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WAVENUMBER K J 3718 IEEE TRANSACTIONS ON MAGNETICS, VOL. MAG-23, NO. 5, SEPTEMBER 1987 CHARACTERIZATION OF MAGNETOSTATIC WAVE DEVICESBY BRILLOUIN LIGHT SCATTERING G. Srinivasan,J. G. Booth, and C.E. Patton. Abstract - Brillouin light scattering (BLS) has been frequency profile for the scattered light, is obtained used for direct detection of backward volume wave (BVW)by analyzing the scattered light with a high contrast excitations in yttrium iron garnet (YIG) filmsin Fabry-Perot interferometer. One observes intensity magnetostatic wave devices. The measurements were peaks at frequencies corresponding to the magnetic carried outon delay line structures andon a excitations. In this study, the BLS technique was used signal-to-noise enhancer device. With an applied fieldto observe BVW excitations and to measure intensity vs. parallel to the film plane and perpendicular to the field profiles and magnon dispersion. The BVW band stripline, backward volume waves are excited in thelimits YIG and dispersion curves were found to be in film by the stripline power. Using the BLS technique,agreement with the theoretical predictions. the BVW excitations in the YIC film were observed over the frequency range2-4 GRz. The observed BVW band limits are in reasonable agreement with theory. There Backward Volume Waves is very good agreement between the experimental and the theoretical dispersion for the BVW excitations. The scattering was foundto be rather weak over the BVW There are two types of magnetic wave excitations for passband, but did reveal considerable structure. in-plane magnetized slabs, backward volume waves (BVW) and surface waves [7]. Workon surface waves has been reported previously [2]-[4]. The magnon dispersion for the volume waves is shown schematically in 1 Fig.for Introduction propagation parallel to the applied magnetic field. The frequencyfk is shown as a function of wavenumber for the various BVW dispersion branches. The upper and Magnetostatic wave (MSW) devices such as delay lower frequency limits for the BVW excitations are lines, power limiters, and signal-to-noise enhancers indicated by f and fH, respectively. These limits B are used for signal processing at microwave frequenciesare given by [l]. In these devices, magnetostatic waves are excited in a yttrium iron garnet (YIG) filman externalby microwave signal. Direct observation of such MSW excitations is possible with the technique of Brillouin light scattering (BLS). This technique has been used and to observe magnetostatic surface waves and parametric spin waves excitedin PIG films in a signal-to-noise f =yH, (2) enhancer device [2], [3]. It was possible to obtain H quantitative information on energy flow profile, magnon dispersion, and spin wave instability processes [4]. Results of recent light scattering studieson backward volume waves (BVW)in PIG filmMSW devices are presented here. The BLS measurements were performedon (i) a signal- to-noise enhancer MSW device with a striplineon an alumina ground plane [5], [6], and(ii) delay line device structures in which stripline transducers are deposited onto silica or sapphire substrates.In both cases, one has a YIG film in contact with the stripline. With an appropriate in-plane static magnetic field applied perpendicular to the stripline, backward volume waves are excited in the YIG film by the striplinepower, In light scattering, the interaction between the incident laser light and the magnetic excitations in the YIG film gives rise to frequency shifted scattered light. The Brillouin spectrum,i.e., the intensity vs. Manuscript received March1, 1987. G. Srinivasan and C.E. Patton are with the Department of Physics, Colorado State University, Fort Collins, CO 80523. ,WAVENUMBER k J. G. Booth is with the Departmentof Pure and Applied Physics, University of Salford, Salford4WT, M5 UK. This work was performed while Professor Booth Fig.was 1. Schematic frequency vs. wavenumber dispersion a Visiting Professor at Colorado State University. branches for backward volume waves (BVW) This work was supported by the Electromagnetics propagating parallel to the in-plane applied fieldin Directorate of the RomeAi.r Development Center, Hanscom thin films. The frequencies fH and fg-correspond to Air Force Base,MA., Contract # F19628-85-K-0002. the lower and upper limits for BVW excltations. 0018-9464/87/0900-3718$01.00@1987 IEEE 3719 where H is the in-plane applied field,4aM is the measurements, even though there was some degradation in saturation induction, and 2ay is the gyromzgnetic the microwave impedance matching characteristics of the ratio. overall device. The dispersion shown inFig. 1 is for the BVW The stripline dimensions and the magnetic parameters excitations with the in-plane wavevectork along fi. for the YIG films used with the various devices are For largek, the excitation profile for the volume given in TableI. The effective 4aM and y values were waves across the film thickness correspondsa toset of determined from ferromagnetic resonazce in-planefor standing waves and the magnon wavenumber perpendicularand out-of-plane fields. The film thickness were to the film plane is quantized accordingkL to= n?r/S, determined by interference techniques, courtesy of where n = 1,2..,and S is the film thickness. Each Westinghouse and Hewlett Packard. branch in Fig.1 corresponds to a particular k solution. The dispersion branches all have nekative so Table I slope, that the group velocity k af /ak is generally negative in the midband region,f < f < fB. This H. negative group velocity is the baslck reason for the Stripline width (w) and YIG film material parameters "BVW" or "backward volume wave" nomenclature. (thickness S, effective saturation induction 4aM and reduced gyromagnetic ratioy) for the signal-to-gAise The objective of this BLS study was to detect (SNE) device and the delay line (DL) device structures directly the BVW excitations which are present in YIGon sapphire and silica. films inMSW device structures and to measure the dispersion properties and scattering profiles for these excitations in actual device configurations. striplinefilm saturation reduced Device width thicknessDevicewidth induction 7 The Device Structures w (pm) S (pm) 4aM (kG)(GHz/kOe) S The signal-to-noiseenhancer (SNE) and delay line (DL) structures which were used for the BLS investigations are shown schematically in Fig.2. The SNE device in Fig.2(a) consists ofa stripline transducer on an alumina ground plane[5], [6]. A YIG film on a gadolinium gallium garnet (GGG) substrate is in contact with the stripline.In the delay line type device in Fig. 2(b), the actual device configuration involved two different structures, one witha sapphire substrate and relatively narrow(22 pm) stripline transducers, and one with a silica substrate and In order to excite backward volume wavesin the relatively broad (560 pm) transducers. These devices, it is necessary to applya d.c. magnetic field substrates were mounted in an open support structureparallel to the YIG film plane and perpendicularto the for which the metal ground plane was removed over stripline.the For suitable bias fields, volume waves are active region of the device. This open structure was excited in the YIG film by the stripline power. These proved to be very useful in order to conveniently BVW excitations propagate parallel H toand carry power access the film with the laser beam for the BLS away from the stripline. In the SNE device, one observes attenuation of the input microwave power when BVW excitations are present,In the DL device, BVW excitations in the YIG film result in coupling of microwave power to the output transducer. Typical dataon the applied field dependence of the measured output power are shown in 3Fig. for the DL device on sapphire, an excitation frequency of4 GHz, and an input power of40 mw. These data will also be plane useful for comparison with the results of BLS measurements described in subsequent sections. For fixed frequency measurements, as in Fig.3, it is convenient to think in terms a offield range, HB < H < -stripline HH, for which BVW excitations are supported. The lower fleld limitHB is simply the valueof H from (1) for GGG f = f H is similarly obtained from (2) with f == TheseBfie18 limits are indicated in 3 alongFig. withfH. Y IG the microwave data for theDL device. silica or The microwave output power vs. field profileFig. in sapphire 3 does show a reasonable correspondence to the BVW I V substrate band, but with several qualifications. First, the power drops off rather rapidly forH > H Second, there is no pronounced cutoff H at= H Bas one might Fig. 2. Diagram showing(a) a signal-to-noise enhancer expect. This is due, presumably, to tEe drop in (SNE) device structure and(b) a delay line (DL) coupling for the higher wavenumber BVW modes as the magnetostatic wave device structure, The yttrium field approachesHH. Third, avery strong coupling is iron garnet film (YIG), the gadolinium gallium garnetevident for fields somewhat below HB. The large peak (GGG) substrate for theYIG, the stripline in intensity for applied fields below the low field BVW transducer(s), and the ground plane support band.edge may be due to surface waves excited in the structures are indicated. YIG film [4]. 3720 - -*O 0 -40 -t a5 5 -60 0 700 900 1100 1300 1500 APPLIEDFIELD H (Oe) Fig. 3. Variation of the measured microwave output power with the applied field 4 atGHz in theDL device structure on sapphire. The input power was 40 mw. The bias field was parallel to the YIG film Fig. 4. Diagram showing (a) the scattering geometry plane and was perpendicular to the portion of the used for direct detection of BVW excitations in the stripline in contact with the YIG film. The field SNE device consisting of an alumina ground (A),plane , H limits for the BVW excitations B and HH, are a stripline (SL), and a yttrium iron garnet film indicated.
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