Damon Eshbach Research Articles

Spin wave propagation detected over [formula omitted] in half-metallic Heusler alloy Co2MnSi

The field of magnon spintronics offers a charge current free way of information transportation by using spin waves (SWs). Compared to forward volume spin waves for example, Damon-Eshbach (DE) SWs need a relatively weak external magnetic field which is suitable for small spintronic devices. In this work we study DE SWs in Co2MnSi, a half-metallic Heusler alloy with significant potential for magnonics. Thin films have been produced by pulsed laser deposition. Integrated coplanar waveguide (CPW) antennas with different distances between emitter and detection antenna have been prepared on a Co2MnSi film. We used a vector network analyzer to measure spin wave reflection and transmission. We observe spin wave propagation up to 100μm, a new record for half-metallic Heusler thin films.

Light scattering by magnons in whispering gallery mode cavities

Brillouin light scattering is an established technique to study magnons, the elementary excitations of a magnet. Its efficiency can be enhanced by cavities that concentrate the light intensity. Here, we theoretically study inelastic scattering of photons by a magnetic sphere that supports optical whispering gallery modes in a plane normal to the magnetization. Magnons with low angular momenta scatter the light in the forward direction with a pronounced asymmetry in the Stokes and the anti-Stokes scattering strength, consistent with earlier studies. Magnons with large angular momenta constitute Damon Eschbach modes are shown to inelastically reflect light. The reflection spectrum contains either a Stokes or anti-Stokes peak, depending on the direction of the magnetization, a selection rule that can be explained by the chirality of the Damon Eshbach magnons. The controllable energy transfer can be used to manage the thermodynamics of the magnet by light.

Computation of Two-Port Parameters in Magnonic Devices Through Circuit-Field Coupling

This paper presents a model for the computation of two-port parameters from the configuration of a typical Magnonic device involving two coplanar waveguides. A Permalloy thin film is saturated using an appropriate dc field. The first waveguide then performs the function of excitation of spin waves by superposition of an RF field. The second waveguide detects the excited wave vector in Damon–Eshbach mode at resonance frequency. The model is solved using the generalized mean residual method for a linearized approximation of the Landau–Lifshitz–Gilbert equation coupled with Maxwell equations. Complex impedance matrices from the two-port network are extracted for both uniformly saturated and non-uniform initial ground states. Findings report low absorption potential and shift in resonance frequency for highly non-uniform ground states. Validation of the model for uniformly saturated ground states is performed against experimental and semi-analytical findings, showing good agreement.

Investigation of non-reciprocal magnon propagation using lock-in thermography

We have investigated the unidirectional spin wave heat conveyer effect in a 200 nm thin yttrium iron garnet (YIG) film using lock-in thermography (LIT). This originates from the non-reciprocal propagation of magnons, which leads to an asymmetric heat transport. To excite the spin waves we use two different respective antenna geometries: a coplanar waveguide (CPW) or a ‘microstrip’-like antenna on top of the YIG. By using the CPW and comparing the results for the Damon–Eshbach and the backward volume modes we are able to show that the origin of the asymmetric heat profile are indeed the non-reciprocal spin waves. Using the ‘microstrip’-like geometry we can confirm these results and we can even observe a distinct excitation profile along the antenna due to small field inhomogeneities.

Direct Observation of Interfacial Dzyaloshinskii-Moriya Interaction from Asymmetric Spin-wave Propagation in W/CoFeB/SiO2 Heterostructures Down to Sub-nanometer CoFeB Thickness.

Interfacial Dzyaloshinskii-Moriya interaction (IDMI) is important for its roles in stabilizing the skyrmionic lattice as well as soliton-like domain wall motion leading towards new generation spintronic devices. However, achievement and detection of IDMI is often hindered by various spurious effects. Here, we demonstrate the occurrence of IDMI originating primarily from W/CoFeB interface in technologically important W/CoFeB/SiO2 heterostructures using Brillouin light scattering technique. Due to the presence of IDMI, we observe asymmetry in the peak frequency and linewidth of the spin-wave spectra in the Damon-Eshbach (DE) geometry at finite k wave-vectors. The DMI constant is found to scale as the inverse of CoFeB thickness, over the whole studied thickness range, confirming the presence of IDMI in our system without any extrinsic effects. Importantly, the W/CoFeB interface shows no degradation down to sub-nanometer CoFeB thickness, which would be useful for devices that aim to use pronounced interface effects.

Magnonic crystals—Prospective structures for shaping spin waves in nanoscale

We have investigated theoretically band structure of spin waves in magnonic crystals with periodicity in one- (1D), two- (2D) and three-dimensions (3D). We have solved Landau–Lifshitz equation with the use of plane wave method, finite element method in frequency domain and micromagnetic simulations in time domain to find the dynamics of spin waves and spectrum of their eigenmodes. The spin wave spectra were calculated in linear approximation. In this paper we show usefulness of these methods in calculations of various types of spin waves. We demonstrate the surface character of the Damon–Eshbach spin wave in 1D magnonic crystals and change of its surface localization with the band number and wavenumber in the first Brillouin zone. The surface property of the spin wave excitation is further exploited by covering plate of the magnonic crystal with conductor. The band structure in 2D magnonic crystals is complex due to additional spatial inhomogeneity introduced by the demagnetizing field. This modifies spin wave dispersion, makes the band structure of magnonic crystals strongly dependent on shape of the inclusions and type of the lattice. The inhomogeneity of the internal magnetic field becomes unimportant for magnonic crystals with small lattice constant, where exchange interactions dominate. For 3D magnonic crystals, characterized by small lattice constant, wide magnonic band gap is found. We show that the spatial distribution of different materials in magnonic crystals can be explored for tailored effective damping of spin waves.

Tunable spin wave properties in [Co/Ni80Fe20]r multilayers with the number of bilayer repetition

Tunability of spin waves in [Co/Ni80Fe20]r multilayer films by varying the number of bilayer repetition (r) is investigated using Brillouin light scattering (BLS) spectroscopy. Conventional magnetometry shows a monotonic increase in the in-plane anisotropy with r. BLS spectra show a number of distinct modes, which were found to be Damon–Eshbach and perpendicular standing spin wave modes confined in several bilayers with different mode numbers by macrospin modelling. The extracted values of the magnetic parameters from the spin wave measurements agree reasonably well with the magnetometry results. A qualitative analysis of the dependence of BLS intensity of PSSW modes on the corresponding thickness has also been performed using a light-scattering theory for confined geometries. The results of the calculations are in agreement with experimental data.

A scenario for magnonic spin-wave traps.

Spatially resolved measurements of the magnetization dynamics on a thin CoFeB film induced by an intense laser pump-pulse reveal that the frequencies of resulting spin-wave modes depend strongly on the distance to the pump center. This can be attributed to a laser generated temperature profile. We determine a shift of 0.5 GHz in the spin-wave frequency due to the spatial thermal profile induced by the femtosecond pump pulse that persists for up to one nanosecond. Similar experiments are presented for a magnonic crystal composed of a CoFeB-film based antidot lattice with a Damon Eshbach mode at the Brillouin zone boundary and its consequences are discussed.

Experimental study of spin-wave dispersion in Py/Pt film structures in the presence of an interface Dzyaloshinskii-Moriya interaction

Brillouin light scattering (BLS), complemented by ferromagnetic resonance (FMR) characterization, has been used for studying spin-wave (SW) propagation in $\mathrm{Py}(L)/\mathrm{Pt}(6\text{\ensuremath{-}}\mathrm{nm})$ bilayers of various Py thicknesses $(4\phantom{\rule{0.16em}{0ex}}\mathrm{nm}\ensuremath{\le}L\ensuremath{\le}10\phantom{\rule{0.16em}{0ex}}\mathrm{nm})$. The FMR measurements allowed determination of the pertinent magnetic parameters and revealed the existence of a normal surface anisotropy. A pronounced asymmetry of Damon-Eshbach (DE) wave frequencies has been evidenced by BLS. Therefore, the difference between Stokes and anti-Stokes DE frequencies has been measured versus SW wave number for all the samples. A detailed discussion about the origin of this frequency difference is reported, which concludes that this is due to interface Dzyaloshinskii-Moriya interaction (IDMI).

Comprehensive Theoretical and Experimental Analysis of Spin Waves in Magnetic Thin Film

Magnetic spin waves (SWs) have been induced and detected in a Ni80Fe20 thin film by two identical copper coplanar waveguides (CPWs). The dc fields up to 25 kA/m have been applied parallel to the CPWs, and magnetostatic surface SWs have been generated by applying voltages through the first CPW (source). The second CPW (pickup), $12~\mu $ m apart from the source, has been used to detect the ensuing SWs. Two independent setup and methods have been applied. Time domain SW measurements were performed by the application of voltage steps and the ensuing SW signal has been measured using a 16 GHz oscilloscope. Frequency domain SW spectroscopy was performed using a two-port vector network analyzer measurement in a frequency range from 10 MHz to 10 GHz. In addition, micromagnetic simulations were performed under the harmonic regime by assuming the induced SWs as perturbations of the saturated ground state excited by a small RF field. The numerically computed dispersion relation closely follows the Damon–Eshbach curve and is in good agreement with the experimental data.

Nonreciprocal dispersion of spin waves in ferromagnetic thin films covered with a finite-conductivity metal

We study the effect of one-side metallization of a uniform ferromagnetic thin film on its spin-wave dispersion relation in the Damon–Eshbach geometry. Due to the finite conductivity of the metallic cover layer on the ferromagnetic film, the spin-wave dispersion relation may be nonreciprocal only in a limited wave-vector range. We provide an approximate analytical solution for the spin-wave frequency, discuss its validity, and compare it with numerical results. The dispersion is analyzed systematically by varying the parameters of the ferromagnetic film, the metal cover layer and the value of the external magnetic field. The conclusions drawn from this analysis allow us to define a structure based on a 30 nm thick CoFeB film with an experimentally accessible nonreciprocal dispersion relation in a relatively wide wave-vector range.

Observation and suppression of quantized spin waves in microfabricated permalloy elements

We report the observation and suppression of quantized spin wave modes in a microfabricated rectangular permalloy (Py) element. The Py element was located on a coplanar waveguide and was connected to a Cu wire. The quantized Damon–Eshbach spin wave and the perpendicular standing spin wave modes appeared in the resonance spectra for the Py elements. Those non-uniform magnetization dynamics were suppressed when the non-uniformity of the radio frequency magnetic field for the excitation was reduced by changing the design of the device structure.

Femtosecond laser excitation of multiple spin waves and composition dependence of Gilbert damping in full-Heusler Co2Fe1−xMnxAl films

Spin-wave dynamics in 30 nm thick Co2Fe1−xMnxAl full-Heusler films is investigated using time-resolved magneto-optical polar Kerr spectroscopy under an external field perpendicular to films. Damon-Eshbach (DE) and the first-order perpendicular standing spin-wave (PSSW) modes are observed simultaneously in four samples with x = 0, 0.3, 0.7, and 1. The frequency of DE and PSSW modes does not apparently depend on composition x, but damping of DE mode significantly on x and reaches the minimum as x = 0.7. The efficient coherent excitation of DE spin wave exhibits the promising application of Co2Fe0.3Mn0.7Al films in magnonic devices.

Impact of eddy currents on the dispersion relation of surface spin waves in thin conducting magnetic films

We propose a rigorous solution to a long-standing problem of the impact of eddy currents on the dispersion relation of surface spin waves propagating in thin conducting magnetic films. Our results confirm the prediction of the Almeida–Mill's exchange-free theory that the inclusion of the eddy-current contribution results in a deviation of the dispersion curve for the fundamental mode from the Damon–Eshbach law and a substantial linewidth broadening in a large wave vector range. We show that the decrease in the spin-wave frequency is due to an increase in the in-plane component of the dynamic magnetic field within the conducting film. The decrease in the frequency is accompanied by a drastic change in the asymmetry of the modal profiles for the waves. This effect is not observable in magneto-insulating films and therefore it is unambiguously attributed to eddy currents that appear in conducting films only. We also show that the wave vector range in which eddy currents affect the dispersion curve is strongly correlated with the value of the film conductivity. This result holds for conducting films with the thickness 10–100 nm, which are considered promising for future magnonic and spintronic applications.

Generation of propagating backward volume spin waves by phase-sensitive mode conversion in two-dimensional microstructures

We present the generation of propagating backward volume (BV) spin waves in a T shaped Ni81Fe19 microstructure. These waves are created from counterpropagating Damon Eshbach spin waves, which are excited using microstrip antennas. By employing Brillouin light scattering microscopy, we show how the phase relation between the counterpropagating waves determines the mode generated in the center of the structure, and prove its propagation inside the longitudinally magnetized part of the T shaped microstructure. This gives access to the effective generation of backward volume spin waves with full control over the generated transverse mode.

Spin wave quantization in continuous film with stripe domains

We investigated the spin wave dynamics of CoFeSiB film, which has a stripe domain structure at a low magnetic field region (<1 kOe). We measured the spin wave excitation spectra by employing Brillouin light scattering. Abnormal field dependence and dispersion relations were observed, and they are similar to spin wave quantization in laterally confined magnetic structures such as arrays of magnetic nanowires. The observed spin wave excitation spectra must be interpreted with spin wave quantization such as Damon–Eshbach mode separation. It was found that the spin wave quantization is related to the stripe magnetic domain structure in continuous film. The physical origin of the quantization is the partial reflection of the propagating spin wave at the periodic stripe domain boundaries.

Magnetic excitations in (SiO 2)Co nano-composite films: Brillouin light scattering study

Behaviour of magnetic excitations in the Damon–Eshbach (DE) and backward volume (BV) geometries in nano-composite (SiO 2) 100− x Co x (50% at< x<80% at) films has been studied by Brillouin light scattering (BLS). It has been shown that it is the structure of Stokes/anti-Stokes BLS lines in the DE geometry that allows reliable identification of dipole-exchange spin waves (SW) and numerical estimation of the value of the effective exchange constant A eff of a super-ferromagnetic nano-granular sample ( x=80% at).

Brillouin light scattering observation of the transition from the superparamagnetic to the superferromagnetic state in nanogranular (SiO2)Co films

Evolution of magnetic excitations from purely magnetostatic modes to dipole-exchange spin waves in the Damon–Eshbach geometry in nanocomposite (SiO2)100−xCox (50 at. %&amp;lt;x&amp;lt;80 at. %) films during a transition from superparamagnetic to superferromagnetic state has been studied by Brillouin light scattering. It has been shown that removal of the degeneracy of backward volume magnetostatic waves due to nonzero exchange constant manifests itself in more pronounced Stokes/anti-Stokes asymmetry. Accompanying static characterization by means of magnetic force microscopy imaging and grazing incidence small angle neutron scattering has demonstrated the presence of weak stripe domains in the superferromagnetic phase.

Intrinsic and non-local Gilbert damping in polycrystalline nickel studied by Ti : sapphire laser fs spectroscopy

The use of femtosecond laser pulses generated by a Ti : sapphire laser system allows us to gain an insight into the magnetization dynamics on time scales from sub-picosecond up to 1 ns directly in the time domain. This experimental technique is used to excite a polycrystalline nickel (Ni) film optically and probe the dynamics afterwards. Different spin-wave modes (the Kittel mode, perpendicular standing spin-wave modes and dipolar spin-wave modes (Damon–Eshbach modes)) are identified as the Ni thickness is increased. The Kittel mode allows determination of the Gilbert damping parameter α extracted from the magnetization relaxation time τα. The non-local damping by spin currents emitted into a non-magnetic metallic layer of vanadium (V), palladium (Pd) and the rare earth dysprosium (Dy) are studied for wedge-shaped Ni films of 1–30 nm. The damping parameter increases from α = 0.045 intrinsic for nickel to α > 0.10 for the heavy materials, such as Pd and Dy, for the thinnest Ni films below 10 nm thickness. Also, for the thinnest reference Ni film thickness, an increased magnetic damping below 4 nm is observed. The origin of this increase is discussed within the framework of line broadening by locally different precessional frequencies within the laser spot region.

Quantization of spin waves in oval-shaped nanorings

Regular arrays of oval-shaped permalloy nanorings have been fabricated using deep ultraviolet lithography and their spin dynamics measured by Brillouin light scattering with the magnetic field applied along long (easy) axes of the rings. The dispersionless behavior of the spin wave modes observed reveals their standing wave nature. Two-dimensional simulations and analytical calculations have been performed for a single isolated nanoring. Results reveal that the observed modes can be interpreted in terms of quantized Damon–Eshbach modes due to lateral confinement in the finite size rings.