LPE growth of iron garnets containing Ge4+ and Si4+ for bubble applications

Mat. R e s . Bull. Vol. I0, pp. 15-ZZ, 1975. in the United S t a t e s .

Pergamon Press,

Inc.

Printed

LPE GROWTH OF IRON GARNETS CONTAINING GeT+h and Si T+ ~ FOR BUBBLE APPLICATIONS W. A. Bonner Bell Laboratories Murray Hill, New Jersey

07974

( R e c e i v e d N o v e m b e r 7, 1974; C o m m u n i c a t e d by N. B. Hannay) AB S TRAC T

2+ 4+ Magnetic garnet films containing Ca plus Ge and/or Si~+ are of interest for bubble device applications. Extensive property variations can be obtained in compositions having a lattice constant essentially the same as GGG and suitable films are easily grown using conventional LPE dipping techniques. Films of Y3_xCaxFes_xGex012, where x = 0.0 to 3.0, have been prepared bn GGG substrates and have bubble diameters from sub-micron sizes to over 150 ~m. Preferred device materials having moments between ~ 200 and 500 gauss with bubble diameters of ~ i to 6 ~m have stable bubble properties from well below -i0 to at least 120°C and mobilities of ~ 2000 cm/sec/0e. The effect of growth conditions and melt composition on bubble properties are discussed. Introduction

Epitaxial, rare earth iron garnet films containing Ca 2+ plus Ge 4+ and/or Si ~ to lower the moment have been prepared from lead borate melts using conventional LPE dipping techniques (1,2). Films so constituted have a Curie temperature (Tc) that is 70-I00°C higher than comparable compositions containing the more familiar GaOr and A I 3 + additions (3). Improved operating temperature stability, resulting from the higher Tc, in conjunction with ease of growth on suitable substrates, suitable moment and high mobility make Ge and Si substituted iron garnets extremely attractive for bubble device applications (4,5). It is the purpose of this paper to discuss the general growth characteristics of Ge and Si substituted rare earth iron garnets prepared by liquid phase epitaxy (LPE) and the effect of growth conditions and melt compositio D on magnetic properties. Particular emphasis is given to Ca2+ + Ge~+ substituted YIG as this combination. does not appreciably effect the lattice match to GGG. Device 15

16

Ge 4 + A N D Si 4+

characteristics for specific applications are included.

Vol. i0, No. 1

compositions,

desirable

for bubble

Film Growth and Melt Preparation LPE growth techniques and details of apparatus for the preparation of magnetic garnet films have been discussed in detail by this author and others (6,7,8,9,10). Films containing Ge and Si are readily prepared using such established techniques from a lead-iron borate solvent. For the present studies, the general apparatus consisted of a Pt-20% Rh wire wound furnace which accommodates a standard form Pt crucible having the capacity to hold ~ 400 gms. of melt. Suitable closures and reflectors provided the necessary low thermal gradient. Substrates were held horizontally using a Pt tricep and axial rotation was employed to obtain uniform thickness. To insure consistency of data, all films were grown at a rate of 2.5 ~m/min, which has been found suitable for the preparation of large numbers of films having similar bubble properties, from the same melt (8). Growth rates of 2-3 ~m/min can be obtained in melts saturated between 950 and I025°C and undercooled 18-30°C when substrate rotation speeds of 200-250 rpm are employed. The molar composition ratios in the melt, defined by Blank and Nielsen (7) have been expanded to include the molar ratios necessary to describe systems containing Ca, Ge and Si. Accordingly, the ratios important for the following discussion are: Fe203 RI ~ Ln2~

(7)

Ge02 R~ ~ Fe203

PbO

Si02

+ Ge02 or Fe203

+ Si02

(7)

R 3 ; B--~3 Fe203 + Ga203 + Ln203 R4 ~ Pb0 + B203 + Fe203 + Ga203 + Ln203 x i00 Fe203 + (Ge02, Si02) RE ~ Pb0 + B203 + Fe203

(7)

+ Ca0 + Ln203

+ (Ge02,Si02)

+ CaO + Ln203

X i00

CaO CaO R 5 ~ Ge02 °r ~ U ~ 2 Films for the present studies were prepared from melts in which the Pb0:B203 portion of the solvent was mixed in a 15:l mole ratio (50:1 wgt ratmo). Assuming the distribution coefficients of the rare earths to be unity, rare earth oxides including Y203 were mixed in the desired stoichiometric proportions. The iron oxide to rare earth oxide molar ratio, R l, was variable between ~ 12 and ~ 28. Ge02 and Si02 were mixed in a suitable molar ratio with Fe203 (R~) to obtain the desired 4~Ms, and in equimolar proportion with-CaO (i.e., R 5 = 1). The ratio, R~, for the growth conditions employed was found to be variable between the values ~ 0.7-1.2. A change in

Vol.

10, N o .

1

G e 4+ A N D Si 4+

17

moment is, however, associated with changing this ratio as shown ia t Fig. i for melts in which R 2 = 4. The moment decreases with increasing R~ to a value of ~ 1.2, at which point an unidentified, difficulty s61uble second phase precipitate may form. Good quality film growth is also difficult from melts in w h i c h R~ ( ~ 0.7 due to) film cracking, apparently related to lattice mismat6h. This mismatch difficulty can be alleviated by the incorporation of large rareearth ions on dodecahedral sites. However, where this alternative is chosen readjustment of R~ must be made to obtain equivalent 4~M s. The m o l a r concentration of crystal component oxides in the melt, R~, necessary to achieve saturation temperatures and undercooling suit~ able for film preparation at growth rates, of ~ 2 ~m/min was 12-16~. Typically, melts were prepared by adding Ca0 plus Si02 and Ge02 in the desired mole ratio with Fe20 q without consideration of R 4. Th~ apparent crystal component concefitration then is somewhat greater for melts containing CaO, Ge02, and Si02, than for standard iron garnet melts. Interestingly, the addition of CaO plus Ge02 without an a c c o m p a n y i n g addition of lead borate solvent to m a i n t a i n R 4 does not appreciably change either saturation or growth temperatures. Ca0 plus Si02 added in the same manner actually lower the saturation temperature. Figure 2 depicts the saturation temperature as a function of Ca0 plus Ge02 and Si02 in a conventional YIG melt initially prepared at R 4 = i0. Saturation temperature is plotted a.s a function of R~ and R~. Consideration of the PbO:Fe20 q (ii), Pb0"GeO. 2 (12), ~nd. Pb0"Si02. . (13) .phase . . equilibria reveaIs all thre6 systems behave similarly in the vmcmnmty of the Pb0 end member. Th~ liquidus temperature decreases toward a 730-740°C eutectic at ~ 18 mole % Fe~0~ or Ge02 and ~ 8 mole % Si0 2" Ge0 2 and Si02 as well a~ Fe20 ~ in ~x~ess of that necessary to stabilize the garnet phase ca~1 be c~nsidered part of the solvent. The somewhat faster rate of decrease of the liquidus temperature in the Pb0:Si02 systems explains the lower saturation temperature of iron garnet systems containing

R4'

60C

I000 |

12

14

16

,

T

,

Co~6e gl

4(

w

900

4o~

k

o

~

CO÷S1

%'%,

qr

o~ 8OO

20C

0.6

I

I

0.8

J.O

I

I

0.2

0.3

i 1.2

Rs(CoO~Se0 e) FIG.

[ 0.1

1

Moment of Ca 2+ + Ge 4+ substituted YIG as a function of R 5, (ca0/Qe02) for = 4.

FIG. 2 Saturation temperature as a function of R~ and R~ for A) Ca0 + G e 0 2 - a n d B ) C a 0 ÷ Si02 additions to YIG.

18

Ge 4+ AND Si4+

Vol. 10, No. l

SiO 2. The addition of GeO 2 and Si02 to iron garnet melts is equivalent to increasing R I which as has previously been shown decreases the saturation temperature if R 4 remains constant (9,10). Results

and Discussion

To lower the moment of iron garnet films to 200-#00 gauss (6-1½ ~m operating bubble dia) generally desirable for device work, and to maintain a T c sufficiently high such that bubble properties are stable over a wide temperature range requires m i n i m u m substitution into octahedral sites by ions capable of lowering the moment. Ga3÷ and A I 3 + commonly employed for this purpose can enter both tetrahedral and octahedral sites in the garnet structure. They predominately enter tetrahedral sites, thereby lowering the moment. As a fair percentage (10-15% depending on the growth temperature) enter octahedral sites they also tend to increase the moment to this extent. The most serious effect of substituting into octahedral sites, however, is a drastic lowering of the T c. This adverse condition can be alleviated by incorporating small highly charged cations which enter tetrahedral sites exclusively so that the decrease in moment is maximized while the decrease in T c is m i n i m i z e d (i). Figure 3 shows the effect, on T c of YIG, for several ion~ employed to decreas'e the moment of iron garnets (3). The T c of Ge ~+ and Si 4 ~ substituted garnets as illustrated is 70-I00°C h i g h e r than for Ga~ + and AIJ + substituted garnets of corresponding moment. V 5+ appears to enter tetrahedral sites exclusively with little change in T c up to one unit substitution per formula, and in fact affords the highest possible T c materials. However, an accompanying requirement of twice the amount of divalent cation for charge compensation tends to limit its usefulness to relatively low concentrations due to grQwth difficulties from the lead borate solvent. In comparison, Sc~+ which enters octahedral rather than tetrahedral site~ lowers T¢ ~ 250°C/unit substitution. Ca ~+ and small amounts of Sr ~+ can be mncorporated on dodecahedral sites to provide th~ necessary charge compensation when used in combination with Ge ~+ and Si ~+. Of the various divalent-tetravalent combinations suitable for substitution in. yttrium iron garnets the moGt interesting are Ca 2+, + Ge ~*, Sr 2+ ~ Si ~+, and Ca 2+ + Si ~+. The Ca ~ + Ge 4+ and Sr 2+ + Si ~+ additions do not appreciably affect the lattice match to GGG over a wide range of compositions. Sr 2+ although, known to substitute in substantial amounts on dodecahedral sites in ceramic samples (14) has only been incorporated in small amounts in films prepared from lead bora~e solvents. Its usefulness therefore is limited. The

o

s

o g

•

,

dodecahedral sites for lattice match to GGG. On the other hand, Ca 2+ ÷ Si ~+ films may be Drepared on a smaller lattice substrate such as Y~Ga~O]~ (12.37T ~). The small advantage gained in Tc, however, ~ust ~ present be traded against the availability and quality of substrates other than GGG. Of particular interest for device studies is the Ca 2+ + Ge 4+ combination. Films of Y~ -Ca Fe~ -Ge 012, where x = 0.0-3.0, have been prepared from le~dXbo~at~-~el~s on GGG. Obviously, a variety of bubble property adjustments can then be made without changing the substrate or as the case m a y be the addition of undesirable components to effect lattice match. Small amounts of rare-earth

Vol.

10, No.

1

Ge 4+.AND

Si 4+

19

coo I X------.×

400

~¢

0

,. ~.

\

.~ 3cc zoo

+~ TETRAHEDRAL SITE PRETERENCE

IOC

vS~ > Sl 4 . > Ge4, > Go3~ > Ai3t > So3,

I

I

I

2

500

X

FIG.

T c as a function -

~ \ \ ~-R,=,o ~--~ ~----~--R,=,e

3 5+

Y3-2xCaxFe5_xMx

of'x for 012,

Y3_xCaxFes_xMx4 +012 , and Y3Fe5 cM~+012~ for various ions ~3).

I

0

I

0.2 0.4 R~(GeO21FezO3+GeOz)

0.6

FIG. 4

M o m e n t o f Ca 2 + + Ge 4 + s u b s t i t u t e d ions, however, can be readily YIG as a function of ! incorporated on dodecahedral sites R 2 for several values of R I. to facilitate control of stress or strain as well as adjust bubble properties to meet device requirements.

Figure ~ depicts the moment of Ge substituted YIG as a function of R§ for several values of RI with R 5 = i. The moment decreases , linearly as R~ increases but decreases for equivalent values of R 2 as R I increases. Melts in which R I = 18 yield films w h i c h have moments corresponding closely to those reported by Geller et al. (3) on ceramics. Films containing Ca ~+ + Si ~+ in sufficient quantities to lower the moment to < 400 gauss have only been prepared from melts in w h i c h R I ~ 16. C o n t i n u a l l y changing lattice parameter of Ca + Si substituted filmS prevents an analysis similar to that for Ca + Ge. Initially, the decrease in moment with increasing R I appears to be related to the growth temperature w h i c h similarly decreases with increasing RI, if a constant growth rate is maintained. From this, it w o u l d appear that the distribution of either Ca or Ge between film and melt changes as a function of the growth temperature. If, however, the growth temperature is varied as a function of the concentration of crystal components R~, b y adding flux (PbO:B20R) to the melt ~ M s changes inversely with temperature, as indicated in Figure 5.

Z0

Ge 4+ .AND Si 4+ 300

Vol.

10, N o .

1

850

200

-- 9 O 0

FIG. e @.

X

I00

95O

,t

0-

5

Moment of Ca 2+ + Ge 4+ substituted YIG as a function growth temperature and crystal component concentration R~, at constant R1, R~, and R 5.

IOOO

Io2 5 5

6 7 8 9 I0 MOLE % CRYSTAL COMPONENTS IN MELT (R4xlO0)

Optimum bubble properties and film quality are exhibited by Ca ÷ Ge substituted films prepared from melts in which RI = 14-20 and R 4 = 8-12% (R~ = 12-16%). These melt ratios allow for growth at a temperature sufficiently high to minimize Pb incorporation (8) yet sufficiently low so as to minimize solvent volatilization. Figure 6 depicts 4~M s and strip width as a function of R~ and x for films of Yq_xCaxFe~_ Ge 0]2 grown on GGG substrates from a melt in which. R I =-i~. -Th~ ~al~e~-of x were estimated from 4~M s data on ceramlcs (3). The ratio of Fe:Ge in the crystal is nearly equivaleht to the normalized Fe2OB:GeO 2 ratio (R~) in the melt, indicating the distribution of Ge between film and melt is close to 2:1. The moment goes through the compensation point at x = ~ 1.1 (R~ = 0.26). At the compensation point there is a reversal in spin alignment due to the change from tetrahedrally to octahedrally dominated fields and 4~M s rises from zero to a value of ~ 420 gauss at x = 1.8. The net moment again decreases to zero beyond this value. As indicated in Fig. 7, the strip width and thus also the bubble size increases as the moment approaches zero. Measurement of the bubble diameter is difficult in the vicinity of the compensation point, however, at moments as low as ~ 40 gauss bubble diameters approaching 150 ~m have been observed. As indicated previously, bubble properties may be adjusted to meet specific device requirements by the incorporation of rare earth ion~ on dodecahedral sites. To increase K u of Y~-xCax Fe5-xGex lO 2, which is intrinsically low Sm, Eu, and Lu m y be employed. Incorpor%tion, though, of even small amounts of large ions requires lattice adjust~nent. Small ions such as Lu 3+ on dodecahedral sites and/or Si 4+ on tetrahedral sites ~ave been employed to obtain the reGuired lattice adjustment. Si ~+ when used in conjunction with EuJ + has the added advantage of providing materials having somewhat higher Q (= Ku/2~M~) values than comparable materials containing only Ge substitution. Nominal film compositions which have been found to possess interesting bubble properties are listed in Table I.

Vol. i0, No. 1

Ge 4+ AND Si 4+

Zl x

)k 1.0

2O E

FIG.

6

4~M s and strip width, as a function of x and R~ for Y3_xCaxFes_xGexOl2 for compositions between x = 0.5 and 2.0.

iO

2.0

0

600

"•

51)0

A

~

400

f - .\

£ q" 3 0 ¢

TET ~'

CT :>TET

~'~,

20C

I

O,Z

!

I

0,4 R 2'

TABLE I 4~M s

Bubble dia

Q

YI.66Eu0.3Luo. ICa0.94Fe4.06Ge0.94012

200

6~

7

YI.75Eu0.3Lu0.1Ca0.85Fe4.15Geo.85012

360

3~

4

YI.9Eu0.3Ca0.8Fe4.2Geo.6Si0.2012

350

3~

8

YI.98Eu0.3Ca0.72Fe4.28Ge0.52Si0.20012

420

1.5~

5

Conclusion The use of Ge 4+ and Si 4+ as opposed to Ga 3÷ or AI 3+ to modify the magnetic properties of iron garnets provides a class of materials having higher T c and greater operating temperature stability for bubble device applications. Ge #+ substituted YIG, when charge compensated with Ca ~+, is especially interesting as lattice match to GGG over a large compositional range allows a range of property adjustments to be made without changing the substrate. Films of the type Y3-'xCa'~Fe~. .j-xGex012~ have been prepared on GGG substrates over the composl~lona± range x = 0.0-3.0. Materials in which the moment is between 200 and 500 gauss (x = 0.75-0.95) have been showr~ to have stable bubble properties and high mobilities. Acknowled~aents I wish to thank A. W. A n d e r s o n for magnetic measurements and L. G. Van Uitert and J. E. Geusic for many helpful discussions.

ZZ

G e 4+ A N D

Si4+

Vol. I0, No.

1

References i.

W. A. Bonner, J. E. Geusic, D. H. Smith, L. G. Van Uitert and G. P. Vella-Coleiro, Mat. Res. Bull. ~, 1223 (1973).

2.

W. A. Bonner, paper IB-3, presented at Nineteenth Conference on Magnetism and Magnetic Materials, Nov. 1973, abstract to be published in AIP Conference Proceedings.

3.

S. Geller, H. J. Williams, G. P. Es~inosa and R. C. Sherwood, Bell System Tech. J. 43, 565 (1964).

4.

J. E. Geusic, D. H. Smith, L. G. Van Uitert and G. P. VellaColeiro, paper IB-4, presented at Nineteenth Conference on Magnetism and Magnetic Materials, Nov. 1973, abstract to be published in AIP Conference Proceedings.

5.

J . W . Nielsen, S. L. Blank, D. H. Smith, G. P. Vella-Coleiro, F. B. Ha~adonn, R. L. Barns, and W. A. Biolsi; J. Elect. Mat.

693 (197 ). 6.

H. J. Levinstein, S. Licht, R. W. Landorf and S. L. Blank, ippl. Phys. ~etters 19, 486 (1971).

7.

S. L. Blank,

8.

W. A. Bonnet, J. E. Geusic, D. H. Smith, L. G. Van Uitert and G. P. Vella-Coleiro, J. Appl. Phys. %__33j3226 (1972).

9.

S. L. Blank, B. S. Hewitt, L. K. Shick and J. W. Nielsen, AIP Conference Proceedings, No. i0, Magnetism and Magnetic Materials Conference 1972 (American Institute of Physics, New York, 1973).

and J. W. Nielsen,

J. Cryst.

i0.

W. A. Bonnet, Mat. Res. Bull. ~ , 8 8 5

ll.

A. J. Mountuala and S. Ravitz,

Growth 17 (1973).

(197%).

J. Am. Ceram.

Soc. ~_~5~786

J. Am. Ceram.

Soc. lhS; 399

(1962). 12.

B. Phillips and M. G. Seroger,

( 965). 13.

R. F. Geller, A. S. Creamer and E. N. Bunting, J. Research Natl. Bur. Standards 1 3 [2] 243 (193%), RP 705.

1%.

B. V. Mill, Dokl ikad Nauk

[USSR] 165, 555

(1965).