A magnetostatic wave tunable resonator which has fine stripe electrodes and bonding electrode films formed on the surface of a YIG thin film supported by a GGG substrate. The stripe electrodes, bonding electrode films and edges of the YIG film are formed by a chemical etching method based on photolithography. The resonator so formed has suppressed spurious resonance of high order modes and enables the use of a magnet of greatly reduced size.

BACKGROUND OF THE INVENTION 
(1) Field of the Invention 
The present invention relates to a frequency tunable magnetostatic wave 
resonator which utilizes magnetic spin resonance of yttrium iron garnet 
(hereinafter referred to as YIG) film. More specifically, the invention 
relates to a wide-band frequency tunable integrated resonator that is 
produced from a thin YIG film grown on a gadolinium gallium garnet 
(hereinafter referred to as GGG) substrate by photolithography, the 
wide-band frequency tunable integrated resonator being adapted to be 
mounted in a small metallic package. 
(2) Description of the Prior Art 
As a magnetostatic wave resonator utilizing the magnetic spin resonance of 
YIG, the prior art has shown a small resonator in which a magnetic bias 
field is applied in the direction of YIG film to utilize magnetostatic 
waves that propagate through the YIG film (see IEEE, Ultrasonic Symposium, 
1984, pp. 164-167). 
The above known resonator resonates on a 12-GHz band using a 4 mm.times.1.4 
mm square YIG/GGG chip as shown in FIG. 2. 
In this case, however, it is not allowed to place on the chip a transducer 
which excites the magnetostatic waves, making it difficult to unitize the 
whole structure and to mount it in a small package. Further, consideration 
has not been given in regard to bringing the chip into alignment with the 
transducers. 
The object of the present invention, therefore, is to realize a resonator 
which can be fabricated maintaining high precision and which requires a 
magnet of a small size. 
SUMMARY OF THE INVENTION 
In order to achieve the above-mentioned object according to the present 
invention, fine stripe electrodes for exciting the magnetostatic waves are 
formed on the surface of the YIG film and straight edges of the YIG film 
for reflecting the magnetostatic waves are constituted on the GGG 
substrate. 
Employment of this structure makes it possible to form the stripe 
electrodes and YIG straight edges by the chemical etching method based 
upon photolithography, contributing to improve the machining precision to 
a degree that is equal to the dimensional precision on the mask surface. 
Formation of a plurality of stripe electrodes on the surface of the YIG 
film helps suppress the spurious resonance of a high order mode. In this 
case, the machining precision on the reflection surface of the YIG 
straight edges may be roughly the same as the width of an electrode 
finger. The magnet which applies a magnetic bias field can be so located 
that the magnetic pole surface thereof comes into contact with the chip, 
enabling the size of the magnet to be greatly reduced.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
An embodiment of the invention will now be described in conjunction with 
FIG. 1, wherein FIG. 1(a) is a side view and FIG. 1(b) is a plan view. A 
YIG film 2 is grown by liquid phase epitaxy on a GGG substrate 1, and fine 
stripe electrodes 3 of thin aluminum film are formed on the surface of the 
YIG film 2 by photolithography. 
The stripe electrodes are much narrower than a pitch P, and both ends 
thereof are connected in parallel, thereby to form a four-terminal device 
in which a terminal A and a ground point receive inputs, and a terminal B 
and a ground point produce outputs. Portions of the thin YIG film 2 more 
remote than the lenths L.sub.1 and L.sub.2 of the straight edges are 
removed by chemical etching at both ends that are in parallel with the 
stripe electrodes 3. 
As a magnetic bias field H.sub.O is applied in the direction of YIG film 2, 
i.e., in the direction of stripe electrode fingers 3, the magnetostatic 
surface wave propagates in the direction of a right-handed thread on the 
YIG film 2 and through a boundary between the YIG film and the GGG 
substrate. The magnetostatic surface wave rotates in the clockwise 
direction at the right end from the surface into the boundary, and from 
the boundary into the surface at the left end, and resonance takes place 
at a frequency at which the total propagation length corresponds to a 
product of an integer number of the wavelength. 
The resonance frequency can be varied by changing the magnetic field 
intensity. Therefore, the lengths L.sub.1 and L.sub.2 must be an interger 
number times 1/2 of the electrode finger pitch P. For the purpose of the 
simplest explanation, this embodiment deals with the case where L.sub.1 
=L.sub.2 =P/2. 
FIG. 3 explains another embodiment of the present invention. The 
magnetostatic wave is reflected by the straight edge cut by a scriber at 
the end of the YIG film. Experiment proved that a sufficiently sharp 
resonance response could be obtained when the coarseness of the straight 
reflection edge was of the order of when it was cut with the scriber, 
i.e., when the coarseness of the straight reflection edge was of the order 
of the electrode finger width, as will be described later. 
The magnetic bias field applied to the YIG film must be as uniform as 
possible. Experiment proves that nonuniformity in the magnetic field 
deteriorates the sharpness of resonance response. It is therefore 
necessary to use a driving coil and a magnet that applies magnetic field 
having a sufficient uniformity of intensity distribution over the YIG 
film. It will be obvious that the structures of both FIGS. 1 and 3 makes 
it possible to reduce the sizes of the magnet and driving coil compared 
with those of the conventional example of FIG. 2. 
FIG. 4 shows an experimental example of the resonator of the invention. 
Three to five aluminum electrode-fingers having a width of 30 .mu.m are 
formed on a 20 .mu.m thick YIG film maintaining a pitch P=300 .mu.m, and a 
magnetic bias field H.sub.O =350 Oe is applied thereto. The thus obtained 
chip is placed on a conductor, and a scattering matrix parameter S.sub.21 
is measured with the terminal A and the conductor 6 shown in FIG. 1 as 
input terminals and the terminal B and the conductor 6 as output 
terminals. A main sharp resonance appears at 2.42 GHz when the loaded Q is 
Q.sub.L =1000. A resonance point in synchronism with the pitch period P is 
also observed at 2.94 GHz, which, however, is smaller than the main 
resonance. The main resonance is very close to a frequency f.sub.c at 
which the propagation of the magnetostatic surface wave is cut off in a 
low-frequency region, 
EQU 2.pi.f.sub.c =.sqroot..gamma.H.sub.i (.gamma.H.sub.i +4.pi.M.gamma.)(1) 
H.sub.i : internal magnetic bias field in the YIG film, 
4.pi.M: saturation magnetization of YIG (1760 G), 
.gamma.: gyro-magnetic ratio 2.pi..times.2.8 MHz/Oe, and it is considered 
that the resonance takes place at a frequency f.sub.R at which the 
propagation length of the surface wave and the boundary wave become equal 
to one wavelength. The frequency f.sub.R is close to the frequency f.sub.c 
and remains nearly constant even when the overall length of the YIG film 
is slightly changed. Error of L.sub.1 =L.sub.2 =P/2 decreases the loaded Q 
of main resonance. The same resonance characteristics are observed even 
when the scattering matrix parameter S.sub.11 is measured by connecting 
the terminal B and conductor 6 of FIG. 1 together, and using the terminal 
A and the conductor as two terminals. When the terminal B and the 
conductor 6 are opened, the resonance response is intensified owing to a 
relationship relative to the characteristic impedance of the measuring 
system. To change the resonance frequency, H.sub.i of the equation (1) 
should be changed, i.e., the external magnetic field H.sub.O should be 
changed. In FIG. 4, the frequency f.sub.R becomes 2.50 GHz if the external 
magnetic field H.sub.O is set to H.sub.O =375 Oe. 
The above experimental results indicate the fact that a plurality of 
electrode fingers suppress the spurious resonances of high-order modes 
other than the resonance in synchronism with the pitch. Therefore, the 
electrode fingers 3 do not need to be periodically arranged. It is desired 
that the electrode strips are symmetrically arranged with respect to a 
center line 4 of FIG. 1. 
Experiments further indicate that areas of the electrode 5 connecting the 
bonding wires determine the magnitude of rejection of the response 
characteristics. It is necessary to optimize the electrostatic capacity of 
the bonding electrodes 5 with respect to ground. 
A further embodiment of the present invention will now be described. The 
external magnetic field is applied in the direction of the surface of the 
YIG film perpendicularly to the stripe electrodes. FIG. 5 shows a 
scattering matrix parameter S.sub.11 of when the magnetic field H.sub.O 
=350 Oe is applied to the same sample as that of FIG. 4, the terminal B is 
connected to the lower conductor, and the terminal A and the conductor are 
used as two terminals. In this case, the magnetostatic backward volume 
wave is excited in the YIG film, and the main resonance takes place at 
f.sub.R =2.26 GHz. A high cut-off frequency of the magnetostatic backward 
volume wave is equal to f.sub.c of the equation (1), and the main 
resonance frequency f.sub.R is lower than f.sub.c and is close to f.sub.c. 
The backward volume waves are reflected by both straight edges of the YIG 
film, and are transformed into standing waves to resonate. 
FIG. 6 shows a futher embodiment of the present invention which deals with 
a resonance that uses magnetostatic backward volume waves by opposing two 
sets of stripe electrode fingers to each other in the staggered form to 
facilitate the impedance matching. FIG. 7 shows a scattering matrix 
parameter S.sub.21 of when the inputs are input to the terminal A and to 
the ground point, and the outputs are produced from the terminal B and the 
ground point. The attenuation quantity of the blocking region is greatly 
improved compared with the case of FIG. 5. Insertion loss increases in the 
pass region, which, however, is improved by matching the input and output 
circuits.