Perpendicular Standing Spin Wave Modes Research Articles

Large Anomalous Frequency Shift in Perpendicular Standing Spin Wave Modes in BiYIG Films Induced by Thin Metallic Overlayers.

Interface-driven effects on magnon dynamics are studied in magnetic insulator-metal bilayers using Brillouin light scattering. It is found that the Damon-Eshbach modes exhibit a significant frequency shift due to interfacial anisotropy generated by thin metallic overlayers. In addition, an unexpectedly large shift in the perpendicular standing spin wave mode frequencies is also observed, which cannot be explained by anisotropy-induced mode stiffening or surface pinning. Rather, it is suggested that additional confinement may result from spin pumping at the insulator-metal interface, which results in a locally overdamped interface region. These results uncover previously unidentified interface-driven changes in magnetization dynamics that may be exploited to locally control and modulate magnonic properties in thin-film heterostructures.

Magnetic Damping Properties of Single-Crystalline Co55Mn18Ga27 and Co50Mn18Ga32 Films

We investigate the structural, static magnetic and damping properties in two Mn-deficient magnetic Weyl semimetal Co-Mn-Ga (CMG) alloy films, i.e., Co55Mn18Ga27 (CMG1) and Co50Mn18Ga32 (CMG2), which were epitaxially grown on MgO (001) substrates. CMG1 has a mixing phase of B 2 and L21, larger saturation magnetization (M s ∼ 760 emu/cm3), stronger in-plane magnetic anisotropy. CMG2 has an almost pure B2 phase, smaller M s (∼ 330 emu/cm3), negligible in-plane magnetic anisotropy. Time-resolved magneto-optical Kerr effect results unambiguously demonstrate an obvious perpendicular standing spin wave (PSSW) mode in addition to the Kittel mode for both of the CMG films. The intrinsic damping constant is about 0.0055 and 0.015 for CMG1 and CMG2, respectively, which are both significantly larger than that of the stoichiometric CMG (i.e., Co2MnGa) film reported previously. In combination with the first-principles calculations, the intrinsic damping properties of the Mn-deficient CMG films can be well explained by considering the increase of density of states at the Fermi level, reduction of M s, and excitation of the PSSW mode. These findings provide a new clue to tuning the magnetic damping of the magnetic Weyl semimetal film through slight off-stoichiometry.

Experimental observation of interlayer perpendicular standing spin wave mode with low damping in skyrmion-hosting [Pt/Co/Ta]10 multilayer

An interlayer perpendicular standing spin wave mode is observed in the skyrmion-hosting [Pt/Co/Ta]10 multilayer by measuring the time-resolved magneto-optical Kerr effect. The observed interlayer mode depends on the interlayer spin-pumping and spin transfer torque among the neighboring Co layers. This mode shows monotonically increasing frequency-field dependence which is similar to the ferromagnetic resonance mode, but within higher frequency range. Besides, the damping of the interlayer mode is found to be a relatively low constant value of 0.027 which is independent of the external field. This work expounds the potential application of the [heavy-metal/ferromagnetic-metal] n multilayers to skyrmion-based magnonic devices which can provide multiple magnon modes, relatively low damping, and skyrmion states, simultaneously.

Magnetic polarization selective spectroscopy of magnetic thin films probed by wideband crossed microstrip circuit in GHz regime.

We have developed a method to obtain wideband magnetic polarization selective spectra of magnetic thin films by using circularly polarized microwaves. The combination of an over-coupled crossed microstrip resonator and a hybrid coupler enables broadband and accurate control of circularly polarized microwaves. The performance of the present method was demonstrated with an yttrium iron garnet thin film, and we detected the magnetic polarization dependence of a Kittel mode and a perpendicular standing spin wave mode in the range of 3-20GHz. In addition, three types of crossed microstrip resonators were systematically studied with experiments and simulations. As a result, we achieved the polarization efficiency of 80% and found how to optimize the crossed microstrip resonator depending on the desired sensitivity, bandwidth, or sample size.

Measuring spin wave resonance in Ni100 x Fe x films: compositional and temperature dependence

We study the composition and temperature dependence of the spin wave resonance in NiFe films, where, x is 40% or below, with a thickness of 100 nm using a temperature variable broadband ferromagnetic resonance (FMR) spectroscopy. From the uniform FMR mode, the saturation magnetization and the Gilbert damping are measured, and using the first perpendicular standing spin wave mode, the exchange stiffness constant is determined. At lower temperatures, we measure an increase of both and A, while α shows a very weak temperature dependence. We find that follows perfectly Bloch’s law, while A is best described by a T 2 law. Additionally, we measure a linear increase in and A with the increase of Fe content in films, while α strongly decreases. The compositional variation of α and A can be ascribed to the increase of in Fe-rich films. Furthermore, we estimate the exchange length () in NiFe films. We find that is temperature independent, however, a strong decrease of is measured in Fe-rich films. This reduction is inversely proportional to .

Noncollinear Remanent Textures Induced by Surface Spin Flop in Synthetic Antiferromagnets with Perpendicular Anisotropy

The surface spin flop, observed in synthetic antiferromagnets (SAFs) with uniaxial anisotropy and strong antiferromagnetic (AF) interlayer exchange coupling, can be considered as a laterally homogeneous, vertical AF domain wall pushed into the SAF from either the top or the bottom in the presence of a strong external vertical magnetic field. As a result, the AF domain wall can be described as a one-dimensional entity. In this work, we present a concept to stabilize laterally homogeneous vertical AF domain walls by local variation of the perpendicular magnetic anisotropy in $\mathrm{Co}/\mathrm{Pt}$-based SAFs. Our approach not only allows the stabilization of the vertical AF domain wall in the absence of any external magnetic field, but furthermore enables a deterministic selection among four different remanent states, each one stable within a broad external magnetic field range of almost one tesla. We also demonstrate an extension to our concept by stabilizing two coexisting vertical AF domain walls, thus yielding a system with a total of six different selectable (and reprogrammable) remanent states. The controlled stabilization of noncollinear AF textures in the form of vertical AF domain walls at remanence could be used as an infrastructure for propagating spin waves within the AF domain wall itself, as well as for tuning the dynamic behavior of perpendicular standing spin wave modes existing vertically across the SAF.

Higher-order ferromagnetic resonances in out-of-plane saturated Co/Au magnetic multilayers

Artificial ferromagnetic (FM)/nonmagnetic multilayers, with large enough FM thickness to prevent the dominance of interface anisotropies, offer a straightforward insight into the understanding and control of perpendicular standing spin wave (PSSW) modes. Here we present a study of the static and dynamic magnetic properties of ${[\mathrm{Co}(3.0\phantom{\rule{0.16em}{0ex}}\mathrm{nm})/\mathrm{Au}(0.6\phantom{\rule{0.16em}{0ex}}\mathrm{nm})]}_{1\ensuremath{\le}N\ensuremath{\le}30}$ multilayers. Magnetometry reveals that the samples exhibit magnetization reversal properties typical of an effective single layer with weak perpendicular anisotropy, with the distinctive thickness-dependent magnetization reorientation transition from uniform in-plane to out-of-plane stripe domains at remanence. However, when such multilayer systems are out-of-plane saturated, the dynamic response reveals the existence of several different ferromagnetic resonances in the form of PSSW modes that strongly depend on the material modulation characteristics along the total thickness. These modes are induced by the layer stacking itself as the effective single layer model fails to describe the observed complex dynamics. In contrast to most systems considered in the past, described by a dynamic model of a single effectively homogeneous thick layer, the specific structures investigated here provide a unique platform for a large degree of tunability of the mode frequencies and amplitude profiles. We argue that the combination of periodic magnetic properties with vertical deformation gradients, arising from heteroepitaxial strain relaxation, converts the Au interlayer regions into a vertical regular array of magnetic pinning planes for the PSSW modes, which promotes the complex dynamics observed in this system.

Ultrafast coherent control of higher-order spin waves in a NiFe thin film by double-pulse excitation

By using a double-pulse excitation scheme, we demonstrate the optical control of higher-order spin waves in a ferromagnetic permalloy thin film. Three spin precession modes are observed in the film under single pulse excitation. Based on a theoretical calculation, these spin precession modes are assigned to the fundamental Kittel mode and the first- and second-order perpendicular standing spin-wave (PSSW) modes. In order to excite the first-order PSSW mode selectively, we use double-pulse excitation at 400 nm. We show that, by choosing an appropriate time interval between the two pump pulses, it is possible to obtain a relatively large amplitude of the first-order PSSW mode while the amplitudes of the other two modes are almost completely suppressed. By analyzing the spin dynamics, it is found that the spin precession motion under the double-pulse excitation condition can be explained by the interference between the spin waves that have been induced by the first and second pulses. Our finding indicates that the selective excitation of a spin precession mode with a high precession frequency in a ferromagnetic metal may be realized by a multiple-pump pulse excitation scheme with more than two pulses.

Field and fluence dependences of laser-induced multiple spin-wave dynamics in Co2FeAl0.5Si0.5 films

Field- and fluence-dependent spin-wave dynamics in full-Heusler Co2FeAl0.5Si0.5 films are studied by using time-resolved magneto-optical polar Kerr spectroscopy. Volume magnetostatic spin-wave (VMSW) and perpendicular standing spin-wave (PSSW) modes are excited in the films with thicknesses of 60 and 100 nm, while only the Kittel mode is observed in the films with thicknesses of 150 and 200 nm. The amplitudes of all three spin-wave modes increase with increasing field and fluence, and the frequencies slightly decrease with increasing fluence as expected. The lifetimes and effective damping are found to be modulated by both the external field and excitation fluence. The effective damping of VMSW and Kittel modes presents strong field dependences that are significantly different from each other and can be attributed to the field-dependent group velocity and magnetic inhomogeneity, respectively. In addition, the effective damping of VMSW and PSSW modes increases with increasing fluence, showing a dominant mechanism of electron–phonon scattering enhancement. However, that of the Kittel mode decreases with increasing fluence and further demonstrates the dominant contribution from magnetic inhomogeneity in this case, which is inferred to suppress the excitation of the VMSW mode.

Discussion of strong pinning effect via nonuniform PSSW mode in Fe/NiFe/Fe multi-layer films with different Fe film thicknesses

Multilayer films with high saturation magnetization and low ferromagnetic resonance linewidth have great potential for various applications in the era of high frequency and miniaturization. In this study, Fe (t nm)/Ni81Fe19 (50 nm)/Fe (t nm) multilayer films were fabricated using electron beam evaporation, and the microstructure, saturation magnetization, coercivity, ferromagnetic resonance (FMR) linewidth and perpendicular standing spin-wave (PSSW) mode are discussed in detail. With increasing Fe film thickness, the saturation magnetization of the multilayer films increased from 10,547 to 14,494 Gs, becoming closer to the theoretical value, and the ferromagnetic resonance linewidth showed an overall increasing trend. It is worth noting that a strong pinning effect has been found via the appearance of the PSSW mode in the FMR spectrum, when the thickness of Fe reaches 4 nm. As the thickness of the Fe film increased from 4 to 15 nm, the perpendicular standing spin-wave field (Hp) increased from 133 to 379 Oe, and the perpendicular standing spin-wave linewidth (ΔHp) increased from 68 to 148 Oe. Remarkably, with 8 nm thick Fe film, the multilayer films achieved excellent comprehensive performances with high saturation magnetization (4πMs, 13,525 Gs), a low in-plane ferromagnetic resonance field (Hr, 725 Oe), and a low FMR linewidth (ΔH, 84.24 Oe).

Role of magnetic anisotropy in the ultrafast magnetization dynamics of Gd-Fe thin films of different thicknesses

The time-resolved magneto-optical Kerr effect technique has been exploited to study the magnetization dynamics over femtosecond to nanosecond timescale for Gd-Fe thin films with different thicknesses. The precessional magnetization dynamics is studied by varying the magnitude and orientation of the bias magnetic field from nearly in-plane to nearly out-of-plane (OOP) direction. The analysis of bias magnetic field dependence of precessional frequency has revealed the development of OOP anisotropy when the film thickness was increased from 20 to 100 nm and this is in good agreement with the static magnetic properties of the films. Moreover, the 100-nm-thick film has shown a signature of perpendicular standing spin wave modes along with the uniform Kittel mode. The evolution of their frequency, decay time, and damping with the magnitude and orientation of bias magnetic field is analyzed in detail, showing possible energy transfer between the perpendicular standing spin wave modes and the uniform Kittel mode. Interestingly, the effective damping could be tuned over a broader range of 0.02 to 0.15, which shows the potential for applications in miniaturized and fast magnetic storage, magnetic memory, and magnonic devices.

Enhancement of intrinsic magnetic damping in defect-free epitaxial Fe3O4 thin films

We have investigated the magnetic damping of precessional spin dynamics in defect-controlled epitaxial grown Fe3O4(111)/Yttria-stabilized Zirconia nanoscale films by all-optical pump-probe measurements. The intrinsic damping constant of the defect-free Fe3O4 film is found to be strikingly larger than that of the as-grown Fe3O4 film with structural defects. We demonstrate that the population of the first-order perpendicular standing spin wave (PSSW) mode, which is exclusively observed in the defect-free film under sufficiently high external magnetic fields, leads to the enhancement of the magnetic damping of the uniform precession (Kittel) mode. We propose a physical picture in which the PSSW mode acts as an additional channel for the extra energy dissipation of the Kittel mode. The energy transfer from the Kittel mode to the PSSW mode increases as in-plane magnetization precession becomes more uniform, resulting in the unique intrinsic magnetic damping enhancement in the defect-free Fe3O4 film.

Resonant magnetization switching conditions of an exchange-coupled bilayer under spin wave excitation

We systematically investigated spin wave-assisted magnetization switching (SWAS) of a L10-FePt/Ni81Fe19 (permalloy; Py) exchange-coupled bilayer by using a pulse-like rf field (hrf) and mapped the switching events in the magnetic field (H)–hrf frequency (f) plane in order to reveal the switching conditions. Switching occurred only in a limited region followed by the dispersion relationship of the perpendicular standing spin wave modes in the Py. The results indicate that SWAS is a resonant magnetization switching process, which is different from the conventional microwave assisted switching and has the potential to be used for selective switching in multilevel recording media.

Simultaneous laser excitation of backward volume and perpendicular standing spin waves in full-Heusler Co2FeAl0.5Si0.5 films

Spin-wave dynamics in full-Heusler Co2FeAl0.5Si0.5 films are studied using all-optical pump-probe magneto-optical polar Kerr spectroscopy. Backward volume magnetostatic spin-wave (BVMSW) mode is observed in films with thickness ranging from 20 to 100 nm besides perpendicular standing spin-wave (PSSW) mode, and found to be excited more efficiently than the PSSW mode. The field dependence of the effective Gilbert damping parameter appears especial extrinsic origin. The relationship between the lifetime and the group velocity of BVMSW mode is revealed. The frequency of BVMSW mode does not obviously depend on the film thickness, but the lifetime and the effective damping appear to do so. The simultaneous excitation of BVMSW and PSSW in Heusler alloy films as well as the characterization of their dynamic behaviors may be of interest for magnonic and spintronic applications.

Perpendicular Standing Spin Wave and Magnetic Anisotropic Study on Amorphous FeTaC Films

Magnetic anisotropy, spin wave (SW) excitation and exchange stiffness constant of amorphous FeTaC ($d$ = 20-200 nm) films were studied as a function of thickness using micro-strip ferromagnetic resonance (MS-FMR) technique. The MS-FMR spectra for in-plane applied magnetic field show the presence of uniform precessional mode ($n$ = 0) along with first perpendicular standing spin wave (PSSW) mode ($n$ = 1) especially for $d$ = 50, 100 and 200 nm films. The angular ($\varphi_{H}$) dependence of resonance field ($H_{r}$) and magnetic field dependence of resonance frequencies ($f_{r}$) in planar configuration for the uniform and PSSW modes were modeled successfully by using dispersion relation which arises from a combination of exchange and dipolar interactions. The relevant parameters such as saturation magnetization ($4\pi M_{S}$), uniaxial anisotropic constant ($K_{u}$), $g$-factor, and exchange stiffness constants ($A_{ex}$) are estimated for different FeTaC film thickness. $A_{ex}$ is found to increase from 1.52(4)$\times$10$^{-7}$ to 5.0(5)$\times$10$^{-6}$ erg/cm as the thickness of film increases from 50 to 200 nm, possibly due to surface pinning effect or significant inhomogeneity especially at higher thickness films.

Resonant switching for an in-plane magnetized L10-FePt | Ni81Fe19 bilayer under spin wave excitation

We report the magnetization dynamics and the magnetization switching mechanism under spin wave excitation in exchange-coupled bilayers. The in-plane magnetized L10-FePt and Ni81Fe19 (permalloy; Py) layers were exchange-coupled through the interface, and the spatially twisted magnetic structure was formed when the external magnetic field was applied owing to the difference in the switching fields (Hsw) between L10-FePt and Py. The ferromagnetic resonance spectra for the microfabricated bilayer element indicated that several kinds of quantized spin wave modes existed. Among the quantized spin wave modes, the perpendicular standing spin wave (PSSW) modes were amplified remarkably when the magnetization dynamics were largely excited by applying the external rf magnetic field (Hrf). Consequently, the microfabricated bilayer element showed clear Hsw reduction due to the assistance of PSSW. In addition, narrow Hsw distribution was observed for the spin wave-assisted magnetization switching. We examined the characteristic tendency of spin wave-assisted magnetization switching using pulse-like Hrf application measurement, and found that the resonant magnetization switching was mainly induced in the conditions exciting the PSSW modes but unexpected off-resonant switching also appeared at the high magnetic field.

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.

Systematic study of the spin stiffness dependence on phosphorus alloying in the ferromagnetic semiconductor (Ga,Mn)As

We study the dependence of the spin stiffness constant on the phosphorus concentration in the ferromagnetic semiconductor (Ga,Mn)(As,P) with the aim of determining whether alloying with phosphorus is detrimental, neutral, or advantageous to the spin stiffness. Time-resolved magneto-optical experiments are carried out in thin epilayers. Laser pulses excite two perpendicular standing spin wave modes, which are exchange related. We show that the first mode is spatially uniform across the layer corresponding to a k≈0 wavevector. From the two frequencies and k-vector spacings we obtain the spin stiffness constant for different phosphorus concentrations using weak surface pinning conditions. The mode assessment is checked by comparison to the spin stiffness obtained from domain pattern analysis for samples with out-of-plane magnetization. The spin stiffness is found to exhibit little variation with phosphorus concentration in contradiction with ab-initio predictions.

Measurements of the exchange stiffness of YIG films using broadband ferromagnetic resonance techniques

Measurements of the exchange stiffness D and the exchange constant A of Yttrium Iron Garnet (YIG) films are presented. YIG films with thicknesses from 0.9 to 2.6 µm were investigated with a microwave setup in a wide frequency range from 5 to 40 GHz. The measurements were performed with the external static magnetic field applied in-plane and out-of-plane. The method of Schreiber and Frait (1996 Phys. Rev. B 54 6473), based on the analysis of the perpendicular standing spin wave mode frequency dependence on the applied out-of-plane magnetic field, was used to obtain the exchange stiffness D. This method was modified to avoid the influence of internal magnetic fields during the determination of the exchange stiffness. Furthermore, the method was also adapted for in-plane measurements. The results obtained using all methods are compared and values of D between (5.18 ± 0.01)· 10−17 T· m2 and (5.40 ± 0.02)· 10−17 T· m2 were obtained for different thicknesses. From this, the exchange constant was calculated to be A = (3.7 ± 0.4) pJ·m−1.

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.