Optical signal processing with magnetostatic waves

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.

Yttrium iron garnet (YIG) thin films were deposited on c-plane sapphire substrates under atmospheric pressure by a mist CVD technique, and their chemical composition and optical properties were examined. The thin films deposited at 450 °C showed an [Y]/[Fe] ratio of 0.57, indicating the deposition of yttrium iron oxide, while the molar ratio of [Y]/[Fe] in the precursor solution was set at 1.5. A thermodynamic model was developed to explain the reaction paths of the YIG thin film fabrication process using thermogravimetric differential thermal analysis (TG-DTA) results. The model indicates that the decomposition rate of yttrium acetylacetonate [Y(acac)3 ∙ nH2O] was much lower than that of iron acetylacetonate [Fe(acac)3], providing a plausible explanation for the large difference between the composition ratio of the thin films and that of the precursor solutions.

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 $1\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}$m, the length scale defined by the exchange length. We show that due to the symmetry of the three-magnon coupling, the decay rate is linear in $\ensuremath{\Delta}H=({H}_{c}\ensuremath{-}H)$ in the vicinity of the threshold field ${H}_{c}$ instead of the steplike ${\ensuremath{\Gamma}}_{\mathbf{k}=0}\ensuremath{\propto}\ensuremath{\Theta}\left(\ensuremath{\Delta}H\right)$ 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 for decays into a sequence of the two-dimensional magnon bands. For yet thicker films, such thresholds merge and crossover to the three-dimensional single-mode behavior: ${\ensuremath{\Gamma}}_{\mathbf{k}=0}\ensuremath{\propto}{|\ensuremath{\Delta}H|}^{3/2}$.

Magnetooptic (MO) interactions between guided-optical waves and magnetostatic waves (MSW) in Yttrium Iron Garnet-Gadolinium Gallium Garnet (YIG-GGG) waveguides has become a subject of increasing interest in recent years. Similar to the guided-wave acoustooptic (AO) interaction in which the surface acoustic wave (SAW) induces a moving optical grating [1], the guided-wave MO interaction results from the moving optical grating induced by the MSW. Specifically, in noncollinear coplanar geometry, as shown in Fig.1, a portion of an incident guided-light wave is Bragg diffracted and mode-converted (TE- to TM-mode and vice versa). We have recently obtained a very large bandwidth and high diffraction efficiency in such interaction geometry with magnetostatic forward volume waves (MSFVW) [2]. For example, a -3dB bandwidth of 1.03GHz centered at 6.0GHz was obtained at the optical wavelength of 1.317μm using a single microstripline transducer together with a fixed homogeneous DC magnetic field of 3500 Oe. As in the AO Bragg diffraction in which the resulting modulator is called the AO Bragg cell, the resulting MO modulator may be called the MO Bragg cell. In a continuing effort we have used the wideband MO Bragg cell to perform, for the first time, wideband light beam deflection and RF spectral analysis, and have obtained large numbers of deflected light spots and frequency channels. In this paper detailed results of both the experiments and related analysis are presented. It should be noted that a number of studies on bulk-wave MO interactions involving an unguided light wave and spin and/or magnetoelastic waves were reported [3-7] in the late 60's and early 70's. To the best of our knowledge, however, neither work on light beam deflection nor on RF spectral analysis using such bulk-wave MO interactions has been reported.

Phase jumps in coupled magnonic waveguides

Abstract. A new technique of inductive measure to determine the initial magnetic permeability (r) of Yttrium Iron Garnet (YIG) thin films has been conceived and developed in our laboratory. The magnetic material is deposited by radio-frequency sputtering between two copper thin layers on alumina substrate. Because the as-deposited films are amorphous and non magnetic, thermal annealing is necessary to make the films crystallize and to obtain satisfactory magnetic properties. After different tests considering the geometrical, morphological and magnetic properties, we have established a protocol permitting to manufacture a prototype in an original design. The performance of the fabricated micro-inductor has been checked using a physical-chemistry characterization. To obtain accurate measurements, we have used a four-point probe test bench and a precision LCR meter. The current sheet method has been validated with thick layers of commercial YIG. The results obtained for thick and thin films of YIG have been compared to the simulation and theoretical ones. Several tests, made for different thickness and different number of loops, have permitted us to evaluate the magnetic permeability of YIG thin films to 32 ± 4.

An efficient finite-element solution procedure is developed for calculating propagation losses of magnetostatic waves in multilayered inhomogeneous waveguides. The final matrix equation is reduced to a standard complex eigenvalue problem whose eigenvalue corresponds to the complex phase constant itself. Thus, iteration procedures are not necessary and the phase and attenuation constants can be directly obtained by solving a standard eigenvalue equation. The validity of the method is confirmed by calculating propagation losses of magnetostatic surface waves in a single yttrium-iron-garnet (YIG)-film structure. Numerical results for a triple-layered YIG film structure are also presented. It is found that, in the triple-layered structure, propagation losses are highly dependent on the linewidth of the film in which the magnetostatic potential is well confined. >

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.

Optical devices using magnetostatic surface waves (MSSWs) have important applications in optical signal processing and optical computing. In this paper we propose the analysis and design of an optical mode conversion device that features improved switching efficiency achieved by the use of a new material with increased magneto-optic effect. The device consists of cerium- and bismuth-doped thin and thick garnet films. The design features 100% mode conversion over an interaction length of about 5 mm for an applied microwave power density in the range 1–10 mW/mm. Cerium- and bismuth-doped Yttrium Iron Garnet (YIG) films are fabricated by rf sputtering and their crystal properties are studied using X-ray microanalyzer and X-ray diffractometer.

Microwave bistability has been obtained for the first time in a magnetostatic wave (MSW) interferometer with external magnetic field feedback. The interferometer contained a magnetostatic surface wave transmission line with a 7.5-/spl mu/m thick epitaxial film of yttrium iron garnet magnetized with an external static magnetic field of 1048 Oe, a variable attenuator in a parallel reference channel, and external feedback in the form of an additional static field derived from the line output. Bistable and multistable output power versus input power responses are derived from the frequency dependence of the interferometer transmission and the MSW wave number dependence on the static magnetic field. Typical bistable and multistable output versus input power hysteresis loops were obtained under constant frequency operation in the range 4.9 to 5.2 GHz, and for input powers from -30 to +10 dBm. The bistable response depends on the frequency of the microwave signal, the attenuation in the reference channel, and the gain of the feedback loop. An analysis based on linear MSW theory agreed with experiment. Applications of MSW microwave bistability include power limiter devices and basic microwave logic elements.

A new high spatial-resolution magnetic-induction probe that measures microwave magnetic field patterns of magnetostatic waves in ferrite thin films has been developed. The probe’s sensing element is either a strand of 25.4-μm-diam gold wire wrapped around a plastic support or an aluminum rectangular loop photolithographically fabricated on glass. The gold wire method yielded a smallest size of 120 μm, and the aluminum loops achieved a highest resolution of 45 μm. The probe is designed for studies of amplitude profiles, dispersion relations, and phase-propagation direction. Investigations of magnetostatic surface waves (MSSW) and backward volume waves (MSBVW) propagating in liquid-phase epitaxially (LPE) grown yttrium-iron-garnet (YIG) were conducted. The LPE-YIG sample used was 4.5 μm thick with the [111] direction perpendicular to the surface of the sample. MSSW and MSBVW of 2–4 GHz microwave frequency were launched in the LPE-YIG, 4.1° from the [01̄1] direction. Studies of MSSW were done under uniform and nonuniform in-plane magnetic bias fields. When contrasted to expected spreading due to diffraction, MSSW in uniform bias fields were found to ‘‘focus’’ as the wave propagated. The ‘‘focusing’’ effect was determined by measurement of the microwave magnetic field profile, transverse to the phase-propagation direction, as a function of propagation distance. As the MSSW propagated, the peak value of the profile increased while the profile width decreased until a maximum profile amplitude occurred, after which the MSSW decreased in amplitude and the width increased. With MSSW and MSBVW in constant uniform bias fields, direct measure of the wavelengths at various microwave frequencies yielded the dispersion relation for each type of wave. Determination of the dispersion relation of MSBVW confirmed the backward nature of these waves.

Spin waves are promising candidates to carry, transport, and process information. Controlling the propagation characteristics of spin waves in magnetic materials is an essential ingredient for designing spin-wave based computing architectures. Here, we study the influence of surface inhomogeneities on the spin-wave signals transmitted through thin films. We use micromagnetic simulations to study the spin-wave dynamics in an in-plane magnetized yttrium iron garnet thin film with a thickness in the nanometre range in the presence of surface defects in the form of locally introduced uniaxial anisotropies. These defects are used to demonstrate that the Backward Volume Magnetostatic Spin Waves (BVMSW) are more responsive to backscattering in comparison to Magnetostatic Surface Spin Waves (MSSWs). For this particular defect type, the reason for this behavior can be quantitatively related to the difference in the magnon band structures for the two types of spin waves. To demonstrate this, we develop a quasi-analytical theory for the scattering process. It shows an excellent agreement with the micromagnetic simulations, sheds light on the backscattering processes and provides a new way to analyze the spin-wave transmission rates in the presence of surface inhomogeneities in sufficiently thin films, for which the role of exchange energy in the spin-wave dynamics is significant. Our study paves the way to designing magnonic logic devices for data processing which rely on a designed control of the spin-wave transmission.

The Brillouin light scattering (BLS) technique has been used for the direct detection of magnetostatic wave (MSW) excitations in a signal-to-noise enhancer MSW device structure. The device consists of a 23 μm wide microstrip transmission line in contact with a 26.6 μm thick single crystal yttrium iron garnet film on a gadolinium gallium garnet substrate, with the applied static magnetic field parallel to the strip-line. By utilizing the wave vector- and frequency-selective capabilities of the BLS spectrometer, it was possible to determine the wave vector distributions of the MSW excitations at both low and high input microwave power levels. At low power levels, the spectra at 3.6 GHz consisted of magnetostatic surface wave excitations with wave number k from about 50 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> to 200 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> and propagation perpendicular to the field and microstrip line direction. At high power levels, parametric half-frequency excitations were observed with k-values greater than 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> .

In a slot‐line‐type signal‐to‐noise enhancer for a magnetostatic wave, it is shown theoretically and confirmed experimentally that enhancement can be improved by the symmetric slot‐line structure with yttrium iron garnet (YIG) films on both sides of the slot line and by the layered structure of high permittivity materials. It is shown that the improvement in the amount of enhancement requires an increased attenuation constant of the slot line caused by the conversion loss of the linear operation to the magnetostatic wave. The magnetostatic wave radiation impedance is then derived by recognizing the contribution of the magnetic flux density normal to the slot plane among the electromagnetic field components traveling along the slot line to the conversion of the magnetostatic wave. By means of this radiation impedance, the attenuation constant is derived for the symmetric slot line and the slot line with a high permittivity layered structure so that improvement to enhancement is clarified.

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&deg;. 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&deg;/cm at the wavelength of 594 nm for an annealing of 740&deg;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.