Pr, Bi and Pb segregation in LPE growth of garnet films for magnetooptical applications

132

Journal of Crystal GrowthPublishing 56 (1982) Company 132—136 North-Holland

Pr, Bi AND Pb SEGREGATION IN LPE GROWTH OF GARNET FILMS FOR MAGNETOOPTICAL APPLICATIONS J.M. DESVIGNES, J.M. CERCEAU, V.B. KRAVTCHENKO

*

and H. LE GALL

CNRS, Laboratoire de Magnétismeet d’Optique des Solides, F-92190 Meudon—Bellevue, France Received 30 April 1980; manuscript received in fmal form 27 January 1981

Pr, Bi and Pb segregation has been analyzed in LPE growth of garnet films having high Faraday rotation for magnetooptical applications in the near infrared and corresponding to the systems YIG:Bi, Pb; YbIG:Bi, Pr, Ga; and (YbGd)IG:Bi,4’~in Pr, Al. In dodeaddition to the usual incorporation of the in the dodecahedral site, it is shown that the model including Pb cahedral and octahedral sites can explain ourPb~ions data.

1. Introduction

[2] on GGG substrates with (111) orientation. The

Materials with high figures of merit for ØF/a, where ØF and a are the Faraday rotation (FR) and the absorption coefficient, respectively, are of inte-

0.6 and 1 inch disks were rotating horizontally at 60 rpm with reversed sense each second which gives growth rate up to 1 .7 pm/mm with PbO—B 203 or PbO—B203 —Bi2 03 flux. A very rapid spinning (1500 rpm) was used for flux droplet removal at low temperature and high Bi203 concentration growth. Film compositions were determined with an automated electron microprobe (Camebax) using an electron beam at 90°incidence to the film plane. The films were considered to be homogeneous in composition as a function of thickness and all the measured films were more than 2 pm thick. Standard materials used were Y3Fe5012, Yb3Fe5012, Gd3Fe5012, Gd3Ga5012, PrF3, PbSe, A1203 single crystals and the metals of Pb, Pt and Bi. The relative precision was ±1%for major constituents as Y, Yb, Gd, Al, Ga, Fe. The precision for Pb, Pt and Bi range from ±1%(for >2% weight) to ±3%(for <1% weight). Data are expressed as atoms per formula unit normalized to a total of 8 cations.

rest for magnetooptical applications. The best materials yet known are the ferrimagnetic garnets corresponding to the YIG—Gd, Ga and YIG—Th, Al tems which present a very low optical absorption in the near infrared range [1]. However, these cornpounds have moderate Faraday rotations between 200 and 400°/cm for the gadolinium and terbium substituted YIG, respectively. The bismuth and praseodymium cations are of special interest, since they induce a strong negative FR in garnets, but due to their large radius, it is difficult to introduce them in high concentrations in the dodecahedral sites of garnets. This films corresponding to the following garnet systems, YIG:Bi, Pb; YbIG:Bi, Pr, Ga and (YbGd)IG: Bi, Pr, Al have been grown on GGG( 111) substrates using the LPE method and analyzed with an electron microprobe. ~

3. Results 2. Experimental procedure About one hundred films were grown by the liquid phase epitaxy process using the dipping technique *

On leave from the Institute of Radiotechnics and Electronics of the Academy of Sciences of the USSR, Moscow.

0022-2048/82/0000—0000/$02.75 © 1982 North-Holland

3.1. YIG:Pb, Bi Epitaxial yttrium iron garnet films are of great interest for compact microwave filter applications. To obtain optimum filter properties some authors

f.M. Desvigneset ci.! Pr, Bland Pb segregation in LPE growth Pb-Pt/Mole

133

Bi/Mole

0.2

0.2

3

0.1

800

I

I

850

900

800

850

900

T

9

Fig. 1. Pb and Pt atoms per garnet formula unit as function of 7’~and Bi203 content: (a) Pb content — pure YIG; (b) Pt content — pure YIG; (1), (3) and (4) Pb content for 0.75, 2.95, and 5.74 mole% Bi203.

[3,4] tried to decrease the resonant linewidths by reducing the epitaxial strain either by controlling the Pb concentration or by introducing La~which requires a slight doping owing to its very large ionic radius. The incorporation of Pb into YIG is a function of two parameters: the saturation temperature (i’s) and the supersaturation L~Tdefined by 1~iT= T~ Tg where 7’~is the growth temperature (fig. 1, curve a). The same phenomenon holds for platinum for which, —

Table 1 Pb, Pt concentrations in atoms/mole in pure YIG asa function of growth temperature

rg

Pb

Pt

917 924 915 900 887 850 830

0.016 0.015 0.012 0.017 0.026 0.068 0.095

810 795

0.146 0.174 0.213

784

Y+

~

Fe+Pt

Pt

— —

0.609 0.611

—

—

0.610

—

(C)

0.005 0.013 0.016

0.615 0.619 0.625 0.633

0.021 0.027 0.029

0.634 0.631 0.639

—

—

5.20 5.23 5.94 6.95 6.44 7.34

deg.C

Fig. 2. Bi atoms as function of Tg and Bi203 in melt: (1) 0.75%; (2) 1.50%; (3) 2.95%; (4) 5.74%.

in addition, the crucible wear is to be considered (fig. I, curve b). In our films the platinum inclusion increases with the solution and crucible ages as reported by Nielsen et al. [5]. The results reported in table 1 are characteristic for new solutions and new crucibles. With older solutions we also obtained values close to those of Davies et al. [61,but this was due to an increase in the Pt concentration. The ratio (Y + Pb)/(Fe + Pt) varies from 0.609 to 0.639. This may be explained by either one of the two following hypotheses: The transfer of a certain quantity of Y~substituting Fe~into the “a” octahedral site. For example a high ratio as 0.639 can be explained by a transfer of 0.12 Y~.Glass et al. [7] demonstrated that this hypothesis is satisfying with respect to the lattice parameter and the magnetic properties. But when Pb/Pt > 1, a positive charge deficiency due to the excess of Pb~implies the presence of Fe* or of oxy—

gen vacancies to maintain the electrical neutrality. 4~ In factdetected were no absorption in YIG:Pb bands [8] attributable or in YIG:Ca, to FeGa films [9] and if Pb2~is charge compensated by oxygen vacancies, the Pb2~concentration should increase as the level of vacancies with increasing Tg. As shown in fig. 1 incorporation of Pb increases with decreasing Tg. The presence of Pb2~in the dodecahedral site (site “c”) and of Pb4~in the “c” and “a” sites —

134

J.M. Desvigneseta!. /Pr, Bland Pb segregation in LPE growth

[9]. This hypothesis developed by Scott et al. [9] is also adequate from magnetic and optical points of view. From the results obtained with films of YIG—Bi, Pb we note that: (1) At a constant L~Tthe Pb concentration decreases with increasing Bi203 content in the melt (fig. 1).

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(2) The Bi concentration increases as a function of ~T, however, with a saturation process (fig. 2). (3) The incorporation of Bi into the film does not change the ratio (Y + Pb + Bi)/(Fe + Pt) which is only dependent on the Pb content. This is confirmed by the other compositions reported in this paper (table 2). If we assume the first hypothesis, the transfer of yttrium into an “a” site is possible due to the cell expansion resulting2~(1.29 from the of a A,incorporation CN8). The same large ion, such asshould Pb thus be found for bismuth type of variation which has a large ionic radius (1.12—1.13 A), but we only found a ratio variation connected with Pb content. Fig. 3 shows a discontinuity in thePb = J~ Bi) curves which can be explained by the presence

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f.M. Desvignes et al. / Pr, Bland Pb segregation in LPE growth

C). The exact substitution of two Pb by one Bi ion is partly shielded by a Y3”’ substitution because it implies a very rapid decrease of lattice parameter, up to 0.0229 A for 0.1 Bi (calculated value with +0.1 Bi site “c”, —0.1 Pb2” site “c”, —0.1 Pbh~site “a”). There is a change in the slope only when new 2” ora Pb4” equilibrium appears in more the cell. The due Pb to the fact concentration decreases slowly, that the Bi3”’ concentration becomes sufficiently high to again expand the dodecahedral site. This change in the slope occurs at higher Bi concentrations when Tg decreases. This may be attributed to a higher Pb4”’ concentration in the solution at low temperatures. 3.2. YbIG:Bi, F~,Ga and YbGdIG:Bi, F~,Al These systems were chosen because they make it possible to change the physical properties of the films to a very large extent. Since we are interested in high FR, we have to use Bi ions which give large negative specific rotations. The Pr ions are also useful for the following two reasons. First, among the rare earths Pr has the largest negative specific FR per ion (900°/cm at 1 .15 pm) and second, it induces an “in plane” easy magnetization in the garnet film. Ytterbium garnets permit the incorporation of large cations without requiring high gallium or aluminium concentrations which decreases the FR when Ga or Al > 0.3—0.5 [1] by reducing the iron contribution. A substitution with gadolinium permits the adjustment of the magnetization by varying only the magnetic moment of the dodecahedral site [10]. The results are reported in table 2. The Pb concentrations remain very low (0.010—0.035) even in films obtained at low temperatures and high supersaturation (60°C).The addition of high concentrations of Bi203 does not increase

135

and Pr contents, we note that the increase of c/(a + d) is much too small to be attributed to a transfer of ytterbium from a dodecahedral to an octahedral site. In addition, the solutions used were particularly rich in B 203 (PbO/(PbO + B203) = 0.93—0.94) and according to Blank et al. oxide [11] which the rarewould earth inhibit would be complexed by boron its incorporation into other than dodecahedral sites. The gallium segregation coefficient in YbIG (1 .7— 1.9) is very close to that of aluminium in YbGdIG (1.1—2) under comparable growth conditions. The difference in the ionic radius of these two ions is compensated by the difference in the lattice parameters of these substituted garnets. Connected with growth temperature we obtained variations of KBI from 0.21 to 0.53, but only from 0.34 to 0.45 for KPr (fig. 4). KBi is calculated as the ratio of the molar contents of Bi in the ifim and in the melt or Bi ~‘ilm 100 melt K Bi = 8~ ~Bi 203 KPr is defined as Pr film ~ R~O3melt KPr = Pr203 We used these special coefficients since Bi203 is not considered only as Bi cation is dodecahedral site but

-3’i

K 0.6

-

Bi’

I

K

Pr

0.5

0.4

.

the solution attack on the crucible. The site ratio

c/(a the to the platinum). + Pr d) content increasing varies0.82) This from Pb increase content to in in0.622 the the ratio (not An including increase related in the from 0.17 to 034 (Pr +is Bi varies from at0.607 constant Pbcell. does not alter the site0.74 ratioto(films No. 300 and 306). Similarly, an increase in the Bi content at constant Pb and Pr (No. 313 and 316) induces only a ratio variation of 0.004. If we compare the fiThis at constant Pb (300, 306, 313, 315, 316), but with a 35% variation in the Bi

0.3 0.2

N

Bi

I

I

I

760

780

800

820

T~

Fig. 4. Dependence of KBj (.) and Kp~(A)on growth tern-

perature.

136

f.M. Desvigneset al. / Pr, Bi and Pb segregation in LPE growth

in addition as a flux constituent. The other distribution coefficients KAI, KGa are determined from usual calculations. The measured Faraday rotations reach 1800°/cm (film No. 316) at 1.15 pm wavelength (12000°/cm at 0.6328 pm) with figures of merit of 50—80 deg dB~at 1.15 pm (1.38 to 2.76 deg dB1 at 0.6328 pm).

4. Conclusion The study of the c/(a + d) ratio obtained from the results of the electron microprobe analysis shows that this ratio only increases with the lead concentration and even a substitution with a large ion as bismuth does not change it. Thus the model of a rare earth transfer into the octahedral site cannot be used more especially since it should imply Fe4” or oxygen vacancies. On the other hand our results can be explained by the presence of Pb2~and Pb4”’. Moreover, films having high Faraday rotations (up to 1800°/cm at 1.15 pm wavelength were grown from LPE on GGG substrates

Acknowledgement

The authors would like to thank Dr. P. Feldmann for useful discussions and Mrs. Rommeluere for electron microprobe analyses.

References [1] M. Torfeh, J.M. Desvignes and H. Le Gall, Intern. Colloq. on Magnetic Films and Surfaces, 1979, p. 127. [2] H.J. Levinstein, S. Licht, R.W. Landorf and S. Blank, Appl. Phys. Letters 19 (1971) 486. [3] H.L. Glass and M.T. Elliott, J. Crystal Growth 34 (1976) 285. [4] M. Nerniroff, IEEE Trans. Magnetics MAG-13 (1977) 1238. [5] J.W. Nielsen, S.J. Licht and C.D. Brandle, IEEE Trans. Magnetics MAG-lO (1974) 474. [6] I.E. Davies, E.A. Giess, J.D. Kuptsis and W. Reuter, J. Crystal Growth 36(1976)191. [7] H.L. Glass and M.T. Elliott, I. Crystal Growth 27 (1974) 253.

[8] Pailhier-Malécot, H. Le Gall, R. Krishnan, TranonKhan Vien and A. Le Intern. Conf. Magnetism, Moscow,

1973, part 2, p. 122. [9] G.B. Scott and J.L. Page, 1. Appl. Phys. 48 (1977) 1342. [101 V.B. Kravtchenko, J.M. Desvignes, H. Le Gall and J.M. Cerceau, Mater. Res. Bull. 14 (1979) 559. [11] chem. S.L. Blank, W.A. Biolsi856. and J.W. Nielsen, J. ElectrOSoc. 123 (1976)