Industrial Applications of Rare Earth Elements - American Chemical

13 Bubble Domain Memory Materials J. W. NIELSEN

Downloaded by UNIV OF CINCINNATI on May 21, 2016 | http://pubs.acs.org Publication Date: September 3, 1981 | doi: 10.1021/bk-1981-0164.ch013

Bell Laboratories, Murray Hill, NJ 07974

The use of c y l i n d r i c a l domains to store information i n a sheet of magnetized material was first reported by Bobeck i n 1967 (1). He observed that c y l i n d r i c a l domains i n a magnetized plate of material were stable over a convenient range of bias field and could be readily moved about i n the plate under the influence of a field gradient. Bobeck called the domains "bubble" domains because their motion i n a perturbing f i e l d looked much like the motion of bubbles on the surface of a liquid. Since Bobeck's discovery, development of bubble domain memories has been carried on i n many laboratories all over the world, and production of bubble domain memories is underway at a few companies. As a result of all the work on bubble domain memories, and materials for them, a voluminous l i t e r a t u r e has been generated (2-10). Here we have space only to outline b r i e f l y the present state of bubble domain memory materials development. The reader may use the references to obtain a more detailed account of memory design and materials selection. M a t e r i a l Requirements A bubble domain i s most s t a b l e under b i a s when i t s diameter i s approximately equal t o , or s l i g h t l y more than, the t h i c k n e s s of the magnetic sheet i n which i t i s s i t u a t e d . Since economical packing d e n s i t i e s r e q u i r e domain diameters of three micrometers or l e s s , i t i s c l e a r that the magnetic medium i n a bubble domain memory must be a t h i n f i l m supported by a s u b s t r a t e . A major breakthrough i n memory development was the d i s c o v e r y by Bobeck et a l (11) that many r a r e e a r t h garnet c r y s t a l s grown from f l u x e s possess s u f f i c i e n t u n i a x i a l a n i s o t r o p y to m a i n t a i n bubble domain s t a b i l i t y . T h i s was s u r p r i s i n g s i n c e r a r e e a r t h magnetic garnets, l i k e the parent compound y t t r i u m - i r o n - g a r n e t , 3Fe50i2> are c u b i c , and i t was soon e s t a b l i s h e d t h a t the a n i s o t r o p y was induced during growth of the c r y s t a l s . v

0097-6156/81/0164-0219$05.00/0 © 1981 American Chemical Society Gschneidner; Industrial Applications of Rare Earth Elements ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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F o l l o w i n g the important d i s c o v e r y of a n i s o t r o p y , Shick et a l (12), and L e v i n s t e i n et a l (13), showed t h a t f i l m s of magnetic garnets could be r e a d i l y deposited by l i q u i d phase e p i t a x y from molten PbO-B203 s o l u t i o n s onto gadolinium g a l l i u m garnet, Gd3Ga 0 » (GGG) s u b s t r a t e s . There f o l l o w e d many s t u d i e s on a l a r g e number of garnet compositions i n the search f o r the optimum bubble domain m a t e r i a l (14). I t soon became apparent that p r o p e r t i e s d e s i r e d i n the garnet f o r best d e v i c e performance, i . e . , h i g h domain m o b i l i t y , low c o e r c i v i t y , h i g h a n i s o t r o p y and h i g h bubble s t a b i l i t y r e q u i r e d a c a r e f u l l y designed compromise i n the s e l e c t i o n of garnet s u b s t i t u e n t s . For example, s p h e r i c a l ions i n the garnet lead to h i g h domain m o b i l i t y and low c o e r c i v i t y , but low a n i s o t r o p y and s t a b i l i t y . Non-spherical i o n s , on the other hand, l e a d to h i g h a n i s o t r o p y and s t a b i l i t y but a t the same time y i e l d low m o b i l i t y and h i g h c o e r c i v i t y . I t i s testimony to the great v e r s a t i l i t y of the garnet system t h a t s u b s t i t u t e d garnets could be designed t h a t not only met the magnetic requirements f o r bubble domain memories but matched the l a t t i c e parameter of the most u s e f u l s u b s t r a t e , GGG, as w e l l (15). A t y p i c a l f i l m composition f o r 3um diameter bubble i s : 5


ELEMENTS

12

Y

1.25

L u

0.45

Sm

0.4

C a

0.9

F e

4.1

G e

0.9

°12"

Substrate P r e p a r a t i o n I t i s f o r t u n a t e t h a t the non magnetic garnet, GGG, w i t h a l a t t i c e parameter (12.383A) most n e a r l y matching those of u s e f u l magnetic garnets, i s a l s o one of the e a s i e s t garnet c r y s t a l s to grow. GGG melts congruently a t 1740°C and i s grown by the w e l l known C z o c h r a l s k i , or p u l l i n g technique. The c r y s t a l s are grown from melts contained i n i r i d i u m c r u c i b l e s under an atmosphere of N£ c o n t a i n i n g 2% 0 (16,17). C r y s t a l s weighing up to 10 kg. w i t h diameters of 75 mm are grown r o u t i n e l y (18). Substrates are prepared f o r f i l m d e p o s i t i o n from the c r y s t a l s by sawing, l a p p i n g and p o l i s h i n g u s i n g techniques s i m i l a r t o those used i n the semiconductor i n d u s t r y . 2

L i q u i d Phase E p i t a x y (LPE)

Garnets

Magnetic garnet f i l m s are deposited on GGG s u b s t r a t e s from s o l u t i o n s of the garnet oxides d i s s o l v e d i n Pb0~B203 m e l t s . The d i p p i n g technique i s used (13), and because of the great s t a b i l i t y of the s o l u t i o n s i n the supersaturated s t a t e , l a r g e numbers of s u b s t r a t e s , up to 30 at a time (19), can be dipped simultaneously under n e a r l y i s o t h e r m a l c o n d i t i o n s . Although the molten s a l t s o l u t i o n s are extremely complex i n that they may c o n t a i n up to n i n e components, they behave i n a s t r a i g h t f o r w a r d manner almost e x a c t l y l i k e the pseudo-ternary Y2O3 - Fe20 - PbO (20). T h i s s i m p l i f i c a t i o n permits great 3

Gschneidner; Industrial Applications of Rare Earth Elements ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

13.

NIELSEN

Bubble Domain Memory

Materials

221

f l e x i b i l i t y i n a d j u s t i n g melt compositions to y i e l d garnet f i l m s meeting v a r i e d device s p e c i f i c a t i o n s . Temperatures f o r garnet LPE may range from 750° to 1100°C, but most experiments, and production runs, are c a r r i e d out near 950°C a t supercoolings of 10° - 40°C.


Rare E a r t h Use i n Bubble Domain Memories Although the garnet compositions used i n memories c o n t a i n r a r e earths, the t o t a l r a r e e a r t h use i n magnetic f i l m s i s not s u f f i c i e n t to make much impact on the r a r e e a r t h market. The use of gadolinium oxide i n s u b s t r a t e c r y s t a l s i s another matter. I t has been estimated that by 1990 the annual use o f Gd203 f o r GGG substrates w i l l reach 40 metric tons, about twice the present r a t e (21). Conclusion The development of the bubble domain memory has been remarkable i n that s i n c e the d i s c o v e r y o f the growth induced anisotropy i n garnets, problems connected w i t h m a t e r i a l s have been r e l a t i v e l y few and not too d i f f i c u l t t o s o l v e . A major reason i s that the d i f f e r e n t s i z e s and magnetic p r o p e r t i e s of the r a r e earths o f f e r a wide range of choices f o r the m a t e r i a l s designer. The major problem s t i l l t o be solved i s development of a high speed m a t e r i a l w i t h bubble diameters of the order one micrometer. In view of the success i n developing garnet m a t e r i a l s so f a r , we can be o p t i m i s t i c about the s o l u t i o n of the small bubble problem (22, 23, 24, 25).

Literature Cited 1. Bobeck, A. H. Bell Syst. Tech. J., 1967, 46, 1901-25. 2. Smith, A. B. "Bubble Domain Memory Devices"; Artech House: Denham, Mass., 1973; p. 258. 3. O'Dell, T. H. "Magnetic Bubbles"; Macmillan: London, 1974; p. 159. 4. Bobeck, A. H.; Della Torre, E. "Magnetic Bubbles"; NorthHolland: Amsterdam, 1975; p. 222. 5. Chang, H. "Magnetic Bubble Technology"; IEEE Press: New York, 1975; p. 699. 6. Bobeck, A. H.; Scovil, H. E. D. Sci. Am., 1971, 224(6), 78-90. 7. Bobeck, A. H.; Bonyhard, P. I.; Geusic, J. E. Proc. IEEE, 1975, 63, 1176. 8. Van Uitert, L. G.; Bonner, W. A.; Grodkiewicz, W. H.; Pictroski, L.; Zydzik, G. J. Mater. Res. Bull., 1970, 5, 825-35. 9. Nielsen, J. W. IEEE Trans. Magnetics, 1976, MAG-12, 327-45.



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ELEMENTS

10. Nielsen, J. W. Am. Rev. Mater. Sci., 1979, 9, 87-121. 11. Bobeck, A. H.; Spencer, E. G.; Van Uitert, L. G.; Abrahams, S. C.; Barns, R. L . ; Grodkiewicz, W. H.; Sherwood, R. C.; Schmidt, P. H.; Smith, D. H.; Walters, E. M. Appl. Phys. Lett., 1970, 17, 131-34. 12. Shick, L. K.; Nielsen, J. W.; Bobeck, A. H.; Kurtzig, A. J.; Michaelis, P. C.; Reekstin, J. P. Appl. Phys. Lett., 1971, 18, 89-91. 13. Levinstein, H. J.; Licht, S. J.; Landorf, R. W.; Blank, S. L. Appl. Phys. Lett., 1972, 19, 486-88. 14. See in particular references 9 and 10. 15. Nielsen, J. W.; Blank, S. L . ; Smith, D. H.; Vella-Colerio, G. P.; Hagedorn, F. B.; Barns, R. L . ; Biolsi, W. A. J.. Electron Mater., 1974, 3, 693-707. 16. Brandle, C. D.; Valentino, A. J. J. Cryst. Growth, 1972, 12, 3-8. 17. Brandle, C. D.; Miller, D. C.; Nielsen, J. W. See Ref. 16, 1972, 195-200. 18. Brandle, C. D. "Crystal Growth, a Tutorial Approach"; Bardsley, W.; Hurle, D. T. J.; Mullin, J. B., Eds. North Holland: Amsterdam, 1979; p. 189-214. 19. Blank, S. L.; Licht, S. J. presented at INTERMAG, Boston, MA, March, 1980. 20. Blank, S. L . ; Nielsen, J. W. J. Cryst. Growth, 1972, 17, 302-11. 21. Arai, Shigeru presented at the First International Conference on Magnetic Bubble Materials, Santa Barbara, CA, January, 1980. 22. Hu, H. L . ; Hatzakis, M.; Geiss, E. A.; Plaskett, T. S. Shift Registers with Submicron Magnetic Bubbles on Epitaxial Garnet Films. Presented at Intermag, Washington, D.C. See also Abstr. 26.5 in Abstr. Dig. for the same conference, 1973. 23. Giess, E. A.; Davies, J . W.; Guerci, C. F.; Hu, H. L. Mater. Res. Bull., 1975, 10, 355-62. 24. Carlo, J. T.; Bullock, D. C.; Johnson, R. E.; Parker, S. G. AIP Conf. Proc., 1976, 29, 105-7. 25. Yamaguchi, K.; Inoue, H.; Asama, K. AIP Conf. Proc., 1976, 34, 160-62. RECEIVED May 7,

1981.


Industrial Applications of Rare Earth Elements - American Chemical

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