We report on the fast relaxation behavior of a high-density magnon gas created by a parametric amplification process. The magnon gas is probed using the technique of spin-wave packet recovery by parallel parametric pumping. Experimental results show a damping behavior which is in disagreement with both the standard model of exponential decay and with earlier observations of non-linear damping. In particular, the inherent magnon damping is found to depend upon the presence of the parametric pumping field. A phenomenological model which accounts for the dephasing of the earlier injected magnons is in good agreement with the experimental data.
Understanding Damping Mechanisms via Spin Diffusion Length in Low-damping Li$_{0.5}$Al$_{1.0}$Fe$_{1.5}$O$_4$ Spinel Ferrite Thin Films
Stanford University（斯坦福大学）；SLAC National Accelerator Laboratory（SLAC国家加速器实验室）；University of California, Irvine（加州大学尔湾分校）；University of California San Diego（加州大学圣地亚哥分校）
We report on broadband ferromagnetic resonance linewidth measurements performed on epitaxial Heusler thin films. A large and anisotropic two-magnon scattering linewidth broadening is observed for measurements with the magnetization lying in the film plane, while linewidth measurements with the magnetization saturated perpendicular to the sample plane reveal low Gilbert damping constants of $(1.5\pm0.1)\times 10^{-3}$, $(1.8\pm0.2)\times 10^{-3}$, and $<8\times 10^{-4}$ for Co$_2$MnSi/MgO, Co$_2$MnAl/MgO, and Co$_2$FeAl/MgO, respectively. The in-plane measurements are fit to a model combining G…
Divergence between long- and short-wavelength magnon damping in spinel ferrites
Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory（斯坦福材料能源科学研究所，SLAC国家加速器实验室）；Department of Physics and Astronomy, University of Iowa（爱荷华大学物理与天文系）；Department of Physics, Stanford University（斯坦福大学物理系）；Geballe Laboratory for Advanced Materials, Stanford University（斯坦福大学先进材料盖伯实验室）；Department of Applied Physics, Stanford University（斯坦福大学应用物理系）；Department of Materials Science and Engineering, Stanford University（斯坦福大学材料科学与工程系）
We present angle dependent measurements of the damping properties of epitaxial Fe layers with MgO, Al and Pt capping layers. Based on the preferential distribution of lattice defects following the crystal symmetry, we make use of a model of the defect density to separate the contribution of two-magnon scattering to the damping from the isotropic contribution originating in the spin pumping effect, the viscous Gilbert damping and the magnetic proximity effect. The separation of the two-magnon contribution, which depends strongly on the defect density, allows for the measurement of a value of th…
The propagation of magnons in temperature gradients is investigated within the framework of an atomistic spin model with the stochastic Landau-Lifshitz-Gilbert equation as underlying equation of motion. We analyze the magnon accumulation, the magnon temperature profile as well as the propagation length of the excited magnons. The frequency distribution of the generated magnons is investigated in order to derive an expression for the influence of the anisotropy and the damping parameter on the magnon propagation length. For soft ferromagnetic insulators with low damping a propagation length in …
Damping mechanisms in magnetic systems determine the lifetime, diffusion and transport properties of magnons, domain walls, magnetic vortices, and skyrmions. Based on the phenomenological Landau-Lifshitz-Gilbert equation, here the effective damping parameter in noncollinear magnetic systems is determined describing the linewidth in resonance experiments or the decay parameter in time-resolved measurements. It is shown how the effective damping can be calculated from the elliptic polarization of magnons, arising due to the noncollinear spin arrangement. It is concluded that the effective dampin…
Magnon systems used in quantum devices require low damping if coherence is to be maintained. The ferrimagnetic electrical insulator yttrium iron garnet (YIG) has low magnon damping at room temperature and is a strong candidate to host microwave magnon excitations in future quantum devices. Monocrystalline YIG films are typically grown on gadolinium gallium garnet (GGG) substrates. In this work, comparative experiments made on YIG waveguides with and without GGG substrates indicate that the material plays a significant role in increasing the damping at low temperatures. Measurements reveal that…
The effects of correlation-induced coupling between spin and charge fluctuations on spin-wave excitations in a band ferromagnet are investigated by including self-energy and vertex corrections within a systematic inverse-degeneracy expansion scheme which explicitly preserves the Goldstone mode. Arising from the scattering of a magnon into intermediate spin-excitation states (including both magnon and Stoner excitations) accompanied with charge fluctuations in the majority spin band, this spin-charge coupling results not only in a substantial reduction of magnon energies but also in anomalous s…
We study theoretically the influence of the temperature and disorder on the spin wave spectrum of the magnonic crystal Fe$_{1-c}$Co$_{c}$. Our formalism is based on the analysis of a Heisenberg Hamiltonian by means of the wave vector and frequency dependent transverse magnetic susceptibility. The exchange integrals entering the model are obtained from the \emph{ab initio} magnetic force theorem. The coherent potential approximation is employed to treat the disorder and random phase approximation in order to account for the softening of the magnon spectrum at finite temperatures. The alloy turn…
Magnon chiral damping beyond the Dzyaloshinskii-Moriya interaction
Rheinland-Pfälzische Technische Universität Kaiserslautern-Landau（莱茵兰-普法尔茨技术大学凯泽斯劳滕-兰道分校）；Indian Institute of Technology (Indian School of Mines)（印度理工学院（印度矿业学院））；Uppsala University（乌普萨拉大学）；University of Münster（明斯特大学）
We report on the impact of nonlinear four-magnon scattering on magnon transport in microstructured Co25Fe75 waveguides with low magnetic damping. We determine the magnon propagation length with microfocused Brillouin light scattering over a broad range of excitation powers and detect a decrease of the attenuation length at high powers. This is consistent with the onset of nonlinear four-magnon scattering. Hence, it is critical to stay in the linear regime, when deriving damping parameters from the magnon propagation length. Otherwise, the intrinsic nonlinearity of magnetization dynamics may le…
Room-temperature magnon-phonon transduction in high-damping Co/Pt structures
Helmholtz-Zentrum Dresden–Rossendorf（德累斯顿-罗斯多夫亥姆霍兹中心）；nextnano Lab（nextnano实验室）；Fritz Haber Institute of the Max Planck Society（马克斯·普朗克学会弗里茨·哈伯研究所）；IT4Innovations, VSB - Technical University of Ostrava（俄斯特拉发技术大学IT4创新研究院）；Charles University（查理大学）；Chemnitz University of Technology（开姆尼茨工业大学）；Dresden University of Technology（德累斯顿工业大学）
Applications based on spin currents strongly profit from the control and reduction of their effective damping and their transport properties. We here experimentally observe magnon mediated transport of spin (angular) momentum through a 13.4 nm thin yttrium iron garnet film with full control of the magnetic damping via spin-orbit torque. Above a critical spin-orbit torque, the fully compensated damping manifests itself as an increase of magnon conductivity by almost two orders of magnitude. We compare our results to theoretical expectations based on recently predicted current induced magnon con…
We study the Landau damping of ferromagnetic magnons in Fe, Co, and Ni as the dimensionality of the system is reduced from three to two. We resort to the \textit{ab initio} linear response time dependent density functional theory in the adiabatic local spin density approximation. The numerical scheme is based on the Korringa-Kohn-Rostoker Green's function method. The key points of the theoretical approach and the implementation are discussed. We investigate the transition metals in three different forms: bulk phases, free-standing thin films and thin films supported on a nonmagnetic substrate.…
The Landau-Lifshitz-Gilbert damping parameter is typically assumed to be a local quantity, independent of magnetic configuration. To test the validity of this assumption we calculate the precession damping rate of small amplitude non-uniform mode magnons in iron, cobalt, and nickel. At scattering rates expected near and above room temperature, little change in the damping rate is found as the magnon wavelength is decreased from infinity to a length shorter than features probed in recent experiments. This result indicates that non-local effects due to the presence of weakly non-uniform modes, e…
We apply time-dependent density functional theory to calculate the transverse magnetic susceptibility of bcc-Cr and Cr$_2$O$_3$, which constitute prototypical examples of antiferromagnets with itinerant and localized magnetic moments respectively. The exchange-correlation kernel is rescaled in order to enforce the Goldstone condition and the magnon dispersion relations are extracted based on a symmetry analysis relying on the generalized Onsager relation. Doing so, our calculations yield the characteristic linear magnon dispersion of antiferromagnets in the long wavelength limit. In the case o…
Kagome antiferromagnetic metal FeSn has become an attracting platform for the exploration of novel electronic states, such as topological Dirac states and the formation of flat bands by localized electrons. Apart from the electronic properties, Dirac magnons and flat magnon bands have also been proposed by applying simplified Heisenberg models to kagome magnetic systems.Inelastic neutron scattering studies on FeSn found well defined magnon dispersions at low energies,but magnons at high energies are strongly dampled, which can not be explained by localized spin models. In this paper, we utiliz…
Dynamic relaxation for nonlinear magnetization excitation is analyzed. For direct processes, such as magnon-electron scattering and two-magnon scattering, the relaxation rate is determined from the linear case simply by utilizing the magnetization oscillation frequency for nonlinear excitation. For an indirect process, such as slow-relaxing impurities, the analysis gives an additional relaxation term proportional to the excitation level. In all cases the effective magnetization damping is increased compared to Landau-Lifshitz-Gilbert damping.
We investigate the physical origin of nonlinear damping due to mode coupling between several auto-oscillatory modes driven by spin-orbit torque in constricted Py/Pt heterostructures by examining the dependence of auto-oscillation on temperature and applied field orientation. We observe a transition in the nonlinear damping of the auto-oscillation modes extracted from the total oscillation power as a function of drive current, which coincides with the onset of power redistribution amongst several modes and the crossover from linewidth narrowing to linewidth broadening in all individual modes. T…
For applications making use of magnonic spin currents damping effects, which decrease the spin conductivity, have to be minimized. We here investigate the magnon transport in an yttrium iron garnet thin film with strongly reduced effective magnetization. We show that in a three-terminal device the effective magnon conductivity can be increased by a factor of up to six by a current applied to a modulator electrode, which generates damping compensation above a threshold current. Moreover, we find a linear dependence of this threshold current on the applied magnetic field. We can explain this beh…
We demonstrate the influence of damping and field-like torques in the magnon-photon coupling process by classically integrating the generalized Landau-Lifshitz-Gilbert equation with RLC equation in which a phase correlation between dynamic magnetization and microwave current through combined Ampère and Faraday effects are considered. We show that the gap between two hybridized modes can be controlled in samples with damping parameter in the order of $10^{-3}$ by changing the direction of the dc current density $J$ if a certain threshold is reached. Our results suggest that an experimental real…
We predict that spin-waves in an easy-plane ferromagnet have a finite lifetime at zero temperature due to spontaneous decays. In zero field the damping is determined by three-magnon decay processes, whereas decays in the two-particle channel dominate in a transverse magnetic field. Explicit calculations of the magnon damping are performed in the framework of the spin-wave theory for the $XXZ$ square-lattice ferromagnet with an anisotropy parameter $λ<1$. In zero magnetic field the decays occur for $λ^*<λ<1$ with $λ^*\approx 1/7$. We also discuss possibility of experimental observation of the p…
The study of the spin excitations in antiferromagnetic (AFM) and ferromagnetic (FM) phases of FeRh is reported. We demonstrate that although the Fe atomic moments are well defined there is a number of important phenomena absent in the Heisenberg description: Landau damping of spin waves, large Rh moments induced by the AFM magnons, the formation of the optical magnons terminated by Stoner excitations. We relate the properties of the spin-wave damping to the features of the Stoner continuum and compare the chirality of the spin excitations in AFM, FM and paramagnetic (PM) systems.
We present a perturbation approach rooted in time-dependent density-functional theory to calculate electron hole (eh)-pair excitation spectra during the non-adiabatic vibrational damping of adsorbates on metal surfaces. Our analysis for the benchmark systems CO on Cu(100) and Pt(111) elucidates the surprisingly strong influence of rather short electronic coherence times. We demonstrate how in the limit of short electronic coherence times, as implicitly assumed in prevalent quantum nuclear theories for the vibrational lifetimes as well as electronic friction, band structure effects are washed o…
We investigate steady-state entanglement (SSE) between two nitrogen-vacancy (NV) centers in distant nanodiamonds on an ultrathin Yttrium Iron Garnet (YIG) strip. We determine the dephasing and dissipative interactions of the qubits with the quanta of spin waves (magnon bath) in the YIG depending on the qubit positions on the strip. We show that the magnon's dephasing effect can be eliminated, and we can transform the bath into a multimode displaced thermal state using external magnetic fields. Entanglement dynamics of the qubits in such a displaced thermal bath have been analyzed by deriving a…
The magnetic damping constant is a critical parameter for magnetization dynamics and the efficiency of memory devices and magnon transport. Therefore, its manipulation by electric fields is crucial in spintronics. Here, we theoretically demonstrate the voltage-control of magnetic damping in ferro- and ferrimagnetic-insulator (FI)/topological-insulator (TI) bilayers. Assuming a capacitor-like setup, we formulate an effective dissipation torque induced by spin-charge pumping at the FI/TI interface as a function of an applied voltage. By using realistic material parameters, we find that the effec…
