Performance and Ferrimagnetic Material Considerations in CryogenicMicrowave DevicesR. L. Comstock and C. E. Fay Citation: Journal of Applied Physics 36, 1253 (1965); doi: 10.1063/1.1714193 View online: http://dx.doi.org/10.1063/1.1714193 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/36/3?ver=pdfcov Published by the AIP Publishing

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JOURNAL OF APPLIED PHYSICS VOLUME 36, NO.3 (TWO PARTS-PART 2) MARCH 1965

Performance and Ferrimagnetic Material Considerations in Cryogenic Microwave Devices

R. L. COMSTOCK*

Lockheed Missiles and SPace Company, Research Laboratories, Palo Alto, California AND

C. E. FAY

Bell Telephone Laboratories, Incorporated, Murray Hill, Nf!'W Jersey

Circulators which can be operated at cryogenic temperatures are discussed. Both symmetrical Y-junction circulators using polycrystalline ferrimagnetic garnets and hybrid-gyrator circulators using single crystals are evaluated. The temperature dependence of the ferromagnetic resonance losses of polycrystalline alumi­num-doped and undoped yttrium iron garnet is reported. Device performance is evaluated in terms of the measured material properties.

1. INTRODUCTION

I N this paper we will discuss the application of ferri­magnetic insulators to low-temperature microwave

transmission devices. Low-temperature microwave mag­netic devices have become important in connection with low-noise microwave preamplifiers, e.g., resonance isolators in traveling-wave masers, and circulators in recent experiments with low-temperature parametric amplifiers. The problems associated with the operation of microwave ferrimagnetic devices at cryogenic tem­peratures are critically related to the properties, e.g., ferromagnetic resonance losses, of the ferrimagnetic garnets at low temperatures (Sec. 2). The low-tempera­ture properties of microwave circulators! are empha­sized in this paper but many of the conclusions con­cerning material properties will be generally applicable to the maser isolator and other low-temperature de­vices. Two broad classes of solutions to the low-tem­perature circulator problem have evolved: symmetrical Y-junction circulators as previously developed for room-temperature operation with polycrystalline ferri­magnetic materials, and gyrator-hybrid circulators using single-crystal ferrimagnets. The properties of Y­junction circulators at low temperatures will be dis­cussed in Sec. 3, single-crystal hybrid-gyrator circula­tors in Sec. 4, and the conclusions summarized in Sec. 5.

Cryogenic Circulator-Parametric Amplifiers

The need for cryogenic temperature circulators to be used in conjunction with cryogenic temperature para­metric amplifiers can be established by considering the noise temperatures of the two microwave receivers outlined in block diagram form in Fig. 1. Each receiver consists of two circulator-coupled reflection-type para­metric amplifiers operated at 4.20 and nOK, respec­tively, and a room-temperature mixer with an effec~ive input noise temperature of 2000°K. In the first recelVer the insertion loss of the circulators, assumed to be

0.4 dB per pass, takes place at the same temperatures as the respective parametric amplifiers, while in the second receiver, the circulators are operated at room temperature (2900 K). The circulators are assumed to have infinite isolation. If thermal noise is the dominant noise contribution, calculations indicate that for gallium arsenide diodes the noise temperatures of the two parametric amplifiers are 1 ° and 20oK, respec­tively,I" The effective input noise temperatures of the receivers are calculated to be 3°K with cooled circula­tors and 300 K with room-temperature circulators. If the room-temperature circulators have 0.2-dB insertion loss, the effective input noise temperature is still 15°K. Uenohara and Josenhans have constructed a two-stage parametric amplifier using a liquid-helium tempera­ture circulator (to be described in Sec. 3) in the first

. stage and a nitrogen temperature circulator in the second stage.l" The receiver was similar to that shown in the block diagram in Fig. 1. The average effective input noise temperature measured on the complete receiver was 6°±3°K. The receiver bandwidth was approximately 3% at 4 Gc/sec. The above noise tem­peratures are competitive with those observed in microwave masers.

The low-noise temperatures available with cryogenic circulator-coupled parametric amplifiers justifies the development of suitable low-temperature circulators. The most critical part of the circulator at low tempera­tures is the ferrimagnetic material. The relevant prop­erties of available materials are discussed in the next section.

2. FERRIMAGNETIC MATERIAL CONSIDERATIONS

The most critical property of ferrimagnetic insulators for use at low temperatures is the ferromagnetic reso­nance loss. For this reason only the ferrimagnetic garnets are considered since they have been observed to have lower linewidths than spinels or other ferrimagnets.

* Formerly with Bell Telephone Laboratories, Murray Hill, h d J J ha S . . of papers of 1& M. Ueno ara an • osen ns, ummanes

New Jersey.· . 'd t k t International Conference on Microwaves, Circuit Theory and 1 An n-port circulator transmits power InCl ent on por .0 Information Theory, p. 133, Inst. Elec. Comm. Engrs., Tokyo,

port (k+1). An isolator can be made from an n=3 port Clr- )

cutator by terminating one port. Japan (1964 . 1253

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1254 R. L. COMSTOCK AND C. E. FAY

FIG. 1. Microwave parametric amplifiers with cooled and uncooled circulators. The theoretical thermal input noise tem­peratures of the two receivers are 3° and 300 K, respectively.

Yttrium Iron Garnet

The narrow ferromagnetic resonance linewidth ( DoH) of yttrium iron garnet (YIG) has made it an important material for room-temperature devices.2 The same is true for low-temperature devices. The resonance losses of moderate purity single crystals of YIG are domi­nated at low temperatures by impurities, the most im­portant of which are silicon and the rare earths. Di­valent iron resulting from charge compensation with tetravalent silicon results in a peak in the single-crystal Iinewidth at r-v50oK (,.....,6 Oe).3.4 For polycrystals the linewidths have contributions from impurities, crystal­line anisotropy, and porosity and are, in general, at least an order of magnitude larger than for the corre­sponding single crystal. In certain cases, the measured ferromagnetic resonance line in polycrystals is asym­metrical. In all the measurements reported here the asymmetry is negligible and the characterization of the losses in terms of a single parameter (the linewidth at the half absorption points) is justified. The results of a measurement of the temperature dependence of DoH for a dense polycrystalline sample of YIG are shown in Fig. 2 (points). All linewidth measurements on the polycrystalline garnets were made at 5.5 Gc/sec. The YIG material was supplied by Trans-Tech, Inc. (G-113) . The samples were unpolished since the surface linewidth is much smaller than the volume porosity linewidth.5 Previous measurements of DoH for 77°< T<300oK on a less-dense sample of polycrystalline YIG showed larger linewidths at all temperatures, but the same temperature dependence as our sample was observed in the above range.6 The peak in~ the line-

2 E. G. Spencer and R. C. LeCraw, Proc. I.E.E. 109, Part B Supp!. 21 (1962).

3 E. G. Spencer and J. P. Remeika, International Conference on Nonlinear Magnetics 12-1-2, Washington, D. C., April 1964.

4 E. G. Spencer, J. P. Remeika, and P. V. Lenzo, App!. Phys. Letters 4, 171 (1964).

6 M. Sparks, Ferromagnetic-Relaxation Theory (McGraw-Hili Book Company, Inc., New York, 1964).

6 P. E. Seiden and J. G. Grunberg, J. App!. Phys. 34, 1696 (1963).

width at ......... 20oK has not been previously reported for polycrystalline YIG and occurs near the temperature at which the linewidth in single-crystal YIG has a similar peak.3 In order to interpret the observed line­width, the two-magnon scattering theory developed by Geschwind and Clogston7 (referred to as GC) will be used. The GC theory predicts an inhomogeneous ferromagnetic resonance linewidth narrowed by dipolar forces having anisotropy,8 and porosity contributions9

DoH = DoHani+DoHpores

= 1.08J(HA2/4?rM) +0.5J4?rMp. (1)

The shape factor J is given in Ref. 7, the anisotropy field HA is given by 21 Kl 11M, and p is the porosity. The porosity of the sample used in obtaining the data for Fig. 2 was determined from the observed and theoretical densities (Pob and Pth) as p= 1- (Poblpth) = 0.6%. The resonance linewidth from these contributions increases with decreasing temperature since the factor J increases with increasing magnetization, especially at low saturating fields, and the anisotropy field in­creases much faster than the magnetization.1o The ob­served linewidth also may have homogeneous contribu­tions which can be measured on single crystals of the same material; this linewidth contribution will be referred to here as DoHs.e .• Previous measurements of Kl (Refs. 11, 12) and M (Ref. 10) for single-crystal YIG lead to predictions of linewidths larger than ob­served; e.g., the anisotropy field measured by Dillon at 1.5°K (2 I Kl 11M = 250 Oe) by itself predicts a line­width at this temperature of 130 Oe. Part of this dis-

14o,----------------------------,

• YIG (MEASURED)

~ 80

:r <l 60

40

THEORETICAL USING . PARAMETERS FOR YIG-0.3%Si

[ dHAN1SOTROPY

+dHPOROS1TY

+ dHS1NGLE CRYSTAL]

°0~~5~0~~1~00~-1~5~0~~200~~25~0~~30~0~~ T(°K}

FIG. 2. Measured (points) and theoreticallinewidth of poly­crystalline YIG at 5.5 Gc/sec. The theoretical curve is based on Dillon's measurements of YIG-O.3% Si.

7 S. Geschwind and A. M. Clogston, Phys. Rev. lOS, 49 (1957). 8 E. Schlomann, J. Phys. Chem. Solids 6,242 (1958). 9 Calculation by E. Schlomann described in Ref. 6. 10 M. A. Gilleo and S. Geller, Phys. Rev. 110, 73 (1954). 11 J. F. Dillon, Jr., Phys. Rev. 105, 759 (1957). 12 G. P. Rodrigue, H. Meyer, and R. V. Jones, J. App!. Phys.

31,375S (1960).

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PER FOR MAN C E AND FER RIM A G NET I C MAT E R I A L CON SID ERA T ION 1255

crepancy may be attributed to the (supposedly) differ­ent amounts of rare-earth impurities in the yttrium oxide from which the two garnets were made. Recently, it has been suggested13 that the reduction of anisot­ropy observed in silicon-doped single-crystal YIG by DiIIon14 may explain the reduction of anisotropy ob­served in polycrystalline YIG. The addition of silicon to the garnet lattice results in ferrous iron by valence compensation, while ferrous iron is present in poly­crystalline YIG because of oxygen deficiency resulting from sintering. The anisotropy contribution to the line­width of polycrystalline YIG using the first term in Eq. (1) with Dillon's measurements13 of Kl in YIG doped with 0.3% silicon is shown in Fig. 3(A). The anisotropy contribution is shown added to the porosity contribution in Fig. 3(B). The single-crystallinewidth measured on the same sample (averaged over the principal axes) is shown in Fig. 3(C). The sum of all three contributions to the linewidth is shown as the curve in Fig. 2. It is not known if the amount of ferrous iron is the same in the polycrystal and single crystal; however, the agreement between the measured and theoretical linewidths suggests that ferrous ions may reduce polycrystalline linewidths as long as the reduc­tion in anisotropy is larger than the increase in homo­geneous relaxation.

Low Magnetization Ferrimagnets

In certain applications, to be described in Sec. 3, ferrimagnetic materials are needed which have smaller values of M than YIG. One successful approach to the problem of obtaining low-magnetization materials has been to replace ferric ions in YIG with nonmagnetic ions. Gilleo and Geller10 have shown that nonmagnetic Ga+ + + and AI+ + + can replace Fe+ + + in YIG in sufficient amounts to reduce the 1.5°K value of 47rM to less than one third the value for YIG. Spencer and LeCraw15 have reported room-temperature Iinewidths

tOOr-~------------------------'

80

20

(8) [t.HANISOTROPY(YtG-O.3% SO

+t.HPOROSITY( YIG)]

(C) t.Hs.c. (YIG-O.3% Si)

°O~~~~~IO~O~~t~~~200~~2~~~~300~~ T(OK)

FIG. 3. Anisotropy linewidth (A), anisotropy linewidth+ porosity linewidth (B), and single-crystal linewidth (C) of silicon-doped YIG based on the dipole-narrowed linewidth theory.

13 J. F. Dillon and R. L. Comstock (unpublished). " J. F. Dillon, Bull. Am. Phys. Soc. 6, 160 (1961). Ii E. G. Spencer and R. C. LeCraw, Bull. Am. Phys. Soc. 5,

58 (1960).

-; t40 o i 120

<l100

80

60

40

20

AWMINUM-DOPED YIG

°0~~4O~~80~~12O~-1~60~~200~~2~40~~~~~320 T("K)

FIG. 4. Measured Iinewidth of aluminum-doped YIG [4n-M (3000K) =500 GJ at 5.5 Gc/sec.

of 1.2 Oe for single-crystal YIG-Ga (4n-M =400 G) as compared to 0.375 Oe for YIG made from starting materials of the same purity. Linewidths of polycrystal­line YIG-Ga at room temperature have been reported by Harrison and Hodges,16 Saunders and Green,11 and Anderson, Cunningham, McDuffie, and Stauder,18 It is found that t:.H for YIG-Ga increases with gallium substitution [25 Oe for YIG to 90 Oe for 20% substitu­tion of Ga+ + + for Fe+ + +].I8 The temperature depend­ence of t:.H in single crystals of YIG--Ga or YIG-AI are both similar to the undoped YIG,15 The temperature de­pendence of t:.H in a dense polycrystalline sample of YIG-AI is shown in Fig. 4. The value of 4n-M is 495 G at 300°K. The linewidths at all temperatures are larger than for YIG because of the smaller dipolar narrowing and the larger anisotropy. In the YIG-Al, the homo­geneous relaxation from the ferrous iron, which un­doubtedly is present, apparently is swamped out by the large anisotropy contribution to the linewidth (the porosity was small).

The magnetization of YIG also can be reduced by substituting magnetic ions on the yttrium site, e.g., relatively dilute concentrations «35%) of Gd. Mag­netization measurements of

3[ (l-x) Y203'xGd20a]5Fe203

from 300° to 4.2°K have been made by Morris and Miller19 for x=0.05, 0.10, 0.15. These measurements indicate a substantial reduction in magnetization from YIG and for x = 0.10 a possible improvement in line­width at 4.2°K over the undoped YIG as evidenced by an improvement in the operation of a maser isolator at 2 Gc/sec.

Other compounds which have lower magnetization than YIG are the recently discovered class of garnets

16 G. R. Harrison and L. R. Hodges, Jr., J. Am. Ceram. Soc. 44, 214 (1961).

17 J. H. Saunders andJ. J. Green, J. App!. Phys. 32, 161S (1961). 18 E. E. Anderson, J. R. Cunningham, Jr., G. E. McDuffie, Jr.,

and F. R. Stauder, J. Phys. Soc. Japan 17, Suppl. B-1, 369 (1962). 19 L. C. Morris and D. J. Miller, I.E.E.E. Trans. on Microwave

Theory and Techniques 12, 421 (1964).

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1256 R. L. COMSTOCK AND C. E. FAY

FIG. 5. Schematic diagram of 3-port Y-junction circulator.

containing no yttrium or rare earths.20 As an example, the calcium-vanadium-bismuth garnet

{ CauBi().5} [Fe2J (Fe1.7uVl.25) 0 12

has 4?rM equal to 600 Gat 4.2°K. Recent measurements by Nilsen and Spencer show narrow linewidths and low anisotropy at low temperatures for a calcium­vanadium-bismuth garnet.21

3. SYMMETRICAL Y-JUNCTION CRYOGENIC CIRCULATORS

Design Considerations

It is the purpose of this section to relate the measure­ments of material parameters discussed in the last sec­tion to the theoretical operation of circulators which use polycrystalline ferrimagnets. The most commonly used circulator for room-temperature operation is the ferrimagnetically loaded symmetrical junction. The most common junctions are strip lines, which are used for frequencies from uhf to low X band (8 Gc/ sec) , and rectangular waveguides which are used at X band and above. Only the strip-line junction circulator has been evaluated at liquid-helium temperature and the detailed discussion will be limited to this case. Ferri­magnetic single crystals can be used in symmetrical junction circulators if they are oriented so as not to destroy the junction symmetry. The results of measure­ments on junction circulators indicate that poly­crystals should be adequate for most applications.

A typical 3-port strip-line circulator is shown sche­matically in Fig. 5. Ferrimagnetic cylinders are placed between the circular center conductor and ground plane of the strip-line junction. The operation of the "Y­junction" circulator can be explained by reference to Fig. 6, which shows the electric and magnetic fields of the dipolar mode which is excited at port # 1, with and without an applied dc magnetic field adjusted for circulation.22 The stationary pattern (fields varying as

00 S. Geiler, G. P. Espinosa, H. J. Williams, R. C. Sherwood, and E. A. Nesbitt, Appl. Phys. Letters 3,60 (1963).

21 W. G. Nilsen and E. G. Spencer (to be published). 52 All of the theoretical discussion in this section is derived from:

C. E. Fay and R. L. Comstock, Trans. I.E.E.E. Trans. Microwave Theory Tech. (to be published).

cost/>, where t/> is the azimuthal coordinate) shown in Fig. 6(A) results from the comhination of the two rotating modes (fields varying as #1<1», whose resonant frequencies are "split" by the internal magnetic field (Hint). The phase angles of the impedances of the two rotating modes are ±300, resulting in a spatial rota­tion of the dipolar mode of 30°, which isolates port # 3. Two cases of circulator operation can be distinguished: "below resonance" (Hint<w/y) in which the pattern is oriented as shown in Fig. 6 and "above resonance" (Hint>wf'y) which results when 30° rotation in the opposite sense obtains and port # 2 is isolated.

In many respects the junction circulator is similar to a symmetrical transmission cavity, e.g., if lossless, it is matched, and the fields are represented by a sta­tionary mode. The loaded Q of the circulator mode is given by

(2)

where d is the ferrite disk thickness, R the disk radius, and GR is the load conductance referred to the edge of the disk. For circulation it is also required that

(3)

where 1) is the fractional splitting of the rotating modes for 30° phase shift. The loaded Q, and hence the splitting 0, is not expected to vary appreciably with temperature [Eq. (2) J. The splitting is given in terms of the Polder tensor components (K, p.) by K/p.=2.460. In order to keep iJ constant with increasing M, which results when the temperature is lowered, it is necessary to decrease the internal field for Hint<wh and increase it for Hmt>w/y. The center frequency of the circulator (wr )

is given by

MAGNETIC WALL

INPUT-

(4)

lal

11 '"

FIG. 6. Dipolar mode used in Y-junction circulator (a) Hint=O or <», (b) HInt for below resonance circulation. The circles indicate the electric fields.

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PER FOR MAN C E AND FER RIM A G NET I C MAT E R I ALe 0 N SID ERA T ION 1257

where J.teff is the scalar microwave permeability. The change in dc field required to maintain a constant value of ~ also changes J.teff and hence the required operating frequency. The resulting operating field and frequency can be found graphically if the change in magnetization is known, by using curves of KIJ.t and J.teff vs Hint and M. In general, for Hint<w/y the required field decreases and the operating frequency increases with increasing M, and for Hint>wl'Y the opposite is true. For below resonance circulators operated at magnetic saturation the required magnetization can be found from the value of KIJ.t with Hint=O,

4?rM = (0.71/Qd (w/y) (5)

at 4 Gclsec with QL= 2 the required value of 4?rM is 500 G. With larger values of 4?rM low field losses may become excessive and with smaller values ferromagnetic resonance losses may become excessive. For above resonance circulators the required 4?rM is larger and the cylinder diameter is reduced.

The magnetic part of the unloaded Q of the circulator mode for below resonance operation is approximately given by

(6)

In most cases the magnetic losses dominate the losses of the circulator, in which case the insertion loss (in dB) is given by 20 10g(1-QL/Qo). The loaded resonator Q is specified by the reverse isolation (VSWR) band­width and is typically in the range 1<QL<4. In order to keep the insertion loss less than 0.5 dB it is neces­sary for Qo> 20QL, resulting in a minimum value for Qo of 20. From Eq. (6) with Qo=20, 4?rM=500 G, at 4 Gclsec, the linewidth is restricted to .dH <400 Oe. The polycrystalline materials described in Sec. 1 (YIG-AI) all have linewidths at 4.2°K smaller than 200 Oe.Similar considerations for the above resonance mode lead to similar restrictions on ferromagnetic resonance linewidth.

Experimental Results

An above-resonance strip-line circulator was designed using the principles outlined in the previous section and a photograph of the assembled circulator is shown

FIG. 7. Photograph of cryogenic V-junction circulator with superconducting magnet.

50

~30 I

Z 2 ~ ..J

~ 20

10

3000 K. H·3860 G 77°K 0

4.2°K "

4330 G 4350 G

CD

0.6 ~ UI UI

0.4 9 ~

0.2 ~ ~

o~----~----~------L-----~o ~ 3.8 4.0 4.2 4.4 4.6

FREQUENCY Ge/SEC

FIG. 8. Experimental results for above resonance circulator at 300°, 77°, and 4.2°K. The insertion loss at 77°K was almost identical to that at 4.2°K,

as Fig. 7. The dc magnetic field was supplied by a superconducting magnet (niobium-zirconium alloy wire) mounted on the circulator body. The ferrimag­netic material was polycrystalline yttrium iron garnet (see Fig. 2) in the form of disks (2R=0.350 in., d= 0.052 in). The circulator was mounted on coaxial stain­less steel transmission lines and the temperature runs were made with a stainless steel double Dewar with the experimental results shown in Fig. 8. The temperature dependence of the operating field and frequency are approximately predicted by the variation in J.teff with KiJ.t constant. The bandwidth of the circulator isolation could be improved with the addition of more sophisti­cated impedance-matching circuits [only a single quarter-wavelength line was used to match the low impedance ( ....... 20 Q) of the circulator mode to the 50-Q input lines]. The measured insertion loss was com­patible with the linewidth measured on spheres of the same material (Fig. 2). The circulator was combined with a parametric amplifier and was operated at 4.2°K. The experimental results were described in the introduction.

Below resonance circulators also have been made with results similar to those obtained with the above resonance mode.23 The below resonance mode of circu­lator operation is preferred to the above resonance mode because of the lower values of magnetic field. However, for Y-junction circulators requiring the low­est possible values of insertion loss it may be preferable to choose the above resonance mode, which allows the use of YIG for frequencies in the vicinity of 4 Gc/sec.

2a J. A. deGruyl, W. E. Heinz, and S. Okwit, Proc. I.E.E.E. (Correspondence) 51,947 (1963).

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1258 R. L. COMSTOCK AND C. E. FAY

FIG. 9. Single-crystal YIG gyrator. The O.072-in. YIG sphere is located between the crossed semiloops.

4. SINGLE-CRYSTAL HYBRID-GYRATOR CIRCULATOR

Multicomponent circulators can be made by com­bining two hybrids and a gyrator in a circuit first de­scribed by Fox.24 Recently, the gyrator element in a multicomponent circulator was made using a small single crystal of YIG.25 Single-crystal YIG of adequate purity is ideally suited to low-temperature applications since its linewidth is lower at 4.2°K than at 300°K.4 The single-crystal gyrator was made using the ferri­magnetically coupled strip-line circuit first described by DeGrasse26 and analyzed in detail by Comstock.27

The strip-line gyrator is shown in Fig. 9. Two half­wave strip-line resonators with centrally located semi­loops are crossed-over and magnetically coupled by the dipolar fields of the ferrimagnetic sphere (d= 0.072 in.) excited in the uniform precession resonance. The resonators are coupled to the external terminals by high impedance quarter-wave lines. The nonre­ciprocal phase shift of the gyrator was measured as ±(900±lS) over a 200-Mc/sec bandwidth centered at 4 Gc/sec. A photograph of the four-port circulator

FIG. 10. Hybrid-gyrator circulator.

24 R. H. Fox, Lincoln Lab. Tech. Rept. No. 68, (1954). ;- 26 R. L. Comstock, Proc. I.E.E.E. (Correspondence) 51, 1768 (1963).

26 R. W. DeGrasse, J. App!. Phys. 30, 155S (1959). 27 R. L. Comstock, in Trans. LE.E.E. Trans. on Microwave

Theory Tech. MTT.12, 599-'1964).

is shown in Fig. 10. The performance of the circulator at 4.2°K is shown in Fig. 11. The reverse isolation from ports 2 to 1 had a bandwidth at the 30-dB isolation points of approximately 50 Me/sec, while the forward insertion loss was less than 1 dB over the same range. With adjustment of impedances near the end of the strip-line resonators all of the four ports operated at the same center frequency. The bandwidth of the circulator is believed to be limited by spurious reflec­tions within the circulator.

5. CONCLUSIONS

The low-temperature properties of ferrimagnets are of increasing technical importance in microwave trans­mission circuits. We have discussed in this paper the cryogenic microwave circulator since for this device detailed comparisons are available relating material properties to device performance Studies have shown that presently available polycrystalline ferrimagnetic materials, principally the magnetic garnets, are ade-

5 INSERTION lOSS I TO 2.

3.~' 3.85 3.90 3.95 4.00 405 4JO 4.15 4.20 425 Ge;...

FIG. 11. Experimental results for single-crystal hybrid-gyrator circulator at 4.2°K.

quate for all but the most severe V-junction circulator applications. For extremely low loss using the below resonance mode of circulator operation it may be neces­sary to develop new low loss materials. These materials will have the following characteristics: (i) magnetiza­tion at 4.2°K less than 500 G, (ii) anisotropy fields (2 I Kl 11M) less than 100 Oe, and (iii) single-crystal linewid ths comparable to YI G « 5 Oe).

An alternative solution to the cryogenic circulator problem consisting of a hybrid-gyrator circulator with a single-crystal gyrator was discussed. This four-port circulator has similar performance to the V-junction circulator but is not believed to possess the potential for wideband operation.

ACKNOWLEDGMENT

The authors wish to acknowledge the assistance of M. Loewy in the construction of the devices and in taking much of the data reported here.

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