Search

The scattering theory of low-energy (slow) electrons has been developed by Evans and Mills [Phys. Rev. B 5, 4126 (1972)]. The formalism is merely based on the electrostatic Coulomb interaction of the scattering electrons with the charge-density fluctuations above the surface and can describe most of the interesting features observed in the high-resolution electron energy-loss spectroscopy experiments. Here we extend this theory by including the spin-orbit coupling in the scattering process. We discuss the impact of this interaction on the scattering cross section. In particular, we discuss cases in which a spin-polarized electron beam is scattered from nonmagnetic surfaces with a strong spin-orbit coupling. We show that under some assumptions one can derive an expression for the scattering cross section, which can be used for numerical calculations of the spin-polarized spectra recorded by spin-polarized high-resolution electron energy-loss spectroscopy experiments., 8 pages, 1 figure

The discovery of two dimensional Van der Waals materials has opened up several possibilities for designing novel devices. Yet a more promising way of designing exotic heterostutures with improved physical properties is to grow a monolayer of these materials on a substrate. For example, in the field of superconductivity it has been demonstrated that the superconducting transition temperature of a monolayer of FeSe grown on some oxide substrates e.g., strontium titanate (STO) is by far higher than its bulk counterpart. Although the system has been considered as a model system for understanding the phenomenon of high-temperature superconductivity, the physical mechanism responsible for this high transition temperature is still highly under debate. Here using momentum and energy resolved high-resolution electron energy-loss spectroscopy we probe the dynamic charge response of the FeSe/STO(001) system and demonstrate that the frequency- and momentum-dependent dynamic charge response is not compatible with a simple film/substrate model. Our analysis reveals the existence of a depletion region at the interface between this Van der Waals monolayer and the substrate. The presence of the depletion layer, accompanied with a considerably large charge transfer from STO into the FeSe monolayer, leads to a strong renormalization of the STO energy bands and a substantial band bending at the interface. Our results shed light on the electronic complexities of the FeSe/oxide interfaces and pave the way of designing novel low-dimensional high-temperature superconductors through interface engineering. We anticipate that the observed phenomenon is rather general and can take place in many two dimensional Van der Waals monolayers brought in contact with dielectric oxides or semiconducting substrates., 12 pages, 3 figures

Electronic surface, interface and edge states are well-known concepts in low-dimensional solids and have already been utilised for practical applications. It is expected that magnons–the bosonic quasiparticles representing the magnetic excitations– shall also exhibit such exotic states. However, how these states are formed in layered magnetic structures is hitherto unknown. Here we bring the topic of magnonic surface and interface states in layered ferromagnets into discussion. We provide experimental examples of synthetic layered structures, supporting our discussions and show that these states can be tailored in artificially fabricated structures. We demonstrate that the magnonic surface or interface states may show peculiar features, including "standing” or "ultrafast” states. We argue that these states can drastically change their electronic and magnonic transport properties. In this way one can design layered ferromagnets which act as magnon conductor, semiconductor and insulator of specific states. Magnons are the quantized spin waves, which describe the collective magnetic excitations and the long-range magnetic order in a solid. Here, the authors describe how to engineer magnonic band structures at the interface and surface of ferromagnetic layered structures and how such magnonic states alter the transport properties.

This topical review presents an overview of the recent experimental and theoretical attempts on designing magnonic crystals for operation at different frequencies. The focus is put on the microscopic physical mechanisms involved in the formation of the magnonic band structure, allowed as well as forbidden magnon states in various systems, including ultrathin films, multilayers and artificial magnetic structures. The essential criteria for the formation of magnonic bandgaps in different frequency regimes are explained in connection with the magnon dynamics in such structures. The possibility of designing small-size magnonic crystals for operation at ultrahigh frequencies (terahertz and sub-terahertz regime) is discussed. Recently discovered magnonic crystals based on topological defects and using periodic Dzyaloshinskii-Moriya interaction, are outlined. Different types of magnonic crystals, capable of operation at different frequency regimes, are put within a rather unified picture.

An immediate need of information technology is designing fast, small and low-loss devices. One of the ways to design such devices is using the bosonic quasiparticles, such as magnons, for information transfer/processing. This is the main idea behind the field of magnonics. When a magnon propagates through a magnetic medium, no electrical charge transport is involved and therefore no energy losses, creating Joule heating, occur. This is the most important advantage of using magnons for information transfer. Moreover the mutual conversion between magnons and the other carriers e.g. electrons, photons and plasmons shall open new opportunities to realize tunable multifunctional devices. Magnons cover a very wide range of frequency, from sub-gigahertz up to a few hundreds of terahertz. The magnon frequency has an important impact on the performance of magnon-based devices (the larger the excitation frequency, the faster the magnons). This means that the use of high-frequency (terahertz) magnons would provide a great opportunity for the design of ultrafast devices. However, up to now the focus in magnonics has been on the low-frequency gigahertz magnons. Here we discuss the feasibility of using terahertz magnons for application in magnonic devices. We shall bring the concept of terahertz magnonics into discussion. We discuss how the recently discovered phenomena in the field of terahertz magnons may inspire ideas for designing new magnonic devices. We further introduce methods to tune the fundamental properties of terahertz magnons, e.g. their eigenfrequency and lifetime.

Utilizing high-resolution electron energy-loss spectroscopy (HREELS) we measure the phonon frequencies of β–FeSe(001), cleaved under ultra-high vacuum conditions. At the zone center ( Γ ¯ –point) three prominent loss features are observed at loss energies of about ≃ 20.5 and 25.6 and 40 meV. Based on the scattering selection rules we assign the observed loss features to the A1g, B1g, and A2u phonon modes of β–FeSe(001). The experimentally measured phonon frequencies do not agree with the results of density functional based calculations in which a nonmagnetic, a checkerboard or a strip antiferromagnetic order is assumed for β–FeSe(001). Our measurements suggest that, similar to the other Fe-based materials, magnetism has a profound impact on the lattice dynamics of β–FeSe(001).

Elementary spin excitations (magnons) play a fundamental role in condensed matter physics, since many phenomena e.g. magnetic ordering, electrical (as well as heat) transport properties, ultrafast magnetization processes, and most importantly electron/spin dynamics can only be understood when these quasi-particles are taken into consideration. In addition to their fundamental importance, magnons may also be used for information processing in modern spintronics. Here the concept of spin excitations in ultrathin itinerant magnets is discussed and reviewed. Starting with a historical introduction, different classes of magnons are introduced. Different theoretical treatments of spin excitations in solids are outlined. Interaction of spin-polarized electrons with a magnetic surface is discussed. It is shown that, based on the quantum mechanical conservation rules, a magnon can only be excited when a minority electron is injected into the system. While the magnon creation process is forbidden by majority electrons, the magnon annihilation process is allowed instead. These fundamental quantum mechanical selection rules, together with the strong interaction of electrons with matter, make the spin-polarized electron spectroscopies as appropriate tools to excite and probe the elementary spin excitations in low-dimensional magnets e.g ultrathin films and nanostructures. The focus is put on the experimental results obtained by spin-polarized electron energy loss spectroscopy and spin-polarized inelastic tunneling spectroscopy. The magnon dispersion relation, lifetime, group and phase velocity measured using these approaches in various ultrathin magnets are discussed in detail. The differences and similarities with respect to the bulk excitations are addressed. The role of the temperature, atomic structure, number of atomic layers, lattice strain, electronic complexes and hybridization at the interfaces are outlined. A possibility of simultaneous probing of magnons and phonons in complex low-dimensional ferromagnetic oxide nanostructures is discussed. The influence of the relativistic spin–orbit coupling on high-energy magnons is addressed. It is shown how the spin–orbit coupling breaks the energy degeneracy of the magnons excited in an ultrathin ferromagnet, and how it influences their lifetime, amplitude, group and phase velocity. A potential application of these new effects in modern spintronics is outlined. It is illustrated how one can take advantage of collective nature of magnons and use these quasi-particles for probing the magnetic exchange interaction at buried interfaces.

The pairing mechanism in iron-based superconductors is believed to be unconventional, i.e. not phonon-mediated. The achieved transition temperatures Tc in these superconductors are still significantly below those of some of the cuprates, with the exception of single layer FeSe films on SrTiO3 showing a Tc between 60 and 100 K, i.e. an order of magnitude larger than in bulk FeSe. This enormous increase of Tc demonstrates the potential of interface engineering for superconductivity, yet the underlying mechanism of Cooper pairing is not understood. Both conventional and unconventional mechanisms have been discussed. Here we report a direct measurement of the electron-boson coupling function in FeSe on SrTiO3 using inelastic electron scattering which shows that the excitation spectrum becomes fully gapped below Tc strongly supporting a predominantly electronic pairing mechanism. We also find evidence for strong electron-phonon coupling of low energy electrons, which is however limited to regions near structural domain boundaries., 10 pages, 7 figures

We report on the phonon spectrum probed at the $\ensuremath{\beta}$-FeSe(001) surface by means of high-resolution electron energy-loss spectroscopy (HREELS). Single crystals of $\ensuremath{\beta}$-FeSe are cleaved under ultrahigh-vacuum conditions and are subsequently measured below and above the nematic transition temperature. In total we observe five phonon modes and a phonon cutoff energy of about 40 meV. We identify the origin of each phonon mode based on the selection rules of HREELS and by comparing the experimental results to those of ab initio density functional calculations. The most prominent phonon modes ${A}_{1g}, {B}_{1g}$, and ${A}_{2u}$ appear at energies of about 20.5 and 25.6 and 40 meV, respectively. These phonon modes disperse rather weakly while changing the momentum from zero up to the zone boundary, indicating that they are mainly of optical nature. A comparison between our results and the results of ab initio calculations indicates that there must be a mutual interplay between magnetism and lattice dynamics in this compound, similar to the other Fe-based superconductors. Finally, we comment on the role of temperature on the phonon modes probed at the $\overline{\mathrm{\ensuremath{\Gamma}}}$ point. It is observed that both the ${B}_{1g}$ and ${A}_{2u}$ phonon modes undergo a downward shift while increasing the temperature from 15 to 300 K. In the case of the ${A}_{2u}$ mode this shift is about 1.5 meV.

We present an efficient methodology to study spin waves in disordered materials. The approach is based on a Heisenberg model and enables calculations of magnon properties in spin systems with disorder of an arbitrary kind and concentration of impurities. Disorder effects are taken into account within two complementary approaches. Magnons in systems with substitutional (uncorrelated) disorder can be efficiently calculated within a single-site coherent potential approximation for the Heisenberg model. From the computation point of view the method is inexpensive and directly applicable to systems like alloys and doped materials. It is shown that it performs exceedingly well across all concentrations and wave vectors. Another way is the direct numerical simulation of large supercells using a configurational average over possible samples. This approach is applicable to systems with an arbitrary kind of disorder. The effective interaction between magnetic moments entering the Heisenberg model can be obtained from first-principles using a self-consistent Green function method within the density functional theory. Thus, our method can be viewed as an ab initio approach and can be used for calculations of magnons in real materials.

Nanoparticles of antiferromagnetic LaFeO3 were prepared by the sol–gel method. An exchange bias effect has been observed and is attributed to the exchange coupling between the ferromagnetic shell and antiferromagnetic core of the particles. The results provide clear evidence of the presence of spontaneous exchange bias in this system. After field cooling from room temperature, the exchange bias increases while the coercivity decreases with decreasing temperature. Taking into account the role of thermal activation, the temperature dependence of exchange bias and coercivity has been interpreted in terms of the spontaneous exchange bias mechanism proposed recently. (Some figures in this article are in colour only in the electronic version)

The spin and orbital magnetism of $8\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ thick ${\mathrm{Fe}}_{2.8}{\mathrm{Si}}_{1.2}$, ${\mathrm{Fe}}_{3}\mathrm{Si}$, and ${\mathrm{Fe}}_{3.2}{\mathrm{Si}}_{0.8}$ films epitaxially grown on MgO(001) was determined experimentally by ferromagnetic resonance and superconducting quantum interference device magnetometry and theoretically by fully relativistic density functional theory calculations. The experimental average spin (orbital) moment of the stoichiometric ${\mathrm{Fe}}_{3}\mathrm{Si}$ $[{\ensuremath{\mu}}_{S(L)}^{\mathrm{av}}=1.38(0.051){\ensuremath{\mu}}_{B}]$ is in reasonable agreement with the theoretical one $[{\ensuremath{\mu}}_{S(L)}^{\mathrm{av}}=1.75(0.029){\ensuremath{\mu}}_{B}]$. Slight increases (reductions) of the Fe content are experimentally found to increase (decrease) the spin and orbital moments as predicted by theory. The results reveal an important step toward tailoring spin and orbital magnetism in the binary Heusler alloys.

This Topical Review presents an overview of the recent experimental results on the quantitative determination of the magnetic exchange parameters in ultrathin magnetic films and multilayers, grown on different substrates. The experimental approaches to probe both the symmetric Heisenberg as well as the antisymmetric Dzyaloshinskii--Moriya exchange interaction in ultrathin magnetic films and at interfaces are discussed in detail. It is explained how the experimental spectrum of magnetic excitations can be used to quantify the strength of these interactions.