Dispersionless propagation of ultra-short spin-wave pulses in ultrathin yttrium iron garnet waveguides
B. Divinskiy, H. Merbouche, K. O. Nikolaev, S. Michaelis de Vasoncellos, R. Bratschitsch, D. Gouere, R. Lebrun, V. Cros, J. Ben Youssef, P. Bortolotti, A. Anane… 展开作者
B. Divinskiy, H. Merbouche, K. O. Nikolaev, S. Michaelis de Vasoncellos, R. Bratschitsch, D. Gouere, R. Lebrun, V. Cros, J. Ben Youssef, P. Bortolotti, A. Anane, S. O. Demokritov, V. E. Demidov
We study experimentally the propagation of nanosecond spin-wave pulses in microscopic waveguides made of nanometer-thick yttrium iron garnet films. For these studies, we use micro-focus Brillouin light scattering spectroscopy, which provides the possibility to observe propagation of the pulses with high spatial and temporal resolution. We show that, for most spin-wave frequencies, dispersion leads to broadening of the pulse by several times at propagation distances of 10 micrometers. However, for certain frequency interval, the dispersion broadening is suppressed almost completely resulting in a dispersionless pulse propagation. We show that the formation of the dispersion-free region is caused by the competing effects of the dipolar and the exchange interaction, which can be controlled by the variation of the waveguide geometry. These conclusions are supported by micromagnetic simulations and analytical calculations. Our findings provide a simple solution for the implementation of high-speed magnonic systems that require undisturbed propagation of short information-carrying spin-wave pulses.
Backscattering Raman spectroscopic investigations were performed on an yttrium-iron garnet single crystal using linearly and circularly polarized light. A terahertz (THz) magnon of the Kaplan-Kittel (KK) exchange resonance was discovered, which had been regarded as unobservable via optical methods. The KK exchange resonance had a 7.8-THz frequency at 80 K, and the polarization selection rule led to an antisymmetric Raman tensor of the $A_{2}$ mode. Moreover, the assignment of all the Raman-active phonon modes, $3A_{1g}$, $8E_{g}$, and $14T_{2g}$, was proposed. This study will stimulate further…
We study the impact of Ga ion exposure on the local and non-local magnetotransport response in heterostructures of the ferrimagnetic insulator yttrium iron garnet and platinum. In particular, we cut the yttrium iron garnet layer in between two electrically separated wires of platinum using a Ga ion beam, and study the ensuing changes in the magnetoresistive response. We find that the non-local magnetoresistance signal vanishes when the yttrium iron garnet film between the Pt wires is fully cut, although the local spin Hall magnetoresistance signal remains finite. This observation corroborates …
We study magnon spectra at finite temperature in yttrium iron garnet from tight-binding model with nearest neighboring exchange interaction. The spin reduction due to thermal magnon excitation are taken into account via the mean field approximation to the local spin and found to be different at two sets of iron atoms. The resulting temperature dependence of the spin wave gap shows good agreement with experiment. We find only two magnon modes are relevant to ferromagnetic resonance.
A platinum (Pt)/yttrium iron garnet (YIG) bilayer system with a well-controlled interface has been developed; spin mixing conductance at the Pt/YIG interface has been studied. Crystal perfection at the interface is experimentally demonstrated to contribute to large spin mixing conductance. The spin mixing conductance is obtained to be $1.3\times10^{18} \rm{m^{-2}}$ at the well-controlled Pt/YIG interface, which is close to a theoretical prediction.
The combination of parametric pumping and spin-transfer torque is a powerful approach that enables high-level control over magnetic excitations in thin-film ferromagnets. The excitation parameters, such as pumping power and external field strength, affect the instabilities of individual magnon modes. We theoretically explore how the simultaneous effects of parametric pumping and spin transfer torque influence these magnetic instabilities in a thin-film ferromagnet. Within the Landau-Lifshitz-Gilbert framework, we perform micromagnetic simulations of magnon excitations in yttrium iron garnet by…
We present an experimental study of spin-wave excitation and propagation in microstructured waveguides patterned from a 100 nm thick yttrium iron garnet (YIG)/platinum (Pt) bilayer. The life time of the spin waves is found to be more than an order of magnitude higher than in comparably sized metallic structures despite the fact that the Pt capping enhances the Gilbert damping. Utilizing microfocus Brillouin light scattering spectroscopy, we reveal the spin-wave mode structure for different excitation frequencies. An exponential spin-wave amplitude decay length of 31 μm is observed which is a s…
We derive and solve quantum kinetic equations describing the accumulation of magnetoelastic bosons in an overpopulated magnon gas realized in a thin film of the magnetic insulator yttrium iron garnet. We show that in the presence of a magnon condensate, there is a non-equilibrium steady state in which incoherent magnetoelastic bosons accumulate in a narrow region in momentum space for energies slightly below the bottom of the magnon spectrum. The results of our calculations agree quite well with Brillouin light scattering measurements of the stationary non-equilibrium state of magnons and magn…
Using femtosecond optical pulses, we have investigated the ultrafast magnetization dynamics induced in a dielectric film of bismuth-substituted yttrium iron garnet (Bi-YIG) buried below a thick Cu/Pt metallic bilayer. We show that exciting the sample from Pt surface launches an acoustic strain pulse propagating into the garnet film. We discovered that this strain pulse induces a coherent magnetization precession in the Bi-YIG at the frequency of the ferromagnetic resonance. The observed phenomena can be explain by strain-induced changes of magnetocristalline anisotropy via the inverse magnetos…
We demonstrate theoretically, and confirm experimentally, that nonlinear spin waves excited in thin yttrium iron garnet films are good candidates for squeezing vacuum quantum noise. The experimental demonstration is in the form of a measurement of spin-wave induced modulation instability (IMI) conducted in the classical regime. The experiment evidences strong phase locking of an idler wave parametrically generated in the film with a deterministic small-signal wave launched into the film from an external source. The theory predicts that the same behaviour will be observed for vacuum quantum noi…
Time-resolved scanning transmission x-ray microscopy (TR-STXM) has been used for the direct imaging of spin wave dynamics in thin film yttrium iron garnet (YIG) with spatial resolution in the sub 100 nm range. Application of this x-ray transmission technique to single crystalline garnet films was achieved by extracting a lamella (13x5x0.185 $\mathrm{μm^3}$) of liquid phase epitaxy grown YIG thin film out of a gadolinium gallium garnet substrate. Spin waves in the sample were measured along the Damon-Eshbach and backward volume directions of propagation at gigahertz frequencies and with wavelen…
The occurrence of Spin-Hall Magnetoresistance (SMR) in platinum (Pt) on top of yttrium iron garnet (YIG) has been investigated, for both in-plane and out-of-plane applied magnetic fields and for different Pt thicknesses [3, 4, 8 and 35nm]. Our experiments show that the SMR signal directly depends on the in-plane and out-of-plane magnetization directions of the YIG. This confirms the theoretical description, where the SMR occurs due to the interplay of spin-orbit interaction in the Pt and spin-mixing at the YIG/Pt interface. Additionally, the sensitivity of the SMR and spin pumping signals on t…
The dependence of the spin pumping efficiency and the spin mixing conductance on the surface processing of yttrium iron garnet (YIG) before the platinum (Pt) deposition has been investigated quantitatively. The ferromagnetic resonance driven spin pumping injects a spin polarized current into the Pt layer, which is transformed into an electromotive force by the inverse spin Hall effect. Our experiments show that the spin pumping effect indeed strongly depends on the YIG/Pt interface condition. We measure an enhancement of the inverse spin Hall voltage and the spin mixing conductance of more tha…
We investigated the spin-wave propagation in a micro-structured yttrium iron garnet waveguide of $40$ nm thickness. Utilizing spatially-resolved Brillouin light scattering microscopy, an exponential decay of the spin-wave amplitude of $(10.06 \pm 0.83)$ $μ$m was observed. This leads to an estimated Gilbert damping constant of $α=(8.79\pm 0.73)\times 10^{-4}$, which is larger than damping values obtained through ferromagnetic resonance measurements in unstructured films. The theoretically calculated spatial interference of waveguide modes was compared to the spin-wave pattern observed experimen…
The spin-Seebeck effect (SSE) in platinum (Pt) and tantalum (Ta) on yttrium iron garnet (YIG) has been investigated by both externally heating the sample (using an on-chip Pt heater on top of the device) as well as by current-induced heating. For SSE measurements, external heating is the most common method to obtain clear signals. Here we show that also by current-induced heating it is possible to directly observe the SSE, separate from the also present spin-Hall magnetoresistance (SMR) signal, by using a lock-in detection technique. Using this measurement technique, the presence of additional…
The spin Hall effect in a heavy metal thin film allows to probe the magnetic properties of an adjacent magnetic insulator via magnetotransport measurements. Here, we investigate the magnetoresistive response of yttrium iron garnet/platinum heterostructures from room temperature to beyond the Curie temperature $T_\mathrm{C, YIG} \approx 560\,\mathrm{K}$ of the ferrimagnetic insulator. We find that the amplitude of the (local) spin Hall magnetoresistance decreases monotonically from $300\,\mathrm{K}$ towards $T_\mathrm{C}$, mimicking the evolution of the saturation magnetization of yttrium iron …
Spin-phonon interaction is an important channel for spin and energy relaxation in magnetic insulators. Understanding this interaction is critical for developing magnetic insulator-based spintronic devices. Quantifying this interaction in yttrium iron garnet (YIG), one of the most extensively investigated magnetic insulators, remains challenging because of the large number of atoms in a unit cell. Here, we report temperature-dependent and polarization-resolved Raman measurements in a YIG bulk crystal. We first classify the phonon modes based on their symmetry. We then develop a modified mean-fi…
In spintronics the propagation of spin-wave excitations in magnetically ordered materials can also be used to transport and process information. One of the most popular materials in this regard is the ferrimagnetic insulator yttrium-iron-garnet due its exceptionally small spin-wave damping parameter. While the small relaxation rate allows for large propagation length of magnetic excitations, it also leads to non-locality of the magnetic properties. By imaging spin waves their band structure is mapped. In doing so wave vector selection is shown to suppress dispersion effects to a large extent a…
Spin waves can probe the Dzyaloshinskii-Moriya interaction (DMI) which gives rise to topological spin textures, such as skyrmions. However, the DMI has not yet been reported in yttrium iron garnet (YIG) with arguably the lowest damping for spin waves. In this work, we experimentally evidence the interfacial DMI in a 7~nm-thick YIG film by measuring the nonreciprocal spin wave propagation in terms of frequency, amplitude and most importantly group velocities using all electrical spin-wave spectroscopy. The velocities of propagating spin waves show chirality among three vectors, i.e. the film no…
We investigated the effect of an external magnetic field on the diffusive spin transport by magnons in the magnetic insulator yttrium iron garnet (YIG), using a non-local magnon transport measurement geometry. We observed a decrease in magnon spin diffusion length $λ_m$ for increasing field strengths, where $λ_m$ is reduced from 9.6$\pm1.2$ $μ$m at 10 mT to 4.2$\pm0.6$ $μ$m at 3.5 T at room temperature. In addition, we find that there must be at least one additional transport parameter that depends on the external magnetic field. Our results do not allow us to unambiguously determine whether t…
The formation of a Bose condensate of magnons in a perpendicularly magnetized film of yttrium iron garnet under radio-frequency pumping in a strip line is studied experimentally. The characteristics of nonlinear magnetic resonance and the spatial distribution of the Bose condensate of magnons in the magnetic field gradient are investigated. In these experiments, the Bosonic system of magnons behaves similarly to the Bose condensate of magnons in the antiferromagnetic superfluid 3He-B, which was studied in detail earlier. Magnonic BEC forms a coherently precessing state with the properties of m…
We report the magnetic properties of yttrium iron garnet (YIG) thin films grown by pulsed laser deposition technique. The films were deposited on Si (100) substrates in the range of 15-50 nm thickness. Magnetic characterizations were investigated by ferromagnetic resonance spectra. Perpendicular magnetic easy axis was achieved up to 50 nm thickness. We observed that the perpendicular anisotropy values decreased by increasing the film thickness. The origin of the perpendicular magnetic anisotropy (PMA) was attributed to the texture and the lattice distortion in the YIG thin films. We anticipate…
We report measurements of the frequency and temperature dependence of ferromagnetic resonance (FMR) for a 15-nm-thick yttrium iron garnet (YIG) film grown by off-axis sputtering. Although the FMR linewidth is narrow at room temperature (corresponding to a damping coefficient $α$ = (9.0 $\pm$ 0.2) $\times 10^{-4}$), comparable to previous results for high-quality YIG films of similar thickness, the linewidth increases strongly at low temperatures, by a factor of almost 30. This increase cannot be explained as due to two-magnon scattering from defects at the sample interfaces. We argue that the …
Conductivities are key material parameters that govern various types of transport (electronic charge, spin, heat etc.) driven by thermodynamic forces. Magnons, the elementary excitations of the magnetic order, flow under the gradient of a magnon chemical potential in proportion to a magnon (spin) conductivity $σ_{m}$. The magnetic insulator yttrium iron garnet (YIG) is the material of choice for efficient magnon spin transport. Here we report an unexpected giant $σ_{m}$ in record-thin YIG films with thicknesses down to 3.7 nm when the number of occupied two-dimensional (2D) subbands is reduced…
Thin Yttrium Iron Garnet (YIG) is a promising material for integrated magnonics. In order to introduce YIG into nanofabrication processes it is necessary to fabricate very thin YIG films with a thickness well below 100 nm while retaining the extraordinary magnetic properties of the material, especially its long magnon lifetime and spin wave propagation length. Here, we give a brief introduction into the topic and we review and discuss the various results published over the last decade in this area. Especially for ultrathin films it turns out that pulsed layer deposition and sputtering are the …
We studied the nonlocal transport behavior of both electrically and thermally excited magnons in yttrium iron garnet (YIG) as a function of its thickness. For electrically injected magnons, the nonlocal signals decrease monotonically as the YIG thickness increases. For the nonlocal behavior of the thermally generated magnons, or the nonlocal spin Seebeck effect (SSE), we observed a sign reversal which occurs at a certain heater-detector distance, and it is influenced by both the opacity of the YIG/heater interface and the YIG thickness. Our nonlocal SSE results can be qualitatively explained b…
We carried out a concerted effort to determine the absolute sign of the inverse spin Hall effect voltage generated by spin currents injected into a normal metal. We focus on yttrium iron garnet (YIG)|platinum bilayers at room temperature, generating spin currents by microwaves and temperature gradients. We find consistent results for different samples and measurement setups that agree with theory. We suggest a right-hand-rule to define a positive spin Hall angle corresponding to with the voltage expected for the simple case of scattering of free electrons from repulsive Coulomb charges.
Bose-Einstain condensation occurs at an appropriate density of bosonic particles, depending on their mass and temperature. We were able to experimentally observe the transition from the spin wave regime to the magnon Bose-Einstein condensed state (mBEC) with increasing magnon density by a microwave pumping. We used optical methods to register the spatial distribution of the magnon density and phase. For the first time, a coherent state of stationary magnons was demonstrated far from the region of their excitation.
Yttrium iron garnet (YIG) is a magnetic insulator with record-low damping, allowing spin-wave transport over macroscopic distances. Doping YIG with gallium ions greatly reduces the demagnetizing field and introduces a perpendicular magnetic anisotropy, which leads to an isotropic spin-wave dispersion that facilitates spin-wave optics and spin-wave steering. Here, we characterize the dispersion of a gallium-doped YIG (Ga:YIG) thin film using electrical spectroscopy. We determine the magnetic anisotropy parameters from the ferromagnetic resonance frequency and use propagating spin wave spectrosc…
Spin information carried by magnons is attractive for computing technology and the development of magnon-based computing circuits is of great interest. However, magnon transport in insulators has been challenging, different from the clear physical picture for spin transport in conductors. Here we investigate the lateral transport properties of thermally excited magnons in yttrium iron garnet (YIG), a model magnetic insulator. Polarity reversals of detected spins in non-local geometry devices have been experimentally observed and are strongly dependent on temperature, YIG film thickness, and in…
Pure spin current, a flow of spin angular momentum without flow of any companying 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-m…
We discuss threshold field-dependence of the decay rate of the uniform magnon mode in yttrium iron garnet (YIG) thin films. We demonstrate that decays must cease to exist in YIG films of thickness less than 1 μm, the lengthscale defined by the exchange length. We show that due to the symmetry of the three-magnon coupling the decay rate is linear in ΔH=(Hc-H) in the vicinity of the threshold field Hc instead of the step-like ΓΘ(ΔH) expected from the two-dimensional character of magnon excitations in such films. For thicker films, the decay rate should exhibit multiple steps due to thresholds fo…
The spin Seebeck effect (SSE) is observed in magnetic insulator|heavy metal bilayers as an inverse spin Hall effect voltage under a temperature gradient. The SSE can be detected nonlocally as well, viz. in terms of the voltage in a second metallic contact (detector) on the magnetic film, spatially separated from the first contact that is used to apply the temperature bias (injector). Magnon-polarons are hybridized lattice and spin waves in magnetic materials, generated by the magnetoelastic interaction. Kikkawa et al. [Phys. Rev. Lett. \textbf{117}, 207203 (2016)] interpreted a resonant enhanc…
We have investigated the unidirectional spin wave heat conveyer effect in sub-micron thick yttrium iron garnet (YIG) films using lock-in thermography (LIT). Although the effect is small in thin layers this technique allows us to observe asymmetric heat transport by magnons which leads to asymmetric temperature profiles differing by several mK on both sides of the exciting antenna, respectively. Comparison of Damon-Eshbach and backward volume modes shows that the unidirectional heat flow is indeed due to non-reciprocal spin-waves. Because of the finite linewidth, small asymmetries can still be …
The features of phonon-magnon interconversion in acoustic resonator determine the efficiency of spin pumping from YIG into Pt that may be detected electrically through the inverse spin Hall effect (ISHE). Based on the methods developed in previous works for calculating resonator structures with a piezoelectric (ZnO) and a magnetoelastic layer in contact with the heavy metal (YIG/Pt), we present the results of numerical calculations of YIG film thickness influence on acoustically driven spin waves. We obtain some YIG film thickness regions with various behavior of dc ISHE voltage $U_{ISHE}$. At…
The formation and properties of envelope dispersive shock wave (DSW) excitations from repulsive nonlinear waves in a magnetic film are studied. Experiments involve the excitation of a spin-wave step pulse in a low-loss magnetic Y$_3$Fe$_5$O$_{12}$ thin film strip, in which the spin-wave amplitude increases rapidly, realizing the canonical Riemann problem of shock theory. Under certain conditions, the envelope of the spin-wave pulse evolves into a DSW that consists of an expanding train of nonlinear oscillations with amplitudes increasing from front to back, terminated by a black soliton. The o…
We study the magnon spin transport in the magnetic insulator yttrium iron garnet (YIG) in a nonlocal experiment and compare the magnon spin excitation and detection for the heavy metal paramagnetic electrodes platinum (Pt|YIG|Pt) and tantalum (Ta|YIG|Ta). The electrical injection and detection processes rely on the (inverse) spin Hall effect in the heavy metals and the conversion between the electron spin and magnon spin at the heavy metal|YIG interface. Pt and Ta possess opposite signs of the spin Hall angle. Furthermore, their heterostructures with YIG have different interface properties, i.…
We present a comprehensive investigation of propagating spin waves in nanometer-thick yttrium iron garnet (YIG) films. We use broadband spin-wave spectroscopy with integrated coplanar waveguides (CPWs) and microstrip antennas on top of continuous and patterned YIG films to characterize spin waves with wave vectors up to 10 rad/$μ$m. All films are grown by pulsed laser deposition. From spin-wave transmission spectra, parameters such as the Gilbert damping constant, spin-wave dispersion relation, group velocity, relaxation time, and decay length are derived and their dependence on magnetic bias …
We studied the transient behavior of the spin current generated by the longitudinal spin Seebeck effect (LSSE) in a set of platinum-coated yttrium iron garnet (YIG) films of different thicknesses. The LSSE was induced by means of pulsed microwave heating of the Pt layer and the spin currents were measured electrically using the inverse spin Hall effect in the same layer. We demonstrate that the time evolution of the LSSE is determined by the evolution of the thermal gradient triggering the flux of thermal magnons in the vicinity of the YIG/Pt interface. These magnons move ballistically within …
Nano resonators in which mechanical vibrations and spin waves can be coupled are an intriguing concept that can be used in quantum information processing to transfer information between different states of excitation. Until now, the fabrication of free standing magnetic nanostructures which host long lived spin wave excitatons and may be suitable as mechanical resonators seemed elusive. We demonstrate the fabrication of free standing monocrystalline yttrium iron garnet (YIG) 3D nanoresonators with nearly ideal magnetic properties. The freestanding 3D structures are obtained using a complex lit…
The exceptional magnetic, optical and phononic properties of Yttrium Iron Garnet (YIG) make it unique for spin-wave based and photonic applications. Yet, nanostructuring crystalline YIG and manipulating its magnetism in a non-destructive way is an outstanding challenge, and so far mostly limited to two-dimensional capabilities. Here, we show that irradiation of single-crystal YIG films with a focused UV laser drives a stable, giant enhancement of the perpendicular magnetic anisotropy, preserving the crystalline quality. This modulation is highly confined at the nanoscale in both the lateral an…
We present a systematic study of the temperature dependence of diffusive magnon spin transport, using a non-local device geometry. In our measurements, we detect spin signals arising from electrical and thermal magnon generation, and we directly extract the magnon spin diffusion length $λ_m$ for temperatures from 2 to 293 K. Values of $λ_m$ obtained from electrical and thermal generation agree within the experimental error, with $λ_m=9.6\pm0.9$ $μ$m at room temperature to a minimum of $λ_m=5.5\pm0.7$ $μ$m at 30 K. Using a 2D finite element model to fit the data obtained for electrical magnon g…
Spin currents are generated within the bulk of magnetic materials due to heat flow, an effect called intrinsic spin-Seebeck. This bulk bosonic spin current consists of a diffusing thermal magnon cloud, parametrized by the magnon chemical potential ($μ_{m}$), with a diffusion length of several microns in yttrium iron garnet (YIG). Transient opto-thermal measurements of the spin-Seebeck effect (SSE) as a function of temperature reveal the time evolution of $μ_{m}$ due to intrinsic SSE in YIG. The interface SSE develops at times < 2 ns while the intrinsic SSE signal continues to evolve at times >…
We experimentally study the transient voltage response of yttrium iron garnet/platinum bilayer samples subject to periodic heating up to gigahertz frequencies. We observe an intrinsic cutoff frequency of the induced thermopower voltage, which characteristically depends on the thickness of the yttrium iron garnet film. The cutoff frequency reaches values of up to 350 MHz in a 50 nm thick yttrium iron garnet film, but drops to below 1 MHz for several-micrometer-thick films. These data corroborate the notion that the magnon spectrum responsible for the spin current emission in the spin Seebeck ef…
Spin-orbit effects [1-4] have the potential of radically changing the field of spintronics by allowing transfer of spin angular momentum to a whole new class of materials. In a seminal letter to Nature [5], Kajiwara et al. showed that by depositing Platinum (Pt, a normal metal) on top of a 1.3 $μ$m thick Yttrium Iron Garnet (YIG, a magnetic insulator), one could effectively transfer spin angular momentum through the interface between these two different materials. The outstanding feature was the detection of auto-oscillation of the YIG when enough dc current was passed in the Pt. This finding …
Magnetoelastic coupling in stripe-domain states of yttrium iron garnet
University College London（伦敦大学学院）；Imperial College London（帝国理工学院）；Japan Atomic Energy Agency（日本原子能机构）；The University of Tokyo（东京大学）；RIKEN Center for Emergent Matter Science (CEMS)（理化学研究所新兴物质科学中心）；WPI-Advanced Institute for Materials Research, Tohoku University（东北大学材料研究高等研究院）
