Design of two-dimensional magnonic crystals using yttrium iron garnets and non-magnetic metals

An insulator‐based spin‐thermoelectric (STE) generator is composed of a thin paramagnetic metal (PM: Pt) layer for generating the STE voltage VSTE via the inverse spin Hall effect and a ferrimagnetic insulator (FMI) film or slab used for producing spin‐wave spin currents due to a temperature gradient . Yttrium iron garnet (YIG) is a key material used for the FMI of STE generators. We examined in detail the ferromagnetic resonance (FMR) properties of YIG (Y3Fe5O12), Bi‐substituted YIG (Y3‐xBixFe5O12) and Bi‐ and Al‐substituted YIG (Y3‐xBixFe5‐yAlyO12) films grown by liquid phase epitaxy (LPE). Bi‐substituted YIG films exhibited the large FMR damping and high STE voltage when the films were incorporated in the STE generator. The LPE‐grown Y2.35Bi0.65Fe5O12 film exhibited the FMR linewidth ΔH of 30 mT and the VSTE of 70 μV for the ∇T of . This paper comprehensively presents the origin of the STE generation in the insulator‐based generators on the basis of the results of FMR and STE measurements. To clarify the origin of STE generation in the generators fabricated from single crystal YIG (YIG, Bi‐substituted YIG and Bi‐ and Al‐substituted YIG) films grown by LPE, three principal factors are explained: (a) thermal energy transfer from the phonon system to the spin system, which strengthens the heat excitation of spin precession, through the spin–orbit coupling enhanced by the growth‐induced magnetic anisotropy of LPE‐grown YIG films, (b) in the YIG films incorporated in the STE generators, the generation of spin‐wave spin currents owing its origin to the ferromagnetic spin exchange interaction acting between neighboring spins due to , and (c) how the spin currents pumped into the Pt layer from the YIG film at the Pt/YIG bilayer interface are affected by the magnetization processes of single crystal YIG films. It is concluded that the STE generation of insulator‐based STE generators can be explained from a viewpoint of the spin dynamics under the effect of in the Pt/YIG bilayer structure. © 2024 Institute of Electrical Engineers of Japan and Wiley Periodicals LLC.

Preparation of two-dimensional yttrium iron garnet magnonic crystal on porous silicon substrate

The anomalous Hall (AH) effect is deemed to be a unique transport property of ferromagnetic metals, caused by the concerted action of spin polarization and spin-orbit coupling. Nevertheless, recent experiments have shown that the effect also occurs in a nonmagnetic metal (Pt) in contact with a magnetic insulator [yttrium iron garnet (YIG)], even when precautions are taken to ensure that there is no induced magnetization in the metal. We propose a theory of this effect based on the combined action of spin-dependent scattering from the magnetic interface and the spin-Hall effect in the bulk of the metal. At variance with previous theories, we predict the effect to be of first order in the spin-orbit coupling, just as the conventional anomalous Hall effect-the only difference being the spatial separation of the spin-orbit interaction and the magnetization. For this reason we name this effect the nonlocal anomalous Hall effect and predict that its sign will be determined by the sign of the spin-Hall angle in the metal. The AH conductivity that we calculate from our theory is in order of magnitude agreement with the measured values in Pt/YIG structures.

We report on an experimental study on the spin-waves relaxation rate in two series of nanodisks of diameter ϕ=300, 500, and 700 nm, patterned out of two systems: a 20 nm thick yttrium iron garnet (YIG) film grown by pulsed laser deposition either bare or covered by 13 nm of Pt. Using a magnetic resonance force microscope, we measure precisely the ferromagnetic resonance linewidth of each individual YIG and YIG|Pt nanodisks. We find that the linewidth in the nanostructure is sensibly smaller than the one measured in the extended film. Analysis of the frequency dependence of the spectral linewidth indicates that the improvement is principally due to the suppression of the inhomogeneous part of the broadening due to geometrical confinement, suggesting that only the homogeneous broadening contributes to the linewidth of the nanostructure. For the bare YIG nano-disks, the broadening is associated to a damping constant α=4 × 10−4. A threefold increase of the linewidth is observed for the series with Pt cap layer, attributed to the spin pumping effect. The measured enhancement allows to extract the spin mixing conductance found to be G↑↓=1.55 × 1014 Ω−1 m−2 for our YIG(20nm)|Pt interface, thus opening large opportunities for the design of YIG based nanostructures with optimized magnetic losses.

Many properties of polycrystalline yttrium iron garnet (YIG) and reduced single-crystal YIG have been attributed to the presence of Fe2+ ions which provide mobile electrons that can diffuse locally through the lattice. It was the purpose of this research to study the modification to the low-temperature anisotropy caused by the presence of Fe2+ ions in silicon-doped YIG, Y3Fe5−δSiδO12 with 0<δ<0.18. Silicon-doped YIG demonstrates a time-dependent induced anisotropy proportional to doping when magnetically annealed to low temperatures. The kinetics and temperature dependence of this induced anisotropy have been studied. It is further found that these doped garnets show a constant, high-field component of rotational hysteresis which is both frequency- and temperature-dependent. The intrinsic, unannealed anisotropy behaves in a novel way. At room temperature the anisotropy coincides with normal YIG, while between 150°K and 300°K | K1 | increases above that of pure YIG Below about 150°K the curves develop asymmetry and appear to be no longer purely cubic. At 4.2°K the easy axis has shifted to the [100] direction (>3% doping) indicative of a change in sign in K1. Silicon-doped lutetium iron garnet and polycrystalline lutetium iron garnet were found to behave in a similar manner. The results are interpreted in terms of an electron-diffusion model which enables the Fe2+ ion to effectively wander among four nonequivalent octahedral sites. A simple kinetic rate theory is at least partially successful in explaining: (1) the time dependence of the torque curves, (2) the rotational hysteresis, and (3) some of the apparent low-temperature deviations from cubic symmetry.

Flexible magnetic composite films based on yttrium iron garnet (YIG) and poly(vinyl alcohol) (PVA) with composition xYIG:(1 − x)PVA (0 ≤ x ≤ 0.63 wt) were successfully obtained by facile casting technique. The structure, magnetic, and microwave absorption properties of YIG/PVA films were characterized by XRD, Raman spectroscopy, FTIR, vibrant sample magnetometry (VSM), and ferromagnetic resonance (FMR) techniques. XRD, Raman and FTIR analysis confirmed the complete synthesis of YIG/PVA composites, without any chemical interaction between particles of YIG and PVA matrix. VSM experiments revealed a ferrimagnetic ordering in composite films with small in-plane anisotropy; their specific magnetization increases linearly with the YIG content. YIG/PVA films exhibit an effective imaginary microwave permeability of $$\mu^{\prime\prime}_{\max }$$ ~ 0.1–0.5, as determined by FMR measurements. Numerical simulations of the FMR spectra showed that the large FMR linewidth of $${\Delta H}_{\text{pp}} \sim$$ 80 kA/m is associated with the shape anisotropy and the interactions between YIG particles. A small effective field of $${H}_{\text{eff}}$$ = 20–25 kA/m was originated by the confinement of YIG particles within a PVA matrix into a two-dimensional geometry. The combination of flexibility, magnetic, and microwave properties makes of YIG/PVA films suitable materials for microwave absorption applications.

The spin rectification effect (SRE), a phenomenon that generates dc voltages from ac microwave fields incident onto a conducting ferromagnet, has attracted widespread attention due to its high sensitivity to ferromagnetic resonance (FMR) as well as its relevance to spintronics. Here, we report the non-local detection of yttrium iron garnet (YIG) spin dynamics by measuring SRE voltages from an adjacent conducting NiFe layer up to 200 nm thick. In particular, we detect, within the NiFe layer, SRE voltages stemming from magnetostatic surface spin waves (MSSWs) of the adjacent bulk YIG which are excited by a shorted coaxial probe. These non-local SRE voltages within the NiFe layer that originates from YIG MSSWs are present even in 200 nm-thick NiFe films with a 50 nm thick SiO2 spacer between NiFe and YIG, thus strongly ruling out the mechanism of spin-pumping induced inverse spin Hall effect in NiFe as the source of these voltages. This long-range influence of YIG dynamics is suggested to be mediated by dynamic fields generated from YIG spin precession near YIG/NiFe interface, which interacts with NiFe spins near the simultaneous resonance of both spins, to generate a non-local SRE voltage within the NiFe layer.

The propagation of magnetostatic waves (MSW) in a waveguide partially loaded with a low-loss ferrite slab is investigated theoretically. The most common low-loss ferrite material used for MSW propagation is epitaxial yttrium iron garnet (YIG). A YIG slab is placed inside and along the guide and not in contact with the sidewalls of the wavegnide. The dc magnetic field is assumed to be parallel to the YIG slab and perpendicular to the direction of propagation. Using the integral equation method, the dispersion relation is found to be an infinitely large determinant equal to zero. Proper truncation of this determinant and numerical analysis to find its roots are carried out in this work. It is seen that in order to obtain high values of group time delay, the YIG slab must be narrow and placed at the bottom of the guide. On the other hand, to maximize the device bandwidth, a narrow YIG slab positioned at the top inside surface of the waveguide is preferred. It is also noticed that there exists a tradeoff between the time delay and the device bandwidth and that maximization of one property leads to a poor value in the other. Thus, some design compromises should be made. It is also observed that the frequency range of operation of the device can be adjusted by an external magnetic bias field. This property of tuning the device to operate in any frequency range adds an extra dimension of flexibility to the operation and also to the design of these devices.

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 and Gilbert damping from the frequency and linewidth of the ferromagnetic resonance (FMR). Next, we use propagating spin wave spectroscopy in the Damon–Eshbach configuration to detect the small spin-wave magnetic fields of this ultrathin weak magnet over a wide range of wavevectors, enabling the extraction of the exchange constant αex=1.32×10−12 J/m. We observe foldover of the FMR with increasing drive power, leading to frequency shifts of the spin-wave modes and a bistable region in the spin-wave spectra. Our results shed light on isotropic spin-wave transport in Ga:YIG and highlight the potential of electrical spectroscopy to map out the dispersion and bistability of propagating spin waves in magnets with a low saturation magnetization.

A millimeter-wave Bell Test using a ferrite parametric amplifier and a homodyne interferometer

Magnons are quasiparticle representations of spin waves, which describe the collective excitations of a magnetic system. They can propagate and carry spin information in electrically insulating and magnetically ordered materials, i.e. magnetic insulators. One of the most famous examples is yttrium iron garnet (YIG) due to its uniquely small magnetic damping and long magnon lifetime. The magnon transport set-up is composed of two heavy metal (HM) strips such as platinum (Pt) or tantalum (Ta) on top of a single crystal YIG film. A small ac current is sent through the first HM strip (the magnon injector), where the spin Hall effect (SHE) transfers the electrical current into a transverse spin current. This leads to an electrical spin accumulation at the HM interface which is in contact with the YIG. Due to the conductivity mismatch at the interface, the mobile electrons themselves cannot go deeply into the insulator. However, the spins can be transferred, which gives rise to magnon generation in YIG. The resulting magnons propagate inside the YIG and they can be picked up by the second HM strip (the magnon detector), where the detected spin currents are transferred back to a charge current via the inverse spin Hall effect (ISHE). Under an open circuit condition, an ISHE voltage is measured. Since current supply and voltage measurement are conducted at two different HM strips, this experiment is referred to as nonlocal measurement. The transport properties of magnons are studied by the distance-dependent behavior of nonlocal signals.

Thin films of Yttrium Iron Garnet (YIG) are grown by radio frequency magnetron non reactive sputtering system. Thin films are crystallised by heat-treatment to obtain magneto-optical properties. On quartz substrate, the network of cracks observed on the annealed samples can be explained by the difference between the thermal expansion coefficient of substrate and YIG. The Faraday rotation of thin films is measured with a classical ellipsometric system based in transmission which allows us to obtained an accuracy of 0.01°. We studied the variation of Faraday rotation versus the applied magnetic field. The variation of the Faraday rotation is the same that this obtained by VSM (vibrating sample magnetometer) analysis. With a quartz substrate, maximum Faraday rotation is observed to be 1900°/cm at the wavelength of 594 nm for an annealing of 740°C. The variation of the Faraday rotation versus the wavelength is studied. The obtained values are comparable to those of the literature for the bulk material. In order to eliminate the stress due to the heat-treatment, we made some films on Gadolinium Gallium Garnet (GGG) which thermal expansion coefficient is near than the YIG one. The material crystallises with no cracks and the Faraday effect is equivalent.

Recent interest in employing single-crystal yttrium iron garnet (YIG) films for miniaturized microwave integrated circuit applications has prompted us to study detaching liquid phase epitaxy (LPE) grown YIG films from their gadolinium gallium garnet (GGG) substrates via crystal ion slicing (CIS). We report studies of magnetization and ferromagnetic resonance (FMR) of narrow linewidth gallium-doped YIG (GaYIG) films in various stages of separation from their 〈111〉 oriented GGG substrates. All samples were diced from the same three-inch wafer of Y3Fe4.6Ga0.4O12 with 4πMeff of 1070 G and 9.45 GHz FMR linewidth of 0.4±0.1 Oe. The CIS separation process involved (1) implantation with helium ions, (2) flash annealing to remove surface damage, and (3) chemical etching to detach the YIG at the implantation damaged layer. The starting films were 10.8-μm thick. Separation occurred 7 to 8 μm from the front surface in the YIG. The implantation (5×1016 cm−2 dosage, 3.8 MeV bias) increased the uniaxial anisotropy of these films. A small cubic anisotropy (∼5 G 60° in-plane periodicity) persisted in both implanted and unimplanted samples. Upon implantation the FMR linewidth increased from 0.4 to 3±0.5 Oe. The detached samples have linewidths ranging from 1.7 to 2.5 Oe. The temperature dependence of the linewidths is roughly exponential decreasing as the temperature is lowered for the unprocessed sample, increasing with reduced temperature for all processed samples. Post-detachment annealing restores the FMR linewidth to 0.55±0.5 Oe.

Magnetic thin films of yttrium iron garnet (YIG) grown from liquid phase epitaxy show very regular strip domains. The width of the domain is a function of the applied magnetic field. For certain substitution for YIG, the magnetization of the domains is practically parallel to the direction of the strip domain. The magnetizations of adjacent domains lying on either side of the magnetic wall are anti-parallel. This leads to the realization of waveguides in the strip domains. It is shown that due to the value of Faraday specific rotation. theta =263 degrees /cm, lambda 0=1152 mu m, for YIG, the different calculations for the propagation of plane monochromatic waves may be limited to the first power of Theta . Under these conditions, the modes are only circularly polarized. The width of strips required to guide these waves is calculated.

The crystallization and magnetic behavior of yttrium iron garnet (YIG) prepared by metallo-organic decomposition (MOD) method are discussed. The chemistry and physics related to synthesis of iron and yttrium carboxylates based on 2-ethylhexanoic acid (2EHA) are studied, since no literature was found which elucidates synthesis of metallo-organic precursor of YIG in spite of the literatures of doped YIG samples such as Bi-YIG. Typically, the metal carboxylates used in preparation of ceramic oxide materials are 2-ethylhexanoate (2EH) solvents. Herein, the synthesis, thermal behavior and solubility of yttrium and iron 2EH used in synthesis of YIG powder by MOD are reported. The crystallization and magnetic parameters, including saturation magnetization and coercivity of these samples, smoothly change as a function of the annealing temperature. It is observed that high sintering temperature of [Formula: see text] to [Formula: see text] promotes the diffraction peaks of YIG, therefore, we can conclude that the formation of YIG in MOD method increases the crystallization temperature. The maximum value of saturation magnetization and minimum value of coercivity and remanence are observed for the sample sintered at [Formula: see text] which are 13.7 emu/g, 10.38 Oe and 1.5 emu/g, respectively. This study cites the drawbacks in chemical synthesis of metallo-organic-based YIG production.