µ(B) Dy(-1) for x = 0.355. X-ray absorption spectra and x-ray magnetic circular dichroism (XMCD) at the Dy M4,5 edge are employed to provide a deeper insight into the magnetic nature of the Dy(3+)-doped films. XMCD, measured in surface-sensitive total-electron-yield detection, gives µ(eff )= 4.2 µ(B) Dy(-1). The large measured moments make Dy-doped films interesting TI systems in which the TRS may be broken via the proximity effect due to an adjacent ferromagnetic insulator.

Influence of current on field-driven domain wall motion in permalloy nanowires from time resolved measurements of anisotropic magnetoresistance.

The motion of magnetic domain walls in permalloy nanowires is investigated by real-time resistance measurements. The domain wall velocity is measured as a function of the magnetic field in the presence of a current flowing through the nanowire. We show that the current can significantly increase or decrease the domain wall velocity, depending on its direction. These results are understood within a one-dimensional model of the domain wall dynamics which includes the spin transfer torque.

Highly spin-polarized room-temperature tunnel injector for semiconductor spintronics using MgO(100).

The spin polarization of current injected into GaAs from a CoFe/MgO(100) tunnel injector is inferred from the electroluminescence polarization from GaAs/AlGaAs quantum well detectors. The polarization reaches 57% at 100 K and 47% at 290 K in a 5 T perpendicular magnetic field. Taking into account the field dependence of the luminescence polarization, the spin injection efficiency is at least 52% at 100 K, and 32% at 290 K. We find a nonmonotonic temperature dependence of the polarization which can be attributed to spin relaxation in the quantum well detectors.

Current-driven excitations in symmetric magnetic nanopillars.

We study experimentally the current-driven magnetic excitations in symmetric Co/Cu/Co nanopillars. In contrast with all the previous observations where the current of only one polarity is capable of exciting a multilayer system saturated by an externally applied magnetic field, we observe that both polarities of the applied current trigger excitations in a symmetric multilayer. This may indicate that in symmetric structures the current propels high-frequency magnetic oscillations in all magnetic layers. We argue, however, that only one layer is excited in our multilayers but, interestingly, currents of opposite polarities excite different layers. This hypothesis is supported by modeling the spin accumulation in symmetric magnetic multilayers.

Optical detection of hot-electron spin injection into GaAs from a magnetic tunnel transistor source.

Injection of spin-polarized hot-electron current from a magnetic tunnel transistor into GaAs is demonstrated by the observation of polarized light emission from a GaAs/In(0.2)Ga(0.8)As multiple quantum well light-emitting diode. Electroluminescence from the quantum wells shows a polarization of approximately 10% after subtraction of a linear background polarization. The polarization shows a strong dependence on the bias voltage across the diode, which may originate from changes in the electron spin relaxation rate in the quantum wells under varying bias conditions.