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Magnetic epitaxial films: Applications and potential
Thin Solid Films, 39 (1976) 185-194 © Elsevier Sequoia S.A., L a u s a n n e ~ r i n t e d in the Netherlands
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M A G N E T I C E P I T A X I A L FILMS : APPLICATIONS A N D P O T E N T I A L * H. LESSOFF AND D. C. WEBB Naval Research Laboratory, Washington, D.C., 20375 (U.S.A.) (Received April 23, 1976; accepted July 30, 1976)
Magnetic films in amorphous, polycrystalline and single-crystal form have shown promise for many years as a method for miniaturizing magnetic components in the design of electronic devices. With the exception of a few specialized applications, e.g. thin film Permalloy memory elements and magnetic tape heads, widespread use of these films has not yet been realized. Recent advances in the preparation of high quality non-metallic single-crystal garnet films have brought a number of important components much closer to reality. The most immediate widespread use of the single-crystal magnetic film is bubble memory devices. These devices exploit recent advances in the magnetic garnet technology and achieve an extremely high data storage density. They are expected to supplant mechanical tape and disc recorders in some applications soon. Another class of single-crystal magnetic film devices with great potential but not as well developed as the bubble devices are the planar yttrium iron garnet microwave devices. Included in this category are limiters, filters and magnetically tunable oscillators, which are compatible with conventional microwave integrated circuits. Magnetostatic surface wave devices represent another class of devices which rely upon high quality single-crystal magnetic films. These devices are capable of carrying out signal processing functions such as fixed, variable and tapped delay, convolution and pulse compression directly at microwave frequencies. This paper concentrates on the material parameters which are most important in the bubble, filter and magnetostatic surface wave applications, on how the parameters can be optimized and on the fundamental limits they place upon device performance.
1. INTRODUCTION
Magnetic films in amorphous, polycrystalline and single-crystal form have shown promise for many years as a means of miniaturizing magnetic components for electronic devices. With the exception of a few specialized applications, e.g. thin Permalloy memory elements and magnetic tape heads, widespread use of these films *Paper presentedat the InternationalConferenceon MetallurgicalCoatings, San Francisco,California, U.S.A., April 5 8, 1976.
186
H. LESSOFF, D. C. WEBB
has not yet been realized. Significant recent advances in the preparation of high quality single-crystal magnetic films have made a number of important devices feasible. We shall explore three classes of these devices: magnetic bubble memories, microwave filters and magnetostatic surface wave (MSSW) delay lines. The operation of each will be reviewed, emphasizing the relationship between important material parameters and the performance that can be achieved. Techniques for growing films with the desired characteristics will be discussed, and finally an assessment of the future of magnetic epitaxial film devices will be made. 2. MAGNETICEPITAXIALFILMDEVICES 2.1. M a g n e t i c bubble m e m o r i e s
The use of ferrite films for memory devices was initially proposed by Pulliam et al. 1 and Archer et al. 2 The structure consisted of an epitaxial film grown on a substrate, then a deposited wire grid and finally an epitaxial film grown on top. The structure acted similarly to a magnetic core array with storage elements occurring at the intersection oforthogonal wires. In 1967, Bobeck 3 suggested and demonstrated the potential of magnetic bubble domains as memory elements. The technology has now evolved to the point where bubble devices are under consideration for many high bit density, medium speed, medium cost, memory applications. Many excellent reviews of bubble devices have recently been given 4' 5 so only a brief description of their operation will be presented here. Physically, bubbles are cylindrical magnetic domains whose magnetization is the reverse of that of the adjacent material. Ones and zeros of a binary stream of data are denoted by the presence or absence of a bubble. A commonly used propagation circuit, the T bar, is shown in Fig. 1. The d.c. bias field is required in order to make the bubbles stable. The in-plane rotating field causes an alternating polarity at the ends of the bars, propelling the bubbles along the circuit. A complete circuit must provide for bubble generation, replication, annihilation and detection as well as propagation. Techniques for realizing all of these functions are now well developed 4' 5. Permalloy
Bars
In~ Plane Rotating
\\\\\\ Magnetic
oarnet GGG ~ Substrate
Fig. l. A T-bar bubble propagation circuit.
~ J ~
t Perpendicular /
Bias Field
MAGNETIC EPITAXIAL FILMS: APPLICATIONS AND POTENTIAL
187
In selecting an optimum bubble memory material composition the most important considerations are bubble stability, speed (average access time) and bit density. The material parameters affecting these properties will be outlined in the following discussion. Bubble stability is determined by the ratio of the anisotropy constant K, to the demagnetizing energy. This important ratio is denoted as the material Q value and is defined by Ku
(1)
O -- 2r~Ms2
Ms is the saturation magnetization of the bubble material. Typically Q is chosen as about 5; this is a compromise between bubble instability and spontaneous nucleation on one hand and ease of generation on the other hand. To minimize power requirements and thermal problems, a minimum in-plane drive field is desired. The material should thus possess a minimum domain wall coercivity Hc (which must be overcome to initiate propagation) and a maximum bubble mobility #. For high speed applications, a maximum bubble propagation velocity is desired. As the drive field intensity is increased, the velocity is found to saturate at a value 247A
(2)
Vp - h(Ku)l/2
where h is the film thickness, usually about half the bubble diameter. Changes in A, K and h affect several other bubble properties and thus cannot be freely changed. However, a "high ~," garnet has been recently reported which raises lip by more than an order of magnitude over conventional bubble materials 6. Another important memory device consideration is the bit density. The optimum (stable) bubble diameter d is approximately
d= 8
(AQ)I/2 Mss
(3)
Q and A cannot be changed significantly so Ms is used to control d. The penalty which must be paid for increasing Ms is that the in-plane drive field must also be increased. In contrast with the microwave devices to be discussed later, the variation of the magnetic properties with temperature is not a significant problem in bubble memories. Here, by utilizing a flexible circuit design and a magnet with a specially selected temperature dependence, moderate temperature variations can be accommodated. From the above discussion it is evident that a great many material parameters must be considered in the design of a bubble memory. A single composition obviously cannot simultaneously optimize all of these, and reasonable compromises, depending upon the application, are necessary.
2.2. Tunable microwave filters Tunable microwave filters consisting of one or more yttrium iron garnet (YIG) spheres in a coaxial, stripline or waveguide structure have been used extensively for
188
H. LESSOFF, D. C. WEBB
both military and test equipment applications for many years. Where low loss, a narrow instantaneous bandwidth and a wide tunable bandwidth are required, this type of filter is used extensively. With recent advances in the epitaxial YIG technology, planar fabrication of high performance multiple filters on a single substrate is now feasible. An example of this type of filter is shown in Fig. 2. Magnetic Film
Magnetic Bias
4 |
/~ ]
~
DielectricSubstrate / / I
Out
Metal Ground Plane
Fig. 2. A two-resonator planar YIG filter.
The important electrical properties of a multiple resonator filter are the individual resonator Q values and resonant frequency, the coupling between individual resonators and the coupling between the bank of resonators and the external circuit (the external Q value). The gyromagnetic resonant frequency tnR of a cylindrically symmetrical body is given by 7 o~ = ?{HA + HK --(NT-- Nz) 4nM~}
(4)
where HA is the applied field, HK the anisotropy field ( = 2l~/Ms) and N T - Nz a geometrical shape (demagnetizing) factor. For a sphere NT--Nz = 0 while for a perpendicularly magnetized disc ART--Nz = 1. For both of these geometries the resonant frequency varies linearly with the applied magnetic field, a factor which greatly facilitates wide band tunability. For YIG sphere filters the resonant frequency can easily be made temperature independent since N T - Nz = 0 and since HK vanishes for certain relative applied field-crystalline axis orientations. Temperature independence is not as simple for the disc resonators since the condition d~_~_ 4rtd~M s
(5)
must be met. Film composition and growth conditions can be adjusted somewhat to accomplish this, but at present it is not clear that adjustments can be made reproducibly without serious degradation of the linewidth. The most significant material parameter for filter applications is the fer-
MAGNETIC EPITAXIAL FILMS: APPLICATIONS AND POTENTIAL
189
romagnetic resonance linewidth AH. A low value of AH has dictated the use of YIG in virtually all applications to date; it is the recent improvements in AH in epitaxial materials which now make planar filters practical. The importance of AH derives from the fact that the loss Lf through a multiple resonator filter is given by s' 9 AH L~ = C 1 7 - -
(6)
Af
All resonators are assumed to have identical values of AH. Cx is a constant determined by the band shape desired; Afis the half-power filter bandwidth. The inter-resonator coupling constants and external Q values (Qe) must be selected in a prescribed manner to achieve an "optimum" frequency response. Qe is inversely proportional to the product o)m ( = ~4rcMs) x Wm (sample volume) where the proportionality constant is a function of circuit parameters only. Some flexibility in circuit design exists, however; for some of the lower values of external Q needed (Q¢ ~ 10) very thick films (approximately 100 lam) are necessary. The interresonator coupling constants are empirically adjusted by the proximity of the adjacent resonators. Another important property of YIG filters is their power-handling capacity. This is determined by non-linearities in the material which occur when the applied r.f. field exceeds a certain threshold value ho.t. In particular (AH) 2 hcrit -
4rtMs
forf
(7a)
and
AH [ AH hcrit = 2 - 1 4 ~ s s ]
/ 1/2 forf>f~
(7b)
where )re = ~m/2 for a perpendicularly magnetized disc 7. For a typical narrow linewidth (approximately 0.50e) Y I G filter, the saturation power is about - 30 dB m f o r f < f ~ and about 0 dB m f o r f > f c . For maximum dynamic range it is desirable to reduce M s; however, a reduction in Ms results in an increase in AH so only a limited change is possible before the loss characteristics are seriously degraded. It is clear that the number of critical material parameters for YIG filters is significantly less than for bubble devices, and the overriding consideration is nearly always the realization of a low linewidth.
2.3. Magnetostatic surface wave devices A final class of devices which we shall consider are the MSSW delay lines. These devices offer the potential of carrying out signal processing, fixed and variable delay and pulse compression, directly at microwave frequencies. A common configuration of this device is shown in Fig. 3. It consists of shorted sections of microstrip transmission line in contact with an epitaxial Y I G film. The bias field is applied in the plane of the film, perpendicular to the direction of propagation. The microwave electromagnetic signal, typically having a frequency of 1-10 GHz, excites an MSSW which propagates in the film to the output microstrip line, which is then reconverted to an electromagnetic wave and which is finally detected.
190
H. LESSOFF, D. C. WEBB
GGG
. J x ~ YIG ~
,/
~
RF Output
ZT"
plan Fig. 3. A magnetostatic surface wave delay line.
The frequency and wavenumber of the MSSW are related by the dispersion equation ~0 G = exp(2kh)
1 1 + (f2s + OH)(1 -- tanh kt) 2(f2s + I2H)+ 1 1 -- (f2s -- f2.) (1 + tanh kt)
(8)
where t is the dielectric substrate thickness and h the magnetic film thickness. s = _ 1 depending on the direction of propagation, I2s = ~/o9m and OH = OgR/Ogm. The group velocity Vg is then given by Vg = -
Re~0G~ff~ lao/& )I
(9)
Vg is typically 106-107 cm S- 1 for films of thickness 1-5 lam. The propagation loss is also found from eqn. (8) by making the substitution HA ---,HA + i AH/2 and employing the relationship = 8.68Im(k) dB c m - 1
(10)
In general ct increases with increasing A H and for frequencies of not less than 3 G H z the loss is11 linearly proportional to AH. Since AHincreases linearly with frequency, the propagation loss does likewise 12. The efficiency and bandwidth of MSSW excitation are complex functions of frequency, applied field, saturation magnetization, film thickness, anisotropy field and circuitry parameters 13. In general a wide excitation bandwidth demands that the width of the microstrip exciter is comparable with the film thickness. This places a lower limit on the group velocity and limits the m a x i m u m delay to about 1 txs. MSSW devices exhibit the same power saturation effects as do Y I G filters except that the critical frequencyf~ is .4 approximately 3.5 GHz. Again an increase in threshold power is possible only at the expense of additional loss. Since all important MSSW properties are complicated functions of Ms and H~, there is no simple technique for stabilizing against temperature fluctuations. Thus the device should be placed in a constant-temperature oven, a temperaturedependent feedback signal should be employed to adjust the bias field or the temperature-induced variations must be tolerated.
MAGNETIC EPITAXIAL FILMS : APPLICATIONS AND POTENTIAL
191
It is evident that the material parameters important for MSSW devices are similar to those of the Y I G filter. The former require somewhat thinner films ( ~< 15 tim) and larger areas. In both devices the linewidth is the most critical parameter, but some adjustment of 4nMs is also often desirable. 3. PREPARATION OF GARNET FILMS The preparation of magnetic oxide films was initially reported by Miller 15 in 1955 ; however, very little work was done in this field until Cech and Alessandrini ~6 prepared oxide layers of iron, cobalt and nickel by chemical vapor deposition (CVD). The decomposition of metal halides in an oxidizing atmosphere was used. Work was then initiated by a number of investigators to prepare epitaxial films for direct replacement of those magnetic components which used bulk materials. Devices considered initially for miniaturization included memory cores, inductors and microwave filters and circulators. An excellent review of the CVD process is given by Mee e t al. ~ 7 Deposition is carried out in a reactor, a simplified representation of which is shown in Fig. 4. The process involves heating anhydrous metal halides, normally chlorides, in an inert gas stream such that the metal halide vapors are carried into a reaction chamber. The vapor stream is mixed with a second stream containing water vapor and oxygen in the reaction zone. The metal halides are oxidized and/or hydrolyzed to form the metal oxides and the corresponding halide and oxyhalide acids. Reaction temperatures are in the range 1000-1300 °C. The reaction process is not simple, and various oxides form in different sections of the reaction chamber. By placing a substrate crystal in the proper segment of the reaction chamber, the desired oxide grows on the substrate. The substrate is normally a non-magnetic oxide having a structure similar to the depositing species. T n~" 7 0 0 ° HejHCl
T2 ~ 9 0 0 °
T 3 ~ 1125"
J
He'O2'H20,I /
Exhaust
/
Source FeCl~
/
j
Source YCI3
7 Subltrote
Fig. 4. A reactor for CVD magnetic oxide growth.
The devices described in this paper all employ garnet films commonly grown on gadolinium iron garnet or other non-magnetic garnet substrates with the lattice parameters tailored to match the magnetic material. Growth of the magnetic oxides by CVD is not simple since source temperature, gas flow rates, reaction temperature and substrate location must all be rigorously controlled. Fortunately an alternative to CVD now exists. Approximately five years after epitaxial growth was achieved by Cech and Alessandrini 16, Linares e t al. ~8 demonstrated that Y I G epitaxial films could be prepared by having a gadolinium gallium garnet ( G G G ) substrate in contact with a fluxed melt containing yttrium
192
H. LESSOFF, D. C. WEBB
oxide and iron oxide. The process, known as liquid phase epitaxy (LPE), is quite simple once the growth conditions are established. It is very similar to processes used in the semiconductor industry for III-V material growth such as GaAs. The flux systems used are PbO-B203-Fe203 (ref. 19), BaO-BaF2-B203 (ref. 20) and BiEOaRO 2 where R is Si, Ge, Ti or Ce (refs. 21, 22). Table I presents some of the advantages and disadvantages of the various systems. TABLE I FLUXES(SOLVENTSUSEDFOR GARNETLPE)
Advantages
Disadvantages
Low temperature
Corrosive Volatile constituent Lead inclusions
BaO-BaF2-B203
No lead inclusions Non-volatile
High growth temperatures Poor solvent removal
Bi2Oa-RO 2
No lead inclusions Low to moderate temperatures
Poor solvent removal Possible inclusion of R (R ~ Si, Ge, Ti, Ce)
PbO-
B203-Fe203
The current growth technique using LPE involves dipping a substrate held either horizontally or vertically into the supercooled melt under isothermal conditions. The degrees of substrate rotation and melt saturation help to determine film uniformity and thickness. Growth occurs in a period of seconds to minutes depending on the flux, temperature and thickness of film required. The substrate is then removed from the melt, often with rapid rotation or pulling in order to remove excess flux on the film. The substrates used for LPE are similar to those used in the CVD method; however, since the number of parameters for growth are less for the LPE system compared with the CVD system, costs appear to be significantly lower. The growth temperatures range from 700°C to 1000 °C for the lead oxide and bismuth oxide melts--considerably lower than in CVD. This results in less strain and diffusion at the surface-film interface and hence higher quality magnetic films. For the above reasons, although both growth techniques yield uniform properties and well-controlled thickness, CVD has b e e n largely replaced by LPE for both bubble and microwave devices. Y I G (Y3FesO1 a) grown on a G G G (Gd3GasO1 a) substrate is most commonly used for filters and MSSW delay lines because of its low ferromagnetic resonance linewidths. Using high quality substrates and adding small amounts of lead to the flux to provide a good lattice match between film and substrate, linewidths superior to bulk (flux-grown) YIG have been achieved. The saturation magnetization is commonly reduced where needed by substituting gallium for iron; this is accompanied by an increase in linewidth, however. YIG also serves as the basic material for most bubble applications--with partial substitution for yttrium and/or iron. One commonly used combination is Y3_~LaxFes_yGayO~v Again gallium is used to reduce Ms, while La is used to adjust the lattice constants. The material has a high wall mobility and a Q value of about 4. Another widely used
M A G N E T I C E P I T A X I A L FILMS ~ A P P L I C A T I O N S A N D P O T E N T I A L
193
bubble material is YSmCaGe garnet with Q,~7 and excellent temperature characteristics 4. For high speed applications, a garnet which incorporates Eu, Ca, Si and Ge into the basic YIG structure looks attractive 6. 4.
A N ASSESSMENT OF E P I T A X I A L M A G N E T I C FILM DEVICES
Bubble memories are envisioned as filling a role intermediate in both cost and speed between semiconductors at the high end of the spectrum and tape, disc and drum memories at the low end. A developmental 100k bit chip has been manufactured which is cost competitive with comparable capacity semiconductor memories. To be a commercial success, however, the bubble device must overcome strong challenges from competing techniques, e.g. charge-coupled devices (which use the well-established silicon technology) and tape and disc storage (whose cost is still decreasing). Low costs are projected for bubble memories compared with semiconductor memories because there are fewer processing steps; however, bubbles also require smaller linewidths for a given bit density~largely eliminating the cost advantage. The commercial success of the bubble memory will probably depend upon the successful development of more compact structures such as the recently proposed bubble lattice and contiguous file systems 4. The first applications of bubble devices will be in areas where the unique properties of the bubble memories are utilized rather than in areas where there is direct competition with a well-established technology. One major field where the bubble memory may have impact is in space and military applications where they can directly replace mechanical storage devices. The bubble offers potentially higher reliability and lower weight and power than current systems using tape storage. The cost of the storage media in such applications is not the prime consideration. Major programs have been funded by N.A.S.A., the Army and the Air Force for bubble replacement for tape storage in satellites. We should also note that even within the bubble area itself epitaxial magnetic films may face stiff competition from amorphous rare earth metal films 21. The amorphous film eliminates the need for costly single-crystal substrates, making mass low cost processing feasible. Further work is required to establish the viability of the new technology, however. Extensive use of the epitaxial Y I G filter will depend on considerations that are quite different from those pertinent to the bubble devices. The YIG filter per se, although used primarily in low volume military applications, is an established device. The film devices seem to offer several advantages over conventional YIG sphere devices--lower loss, cheaper fabrication and assembly, the possibility of lower frequency operation and microwave integrated circuit (MIC) compatibility. These advantages are gained only at the expense of temperature stability and flexibility in obtaining the desired frequency characteristics. Since the main applications are military where only moderate quantities are involved (and thus cost is not an overriding consideration) and where high performance is essential, the epiYIG filter will probably not make significant inroads into the conventional YIG filter market in the near future. Where variable or tapped delay directly at microwave frequencies is required, MSSW devices should prove useful. However, the limited bandwidth capability
194
H. LESSOFF, D. C. WEBB
(about 250 MHz at present), the low power saturation level, poor temperature characteristics and relatively short delay (approximately 1 ~ts) greatly restrict the number of applications that we can foresee. High frequency ( > 10 GHz), small bandwidth applications are well suited to MSSW devices because of their ease of excitation and their low propagation loss compared with the most widely used bulk acoustic wave devices. In general, MSSW devices will be used in specialized areas where there are no viable substitutes. In certain specialized areas all the epi-film devices discussed here--bubbles, microwave filters and MSSW delay lines--will be useful, if not essential, because of their unique properties. However, where they compete directly with better financed, firmly entrenched technologies they will most likely be the loser. REFERENCES 1 G . R . Pulliam, J. E. Mee, J. L. Archer and R. G. Warren, Proc. Natl. Aerospace Electron. Conf., Dayton, Ohio, 1965, p. 241. 2 J. L. Archer, G. R. Pulliam, R. G. Warren and J. E. Mee, in H. Peisner (ed.), Crystal Growth, Pergamon, New York, 1967, p. 337. 3 A . H . Bobeck, Bell Syst. Tech. J., 46 (1967) 1901. 4 A . H . Bobeck, P. I. Bonyhard and J. E. Geusic, Proc. IEEE, 63 (1975) 1176.
5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
M.S. CohenandH. Chang, Proc. IEEE, 63(1975) l196. R . C . LeCraw and S. L. Blank, Appl. Phys. Lett., 26 (1975) 402. B. Fox and K. J. Button, Microwave Ferrites and Ferrimagnetics, New York, McGraw-Hill, 1962. P.S. Carter, IEEE Trans. Microwave Theory Tech., 13 (1965) 306. G . L . Matthaei, 1EEE Trans. Microwave Theory Tech., 13 (1965) 203. W . L . Bongianni, J. Appl. Phys., 43 (1972) 2541. C. Vittoria and N. D. Wilsey, J. Appl. Phys., 45 (1974) 414. D . C . Webb, C. Vittoria, P. Lubitz and H. Lessoff, IEEE Trans. Magn., 11 (1975) 1259. A . K . Ganguly and D. C. Webb, IEEE Trans. Microwave Theory Tech., 23 (1975) 998. J. D. Adams, Z. M. Bardai, J. H. Collins and J. M. Owens, AlP Conf. Proc., Magnetism and Magnetic Materials, 24 (1974) 495. R.J. Miller, U.S. Naval Ord. Lab. Rep. 3962, 1955. R . E . Cech and E. I. Alessandrini, Trans. Am. Soc. Met., 50 (1959) 150. J.E. Mee, G.R. Pulliam, J.L. ArcherandP. J. Besser, IEEETrans. Magn.,5(1969) 717. R . C . Linares, R. B. McGraw and J. B. Shroeder, J. Appl. Phys., 36 (1964) 2884. J . W . Nielsen and E. F. D. Dearborn, J. Phys. Chem. Solids, 5 (1958) 202. R. Hiskes and R. A. Burmeister, AlP Conf. Proc., 10 (1972) 304-308. J . M . Robertson and J. C. Brice, J. Cryst. Growth, 31 (1975) 371. J . M . Robertson, P. K. Larsen and P. F. Bongers, IEEE Trans. Magn., 11 (1975) 1112.
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M A G N E T I C E P I T A X I A L FILMS : APPLICATIONS A N D P O T E N T I A L * H. LESSOFF AND D. C. WEBB Naval Research Laboratory, Washington, D.C., 20375 (U.S.A.) (Received April 23, 1976; accepted July 30, 1976)
Magnetic films in amorphous, polycrystalline and single-crystal form have shown promise for many years as a method for miniaturizing magnetic components in the design of electronic devices. With the exception of a few specialized applications, e.g. thin film Permalloy memory elements and magnetic tape heads, widespread use of these films has not yet been realized. Recent advances in the preparation of high quality non-metallic single-crystal garnet films have brought a number of important components much closer to reality. The most immediate widespread use of the single-crystal magnetic film is bubble memory devices. These devices exploit recent advances in the magnetic garnet technology and achieve an extremely high data storage density. They are expected to supplant mechanical tape and disc recorders in some applications soon. Another class of single-crystal magnetic film devices with great potential but not as well developed as the bubble devices are the planar yttrium iron garnet microwave devices. Included in this category are limiters, filters and magnetically tunable oscillators, which are compatible with conventional microwave integrated circuits. Magnetostatic surface wave devices represent another class of devices which rely upon high quality single-crystal magnetic films. These devices are capable of carrying out signal processing functions such as fixed, variable and tapped delay, convolution and pulse compression directly at microwave frequencies. This paper concentrates on the material parameters which are most important in the bubble, filter and magnetostatic surface wave applications, on how the parameters can be optimized and on the fundamental limits they place upon device performance.
1. INTRODUCTION
Magnetic films in amorphous, polycrystalline and single-crystal form have shown promise for many years as a means of miniaturizing magnetic components for electronic devices. With the exception of a few specialized applications, e.g. thin Permalloy memory elements and magnetic tape heads, widespread use of these films *Paper presentedat the InternationalConferenceon MetallurgicalCoatings, San Francisco,California, U.S.A., April 5 8, 1976.
186
H. LESSOFF, D. C. WEBB
has not yet been realized. Significant recent advances in the preparation of high quality single-crystal magnetic films have made a number of important devices feasible. We shall explore three classes of these devices: magnetic bubble memories, microwave filters and magnetostatic surface wave (MSSW) delay lines. The operation of each will be reviewed, emphasizing the relationship between important material parameters and the performance that can be achieved. Techniques for growing films with the desired characteristics will be discussed, and finally an assessment of the future of magnetic epitaxial film devices will be made. 2. MAGNETICEPITAXIALFILMDEVICES 2.1. M a g n e t i c bubble m e m o r i e s
The use of ferrite films for memory devices was initially proposed by Pulliam et al. 1 and Archer et al. 2 The structure consisted of an epitaxial film grown on a substrate, then a deposited wire grid and finally an epitaxial film grown on top. The structure acted similarly to a magnetic core array with storage elements occurring at the intersection oforthogonal wires. In 1967, Bobeck 3 suggested and demonstrated the potential of magnetic bubble domains as memory elements. The technology has now evolved to the point where bubble devices are under consideration for many high bit density, medium speed, medium cost, memory applications. Many excellent reviews of bubble devices have recently been given 4' 5 so only a brief description of their operation will be presented here. Physically, bubbles are cylindrical magnetic domains whose magnetization is the reverse of that of the adjacent material. Ones and zeros of a binary stream of data are denoted by the presence or absence of a bubble. A commonly used propagation circuit, the T bar, is shown in Fig. 1. The d.c. bias field is required in order to make the bubbles stable. The in-plane rotating field causes an alternating polarity at the ends of the bars, propelling the bubbles along the circuit. A complete circuit must provide for bubble generation, replication, annihilation and detection as well as propagation. Techniques for realizing all of these functions are now well developed 4' 5. Permalloy
Bars
In~ Plane Rotating
\\\\\\ Magnetic
oarnet GGG ~ Substrate
Fig. l. A T-bar bubble propagation circuit.
~ J ~
t Perpendicular /
Bias Field
MAGNETIC EPITAXIAL FILMS: APPLICATIONS AND POTENTIAL
187
In selecting an optimum bubble memory material composition the most important considerations are bubble stability, speed (average access time) and bit density. The material parameters affecting these properties will be outlined in the following discussion. Bubble stability is determined by the ratio of the anisotropy constant K, to the demagnetizing energy. This important ratio is denoted as the material Q value and is defined by Ku
(1)
O -- 2r~Ms2
Ms is the saturation magnetization of the bubble material. Typically Q is chosen as about 5; this is a compromise between bubble instability and spontaneous nucleation on one hand and ease of generation on the other hand. To minimize power requirements and thermal problems, a minimum in-plane drive field is desired. The material should thus possess a minimum domain wall coercivity Hc (which must be overcome to initiate propagation) and a maximum bubble mobility #. For high speed applications, a maximum bubble propagation velocity is desired. As the drive field intensity is increased, the velocity is found to saturate at a value 247A
(2)
Vp - h(Ku)l/2
where h is the film thickness, usually about half the bubble diameter. Changes in A, K and h affect several other bubble properties and thus cannot be freely changed. However, a "high ~," garnet has been recently reported which raises lip by more than an order of magnitude over conventional bubble materials 6. Another important memory device consideration is the bit density. The optimum (stable) bubble diameter d is approximately
d= 8
(AQ)I/2 Mss
(3)
Q and A cannot be changed significantly so Ms is used to control d. The penalty which must be paid for increasing Ms is that the in-plane drive field must also be increased. In contrast with the microwave devices to be discussed later, the variation of the magnetic properties with temperature is not a significant problem in bubble memories. Here, by utilizing a flexible circuit design and a magnet with a specially selected temperature dependence, moderate temperature variations can be accommodated. From the above discussion it is evident that a great many material parameters must be considered in the design of a bubble memory. A single composition obviously cannot simultaneously optimize all of these, and reasonable compromises, depending upon the application, are necessary.
2.2. Tunable microwave filters Tunable microwave filters consisting of one or more yttrium iron garnet (YIG) spheres in a coaxial, stripline or waveguide structure have been used extensively for
188
H. LESSOFF, D. C. WEBB
both military and test equipment applications for many years. Where low loss, a narrow instantaneous bandwidth and a wide tunable bandwidth are required, this type of filter is used extensively. With recent advances in the epitaxial YIG technology, planar fabrication of high performance multiple filters on a single substrate is now feasible. An example of this type of filter is shown in Fig. 2. Magnetic Film
Magnetic Bias
4 |
/~ ]
~
DielectricSubstrate / / I
Out
Metal Ground Plane
Fig. 2. A two-resonator planar YIG filter.
The important electrical properties of a multiple resonator filter are the individual resonator Q values and resonant frequency, the coupling between individual resonators and the coupling between the bank of resonators and the external circuit (the external Q value). The gyromagnetic resonant frequency tnR of a cylindrically symmetrical body is given by 7 o~ = ?{HA + HK --(NT-- Nz) 4nM~}
(4)
where HA is the applied field, HK the anisotropy field ( = 2l~/Ms) and N T - Nz a geometrical shape (demagnetizing) factor. For a sphere NT--Nz = 0 while for a perpendicularly magnetized disc ART--Nz = 1. For both of these geometries the resonant frequency varies linearly with the applied magnetic field, a factor which greatly facilitates wide band tunability. For YIG sphere filters the resonant frequency can easily be made temperature independent since N T - Nz = 0 and since HK vanishes for certain relative applied field-crystalline axis orientations. Temperature independence is not as simple for the disc resonators since the condition d~_~_ 4rtd~M s
(5)
must be met. Film composition and growth conditions can be adjusted somewhat to accomplish this, but at present it is not clear that adjustments can be made reproducibly without serious degradation of the linewidth. The most significant material parameter for filter applications is the fer-
MAGNETIC EPITAXIAL FILMS: APPLICATIONS AND POTENTIAL
189
romagnetic resonance linewidth AH. A low value of AH has dictated the use of YIG in virtually all applications to date; it is the recent improvements in AH in epitaxial materials which now make planar filters practical. The importance of AH derives from the fact that the loss Lf through a multiple resonator filter is given by s' 9 AH L~ = C 1 7 - -
(6)
Af
All resonators are assumed to have identical values of AH. Cx is a constant determined by the band shape desired; Afis the half-power filter bandwidth. The inter-resonator coupling constants and external Q values (Qe) must be selected in a prescribed manner to achieve an "optimum" frequency response. Qe is inversely proportional to the product o)m ( = ~4rcMs) x Wm (sample volume) where the proportionality constant is a function of circuit parameters only. Some flexibility in circuit design exists, however; for some of the lower values of external Q needed (Q¢ ~ 10) very thick films (approximately 100 lam) are necessary. The interresonator coupling constants are empirically adjusted by the proximity of the adjacent resonators. Another important property of YIG filters is their power-handling capacity. This is determined by non-linearities in the material which occur when the applied r.f. field exceeds a certain threshold value ho.t. In particular (AH) 2 hcrit -
4rtMs
forf
(7a)
and
AH [ AH hcrit = 2 - 1 4 ~ s s ]
/ 1/2 forf>f~
(7b)
where )re = ~m/2 for a perpendicularly magnetized disc 7. For a typical narrow linewidth (approximately 0.50e) Y I G filter, the saturation power is about - 30 dB m f o r f < f ~ and about 0 dB m f o r f > f c . For maximum dynamic range it is desirable to reduce M s; however, a reduction in Ms results in an increase in AH so only a limited change is possible before the loss characteristics are seriously degraded. It is clear that the number of critical material parameters for YIG filters is significantly less than for bubble devices, and the overriding consideration is nearly always the realization of a low linewidth.
2.3. Magnetostatic surface wave devices A final class of devices which we shall consider are the MSSW delay lines. These devices offer the potential of carrying out signal processing, fixed and variable delay and pulse compression, directly at microwave frequencies. A common configuration of this device is shown in Fig. 3. It consists of shorted sections of microstrip transmission line in contact with an epitaxial Y I G film. The bias field is applied in the plane of the film, perpendicular to the direction of propagation. The microwave electromagnetic signal, typically having a frequency of 1-10 GHz, excites an MSSW which propagates in the film to the output microstrip line, which is then reconverted to an electromagnetic wave and which is finally detected.
190
H. LESSOFF, D. C. WEBB
GGG
. J x ~ YIG ~
,/
~
RF Output
ZT"
plan Fig. 3. A magnetostatic surface wave delay line.
The frequency and wavenumber of the MSSW are related by the dispersion equation ~0 G = exp(2kh)
1 1 + (f2s + OH)(1 -- tanh kt) 2(f2s + I2H)+ 1 1 -- (f2s -- f2.) (1 + tanh kt)
(8)
where t is the dielectric substrate thickness and h the magnetic film thickness. s = _ 1 depending on the direction of propagation, I2s = ~/o9m and OH = OgR/Ogm. The group velocity Vg is then given by Vg = -
Re~0G~ff~ lao/& )I
(9)
Vg is typically 106-107 cm S- 1 for films of thickness 1-5 lam. The propagation loss is also found from eqn. (8) by making the substitution HA ---,HA + i AH/2 and employing the relationship = 8.68Im(k) dB c m - 1
(10)
In general ct increases with increasing A H and for frequencies of not less than 3 G H z the loss is11 linearly proportional to AH. Since AHincreases linearly with frequency, the propagation loss does likewise 12. The efficiency and bandwidth of MSSW excitation are complex functions of frequency, applied field, saturation magnetization, film thickness, anisotropy field and circuitry parameters 13. In general a wide excitation bandwidth demands that the width of the microstrip exciter is comparable with the film thickness. This places a lower limit on the group velocity and limits the m a x i m u m delay to about 1 txs. MSSW devices exhibit the same power saturation effects as do Y I G filters except that the critical frequencyf~ is .4 approximately 3.5 GHz. Again an increase in threshold power is possible only at the expense of additional loss. Since all important MSSW properties are complicated functions of Ms and H~, there is no simple technique for stabilizing against temperature fluctuations. Thus the device should be placed in a constant-temperature oven, a temperaturedependent feedback signal should be employed to adjust the bias field or the temperature-induced variations must be tolerated.
MAGNETIC EPITAXIAL FILMS : APPLICATIONS AND POTENTIAL
191
It is evident that the material parameters important for MSSW devices are similar to those of the Y I G filter. The former require somewhat thinner films ( ~< 15 tim) and larger areas. In both devices the linewidth is the most critical parameter, but some adjustment of 4nMs is also often desirable. 3. PREPARATION OF GARNET FILMS The preparation of magnetic oxide films was initially reported by Miller 15 in 1955 ; however, very little work was done in this field until Cech and Alessandrini ~6 prepared oxide layers of iron, cobalt and nickel by chemical vapor deposition (CVD). The decomposition of metal halides in an oxidizing atmosphere was used. Work was then initiated by a number of investigators to prepare epitaxial films for direct replacement of those magnetic components which used bulk materials. Devices considered initially for miniaturization included memory cores, inductors and microwave filters and circulators. An excellent review of the CVD process is given by Mee e t al. ~ 7 Deposition is carried out in a reactor, a simplified representation of which is shown in Fig. 4. The process involves heating anhydrous metal halides, normally chlorides, in an inert gas stream such that the metal halide vapors are carried into a reaction chamber. The vapor stream is mixed with a second stream containing water vapor and oxygen in the reaction zone. The metal halides are oxidized and/or hydrolyzed to form the metal oxides and the corresponding halide and oxyhalide acids. Reaction temperatures are in the range 1000-1300 °C. The reaction process is not simple, and various oxides form in different sections of the reaction chamber. By placing a substrate crystal in the proper segment of the reaction chamber, the desired oxide grows on the substrate. The substrate is normally a non-magnetic oxide having a structure similar to the depositing species. T n~" 7 0 0 ° HejHCl
T2 ~ 9 0 0 °
T 3 ~ 1125"
J
He'O2'H20,I /
Exhaust
/
Source FeCl~
/
j
Source YCI3
7 Subltrote
Fig. 4. A reactor for CVD magnetic oxide growth.
The devices described in this paper all employ garnet films commonly grown on gadolinium iron garnet or other non-magnetic garnet substrates with the lattice parameters tailored to match the magnetic material. Growth of the magnetic oxides by CVD is not simple since source temperature, gas flow rates, reaction temperature and substrate location must all be rigorously controlled. Fortunately an alternative to CVD now exists. Approximately five years after epitaxial growth was achieved by Cech and Alessandrini 16, Linares e t al. ~8 demonstrated that Y I G epitaxial films could be prepared by having a gadolinium gallium garnet ( G G G ) substrate in contact with a fluxed melt containing yttrium
192
H. LESSOFF, D. C. WEBB
oxide and iron oxide. The process, known as liquid phase epitaxy (LPE), is quite simple once the growth conditions are established. It is very similar to processes used in the semiconductor industry for III-V material growth such as GaAs. The flux systems used are PbO-B203-Fe203 (ref. 19), BaO-BaF2-B203 (ref. 20) and BiEOaRO 2 where R is Si, Ge, Ti or Ce (refs. 21, 22). Table I presents some of the advantages and disadvantages of the various systems. TABLE I FLUXES(SOLVENTSUSEDFOR GARNETLPE)
Advantages
Disadvantages
Low temperature
Corrosive Volatile constituent Lead inclusions
BaO-BaF2-B203
No lead inclusions Non-volatile
High growth temperatures Poor solvent removal
Bi2Oa-RO 2
No lead inclusions Low to moderate temperatures
Poor solvent removal Possible inclusion of R (R ~ Si, Ge, Ti, Ce)
PbO-
B203-Fe203
The current growth technique using LPE involves dipping a substrate held either horizontally or vertically into the supercooled melt under isothermal conditions. The degrees of substrate rotation and melt saturation help to determine film uniformity and thickness. Growth occurs in a period of seconds to minutes depending on the flux, temperature and thickness of film required. The substrate is then removed from the melt, often with rapid rotation or pulling in order to remove excess flux on the film. The substrates used for LPE are similar to those used in the CVD method; however, since the number of parameters for growth are less for the LPE system compared with the CVD system, costs appear to be significantly lower. The growth temperatures range from 700°C to 1000 °C for the lead oxide and bismuth oxide melts--considerably lower than in CVD. This results in less strain and diffusion at the surface-film interface and hence higher quality magnetic films. For the above reasons, although both growth techniques yield uniform properties and well-controlled thickness, CVD has b e e n largely replaced by LPE for both bubble and microwave devices. Y I G (Y3FesO1 a) grown on a G G G (Gd3GasO1 a) substrate is most commonly used for filters and MSSW delay lines because of its low ferromagnetic resonance linewidths. Using high quality substrates and adding small amounts of lead to the flux to provide a good lattice match between film and substrate, linewidths superior to bulk (flux-grown) YIG have been achieved. The saturation magnetization is commonly reduced where needed by substituting gallium for iron; this is accompanied by an increase in linewidth, however. YIG also serves as the basic material for most bubble applications--with partial substitution for yttrium and/or iron. One commonly used combination is Y3_~LaxFes_yGayO~v Again gallium is used to reduce Ms, while La is used to adjust the lattice constants. The material has a high wall mobility and a Q value of about 4. Another widely used
M A G N E T I C E P I T A X I A L FILMS ~ A P P L I C A T I O N S A N D P O T E N T I A L
193
bubble material is YSmCaGe garnet with Q,~7 and excellent temperature characteristics 4. For high speed applications, a garnet which incorporates Eu, Ca, Si and Ge into the basic YIG structure looks attractive 6. 4.
A N ASSESSMENT OF E P I T A X I A L M A G N E T I C FILM DEVICES
Bubble memories are envisioned as filling a role intermediate in both cost and speed between semiconductors at the high end of the spectrum and tape, disc and drum memories at the low end. A developmental 100k bit chip has been manufactured which is cost competitive with comparable capacity semiconductor memories. To be a commercial success, however, the bubble device must overcome strong challenges from competing techniques, e.g. charge-coupled devices (which use the well-established silicon technology) and tape and disc storage (whose cost is still decreasing). Low costs are projected for bubble memories compared with semiconductor memories because there are fewer processing steps; however, bubbles also require smaller linewidths for a given bit density~largely eliminating the cost advantage. The commercial success of the bubble memory will probably depend upon the successful development of more compact structures such as the recently proposed bubble lattice and contiguous file systems 4. The first applications of bubble devices will be in areas where the unique properties of the bubble memories are utilized rather than in areas where there is direct competition with a well-established technology. One major field where the bubble memory may have impact is in space and military applications where they can directly replace mechanical storage devices. The bubble offers potentially higher reliability and lower weight and power than current systems using tape storage. The cost of the storage media in such applications is not the prime consideration. Major programs have been funded by N.A.S.A., the Army and the Air Force for bubble replacement for tape storage in satellites. We should also note that even within the bubble area itself epitaxial magnetic films may face stiff competition from amorphous rare earth metal films 21. The amorphous film eliminates the need for costly single-crystal substrates, making mass low cost processing feasible. Further work is required to establish the viability of the new technology, however. Extensive use of the epitaxial Y I G filter will depend on considerations that are quite different from those pertinent to the bubble devices. The YIG filter per se, although used primarily in low volume military applications, is an established device. The film devices seem to offer several advantages over conventional YIG sphere devices--lower loss, cheaper fabrication and assembly, the possibility of lower frequency operation and microwave integrated circuit (MIC) compatibility. These advantages are gained only at the expense of temperature stability and flexibility in obtaining the desired frequency characteristics. Since the main applications are military where only moderate quantities are involved (and thus cost is not an overriding consideration) and where high performance is essential, the epiYIG filter will probably not make significant inroads into the conventional YIG filter market in the near future. Where variable or tapped delay directly at microwave frequencies is required, MSSW devices should prove useful. However, the limited bandwidth capability
194
H. LESSOFF, D. C. WEBB
(about 250 MHz at present), the low power saturation level, poor temperature characteristics and relatively short delay (approximately 1 ~ts) greatly restrict the number of applications that we can foresee. High frequency ( > 10 GHz), small bandwidth applications are well suited to MSSW devices because of their ease of excitation and their low propagation loss compared with the most widely used bulk acoustic wave devices. In general, MSSW devices will be used in specialized areas where there are no viable substitutes. In certain specialized areas all the epi-film devices discussed here--bubbles, microwave filters and MSSW delay lines--will be useful, if not essential, because of their unique properties. However, where they compete directly with better financed, firmly entrenched technologies they will most likely be the loser. REFERENCES 1 G . R . Pulliam, J. E. Mee, J. L. Archer and R. G. Warren, Proc. Natl. Aerospace Electron. Conf., Dayton, Ohio, 1965, p. 241. 2 J. L. Archer, G. R. Pulliam, R. G. Warren and J. E. Mee, in H. Peisner (ed.), Crystal Growth, Pergamon, New York, 1967, p. 337. 3 A . H . Bobeck, Bell Syst. Tech. J., 46 (1967) 1901. 4 A . H . Bobeck, P. I. Bonyhard and J. E. Geusic, Proc. IEEE, 63 (1975) 1176.
5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
M.S. CohenandH. Chang, Proc. IEEE, 63(1975) l196. R . C . LeCraw and S. L. Blank, Appl. Phys. Lett., 26 (1975) 402. B. Fox and K. J. Button, Microwave Ferrites and Ferrimagnetics, New York, McGraw-Hill, 1962. P.S. Carter, IEEE Trans. Microwave Theory Tech., 13 (1965) 306. G . L . Matthaei, 1EEE Trans. Microwave Theory Tech., 13 (1965) 203. W . L . Bongianni, J. Appl. Phys., 43 (1972) 2541. C. Vittoria and N. D. Wilsey, J. Appl. Phys., 45 (1974) 414. D . C . Webb, C. Vittoria, P. Lubitz and H. Lessoff, IEEE Trans. Magn., 11 (1975) 1259. A . K . Ganguly and D. C. Webb, IEEE Trans. Microwave Theory Tech., 23 (1975) 998. J. D. Adams, Z. M. Bardai, J. H. Collins and J. M. Owens, AlP Conf. Proc., Magnetism and Magnetic Materials, 24 (1974) 495. R.J. Miller, U.S. Naval Ord. Lab. Rep. 3962, 1955. R . E . Cech and E. I. Alessandrini, Trans. Am. Soc. Met., 50 (1959) 150. J.E. Mee, G.R. Pulliam, J.L. ArcherandP. J. Besser, IEEETrans. Magn.,5(1969) 717. R . C . Linares, R. B. McGraw and J. B. Shroeder, J. Appl. Phys., 36 (1964) 2884. J . W . Nielsen and E. F. D. Dearborn, J. Phys. Chem. Solids, 5 (1958) 202. R. Hiskes and R. A. Burmeister, AlP Conf. Proc., 10 (1972) 304-308. J . M . Robertson and J. C. Brice, J. Cryst. Growth, 31 (1975) 371. J . M . Robertson, P. K. Larsen and P. F. Bongers, IEEE Trans. Magn., 11 (1975) 1112.