Polycrystalline Yttrium Iron Garnet Research Articles

Microstructure characterization of polycrystalline yttrium iron garnets (YIGs) sintered from the calcined sol-gel powder

We present the results of microstructural characterization of the polycrystalline yttrium iron garnet (YIG) during high temperature sintering. Three YIG samples, compacted from the calcined sol-gel powder, are annealed at 1400 °C for 350 min, 1050 min and 1750 min, respectively. During the annealing, the polycrystalline YIG coarsens via normal grain growth, developing self-similar steady-states in grain size distribution, grain texture, and grain boundary texture. However, the 1050-min and 1750-min samples feature x-ray peaks with narrower full width at half-maximum (FWHM), and smaller local misorientation than the 350-min one, signifying that the strain still decreases after annealing for 350 min at 1400 °C. EPMA analyses show that more uniform distribution of the elements is found in 1750-min sample than in 350-min sample. Notably, little segregation of metal elements is observed on the grain boundary of the current YIG samples, as opposed to previously reported conventionally sintered YIGs using yttrium-oxide and iron-oxide powders, often featuring either second phases or segregation of Fe on YIG grain boundaries. • The high-density polycrystalline yttrium iron garnet (YIG) is sintered using calcined sol-gel powders. • Microstructure coarsens via normal grain growth, maintaining the self-similarity in grain size distribution, grain texture, and grain boundary misorientation and texture. • X-ray and KAM measurements suggest that the lattice strain reduces during annealing. • Considering observed slow growth kinetics, both sharpening of the X-ray peaks and decrease in the KAM may result from the homogenization of elements during annealing, producing the concurrent reduction in the lattice strain.

Dual-band composite right/left-handed metamaterial lines with dynamically controllable nonreciprocal phase shift proportional to operating frequency

Abstract Dual-band composite right/left-handed transmission lines with the nonreciprocal phase shift approximately proportional to the operating frequency are proposed and demonstrated by using normally magnetized ferrite microstrip lines. The nonreciprocal phase shift can be dynamically controlled by changing the externally applied dc magnetic field. Dynamic change in the nonreciprocal phase gradient along the line enables us to realize dual-band and unidirectional beam-scanning leaky-wave antennas without suffering from frequency-dependent change in the beam direction. A prototype nonreciprocal metamaterial line using polycrystalline yttrium iron garnet was fabricated and measured for verification of our basic concept.

Parallel-pumped nonlinear loss effects for polycrystalline yttrium iron garnet in the high-power microwave environment

In this study, the parallel-pumped nonlinear (NL) loss effects of polycrystalline Yttrium Iron Garnet (YIG) material caused by the high-power microwave (HPM) were explored for the first time. In terms of the coupling process of spin waves and electromagnetic waves, the improved ferrite NL theory encompassing the input microwave waveform parameters (pulse duration and power) was developed. The theoretical and experimental results indicated that the critical threshold power of the NL effect is approximately inversely proportional to incident microwave pulse with a duration below 80 ns, demonstrating the features of the energy threshold. Furthermore, the theoretical calculation and the high-power measurements show that the polycrystalline YIG sample exhibits a relatively stable microwave power loss trend in the HPM environment with input power of 160 W–400 W. The influence of the external magnetic bias field on the YIG nonlinear effects was also carried out for comparative analysis. This research not only expanded the NL loss theory of ferrite in the nanosecond-level microwave environment but also offered a potential limiting protection application against HPM electromagnetic environment threats.

Fabrication and broadband ferromagnetic resonance studies of freestanding polycrystalline yttrium iron garnet thin films

Flexible magnetic materials and devices with basic functional properties are highly desirable for flexible electronic applications in wearable products and implantable systems. In this work, a freestanding polycrystalline yttrium iron garnet (YIG) thin film with strong magnetism has been synthesized by pulsed laser deposition using a water-dissolvable Sr3Al2O6 sacrificial layer, and the magnetism of the resulting freestanding film was confirmed by a vibrating sample magnetometer and broadband ferromagnetic resonance spectroscopy. When transferred to a support layer using a thermal release tape, the flexible polycrystalline YIG thin film exhibits a lower damping constant α and larger magnetocrystalline anisotropy, in contrast to the polycrystalline heterostructure of YIG, which arises due to stress release. Thus, this work offers a viable solution for flexible YIG thin films that can be used in a number of applications.

Электрофизические и диэлектрические свойства поликристаллов железо-иттриевого феррита-граната, полученных по технологии радиационно-термического спекания

In this paper the electrical and dielectric properties of polycrystalline yttrium iron garnet, obtained by the radiation-thermal sintering technology in a fast electron beam were investigated. Spectra of complex dielectric constant, dielectric loss tangent and conductivity were measured in the frequency range 25 – 1∙106 Hz. For comparison, DC resistance measurements were also performed. The temperature dependences of the above parameters were measured at frequencies of 1 kHz, 100 kHz in the range 25 - 300 °C. It is shown, that conduction activations energy, permittivity, loss tangent and resistance vary significantly from sintering temperature in the range of 1300 to 1450 °C. It is found that with an increase in the sintering temperature to 1450 °C, dielectric properties are the same as samples made by the traditional ceramic technology.

Features of the Magnetic Structure of Y3Fe5O12 Polycrystals Synthesized by Radiation Thermal Sintering

Samples of polycrystalline yttrium-iron garnet synthesized using radiation-thermal sintering and ceramic processing have been studied by the Mossbauer spectroscopy method. The best decomposition of the Mossbauer spectroscopy spectra of the research objects, which is a simulation of the experimental spectrum with five sextets, has been selected. An additional fifth sextet is caused by Fe3+ ions, which are surrounded by oxygen vacancies leading to distortion of Fe-tetrahedra, which is reflected in an increase in the quadrupole splitting of Fe3+. An increase in the density of s-electrons on Fe ions in distorted tetrahedra has been found, resulting in a decrease in the isomeric chemical shift δ of Fe ions to a value close to the δ value for Fe4+ ions. It has been shown that the optimal crystal structure is realized in Y3Fe5O12 polycrystals when they are sintered for 40–60 min in the temperature range of 1350–1400°C by the radiation-thermal sintering method.

Influence on the Gilbert damping of yttrium-iron-garnet films by the spin-pumping effect

The effect of a platinum (Pt) capping layer on the magnetization dynamics in polycrystalline yttrium iron garnet (YIG) films of various thicknesses has been studied in order to understand the influence on the Gilbert damping of YIG films by the spin-pumping at the YIG/Pt interface. The Gilbert damping in each YIG film with the Pt capping layer is found to be much higher than that of the YIG film without the Pt capping layer, as expected due to the spin-pumping effect. In comparison with the samples without capping layer, an enhanced value of interfacial spin mixing conductance can also be observed for films with the capping layer. These results demonstrate the existence of spin-pumping effect at the YIG/Pt interface which plays an important role for the enhancement of Gilbert damping.

E-tunable magnetic susceptibility and reversible magnetization switching in YIG/Pt/PMN-PZT/Pt heterostructure by low electric and magnetic fields

The electric field (E) controlled magnetism in the multiferroic heterostructure comprising polycrystalline yttrium iron garnet (YIG) film, Pt electrodes, and lead magnesium niobate-lead zirconate titanate (PMN-PZT) ceramic is presented in this work. The electric-field-dependent magnetization and susceptibility of YIG film reveal the strain-mediated transformation of magnetocrystalline anisotropy. A strong converse magnetoelectric (CME) effect has been observed in the YIG/Pt/PMN-PZT/Pt heterostructure, and the CME coefficient can reach 17 × 10−8 s/m at 0 Oe and 4.2 kV/cm. The reversible magnetization switching by means of a low voltage pulse (±4.6 kV/cm) can be realized at 0 Oe in the YIG/Pt/PMN-PZT/Pt heterostructure, and the E-tunable susceptibility can reach Δχ/χ0+ = 55.5% at 20 Oe and 4 kV/cm. These results show great potential in power-efficient magnetoelectric components for information storage and tunable devices.

Spin‐thermoelectric voltage in longitudinal SSE elements incorporating LPE YIG films and polycrystalline YIG slabs with an ultrathin Pt layer

AbstractA spin‐thermoelectric (STE) voltage is generated when a temperature gradient ∇T is applied to an element having a thin Pt layer coated on a magnetic substance. In this study, yttrium iron garnet (YIG) ferrimagnetic films prepared by liquid phase epitaxy (LPE) were tested as magnetic insulators. In addition, polycrystalline YIG slabs were tested to compare the STE voltages of film and slab samples. In a Pt coating and YIG film bilayer structure made by an ultrathin Pt layer of 1‐4 nm thickness and an LPE film of approximately 10 µm thickness, a large STE voltage of 600 µV was observed at a probe distance of 5 mm with a temperature difference ∆T of 30 K. On the other hand, the STE voltage of a Pt layer and YIG slab bilayer structure was 340 µV, which is roughly half of that of the Pt/YIG‐film element. The cause of the large voltage observed experimentally for the longitudinal spin Seebeck effect element incorporating an LPE YIG film was discussed mainly from the viewpoint of the Pt layer resistivity and the effects of YIG specimen surface conditions on crystallinity and the magnetization process.

Bulk-like magnetization and improved microwave properties of polycrystalline YIG (Y3Fe5O12) films grown on quartz substrates by pulsed laser deposition

Yttrium iron garnet (YIG) thin films with thickness 110 nm ≤ t ≤ 530 nm were grown on amorphous quartz substrates using pulsed laser deposition (PLD). The films show an improved magnetic and microwave performance. The value of saturation magnetization (4πMS) and ferromagnetic resonance (FMR) linewidth were found to be dependent on the thickness (t) of the film. The film with t = 530 nm shows a 4πMS value of 1725 (98% of the bulk value of YIG) and a minimum value for FMR linewidth of 57 Oe and 78 Oe respectively for the magnetic field applied perpendicular and parallel to the film plane. An almost linear relation between linewidth and coercivity is observed, which suggests a positive correlation between magnetic anisotropy, coercivity, and linewidth. The results demonstrated that YIG films prepared on quartz substrates can be beneficial to the application of YIG films to semiconductor integrated devices.

Spin Seebeck effect in polycrystalline yttrium iron garnet pellets prepared by the solid-state method

We study the properties of polycrystalline bulk yttrium iron garnet (YIG) pellets prepared by the solid-state method, where the choice of the sintering temperature can lead to mixed phases of yttrium iron perovskite (YIP) and YIG or single phase YIG. Magnetometry shows multiple switching regimes in the mixed-phase pellets where the saturation magnetization is dominated by the proportion of YIG present. Ferromagnetic resonance was used to corroborate the saturation magnetization from magnetometry and to extract the spin wave damping α. The lowest damping was observed for the YIG pellet, which resulted in a spin Seebeck effect (SSE) coefficient that was approximately 55% of single crystal YIG. This demonstrates that macroscale crystallization does not play a major role in the SSE and paves the way for utilising polycrystalline samples for thermomagnetic applications.

Investigation of Rare Earth Garnet and its Physical Properties Synthesized by Facile Sol-Gel Method

To meet the demand of electronic devices pure polycrystalline yttrium iron garnet with cubic structure is investigated by sol gel method. The crystalline information and identification of garnet phases is confirmed using X-ray Diffraction. The X-ray diffraction pattern is analyzed by utilizing Rietveld Refinement method using Fullprof Software suite. Well defined domain morphology is observed in the High Resolution Scanning Electron Microscope image (HR-SEM). Pure YIG is inspected by Impedance Spectroscopy (IS) using Cole-Cole plot and Bode plot. It is interpreted using electrical circuit which is the combination of resistors and capacitors. The effect of g value of the sample is measured using Electron Spin Resonance (ESR).

Fabrication of yttrium–iron–garnet/Pt multilayers for the longitudinal spin Seebeck effect

For longitudinal spin Seebeck effect (LSSE) devices, a multilayer structure comprising ferromagnetic and nonmagnetic layers is expected to improve their thermoelectric power. In this study, we developed a fabrication method for alternately stacked yttrium–iron–garnet (YIG)/Pt multilayer films on a gadolinium gallium garnet (GGG) (110) substrate, GGG/[YIG(49 nm)/Pt(4 nm)]n (n = 1–5) based on room-temperature sputtering and ex-situ post-annealing methods and we evaluated their structural and LSSE properties. The fabricated [YIG/Pt]n samples show flat YIG/Pt interfaces and almost identical saturation magnetization Ms although they contain polycrystalline YIG layers on Pt layers as well as single-crystalline YIG layers on GGG. In the samples, we observed clear LSSE signals and found that the LSSE thermoelectric power factor (PF) increases monotonically with increasing n; the PF of the [YIG/Pt]5 sample is enhanced by a factor of ∼28 compared to that of [YIG/Pt]1. This work may provide a guideline for developing future multilayer-based LSSE devices.

Effect of Annealing on the Structural and FMR Properties of Epitaxial YIG Thin Films Grown by RF Magnetron Sputtering

Effect of Annealing on the Structural and FMR Properties of Epitaxial YIG Thin Films Grown by RF Magnetron Sputtering

Electric field tuning of magnetism in heterostructure of yttrium iron garnet film/lead magnesium niobate-lead zirconate titanate ceramic

In this paper, the converse magnetoelectric (CME) effect by electric field tuning of magnetization in an original heterostructure composed of a polycrystalline yttrium iron garnet (YIG) film and a lead magnesium niobate-lead zirconate titanate (PMN-PZT) ceramic is presented. The magnetic performances of the YIG films with different thicknesses under a DC electric field applied to the PMN-PZT ceramics and a bias magnetic field are investigated. All the magnetization-electric field curves are found to be in good agreement with the butterfly like strain curve of the PMN-PZT ceramic. Both the sharp deformation of about 2.5‰ of PMN-PZT and the easy magnetization switching of YIG are proposed to be the reasons for the strongest CME interaction in the composite at the small electric coercive field of PMN-PZT (4.1 kV/cm) and the small magnetic coercive field of YIG (20 Oe) where the magnetic susceptibility reaches its maximum value. A remarkable CME coefficient of 3.1 × 10−7 s/m is obtained in the system with a 600 nm-thick YIG film. This heterostructure combining multiferroics and partially magnetized ferrite concepts is able to operate under a small or even in the absence of an external bias magnetic field and is more compact and power efficient than the traditional magnetoelectric devices.

SQUID-detected FMR: Resonance in single crystalline and polycrystalline yttrium iron garnet

Here two new techniques for the detection of broadband (100 MHz-20 GHz) ferromagnetic resonance (FMR)/ferrimagnetic resonance in single and poly-crystalline materials, which rely on SQUID-based gradiometry detection of small changes in the magnetisation, are developed. In the first method, small changes in the along-the-applied-field projection of the coupled magnetic moment (Δmz) are detected as the material is driven into resonance. Absolute measurement of the longitudinal component of the magnetisation and the resonance induced lowering of this moment makes estimation of the precession cone angle accessible, which is typically difficult to extract using conventional cavity or stripline based detection methods. The second method invokes the change in Δmz with the resonance-induced thermal heating dmzdT. Magnetisation dynamics in bulk Y3Fe5O12 are observed over a broad range of experimental temperatures (4 K-400 K) and fields (10-500 mT). The inhomogeneous microwave excitation allows for the observation of higher magnetostatic modes and the convenient tracking of very broad resonances. The two SQUID-detection techniques when combined with conventional broadband vector network analyser-FMR, low-frequency magnetic susceptibility, and DC magnetometry, all easily realised, essentially concurrently, using the same module, greatly expand the amount of static and dynamic information accessible.

Dependence of magnetic and microwave loss on evolving microstructure in yttrium iron garnet

The parallel magnetic and microwave loss dependence on microstructural evolutions in several polycrystalline yttrium iron garnet samples were studied in detail, focusing on the attendant occurrence of their relationships. In this study, polycrystalline YIG samples were synthesized by employing the mechanical alloying technique and sintering toroidal compacts at temperatures from 600 to 1400 °C. The samples were characterized for their evolution in crystalline phases, structure, microstructure, magnetic hysteresis parameters, microwave losses and electrical resistivity. The results showed an increasing tendency of the saturation magnetization with grain size, which is attributed to crystallinity increase in the grains. The M–H hysteresis loop results showed a transition from disordered-to-ordered magnetism which belongs to different magnetically dominant stages of formation. The starting appearance of room temperature ferromagnetic order suggested by the sigmoid-shaped loops seems to be dependent on crystallinity, phase purity and a sufficient number of large enough magnetic domain-containing grains having been formed in the microstructure. An increasing trend of transmission loss with grain size may be attributed to increment of loss contribution from hysteresis and domain wall resonance of the samples. The changes in crystallinity and microstructure, and the associated processes of microwave resonance and relaxation due to domain wall movements and damping of spin rotation contributes to the variations in transmission loss and ferromagnetic linewidth of the samples. The increased electrical resistivity while the microstructure was evolving is believed to strongly indicates improved phase purity and compositional stoichiometry.

Effect of a Platinum Buffer Layer on the Magnetization Dynamics of Sputter Deposited YIG Polycrystalline Thin Films

The present investigation deals with the effect of thickness and a platinum (Pt) buffer layer on the dynamic magnetic properties of polycrystalline yttrium iron garnet (YIG) films fabricated after post-annealing to recrystallize from the amorphous state. The dynamic magnetic properties of films without a buffer layer deteriorate as the thickness decreases as indicated by the increased line-widths in the ferromagnetic resonance spectra. On the other hand, a strong rejuvenation in the static and dynamic magnetic properties of the films is observed when a Pt buffer layer is used. The Pt buffer layer decreases the Gilbert damping parameter from 11.4×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> and 3.5×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> for a 100 nm-thick YIG film. These observations mainly originate from controlling the microstructure or the surface morphology of the films.

Robust spin current generated by the spin Seebeck effect

Spin pumping (SP) and the spin Seebeck effect (SSE) are widely used to generate a spin-wave spin current from ferromagnetic insulators. In this work, the authors show that while SP is significantly reduced in a polycrystalline yttrium iron garnet (YIG), the SSE is insensitive to the crystal structure. This discovery not only offers new insights into the mechanisms between the coherently driven SP and the noncoherently excited SSE but also demonstrates that the robust spin current generated by the SSE is a uniquely powerful tool to study the physics of the pure spin current in spintronics.

Influence of static and dynamic dipolar fields in bulk YIG/thin film NiFe systems probed via spin rectification effect

The characteristics of the static and dynamic components of the dipolar fields originating from a bulk polycrystalline yttrium iron garnet (YIG) substrate are probed by depositing a NiFe (Permalloy) layer on it, which acts as a detector. By measuring dc voltages generated via spin rectification effect (SRE) within the NiFe layer under microwave excitation, we characterize the influence of dipolar fields from bulk YIG on the NiFe layer. It is found that the dynamic YIG dipolar fields modify the self-SRE of NiFe, driving its own rectification voltages within the NiFe layer, an effect we term as non-local SRE. This non-local SRE only occurs near the simultaneous resonance of both YIG and NiFe. On the other hand, the static dipolar field from YIG manifests itself as a negative anisotropy in the NiFe layer which shifts the latter’s ferromagnetic resonance frequency.