Yttrium Iron Garnet Films Research Articles

Ferromagnetism in the Ferromagnetic Yttrium Iron Garnet Film/Ferromagnetic Intermetallic Compound Heterostructure

We report on the investigations of the magnetic properties and ferromagnetic resonance in the heterostructure consisting of an epitaxial yttrium iron garnet (Y3Fe5O12) film and a nanometer rare-earth intermetallic superlattice comprised of the (TbCo2/FeCo)n exchange-coupled layers. The (TbCo2/FeCo)n superlattice exhibits the giant magnetostriction and the controlled magnetic anisotropy induced by a magnetic field or elastic stresses. The magnetic interaction of the films in the (TbCo2/FeCo)n/Y3Fe5O12 heterostructure has been experimentally established and the spin current flowing through their interface has been detected.

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.

Magnons parametric pumping in bulk acoustic waves resonator

We report on the experimental observation of excitation and detection of parametric spin waves and spin currents in the bulk acoustic wave resonator. The hybrid resonator consists of a ZnO piezoelectric film, yttrium iron garnet (YIG) films on a gallium gadolinium garnet substrate, and a heavy metal Pt layer. Shear bulk acoustic waves are electrically excited in the ZnO layer due to piezoeffect at the resonant frequencies of the resonator. The magnetoelastic interaction in the YIG film emerges magnons (spin waves) excitation by acoustic waves either on resonator's eigenfrequencies or the half-value frequencies at supercritical power. We investigate the acoustic pumping of magnons at the half-value frequencies and acoustic spin pumping from parametric magnons, using the inverse spin Hall effect in the Pt layer. The constant electric voltage in the Pt layer, depending on the frequency, the magnetic field, and the pump power, was systematically studied. We explain the low threshold obtained (∼0.4 mW) by the high efficiency of electric power transmission into the acoustic wave in the resonator.

Wannier-Stark ladder spectrum of Bloch oscillations of magneto-dipole spin waves in graded 1D magnonic crystals

We have used the method of Wannier functions to calculate the frequencies and profiles of spin waves localized in one-dimensional magnonic crystals due to a gradient in the bias magnetic field. This localization of spin waves is analogous to the phenomenon of Bloch oscillations of quantum-mechanical electrons in crystals in a uniform electric field. As a convenient yet realistic model, we consider backward volume magnetostatic spin waves in a film of yttrium-iron garnet in a bias magnetic field comprising spatially uniform, cosine and gradient contributions. The spin-wave spectrum is shown to have the characteristic form of a Wannier–Stark ladder. The analytical results are verified using those obtained using numerical micromagnetic simulations. The physics of spin-wave Bloch oscillations combines the topics of magnonic crystals and graded magnonic index — the two cornerstones of modern magnonics.

Exchange spin waves and their application for diagnostics of the layered structure of epitaxial YIG films

The results of modelling of the excitation and propagation processes of exchange spin waves (ESW) in epitaxial films of yttrium iron garnet (YIG) are presented. It is shown that the origin of the efficient ESW excitation is magnetic inhomogeneity, which is always present in a thin transition layer at the interface between a magnetic film and a nonmagnetic substrate. It is found that the parameters of magnetic inhomogeneity significantly affect wave characteristics of the emitted ESWs. A method was proposed for employing ESWs to measure the magnetization distribution parameters over the thickness of the YIG film and the ESW dissipation parameter. The measured parameters were used to calculate the dispersion and attenuation of the ESWs, define their amplitude characteristics in continuous and pulsed regimes, and estimate their phase losses. It is shown that dissipative losses of ESWs are not significant. More significant losses can occur due to phase dispersion on non-parallel reflective surfaces of the YIG film. It is demonstrated that such phase losses can be significantly reduced by precision optical polishing of the YIG film samples.

Spin waves in meander shaped YIG film: Toward 3D magnonics

We report on the experimental observation of a spin wave (SW) propagation in a magnetic film lying on a developed surface—a three-dimensional (3D) magnonic structure. The 3D magnonic structure was fabricated by deposition of a 180 nm thick yttrium iron garnet film on the patterned substrate with a one-dimensional array of 1.5 μm deep grooves with the period of 20 μm using ion beam sputtering. We experimentally explored SW propagation in such a periodic structure for Bragg and Laue geometries. Transmission characteristics showed that for the Bragg geometry, the examined structure works more like a system of coupled resonators that are formed by the flat segments of the 3D structure, whereas for the Laue geometry, the features related to a formation of width SW modes and to the discrete diffraction of the SW were observed.

Low Current Densities Toggle Optical Polarization Switching in Pt/Yttrium Iron Garnet Magnetic Heterostructures Using Energy Resolution

Spin current, which is converted from the charge current in heavy metals, is widely used to manipulate magnetization in heavy metal/magnetic insulator systems. However, there is scant research on how the spin‐polarized density of states (DOS) is affected by the absorption of spin current. This study investigates the spin‐current‐induced magnetic variation of an yttrium iron garnet (YIG) film driven by charge current in an overlayer of Pt that uses energy‐resolved magnetic circular dichroism (MCD) spectra as an example. MCD intensity can be manipulated reversibly using an applied current at near d–d transitions. Furthermore, the MCD sign, which is associated with an elliptically polarized direction, is reversed by a relatively low current density (≈108 A m−2) near YIG charge transfer transitions, even without the assistance of a magnetic field. The spin accumulation in Fe orbits and the corresponding change in spin‐polarized DOS explain the observed phenomenon. This paves the way to low‐dissipation spintronics and electrically controlled magneto‐optical devices based on magnetic insulators.

Magnetoelastic Properties of Yttrium–Iron Garnet Films Manufactured by Means of Ion-Beam Sputtering onto Si and GaAs Substrates

The effect is studied of tensile deformations on the spectrum of the ferromagnetic resonance of submicron yttrium–iron garnet polycrystalline films manufactured by means of ion-beam sputtering onto silicon and gallium arsenide substrates. Magnetoelastic constants of films on both substrates, the values of which did not exceed 16% of that of the bulk polycrystalline garnet, were calculated from the magnitude of the frequency shift of the absorption maximum in the ferromagnetic resonance spectrum. An approach is proposed to increase the efficiency of electrical frequency tuning in composite multiferroid structures by means of combining static and dynamic (caused by piezoelectric effect) tensile deformations.

Enhanced Spin Seebeck Effect in Monolayer Tungsten Diselenide Due to Strong Spin Current Injection at Interface

AbstractThe spin current is significantly limited by the spin‐orbit interaction strength, material quality, and spin‐mixing conductance at material interfaces. Such limitations lead to spin current decay at the interfaces, which severely hinders potential applications in spin‐current‐generating thermoelectric devices. Thus, methodical studies on the enhancement of spin currents are indispensable. Herein, a novel approach for enhancing the spin current injected into a normal metal, Pt, using interface effects with a ferromagnetic insulator, yttrium iron garnet (YIG), is demonstrated. This is accomplished by inserting atomically thin monolayer (ML), tungsten diselenide (WSe2) between Pt and YIG layers. A comparative study of longitudinal spin Seebeck effect (LSSE) measurements is conducted. Two types of ML WSe2 (continuous and large‐area ML WSe2 and isolated ML WSe2 flakes) are used as intermediate layers on YIG film. Notably, the insertion of ML WSe2 between the Pt and YIG layers significantly enhances the thermopower, VLSSE/ΔT by a factor of approximately 5.6 compared with that of the Pt/YIG reference sample. This enhancement in the measured LSSE voltages in the Pt/ML WSe2/YIG trilayer can be explained by the increased spin‐to‐charge conversion at the interface owing to the large spin‐orbit coupling and improved spin mixing conductance with the ML WSe2 intermediate layer.

Terahertz faraday rotation of magneto-optical films enhanced by helical metasurface

The Faraday rotation effect of both the La: yttrium iron garnet (YIG) film and the YIG metasurface were experimentally and numerically investigated in the terahertz (THz) region. A THz magneto-optic polarization measurement system was used to observe the transmission, resonance, and magneto-optical effect of the La:YIG film and YIG metasurface. The THz artificial chirality and resonance localization of the helical metasurface generate the superchiral THz field, which enhances the THz magneto-optical effect of the YIG film. The results show that the Faraday effect of the YIG metasurface is about three times that of the pure YIG film, whose differential rotation angle increases from 8° to over 24°. This work achieves more sensitively active polarization control of THz waves, which is of great significance for THz polarization conversion, sensing, and non-reciprocal transmission.

Mode conversion characteristics of multimode magneto-optical Bragg diffraction cells

We study the dependence of magneto-optical (MO) coupling coefficients on the modal fields for the design of multi-mode MO Bragg cells. The mode conversion and diffraction efficiencies between the same order TM and TE modes can theoretically be up to about 98% by adjusting the excitation frequency of the fundamental-mode magneto-static forward volume wave (MSFVW) in the bismuth-substituted yttrium iron garnet film at the optimized thickness and length. The different order mode conversion is also available by means of the higher-mode MSFVWs. The resulting mode conversion-based switch function is discussed.

Octave-Tunable Magnetostatic Wave YIG Resonators on a Chip.

We have designed, fabricated, and characterized magnetostatic wave (MSW) resonators on a chip. The resonators are fabricated by patterning single-crystal yttrium iron garnet (YIG) film on a gadolinium gallium garnet (GGG) substrate and excited by loop-inductor transducers. We achieved this technology breakthrough by developing a YIG film etching process and fabricating thick aluminum coplanar waveguide (CPW) inductor loop around each resonator to individually address and excite MSWs. At 4.77 GHz, the 0.68-mm2 resonator achieves a quality factor ( with a bias field of 987 Oe. We also demonstrate YIG resonator tuning by more than one octave from 3.63 to 7.63 GHz by applying an in-plane external magnetic field. The measured quality factor of the resonator is consistently over 3000 above 4 GHz. The micromachining technology enables the fabrication of multiple single- and two-port YIG resonators on the same chip with all resonators demonstrating octave tunability and high Q .

Surface-sensitive magnetic characterization technique for ultrathin ferromagnetic film with perpendicular magnetic anisotropy

Ultrathin ferromagnetic films display a wealth of spectacular characteristics, one of which is that two-dimensional ferromagnetism is stabilized exclusively by magnetic anisotropies. In this work, we experimentally demonstrated the realization of ultrathin yttrium iron garnet (Y3Fe5O12) films with perpendicular magnetic anisotropy by pulsed laser deposition. A labyrinthine magnetic domain configuration in Y3Fe5O12 was identified by magnetic force microscopy, qualitatively determining that the magnetic moment possesses an out-of-plane component. Magnetic moments in the Y3Fe5O12 film were investigated by using a vibrating sample magnetometer and magnetic circular dichroism spectroscopy. Due to the surface sensitivity, the characterization technique based on magnetic circular dichroism spectroscopy presents as a high-resolution magneto-optical method to determine the magnetic property quantitatively, which has an advantage over the vibrating sample magnetometer technique commonly used for bulk crystals or powders, especially when the paramagnetic background from the supporting substrate is non-negligible. Our work introduces the burgeoning material synthesis and processing technology, and miscellaneous characterization techniques into the study of conventional magnetic oxide films, providing powerful supplements to the detection and modulation of magnetism at the nanoscale.

Differences in the magnon diffusion length for electrically and thermally driven magnon currents in Y3Fe5O12

Recent demonstration of efficient transport and manipulation of spin information by magnon currents have opened exciting prospects for processing information in devices. Magnon currents can be driven both electrically and thermally, even in magnetic insulators, by applying charge currents in an adjacent metal layer. Earlier reports in thin yttrium iron garnet (YIG) films suggested that the diffusion length of magnons is independent on the biasing method, but different values were obtained in thicker films. Here, we study the magnon diffusion length for electrically and thermally driven magnon currents in the linear regime in a 2-$\mu$m-thick YIG film as a function of temperature and magnetic field. Our results show a decrease of the magnon diffusion length with magnetic field for both biasing methods and at all temperatures from 5 to 300 K, indicating that sub-thermal magnons dominate the long-range transport. Moreover, we demonstrate that the value of the magnon diffusion length depends on the driving mechanism, suggesting that different non-equilibrium magnon distributions are biased for each method. Finally, we demonstrate that the magnon diffusion length for thermally driven magnon currents is independent of the YIG thickness and material growth conditions, confirming that this quantity is an intrinsic parameter of YIG.

Nanometer-thick YIG-based magnonic crystals: Bandgap dependence on groove depth, lattice constant, and film thickness

We report on bandgap tuning in magnonic crystals made of nanometer-thick yttrium iron garnet (YIG) films with CoFeB-filled grooves via a variation of the groove depth, lattice constant, and film thickness. Using broadband spin-wave spectroscopy, we demonstrate bandgap widening in a 260-nm-thick YIG crystal when the grooves are deepened from half to full film thickness. Importantly, low-loss spin-wave transmission in the allowed bands of the magnonic crystal is almost unaffected by the patterning of fully discrete YIG stripes. Downscaling of the YIG film thickness to 35 nm decreases the bandgap size through a flattening of the spin-wave dispersion relation. We show that a reduction in the lattice constant effectively compensates for this trend. Our experimental results are corroborated by micromagnetic simulations, providing relevant information for the design of ultrathin YIG-based magnonic crystals with optimized bandgaps and spin-wave transmission properties.

Spin-wave filters based on thin Y3Fe5O12 films on Gd3Ga5O12 and Si substrates for microwave applications

Spin-wave bandpass filters with small sizes on the basis of thin Y3Fe5O12 [yttrium-iron garnet (YIG)] films on Gd3Ga5O12 [gadolinium-gallium garnet (GGG)] and SiO2/Si substrates have been constructed. A thin YIG film with a thickness of 0.8μm on SiO2/Si substrates has been grown by the ion-beam sputtering. It is found that the coplanar antenna structure can be successfully used to construct tunable small-sized narrowband bandpass filters based on spin waves. The filter transmission characteristic has a nonreciprocity when the direction of energy propagation changes. Spin-wave filter characteristics are analyzed using the transmission-matrix formalism.

Effect of interface magnetization depinning on the frequency shift of ferromagnetic and spin wave resonance in YIG/GGG films**Project supported by the National Natural Science Foundation of China (Grant Nos. 11674187 and 51871127) and Technology on Electronic Test & Measurement Laboratory (Grant No. 6142001180103).

Nowadays the yttrium iron garnet (Y3Fe5O12, YIG) films are widely used in the microwave and spin wave devices due to their low damping constant and long propagation distance for spin waves. However, the performances, especially the frequency stability, are seriously affected by the relaxation of the interface magnetic moments. In this study, the effect of out-of-plane magnetization depinning on the resonance frequency shift (Δfr) was investigated for 3-μm YIG films grown on Gd3Ga5O12 (GGG) (111) substrates by liquid-phase epitaxy. It is revealed that the ferromagnetic resonance (FMR) and spin wave propagation exhibit a very slow relaxation with relaxation time τ even longer than one hour under an out-of-plane external magnetic bias field. The Δfr span of 15.15–24.70 MHz is observed in out-of-plane FMR and forward volume spin waves. Moreover, the Δfr and τ depend on the magnetic field. The Δfr can be attributed to that the magnetic moments break away from the pinning layer at the YIG/GGG interface. The thickness of the pinning layer is estimated to be about 9.48 nm to 15.46 nm according to the frequency shifting. These results indicate that Δfr caused by the pinning layer should be addressed in the design of microwave and spin wave devices, especially in the transverse magnetic components.

The microstructure and magnetic properties of yttrium iron garnet film prepared using water-alcohol solvents

This study reports on the preparation of yttrium iron garnet thin films using three different solvents via a sol-gel method, followed by a spin coating technique and an annealing process at 750 °C for 2 h in the air. The three different solvents were water-ethanol, water-methanol and water. The microstructure analysis using an X-ray diffractometer revealed that the films prepared using a water-alcohol solvent crystallized into YIG single phase at a low annealing temperature (750 °C). The film thicknesses are in the range of 200 nm to 400 nm. The film prepared using a water-ethanol solvent has the smallest crystallite size. All films are soft ferrimagnetic at room temperature and the film prepared using a water-ethanol solvent exhibits the highest saturation magnetization value (162 emu/cm3).

Ultra Thin Films of Yttrium Iron Garnet with Very Low Damping: A Review

Thin film Yttrium Iron Garnet (YIG) is a promising material for integrated magnonics. 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. Herein, a brief introduction into the topic is given and the various results published over the last decade in this area are reviewed and discussed. Especially for ultrathin films it turns out that pulsed layer deposition and sputtering are the most promising candidates. In addition, the application of room‐temperature deposition and annealing for lift‐off based nanopatterning is discussed, as well as the properties of nanostructures obtained by this method over the past years.

Synthesis of yttrium iron garnet/bismuth quantum dot heterostructures with localized plasmon enhanced magneto-optical performance

Interactions between light and magnetic matter attracted great attention lately due to their potential applications in nanophotonics, spintronics, and high-accuracy sensing. Here, we grew bismuth quantum dots (Bi–QDs) with strong spin–orbit coupling on a magnetic insulator yttrium iron garnet (YIG) via molecular beam epitaxy. The YIG/Bi–QDs material shows an enhanced magneto-optical Kerr rotation up to 130% compared with that of a bare YIG film. The Bi–QDs were also introduced onto a lutetium–bismuth co-doped YIG film to form a hybrid system with remarkably enhanced Kerr rotation (from 1626 to 2341 mdeg). Ferromagnetic resonance measurements showed an increased effective magnetization as well as interfacial spin–orbit field in the YIG/Bi–QD heterostructures. Localized plasmons were mapped using electron energy loss spectroscopy with high spatial resolution, revealing enhanced plasmon intensity at both the Bi–QD surface and YIG/Bi–QD interface. Introducing Bi-QDs onto the YIG film enhanced Kerr rotation owing to the attenuated optical reflection and increased effective magnetization. The Bi–QD-enhanced magneto-optical effect enables development of efficient nanoscale light switching, spintronics, and even plasmonic nano-antennas.