LPE growth of YLaTm and YLaEu garnet films

Journal of Crystal Growth 31(1975) 366—370 © North-Holland Publishing Company

LPE GROWTH OF YLaTm AND YLaEu GARNET FILMS B.F.STEIN, Sperry Univac, Blue Bell, Pennsylvania 19422, U S.A. and

M. KESTIGIAN Sperry Research Center, Sudburv, Massachusetts 01776, U.S.A.

The growth of films of the composition Y

3(x+y)LaxR),Fes_zGazOl2~where R = Tm or Eu, on GGG substrates by liquid phase epitaxy from a PbO—B203 fluxed melt is described. Highly uniform, low defect density films have been grown by means of horizontal dipping using intermittent substrate rotation in conjunction with a novel substrate holder. A furnace with three separate temperature zones was used to optimize the vertical temperature profile. Incorporation of La into the films allows compositions containing Tm to be made whose lattice parameters are matched to GGG and compositions which contain Eu to be made with smaller Eu concentrations than were previously possible. The compositions grown, which contain only trivalent cations, exhibit growth induced anisotropy, high mobility, low coercivity, and sufficient magnetostriction to allow ion implantation to effectively suppress hard bubble formation. Detailed measurements of saturation temperature as a function of oxide concentration show that the La ion contributes differently to the garnet phase than do other commonly used rare earth cations. The kinetics of melt equilibration have been studied and procedures to equilibrate the melt to ensure reproducible film growth are described. Electron microprobe analysis has been used to determine the La distribution coefficient as a function of La concentration, the growth rate of the film, and the melt temperature.

1. Introduction

garnet and provide a lattice match to GGG. Substitution of lanthanum has previously been used to lattice match Y gallium—iron garnet to GGG [2—5]. A small amount of La substituted for Eu in YEu iron—gallium garnet results in a larger negative magnetostriction coefficient and enables successful hard bubble suppression. Some magnetic characteristics of YLaTm garnet have been described by Smith et al. [6]. The distribution coefficient of La in both the YLaTm and YLaEu garnet systems is small and melt equilibration procedures have been developed to assure reproducible film growth.

Many film compositions have been reported which are useful for bubble domain devices. The general aim has been to develop materials which exhibit high domain wall velocity, low coercivity, and which can be used over a large temperature range. Both the YTrn and YEu iron—gallium garnet systems are attractive because they contain only low damping cations. These systems have certain disadvantages, however. Both Y and Tm iron garnets have smaller lattice parameters than the most commonly used non-magnetic garnet substrate, gadolinium gallium garnet (GGG). YEu iron—gallium garnet has too small a magnetostriction coefficient for reliable suppression of hard bubbles to be achieved by ion implantation [1]. The addition of the cation La alleviates both of these problems. La is a large ion and can be used to increase the lattice parameter of YTm iron—gallium

2. Experimental Kanthal wound furnaces with either a single heated zone or three heated zones were used. The temperature profile in a single zone furnace is largely deter366

BE Stein, M. Kestigian

/ LPE growth of YLaTm and

YLaEu garnet films

367

mined by furnace geometry, conduction losses from the ends of the furnace, and by the position of baffles which minimize convection currents. A three zone arrangement allows the temperature profile to be optimized by supplying different power to each zone, Temperature control in a three zone furnace is more complex, however, because of the interaction between zones, The principal differences between the three zone and single zone furnaces were the length of the uniform temperature zone and the temperature profile above the melt. A zone uniform in temperature to ±1°C was 12.5 cm long in the three zone furnace and 6cm long in the single zone furnace. The temperature

thickness uniformity of the upper film surface. Films were grown on (111) GGG substrates by the liquid phase epitaxy technique previously described [7,8] except that the substrates were rotated intermittently with a period of 3 sec at a rate of 60 rpm [9]. Rotation at rates less than 30 rpm or greater than 120 rpm led to a degradation of the thickness uniformity. The film thickness was uniform to 0.13 jim to within 1 mm of the edge on the lower film surface and to within 3 mm of the edge on the upper film surface for both 25 and 38 mm diameter substrates. Typical melt and film compositions are given for Y171La044Tin085Fe38Ga12O12 (YLaTm garnet)

at 5 cm above the crucible decreased by 5°Cin the single zone furnace. At the same distance, the ternperature was adjusted to increase by 2°Cin the three zone furnace. A higher temperature above the melt has the dual advantages of reducing convection currents in the solution and of assuring that the substrate is sufficiently pre-heated to prevent spurious nucleation on insertion into the melt. The crucible was 6 cm in diameter and 7 cm high. In the melt, which was 4 cm m depth, the temperature was uniform to ±0.25°C. The substrate holder shown in fig. 1 was used. It consists of a platinum disc of the same diameter as the substrate to which three Pt—5%Au wire legs 1 mm in diameter and 2 cm in length are attached. The curvature of the legs was needed to achieve satisfactory

and for Y2 38La0 09Eu0 53Fe3 9Ga1 1012 (YLaEu garnet) in table 1.

3. Results 3.1. Saturation temperature The saturation temperature T5 was defined for each melt as the temperature at which the growth rate was less than 0.05 jim/mm for a 10 mm growth time. T5 determined in this manner is reproducible to ±2.5°C. We have found that in garnet systems which do not contain La the saturation temperature is a strong function of the rare earth oxide concentration [7,8]. This is not the case for La203. No change in T5 was

ALUMINA ROD

~

Table 1 Typical melt compositions for growth of YLaTm and YLaEu garnets; the values are given in mole% YLaTm garnet a

PLATINUM HOLDER

SUBSTRATE

La103 Tm203 Eu203 Ga203 Fe203 PbO B203

0.30 0.43 0.17 —

1.53 10.90 81.23 5.44

a Y171La044Tm085Fe38Ga12O12. Fig. 1. Substrate holder

b Y238La009Eu053Fe39Ga1 .1012

YLaEu garnet b 0.43 0.05 —

0.10 1.36 9.76 82.95 5.35

B. F Stein, M. Kestigian / LPE growth of YLaT,n and YLaEu garnet films

368 Table 2

170°Cin T5 which would have occurred if La203

Saturation temperatures for melts containing La203 under several assumptions as to the extent of the La203 contribution (See text)

behaved as other rare earth oxides in the melt.

3.2. Distribution coefficient

_____

T5 z~T(La203 contribu(°C) tion not included) 800 895 920

940

960 980 1000

.~T(La203 contribution included)

The maximum La substitution in YIG has been shown to be approximately 0.40 by Espinosa et al.

20 5 0 10 25

50 55 80 170 150

[11]. The radius of the La ion is large and therefore the ion cannot be easily accommodated in the dodecahedral site in iron garnets. As a consequence, the

—5

130 150

limiting value. The distribution coefficient is defined as [12] [La/(La + R)]film aLa = [La/(La+ R)]meit~

s

~--

distribution coefficient is small, especially as the

lanthanum concentration in the film approaches the

-~------—-

found when La203 additions were made toa melt which increased the La203 concentration from 0.12 to 0.40 mole%. An increase in the Y203 concentration by that amount, for example, would have increased the saturation temperature by more than 200°C.It probably would have also decreased the iron oxide to rare earth oxide ratio to the extent that garnet would not be the primary phase. We can also see this in another way. In a previous study of the YEu iron—gallium garnet system we gave an experimentally determined plot of the saturation temperature as a function of both rare earth and iron—gallium oxide concentrations [7]. The T~ isotherms from this plot give the saturation temperature of many garnet system melts to ±10°C. Table 2 gives the temperature difference ~Tbetween the observed T5 and that which is expected from the plot under two extreme cases in which (a) all of the La203 present in the nielt contributes to T5, and (b) none of the La203 present in the melt contributes to T5. Melts were formulated with saturation temperatures between 800 C and 1000 C. For these melts, the observed T5 agree quite well (~Tis small) with the values expected from the plot for the assumption that no La203 contributes to T5. Bonner [10] has shown that in the yttrium samariurn calcium germanium system excess Ge02 acts as flux and reduces T5. The decrease in T5 for the largest La203 concentration which we have used would be only 1°Cif all of the La203 acted as flux. This is too

small to compensate for the increases of from 50

Film compositions were determined by means of electron microprobe analysis [13]. Pressed rare earth oxide powders were used as standards. The distribution coefficients of Y, Eu, and Tm were 1.00, 1.00 and 0.86, respectively and were unchanged from previous measurements in garnets which did not contain La. Compositions were also found from the X-ray lattice parameter and the known distribution coefficients of the other rare earths. Lead incorporation, which could cause anomalously large lattice constants, is negligible at the usual growth temperature of 990°C. Direct microprobe and lattice parameter determinations of the La composition agreed to ±2%for YLaTrn and to ±5%for YLaEu. The behavior of aLa as a function of composition is given in table 3 for a growth rate of 0.6 jim/mm in both YLaTm and in YLaEu garnets. aLa is independent of the Ga concentration up to 1.2 moles of Ga. The effect on the incorporation of a particular cation in one site of the garnet structure is dependent on the size of the cations which occupy the remaining sites. Since the ionic radii of trtvalent gallium and trivalent iron do not differ appreciably, and since only small amounts of gallium are substituted, this result is not surprising. The difference between the YLaTrn value of 0.19 and the YLaEu value of 0.31 arises because of the much smaller lanthanum concentration in YLaEu. In this case, the constraint of roughly 0.4 maximum moles of lanthanum in the film only minimally affects

369

BE Stein, M. Kestigian /LPE growth of YLaTm and YLaEu garnet films

temperature is maintained at 50°aboveT~.Film growth typically takes place with 10—15°C super-

Table 3 Lanthanum distribution coefficient in both YLaTm and YLaEu iron—gallium garnets

cooling. Between sequential YLaTm film growth exLa

___________

Yi.

7iLao.44Tno.85Fe5_~Ga~012 -

~‘2 38La0 09Eu0 53Fe3 9Ga1 1012

0

0.19

0.7 1.2

0.19 0.18

—

0.31

periments thestirred. solution must be heated to 50°Cabove T~and again Otherwise, film growth conditions will not be reproduced. For example, without this re-equilibration procedure the YLaTm film growth rate decreased by 0.1 pm/mm for the second

The dependence of aLa on the growth rate for YLaTm is shown in fig. 2. The increase of aLa with increasing growth rate is consistent with thermodynamic arguments that all distribution coefficients approach unity as the growth rate approaches infinity

film as compared to the first, with an attendant change in magnetic properties. At the end of each day, the system is again recycled to 150°Cabove T5 for 8 hr and again stirred. Stirring is not necessary for sequential film growth of YLaEu probably because of the considerably smaller amount of La2O3 present in the melt as compared to the YLaTm melt. The dependence of growth rate on supercooling

[14]. We have found aLa to be insensitive to temperature between 920°Cand 990°C.This is in contrast to aGa

also is related to the equilibration technique. In solutions prepared as described above, the growth rate is approximately 0.050 jim/min°C.If the solution is

which has a strong temperature dependence and again is related to the limited La incorporation in the garnet structure,

not heated and stirred between film growth experiments in YLaTrn, a 0.025 pm/min°Cgrowth rate is observed. These values can be compared to growth rates of 0.050 jim/min°Cand 0.075 jim/min°Cfor

4. Growth kinetics Solution equilibration in YLaTm is less straightforward than in other garnets which do not contain La. The mixed oxide powders are pre-melted and then heated to a temperature of 150°Cabove T5 for 16 hr. The solution is then stirred for 10—20 mm while the 022

YLaEu and YEu garnet respectively. Many garnet films of both the YLaTm and YLaEu compositions have been grown for use in bubble domain devices. The reproducibility of the growth process can be illustrated by two sets of ten YLaTm films prepared from separate melts at different times. The melt equilibration technique which has been de-

scribed above was carefully followed to achieve these results. Film thicknesses of 5.95 ±0.36 jim and 6.02 ±0.30 pm, respectively were obtained for the two sets. Two sets of eight YLaEu films were pre-

0.21

-

020

-

0La0.19 018

-

017

-

016

-

..

pared in a similar manner and film thicknesses of 5.96 ±0.31 jim and 5.93 ±0.24 pm were obtained, respectively. These variations are similar to those which iron gallium we have garnets observed which in do preparing not contain other La. rare earth

•

-

•,•

5. Discussion 0 ~

I 0

01

0.2

I

I

I

0.3 04 0.5 0.6 GROWTH RATE (/.~/mnI

I

I

07

08

09

The lanthanum ion is unique among the rare earth ions commonly used in liquid phase epitaxial growth

of gamets since additions of La 203 (as high as 0.43

Fig. 2. Distribution coefficient of lanthanum in YLaTm garnet as a function of growth rate,

mole%) to the melt do not increase the saturation temperature. Similar behavior has been noted previous-

370

B. F. Stein, M Kestigian / LPE growth of YI,aTm and YLaEu garnet films

ly for small La203 additions (0.08—0.16 mole%) to melts in which the films were grown isothernially without substrate rotation [2,3]. It is possible that the saturation temperature is independent of the La203 concentration because there is no stable lanthanum iron—gallium garnet (although small amounts of lanthanum can be incorporated into other rare earth garnets). A wide variety of lanthanum orthoferrites are stable, however. This implies that La203 additions contribute to a possible lanthanum orthoferrite phase in the melt and thus

do not contribute to the effective garnet oxide concentration or to T~.However, no orthoferrite precipitation has been noted under our growth conditions with the concentrations of La2O3 which we have

used. We observed an increase in aLa of approximately 50% for a fourfold increase in the film growth rate. These results are in agreement with those of both Tolksdorfet a]. [4] and Morgan [15] who studied statically grown films and are consistent with thermo-

earth oxide concentrations and may result from the absence of a stable lanthanum iron garnet phase. Procedures to equilibrate the melt have been described which result in a growth process whose reproducibility compares favorably with that obtained in other garnet systems.

Acknowledgments The authors thank A. Baltz, W. Bekebrede, W.D.

Doyle, R. Josephs, and A.B. Smith for many helpful technical discussions and F. Bradlee, C. Burilla, F. Garabedian, and J. Goodroe for expert experimental assistance.

References [~

dynamic considerations. Morgan [15] has shown that the Ghez and Giess

R. Wolfe, J.C. North and Y.P. Lai, AppI. Phys. Letters 22(1973)683. 121 W. Tolksdorf, G. Bartels, P. HoIst and W.T. Stacy, J. Crystal Growth 26 (1974) 122. [3] J.M. Robertson,W. Tolksdorf and H.D. Jonker, J. Crystal Growth 27 (1974) 241.

model for liquid phase epitaxial growth [16] which

141

was developed for growth with continuous axial substrate rotation applies to static vertical growth. This is a consequence of convective stirring. Our results suggest that the model also applies in the case of intermittent substrate rotation.

6. Conclusions Both YLaTm and YLaEu garnet compositionshave been developed which exhibit desirable properties for bubble domain devices. The La distribution coefficient is small and increases with increasing growth rate. The saturation temperature is independent of

W. Tolksdorf, G. Bartels, G.P. Espinosa, P. Holst, D. Mateika and F. Welz, J. Crystal Growth 17 (1972) 322.

[5] J. Haisma, G. Bartels and W. Tolksdorf, Philips Res. Rept. 29 (1974) 493. [6] Res. A.B. Bull. Smith, Kestigian 10M. (1975) 303. and W.R. Bekebrede, Mater. 171 B.F. Stein, AlP Conf. Proc. 18 (1974) 48. [8] B.F. Stein and R.M. Josephs, AlP Conf. Proc. 10 (1973) 329.

[9] R.G. Warren, i.E. [10] [ill [12] [13] [14]

Mee, F.S. Stearns and E.C. Whitcomb, AlP Conf. Proc. 18 (1973) 63. W.A. Bonner, Mater. Res. Bull. 10 (1973) 15. G.P. Espinosa, J. Chem. Phys. 37 (1962) 2344. S.L. Blank and J.W. Nielsen, J. Crystal Growth 17 (1972) 302. A. Baltz, private communication. J.A. Burton, R.C. Prim and W.P. Slichter, J. Chem. Phys. 21(1953)1987.

the lanthanum oxide concentration. This is in con-

115] A.E. Morgan, J. Crystal Growth 27 (1974) 266.

trast to th~strong dependence of T~on other rare

[16] R. Ghez and E.A. Giess, Mater. Res. Bull. 8 (1973) 31.