Intrinsic Damping Parameter Research Articles

Dynamic magnetic properties of amorphous Fe80B20 thin films and their relation to interfaces

We present a ferromagnetic resonance study of the dynamic properties of a set of amorphous Fe-B films deposited on Corning Glass® and MgO (001) substrates, either with or without capping. We show that the in plane anisotropy of the MgO grown films contains both uniaxial and biaxial components whereas it is just uniaxial for those grown on glass. The angular dependence of the linewidth strongly differs in terms of symmetry and magnitude depending on the substrate and capping. We discuss the role of the interfaces on the magnetization damping and the generation of the anisotropy. We obtained values of the intrinsic damping parameters comparable to the lowest ones reported for amorphous films of similar compositions.

Tuning of the intrinsic magnetic damping parameter in epitaxial CoNi(001) films : Role of the band-filling effect

The band-filling effect, which is due to tuning the electron occupation number near the Fermi level, on the intrinsic magnetic damping parameter is demonstrated by employing high-quality epitaxial ${\mathrm{Co}}_{1\ensuremath{-}x}{\mathrm{Ni}}_{x}$ alloy films, in which the Fermi level and the density of states can be continuously tuned by varying the Ni concentration $x$. The intrinsic magnetic damping parameter, measured by the time-resolved magneto-optical Kerr effect, changes weakly at small $x$ and increases sharply for $x>0.80$. The experimental results are well reproduced by the density functional theory calculation. More interestingly, the magnetic damping parameter and the density of states near the Fermi level share similar variation trends, demonstrating their correlation. The band-filling effect in $3d$ magnetic transition metal alloys provides a way to tune the magnetic damping parameter as a key element in the controlg of energy loss and speed of spintronic devices.

Microstructural and magnetic properties of rGO/MnFe2O4 nanocomposites; relaxation dynamics

In the present investigation, nanocomposite (NC) of reduced graphene oxide (rGO) and manganese ferrite (MnFe2O4) with three different (rGO/MnFe2O4) weight-ratio of rGO have been synthesized using a simple and efficient one-step hydrothermal route. The crystallinity, morphology and magnetic studies were conducted using X-ray diffraction (XRD), confocal Raman imaging and vibrating sample magnetometry (VSM).Detailed ferromagnetic resonance (FMR) studies were performed in the field-sweep mode at different applied frequencies ranging from 5 to 30 GHz using a vector network analyzer (VNA). We have realized a nano-composite layer of rGO/MnFe2O4 NC on top of coplanar waveguide (CPW) transmission lines to perform FMR experiments. The largest ferromagnetic resonance (FMR) absorption of −28 dB was observed for 0.06 g GO doped MnFe2O4 at the 22 GHz frequency. This absorption at FMR is found to be tunable by rGO concentration in manganese ferrite. We have also evaluated various relaxation/damping parameters of the nanocomposites. The experimental values of FMR linewidths were fitted using two micromagnetic models, and hence intrinsic and extrinsic damping parameters were obtained for nanocomposite samples. The damping parameters were observed to influenced by the rGO weight ratio. Therefore the developed nanocomposite can find various applications in the area of microwave monolithic devices and circuits like filters, phase shifters, circulators, etc.

Co/Ni] multilayers with robust post-annealing performance for spintronics device applications

The effect of annealing on the diffusion induced changes in the magnetic and magnetodynamic properties of (Co/Ni) multilayers were investigated. Here, we report, all aspects of post annealing induced effect on perpendicular magnetic anisotropy (PMA), domain morphology, switching mechanism and high frequency magnetization dynamics to understand spin relaxation. (Co/Ni) multilayers of this study exhibited a loop squareness of 95%, a slight drop in saturation magnetization from 650 emu/cc to 620 emu/cc and high magnetic anisotropy (Ku ~ 3 × 105 J m−3) even after annealing at temperatures as high as 400 °C. Successive domain mapping by magnetic force microscopy revealed characteristics of multi-domains stripe that feature at all stages of annealing except at 500 °C, attributed to the loss of PMA, driven by interfacial diffusion. High frequency broadband ferromagnetic resonance measurements showed little effect of annealing on intrinsic damping parameter (α = 0.019–0.024) while inhomogeneous linewidth broadening varied without a clear trend. These results are discussed considering crystallographic modifications and it was found that fcc (1 1 1) texture was retained in the samples until 400 °C post annealing. The retention of a high anisotropy, high magnetization and low damping constant after high temperature annealing makes these Co/Ni multilayers suitable for designing spin torque based memory devices.

Exchange-coupled hard-soft ferrites; A new microwave material

During the past decade, hexaferrite has become an important candidate for a variety of microwave and millimeter-wave devices due to their large uniaxial magneto-crystalline anisotropy and high saturation magnetization. The goal of this investigation is to synthesize BaFe12O19/Y3Fe5O12 (Barium hexaferrite/Yttrium Iron Garnet - BaM/YIG) hard-soft ferrite nanocomposites (NCs) as an advanced microwave material. A detailed microstructure and magnetic properties of these nano-composites were investigated using various techniques; viz. X-ray diffractometry (XRD), vibrating sample magnetometry (VSM), Raman spectroscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM).With recent advances in nanostructure synthesis techniques, we have realized a thin nano-composite layer on top of coplanar waveguide (CPW) planar transmission lines to design microwave notch-filters. Using a vector network analyzer we have checked the device operation from X-to Ka-band through ferromagnetic resonance (FMR) spectroscopy. The FMR linewidth (device bandwidth) is observed to be tunable with (BaM)1-x (YIG)x nanocomposite from 2 kOe (900 MHz) for pure BaM (x = 0) to 0.2 kOe (90 MHz) for pure YIG (x = 1). Various mathematical models were employed to fit the experimentally observed FMR linewidth (ΔH) data to yield intrinsic and extrinsic damping parameters as a function of ‘x’ value in (BaM)1-x (YIG)x nanocomposite.

Room-temperature helimagnetism in FeGe thin films

Chiral magnets are promising materials for the realisation of high-density and low-power spintronic memory devices. For these future applications, a key requirement is the synthesis of appropriate materials in the form of thin films ordering well above room temperature. Driven by the Dzyaloshinskii-Moriya interaction, the cubic compound FeGe exhibits helimagnetism with a relatively high transition temperature of 278 K in bulk crystals. We demonstrate that this temperature can be enhanced significantly in thin films. Using x-ray scattering and ferromagnetic resonance techniques, we provide unambiguous experimental evidence for long-wavelength helimagnetic order at room temperature and magnetic properties similar to the bulk material. We obtain αintr = 0.0036 ± 0.0003 at 310 K for the intrinsic damping parameter. We probe the dynamics of the system by means of muon-spin rotation, indicating that the ground state is reached via a freezing out of slow dynamics. Our work paves the way towards the fabrication of thin films of chiral magnets that host certain spin whirls, so-called skyrmions, at room temperature and potentially offer integrability into modern electronics.

BaM/YIG nano-composites: A microwave material for C to U band application

Hexaferrites have become important candidates for a variety of microwave and millimeter wave devices due to their large uniaxial magneto-crystalline anisotropy and high saturation magnetization. The goal of the present investigation is to synthesize Barium hexaferrite/Yttrium Iron Garnet (BaFe12O19/Y3Fe5O12): (BaM/YIG) Nano-Composites (NCs) to be used in broad band microwave frequency range applications, especially as microwave absorber. X-ray diffractometry, Vibrating Sample Magnetometer (VSM), and ferromagnetic resonance (FMR) techniques were used to characterize these NCs. Using a Cu coplanar wave guide and a Vector Network Analyzer, broadband (C to U) microwave absorption were investigated by placing the bulk sample in flip chip mode. Various mathematical models were employed to fit the experimental data to yield intrinsic and extrinsic damping parameters.

Magnetocrystalline anisotropy and Gilbert damping of Co2MnAl films epitaxially grown on GaAs

The thickness dependence of both the magnetocrystalline anisotropy and Gilbert damping are investigated for L21-ordered Co2MnAl films by time-resolved magneto-optical Kerr (TR-MOKE) measurements. The intrinsic damping parameter of 0.0039 is evaluated by applying both the in-plane and out-of-plane external magnetic field. The magnetocrystalline anisotropy and intrinsic damping parameter are found to show the similar dependence on film’s thickness, revealing their same physical origination related to the spin–orbit coupling. Our experimental findings provide essential information for the dynamic magnetization property of Co2MnAl films for its promising application in spintronic devices.

Ferromagnetic resonance and magnetic damping in C-doped Mn5Ge3

Ferromagnetic resonance (FMR) was used to investigate the static and dynamic magnetic properties of carbon-doped Mn5Ge3 thin films grown on Ge(1 1 1). The temperature dependence of magnetic anisotropy shows an increased perpendicular magneto-crystalline contribution at 80 K with an in-plane easy axis due to the large shape contribution. We find that our samples show a small FMR linewidth (corresponding to an intrinsic magnetic damping parameter ) which is a measure of the spin relaxation and directly related with the magnetic and structural quality of the material. In the perpendicular-to-plane geometry, the FMR linewidth shows a minimum at around 200 K for all the samples, which seems to be not correlated to the C-doping. The magnetic relaxation parameters have been determined and indicate the two-magnon scattering as the main extrinsic contribution. We observe a change in the main contribution from scattering centres in Mn5 Ge3C0.2 at low temperatures, which could be related to the minimum in linewidth.

Tuning Effects of Spin–Orbit Coupling in L10Ordered and Disordered FePdPt Films

Effects of spin–orbit coupling (SOC) on magnetocrystalline anisotropy, magneto-optical Kerr effect (MOKE), magnetic damping parameter and anomalous Hall effect (AHE) have been studied in theory extensively. In contrast, few experimental reports have appeared on these issues. In this review paper, we introduce our recent experimental investigation on the SOC tuning effects on the perpendicular magnetic anisotropy, polar MOKE, intrinsic magnetic damping parameter and AHE in L10ordered [Formula: see text] ternary alloys. We also outline the SOC effects in polycrystalline disordered [Formula: see text] films for comparison.

Perpendicular magnetic anisotropy and magnetization dynamics in oxidized CoFeAl films.

Half-metallic Co-based full-Heusler alloys with perpendicular magnetic anisotropy (PMA), such as Co2FeAl in contact with MgO, are receiving increased attention recently due to its full spin polarization for high density memory applications. However, the PMA induced by MgO interface can only be realized for very thin magnetic layers (usually below 1.3 nm), which would have strong adverse effects on the material properties of spin polarization, Gilbert damping parameter, and magnetic stability. In order to solve this issue, we fabricated oxidized Co50Fe25Al25 (CFAO) films with proper thicknesses without employing the MgO layer. The samples show controllable PMA by tuning the oxygen pressure (PO2) and CFAO thickness (tCFAO), large perpendicular anisotropy field of ~8.0 kOe can be achieved at PO2 = 12% for the sample of tCFAO = 2.1 nm or at PO2 = 7% for tCFAO = 2.8 nm. The loss of PMA at thick tCFAO or high PO2 results mainly from the formation of large amount of CoFe oxides, which are superparamagnetic at room temperature but become hard magnetic at low temperatures. The magnetic CFAO films, with strong PMA in a relatively wide thickness range and small intrinsic damping parameter below 0.028, would find great applications in developing advanced spintronic devices.

Magnetic thin-film insulator with ultra-low spin wave damping for coherent nanomagnonics.

Wave control in the solid state has opened new avenues in modern information technology. Surface-acoustic-wave-based devices are found as mass market products in 100 millions of cellular phones. Spin waves (magnons) would offer a boost in today's data handling and security implementations, i.e., image processing and speech recognition. However, nanomagnonic devices realized so far suffer from the relatively short damping length in the metallic ferromagnets amounting to a few 10 micrometers typically. Here we demonstrate that nm-thick YIG films overcome the damping chasm. Using a conventional coplanar waveguide we excite a large series of short-wavelength spin waves (SWs). From the data we estimate a macroscopic of damping length of about 600 micrometers. The intrinsic damping parameter suggests even a record value about 1 mm allowing for magnonics-based nanotechnology with ultra-low damping. In addition, SWs at large wave vector are found to exhibit the non-reciprocal properties relevant for new concepts in nanoscale SW-based logics. We expect our results to provide the basis for coherent data processing with SWs at GHz rates and in large arrays of cellular magnetic arrays, thereby boosting the envisioned image processing and speech recognition.

Magnetization dynamics and Gilbert damping in a hybrid Fe/GaAs heterostructure

In this paper, we studied the ultrafast magnetization dynamics and damping process in a hybrid epitaxial Fe/GaAs structure by using time-resolved magneto-optical measurements over various temperatures. The phenomenological Gilbert damping constant depended strongly on both the direction and strength of the applied magnetic field. The damping process was revealed to be dominated by incoherent spin-flip scattering, with involvement of the Fe/GaAs interfacial defects mediated two-magnon scattering. The intrinsic damping parameter was extracted at sufficiently high magnetic fields (or magnetization precession frequencies). Our results allow for better understanding of magnetization relaxation of Fe/GaAs heterostructure for its potential application in novel spintronic devices.

Co2FeAl Heusler thin films grown on Si and MgO substrates: Annealing temperature effect

10 nm and 50 nm Co2FeAl (CFA) thin films have been deposited on MgO(001) and Si(001) substrates by magnetron sputtering and annealed at different temperatures. X-rays diffraction revealed polycrystalline or epitaxial growth (according to CFA(001)[110]//MgO(001)[100] epitaxial relation) for CFA films grown on a Si and on a MgO substrate, respectively. For these later, the chemical order varies from the A2 phase to the B2 phase when increasing the annealing temperature (Ta), while only the A2 disorder type has been observed for CFA grown on Si. Microstrip ferromagnetic resonance (MS-FMR) measurements revealed that the in-plane anisotropy results from the superposition of a uniaxial and a fourfold symmetry term for CFA grown on MgO substrates. This fourfold anisotropy, which disappears completely for samples grown on Si, is in accord with the crystal structure of the samples. The fourfold anisotropy field decreases when increasing Ta, while the uniaxial anisotropy field is nearly unaffected by Ta within the investigated range. The MS-FMR data also allow for concluding that the gyromagnetic factor remains constant and that the exchange stiffness constant increases with Ta. Finally, the FMR linewidth decreases when increasing Ta, due to the enhancement of the chemical order. We derive a very low intrinsic damping parameter (1.1×10−3 and 1.3×10−3 for films of 50 nm thickness annealed at 615 °C grown on MgO and on Si, respectively).

Tuning the Magnetization Dynamics in Sputtered ${\hbox{Co}}_{2}{\hbox{FeAl}}_{0.5}{\hbox{Si}}_{0.5}$ Heusler Alloy Thin Film by Gas Pressure

The influences of sputtering gas pressure on the high frequency magnetization dynamics properties of sputtered Co2FeAl0.5Si0.5 (CFAS) Heusler alloy thin films have been systematic studied. Results show that the surface roughness, grain size, anisotropy field and dynamic magnetic properties can be tailored by changing sputtering gas pressures. The inhomogeneous linewidth broadening of the sputtered CFAS Heusler thin films is monotonously enhanced with the pressure increasing, similar as with the anisotropy field. A low value intrinsic damping parameter is extracted as 0.0073 with a low sputtering pressure, 1.58 ×10-4 mbar. The intrinsic damping parameter increases monotonously up to 0.0122 with the pressure increasing. The high tunability of the damping constant indicates that controlling the sputtering gas pressure could be an effective method to tune the magnetization dynamics in sputtered CFAS thin film.

Effect of the annealing temperature on dynamic and structural properties of Co2FeAl thin films

10 nm and 50 nm thick Co 2 FeAl (CFA) thin films have been deposited on thermally oxidized Si(001) substrates by magnetron sputtering using a Tantalum cap layer and were then ex-situ annealed at 415°C, 515°C and 615°C during 15 minutes in vacuum. X-rays diffraction indicates that films CFA are polycrystalline and exhibit an in-plane isotropy growth. Ferromagnetic resonance measurements, using a microstrip line (MS-FMR), reveal a huge interfacial perpendicular magnetic anisotropy and small in-plane uniaxial anisotropy both annealing temperature-dependent. The MS-FMR data also allow concluding that the gyromagnetic factor remains constant and that the exchange stiffness constant increases with annealing temperature. Finally, the FMR linewidth decreases with increasing annealing temperature due to the enhancement of the chemical order, and allow deriving a very low intrinsic damping parameter (1.3×10 −3 at 615°C).

Roughness induced magnetic inhomogeneity in Co/Ni multilayers: Ferromagnetic resonance and switching properties in nanostructures

We report on the effect roughness has on the magnetic properties of Co/Ni multilayers with perpendicular anisotropy. We can systematically vary the surface roughness and grain size by the variation in the Cu seed layer thickness. The roughness has a significant effect on the lateral anisotropy variation across the material, which was studied through the size dependence of the switching field and switching field distribution in nanostructures. Ferromagnetic resonance measurements show a large dependence of the linewidth on the topography. However, only the inhomogeneous linewidth broadening varied significantly with roughness with little to no effect on the intrinsic damping parameter of α=0.015–0.018.

Spin dynamics of polycrystalline Ni films on Si substrate

We have prepared thin Ni films by direct electrodeposition onto Si substrates and investigated the magnetization dynamics up to 9 GHz. Films with typical thickness of 200 nm show good adhesion to the substrate. Experimental absorption spectra were fitted using a model, which combines the magnetization dynamics according to Landau–Lifshitz–Gilbert and eddy current contribution. Damping parameters ranging from 0.12 to 0.08 were obtained. These values are larger than the intrinsic damping parameter of 0.064 for Ni, which indicates that the effect of eddy current is the prime contribution to line broadening in the electrodeposited Ni/Si structure. The dependence of damping parameter on applied field also supports this idea, the larger the applied field the smaller the damping parameter is.

Ac-induced damping of a fluxon in a long Josephson junction

We present a theoretical and experimental study of Josephson vortex (fluxon) moving in the presence of spatially homogeneous dc and ac bias currents. By mapping this problem to the problem of calculating the current-voltage characteristic of a small Josephson junction, we derive the dependence of the average fluxon velocity u on the dc bias current \ensuremath{\gamma}. In particular we find that the low frequency ac bias current results in an additional nonlinear damping of fluxon motion. Such ac induced damping crucially depends on the intrinsic damping parameter $\ensuremath{\alpha}$ and increases drastically as $\ensuremath{\alpha}$ is reduced. We find a good agreement of the analysis with both the direct numerical simulations and the experimentally measured $\ensuremath{\gamma}(u)$ curves of a long annular Josephson junction with one trapped fluxon.

Ferromagnetic resonance linewidth of Fe ultrathin films grown on a bcc Cu substrate

The ferromagnetic resonance (FMR) linewidth ΔH is a parameter used to characterize ultrathin film quality. The frequency-independent part, ΔH(0), reflects the contribution of magnetic inhomogeneities. The linear part is caused by the intrinsic Gilbert damping mechanism. Recently we have found that the lattice reconstructions of substrate templates strongly affects the FMR linewidth. bcc Fe(001) films (∼10 ML) were grown on unreconstructed fcc Ag(001), bcc Cu(001), and on surface reconstructed bcc Cu(001) templates. Fe layers grown on unreconstructed Cu templates exhibited an isotropic FMR linewidth. Fe layers grown on reconstructed Cu templates, on the contrary, showed an anisotropic behavior in both the zero-frequency FMR linewidth and the intrinsic damping parameter. Other magnetic parameters such as uniaxial and 4-fold anisotropies are also discussed.