Characterization of ferromagnetic perovskites for magnetically tunablemicrowave superconducting resonatorsJ. Wosik, L.-M. Xie, M. Strikovski, P. Przyslupski, M. Kamel et al. Citation: J. Appl. Phys. 91, 5384 (2002); doi: 10.1063/1.1459600 View online: http://dx.doi.org/10.1063/1.1459600 View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v91/i8 Published by the AIP Publishing LLC. Additional information on J. Appl. Phys.Journal Homepage: http://jap.aip.org/ Journal Information: http://jap.aip.org/about/about_the_journal Top downloads: http://jap.aip.org/features/most_downloaded Information for Authors: http://jap.aip.org/authors

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JOURNAL OF APPLIED PHYSICS VOLUME 91, NUMBER 8 15 APRIL 2002

Characterization of ferromagnetic perovskites for magnetically tunablemicrowave superconducting resonators

J. Wosik,a) L.-M. Xie, M. Strikovski, P. Przyslupski,b) M. Kamel, V. V. Srinivasu,and S. A. LongTexas Center for Superconductivity, Department of Electrical and Computer Engineering,University of Houston, Texas 77204-5932

~Received 20 July 2001; accepted for publication 17 January 2002!

An investigation of electrical, magnetic, and microwave properties is presented forNd12xSrxMnO32y ~NSMO! thin films. The NSMO thin films were deposited on~100!-orientedLaAlO3 substrates using both high-pressure sputtering and laser-ablation methods. Several filmswith different doping concentration ranging from 0.17 to 0.33 were tested for microwave loss andtheir frequency dependence on the dc magnetic field. The films exhibited Curie temperaturesranging from 220 to 60 K, and saturation magnetization from 0.3 to 0.1 T. The feasibility ofapplications of magnetic perovskites for magnetic tuning of resonators is analyzed and discussed.© 2002 American Institute of Physics.@DOI: 10.1063/1.1459600#

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I. INTRODUCTION

Planar superconducting filters have emerged as onthe primary applications of high-temperature supercondors ~HTS!. The major benefits of using superconducting mterials for microwave-passive electronics include a signcant reduction in the size of the components and highresonator-Q values than those normally obtained usitraditional technologies. Conventional materials cannot pvide such specifications without exceeding the sizeweight of superconducting filters.1

If tunable superconducting filters can be designedfabricated, more opportunities for applications with greautility would be possible. There are several reports of fabcation of superconducting filters, which are tuned eitherelectric or magnetic fields using ferroelectric or ferromanetic materials, respectively.2,3 Electric-field tuning, even ifsatisfactory in terms of tuning range and speed, often dnot meet the high-Q criteria because of the high dielectrlosses in the ferroelectric material, which limits the qualfactor of the filter. The most significant work concerninmagnetic tuning has been done by Oates and co-work4

who demonstrated the magnetic-field tuning of supercducting resonators and filters that consists of microwavecuits coupled to low-loss ferrite substrates. Single-crysyttrium–iron–garnet ~YIG! ~Ref. 5! and polycrystallineferrite6 were used either as a substrate for the superconduor in a flip–flop configuration. However, because ferrite sustrates are not crystallographically compatible with HTS mterials and practically can only be used in hybrid microwastructures, there is a need for an alternative to polycrystalferrite substrate materials that would be compatible wmonolithic HTS thin-film technology. Interesting candidatfor this application are ferromagnetic perovskite oximaterials.7,8

a!Electronic mail: jarek@uh.edub!Present address: Institute of Physics of Polish Academy of Sciences,

00-668 Warszawa, Poland.

5380021-8979/2002/91(8)/5384/7/$19.00

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In recent years, manganese-based rare-earth perovshave attracted considerable interest due to their potentiause in device applications. Manganese perovskites, witformula of the form A12xBxMnO3 ~where A are trivalentrare-earth ions and B are divalent alkaline-earth eleme!have complex structural, transport, and magnetic properThese materials exhibit a paramagnetic-to-ferromagn~FM! transition accompanied by an insulator–metal trantion for a wide range of doping. One of the theories explaing this behavior is based on Zener’s9 theory of double ex-change~DE! of electrons between Mn31 and Mn41 ions.According to this theory, ionic states of trivalent and tetravlent manganese ions fluctuate due to the electron tranbetween them. As a result of such electron hoppimanganese-localized spins are aligned and both magnetion and metallic conductivity appear simultaneously. TDE model provides an explanation for the coexistencestrong charge hopping and alignment of localized momeSuch systems exhibit a colossal magnetoresistance~CMR!,10

because of the strong magnetic-field dependence of the Ctemperature, allowing the system to be switched betwhigh- and low-conductivity states by applying a dc magnefield.

It is clear that such a system optimized for the CMeffect will exhibit a high microwave loss while it is in ametallic state, and thus, will not be suitable for any micrwave applications. For such applications a ferromagnperovskite, highly resistive~insulating! at the temperature ointerest, is needed. It is well known that for magnetic povskites, the doping may significantly change the groustate of the system. For example, Ca doping11,12 inLa12xCaxMnO3 can change the system from anti-FM aninsulating ~for x.0.5 doping! through FM and conducting(x50.33) to FM and insulating~for x,0.2!. Here, we de-scribe the investigations of the field-and temperatudependent microwave properties of Nd12xSrxMnO3 ~NSMO!thin films with different doping concentrations of Sr ion

4 © 2002 American Institute of Physics

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looking for suitable material parameters for microwave aplications.

Despite a long history of investigations of these mateals, there have been only limited studies of their higfrequency properties. As a result the understanding of micwave losses in these perovskites is far from complete. Itshown13,14 that below the Curie temperature (Tf) the high-frequency absorption in a dc magnetic field B5m0H ~in thisarticles, the SI, Sommerfeld system of units is used! is domi-nated by the dependency ofm on B. The resonance frequency shift due to the change ofm was also observed at lowfrequencies in both bulk and single-crystal materials.15,16Forthin films, which are important in microwave device appcations, although microwave absorption data are availabsystematic investigation of the frequency dependence onstill lacking.

II. EXPERIMENT

A. Sample preparation

We deposited the NSMO films on~100!-orientedLaAlO3 substrates using a high-pressure, on-axis dc spuing deposition system.17 The sputtered films were depositeat 760 °C in 3 mbar oxygen pressure, and were oxygenateinsitu at 460 °C for 20 min. Three different targets with dopinx50.17, 0.21, and 0.33, respectively, were used for deption of typically 100–200-nm-thick films. The NSMO filmwith a standard dopingx50.33 exhibited a peak in thetemperature-dependent resistivity~insulator-to-metal transi-tion! at a temperatureTp5210 K. The Curie temperatureTf

of such NSMO films is about 200 K.18 Films with two otherSr concentrations showedTf temperatures around 125 an80 K, respectively.

Some of the NSMO films investigated in this study wegrown by pulsed-laser deposition~PLD! using a LambdaPhysics KrF excimer laser operating at 10 Hz. Conditionsthe film growth were optimized according to a previousreported procedure19 that produces a high insulator–mettransition temperature in the as-deposited films. Optimgrowth rate at an ambient oxygen pressure of 400 mTorrabout 0.075 nm/pulse.

B. Measurement techniques

The microwave measurements were carried out ustwo types of resonators. The first one was a 13 GHz shieldielectric cavity@shown schematically in Fig. 1~a!#, whichhas been described previously in detail.20 In order to charac-terize the NSMO film losses, the film was attached to aelectric sapphire disk, which was sandwiched betweencopper plates. In such a configuration, the TE01d mode wasexited in the cavity. Two coupling probes, which enablether reflection or transmission measurements ofQ-factor andresonant frequency, are also schematically shown in1~a!. For simplicity, the metal shield is not shown. Dc manetic field B5m0H and also magnetization M are parallela surface of the NSMO film. The rf magnetic field Brf

5m0H rf is radially distributed in the sample. However, onrf magnetic-field components, which are perpendicular todc magnetic field, take part in the tuning mechanism. T

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second resonator, a planar S-shapednl/2 microstrip resona-tor @shown schematically in Fig. 1~b!#, was fabricated eitherfrom a double-sided gold on alumina substrate or froYBCO films deposited on LaAlO3 . The characteristic im-pedance was 50V for both the gold and YBCO resonatorThe microstrip was capacitively coupled by etching gapsthe transmission line. For designing microstrip resonatowe have selected a frequency close to the frequency raused in wireless communication and to the frequency apppriate for a simple S-shaped resonator. Since the resofrequency is inversely proportional to the strip length aproportional to the propagation velocity, further lowering tfrequency would require making a meander line microstresonator. Therefore, it could complicate the experiment. Tgold and superconducting microstrip resonators used inmeasurements were resonating around 3 and 2 GHz, restively. In the measurements, the NSMO film on a LaAlO3

substrate was clamped to a gold or YBCO microstrip.

FIG. 1. ~a! Standard dielectric resonator technique modified for measuments of the magnetic materials is showed. Dielectric sapphire disk~9 mmin diameter by 5 mm in height! is sandwiched between normal metal uppand bottom plates. Magnetic material under investigation is placed betwthe bottom metal plate and sapphire disk.~b! Simple S-shaped microstripresonator is made comprised of a superconducting or metal strip on adielectric ~LAO! substrate having a ground plane made out of the samaterial as the top strip. On the top of the structure, a NSMO film deposon a LAO substrate was placed in a flip–flop configuration. Dc magnfield B5m0H ~thus, also magnetizationM! and rf magnetic fieldBrf

5m0H rf are parallel to the plane of the film, but perpendicular to oneother in the investigated film volume.

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The measurement system consisted of a HP 8250Ctor analyzer, a Janis cold head cryostat, and a Varian elemagnet with a four-quadrant power supply. The change imwas obtained by placing the resonator confining the NSMfilm in a uniform dc magnetic field between two poles of tVarian electromagnet. In the case of shielded dielectric city, a dc magnetic field was applied parallel to the NSMfilm surface and in the case of the microstrip, the field wapplied parallel to the propagation direction of the wave treling on the microstrip. Measurements of theQ-factor andcenter frequency of the resonator, as functions of tempture, were first carried out at zero-magnetic field. Then,magnetic-field dependence ofQ was measured at several slected temperatures. The unloaded quality factor (Qu) wasmeasured in one-or two-port configuration using softwemploying the Ginzton–Kajfez method21 with a refined losscalibration procedure.

The temperature dependence of the film’s resistivr(T) was measured by four-probe Van der Pauw configution using a helium-flow cryostat and a bias current as low10 nA. The effective magnetizationMeff(T) of each film in-vestigated was extracted from the ferromagnetic resona~FMR! measurements. These measurements were carrieusing a conventional Bruker ESP 300 spectrometer operaat 9.4 GHz. An Oxford Instruments continuous-gas-flow costat was used to obtain temperatures ranging from 4.300 K. Thin-film samples were placed in the center of tTE102 microwave cavity of a Bruker’s spectrometer. Thstatic magnetic field was horizontally aligned, while the mcrowave field was always perpendicular to the dc field. Tdc magnetic field was modulated at 100 kHz. Values of mnetization obtained by this method were very close to thmeasured with a superconducting quantum interferencevice magnetometer for the same samples.

III. RESULTS AND DISCUSSION

Figure 2 shows a plot of the microwave absorption vsus the dc magnetic field for a Nd12xSrxMnO3 (x50.33)laser-ablated thin film at different temperatures ranging frroom temperature to 80 K. Transition from paramagnetic-ferromagnetic resonance is clearly seen and change oferromagnetic resonance field is the measure of the film mnetization.

In Fig. 3 the effective magnetization and dc resistivityNd12xSrxMnO3 films are plotted versus temperature for Sion doping levels of~x50.17, 0.21, and 0.33!. The plot ofthe effective magnetization versus temperature shown in3 was obtained from the FMR method presented in Fig. 2clear shift of the resistivity maximum to lower temperaturand a reduction of the drop in resistivity, with decreasiconcentration of Sr, is observed. For a doping ofx50.33 thefilm exhibits a paramagnetic–ferromagnetic transition at 2K; whereas forx50.21 andx50.17 such a transition can bseen at 125 and 80 K, respectively.

To characterize the microwave loss of the films in tferromagnetic state, we have measured the quality factorQ,of the sapphire dielectric cavity versus temperature w(Qu) and without (Qr) the NSMO film. A transmission line

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model was used to extract the microwave loss of the NSMfilm. Such a microwave loss is inversely proportional to tquality factor of the ferromagnetic film. In Fig. 4 (Qr /Qu)21 of the NSMO films, extracted from measured overcavity Q, is plotted as a function of temperature. As the teperature is reduced below the transition temperature forthree samples withx50.33, 0.21, and 0.17, the loss increasand levels off slowly. This can be attributed to the loss of tspin system. Thus, a significant component of this losscrease is related to the ferromagneticTf transitiontemperature.22 Such a conclusion can be supported by tincrease of the microwave loss belowTf for the x50.17curve, for which the loss begins to increase below 80 K, ethough no resistivity drop is taking place at this temperat

FIG. 2. Plot of microwave absorption vs dc magnetic field fNd12xSrxMnO3 (x50.33) measured at different temperatures ranging frroom temperature to 80 K. Standard electron spin resonance~ESR! spec-trometer with a TE102 rectangular 9.4 GHz microwave cavity was used fthese measurements.

FIG. 3. Plot of magnetization~closed markers, right scale! and resistivity~open markers, left scale! vs temperature for Nd12xSrxMnO3 100-nm-thickfilms with dopingx50.33~square!, 0.21~triangle!, and 0.17~circle!, respec-tively. Magnetization data points are calculated from the ESR spectrommeasurements.

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5387J. Appl. Phys., Vol. 91, No. 8, 15 April 2002 Wosik et al.

~not shown in Fig. 3!. However, the microwave loss for thx50.17 case reaches a maximum around 40 K, and tstarts to decrease. Two possible explanations can be usaccount for this decrease. The first one is that below 40 Kanisotropic fieldBan is increasing with decreasing temperture, resulting in smaller zero-field loss. The second possity is that the damping parametera begins to decrease whethe temperature drops below 40 K. The former is perhmore likely, however, further investigation is needed to fuexplain the observed microwave loss behavior.

NSMO films of compositionx50.21 have both the highresistance and sufficient magnetization, as can be seen in3. Thex50.21 films were selected for further testing using3 GHz gold or YBCO microstrip resonator. Thex50.33films could not be used in the microstrip experiments,cause they were exhibiting too high losses. Thex50.17 ex-hibited the smallest microwave loss, but delivered less tunthan the films withx50.21 composition.

In order to describe the above results, an introductionmagnetic tuning theory is needed. Magnetic-field tuning omicrowave resonator that employs a ferromagnetic insulamaterial~for example, in microstrip geometry!, can be real-ized by utilizing changes in the effective permeability,m(B),of the material as the applied magnetic field is changed.change inm(B) then alters the resonant frequency of tmicrowave device. The Landau–Lifschitz–Gilbert dynampermeability formula can be written in the following form:

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FIG. 4. Plot of microwave losses (Q0 /Q)21 vs temperature for 100-nmthick Nd12xSrxMnO3 films with three different concentration of Sr~x50.33, x50.21 andx50.17!. Q0 is the Q without sample. A 13 GHz di-electric cavity was used.

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Here,B5m0H, M5m0M , andBar5m0Han are the ap-plied field, the saturation magnetization, and the anisotrofield, respectively. These parameters are expressed in tOther quantities includea and g, which are the Gilbertdamping parameter and the gyromagnetic factor, resptively. Equation ~1! is derived for the case where the dmagnetic fieldB and the rf magnetic fieldBrf5m0H rf areparallel to the plane of the film, but perpendicular to eaother.

Figure 5 shows the variation of real and imaginary paof the dynamic permeability,m5m82 j m9, calculated usingEq. ~1!, assuming that the frequency is equal to 3 GHz,M50.06 T, a50.2, andBan50. The dispersive properties othe material are described by the real part,m8. The absorp-tion loss component determines the imaginary part,m9. Themaximum absorption occurs at 0.08 T, which is the fermagnetic resonance fieldBr5m0Hr . The inflection points ofthe absorption line correspond to the maximum and mmum points of the dispersion curve.

Thus, the differenceDB between the maximum ofm9and the maximum ofm8 determines the half width of theabsorption line, which is proportional toa (DB5va/g) andcan be considered as a measure of the quality of the mateIn Fig. 5, three magnetic-field regions are marked, in whthe real part ofm depends significantly on the dc magnefield. Region ‘‘a,’’ which covers the lower-magnetic-fielrange, shows sufficientm8 change and low-microwaveabsorption loss. In this range the magnetization is usuonly partially saturated. If the FMR field is high enough, this an attractive region for use in tuning. This is a result of tvery low microwave loss and relatively largem8 change dueto variation of effectiveM with respect to the increase of a dapplied field.

The region marked as ‘‘b’’ is characterized by vestrong field dependence ofm8; but since the FM resonancoccurs in this region, it is characterized by very large abso

FIG. 5. Calculated complex permeability components vs dc magnetic fifor saturation magnetizationM50.06 T anda50.2, which corresponds to ahalf linewidthDB50.022 T. The frequency was assumed to be 3 GHz. Abias fieldBb , the maximum of the figure-of-meritF is determine by boththe slope ofm8 and the value ofm9.

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tion loss, which makes it unsuitable for tuning. In the ‘‘cregion, which is above the FMR field,m8 has a large valuethat decreases steadily with increasing field while the abstion loss decreases rapidly. This region can be used foring; however, an applied magnetic bias fieldBb5m0Hb willbe needed.

A set of data for a 3 GHz microstrip made out of goland coupled to the NSMO (x50.21) thin film is presented inFig. 6. The measurements presented in Fig. 6 were carout at 118 K. At this temperature the Cu/NSMO microstsandwich exhibited the maximum frequency shift. Twbranches of frequency shift can be identified: one nearzero field where theQ factor is the lowest, marked as regiob in Fig. 6, and the other at a higher field, region c, withmuch largerQ, which indicates lower losses. These two rgions, b and c, correspond to the same two regions prously shown in Fig. 5; the only difference here is that duethe large saturation magnetization coupled with an antropic field Ban, the FMR peak has been pushed near tozero field, making region a inaccessible. It is simulatedFig. 7, where by using Eq.~1! the two maxima ofm(B)8 andm9(B), are fitted to the two corresponding minima ofD f (B)andQ(B), respectively, in Fig. 6. In addition to the saturtion magnetizationM50.28 T, an additional anisotropifield, Ban50.02 T, was added to fit the data. In this cathere is no a region and region c is the only viable choicetuning; however, it requires a magnetic bias field,Bb , tomove the system into region c.

The sample withx50.21 was then coupled to a 2 GHzmicrostrip of YBCO, at 65 K, below the superconductincritical temperatureTc588 K of YBCO. The data measureat 65 K are shown in Fig. 8. Comparing the data with thatFig. 7, shows that the behavior of the curves are similathat in region b. The measurements did not go beyondboundary of region c.

To compare the experimental results with calculated vues of the resonant frequency change of the microscoupled to the thin ferromagnetic film, a perturbation meth

FIG. 6. Plot of frequency change~closed squares, right scale! and qualityfactor Q ~open circles, left scale! vs applied field for a gold 3 GHz microstrip resonator on an Al2O3 substrate coupled to a Nd12xSrxMnO3 film (x50.21) deposited on a LaAlO3 substrate. The measurements were carrout at 118 K.

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will be used. Such an approach can be justified by the smsizes of the samples used in our experiments. The changthe resonance frequency for such a case can be express

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Here,V is the entire volume of the medium where thtotal microwave energyW is stored,v is the portion ofVwherem can be changed, andj is the filling factor defined asthe ratio of the field energy in the ferromagnetic samplethe total field energy. The filling factor can be also appro

FIG. 7. Variation of complex permeability components vs dc magnetic ficorresponding to the experimental data shown in Fig. 6. The simulationdone for an operating frequency of 3 GHz; assuming a saturation magzation M50.28 T, and existence of an additional anisotropy field ofBan

50.02 T. The value of a linewidthDB equal to 0.03 T was taken fromFig. 6.

FIG. 8. Plot of frequency change~open circles, left scale! and quality factorQ ~closed squares, right scale! vs applied dc magnetic field for a YBCOS-shaped microstrip resonator on a LaAlO3 substrate coupled to aNd12xSrxMnO3 film (x50.21). Data were taken at 2 GHz.

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5389J. Appl. Phys., Vol. 91, No. 8, 15 April 2002 Wosik et al.

mately calculated as a ratio of the sample volume to effecvolume of either the dielectric or microstrip resonator:j;v/V. Therefore, the frequency shift is proportional nonly to Dm, but also to the filling factor of the ferromagnetmaterialj. For example, if the resonance frequency of a mcrostrip resonator coupled with a 0.5-mm-thick ferromanetic material can be changed by 1 GHz when a dc magnfield is applied, it can be shown that this shift will be reducto about 2 MHz when the thickness is reduced to 1mm. Themicrowave loss due to the change of permeability is proptional to its imaginary partm9. This magnetic loss,Pm , canbe expressed as

Pm52Svm0m2H2vm9, ~3!

whereS is the sample shape factor. From the measured qity factor of the resonator coupled to the ferromagnetic mterial, the partialQ-factorQm of the magnetic material itselcan be extracted. For a dielectric cavity a transmissionmodel can be used, whereas the microstrip requires a dient approach. The quality factor,Qm , is related to the magnetic lossPm in the following way:

1

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The basic principle for designing a tuning device isoptimize the frequency shift and minimize the absorptloss. A figure of merit,F, is defined23 as follows:

F5D f Qu

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whereQu is the unloadedQ of the resonator with a ferromagnetic film, andD f is a frequency shift due to tuning.Qu

can be expressed in terms ofQr , the quality factor of theempty resonator, andQm @Eq. ~4!#, the quality factor of themagnetic material, in the following way:

Qu5Qr /~11Qr /Qm!. ~6!

Substituting thisQu into Eq. ~5! and from Eqs.~2! and ~4!,one obtains:

F5maxH F~B!52jDm8~B!Qr

114jQr@2m9~B!#UBr,B,`J . ~7!

In Fig. 9, three plots ofF versus peak-to-peak FMRlinewidth DB are shown. They were calculated from Eqs.~1!and ~7! for two values ofQr , 175 and 2000. TheQr valuescorrespond to measuredQ values of the copper and YBCOmicrostrip coupled to the NSMO (x50.21) film. The param-etersj andM were assumed to be equal to 0.00045 and 0T, respectively. The top curve is also calculated forQ52000, however, ten times larger filling factorj was usedfor the calculation. As expected, the figure of meritF in-creases either with decreasingDB or with increasing ofj.For large values ofj and/orQr theF will be a constant. Theinset of Fig. 9 shows dependence ofF on B, calculated fromEq. ~7!. The F value within theb region is very small, andhas its maximum in region c. A maximum value ofF occursat a certain fieldBb , where the combination of a large slopof m8 and a small value ofm9, see Fig. 5, is compromised

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Values of F obtained from the experimental data prsented in Figs. 6 and 8 are shown as an open squarecircle, respectively in Fig. 9. Both are very close to the cresponding theoretical values and neither is larger thanThere are two reasons for such a relatively small shift inmicrostrip frequency measured for NSMO thin films. Thfirst one is related to the small volume of the NSMO thfilm used in our measurements, which results in a smalling factorj. The other reason is related to the large linewidDBpp of the FMR resonance. In the investigated films, tsmallest measured linewidth value was equal to 0.06 T. Sa broad linewidth of the absorption peak, we believe, isan intrinsic property, but indicates a large magnetic inhomgeneity, which can possibly be reduced either using differprocessing methods or by selection of a differemanganite.22

From Fig. 9 we can estimate that as the half-width, peto-peak value of the material is reduced to 0.01 T it canincreased toF515 for Q52000. If the volume of the sampleis increased, it could exceed 20. SuchF values are comparable with those obtained from ferroelectrically tuned supconducting filters.4 However, ferrites working at liquid-helium temperature and in the a region~see Fig. 5! candeliver much higherF values~of an order of hundreds!. Thisis due to the very narrow FMR line, which leads to smzero-field loss.

IV. CONCLUSIONS

In searching for a material, which can be an alternatto ferrites for tuning purposes but crystallographically copatible with HTS, we have found that ferromagnetic perokites such as NSMO would be useful if their properties cobe optimized to combine both a significant magnetizatand a high~insulating! dc resistivity. Presumably, a goocandidate for a tuning material would be an underdoped povskite film that shows a transition from the paramagne

FIG. 9. Plot of figure-of-meritF vs FMR linewidthDBpp . Curves are cal-culated forM50.28 T. Bottom and the middle curves are calculated wthe same filling factorj50.00045 forQ5175 and 2000, respectively. Thupper curve represents simulation forQ52000 andj50.0045. The opensquare and circle symbols mark theF values obtained from experimentadata presented in Figs. 6 and 8, respectively. Inset shows an example oF on B dependence. The maximum value ofF is marked at theBb field inthe main figure as a solid circle.

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insulator to the ferromagnetic insulator state. We have fothat for films with a Sr doping ofx50.2 the microwavelosses are significantly reduced while still maintaining a snificant magnetization level.

Using the field-dependent permeabilitym(B) of NSMOfilms opens the prospects for the fabrication of monolitmagnetically tunable microwave devices. However, in orto use these materials instead of ferrites for the magntuning of resonators, a better understanding of the micwave loss mechanisms in perovskite materials is neededthe investigated films the FMR was broad, indicating a snificant magnetic inhomogeneity. For an adequate tunrange, thin films with narrow FMR lines of the order of 1mT are required.

ACKNOWLEDGMENTS

This work was supported, in part, by the Texas HighEducation Coordinating Board Advanced Research Progand the Advanced Technology Program, by the Texas Cefor Superconductivity at the University of Houston, andthe Committee for Scientific Research~KBN! Grant No. 5PO3B 06220.

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