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New Self Biased Circulators
High internal (anisotropy) magnetic jields in fem'tes are
exploited to design microstrip and waveguide junction circulators
near 31 GHz which provide over 20 dB isolation, below 1 dB loss,
yet require no external magnets.
Jerald A. Weiss Massachusetts Znstitute of Technology Lincoln
Laboratoiy Lexington, Massachusetts and The Department of Physics
Worcester PoEytechnic Znstitute Worcester; Massachusetts
Nigel G. Watson Gerald F. Dionne Lincoln Laboratoiy
his paper describes our development of mil- limeter wave
junction circulators using as T the nonreciprocal (gyrotropic)
element ma-
terials of the magnetoplumbite (or hexagonal-fer- rite, or
uniaxial-ferrite) type. Our principal objec- tive has been to
explore the feasibility of eliminating altogether the need for an
external dc bias magnet. This idea has been appealing and dis-
cussed in the literature as long as these distinctive materials
themselves have been available for micro- wave applications, since
the 1950's.
Today, system design has been transformed by the introduction of
microwave integrated circuits with components on a miniature scale.
But success in the miniaturization of nonreciprocal devices has
lagged far behind, to the point where the circula- tors, for
example, now tend to appear as hulking monsters beside the
intricate but miniature circuits they are intended to serve.
Systems designed today use miniature scale integrated circuits
but the miniaturization of nonreciprocal devices has lagged far
behind.
74 APPLIED MICROWAVE Fall 1990
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To be sure, some inexorable physical laws stand in the way -
such as the elementary relation be- tween frequency and wavelength.
Plenty of room still exists, however, for design efficiency and
inge- nuity within those constraints. Indeed, great pro- gress has
been achieved and is continuing in the effort to minimize size and
weight, as may be seen in modern products of the ferrite device
industry.
Elimination of the external magnet is a significant step toward
more compact circulators.
Nonetheless, elimination of the external magnet wves as a very
significant step toward more com- pact device designs. In this
paper we describe the principles underlying this idea and the key
relations governing the ferrimagnetic resonance effect on which the
self biased circulator depends.
We discuss the relevant properties of the materi- als -
specifically, high magnetocrystalline aniso- tropy and high
coercivity, as well as other qualifica- tions which the materials
must have for microwave use. Two device configurations have been
built, rec- tangular-waveguide and microstrip junction circu-
lators for millimeter wave use.
This work has also demonstrated the unexpected possibility of
adopting for millimeter-wave applica- tions certain sintered
hexagonal ferrites which are widely marketed today for a variety of
commercial purposes, but were not formulated with microwave
applications in mind.
This work demonstrated the unexpected possibility of adopting
certain fem'tes not formulated with microwave applications in
mind.
Historical Background Byway of historical background references
[l-161
represent a selection keyed to this paper. There has been much
more written about this subject, and we would be happy to share our
extended bibliography with interested readers.
Polder's theoretical analysis of resonance in ma- terials having
gyromagnetic properties, including definition of the permeability
tensor now known by his name, was published in 1949. Rathenau
pre-
sented details of the basic magnetic properties of barium
ferrite and of derivative magnetoplumbites in 1953. In 1955 his
associates Smit and Beljers at the Philips laboratories in
Eindhoven, the Nether- lands, published a thorough analysis of the
theory of magnetic resonance for partially and fully mag- netized
anisotropic materials, together with experi- mental measurements on
barium ferrite. That same year, M.T. Weiss and P.W. Anderson at
Bell Tele- phone Laboratories briefly presented resonance re- sults
on Ferroxdure (a Philips tradename), together with a similar
formulation of the theory.
In 1959, Casimir reviewed ferrite research at Philips in a
report that included important data on the temperature dependence
of magnetic param- eters of the magnetoplumbites. Progress in the
areas of materials and device research and develop- ment was
reviewed at successive stages by Rodri- gue, Pippin and Wallace in
1962, by Wijn in 1970, by Okazaki, Horiguchi and Akaiwa in 1974, by
Winckler and Dotsch in 1979, and by Harrison in 1981.
Perhaps the earliest device exploiting oriented hexagonal
ferrite, a millimeter wave resonance iso- lator, was reported by
Kravitz and Heller of MIT Lincoln Laboratory in 1959. They used
Indox V, one of the commercial materials similar to those on which
we are reporting. A dc magnetic field of 8 kiloersteads was
required.
A 73.5 GHz waveguide junction circulator with- out a magnet was
presented by Akaiwa and Oka- zaki in 1974. So far as we know,
theirs was the first reported successful embodiment of the idea of
eliminating the external magnet.
Present Approach Our waveguide circulator, at 31 GHz, is
adapted
from a design due to Piotrowski and Raue, 1976, chiefly by
replacing the magnetically isotropic ele- ment by a hexagonal
ferrite. Our microstrip junc- tion layout takes its design style
from the type ex- emplified by G.P. Riblet in 1980 and by Xu and
Miao in 1988.
The relevant design principles are contained in Appendix A. This
gives in conventional notation, a simplified form of the magnetic
energy density G (properly, the free enthalpy density) of the
speci- men, including the anisotropy energy for the uniax- ial case
(KA is the anisotropy energy constant); magnetic field energy in
terms of the applied field Ho and magnetization M; and the
demagnetization correction to the magnetic field energy, in terms
of M and the three principal demagnetizing factors N.
APPLIED MICROWAVE Fall 1990 75
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Hamilton’s equations are applied to derive the equations of
motion for small oscillations due to stimulation by a microwave
electromagnetic field, and to solve for the condition of
ferrimagnetic reso- nance. Here nu is the resonance frequency
(Hertz) and gamma is the electron gyromagnetic ratio, the value of
which is shown in Appendix A.
We limit ourselves to the simplest cases which are relevant to
the circulators of interest. Here, the orientations of both the
easy axis of anisotropy and the resultant magnetization are in the
direction of the axis of cylindrical symmetry of the specimen, that
is Nx = Ny = Nt (transverse).
Illustrating with a typical example, we find for barium ferrite
with no dc field (Ho = 0), for a thin disk the resonance frequency
is 36.4 GHz and for an ”equilateral” cylinder it is 47.6 GHz. These
reso- nance frequencies are above the operating frequen- cy of 31
GHz for our devices - a relationship termed ”above resonance
operation”.
However the nonreciprocal performance of the devices depends on
the parameters mu and kappa of the Polder permeability tensor,
whose influence ranges far above and below resonance. We believe
that our circulators are operating in the “above resonance” regime;
that is, with the effective inter- nal dc field somewhat on the
high side of the reso- nance value for the frequency of
operation.
A photograph of our waveguide three port junc- tion circulator
is shown assembled (left) and open (halves at right) in Figure 1..
The ferrite elements are two cylinders of a commercial strontium
ferrite
made by the D.M. Steward Co., called F-520, with orientation
axis aligned with the junction symmetry axis.
The structure, an adaptation of that by Pio- trowski and Raue,
is shown in Figure 2. wherein nickel ferrite used by those authors
has been re- placed by the magnetoplumbite F-520. There are two
ferrite cylinders separated by a metal septum, and an arrangement
of dielectric spacers and metal matching transformer. After
assembly, the material is brought to the remanent state with use of
a labo- ratory electromagnet, following which the circula- tor is
then removed from the electromagnet and operated with no external
magnet of its own.
A somewhat different structure was used by Akaiwa and Okazaki in
their barium-ferrite wave- guide circulator at 73.5 GHz with no
magnet. The combination of parameters suggests that their de- vice
operated in the “below resonance” regime, a different mode compared
to ours which could have favorable significance with regard to
bandwidth and temperature stability for some applications.
A network analyzer plot showing good isolation peaks for the
three ports at about 30.7 GHz is shown in Figure 3. Other data we
have obtained for nominally the same structure show slightly poorer
symmetry but better insertion loss, less than 0.5 dB over about a
1.5 percent bandwidth.
Consider the oriented hexagonal ferrites. In Fig- ure 4 we have
an example of a hysteresis loop for a highly oriented
barium-strontium ferrite made by the Stackpole Corp., called S-7.
This is an admira-
Figure l. Photograph of the assembled (len) and disassembled
three port, Self biased, waveguide circulator.
76 APPLIED MICROWAVE Fall 1990
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Figure 2. Plan view of the self biased waveguide circulator
(after Piotrowski & Raue).
0
29.8 30.4 31.0 31.6 FREOUENCY (QHr)
Figure 3. Measured insertion loss and isolation at 3 ports for
the waveguide circulator.
Oe
Figure 4. The hysteresis loop for a highly oriented bari-
um-strontium ferrite, Stackpole 5-7.
APPLIED MICROWAVE Fall 1990 77
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bly square loop with high coercive field Hc of about 3700 Oe;
the remanent magnetization Mr is3475 G.
Note the part of the loop in the second (upper left) quadrant,
which permanent magnet specialists call the “demagnetizing curve”.
This is shown in detail in Figure 5. for the Stackpole S-7
material.
The demagnetizing line indicated in Figure 5 has a slope of -1,
appropriate for a thin disk; magnetiz- ation is about 3150 G and
the demagnetizing field is of equal magnitude, 3150 Oe. This
material has anisotropy field HA = 21 kOe (Compared to 17 kOe for
barium ferrite). Resistivity is at least 1 million ohm-cm, probably
substantially higher.
The density is about 90% compared to the nomi- nal X-ray value
of 5.14 gramsicubic centimeter for this composition. The degree of
orientation must be very high, judging by the squareness of the
hyster- esis loop. All these properties are favorable for mi-
crowave applications and lend themselves particu- larly well to
planar circuit designs.
In Figure 6 is shown the demagnetizing curve for the D.M.
Steward Co. material called F-520 which was used in the both of the
circulators presented in this paper. The intrinsic remanence is
high, about 4000 Gauss; the coercivity is 1280 Oe. The intersec-
tion of the curve with the diagonal demagnetizing line represents
the remanent state of the specimen. The slope is -3, appropriate
for an “equilateral” cylinder.
0
Figure 5. Detail view of the “demagnetizing curve’’ (up- per
left quadrant of the hysteresis loop) for Stackpele s-7
material.
78 APPLIED MICROWAVE Full 1990
Figure 6. Demagnetizing curve for the Steward F-520 ma- terial
used for the waveguide and microstrip experimen- tal
circulators.
This shows a stable state of high magnetization, about 3500 G,
with an internal reverse field of 1270 Oe. The waveguide junction
design calls for a pair of such cylinders close together on a
common axis which, being axially longer, actually confers a some-
what more favorable magnetic state (that is, weaker internal
demagnetizing field) than this demagnetiz- ing line would
indicate.
Since temperature stability is an important sys- tem
requirement, we must consider the tempera- ture dependence of the
magnetic parameters, illus- trated for barium ferrite in Figure 7.
The graph, showing saturation magnetization, anisotropy ener- gy
parameter, and effective anisotropy field, is from the work of
Casimir as cited in the handbook of microwave ferrites by von
Aulock. Although 4sMs and K1 are steeply declining functions of
tempera- ture, the anisotropy field HA, which is equal to the ratio
KI/Ms tends to he relatively temperature sta- ble.
Isolation was better than 20 dB from 0 to 50 degrees C, but at
-20 C it was poox
The net influence on the waveguide circulator performance is
shown in Figure 8. Over the range from 0 to 50 degrees Centigrade,
isolation remains
-
- 6
2 - - 4 Lo
7 P
- 2
. - 0
0 200 400 600 800 TEMPERATURE (K)
Figure 7. Temperature dependence of the magnetic pa- rameters
for the barium ferrite.
greater than 20 dB, but at -20 C it is poor. Insertion loss
remains better than 2 dB over the entire tem- perature range
investigated.
The photograph of our microstrip 31 GHz circu- lator which also
operates with no magnet is shown in Figure 9. Actually, the
junction overlay pattern is not fully shown here. Figure 10. shows
the layout together with a sectional view. Note that, because we
were working with the relatively low-coercivity D.M. Steward
material, we obtained the benefit of a lower demagnetizing field by
making a socket in
301 V 40 I
25x6 30.4 a1.0 31.6 FREOUENCY (GHz)
Figure 8. Insertion loss and isolation measured over temperature
for the waveguide circulator.
82 APPLIED MICROWAVE Fall 1990
Figure 9. Photograph of the microstrip 31 Ghz circulator.
the ground plane and inserting one of our equilat- eral cylinder
specimens. The substrate is 10 mils thick; the ferrite is 60 mils
in both axial length and diameter.
The merit of this strategy to overcome the limita- tion of low
coercive field is thus demonstrated; we also hope to show that
substitution of a suitable oriented material of sufficiently high
coercivity, at least equal to remanent magnetization 47rMr, would
allow the use of a thin disk. The overlay pattern illustrated here
is of the type described by G.P. Riblet. We are also working with
the type of pattern, of Xu and Mido, shown in Figure II.
In some devices symmetly and insertion loss were problems, but
we believe they stem from mechanical defects of the models, not
fundamental limitations.
Representative performance for the microstrip circulator is
shown in Figure 12. The curves of in- sertion loss and isolation
show best performance at about 31 GHz. In some of our experimental
de- vices, symmetry and insertion loss were problems, but we
believe they stem from defects in the me- chanical structure of the
experimental models and are not fundamental to the approach or
materials.
Expectations Self biased circulators such as this one but at
different frequencies could be scaled provided the disc size is
appropriately adjusted - inversely pro- portional to frequency
together with a correction for any wavelength change due to a
different effec-
-
JUNCTION OVERLAY
STRIP. / K CONNECTORS
GROUND PL ENT
JUNCTION: CONDUCTOR CONFIGURATION
Figure 10. Layout drawings of the overlay pattern and assembly
section for the microstrip circulator.
Y Circulator Strlpllne Figure 11. An alternate microstrip center
conductor pat- lern being explored.
40 29.0 30.0 31.0 32.0 33.0
FREQUENCY (GHZ)
Figure 12. Insertion loss (upper curve) and isolation (lower
curve) for the microstrip circulator.
APPLIED MICROWAVE Fall 1990 83
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tive permeability and dielectric constant. Further- more, the
effective internal magnetic bias field must be suitably adjusted.
This adjustment involves the magnetocrystalline anisotropy field
(HA), the saturation magnetization and other properties of the
material.
We believe that these prototype devices have only just begun to
indicate the possibilities which might be achieved. Adaptation to
higher and lower frequencies and improvements in bandwidth, inser-
tion loss, and isolation, combined with more com- pact and more
light-weight designs, are feasible. The promise of materials of the
magnetoplumbite class for the future of the microwave art appears
to be at least as bright as ever.
This work was sponsored by the U.S. Air Force.
References 1. Polder, D.: On the theoq of ferromagnetic
resonance;
Phil. Mag. 40 (1949) p. 99. 2. Rathenau, G.W.: Saturation and
magnetization of hexag-
onal iron oxide compounds; Rev. Mod. Phys. 25 (1953) p. 279.
3. Smit, J. & Beljers, H.G.: Ferromagnetic resonance absorp-
tion in BaFel2019, a highly anisotropic crystal; Philips Res. Rep.
10 (1955) pp. 113-130.
4. Weiss, M.T. &Anderson, P.W.: Ferromagnetic resonance in
Ferroxdure; Phys. Rev. 98 (May 15, 1955) pp. 925-926.
5. Casimir, H.B.G. et al.: Rapport sur quelques recherches dans
le domaine du magnetisme aux Laboratoires Philips; J. Phys. Radium
20 (1959) p. 360.
6. Rodrigue G.P., Pippin, J.E. &Wallace, M.E.: Hexagonal
ferrites for use at X to V hand frequencies; J. Appl. Phys. 33s
(Mar. 1962) pp. 1366-1368.
7. Wijn, H.P.J.: Hexagonal ferrites; Landolt-Boernstein, Group
111, vol. 4h, Sec. 7, p. 547 (1970). [8 ] Okazaki, T., Horiguchi,
Y. & Akaiwa, Y.: Characteristics of polycrystal- line hexagonal
ferrites for use at millimeter-wave frequen- cies: Elec. Comm.
Japan 57-C (July 1974) pp. 128-136.
9. Winkler, G. & Doetsch, H.: Hexagonal ferrites at millime-
ter waves; Ninth European Microwave Conf. (1979).
10. Harrison, G.R.: Hexagonal ferrites for millimeter-wave ap-
plications; SPIE - Intl. Soc. for Opt. Eng. 317 (Integrated Optics
and Millimeter and Microwave Integrated Cir- cuits); Huntsville AL,
Nov. 16-19, 1981; pp. 251-261.
11. Kravitz, L.C. & Heller, G.S.: Resonance isolator at 70
KMC; Proc. IRE 47 (1959) p. 331.
12. Akaiwa. Y. & Okazaki. T.: An aoolication of a
hexaeonal
13.
14.
ferrite to a millimeter wave Y ‘iirculator; IEEE frans. MAG-I0
(1974) pp. 374-378. Piotrowski, W.S. & Raue, J.E.: Low-loss
broad-band EHF circulator; IEEE Trans. MTT-24 (1976) pp. 863-866.
Rihlet, G.P.: Techniques for broad-handing above-reso- nance
circulator junctions without the use of external matching networks;
IEEE Trans. MTT-28 (1980) pp. 125- 174
15. Y. & Miao. J.: Theorv and desien of stub tuned
nonrecioro- cal stripline junctidns and hrculators; Microw.&
Opt. Tech. Let. 1 (19881 351-356.
16. von Aulock, W.H.: Handbook of Microwave Ferrite Mate- rials;
Academic Press, 1965: Sec. 4.
17. Weiss, Watson & Dionne: This work was described in “New
Uniaxial-Ferrite Millimeter-Wave Junction Circula- tors”; IEEE
MTT/S Intl. Microwave Symp., Long Beach CA, June 13, 1989.
84 APPLIED MICROWAVE Fall 1990
~ ~~
Jerald A. Wciss received the BS and MS degrees in physics from
the Ohio State University in 1949 and the PhD degree inphysics,
also form Ohio Stare, in 1953. From 1953 to 1960, he was a member
of the technical staffof Bell Telephone Laboratories (now AT&T
Bell Labs) at Murray Hill, NJ, where he engaged in microwave com-
ponent development with applications of magnetic materials. He
joined in the founding of Hylerronics Cop. for design and
manufacture of micro- wave devices and subsystems in 1960. I n 1962
he joined the faculty of the Physics Department at Worcester
Potytechnic Institute, and that year he was also appointed as
consultant at MIT Lincoln Laboratory in work relating 10 array
radar development.
Professor Wei.s.7 has conducted academic research in a number
offields and has been active in physics education and educational
innovation. He has published on subjects including transmis.sion
line theory; instrumentation, measuremenr.s and applications of
magneti.sm; and microwave and millimeter wave !he09 measure- ments
and devices.
Nigel Watson joined the MITLin- coln Laboratory in 19870s a
member of the Space Radar Technology Group. He has been involrsed
in the development of lighnveigtit TIR mod- ule design al Rayrheon
Equipment Di- vi.7ion and Spew Coporation. Mr Watson received a BS
in physics fmm CIarkson College of Technology in 1979 and an
MSEEfrom the Univer- siw of Massachusett.7, Amherst, in 1981. He is
a member of the IEEE and the MTT-S.
Gerald F. Dionne received a BSc degee from Concordia University
in 1956, a BEngfrom McGill University in 1958, and an MS in physics
from Carnegie-Mellon University in 1959. A f m two years of
semiconductor de- vice development work with IBMand GTE, he
returned to McCiil for doc- toral work in physics, with a thesis in
electron paramagnetic resonance. From 1964 lo 1966, he carried out
research in electron emission and sur- face ionization for
thermionic energy conversion at United Technologies.
Since 1966 he has been a member of the technical staff at MIT
Lincoln Laboratory, where he has made nu- merous contributions of
basic and applied research in a broad spectrum of filds that
include magnetism theory, ferrimagnetic materiak for microwave and
millimeter wave applications, secon- dary electron emission for
cold cathodes, submillimeter wave spec- troscopy and radiometry,
and magneto-optic materials and devices forfiber-optic systems.
Most recent& he has been developing theor- ies for
high-temperature superconductivity. DK Dionne i.7 a Senior Member
of the IEEE and a member of
the American Physical Society and Sigma Xi.
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Appendix
The magnetic free energy density G:
ANISOTROPY + APPLIED-FIELD + DEMAGNETIZATION 1 KA sin2 8 - HoM
cos2 8 + ,M2(NS sin2 8 cos2 e+N, sin2 8 sin2 d+N, cos2 8 )
Resonance: for If0 applied parallel to the easy axis
(z-axis),
[fl = [(HO t H A ) + (Nz - Nz)M] [(HO + R A ) + (Nv - Nz)M]
where 7 = 2.80 MHz Oe-' , H A = % For cylindrical symmetry (N, = N,
= Nt):
Y - = [(HO + H A ) + (Nt - Nz)M] 7
Example: Let El0 = 0, H A = 17 kOe, 4rM = 4000 G (Ba
ferrite):
Thin disk (Nt = O,N, = 4r) v = 36.4 GHz "Equilateral" cylinder
(Nt = N, = 9) v = 47.6 GHz
2 .
APPLIED MICROWAVE Fall 1990 85