Spin Wave Linewidth Research Articles

Substitution modification mechanism of YIG ferrite with a high spin-wave linewidth via fast relaxation Dy3+ ion doping and sintering modulation

YIG ferrites are often used as nonreciprocal materials in microwave ferrite devices because of their low microwave magnetic and dielectric losses. To comply with the trend of planarization and integration of microwave systems, YIG ferrites are required to withstand higher powers. In this study, we investigated the variation in material microwave properties with material composition and microstructure. The experimental results show that doping the material with high-relaxation ions Dy3+ combined with modulating the sintering holding time can effectively improve the material’s high-power microwave signal withstanding capability, and YIG ferrite with a spin-wave line width of 18.7 Oe is obtained. In addition, when the average grain size of the material is controlled within 5 μm, the effect of the average grain size on the spin-wave linewidth is more noticeable than that of Dy3+ substitution.

Lowered ferromagnetic resonance linewidth and enhanced spin wave linewidth of nickel-based ferrite ceramics using hot-pressing method

Previous investigations commonly employed solid-state sintering to fabricate Ni-based ferrite ceramics to improve magnetic performance. A few researchers used the hot-pressing method to prepare Ni-based ferrites, but their porosity remained relatively high values (>2%), resulting in high ferromagnetic resonance line widths (ΔH). To date, the influence of high-temperature and high-pressure forming processes on ΔH and spin wave line width (ΔHk) in Ni-based ferrite ceramics has been rarely reported using hot pressing. This study utilized a uniaxial hot-pressing method and systematically investigated the effect of porosity, grain size and grain uniformity on the magnetic properties of Ni-based ferrite ceramics. Porosity played a crucial role in determining ΔH at different sintering temperatures. As the sintering temperature increased, the reduced porosity resulted in a decrease in ΔH. Under varying sintering pressures, the grain uniformity primarily affected ΔH. Increasing pressures led to more uniform grains, causing a decrease in ΔH. The ΔHk was directly associated with the grain sizes. With the increase of sintering temperatures, the grain sizes increased, leading to a decrease in ΔHk. Conversely, as the pressure increased, the decreased grain sizes brought the increased ΔHk. Compared to other hot-pressing results on the Ni-based ferrite ceramics, the optimized high-temperature and high-pressure processing with annealing significantly reduced the porosity of Ni-based ferrite ceramics (0.11%). In comparison with the solid-state sintering without pressures, the optimized hot-pressed Ni-based ferrite ceramics exhibited a 7% increase in saturation magnetization, a 63% reduction in ΔH and a 6.5% increase in ΔHk. The striking reduction of ΔH suggests that microwave devices assembled from Ni-based ferrite ceramics could withstand high power levels while minimizing lower electromagnetic losses, predicting promising applications for Ni-based ferrite materials in high-power microwave ferrite devices.

Verification of spinwave excitation in coaxial gyromagnetic nonlinear transmission lines.

The knowledge of physical mechanism of microwave generation in coaxial gyromagnetic nonlinear transmission lines (GNLTLs) is not complete up until now, especially the action of spinwave excitation during this process. In this paper, control experiments on different groups of GNLTLs with a single variable of NiZn ferrite material spinwave linewidth ΔHk are proposed as an indirect way to demonstrate this microscopic process. Comparative analyses of different groups of GNLTL experimental results are conducted to clarify the existence and effect of spinwave excitation. Theoretical treatment of conditions of spinwave excitation in GNLTLs is derived to explain the experimental results. It is illustrated that spinwave can be excited when the synchronism condition between the working frequency of GNLTL and the spinwave frequency spectrum is satisfied. The unstable spinwave excitation will consume the RF energy of GNLTLs heavily and cause a rapid decrease in RF oscillation.

Vacancy-engineered Gd3+-substituted yttrium iron garnet with narrow ferrimagnetic linewidth and high Curie temperature

With the urgent need for high-power low-loss microwave systems, next-generation wireless communications call for high-performance yttrium iron garnet with narrow ferrimagnetic resonance (FMR) linewidth, large spin-wave linewidth, and high Curie temperature, which can be tailored by cation substitution and vacancy engineering. Here, the electromagnetic properties of vacancy-engineered Gd3+-substituted garnet ferrites are reported. Cation vacancies on the dodecahedral sites of the garnet crystal structure are introduced by deficient Gd3+ substitution for the Y3+ ions. The polycrystalline microstructure, magnetic properties, and electrical properties of both the Gd3+-sufficient and Gd3+-deficient samples are comprehensively examined and compared. The deficient Gd3+ substitution promotes the densification process and grain growth during the sintering stage. Without detriment to the spin-wave linewidth and electrical resistivity, the vacancies are beneficial to reducing the FMR linewidth and increasing the Curie temperature. With appropriate vacancy concentrations, the garnet ferrite samples exhibit a 58% reduction in the FMR linewidth and a 13% increase in the Curie temperature. These results reveal the great potential of the vacancy-engineered Gd3+-substituted garnet ferrites for high-power low-loss microwave communication systems.

Spin-Wave Linewidth Measurement of Microwave Gyromagnetic Materials in a Low RF Power

In this paper, a novel test system for spin-wave linewidth has been proposed. As the traditional test system requires a high-power RF source, a high Q cylindrical resonator with efficiency compression of microwave magnetic field is designed by a partially filled dielectric in this paper. Experimental results show that this method can achieve spin-wave linewidth measurement with lower RF power compare to the original rectangular cavity. Simultaneous, replacing the traditional tuned resonant cavity with the frequency adjustable signal source can reduce the system interference to cavity field distribution and lays the foundation for fully automated testing. Finally, the wide linewidth spin-wave linewidth test is realized in a low RF power environment, and the test uncertainty for the system is estimated within ±10 %.

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).

Improved reflection technique for the permittivity measurement of ferrites using slotted coaxial samples

This paper presents the effect of magnetic permeability on the permittivity measurement using coaxial line reflection technique. It discusses in detail about the design aspects, analysis and simulation of the slotted co-axial sample and fixture for dielectric characterization of the ferrites and garnets possessing both dielectric and magnetic properties. This technique makes use of improved coaxial fixture and APC 07 compatible slotted coaxial sample geometry to measure the complex permittivity of the ferrite material. Permittivity is calculated from the impedance of sample loaded fixture, which is independent of magnetic permeability, because of air slot cut in the coaxial hollow cylindrical sample along the length. Characterization data of dielectrics and ferrites is used in design of ferrite circulators, and pulse magnets for particle accelerator applications. This is also used in many other RF applications including design of dielectric resonator for spin wave line width measurement of ferrites. Design work is aimed at accurate measurement of complex permittivity; using reflection measurement in coaxial fixture over a wide frequency range of 20 kHz to 1 GHz. Reflection measurement technique is further explored for accurate measurement of complex permittivity.

Spin wave linewidth measurement of microwave ferrite materials using rectangular resonant cavity with adjustable frequency

The traditional method to measure the spin wave linewidth of ferrite material uses a high-power microwave source with adjustable frequency and a rectangular cavity with fixed frequency. However, the microwave source with adjustable is more expensive and inaccurate. In this paper, we propose a new and economic method to tackle these problems. A microwave source with a fixed frequency instead of the adjustable one and an adjustable cavity are employed. The resonant frequencies of the cavity loaded with different samples are adjusted to the same value with the frequency of the microwave source. This means the proposed system has better performance in terms of accuracy and operation. Besides, an adjustable high-power attenuator and an isolator are employed to replace the power divider and high-power termination in other systems, which makes the operation more convenient. Experiments verify that a wider linewidth ranging from 1–35Oe can be measured accurately and the uncertainty of the linewidth can be estimated within ±15%.

Standing spin-wave mode structure and linewidth in partially disordered hexagonal arrays of perpendicularly magnetized sub-micron Permalloy discs

Standing spin wave mode frequencies and linewidths in partially disordered perpendicular magnetized arrays of sub-micron Permalloy discs are measured using broadband ferromagnetic resonance and compared to analytical results from a single, isolated disc. The measured mode structure qualitatively reproduces the structure expected from the theory. Fitted demagnetizing parameters decrease with increasing array disorder. The frequency difference between the first and second radial modes is found to be higher in the measured array systems than predicted by theory for an isolated disc. The relative frequencies between successive spin wave modes are unaffected by reduction of the long-range ordering of discs in the array. An increase in standing spin wave resonance linewidth at low applied magnetic fields is observed and grows more severe with increased array disorder.

Effects of Gd<sup>3+</sup>-Substitution on the Structure and Magnetic Properties of Multi-Doped YIG Ferrites

Y2.20-xCa0.80GdxV0.40In0.15Fe4.45O12 garnet ferrite samples were prepared by solid-state reaction method, the effect of Gd-substitution on the microstructure and electromagnetic properties of YIG were systematically investigated. The results show that the substitution of Gd3+ makes the Ms~T curve more flat which improve the temperature stability of saturation magnetization; meanwhile, it appreciably increases the ferromagnetic resonance linewidth and spin wave linewidth.

Surface spin waves of fcc cobalt films on Cu(100): High-resolution spectra and comparison to theory

With the help of a recently developed electron energy-loss spectrometer we have studied the surface spin waves on an eight-monolayer cobalt film deposited on Cu(100) surfaces with unprecedented energy resolution. Standing waves of the bulk of the film are discovered in the region of small wave vectors ${q}_{\ensuremath{\parallel}}l0.35$ \AA{}${}^{\ensuremath{-}1}$. The dispersion of surface spin waves is isotropic even for large ${q}_{\ensuremath{\parallel}}$. Because of the high-energy resolution and the complete characterization of the electron optical properties of the spectrometer reliable data for the linewidth of the surface spin waves are obtained. As a byproduct the dispersion of the Rayleigh surface phonon was measured. Data are compared to theoretical spin wave spectra extracted from calculations of the transverse spin susceptibility based on an ab initio electronic structure that incorporates both the metallic substrate and the magnetic film. The calculation takes fully into account the itinerant nature of the electrons responsible for the magnetic moments. The agreement between theoretical and experimental spin wave energies and linewidths is remarkably good.

Ferromagnetic resonance saturation and second order Suhl spin wave instability processes in thin Permalloy films

High power ferromagnetic resonance (FMR) loss versus static field H profiles and the corresponding spin wave instability threshold microwave field amplitude hcrit vs H butterfly curves were measured for in-plane magnetized thin Permalloy films of thicknesses 35, 57, 74, 104, and 123nm at a nominal pumping frequency of 9.11GHz. Cavity loading and calibration issues that proved to be problematic in past attempts to obtain accurate resonance saturation data over the full FMR profile in ferrites and metal films were resolved through a careful decoupling of the pump field and a full cavity response calibration. The FMR profiles show a drop in the loss peak, a shift in the peak to lower field, a broadening, and the development of a foldover-like asymmetry as the power is increased. The butterfly curves show a minimum hcrit at the low power FMR field and a smooth rounded increase on either side, except for a small kink on the low field side associated with the shift and asymmetry development. Apart from the kink, the second order Suhl spin wave instability theory, suitably modified for thin films, provided good fits to the butterfly curve data through the use of a single spin wave linewidth ΔHk value for each data set. The ΔHk values ranged from 16to35Oe, with the implied critical mode in-plane wave vectors always directed parallel to the static field. These spin wave linewidths translate into Gilbert damping parameter αk values in the 0.002–0.005 range, the same order as expected for intrinsic magnon-electron scattering losses in metal ferromagnets. These αk values are about a factor of 2 smaller than those implied by the low power FMR linewidths. The FMR in-plane precession cone angles at threshold were on the order of 3°–6°.

Microstructure-related relaxation and spin-wave linewidth in polycrystalline ferromagnets

The theoretical approach of two-magnon scattering in polycrystalline ferrites by Sparks, Loudon, and Kittel [Phys. Rev. 122, 791 (1961)] aiming at the microstructure-related relaxation of the uniform precession mode is extended to describe the relaxation of spin-wave modes. Within this framework we introduce a unified model that describes the influence of pores and microdomains on the relaxation of both the uniform precession and spin-wave modes. It is shown that the spin-wave linewidth does not only depend on the wave number---as assumed in conventional theory---but also on the propagation direction of the spin waves. This will, in particular, affect the damping of the critical modes at spin-wave instabilities, when probed in different experimental configurations. The effect of the number and size of pores as well as the influence of domain size and anisotropy is studied in detail, and as will be shown, this results in quite different parameter dependencies. Good qualitative and even quantitative agreement with previous experimental data is achieved without using any fit parameter.

Exchange narrowing of magnetic resonance linewidths in inhomogeneous ferromagnets

The theory of exchange narrowing of ferromagnetic and spin-wave resonance linewidths in ferromagnets with the randomly inhomogeneous magnetic anisotropy is developed. The calculation is carried out by the method of averaged Green functions in the framework of the coherent potential approximation (CPA). One- and three-dimensional inhomogeneities with an arbitrary correlation wave number ${k}_{c}$ are considered (${r}_{c}={k}_{c}^{\ensuremath{-}1}$ is the correlation radius of the inhomogeneities). The method of the approximate solution of the CPA equation is proposed which has the quick convergence. Effects of exchange narrowing of the magnetic resonance lines and shifts of these lines are calculated in the whole region of the values of the correlation wave number ${k}_{c}$. The approximate analytical expressions for the linewidth of the FMR in the limiting cases of the small and large ${k}_{c}$ are obtained. Large narrowing of the FMR and spin-wave linewidths with the decrease of the correlation radius of inhomogeneities is the substantiation of the main advantage of nanocrystalline and amorphous materials over usual polycrystals when they are used at high frequency devices.

The effect of cordierite additives on microwave magnetic properties of LiZn ferrites

The influence of firing temperature and cordierite additive on the microstructures and hysteresis loop and microwave magnetic properties of LiZn ferrites has been investigated. The addition of cordierite was found to help densification and grain growth of the LiZn ferrite, depending on firing temperature and the content of cordierite. Microstructural observations revealed that the cordierite tended to segregate at grain boundaries more apparently with increasing the content of cordierite, and not to crystallize even at a relatively high temperature of 1150 °C. Several magnetic parameters including coercive force H c, spin wave line width Δ H k and 3dB resonance line width Δ H 3dB were believed to depend on microstructural characteristics such as grain size, porosity and grain boundary segregation. When the sample was fired at 1000 °C, for example, Δ H 3dB showed a minimum value of ∼520 Oe with the addition of 0.5 wt.% cordierite and gradually increased with further additions presumably due to decreased grain size and more glass segregation at grain boundaries.

High power ferromagnetic resonance and spin wave instability processes in Permalloy thin films

The high power ferromagnetic resonance (FMR) response, as well as butterfly curves of the spin wave instability threshold microwave field amplitude hcrit versus in-plane static field H profiles, have been measured for Permalloy films with thicknesses of 104, 128, and 270nm at a nominal pumping frequency of 9.37GHz. The hcrit values range from about 1 to 7Oe. Both the resonance saturation response at the FMR field and the subsidiary absorption (SA) response for static fields below the FMR field are similar in appearance to those for bulk ferrites. Butterfly curves over the SA response region, while similar to those for ferrites, exhibit a film thickness dependent band edge cutoff effect not found in bulk ferrites. The SA butterfly curve data were analyzed on the basis of a spin wave instability theory adapted to thin films. The observed shift in the SA band edge cutoff with thickness agrees with calculations based on the thin film dispersion response and the assumption of first order instability processes with critical modes at one half the pumping frequency. The fitted SA spin wave linewidths give values consistent with metallic relaxation processes, but indicate critical modes with wave vectors that always make relatively small 0°–20° angles with the static field, very different from the critical modes for bulk ferrites. Three key conclusions from this work are (1) the nonlinear microwave FMR response in Permalloy films is a threshold effect related to well established spin wave instability processes, (2) the details of the SA response are controlled largely by the thin film spin wave dispersion, and (3) these nonlinear processes occur for very small precession angles.

Near theoretical microwave loss in hot isostatic pressed (hipped) polycrystalline yttrium iron garnet

The ferromagnetic resonance (FMR) linewidth, the field dependent effective linewidth, and the parallel pump spin wave linewidth were measured for spheres and disks prepared from a block of hot isostatic pressed (hipped) polycrystalline yttrium iron garnet (YIG). All linewidths as well as static magnetization data indicate close to 100% density. Vibrating sample magnetometer measurements give an average saturation induction 4πMs of 1825 G. The FMR half-power linewidths for the spheres at 9.5 GHz were 13 Oe. Linewidths measured over the 9.5–18 GHz frequency range show a small but distinct drop and agree with Schlömann’s theory of anisotropy-dominated two-magnon scattering for polycrystalline ferrites. The effective linewidth versus field data at 10 GHz show a region of strong absorption that corresponds to the width of the spin wave manifold for low wave numbers and a high field value of about 2 Oe. Parallel pumping measurements give minimum spin wave linewidths of 1.2 and 0.6 Oe at 9 and 16.7 GHz, respectively. The 16.7 GHz spin wave linewidths correspond to half-frequency spin waves at 8.35 GHz. The extrapolated linewidths at zero wave number are about 0.5 Oe and match the established intrinsic linewidths expected for YIG single crystals at 8–9 GHz. The spin wave linewidths increase linearly with wave number and are consistent with a transit time scattering process with scattering lengths that are about ten times greater than the average grain size.

Growth and characterization of high purity epitaxial yttrium iron garnet films grown from BaO–B2O3–BaF2 flux

The growth of yttrium iron garnet (YIG) epitaxial films of excellent purity from a lead-free BaO-B 2 O 3 -BaF 2 flux and their characterization by the magnetic resonance methods are reported. The films were prepared by the standard dipping liquid-phase epitaxy onto gadolinium gallium garnet substrates of three crystallographic orientations (111), (110), and (100). The objective of utilization of a lead-free flux was to reduce the content of impurities and intrinsic defects in the garnet films and to obtain samples with minimum microwave losses. The samples exhibited very narrow NMR and spin wave linewidths. The diamagnetic and paramagnetic doping of YIG with ions of different valency and segregation of these ions between the garnet layer and the barium melt were also studied and the relevant distribution coefficients for doping into the tetrahedral (Ga, Al, Si Ge), octahedral (In) and dodecahedral (La, Ca) sites were determined.

Effect of large magnetocrystalline anisotropy on the spin wave linewidth in Zn–Y hexagonal ferrite

The oblique pumping spin wave instability threshold microwave field amplitude hcrit was measured as a function of the static field for both in-plane and out-of-plane static fields for a thin disk of Mn substituted Zn–Y (Ba2Zn2Fe12O22) easy plane hexagonal ferrite at 16.7 GHz. Theoretical fits to the in-plane static field data gave a single value of 12.8 Oe for the spin wave linewidth ΔHk. The best overall fits were for a critical spin wave mode wave number k close to zero and there was no correlation with the critical mode propagation directions. The fitted ΔHk for the out-of-plane static field geometry was found to increase with the out-of-plane magnetization angle, but showed no correlation with the critical mode propagation directions. These critical modes had k≈0 as well. The fitted ΔHk increased from the in-plane field value of 12.8 Oe to a value of 21 Oe at the maximum accessible magnetization angle of 70° relative to the disk plane. This increase in spin wave damping with the out-of-plane angle is attributed to spin orbit coupling.

On the instability threshold of cobalt substituted Ni–Al ferrite at high-microwave-power levels

Parallel-pump spin-wave instability in polycrystalline Ni–Al ferrite has been measured at 9.384 GHz. The ground degeneracy of Co 2+ ions with concentration between 0.01 and 0.025 ions/f.u. added stoichiometrically, gives minimum losses and high-instability threshold values. Generalized instability theory for oblique pumping is used to reproduce the experimental butterfly curves. Results show a strong dependence of spin-wave linewidth on wave vector and crystalline anisotropy of cobalt.