Surface Magnons Research Articles

Controlling spins with surface magnon polaritons

Polaritons in metals, semimetals, semiconductors, and polar insulators can allow for extreme confinement of electromagnetic energy, providing many promising opportunities for enhancing typically weak light-matter interactions such as multipolar radiation, multiphoton spontaneous emission, Raman scattering, and material nonlinearities. These extremely confined polaritons are quasielectrostatic in nature, with most of their energy residing in the electric field. As a result, these ``electric'' polaritons are far from optimized for enhancing emission of a magnetic nature, such as spin relaxation, which is typically many orders of magnitude slower than corresponding electric decays. Here, we take concepts of ``electric'' polaritons into magnetic materials, and propose using surface magnon polaritons in negative magnetic permeability materials to strongly enhance spin relaxation in nearby emitters. Specifically, we provide quantitative examples with ${\mathrm{MnF}}_{2}$ and ${\mathrm{FeF}}_{2}$, enhancing spin transitions in the THz spectral range. We find that these magnetic polaritons in 100-nm thin films can be confined to lengths over 10 000 times smaller than the wavelength of a photon at the same frequency, allowing for a surprising 12 orders of magnitude enhancement in magnetic dipole transitions. This takes THz spin-flip transitions, which normally occur at timescales on the order of a year, and forces them to occur at sub-ms timescales. Our results suggest an interesting platform for polaritonics at THz frequencies, and more broadly, a way to use polaritons to control light-matter interactions.

Three-dimensional topological magnon systems

We propose a class of models for a magnonic analog of topological insulators in three dimensions. The models have pseudo-time-reversal symmetry which ensures the existence of bosonic Kramers pairs. We define a set of $\mathbb{Z}_2$ topological invariants that characterizes different topological phases and determines the presence or absence of surface Dirac cones. This is demonstrated by considering a bosonic counterpart of the Fu-Kane-Mele model on a diamond lattice. The model is found to exhibit three distinct phases analogous to strong topological, weak topological, and trivial insulator phases of the original fermionic model. We also discuss a possible realization of the thermal Hall effect of surface magnons in the presence of a magnetic field in proximity to a normal ferromagnet.

Optimal mode matching in cavity optomagnonics

Inelastic scattering of photons is a promising technique to manipulate magnons but it suffers from weak intrinsic coupling. We theoretically discuss an idea to increase optomagnonic coupling in optical whispering gallery mode cavities, by generalizing previous analysis to include the exchange interaction. We predict that the optomagnonic coupling constant to surface magnons in yttrium iron garnet (YIG) spheres with radius $300\,\mathrm{\mu}$m can be up to $40$ times larger than that to the macrospin Kittel mode. Whereas this enhancement falls short of the requirements for magnon manipulation in YIG, nanostructuring and/or materials with larger magneto-optical constants can bridge this gap.

Surface dynamics of rough magnetic films

The chirality of Damon-Eshbach (DE) magnons affects the transport of energy and angular momentum at the surface of magnetic films and spheres. We calculate the surface-disorder-limited dephasing and transport lifetimes of surface modes of sufficiently thick high-quality magnetic films such as yttrium iron garnet. In spite of their chirality surface magnons are not protected, but interact strongly with smooth surface roughness. Nevertheless, for long-range disorder the transport is much less affected by the suppressed back scattering (vertex correction). Moreover, in the presence of roughness ferromagnetic resonance under a \textit{uniform} microwave field can gennerate a considerable amount of surface magnons.

The discrete noise of magnons

We report experimental results, which show that the low-frequency noise of magnonic devices is dominated by the random telegraph signal noise rather than 1/f noise—a striking contrast to many electronic devices (f is a frequency). It was also found that the noise level of surface magnons depends strongly on the power level, increasing sharply at the on-set of nonlinear dissipation. The presence of the random telegraph signal noise suggests that the current fluctuations involve random discrete macro events caused by an individual macro-scale fluctuator. We anticipate that our results will help in developing the next generation of magnonic devices for information processing and sensing.

Effect of Chemical Composition on Volume and Surface Magnon Creation in Multilayer CoxPt1−x/Pt

The existence of two types of magnon populations created in volume and in the surfaces of the CoxPt1−x/Pt multilayer was demonstrated. A band structure of these magnons was evidenced by the shape of the calculated excitation spectra. Two bands of energy allowed for a creation of these magnetic excitations are obtained. “Creation gaps” and associated lifetimes are also calculated. The comparison of the calculated relaxation times with experimental results is more than satisfactory.

Chiral topological insulator of magnons

We propose a magnon realization of 3D topological insulator in the AIII (chiral symmetry) topological class. The topological magnon gap opens due to the presence of Dzyaloshinskii-Moriya interactions. The existence of the topological invariant is established by calculating the bulk winding number of the system. Within our model, the surface magnon Dirac cone is protected by the sublattice chiral symmetry. By analyzing the magnon surface modes, we confirm that the backscattering is prohibited. By weakly breaking the chiral symmetry, we observe the magnon Hall response on the surface due to opening of the gap. Finally, we show that by changing certain parameters the system can be tuned between the chiral topological insulator (mcTI), three dimensional magnon anomalous Hall (3D-mAH), and Weyl magnon phases.

Non-reciprocity and topology in optics: one-way road for light via surface magnon polariton

We show how non-reciprocity and topology are used to construct an optical one-way waveguide in the Voigt geometry. First, we present a traditional approach of the one-way waveguide of light using surface polaritons under a static magnetic field. Second, we explain a recent discovery of a topological approach using photonic crystals with the magneto-optical coupling. Third, we present a combination of the two approaches, toward a broadband one-way waveguide in the microwave range.

A corrugated perfect magnetic conductor surface supporting spoof surface magnon polaritons.

In this paper, we demonstrate that spoof surface magnon polaritons (SSMPs) can propagate along a corrugated perfect magnetic conductor (PMC) surface. From duality theorem, the existence of surface electromagnetic modes on corrugated PMC surfaces are manifest to be transverse electric (TE) mode compared with the transverse magnetic (TM) mode of spoof surface plasmon plaritons (SSPPs) excited on corrugated perfect electric conductor surfaces. Theoretical deduction through modal expansion method and simulation results clearly verify that SSMPs share the same dispersion relationship with the SSPPs. It is worth noting that this metamaterial will have more similar properties and potential applications as the SSPPs in large number of areas.

Magnetic fluctuations in topological insulators with ordered magnetic adatoms: Cr on Bi2Se3from first principles

We consider a system consisting of a topological insulator with an array of magnetic adatoms interacting with the surface states of electrons. We find that the indirect coupling of the magnetic impurities results in a ferromagnetic ordering of the magnetic moments and is also responsible for the unusual linear dispersion of the surface magnons. We also show that the interaction of magnons with surface electrons essentially renormalizes the electron energy spectrum. The renormalized spectrum is nonlinear and can be characterized by a negative effective mass of electrons and holes for any $k\ensuremath{\ne}0$. The electron velocity near the Dirac point depends on the electron-magnon coupling.

Surface magnons probed by spin-polarized electron energy loss spectroscopy

Short-wavelength magnons at ferromagnetic surfaces can be probed by electrons. The unique property of electrons, i.e. having a very strong interaction with the surface together with the spin degree of freedom enables one to investigate the spin dependent quasi-particles, e.g. magnons at magnetic surfaces.We review the experimental results of short-wavelength magnons probed at ferromagnetic Co(0001) and Fe(110) surfaces by spin-polarized electron energy-loss spectroscopy. The differences and similarities to their bulk counterpart are discussed in detail. Although in the case of Co(0001) surface magnons behave similar to the ones in bulk Co, in the case of Fe(110) they possess a smaller exchange stiffness meaning that the effective exchange coupling is smaller at the surface. In both cases, surface magnons have an extremely short lifetime being in the order of a few tens of femtosecond.

NEW CLASS OF SURFACE MAGNON POLARITONS IN ENANTIOMERIC ANTIFERROMAGNETIC STRUCTURES

A novel class of surface magnon polaritons supported in identical enantiomeric antiferromagnetic structures is presented. The surface waves arise due to bianisotropy. The existence of two distinct surface modes with unusual dispersion and polarization properties is predicted. The role of losses is investigated and the propagation length of the surface waves is determined.

Elementary Excitations at Magnetic Surfaces and Their Spin Dependence

The elementary surface excitations are studied by spin-polarized electron energy loss spectroscopy on a prototype oxide surface [an oxygen passivated Fe(001)-p(1×1) surface], where the various excitations coexist. For the first time, the surface phonons and magnons are measured simultaneously and are distinguished based on their different spin nature. The dispersion relation of all excitations is probed over the entire Brillouin zone. The different phonon modes observed in our experiment are described by means of ab initio calculations.

Inelastic electron tunneling spectrum from surface magnon and magnetic impurity scatterings in magnetic tunnel junctions

Analytic expressions for contributions to the inelastic electron tunneling spectrum (IETS) from surface magnon scattering and magnetic impurity scattering are obtained. It is shown that surface magnon scattering alone does not lead to peaks in the IETS. The peaks at small bias often observed in the IETS of magnetic junctions are due to magnetic impurity scattering, in agreement with the traditional model for zero-bias anomaly. These impurity resonance peaks can sometimes split due to the impurities' magnetic coupling to the electrodes. Measurements of AlO and MgO barrier junctions yield excellent agreement to the theory. The experiment further shows that the magnetic impurities in MgO barriers are strongly coupled to the electrodes but those in AlO barriers are not magnetically coupled to the electrodes.

Inelastic tunneling spectra of MgO barrier magnetic tunneling junctions showing large magnon contribution

We investigated bias-voltage and temperature dependence of conductivity arising from the magnon contribution in MgO-based magnetic tunneling junctions (MTJs) with different ferromagnetic electrodes. Second derivative conductance curves showed broad peak structure, which extends from 5 to 200 mV, accompanied by additional peaks at around 23, 54, and 85 mV. The peak intensities were larger for antiparallel configuration than for parallel configuration except that at 85 mV. This difference in the peak intensity was observed to be larger for the MTJs having higher tunneling magnetoresistance ratio, indicating a magnetic origin of these peaks. Abrupt increase in the second derivative conductance at very low biasing voltage in the antiparallel configuration suggests the important role of the surface magnon excitation.

Impact of interface crystallization on inelastic tunneling in Al∕AlOx∕CoFeB

We report the change in inelastic electron tunneling spectra (IETS) for Al∕AlOx∕CoFeB∕Al junctions when the structure of CoFeB at its interface with AlOx is intentionally changed from quasiamorphous to highly textured fcc. While for the quasiamorphous interface there are signs of the size quantization of magnons, the spectra for the fcc interface show distinct excitations at bias voltages associated with known surface magnon modes in fcc Co. These results demonstrate that IETS can be used as a tool to probe distinct structural changes of the magnetic electrode in tunnel junctions.

Electron spectrometers for inelastic scattering from magnetic surface excitations

AbstractThis paper compares the performance of two different designs for electron spectrometers to be used for inelastic scattering of spin‐polarized electrons from surface magnons and Stoner excitations. The recently proposed combination of monochromators with 90° and 180° deflection angles ensures a maximum spin asymmetry signal while providing still a good quality of the monochromatic beam. Higher intensities, however, are obtained with the classical 146° /146° combination, which features a spin polarization longitudinal to the beam direction at the target. We present calculations for the monochromatic current for both combinations and argue that the use of the classical spectrometer type may be advantageous. Copyright © 2006 John Wiley & Sons, Ltd.

Bulk and surface spin excitations in thin films of manganites

A review of experimental studies of magnon excitations in manganites is presented. A microwave resonance technique, in particular a spin-wave resonance (SWR) technique, is shown to allow the study of not only bulk properties but also surface properties of manganite films. There are two main theoretical approaches used to interpret the SWR spectra: the volume inhomogeneity and the surface inhomogeneity models. The latter, introduced by Puszkarski, allows the interpretation of the observed surface magnon excitations in thin films. It is found that SWR spectra show evidence of manganites, besides bulk spin-wave modes, and also of surface spin-waves. This observation is used to determine the surface magnetic anisotropy. The effect of surface anisotropy on the spin-wave excitation conditions in epitaxial La 0.7 Mn 1.3 O 3-δ films is investigated in detail. SWR data give indirect evidence of periodic charge-ordered stripe formation. A strong effect of thermal annealing on stripe structure is observed and discussed.

Magnon Excitations in Manganites

A review of experimental studies of magnon excitations in manganites is presented. Two kinds of techniques: the inelastic neutron scattering (in bulk materials) and the microwave resonance (in thin fllms) are considered. Experimental studies of spin dynamics by inelastic neutron scattering in metallic ferromagnetic manganites have shown that at low temperature for small wave vectors k ! 0 the dispersion relation has a quadratic shape similar to that observed in Heisenberg ferromagnets. However, the above technique although very informative can be used only for su‐ciently large samples of bulk materials. A complementary microwave resonance technique allows studying not only bulk properties, but also surface properties. There are two main theoretical approaches used to interpret the spin wave resonance spectra: the volume inhomogeneity and the surface inhomogeneity models. The last one introduced by Puszkarski has allowed for interpretation of the observed surface magnon excitations in thin fllms.

Influence ofs−dinterfacial scattering on the magnetoresistance of magnetic tunnel junctions

We propose the two-band s-d model to describe theoretically a diffuse regime of the spin-dependent electron transport in magnetic tunnel junctions (MTJ's) of the form F/O/F where F's are 3d transition metal ferromagnetic layers and O is the insulating spacer. We aim to explain the strong interface sensitivity of the tunneling properties of MTJ's and investigate the influence of electron scattering at the nonideal interfaces on the degradation of the TMR magnitude. The generalized Kubo formalism and the Green's functions method were used to calculate the conductance of the system. The vertex corrections to the conductivity were found with the use of "ladder" approximation combined with the coherent-potential approximation (CPA) that allowed to consider the case of strong electron scattering. It is shown that the Ward identity is satisfied in the framework of this approximation that provides the necessary condition for a conservation of a tunneling current. Based on the known results of ab-initio calculations of the TMR for ballistic junctions, we assume that exchange split quasi-free s-like electrons with the density of states being greater for the majority spin sub-band give the main contribution to the TMR effect. We show that, due to interfacial inter-band scattering, the TMR can be substantially reduced even down to zero value. This is related to the fact that delocalized quasi-free electrons can scatter into the strongly localized d sub-band with the density of states at the Fermi energy being larger for minority spins compared to majority spins. It is also shown that spin-flip electron scattering on the surface magnons within the interface leads to a further decrease of the TMR at finite temperature.