Intrinsic Magnetic Damping Research Articles

Spin-wave enhancement using feedback-ring structure

Magnonic mode interconversion has paved the way for the integration of various developed magnonic functionalities, such as logic gates, switches, and multiplexers; however, it is limited by intrinsic magnetic damping. Therefore, this study proposes a potential amplification method to integrate spin-waves into magnonic circuits. The phase-matching conditions were tuned by introducing a feedback-ring structure. The results of microfocused Brillouin light scattering spectroscopy and micromagnetic simulations demonstrate the effectiveness of the spin-wave enhancement of feedback-ring structure. Consequently, spin-wave enhancement preserving phase information can be developed to realize integrated magnonic circuits.

Magnetization Precession at Sub-Terahertz Frequencies in Polycrystalline Cu2 Sb-Type (Mn-Cr)AlGe Ultrathin Films.

A ferromagnetic metal nanolayer with a large perpendicular magnetic anisotropy, small saturation magnetization, and small magnetic damping constant is a crucial requirement for high-speed spintronic devices. Fabrication of these devices on Si/SiO2 amorphous substrates with polycrystalline structure is also strongly desired for the mass production industry. This study involves the investigation of sub-terahertz (THz) magnetization precessional motion in a newly developed material system consisting of Cu2 Sb-type MnAlGe and (Mn-Cr)AlGe films by means of an all-optical pump-probe method. These materials exhibit large perpendicular magnetic anisotropy in regions of a few nanometers in size. The pseudo-2D crystal structures are clearly observed in the high-resolution transmission electron microscopy (TEM) images for the film samples grown on thermally oxidized silicon substrates. The TEM images also show a partial substitution of Cr atoms for the Mn sites in MnAlGe. A magnetization precession frequency of 0.164 THz with a relatively small effective magnetic damping constant of 0.012 is obtained for (Mn-Cr)AlGe. Theoretical calculation infers that the modification of the total density of states by Cr substitution decreases the intrinsic magnetic damping constant of (Mn-Cr)AlGe.

Thermal Control of the Intrinsic Magnetic Damping in a Ferromagnetic Metal

We report experiments on the control of intrinsic magnetic damping by thermal torque effects produced by the spin Seebeck effect and the anomalous Nernst effect in a thin layer of a ferromagnetic metal (Permalloy (Py), ${\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$). Damping is measured in ferromagnetic resonance (FMR) experiments on a sample excited by microwave radiation and detected by the dc-voltage-generated spin rectification and magnonic charge pumping in the Py film. Application of a temperature gradient in the longitudinal configuration increases or decreases the dc-voltage line width, depending on the sign of the thermal gradient, demonstrating that the magnetic damping in Py is controlled by currents generated by thermal effects. The absolute values of the line-width changes in Py may reach 1 order of magnitude larger than in the ferrimagnetic insulator yttrium iron garnet. The large change in magnetic damping is interpreted as a superposition of different phenomena in the same metallic ferromagnet.

Resonant enhancement of magnetic damping driven by coherent acoustic phonons in thin Co2FeAl film epitaxied on GaAs

The magnetic dynamics of a thin Co2FeAl film epitaxially grown on GaAs substrate was investigated using the time- resolved magneto-optical Kerr measurement under an out-of-plane external field. The intrinsic magnetic damping constant, which should do not vary with the external magnetic field, exhibits an abnormal huge increase when the precession frequency is tuned to be resonant with that of the coherent longitudinal acoustic phonon in the Co2FeAl/GaAs heterostructure. The experimental finding is suggested to result from the strong coherent energy transfer from spins to acoustic phonons via magnetoelastic effect under a resonant coupling condition, which leads to a huge energy dissipation of spins and a greatly enhanced magnetic damping in Co2FeAl. Our experimental findings provide an experimental evidence of spin pumping-like effect driven by propagating acoustic phonons via magnetoelastic effect, suggesting an alternative approach to the possible long-range spin manipulation via coherent acoustic waves.

Auto-oscillations for the coupling between breathing mode and chiral switching in magnetic skyrmions

Magnetic skyrmions are topological protected nanometric swirling spin structures, and usually possess a fixed chiral feature. Herein, we theoretically demonstrate a dynamic switching for the chirality in magnetic skyrmions by optimizing the spin-transfer torque (STT). Such a switching process is coupled with the breathing mode, presented as an auto-oscillation phenomenon. In this case, the STT plays a role as the compensation of the intrinsic magnetic damping over a period of oscillation. The oscillation frequency could be tuned by the applied current density and type, and the material parameters such as the anisotropy constant and ferromagnetic exchange coefficient. Our results would be significant for designing nanoscale spin-transfer nano-oscillators.

Spin-Wave Relaxation by Eddy Currents in Y3Fe5O12/Pt Bilayers and a Way to Suppress It

Because of their record-low intrinsic magnetic damping properties, single-crystal yttrium-iron-garnet (YIG) films serve as an excellent model medium for studying magnon-induced spintronic phenomena such as spin pumping and the spin-orbit torque effect. For this purpose, YIG films are covered with sub-skin-depth layers of nonmagnetic heavy metals with strong spin-orbit coupling. In the present work, we show experimentally and theoretically that ohmic losses of spin-wave-induced microwave eddy currents in the heavy-metal layer deliver a strong contribution to spin-wave damping in these hybrid structures. We demonstrate that this adverse effect can be controlled and largely eliminated by placing a highly conducting metal plate near to the surface of the YIG/$\mathrm{Pt}$ structures. These findings are of value for a proper interpretation of experiments on the magnon spintronic effects and for the design of future magnon spintronic devices.

High spin-wave propagation length consistent with low damping in a metallic ferromagnet

We report ultralow intrinsic magnetic damping in Co25Fe75 heterostructures, reaching the low 10-4 regime at room temperature. By using a broadband ferromagnetic resonance technique in out-of-plane geometry, we extracted the dynamic magnetic properties of several Co25Fe75-based heterostructures with varying ferromagnetic layer thicknesses. By measuring radiative damping and spin pumping effects, we found the intrinsic damping of a 26 nm thick sample to be α 0 ≲ 3.18 × 10-4. Furthermore, using Brillouin light scattering microscopy, we measured spin-wave propagation lengths of up to (21 ± 1) μm in a 26 nm thick Co25Fe75 heterostructure at room temperature, which is in excellent agreement with the measured damping.

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.

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.

Magnetic anisotropy manipulation of the femtosecond laser-excited spin wave modes in full-Heusler Co2Fe1−xMnxAl films

Spin-wave dynamics in full-Heusler Co2Fe1−xMnxAl films have been investigated using all-optical pump-probe magneto-optical polar Kerr spectroscopy. We find magnetic damping and anisotropy can be modulated by composition x. Damon–Eshbach (DE) spin wave occurs only in the samples which present intrinsic magnetic damping and have huge uniaxial magnetic anisotropy, implying that intrinsic magnetic damping and huge uniaxial magnetic anisotropy is the necessary conditions to excite coherent DE spin wave. Kittel spin wave appears in low uniaxial magnetic anisotropic samples and presents extrinsic magnetic damping. Therefore, laser-excited spin-wave modes can be manipulated by magnetic anisotropy, whose physical picture is discussed phenomenologically.

Tunable magnetization relaxation of Fe2Cr1−xCoxSi half-metallic Heusler alloys by band structure engineering

We report a systematic investigation on the magnetization relaxation properties of iron-based half-metallic Heusler alloy ${\mathrm{Fe}}_{2}{\mathrm{Cr}}_{1\ensuremath{-}x}\mathrm{Co}{}_{x}\mathrm{Si}$ (FCCS) thin films using broadband angular-resolved ferromagnetic resonance. Band structure engineering through Co doping ($x$) demonstrated by first-principles calculations is shown to tune the intrinsic magnetic damping over an order of magnitude, namely $1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}2}--8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$. Notably, the intrinsic damping constants for samples with high Co concentration are among the lowest reported for Heusler alloys and even comparable to magnetic insulator yttrium iron garnet. Furthermore, a significant reduction of both isotropic and anisotropic contributions of extrinsic damping of the FCCS alloys was found in the FCCS films with $x=0.5--0.75$, which is of particular importance for applications. These results demonstrate a practical recipe to tailor functional magnetization for Heusler alloy-based spintronics at room temperature.

Magnetic damping in poly-crystalline Co25Fe75: Ferromagnetic resonance vs. spin wave propagation experiments

We report on the investigation of the magnetic damping of a 10 nm thin, poly-crystalline Co25Fe75 film grown by molecular beam epitaxy. Ferromagnetic resonance (FMR) measurements reveal a low intrinsic magnetic damping αintFMR=(1.5±0.1)×10−3. In contrast, in patterned micrometer wide stripes, spin wave (SW) propagation experiments performed by time resolved scanning magneto-optical Kerr microscopy yield attenuation lengths on the order of 5–8 μm. From this quantity, we deduce an effective magnetic SW damping αeffSW, exp =(3.9±0.3)×10−3. For the system studied, this significant difference between both damping parameters is attributed to the non-negligible extrinsic contributions (local inhomogeneities and two-magnon scattering) to the magnetic losses which manifest themselves as a distinct inhomogeneous FMR linewidth broadening. This explanation is supported by micromagnetic simulations. Our findings prove that poly-crystalline Co25Fe75 represents a promising binary 3d transition metal alloy to be employed in magnonic devices with much longer SW attenuation lengths compared to other metallic systems.

Lack of dependence between intrinsic magnetic damping and perpendicular magnetic anisotropy in Cu(tCu)/[Ni/Co]N multilayers

The correlation between magnetic damping and perpendicular magnetic anisotropy has been investigated in Cu(tCu)/[Ni/Co]N multilayers by time-resolved magneto-optical Kerr effect. The uniaxial magnetic anisotropy constant Ku is varied in the range of 3.0–3.6 Merg/cm3 by tuning either multilayer repetition number N or Cu thickness tCu. It is found that the PMA strength Ku increases with the increase of N, while the damping constant α0 keeps nearly a constant of 0.025, implying the intrinsic damping is independent of the Ku tuned by N. In contrast, as tCu increases from 2.5 to 20nm, the α0 value rises continuously up to 0.040, in spite of the rather weak enhancement in Ku and its non-monotonic variation behavior. We consider the constant α0 with N is due to the unchanged spin-orbit coupling strength at each Co/Ni interface, while the obvious enhancement in α0 with tCu results mainly from the increased degree of spin disordering at the rougher Cu/Ni interface.

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.

Materials and Physics Challenges for Spin Transfer Torque Magnetic Random Access Memories

Spin transfer torque (STT) is a process by which a dc current of spin polarized electrons exerts a torque on the magnetization density in a magnetic medium [1]. STT can make the magnetization in a patterned structure unstable and switch between two stable magnetization states. The basic principle behind STT random access memories (STT-MRAM) is to use magnetic tunnel junctions (MTJs) with two thin magnetic layers separated by a thin insulating layer. In one magnetic layer, (the fixed layer) the magnetization has a fixed direction. In the other layer, the free layer or storage layer, the magnetization can be switched between parallel and antiparallel with the magnetization in the fixed layer with corresponding low and high resistance through the tunneling magnetoresistive effect.It has long been recognized that STT-MRAMs can be used as storage-class memory [2]. By using STT to write, stray fields causing unwanted writes on neighboring devices are eliminated. There is nothing that fundamentally prohibits MTJs to be scaled down to below 20 nm size, and the processing is compatible with Si technology. 64 Mbyte STT-MRAM chips are indeed now manufactured by Everspin [3], but there are great challenges that need to be overcome to realize STT-MRAM storage-class memories. The purpose of this talk is to highlight some of these challenges from a materials and physics perspective.One challenge is related to the current that is needed to write. Enough current has to be supplied by a Si-MOSFET transistor. As the current scales with the channel width, a large current requires a large transistor, which sets a lower limit on the footprint of the device. A large write current also impacts the reliability of the device: a moderately large write current degrades the device over time, while a much larger current still may cause a dielectric breakdown in the thin insulating barrier in the MTJ [4]. Therefore, a substantial effort has gone into either device designs or materials development that may allow for a low enough write current that the device can operate reliably while writing with certainty. As an example, dual-MTJs consisting of two insulating barriers have been proposed as means of reducing the required write current [5]. An issue here is that it is very difficult to manufacture arrays of dual MTJs with sufficiently narrow distributions of resistance in both high and low resistance states because of the way the materials in an MTJ grow on one another, and the restrictions on materials sets and crystallinity required for sufficiently high magnetoresistive ratios.Current available STT-MRAM use shape anisotropy – elliptical devices – with the magnetization in the plane of the ellipse to produce stable magnetization states along the long axes of the ellipse. This leads to large variabilities due to process variations. One way to decouple the energy barrier that gives stability from process variation is to use perpendicular anisotropy [6,7], in which the magnetization points out of the plane of the layers (Fig. 1). This gives rise to other challenges such as finding materials with sufficient anisotropy that can be manufactured, are stable, and give rise to high enough tunneling magnetoresistance.In addition to these, and other, materials challenges, there are issues that are beginning to emerge for small magnetic elements. The required write current is in a simple theory proportional to the intrinsic magnetic damping, so reducing the damping reduces the write current. Recent research [8] has shown that damping in nanoscale magnetic elements is highly dependent on size and the dynamical mode of the magnetization. This necessitates a fundamental understanding of the actual inhomogeneous and incoherent motion of the magnetization during reversal, and in addition also a better understanding of the fundamental coupling between spin-polarized conduction electrons and inhomogeneous magnetization motion or spin wave modes.Part of this work was supported by the U.S. Department of Energy, Office of Science, Materials Sciences and Engineering Division. Argonne National Laboratory is operated under Contract No. DE-AC02-06CH11357 by UChicago Argonne, LLC.