Showing papers on "Spin wave published in 2016"

Room-temperature chiral magnetic skyrmions in ultrathin magnetic nanostructures

Abstract: Magnetic skyrmions are chiral spin structures with a whirling configuration. Their topological properties, nanometre size and the fact that they can be moved by small current densities have opened a new paradigm for the manipulation of magnetization at the nanoscale. Chiral skyrmion structures have so far been experimentally demonstrated only in bulk materials and in epitaxial ultrathin films, and under an external magnetic field or at low temperature. Here, we report on the observation of stable skyrmions in sputtered ultrathin Pt/Co/MgO nanostructures at room temperature and zero external magnetic field. We use high lateral resolution X-ray magnetic circular dichroism microscopy to image their chiral Neel internal structure, which we explain as due to the large strength of the Dzyaloshinskii–Moriya interaction as revealed by spin wave spectroscopy measurements. Our results are substantiated by micromagnetic simulations and numerical models, which allow the identification of the physical mechanisms governing the size and stability of the skyrmions.

Magnetic domain walls as reconfigurable spin-wave nanochannels.

Abstract: Magnetic domain walls in a permalloy sample can be arranged to define waveguides for the transmission of information via magnons. In the research field of magnonics1,2,3,4,5,6,7, it is envisaged that spin waves will be used as information carriers, promoting operation based on their wave properties. However, the field still faces major challenges. To become fully competitive, novel schemes for energy-efficient control of spin-wave propagation in two dimensions have to be realized on much smaller length scales than used before. In this Letter, we address these challenges with the experimental realization of a novel approach to guide spin waves in reconfigurable, nano-sized magnonic waveguides. For this purpose, we make use of two inherent characteristics of magnetism: the non-volatility of magnetic remanence states and the nanometre dimensions of domain walls formed within these magnetic configurations. We present the experimental observation and micromagnetic simulations of spin-wave propagation inside nano-sized domain walls and realize a first step towards a reconfigurable domain-wall-based magnonic nanocircuitry.

Generation of coherent spin-wave modes in yttrium iron garnet microdiscs by spin-orbit torque.

Abstract: In recent years, spin-orbit effects have been widely used to produce and detect spin currents in spintronic devices. The peculiar symmetry of the spin Hall effect allows creation of a spin accumulation at the interface between a metal with strong spin-orbit interaction and a magnetic insulator, which can lead to a net pure spin current flowing from the metal into the insulator. This spin current applies a torque on the magnetization, which can eventually be driven into steady motion. Tailoring this experiment on extended films has proven to be elusive, probably due to mode competition. This requires the reduction of both the thickness and lateral size to reach full damping compensation. Here we show clear evidence of coherent spin-orbit torque-induced auto-oscillation in micron-sized yttrium iron garnet discs of thickness 20 nm. Our results emphasize the key role of quasi-degenerate spin-wave modes, which increase the threshold current.

Antiferromagnetic Domain Wall Motion Driven by Spin-Orbit Torques.

Abstract: We theoretically investigate the dynamics of antiferromagnetic domain walls driven by spin-orbit torques in antiferromagnet-heavy-metal bilayers. We show that spin-orbit torques drive antiferromagnetic domain walls much faster than ferromagnetic domain walls. As the domain wall velocity approaches the maximum spin-wave group velocity, the domain wall undergoes Lorentz contraction and emits spin waves in the terahertz frequency range. The interplay between spin-orbit torques and the relativistic dynamics of antiferromagnetic domain walls leads to the efficient manipulation of antiferromagnetic spin textures and paves the way for the generation of high frequency signals from antiferromagnets.

Spin Nernst effect of magnons in collinear antiferromagnets

Abstract: In a collinear antiferromagnet with easy-axis anisotropy, symmetry guarantees that the spin wave modes are doubly degenerate. The two modes carry opposite spin angular momentum and exhibit opposite chirality. Using a honeycomb antiferromagnet in the presence of the Dzyaloshinskii-Moriya interaction, we show that a longitudinal temperature gradient can drive the two modes to opposite transverse directions, realizing a spin Nernst effect of magnons with vanishing thermal Hall current. We find that magnons around the $\mathrm{\ensuremath{\Gamma}}$ point and the $K$ point contribute oppositely to the transverse spin transport, and their competition leads to a sign change of the spin Nernst coefficient at finite temperature. Possible material candidates are discussed.

A first theoretical realization of honeycomb topological magnon insulator.

Abstract: It has been recently shown that in the Heisenberg (anti)ferromagnet on the honeycomb lattice, the magnons (spin wave quasipacticles) realize a massless two-dimensional (2D) Dirac-like Hamiltonian. It was shown that the Dirac magnon Hamiltonian preserves time-reversal symmetry defined with the sublattice pseudo spins and the Dirac points are robust against magnon-magnon interactions. The Dirac points also occur at nonzero energy. In this paper, we propose a simple realization of nontrivial topology (magnon edge states) in this system. We show that the Dirac points are gapped when the inversion symmetry of the lattice is broken by introducing a next-nearest neighbour Dzyaloshinskii-Moriya (DM) interaction. Thus, the system realizes magnon edge states similar to the Haldane model for quantum anomalous Hall effect in electronic systems. However, in contrast to electronic spin current where dissipation can be very large due to Ohmic heating, noninteracting topological magnons can propagate for a long time without dissipation as magnons are uncharged particles. We observe the same magnon edge states for the XY model on the honeycomb lattice. Remarkably, in this case the model maps to interacting hardcore bosons on the honeycomb lattice. Quantum magnetic systems with nontrivial magnon edge states are called topological magnon insulators. They have been studied theoretically on the kagome lattice and recently observed experimentally on the kagome magnet Cu(1-3, bdc) with three magnon bulk bands. Our results for the honeycomb lattice suggests an experimental procedure to search for honeycomb topological magnon insulators within a class of 2D quantum magnets and ultracold atoms trapped in honeycomb optical lattices. In 3D lattices, Dirac and Weyl points were recently studied theoretically, however, the criteria that give rise to them were not well-understood. We argue that the low-energy Hamiltonian near the Weyl points should break time-reversal symmetry of the pseudo spins. Thus, recovering the same criteria in electronic systems.

Spin Transport at Interfaces with Spin-orbit Coupling: Formalism

Abstract: Spin transport at interfaces between nonmagnets and ferromagnets plays an important role in spintronic devices. Lately, there is an increasing suspicion that spin-orbit coupling, which couples the spin and momentum of carriers, might contribute significantly to this process. Unfortunately, the existing description of spin transport at such interfaces, magnetoelectronic circuit theory, is not valid when spin-orbit coupling is present at the interface. This paper presents a generalization of magnetoelectronic circuit theory to interfaces with spin-orbit coupling. Like the original theory, this generalization describes spin transport in terms of drops in spin and charge accumulations across the interface, but also includes responses to in-plane electric fields and offsets in spin accumulations. The most important result is a description of the way in-plane electric fields generate spin accumulations, spin currents, and torques at the interface. The effects described by this generalized circuit theory impact the interpretation of experiments involving spin-orbit torques, spin pumping, spin memory loss, the Rashba-Edelstein effect, and spin Hall magnetoresistance.

Magnetic vortex cores as tunable spin-wave emitters

Abstract: The magnetic field-driven dynamics of nanosized magnetic vortex cores can be used to generate propagating short-wavelength spin waves in heterostructures with antiferromagnetically coupled layers.

Spin-current probe for phase transition in an insulator

Abstract: Spin fluctuation and transition have always been one of the central topics of magnetism and condensed matter science. Experimentally, the spin fluctuation is found transcribed onto scattering intensity in the neutron-scattering process, which is represented by dynamical magnetic susceptibility and maximized at phase transitions. Importantly, a neutron carries spin without electric charge, and therefore it can bring spin into a sample without being disturbed by electric energy. However, large facilities such as a nuclear reactor are necessary. Here we show that spin pumping, frequently used in nanoscale spintronic devices, provides a desktop microprobe for spin transition; spin current is a flux of spin without an electric charge and its transport reflects spin excitation. We demonstrate detection of antiferromagnetic transition in ultra-thin CoO films via frequency-dependent spin-current transmission measurements, which provides a versatile probe for phase transition in an electric manner in minute devices.

Generation of coherent spin-wave modes in yttrium iron garnet microdiscs by spin-orbit torque

Abstract: In recent years, spin–orbit effects have been widely used to produce and detect spin currents in spintronic devices. The peculiar symmetry of the spin Hall effect allows creation of a spin accumulation at the interface between a metal with strong spin–orbit interaction and a magnetic insulator, which can lead to a net pure spin current flowing from the metal into the insulator. This spin current applies a torque on the magnetization, which can eventually be driven into steady motion. Tailoring this experiment on extended films has proven to be elusive, probably due to mode competition. This requires the reduction of both the thickness and lateral size to reach full damping compensation. Here we show clear evidence of coherent spin–orbit torque-induced auto-oscillation in micron-sized yttrium iron garnet discs of thickness 20 nm. Our results emphasize the key role of quasi-degenerate spin-wave modes, which increase the threshold current.

Approaching soft X-ray wavelengths in nanomagnet-based microwave technology

Abstract: Seven decades after the discovery of collective spin excitations in microwave-irradiated ferromagnets, there has been a rebirth of magnonics. However, magnetic nanodevices will enable smart GHz-to-THz devices at low power consumption only, if such spin waves (magnons) are generated and manipulated on the sub-100 nm scale. Here we show how magnons with a wavelength of a few 10 nm are exploited by combining the functionality of insulating yttrium iron garnet and nanodisks from different ferromagnets. We demonstrate magnonic devices at wavelengths of 88 nm written/read by conventional coplanar waveguides. Our microwave-to-magnon transducers are reconfigurable and thereby provide additional functionalities. The results pave the way for a multi-functional GHz technology with unprecedented miniaturization exploiting nanoscale wavelengths that are otherwise relevant for soft X-rays. Nanomagnonics integrated with broadband microwave circuitry offer applications that are wide ranging, from nanoscale microwave components to nonlinear data processing, image reconstruction and wave-based logic.

A reconfigurable waveguide for energy-efficient transmission and local manipulation of information in a nanomagnetic device.

Abstract: Spin-wave-based devices promise to usher in an era of low-power computing where information is carried by the precession of the electrons' spin instead of dissipative translation of their charge. This potential is, however, undermined by the need for a bias magnetic field, which must remain powered on to maintain an anisotropic device characteristic. Here, we propose a reconfigurable waveguide design that can transmit and locally manipulate spin waves without the need for any external bias field once initialized. We experimentally demonstrate the transmission of spin waves in straight as well as curved waveguides without a bias field, which has been elusive so far. Furthermore, we experimentally show a binary gating of the spin-wave signal by controlled switching of the magnetization, locally, in the waveguide. The results have potential implications in high-density integration and energy-efficient operation of nanomagnetic devices at room temperature.

Thermal spin dynamics of yttrium iron garnet

Abstract: The magnetic insulator yttrium iron garnet can be grown with near perfection and is therefore and ideal conduit for spin currents. It is a complex material with 20 magnetic moments in the unit cell. In spite of being a ferrimagnet, YIG is almost always modeled as a simple ferromagnet with a single spin wave mode. We use the method of atomistic spin dynamics to study the temperature evolution of the full spin wave spectrum, in quantitative agreement with neutron scattering experiments. The antiferromagnetic or optical mode is found to suppress the spin Seebeck effect at room temperature and beyond due to thermally pumped spin currents with opposite polarization to the ferromagnetic mode.

Curvature-Induced Asymmetric Spin-Wave Dispersion.

Abstract: In magnonics, spin waves are conceived of as electron-charge-free information carriers. Their wave behavior has established them as the key elements to achieve low power consumption, fast operative rates, and good packaging in magnon-based computational technologies. Hence, knowing alternative ways that reveal certain properties of their undulatory motion is an important task. Here, we show using micromagnetic simulations and analytical calculations that spin-wave propagation in ferromagnetic nanotubes is fundamentally different than in thin films. The dispersion relation is asymmetric regarding the sign of the wave vector. It is a purely curvature-induced effect and its fundamental origin is identified to be the classical dipole-dipole interaction. The analytical expression of the dispersion relation has the same mathematical form as in thin films with the Dzyalonshiinsky-Moriya interaction. Therefore, this curvature-induced effect can be seen as a ``dipole-induced Dzyalonshiinsky-Moriya-like'' effect.

Observation of magnon-mediated current drag in Pt/yttrium iron garnet/Pt(Ta) trilayers

Abstract: Pure spin current, a flow of spin angular momentum without flow of any accompanying net charge, is generated in two common ways. One makes use of the spin Hall effect in normal metals (NM) with strong spin-orbit coupling, such as Pt or Ta. The other utilizes the collective motion of magnetic moments or spin waves with the quasi-particle excitations called magnons. A popular material for the latter is yttrium iron garnet, a magnetic insulator (MI). Here we demonstrate in NM/MI/NM trilayers that these two types of spin currents are interconvertible across the interfaces, predicated as the magnon-mediated current drag phenomenon. The transmitted signal scales linearly with the driving current without a threshold and follows the power-law T(n) with n ranging from 1.5 to 2.5. Our results indicate that the NM/MI/NM trilayer structure can serve as a scalable pure spin current valve device which is an essential ingredient in spintronics.

Snell’s Law for Spin Waves

Abstract: We report the experimental observation of Snell's law for magnetostatic spin waves in thin ferromagnetic Permalloy films by imaging incident, refracted, and reflected waves. We use a thickness step as the interface between two media with different dispersion relations. Since the dispersion relation for magnetostatic waves in thin ferromagnetic films is anisotropic, deviations from the isotropic Snell's law known in optics are observed for incidence angles larger than 25° with respect to the interface normal between the two magnetic media. Furthermore, we can show that the thickness step modifies the wavelength and the amplitude of the incident waves. Our findings open up a new way of spin wave steering for magnonic applications.

Frequency nonreciprocity of surface spin wave in permalloy thin films

Abstract: One of the most peculiar properties of spin waves in magnetic thin films is nonreciprocal wave propagation, i.e., a dependence of the wave properties on the propagation direction. The authors study this effect in permalloy films of thickness varying from 6 to 40 nm and find that the difference in the frequencies can reach several tens of MHz. The authors show that this frequency nonreciprocity is mainly caused by asymmetric magnetic anisotropies at the two surfaces of the ferromagnetic film. This result is important to understand recent works on the interfacial Dzyaloshinskii-Moriya interaction, an unconventional chiral magnetic interaction that generates a similar frequency nonreciprocity. The authors demonstrate that while these contributions usually occur at the same time, the two can be distinguished via the difference in their dependence on the film thickness.

Magnetochiral nonreciprocity of volume spin wave propagation in chiral-lattice ferromagnets

Abstract: In magnetic materials with chiral crystal structure, it has been predicted that quasiparticle flows propagating parallel and antiparallel to the external magnetic field can show different propagating character, with its sign of nonreciprocity dependent on the chirality of the underlying bulk crystal lattice. This unique phenomenon, termed magnetochiral nonreciprocity, has previously been demonstrated for the propagating light and conduction electrons but seldom for other quasiparticles. In this study, we report the experimental observation of magnetochiral nonreciprocity of propagating magnons for a chiral-lattice ferromagnet ${\mathrm{Cu}}_{2}{\mathrm{OSeO}}_{3}$ by employing the spin wave spectroscopy. We found that the sign of nonreciprocity is reversed for the opposite chirality of crystal, and also directly identified the wave-number-linear term in the spin wave dispersion associated with the Dzyaloshinskii-Moriya (DM) interaction as the origin of observed nonreciprocity. Our present results pave a route for the design of efficient spin wave diode based on the bulk crystallographic symmetry breaking and also offer a unique method to evaluate the magnitude of DM interaction in chiral-lattice bulk compounds.

Macrospin dynamics in antiferromagnets triggered by sub-20 femtosecond injection of nanomagnons.

Abstract: The understanding of how the sub-nanoscale exchange interaction evolves in macroscale correlations and ordered phases of matter, such as magnetism and superconductivity, requires to bridging the quantum and classical worlds. This monumental challenge has so far only been achieved for systems close to their thermodynamical equilibrium. Here we follow in real time the ultrafast dynamics of the macroscale magnetic order parameter in the Heisenberg antiferromagnet KNiF3 triggered by the impulsive optical generation of spin excitations with the shortest possible nanometre wavelength and femtosecond period. Our magneto-optical pump-probe experiments also demonstrate the coherent manipulation of the phase and amplitude of these femtosecond nanomagnons, whose frequencies are defined by the exchange energy. These findings open up opportunities for fundamental research on the role of short-wavelength spin excitations in magnetism and strongly correlated materials; they also suggest that nanospintronics and nanomagnonics can employ coherently controllable spin waves with frequencies in the 20 THz domain.

Excitation of coherent propagating spin waves by pure spin currents.

Abstract: Utilization of pure spin currents not accompanied by the flow of electrical charge provides unprecedented opportunities for the emerging technologies based on the electron's spin degree of freedom, such as spintronics and magnonics. It was recently shown that pure spin currents can be used to excite coherent magnetization dynamics in magnetic nanostructures. However, because of the intrinsic nonlinear self-localization effects, magnetic auto-oscillations in the demonstrated devices were spatially confined, preventing their applications as sources of propagating spin waves in magnonic circuits using these waves as signal carriers. Here, we experimentally demonstrate efficient excitation and directional propagation of coherent spin waves generated by pure spin current. We show that this can be achieved by using the nonlocal spin injection mechanism, which enables flexible design of magnetic nanosystems and allows one to efficiently control their dynamic characteristics.

All-Electrical Measurement of Interfacial Dzyaloshinskii-Moriya Interaction Using Collective Spin-Wave Dynamics

Abstract: Dzyaloshinskii-Moriya interaction (DMI), which arises from the broken inversion symmetry and spin–orbit coupling, is of prime interest as it leads to a stabilization of chiral magnetic order and provides an efficient manipulation of magnetic nanostructures. Here, we report all-electrical measurement of DMI using propagating spin wave spectroscopy based on the collective spin wave with a well-defined wave vector. We observe a substantial frequency shift of spin waves depending on the spin chirality in Pt/Co/MgO structures. After subtracting the contribution from other sources to the frequency shift, it is possible to quantify the DMI energy in Pt/Co/MgO systems. The result reveals that the DMI in Pt/Co/MgO originates from the interfaces, and the sign of DMI corresponds to the inversion asymmetry of the film structures. The electrical excitation and detection of spin waves and the influence of interfacial DMI on the collective spin-wave dynamics will pave the way to the emerging field of spin-wave logic dev...

Low-Energy Spin Dynamics of the Honeycomb Spin Liquid Beyond the Kitaev Limit.

Abstract: We investigate the generic features of the low energy dynamical spin structure factor of the Kitaev honeycomb quantum spin liquid perturbed away from its exact soluble limit by generic symmetry-allowed exchange couplings. We find that the spin gap persists in the Kitaev-Heisenberg model, but generally vanishes provided more generic symmetry-allowed interactions exist. We formulate the generic expansion of the spin operator in terms of fractionalized Majorana fermion operators according to the symmetry enriched topological order of the Kitaev spin liquid, described by its projective symmetry group. The dynamical spin structure factor displays power-law scaling bounded by Dirac cones in the vicinity of the Γ, K, and K^{'} points of the Brillouin zone, rather than the spin gap found for the exactly soluble point.

Transformation of spin current by antiferromagnetic insulators

Abstract: It is demonstrated theoretically that a thin layer of an anisotropic antiferromagnetic (AFM) insulator can effectively conduct spin current through the excitation of a pair of evanescent AFM spin wave modes. The spin current flowing through the AFM is not conserved due to the interaction between the excited AFM modes and the AFM lattice and, depending on the excitation conditions, can be either attenuated or enhanced. When the phase difference between the excited evanescent modes is close to $\ensuremath{\pi}/2$, there is an optimum AFM thickness for which the output spin current reaches a maximum, which can significantly exceed the magnitude of the input spin current. The spin current transfer through the AFM depends on the ambient temperature and increases substantially when temperature approaches the N\'eel temperature of the AFM layer.

High-efficiency control of spin-wave propagation in ultra-thin yttrium iron garnet by the spin-orbit torque

Abstract: We study experimentally with submicrometer spatial resolution the propagation of spin waves in microscopic waveguides based on the nanometer-thick yttrium iron garnet and Pt layers. We demonstrate that by using the spin-orbit torque, the propagation length of the spin waves in such systems can be increased by nearly a factor of 10, which corresponds to the increase in the spin-wave intensity at the output of a 10 μm long transmission line by three orders of magnitude. We also show that, in the regime, where the magnetic damping is completely compensated by the spin-orbit torque, the spin-wave amplification is suppressed by the nonlinear scattering of the coherent spin waves from current-induced excitations.

Ultrafast domain wall dynamics in magnetic nanotubes and nanowires.

Abstract: The dynamic properties of magnetic domain walls in nanotubes and in cylindrical nanowires can be significantly different from the well known domain wall dynamics in thin films and in flat thin strips. The main differences are the occurrence of chiral symmetry breaking and, perhaps more importantly, the possibility to obtain magnetic domain walls that are stable against the usual Walker breakdown. This stability enables the magnetic field-driven propagation of the domain walls in nanotubes and nanocylinders at constant velocities which are significantly higher than the usual propagation speeds of the domain walls. Simulations predict that the ultrafast motion of magnetic domain walls at velocities in a range above 1000 m s-1 can lead to the spontaneous excitation of spin waves in a process that is the magnetic analog of the Cherenkov effect. In the case of solid cylindrical wires, the domain wall can contain a micromagnetic point singularity. We discuss the current knowledge on the ultrafast dynamics of such Bloch points, which remains still largely unexplored.

Origin of inverse Rashba-Edelstein effect detected at the Cu/Bi interface using lateral spin valves

Abstract: Spin-orbitronics, which exploits the coupling between the spin and the orbital momentum of electrons, relies on the possibility to electrically create and detect pure spin currents without need of ferromagnetic elements. An efficient way to achieve this spin-to-charge conversion (and vice versa) is expected by exploiting the Rashba-Edelstein effect. This phenomenon is related to the well-known spin Hall effect, but in the former, the spin-to-charge current conversion occurs at the interface of materials with a strong spin splitting of the surface states, instead of the bulk. This paper reports an observation of the inverse Rashba-Edelstein effect, i.e., conversion of a spin current into a charge current, at a bismuth/copper interface, by using spin absorption with lateral spin valves. The induced charge current changes sign with temperature, a phenomenon that the authors can explain theoretically owing to the complex spin structure and dispersion of the surface states at the Fermi energy.

Stoner versus Heisenberg: Ultrafast exchange reduction and magnon generation during laser-induced demagnetization

Abstract: The excitation of a ferromagnetic film by a femtosecond laser pulse causes an unexpectedly fast quenching of the film's magnetization on subpicosecond time scales. The microscopic physical mechanisms responsible for this remain a scientific puzzle. The authors employ femtosecond extreme ultraviolet pulses produced by high harmonic generation to follow how the magnetization of a thin cobalt film evolves after the excitation by a 40-fs laser pulse. By measuring the time-, energy-, and angle-resolved magneto-optical response of the Co films across the ${M}_{2,3}$ absorption edge, they obtain a set of time-lapsed magnetic asymmetry spectra, which contain a wealth of information about the different mechanisms at work. When combined with advanced ab initio magneto-optical calculations, they identify two dominant contributions: first, a transient reduction of exchange splitting, and second, magnon excitation. This work thus distinguishes between two fundamental models of magnetism, the Stoner and Heisenberg models, which ascribe magnetization dynamics to an exchange splitting reduction and spin wave excitations, respectively.

Spin pumping in strongly coupled magnon-photon systems

Abstract: We experimentally investigate magnon polaritons arising in ferrimagnetic resonance experiments in a microwave cavity with a tunable quality factor. To this end, we simultaneously measure the electrically detected spin pumping signal and the microwave reflection (the ferrimagnetic resonance signal) of a yttrium iron garnet (YIG)/platinum (Pt) bilayer in the microwave cavity. The coupling strength of the fundamental magnetic resonance mode and the cavity is determined from the microwave reflection data. All features of the magnetic resonance spectra predicted by first principle calculations and an input-output formalism agree with our experimental observations. By changing the decay rate of the cavity at constant magnon-photon coupling rate, we experimentally tune in and out of the strong coupling regime and successfully model the corresponding change of the spin pumping signal and microwave reflection. Furthermore, we observe the coupling and spin pumping of several spin wave modes and provide a quantitative analysis of their coupling rates to the cavity.