Among of rare-earth (RE) hexaborides only two compounds SmB6 and YbB6 are discussed in literature to be members of a new class of 3D topological insulators. However, their ground states originate due to different physical mechanisms, including Kondo 4f-5d hybridization and 5d-2p band inversion, respectively. Here we report a comparative study of magnetotransport (resistivity and transverse magnetoresistance) measured on high quality single crystals of YbxSm1-xB6 and EuxSm1-xB6 solid solutions (x ≤ 0.05) at temperatures 1.7 − 300 K in magnetic fields up to 82 kOe. The choice of dopant was determined by the fact that the presence of magnetic/nonmagnetic (Eu2+/Yb2+) impurity in parent SmB6 matrix should lift/not lift the topological protection of surface states. Based on the two-gap paradigm the x-evolution of electron spectra in YbxSm1-xB6 and EuxSm1-xB6 was studied. Our data show that both the 4f lattice coherence (Eg) and the intrinsic gap (Ea) related to many-body states survive under RE doping at least for x ≈ 0.02 − 0.024. We also suggest that the point x(Eu) = 0.05 can be treated as an upper limit of the small gap closing in EuxSm1-xB6 materials. In YbxSm1-xB6 family a negative linear transverse magnetoresistance (TMR) was detected for the first time in the regime of surface conductivity (T < T* ≈ 5 K). The TMR anomaly at T* caused possibly by the topological protection of surface states in SmB6 is found to survive in Eu-doped compounds but disappears almost completely for Yb-doped compositions in the same fixed magnetic fields. This paradoxical observation is not consistent with general predictions of the topological Kondo insulator (TKI) model.

We present a study of low temperature electron transport (resistivity, magnetoresistance and Hall coefficient) in SmB6 single crystals having different polar (100) and nonpolar (110) and (111) surfaces after mechanical polishing and chemical etching. The estimation of effective parameters for surface and bulk charge carriers allows us to conclude that surface conductivity is very sensitive to the method of surface treatment. The most pronounced change is observed for the polar (100) surface, for which the related Hall concentration of charge carriers at 2 K decreases more than by 2 orders of magnitude and the Hall mobility increases by a factor of 15 after etching these faces in diluted nitric acid. We suggest that the strong dependence of surface properties on the type of treatment may result from both topological protection, which is influenced by intrinsic defects or surface reconstruction, and band bending effects, which modulate the properties of the surface conduction layer in case of polar faces.

The galvanomagnetic characteristics of SmB6 single crystals are studied within the temperature range of 1.9‒3.6 K at different orientations of the crystal faces. As a result, the electrical resistivities of the surfaces corresponding to the (100), (110), (111), and (211) crystallographic planes are determined. It is shown that the effective parameters of charge carriers, which determine the surface conductivity in SmB6, depend both on the orientation of the surface and on the method of its processing. It is found that the etching of polished polar surfaces formed by (100) planes leads at 1.9 K to a decrease in the density and to an increase in the mobility of surface n-type charge carriers from 113/a2 and 1.12 cm2/(V s) to 0.76/a2 and 18 cm2/(V s), respectively (lattice parameter a ≈ 4.134 Å). For etched nonpolar surfaces corresponding to the (110) and (111) planes, the maximum density of surface charge carriers (per unit area of the surface Brillouin zone) is found to increase by factors of 2.3 and 3.9, respectively. It is proposed to use this parameter as a simple criterion to identify the features of electron transport due to the nontrivial topology of the band structure of SmB6.

We present the results of experimental studies on the synthesis by molecular-beam epitaxy of AlGaAs nanowires with InAs quantum dots. The morphological, structural, and optical properties of the grown nanostructures have been studied. It is important to note that the emission from quantum dots is observed in the wavelength range from 750 to 970 nm. Assumptions about the nature of short-wavelength emission from quantum dots are formulated. In particular, one of the reasons may be a significant desorption of indium atoms and the presence of gallium atoms in catalyst drops during the growth at a substrate temperature of 510◦C. The proposed technology opens up new possibilities for integration direct-gap III−V materials with a silicon platform for various applications in photonics and quantum communications.

Magnonics addresses the physical properties of spin waves and utilizes them for data processing. Scalability down to atomic dimensions, operation in the GHz-to-THz frequency range, utilization of nonlinear and nonreciprocal phenomena, and compatibility with CMOS are just a few of many advantages offered by magnons. Although magnonics is still primarily positioned in the academic domain, the scientific and technological challenges of the field are being extensively investigated, and many proof-of-concept prototypes have already been realized in laboratories. This roadmap is a product of the collective work of many authors, which covers versatile spin-wave computing approaches, conceptual building blocks, and underlying physical phenomena. In particular, the roadmap discusses the computation operations with the Boolean digital data, unconventional approaches, such as neuromorphic computing, and the progress toward magnon-based quantum computing. This article is organized as a collection of sub-sections grouped into seven large thematic sections. Each sub-section is prepared by one or a group of authors and concludes with a brief description of current challenges and the outlook of further development for each research direction.

Time-resolved scanning transmission x-ray microscopy has been used for the direct imaging of spin-wave dynamics in a thin film yttrium iron garnet (YIG) with sub-200 nm spatial resolution. Application of this x-ray transmission technique to single-crystalline garnet films was achieved by extracting a lamella (13×5×0.185 μm3) of the liquid phase epitaxy grown YIG thin film out of a gadolinium gallium garnet substrate. Spin waves in the sample were measured along the Damon-Eshbach and backward volume directions of propagation at gigahertz frequencies and with wavelengths in a range between 200 nm and 10 μm. The results were compared to theoretical models. Here, the widely used approximate dispersion equation for dipole-exchange spin waves proved to be insufficient for describing the observed Damon-Eshbach type modes. For achieving an accurate description, we made use of the full analytical theory taking mode-hybridization effects into account.

The evolution of wide-temperature range (4.2?300 K) electron transport in niobium-carbon nanocomposites was studied at niobium concentration range 0.15?0.35. It was found that electron transport in the nanocomposites has the features of universality, being expressed in the existence of two characteristic temperature intervals on the temperature dependences of conductivity. The crossover temperature between the intervals is in the range 20?30 K. Within each temperature interval, corrections to the conductivity are found to be as power-like ones. Power exponent p is characterized by the non-monotonic dependences on niobium concentration and varies in the ranges 0.5?1.4 and 0.2?1.4 in the low- and high-temperature intervals with a minimum at 0.30 and 0.27 of Nb content, respectively. The satisfactory description of electron transport in niobium-carbon nanocomposites was achieved within the model of the inelastic tunneling of the electrons between the metal grains in the framework of the effective medium approximation.

Nowadays, the interaction between phonon and magnon subsystems of a magnetic medium is a hot topic of research. The complexity of phonon and magnon spectra, the existence of both bulk and surface modes, the quantization effects, and the dependence of magnon properties on applied magnetic field, make this field very complex and intriguing. Moreover, the recent advances in the fields of spin-caloritronics and magnon spintronics as well as the observation of the spin Seebeck effect (SSE) in magnetic insulators points on the crucial role of magnons in spin-caloric transport processes. In this review, we collect the variety of different studies in which magnon-phonon interaction play important role. The scope of the paper covers the wide range of phenomena starting from the interaction of the coherent magnons with surface acoustic wave and finishing with the formation of magnon supercurrents in the thermal gradients.

We present the results of precise magnetoresistance Δρ/ρ measurements of HoB12 dodecaboride with dynamic charge stripes for various orientations of the external magnetic field up to 80 kOe relative to the crystal structure and for different temperatures in the antiferromagnetic phase. The performed analysis allowed us to separate different Δρ/ρ contributions and find coefficients attributed to the linear positive and quadratic negative magnetoresistance components and their changing with temperature and magnetic field intensity and direction. It is deduced that strong Maltese-cross type anisotropy in charge carriers scattering is mainly attributed to the renormalization of RKKY-exchange interaction due to high-frequency oscillations of charge carriers in dynamic charge stripes.

Nowadays, the interaction between phonon and magnon subsystems of a magnetic medium is a hot topic of research. The complexity of phonon and magnon spectra, the existence of both bulk and surface modes, the quantization effects, and the dependence of magnon properties on applied magnetic field, make this field very complex and intriguing. Moreover, the recent advances in the fields of spin caloritronics and magnon spintronics as well as the observation of the spin Seebeck effect in magnetic insulators points on the crucial role of magnons in spin-caloric transport processes. In this review, we collect the variety of different studies in which magnon-phonon interaction play important role. The scope of the paper covers the wide range of phenomena starting from the interaction of the coherent magnons with surface acoustic wave and finishing with the formation of magnon supercurrents in the thermal gradients.

Comparative analysis of Hall effect in substitutional solid solutions Ho0.5Lu0.5B12, Mn1 – xFexSi (0 ≤ x ≤ 1) and Eu1 – xGdxB6 (x < 0.04) is carried out at temperatures 2–300 K in magnetic fields up to 8 T. Anomalous contribution to the Hall effect $$\rho _{{xy}}^{{\text{A}}}$$ ~ $${{\rho }_{{xx}}}M$$ has been identified for systems with various types of magnetic frustration. The linear scaling $$\rho _{{xy}}^{{\text{A}}}$$ ~ ρxx is detected in the resistivity range ρxx ~ 0.01–1 mΩ cm lying outside the range of applicability of the classical model of asymmetric scattering. It is associated with the increase in the amplitude of spin fluctuations in the paramagnetic phase of the investigated compounds with noncollinear magnetic structure. The topological contribution to the Hall effect is extracted for Ho0.5Lu0.5B12 and Eu1 – xGdxB6. Its amplitude is found to vary from 80 nΩ  cm (Ho0.5Lu0.5B12) to 7.5 μΩ  cm (Eu0.97Gd0.03B6).

In the emerging field of magnonics, spin waves are foreseen as signal carriers for future spintronic information processing and communication devices, owing to both the very low power losses and a high device miniaturization potential predicted for short-wavelength spin waves. Yet, the efficient excitation and controlled propagation of nanoscale spin waves remains a severe challenge. Here, we report the observation of high-amplitude, ultrashort dipole-exchange spin waves (down to 80 nm wavelength at 10 GHz frequency) in a ferromagnetic single layer system, coherently excited by the driven dynamics of a spin vortex core. We used time-resolved x-ray microscopy to directly image such propagating spin waves and their excitation over a wide range of frequencies. By further analysis, we found that these waves exhibit a heterosymmetric mode profile, involving regions with anti-Larmor precession sense and purely linear magnetic oscillation. In particular, this mode profile consists of dynamic vortices with laterally alternating helicity, leading to a partial magnetic flux closure over the film thickness, which is explained by a strong and unexpected mode hybridization. This spin-wave phenomenon observed is a general effect inherent to the dynamics of sufficiently thick ferromagnetic single layer films, independent of the specific excitation method employed.

Magnon Bose-Einstein Condensates (BECs) and supercurrents are coherent quantum phenomena, which appear on a macroscopic scale in parametrically populated solid state spinsystems. One of the most fascinating and attractive features of these processes is the possibility of magnon condensation and supercurrent excitation even at room temperature. At the same time, valuable information about a magnon BEC state, such as its lifetime, its formation threshold, and coherency, is provided by experiments at various temperatures. Here, we use Brillouin Light Scattering (BLS) spectroscopy for the investigation of the magnon BEC dynamics in a single-crystal film of yttrium iron garnet in a wide temperature range from 30 K to 380 K. By comparing the BLS results with previous microwave measurements, we re-vealed the direct relation between the damping of the condensed and the parametrically injected magnons. The enhanced supercurrent dynamics was detected at 180 K near the minimum of BEC damping.

Magnon systems used in quantum devices require low damping if coherence is to be maintained. The ferrimagnetic electrical insulator yttrium iron garnet (YIG) has low magnon damping at room temperature and is a strong candidate to host microwave magnon excitations in future quantum devices. Monocrystalline YIG films are typically grown on gadolinium gallium garnet (GGG) substrates. In this work, comparative experiments made on YIG waveguides with and without GGG substrates indicate that the material plays a significant role in increasing the damping at low temperatures. Measurements reveal that damping due to temperature-peak processes is dominant above 1 K. Damping behaviour that we show can be attributed to coupling to two-level fluctuators (TLFs) is observed below 1 K. Upon saturating the TLFs in the substrate-free YIG at 20 mK, linewidths of 1.4 MHz are achievable: lower than those measured at room temperature.

The existence of backscattering-immune spin-wave modes is demonstrated in magnetic thin films of nanoscale thickness. Our results reveal that chiral magnetostatic surface waves (CMSSWs), which propagate perpendicular to the magnetization direction in an in-plane magnetized thin film, are robust against backscattering from surface defects. CMSSWs are protected against various types of surface inhomogeneities and defects as long as their frequency lies inside the gap of the volume modes. Our explanation is independent of the topology of the modes and predicts that this robustness is a consequence of symmetry breaking of the dynamic magnetic fields of CMSSWs due to the off-diagonal part of the dipolar interaction tensor, which is present both for long- (dipole-dominated) and short-wavelength (exchange-dominated) spin waves. Micromagnetic simulations confirm the robust character of the CMSSWs. Our results open a new direction in designing highly efficient magnonic logic elements and devices employing CMSSWs in nanoscale thin films.

A comparative analysis of Hall effect in substitutional solid solutions Ho0.5Lu0.5B12, Mn1-xFexSi (0≤ x≤1) and Eu1-xGdxB6 (x<0.04) is carried out at temperatures 2-300 K in magnetic fields up to 8 T. Anomalous Hall effect rhoxyA~rhoxxM is identified for the systems with different kinds of magnetic frustration. Linear scaling rhoxyA~rhoxx, which is detected at resistivity rhoxx~0.01-1 mOmega·cm out from the region of applicability of classical skew-scattering model, is associated with the increase of spin fluctuations amplitude in paramagnetic phases of the investigated compounds with noncollinear magnetic structure. Topological contribution to Hall effect is extracted for Ho0.5Lu0.5B12 and Eu1-xGdxB6, its amplitude varying from 80 nOmega·cm (for Ho0.5Lu0.5B12) up to 7.5 μOmega·cm (for Eu0.97Gd0.03B6).

L. MIHALCEANU, D.A. BOZHKO, 2 V.I. VASYUCHKA, A.A. SERGA, B. HILLEBRANDS, A. POMYALOV, V.S. L’VOV, V.S. TIBERKEVICH 1 Fachbereich Physik and Landesforschungszentrum OPTIMAS, Technische Universität Kaiserslautern (67663 Kaiserslautern, Germany; e-mail: mihalcea@rhrk.uni-kl.de) 2 James Watt School of Engineering, University of Glasgow (Glasgow G12 8LT, United Kingdom; e-mail: dbozhko@uccs.edu) 3 Department of Chemical and Biological Physics, Weizmann Institute of Science (Rehovot 76100, Israel; e-mail: Anna.Pomyalov@weizmann.ac.il) 4 Department of Physics, Oakland University (Rochester MI 48309, United States; e-mail: tyberkev@oakland.edu)

We review the development trends in microwave magnetism and superconductivity over the last five decades. The review contains the key results of recent studies related to the promising areas of modern magnetism and applied physics – spintronics, magnonics, magnon caloritronics, physics of magnonic crystals, spin-wave logic, and the development of novel micro- and nano-scale magnetic devices. The main achievements in these fields of physics are summarized and generalized.

The wide-temperature-range (4.2–300 K) electron transport had being studied in tungsten–carbon nanocomposites in tungsten concentration interval 0.1–0.45. It is shown that electron transport in the nanocomposites possesses the features of the universality, manifested in the form of power-law dependences of the conductivity on temperature in the two characteristic temperature intervals. The critical temperature separating the intervals is about 25–30 K and has no appreciable dependence on the value of tungsten concentration in nanocomposites. The power exponents of the temperature dependences of the conductivity in both temperature intervals are the non-monotonic functions of the tungsten concentration and vary in the range 0–2 with a wide minimum at 0.2 and 0.25 of tungsten content in the high- and low-temperature intervals, respectively. The observed power-law temperature corrections to the conductivity are simulated and discussed within the effective medium approximation in the framework of the model of the inelastic tunneling of the electrons between the conducting clusters in the tungsten–carbon nanocomposites.