Contains research objectives and summary of research on two research projects.Joint Services Electronics Program (Contract DAAB07-76-C-1400

Kyhl, Robert L.

Morgenthaler, Frederic R.

Platzker, Aryeh

Zeskind, Dale A.

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XI. MICROWAVE DEVICES EMPLOYING MAGNETIC WAVESAcademic and Research StaffProf. Frederic R. Morgenthaler Dr. Aryeh PlatzkerProf. Robert L. Kyhl Dale ZeskindGraduate StudentsJohn J. Cooley Peter N. HorowitzLance A. Glasser James A. Spreen1. MAGNETOSTATIC RACETRACK RESONATOR -NEW TYPE OFHIGH-Q MICROWAVE FILTERJoint Services Electronics Program (Contract DAAB07-76-C-1400)Frederic R. Morgenthaler, Robert L. Kyhl, Dale ZeskindWe have undertaken a comprehensive program directed at developing and perfectinga novel class of microwave yttrium iron garnet (YIG) filters based upon our recentobservations (in the Microwave and Quantum Mechanics Group of the Department ofElectrical Engineering and Computer Science) of extremely sharp localized resonancein single crystals biased with spatially nonuniform fields. Such magnetically tunablefilters would employ photolithographically derived planar microwave circuit elementsfor coupling energy into and out of the crystal, and it appears that multipole filters canbe integrated within the same crystal to provide economy of scale and hence precludecostly time-consuming hand-alignment procedures.Magnetic resonance is normally very much broadened if a ferrimagnetic sample isimmersed in a spatially nonuniform field, and experimentalists measuring fundamentalresonance parameters take great pains to employ ellipsoidal sample shapes (usuallysmall spheres) that are positioned in fields of very high uniformity. Therefore it isvery remarkable that in our work in the Microwave and Quantum Magnetics Group werecently observed extremely sharp resonances of a highly localized character in single-crystal YIG rectangular slabs that encounter highly nonuniform demagnetization fieldsbecause of the nonellipsoidal shape. One view of the newly discovered high-Q resonanceis that magnetostatic mode patterns are formed for which the resonant energies arehighly confined to certain regions or " tracks" within the crystal that allow wave prop-agation around them. If the mode amplitudes are very small at the edges and cornersof the sample, the surface scattering (which would be expected to be enormous) canlargely be prevented; consequently, the Q of the resonance is governed primarily bythe intrinsic linewidth of the bulk crystal together with normal circuit-loading consider-ations. In effect, appropriately designed magnetic field profiles create surfaces of dis-continuity where there are no actual surfaces; surface wave propagation at suchPR No. 119(XI. MICROWAVE DEVICES EMPLOYING MAGNETIC WAVES)" surfaces" should be free of many of the drawbacks and loss mechanisms encounteredat true surfaces. It is our intent to learn how to characterize, control, and efficientlycouple to such modes so that we may create a new class of microwave magneticallytunable resonance filters.This research effort was motivated by observations reported by Dale Zeskind I ofnew spatially localized modes of ferromagnetic resonance in rectangular slabs of single-crystal yttrium iron garnet (YIG), with dc magnetic field applied perpendicular to theplane of the slabs and microwave coupling provided between two closely spaced striplineantennas on the same surface of the YIG slab. With crystal dimensions of 0. 190" X0. 161" X 0. 016" and antenna separation of 0. 0125" (antennas located along the centerline of the slab) the resonance is characterized by a half-power bandwidth of 1. 8 MHzat 2. 5 GHz. The resonance center frequency is tunable over the range 2. 2-3. O0 GHz bychanging dc bias field magnitude.A hybrid stripline multiantenna device was constructed using microwave integratedcircuit techniques. This device was used to plot the spatial distribution of the surfacecoupling phenomenon across the face of the crystal. Spatially localized points of reso-nance exhibit symmetry about the middle of the crystal with maximum responseoccurring at the midplane. Half-power spatial linewidths are approximately 0. 0025".A new sample holder with microstripline probes is being constructed and will be usedin an attempt to provide definitive determinations of the high-Q mode distributions inexisting crystals and in new crystals of both rectangular slab and circular disk geom-etries.We have also initiated a program of device synthesis aimed at tailoring modes withcharacteristics that are best suited to high-Q filter applications and will still allow rea-sonably efficient coupling to input and output circuits. Two separate methods of creating"tracks" to support and guide surfacelike magnetostatic waves of prespecified groupvelocity in YIG will be compared.In the first, spatially nonuniform magnetic fields of prespecified characteristicswill be synthesized inside the crystal so as to form effective surfaces away from theactual crystal boundaries.In the second, they will be formed by mechanical means (i. e., etched grooves, slots,etc.) followed by argon ion beam milling to remove damaged surface layers.In both cases, control of the wave group velocities will be exercised so as to keepenough electromagnetic character to allow reasonable coupling efficiencies and stillprovide magnetostatic wavelength reductions and tunability, especially at lower micro-wave frequencies. Special emphasis is being given to creating modes that can consti-tute multisection filters of even higher selectivity and to configurations that will lendthemselves to planar fabrication and integration techniques.We have already made substantial progress in determining theoretically thePR No. 119(XI. MICROWAVE DEVICES EMPLOYING MAGNETIC WAVES)conditions necessary to cause a magnetostatic wave to be bound or confined by a dc mag-netic field gradient located near the surface of a slab of ferrimagnetic crystal when thewave propagation is both parallel to the surface and perpendicular to the dc field direc-tion.In the course of this investigation we have found a class of special dc field profilesthat can bind magnetic waves having simple analytical properties characterized byclosed-form expressions for mode pattern, group velocity, and the like.By proper choice of the gradient, the wave energy can be forced to occur eitherlargely within or predominantly outside the crystal.Of equal importance is the fact that the bound wave frequencies and group velocitiescan be gradient-controlled to a considerable extent. Therefore, in contrast with theDamon-Eshbach surface mode propagating on a thick substrate with negligible speed,which in YIG must lie above 2. 5 GHz, frequencies can be made considerably lower andvelocities much higher, if desired.When no boundary is present in the vicinity of the gradient region, bound waves canstill exist but the transverse mode pattern is more complicated. We have also consid-ered situations in which the dc magnetic field profile is "well-shaped" with either pos-itive or negative curvature, as required.References1. D. A. Zeskind, "Localized Ferromagnetic Resonance in Non-uniform MagneticFields," S.M. and E.E. Thesis, Department of Electrical Engineering and ComputerScience, M. I. T., February 1976; also available as Microwave and Quantum Mag-netics Group Technical Report No. 36.2. MICROWAVE DEVICES EMPLOYING CONTROLLED FOCUSINGOF MAGNETOSTATIC WAVESJoint Services Electronics Program (Contract DAAB07-76-C-1400)Frederic R. Morgenthaler, Aryeh PlatzkerOur recently reported magnetic field synthesis procedurel has been utilized byPlatzker and Morgenthaler 2 to construct a series of broadband, low-loss linearlydispersive magnetoelastic microwave delay lines that have yielded the highest levelsof performance achieved with this class of device.The most striking features are the very broad instantaneous bandwidth, coupled witha very high degree of linearity of the delay time/frequency relation. These highlydesirable features have been achieved without sacrifice of insertion loss, which remainsrelatively low and uniform across the operating frequency band.Although the synthesis to exacting standards of prespecified magnetic field profilesPR No. 119(XI. MICROWAVE DEVICES EMPLOYING MAGNETIC WAVES)has made these achievements possible, it appears that extension to higher frequenciesis feasible. Moreover, completely new types of linear and nonlinear filters can bedeveloped, provided that proper control of focusing and defocusing characteristics ofmagnetostatic and/or magnetoelastic wave packets is exercised. Consequently, theMicrowave and Quantum Magnetics Group of the Department of Electrical Engineeringand Computer Science has initiated fundamental studies of controlled focusing. Experi-mentally, this has suddenly become feasible because of the magnetic field synthesiscapability.In the undergraduate thesis research of Syed H. Nawab, now nearing completion,an improved synthesis procedure for creating a prespecified magnetic field profile H(z)inside a single-crystal YIG cylinder has been applied. In this case, the field necessaryfor operating a microwave magnetoelastic delay line has linear frequency dispersionover the 1-2 GHz frequency band. The measured characteristics of the synthesizeddevice will be correlated with those expected from the theoretical model and the latterwill be used to determine the actual dc field profile within the crystal.When this first phase is completed, we shall turn to the task of generating a specialclass of field profiles that have emerged from our analysis of magnetostatic wavefocusing and defocusing. These "constant Q" profiles render the focusing parameter,2HM 3 (H+M)H"t(H+M)2 (H') 2independent of position. Here M is the saturation magnetization and H' and H" are thefirst and second derivatives of H with respect to z. We are particularly interested inthe predicted range 0 <Q < 1 for providing optimum focusing.Referencesi. F. R. Morgenthaler, "Synthesis of Cylindrically Symmetric Static Magnetic Fieldsin a Locally Saturated Ferromagnet," AIP Conference Proceedings No. 28, Mag-netism and Magnetic Materials - 1974 (20th Annual Conference, San Francisco),pp. 503-504; available with greater detail as Microwave and Quantum MagneticsGroup Technical Report No. 34, December 1974.2. A. Platzker and F. R. Morgenthaler, "Synthesized Linearly Dispersive MicrowaveYIG Delay Line with Wide Instantaneous Bandwidth," AIP Conference ProceedingsNo. 29, Magnetism and Magnetic Materials - 1975 (21st Annual Conference, Phila-delphia), pp. 645-64-6PR No. 119

Microwave Devices Employing Magnetic Waves

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Microwave Devices Employing Magnetic Waves

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