Giant transition-state enhancement of quasiparticle spin-Hall effect in an exchange-spin-split superconductor detected by non-local magnon spin-transport
Kun-Rok Jeon, Jae-Chun Jeon, Xilin Zhou, Andrea Migliorini, Jiho Yoon, Stuart S. P. Parkin
Although recent experiments and theories have shown a variety of exotic transport properties of non-equilibrium quasiparticles (QPs) in superconductor (SC)-based devices with either Zeeman or exchange spin-splitting, how QP interplays with magnon spin currents remains elusive. Here, using non-local magnon spin-transport devices where a singlet SC (Nb) on top of a ferrimagnetic insulator (Y3Fe5O12) serves as a magnon spin detector, we demonstrate that the conversion efficiency of magnon spin to QP charge via inverse spin-Hall effect (iSHE) in such an exchange-spin-split SC can be greatly enhanced by up to 3 orders of magnitude compared with that in the normal state, particularly when its interface superconducting gap matches the magnon spin accumulation. Through systematic measurements with varying the current density and SC thickness, we identify that superconducting coherence peaks and exchange spin-splitting of the QP density-of-states, yielding a larger spin excitation while retaining a modest QP charge-imbalance relaxation, are responsible for the giant QP iSHE. The latter exchange-field-modified QP relaxation is experimentally proved by spatially resolved measurements with varying the separation of electrical contacts on the spin-split Nb.
The fermionic self-energy on the surface of a topological insulator proximity coupled to ferro- and antiferromagnetic insulators is studied. An enhanced electron-magnon coupling is achieved by allowing the electrons on the surface of the topological insulator to have a different exchange coupling to the two sublattices of the antiferromagnet. Such a system is therefore seen as superior to a ferromagnetic interface for the realization of magnon-mediated superconductivity. The increased electron-magnon-coupling simultaneously increases the self-energy effects. In this paper we show how the inver…
Spin waves involved in three-magnon splitting in synthetic antiferromagnets
Université Paris-Saclay, CNRS, Centre de Nanosciences et de Nanotechnologies（巴黎萨克雷大学，法国国家科学研究中心，纳米科学与技术中心）；SPEC, CEA, CNRS, Université Paris-Saclay（物质科学研究中心，法国原子能和替代能源委员会，法国国家科学研究中心，巴黎萨克雷大学）
We report on nonlocal transport in multiterminal superconductor-ferromagnet structures, which were fabricated by means of e-beam lithography and shadow evaporation techniques. In the presence of a significant Zeeman splitting of the quasiparticle states, we find signatures of spin transport over distances of several μm, exceeding other length scales such as the coherence length, the normal-state spin-diffusion length, and the charge-imbalance length. The relaxation length of the spin signal shows a nearly linear increase with magnetic field, hinting at a freeze-out of relaxation by the Zeeman …
Alignment-Dependent Gapless Chiral Split Magnons in Altermagnetic Domain Walls
School of Physics and State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China（电子科技大学物理科学与技术学院及电子薄膜与集成器件国家重点实验室）
We report on nonlocal spin transport in mesoscopic superconducting aluminum wires in contact with the ferromagnetic insulator europium sulfide. We find spin injection and long-range spin transport in the regime of the exchange splitting induced by europium sulfide. Our results demonstrate that spin transport in superconductors can be manipulated by ferromagnetic insulators, and opens a new path to control spin currents in superconductors.
Magnon spectrum of altermagnets beyond linear spin wave theory: Magnon-magnon interactions via time-dependent matrix product states vs. atomistic spin dynamics
We present a combined numerical, theoretical and experimental study on stimulated three-magnon splitting in a magnetic disk in the vortex equilibrium state. Our micromagnetic simulations and Brillouin-light-scattering results confirm that three-magnon splitting can be triggered even below threshold by exciting one of the secondary modes by magnons propagating in a waveguide next to the disk. The experiments show that stimulation is possible over an extended range of excitation powers and a wide range of frequencies around the eigenfrequencies of the secondary modes. Rate-equation calculations …
A theory of spin transport and spin detection in Zeeman-split superconducting films at low temperatures is developed. It is shown that an injection of spin-unpolarized quasiparticles into a Zeeman-split superconductor gives rise to a spin imbalance. The relaxation length of such a spin signal is determined by the energy relaxation length and can be extremely large as compared to the relaxation length of spin-polarized quasiparticles. There can exist two types of signals: due to nonthermalized quasiparticle distribution and due to thermalized overheated electron distribution. They have differen…
Non-equilibrium studies of two-dimensional (2D) superconductors (SCs) with Ising spin-orbit coupling are prerequisite for their successful application to equilibrium spin-triplet Cooper pairs and, potentially, Majorana fermions. By taking advantage of the recent discoveries of 2D SCs and their compatibility with any other materials, we fabricate here non-local magnon devices to examine how such 2D Ising superconductivity affects the conversion efficiency of magnon spin to quasiparticle charge in superconducting flakes of 2H-NbSe2 transferred onto ferrimagnetic insulating Y3Fe5O12. Comparison w…
In superconductors spin-split by an exchange field, thermal effects are coupled to spin transport. We show how an oscillating electromagnetic field in such systems creates spin imbalance, that can be detected with a spin-polarized probe. The sign and magnitude of the probe signal result from a competition between processes converting field induced spin energy imbalance to spin imbalance, dominant at low frequencies, and microwave-driven pair breaking at high frequencies. In the presence of spin-flip scattering, we show that ac excitation also leads to multistabilities in the superconducting st…
International Quantum Academy（国际量子学院）；Beihang University（北京航空航天大学）；Universidad Técnica Federico Santa María（弗雷德里科·Santa María 技术大学）；Cnr-Istituto Officina dei Materiali（意大利国家研究委员会材料研究所）
We present an experimental and numerical study of three-magnon splitting in a micrometer-sized magnetic disk with the vortex state strongly deformed by static in-plane magnetic fields. Excited with a large enough power at frequency $f_\mathrm{RF}$, the primary radial magnon modes of a cylindrical magnetic vortex can decay into secondary azimuthal modes via spontaneous three-magnon splitting. This nonlinear process exhibits selection rules leading to well-defined and distinct frequencies $f_\mathrm{RF}/2\pm Δf$ of the secondary modes. Here, we demonstrate that three-magnon splitting in vortices…
In the weak ferromagnetic MnCO$_3$ system, a low-frequency collective spin excitation (magnon) is the hybridized oscillation of nuclear and electron spins coupled through the hyperfine interaction. By using a split-ring resonator, we performed transmission spectroscopy measurements of MnCO$_3$ system and observed, for the first time, avoiding crossing between the hybridized nuclear-electron magnon mode and the resonator mode in the NMR-frequency range. The splitting strength is quite large due to the large spin density of $^{55}$Mn, and the cooperativity value $C=0.2$ (magnon-photon coupling p…
Microfocused Brillouin light scattering (BLS) and microwave absorption (MA) are used to study magnon-photon coupling in a system consisting of a split-ring microwave resonator and a yttrium iron garnet (YIG) film. The split-ring resonantor is defined by optical lithography and loaded with a 1 $μ$m-thick YIG film grown by liquid phase epitaxy. BLS and MA spectra of the hybrid system are simultaneously recorded as a function of the applied magnetic field magnitude and microwave excitation frequency. Strong coupling of the magnon and photon modes is found with a coupling strength of $g_\text{eff}…
D-Wave Phonon Angular Momentum Texture in Altermagnets by Magnon-Phonon Hybridization
Institute of Physics, Martin Luther University Halle-Wittenberg and Halle-Berlin-Regensburg Cluster of Excellence “Center for Chiral Electronics”（马丁·路德哈雷-维滕贝格大学物理研究所）；Institute of Physics, Otto von Guericke University Magdeburg（奥托·冯·格里克马格德堡大学物理研究所）；Institute of Solid State Theory, University of Münster（明斯特大学固体理论研究所）；Institute of Physics, Johannes Gutenberg University Mainz（约翰内斯·古腾堡美因茨大学物理研究所）
The coupling of fermionic quasiparticles to magnons is essential for a wide range of processes, from ultrafast magnetization dynamics in ferromagnets to Cooper pairing in superconductors. Although magnon energies are generally much larger than phonon energies, up to now their electronic band renormalization effect in ferromagnetic metals suggests a significantly weaker quasiparticle interaction. Here, using spin- and angle-resolved photoemission, we show an extraordinarily strong renormalization leading to replica-band formation of an iron surface resonance at ~200 meV. Its strong magnetic lin…
The Doppler shift of the quasiparticle dispersion by charge currents is responsible for the critical supercurrents in superconductors and instabilities of the magnetic ground state of metallic ferromagnets. Here we predict an analogous effect in thin films of magnetic insulators in which microwaves emitted by a proximity stripline generate coherent chiral spin currents that cause a Doppler shift in the magnon dispersion. The spin-wave instability is suppressed by magnon-magnon interactions that limit spin currents to values close to but below the threshold for the instability. The spin current…
On the basis of the Schwinger-Keldysh formalism, we have closely investigated the temperature dependence of quantum spin pumping by electron spin resonance. We have clarified that three-magnon splittings excite non-zero modes of magnons and characterize the temperature dependence of quantum spin pumping. Our theoretical result qualitatively agrees with the experiment by Czeschka et al. that the mixing conductance is little influenced by temperature [F. D. Czeschka et al., Phys. Rev. Lett., 107, 046601 (2011)].
We investigate the influence of magnons on the temperature-dependence and the anisotropy of the spin-orbit torque (SOT). For this purpose we use 3rd order perturbation theory in the framework of the Keldysh formalism in order to derive suitable equations to compute the magnonic SOT. We find several contributions to the magnonic SOT, which depend differently on the spin-wave stiffness $\mathcal{A}$ and on the temperature $T$, with the dominating contribution scaling like $T^{2}/\mathcal{A}^{2}$. Based on this formalism we compute the magnonic SOT in the ferromagnetic Rashba model. For large Ras…
We report the experimental demonstration of an abnormal, opposite anti-crossing effect in a photon-magnon-coupled system that consists of an Yttrium Iron Garnet film and an inverted pattern of split-ring resonator structure (noted as ISRR) in a planar geometry. It is found that the normal shape of anti-crossing dispersion typically observed in photon-magnon coupling is changed to its opposite anti-crossing shape just by changing the position/orientation of the ISRR's split gap with respect to the microstrip line axis along which ac microwave currents are applied. Characteristic features of the…
We experimentally and theoretically demonstrate that nonlinear spin-wave interactions suppress the hybrid magnon-photon quasiparticle or "magnon polariton" in microwave spectra of an yttrium iron garnet film detected by an on-chip split-ring resonator. We observe a strong coupling between the Kittel and microwave cavity modes in terms of an avoided crossing as a function of magnetic fields at low microwave input powers, but a complete closing of the gap at high powers. The experimental results are well explained by a theoretical model including the three-magnon decay of the Kittel magnon into …
We use vector Hamiltonian formalism (VHF) to study theoretically three-magnon parametric interaction (or three-wave splitting) in a magnetic disk existing in a magnetic vortex ground state. The three-wave splitting in a disk is found to obey two selection rules: (i) conservation of the total azimuthal number of the resultant spin-wave modes, and (ii) inequality for the radial numbers of interacting modes, if the mode directly excited by the driving field is radially symmetric (i.e. if the azimuthal number of the directly excited mode is $m = 0$). The selection rule (ii), however, is relaxed in…
Magnons dominate the magnetic response of the recently discovered insulating ferromagnetic two dimensional crystals such as CrI$_3$. Because of the arrangement of the Cr spins in a honeycomb lattice, magnons in CrI$_3$ bear a strong resemblance with electronic quasiparticles in graphene. Neutron scattering experiments carried out in bulk CrI$_3$ show the existence of a gap at the Dirac points, that has been conjectured to have a topological nature. Here we propose a theory for magnons in ferromagnetic CrI$_3$ monolayers based on an itinerant fermion picture, with a Hamiltonian derived from fir…
Magnon-polarons are coherently mixed quasiparticles that originate from the strong magnetoelastic coupling of lattice vibrations and spin waves in magnetic-ordered materials. Recently, magnon-polarons have attracted a lot of attention since they provide a powerful tool to manipulate magnons, which is essential for magnon-based spintronic devices. In this work, we report the experimental observation of facet-dependent magnon-polarons in epitaxial ferrimagnetic Fe3O4 thin films via spin Seebeck effect measurement. The critical magnetic fields for the magnon-polarons in the (110)- and (100)-orien…
A leading nonlinear effect in magnonics is the interaction that splits a high-frequency magnon into two low-frequency ones with conserved linear momentum. Here, we report experimental observation of nonlocal three-magnon scattering between spatially separated magnetic systems, viz. a CoFeB nanowire and an yttrium iron garnet (YIG) thin film. Above a certain threshold power of an applied microwave field, a CoFeB Kittel magnon splits into a pair of counter-propagating YIG magnons that induce voltage signals in Pt electrodes on each side, in excellent agreement with model calculations based on th…
Magnon-polaritons are hybrid light-matter quasiparticles originating from the strong coupling between magnons and photons. They have emerged as a potential candidate for implementing quantum transducers and memories. Owing to the dampings of both photons and magnons, the polaritons have limited lifetimes. However, stationary magnon-polariton states can be reached by a dynamical balance between pumping and losses, so the intrinsical nonequilibrium system may be described by a non-Hermitian Hamiltonian. Here we design a tunable cavity quantum electrodynamics system with a small ferromagnetic sph…
Double anticrossings induced by nonlinear magnon interactions
RIKEN Center for Emergent Matter Science (CEMS)（理化学研究所物质结构科学研究中心）；Advanced Institute for Materials Research, Tohoku University（东北大学物质材料研究高等研究院）；Institute for AI and Beyond, The University of Tokyo（东京大学AI与未来研究所）
Altermagnetic Magnons in Dipolar Nanomagnet Arrays
Universität Münster（明斯特大学）；Rheinland-Pfälzische Technische Universität Kaiserslautern-Landau（莱茵兰-普法尔茨凯泽斯劳滕-兰道技术大学）；Radboud University（拉德堡德大学）；Johannes Gutenberg University Mainz（约翰内斯·古腾堡美因茨大学）
Time-resolved observation of magnon splitting into vortex gyration and Floquet spin waves
Université Paris-Saclay, CNRS, Centre de Nanosciences et de Nanotechnologies（巴黎萨克雷大学）；Helmholtz-Zentrum Dresden–Rossendorf（德累斯顿-罗森多夫亥姆霍兹中心）；International Iberian Nanotechnology Laboratory（伊比利亚纳米技术国际实验室）
Owing to its charge-free property, magnon is highly promising to achieve dissipationless transport without Joule heating and thus potentially applicable to energy-efficient devices. Moreover, a kagome lattice, as stacking layers of many magnon ferromagnets, also exhibits valley structure in quasiparticle spectra, which are likely to add a new dimension to magnon excitation. Here, we investigate valley magnon and associated valley modulation in a kagome lattice, with staggered exchange interaction and Dzyaloshinskii-Moriya interaction. The staggered exchange interaction breaks spatial inversion…
Exploiting inversion symmetry breaking (ISB) in systems with strong spin-orbit coupling promises control of spin through electric fields - crucial to achieve miniaturization in spintronic devices. Delivering on this promise requires a two-dimensional electron gas with a spin precession length shorter than the spin coherence length and a large spin splitting so that spin manipulation can be achieved over length scales of nanometers. Recently, the transition metal oxide terminations of delafossite oxides were found to exhibit a large Rashba spin splitting dominated by ISB. In this limit, the Fer…
One recent breakthrough in the field of magnonics is the experimental realization of reconfigurable spin-wave nanochannels formed by magnetic domain wall with a width of $10-100$ nm [Wagner \emph{et al}., Nat. Nano. \textbf{11}, 432 (2016)]. This remarkable progress enables an energy-efficient spin-wave propagation with a well-defined wave vector along its propagating path inside the wall. In the mentioned experiment, a micro-focus Brillouin light scattering spectroscopy was taken in a line-scans manner to measure the frequency of the bounded spin wave. Due to their localization nature, the co…
We investigate entanglement dynamics in the antiferromagnetic Heisenberg model in two dimensions following a spatially anisotropic quench of the exchange interactions. Opposed to established results in one dimension, the magnon quasiparticles show an initial growth of entanglement dynamics that does not depend on the system size and is governed by the oscillation period of the exchange interaction. We ascribe this to the dominance of the intrinsic entanglement of short wavelength non-propagating magnon-pairs, which also leads to a competition between area-law and volume-law contribution in the…
