t Solid State Coninunications, Vol.31, pp.403—408.

Pergamon Press Ltd. 1979. Printed in Great Britain.

FACTORSAFFECTING COERCIVITY IN (Y,Sm,Tm)3(FeGa)5012

AND (Y,Sm,Lu,Ca)3(FeGe)5012 LPE FILMS

S. G. Parker

Texas Instruments Incorporated, Physical Sciences Research LaboratoryDallas, Texas 75265, USA

Received 26 February 1979 A.G. Chynoweth

Factors which Influence coercivity, H~, in (Y,Sm,Lu,Ca)3(FeGe)5012

films and in (Y,Sm,Tm)3(FeGa)5012 films grown by LPE have been

identified. An anomalous layer at the film—substrate interfaceexhibits coercivity values different from that of the middle, bulk,portion of the film. The contribution of the transient layer atthe substrate interface could be reduced by Increasing the rota-tion rate while immersing the substrate into the melt. Filmscontaining Ga show lower coercivities than films containingCa—Gepossibly because films with Ga are more uniform in

composition. Films with Ga show increased coercivities withincreasing growth rates and with increasing Sm content through-

out the film.

One of the requirements of garnet films reported thickness dependence of coercivity infor magnetic bubble memory devices is that the an EuGa garnet film. More recently Keszei andcoercivity, which is broadly defined as a Pardavi-Horvath

11 showed how the coercivity ofmeasure of film perfection, be as low as LPE (Y,Sm,Ca)

3(FeGe)5012 films was influencedpossible. Various theories have been advanced by growth rate and by Ge content of the melt.for the source or cause of coercivity. Davies Further, the overall coercivity was due to aand Giess

1 observed that coercivity appeared low-value, bulk coercivity and a higher value,to scale with anisotropy. Nielsen, et al2 transient layer coercivity.noted that garnets containing ions with a From the above it is apparent that thelarge orbital momentum possess the highest cause and control of coercivity are not clearlycoercivitles but did not explain why this is defined. Actually coercivity seems to betrue. Increases in coercivity for dependent upon several factors such as film(Y,Eu,Yb)

3(FeGa)5012 films with Increasing Ga composition and growth conditions so that nocontent was attributed to lattice mismatch by one factor can probably be cited as the sourceMoody, et al

3. For (Y,Sm,Ca)3(FeGe)5012 films of coercivity. Here, the influence of growth

Kestlylan, et alk concluded that high coercivi— rate and growth conditions ofty was related to the Sm content rather than (Y,Sm,Tm)3(FeGa)5012 andlattice mismatch. However, Bonner, et al

5 (Y,Sm,Lu,Ca)3(FeGe)5012 films grown by LPE

found high coercivity in (Y,Lu,Ca)3(FeGe)5012 gives some insight into the causes offilms in which. there was no Sm. Sumner and coercivity.Cox

6 thought that high coercivity of Experimental Conditions:(y,sm,Lu,Ca)

3(FeGe)5o12 films was related to Both (Y,Sin,Tm)3(FeGa)5012 anda Ca-Ge Imbalance. Parker and Cox

7 obtained (V,Sm,Lu,Ca)3(FeGe)5012 garnet films were

additional Information that suggested a Ca-Ge grown on either one or two inch diameterimbalance contributed to high coercivity but Gd3Ga5O12 (GGG) substrates by liquid phaseother factors such as growth rate, Ge/Sm ratio epitaxy from supercooled, isothermal meltsand lattice mismatch influenced coercivity as using ap~aratus and procedures previouslywell, reported ,12, Typical melt compositions are

Mlkami8 proposed on the basis of anneal- given in Table I.

ing studies on CaGe garnet films grown by The melts were prepared by meltingliquid phase epitaxy that coercivity was together in a Pt crucible the desired amountscaused by cation and oxygen vacancies. Moore, of oxides at 1100°C for several hours whileet a19 observed that there are two contribu— stirring. The solutions were held at ~5O°Ctions to the net coercivity of above the saturation temperatures between(y,sm,Lu,ca)

3(FeGe)5o12 films; one is growth runs. Periodic additions of PbO andassociated with the film/GGG substrate garnet oxides were made to replenish that lostinterface and the other with the bulk of the by volatilization and deposition.film. The coercivity was greater in the The effects of growth rate and melt compo-fIim/GGG interface layer or transient layer sition upon coercivity were studied together withthan in the bulk of the film. Brown

1° also growth conditions. Growth rate is determined by

403

404 FACTORS AFFECTING COERCIVITY IN LPE FILMS Vol. 31, No. 6

However, for this work we elected to charac-Table I. terize these films using static coercivity as a

Composition of Melts Used for LPE Growth material parameter because It was more readilyA. (Y,Sm,Lu,Ca)

3(FeGe)5012 Material and obtained. We also felt that the static coercivltyB. (Y,Sm,Tm)3(FeGa)5012 Material, method would be more accurate for very thin films

__________________________________________ (<2iim thick) than the dynamic coercivity methodA B which requires a visible assessment of bubble move-

Oxide Weight (g) Oxide Weight (g) sent. We feel our coercivity measurements areaccurate to +0.05 Oe while the dynamic coercivity

5.253 Y203 2.012 measurements were reported by Walling to be

Sm203 0.541 Sm203 1.570 accurate to +30%. Our primary Interest is intrends of coercivity as related to film composi-Lu203 0.613 Tm2O3 3.580 tion. Because static coercivity scales with dy-

CaO 3.712 Ga 0 10 500 namic coercivity and is readily measured, we felt2 3 that it would be more suitable for our needs,

GeO2 5.774 Fe203 121 .500 Results:

Fe2O3 ‘~5.O0O PbO 1500.000 Measurements of coercivity, Hc~after thematerial was etched away in small increments inPbO 500.000 B2O3 30.000 H3POk at 11+0°C showed increases for (Y,Sm,Lu,Ca)3

B 0 10 000 (FeGe)5012 films as the substrate is approached.2 3 As illustrated by Curve A In Figure 1, the coer-

civity in the bulk of the film was rather low butthe degree of supercoolfng below the saturation attained values greater than 1 Oe near the sub—temperature, and by the rotation rate of the sub— strate. Others have reported similar results

9’11’18strate during growth. Because garnet films con- but here the transient layer, which contributestaming CaGe usually have greater coercivities most to the coercivity, is much thicker thanthan Ga—containing films13, both types of films previously reported. The thickness of this tran-were grown for comparison. The procedures used sient layer and hence the coercivity could befor introducing the slices into the melt also changed by changing the rotation rate of the sub-were studied, since these had an effect on coer- strate while inserting the substrate into thecivity. melt as shown by Curve B in fig. 1. Growth of

Film thickness was measured by the inter- the film of Curve A was at 60 rpm, but there wasference fringe technique1~ while the material no rotation of the substrate as it was loweredlength, I, and saturation magnetization, Li,rM, into the melt until a specific depth was reachedwere obtained from microscope measurements of which required about three minutes. For thebubble collapse field, H

0, and demagnetized material of Curve B the substrate was rotated atstripe period using the method of Fowlis and 60 rpm while inserting into the melt and duringCopeland

11’. The unlaxial anisotropy field, HK, growth. The coercivlty of fIlm B remained ratherwas calculated from ferromagnetic resonance data15. small until about O.2um from the substrate. FromThe lattice match between the GGG substrates and these curves the thickness of the transient layersthe epitaxial film, ~a

0 = a substrate -a film, was deduced to be approximately 0.Zpm and l.0~m forwas determined by x—ray dif?raction. E1e~tron rotation of the substrate going into the melt andbeam microprobe analysis was used to obtain film no rotation respectively.compositions. Relative concentrations of the Changes in lattice mismatch, I~a , were deter—elements in the films at various depths were mined for the film used, in Curve A ~?fig. I. Thesemeasured by SIMS or ion probe analysis which changes in i~a as material was etched off is givenconsists of volatilization of the film’s components by Curve C in°fig. 1. Because these materials obeyas ions by bombardment with He ions. The ionized Vegard’s law, the lattice constant can be taken ascomponents are then determined by mass spectro- an indication of composition. From Curve C it canscopy. be seen that the film does not appear to be uniform

The domain wall (static) coercivity, Hc, was in composition and that large changes in Aa0 beganmeasured with an optical magnetometer’

6. This to occur about 2~im from the substrate also. Thetechnique employs an ac—bias field to move domain lattice match of film and substrate for the mater-walls. Another technique which gives the dynamic ial of Curve B showed essentially no change asor propagation coercivity involves measurements material was etched away which Indicated that thisin pulsed bubble propagation experiments. Both material was rather homogeneous.techniques are widely used and, since the whole Ion probe analysis of a film grown under con-field of coercivity measurements and their inter- ditions similar to that for the Curve A film inpretation is in a state of evolution, it should be fig I also showed compositional variations,. Thisemphasized that each technique yields a physically is illustrated in Figure 2 in which the relativedifferent though related quantity. Dynamic coer- signal intensity of the various elements in thecivity is favored by some because bubble transla- film were determined at different depths into thetion best approximates conditions encountered in film, It can be seen that only minor changes occurdevice operation. Static coercivity on the other in the Ca, Fe, and V content throughout the ‘film;hand involves size changes in domains and includes however, there are large changes in the Sm, Lu,magnetic restoring forces. Walling’7 has derived and Ge content. It is interesting to note thatan equation relating a special case of static and the non—uniformity in the transient layer extendsdynamic coercivity and has measured both types of about l-l.5um into the bulk of the film from thecoercivity on the same films. In all cases it was substrate. Ga and Gd also began to appear In thefound that the dynamic coercivity was higher. We epi film about 1Mm from the substrate. Thishave obtained similar results with both techniques suggests that melt back of the GGGsubstrate occurson numerous films, and that Gd and Ga in the boundary layer above the

Vol. 31, No. 6 FACTORSAFFECTING COERCIVITY IN LPE FILMS 405

substrate become incorporated into the epitaxial where the relative signal Intensities of thefilm as it grows. elements present are plotted for different

Ion probe analysis of a film grown under depths in the film. In this film theconditions similar to that for Curve B film of composition is rather uniform throughout forFigure 1 showed that the material was rather all elements. Ga and Gd began to be detectedhomogeneous. This is illustrated In Figure 3 about 0.1+ um from the substrate which is less

SUBSTRATE

FILM SURFACE/ IN

-a — • - 3.0

I_~_—

I- IU) I =° ‘25

—U)

o I• I

I.)=

U.’~ -3- I

1.50IU)

—2- I \ I

I.-

—1 - A•—• ‘ 0.5

•—

_1 .L~::____.. .—. ~ ‘~,_~

0 1 2 3 4 5 6

POSITION, pm

Figure 1. Changes in coercivity and lattice match of film andsubstrate as LPE film was etched. Curves A and C,no rotation of substrate while inserting into melt.Curve B, rotation of substrate at 60 rpm whileinserting into the melt. (Y,Sm,Lu,Ca)

3(Fe,Ge)5O,2

garnet film.

Ga & GdL— FILM SURFACE PRESENTI Fe

10 ,~_____•,__• •

~ :~—~--“ ~ ___.o__.__ .,,~0,__..o<‘N SUBSTRATEINTERFACE54\G~7Lu

3 - O—~I.1 £U.’ Ca

0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5POSITION, pm

Figure 2. ion probe analysis of LPE (Y,Sm,Lu,Ca)3(FeGe)5O,2film grown with no rotation while inserting intothe melt.

406 FACTORS AFFECTING COERCIVITY IN LPE FILMS Vol. 31, No. 6

FILM SURFACE Gd & Ga

Fe PRESENT10 ~__•__ —. . .

~ o__~.—o———a _______ I0 —0

a— I Sm! 7o~

a • - ~o~’~° ~~~STRATECa

°No~o~0~5-

—4-Lu

____ __________

21—Ge

I I I I0.4 0.2 1.2 1.6 2.0 2.4

POSITION. pm

Figure 3. Ion probe analysis of LPE (Y,Sm,Lu,Ca)3(FeGe)5O12film grown with 60 rpm rotation going into the melt.

FILM SURFACE

15 \ SUBSTRATE 0

Tm /)- .U) — Fea

— F~\.__.____...___.~_U __._U~ 10 ______U.’ ~ C \U.’ y \.5

a

I I I I I I0.5 1.0 1.5 2.0 2.5 3.0

DISTANCE, pm

Figure 4. Ion probe analysis of LPE (Y,Sm,Tm)3(FeGa)5O32film grown with rotation at 60 rpm going into themelt.

Vol. 31, No. 6 FACTORSAFFECTING COERCIVITY IN LPE FILMS 407

than for a film in which there is no rotation substrate at 60 rpm at all times showed littleof the substrate whIle going Into the melt. or no changes as the film was etched away InThe coercivity of this film was low like that small Increments.of the Curve B film of Figure 1. The effects of rotation rate and

The above results suggest that the undercoollng upon coercivity was alsoIncreased coercivity which occurs in the determined for (V,Sm,Tm)

3(FeGa)5012 films. Astransient layers is caused by compositional shown in Table 2, the coercivlty increased asnon-uniformities. Competition of the various the rotation rate increased; these results areions for different sites in the lattice opposite to those reported by Keszel andpossibly lead to this non—uniformity as well Pardavi—Horvath

11 for (Y,Sm,Lu,Ca)3(FeGe)5012

as the formatIon of vacancies and/or films.Interstitials which result in crystalline Changes in rotation rate would influenceimperfections. Change in magnetic forces the growth rate and tend to make the segrega-around these defects probably manifest tion coefficients for the various ionsthemselves as the coerclvity. approach unity as the rotation rate increased.

The growth of (V,Sm,Tm)3(FeGa)5012 garnet As an example in the Ga melt the segregationfilms under conditions similar to those used coefficient of Sm was determined to be 0.85.for the growth of (Y,Sm,Lu,Ca)3(FeGe)5012 Thus, the Sm content of a Ga—containing filmfilms always gave lower values of coercivity. should increase as the rotation rate increasedThe coerclvity of Ga-containing films usually and perhaps cause an increase in coercivity

4.ranged from 0-0.20 Oe but was never greater Ion probe analysis showed that film numberthan 0.1+ Dc. The coercivity of Ca, Ge—films 1963 grown at 141+ rpm indeed contained 7 ½%ranged from 0.1-1.0 Dc and was sometimes more Sm throughout the entire film thicknessgreater than 3.0 Qe. The higher coercivity than film number 1956 grown at 30 rpm. Inin CaGe films Is believed to be related to Table 2, it can also be seen that the latticethe difficulties in maintaining the proper Ca match, t~ao,and the unlaxial anisotropy, HK,to Ge ratio In the transIent layer In the also increase with increasing rotation rate;presence of Ga which comes from the melt back increases in these properties are most likelyof the substrate. The Ga and Ge are competing caused by increases in Sm concentration.for the same sites in the garnet structure. The effects of undercooling at constantIn the Ga melts, the Ga present tends to rotation rate upon coercivity of Y, Sm, Tm,restrict melt back so that Gd begins to be Ga, Fe garnet films were also studied asdetected in a film about 0.2 pm from the illustrated in Table 3, the coercivity, Hc,substrate as contrasted to 0.4 pm for a CaGe tends to increase as the degree offilm grown under the same conditions as the undercooling or growth rate used during filmGa film. Further as shown in Figure 1+, the deposition increases.uniformity of a (Y,Sm,Tm)

3(FeGa)5O12 film as It can also be seen in Table 3 thatdetermined by ion probe is rather uniform ~a0 and HK increase with increasingthroughout the entire bulk of film thickness. supercooling. All this suggests increases inMeasurements of Hc and ~a0 of V,Sm,Tm,Fe,Ga growth rate either by increased rotation rategarnet films grown by rotation of the or by a greater degree of supercool ing causes

Table 2

Effects of Rotation Rate Upon Various FilmProperties for Y, Sm, Tm. Ga, Fe Garnet Films

Film Growth Rotation t, Hc, ~a0, HK,No. Temp, °C Rate,um/mln Rate, rpm pm Oe mA Dc

1956 955 0.32 30 0.22 0.10 -5.31 1656

1962 955 0.53 50 0.21 0.25 -6.89 1951

1963 955 0.85 144 0.21 0.40 -8.03 2108

Table 3Effects of Degree of Supercoollng Upon

Various Epitaxial Film Properties

Film Growth Rotation L, H~, 1a0, HK,No. Temp. ~C Rate,pm/mln Rate,rpm urn Oe mA Dc

1940 960 0.35 100 0.21 0.1 -3.70 16501949 950 0.91 100 0.21 0.2 -9,06 23251948 940 1.26 100 0.21 0.3 -11.93 2600

408 FACTORS AFFECTING COERCIVITY IN LPE FILMS Vol. 31, No. 6

the Sm content of the films to increase with content and hence the coercivity of theseattendent Increases in Hc, ~a0, and HK. Films films is determined by the growth rate.No. 1949 and 1963 which have similar growth Increases in growth rate tend to giverates do not have the same properties because increases in coercivity for these Gathey were grown from different solutions. containing films. Keszel and Pardavi—Horvath

11Discussion: observed increases in coercivities with

From the data presented here as well as increases in growth rate due to the degree ofthat of others1”~3 it appears that coercivity supercool ing but found decreases in coercivityof LPE garnet films may be caused by several with increasing rotation rate which would also

factors. Our data supports the findings of increase the growth rate. In their CaGeMoore, et al9, Keszei, et al11, and Brown10 material the reduction in Hc with increasingthat coercivity in some films consists of a rotation rate apparently involves reducing thelow value bulk coercivity and a higher value thickness of the transIent layer so that thecoercivity in a transient layer at the non-uniformity contribution to the overa!1 Hcsubstrate film interface. The thickness of is less. Their increase in Hc with increasingthis transient layer which contributes to a undercooling at a constant rotational ratehigher coercivity can be controlled by the probably is due to the composition of the bulkrotation rate used during growth especially of the film.during the initial growth period. In The results reported have indicated thataddition, the thickness of this transient coercivity of LPE films Is related to thelayer is influenced by the melt composition chemical composition in both the bulk and theused and by melt back of the substrate. CaGe transient layer. In CaGe films the chemicalmelts give thicker transient layers with composition of the transient layer seems togreater coercivities than films grown from Ga contribute most to the coercivity, while in Gamelts under the same conditions. Higher films the chemical composition of the bulkcoercivity, of the CaGe films seems to be layer determines the coercivity. The chemicalrelated to compositional non—uniformity in the composition of the LPE films is determined bytransient layer. Ion probe date in this study the melt composition and the growth conditions.together with compositional analysis of CaGefilms7 tend to confirm that high coercivity is Acknowledgement - The author wishes to thankrelated to a Ca and Ge imbalance. Keszei and Drs. C.T.M. Chang, R. Korenstein and D.W. ShawPardavl~HorvathU showed that the Ca/Ge ratio for helpful discussions and coninents.in the melt had a large influence upon Acknowledgement Is also made to N.E. Tidwellresulting coercivitles of grown films, for growth of the epitaxial films and to

For V, Sm, Tm, Ga, .Fe garnet films the Drs. C.A. Castro, R.D. Dobrott andcoercivlty seems to be related to the Sm J.H. Tregilgas for measurement of filmcontent throughout the entire film. The Sm properties.

REFERENCES

1. J.E. Davies and E.A. Giess, J. Materials 10. B.R. Brown, AlP Conference Proceedings, 29,Science 10, 2156 (1975). 69 (l97~.

2. J.W. Nielsen, S.L. Blank, D.H. Smith, G.P. 1’. B. Keszei and M. Pardavi—Horvath, IEEEVella—Coleiro, F.B. Hagedorn, R.L. Barns, Transaction Magnetics Mag-l4, 605 (1978).and W.A. Biolsi, .J. Electronic Materials, 12. R. Ghez and E.A. Giess, J. Crystal Growth,3, 693 (1974). 27, 221 (1974).

3. J.W. Moody, R.W. Shaw, R.M. Sandfort, R.L. 13. G.P. Vella—Coleiro, F.B. Hagedorn, S.L. Blank,Stermer, Materials Research Buljetin, 9, and L.C. Luther, Presented at 3M Conference527 (1974). Cleveland, Ohio, November 1978.

4. M. Kestigian, A.B. Smith, W.R. Bekebrede, 14. D.C. Fowlis and J.A. Copeland, AlP ConferenceMaterials Research Bulletin, II, 773 (1976). Proceedings Series 5, 240 (1971).

5. W.A. Bonner, J.E. Geusic, D.H. Smith, L.G. 15. R.C. LeCraw and R.D. Pierce, AlP ConferenceVan Uitert, G.R. Vella—Coleiro, Materials Proceedings No. 5, Magnetism and MagneticResearch Bulletin, 8, 1223 (1973). Materials, 1971 (Amerlcal Institute Physics,

6. G.G. Sumner and W.R. Cox, AlP Conference New York, 1973) p. 200.Proceedings No. 31+, 157 (1976). 16. W.R. Cox and S.G. Parker, Materials Research

7. S.G. Parker and W.R. Cox, J. Crystal Growth, Bulletin 13, 501 (1978).42, 334 (1977). 17. J.C. Walling, Presented at 3M Conference

8. M. Mikami, Materials Research Bulletin 13, Cleveland, Ohio, November 1978681 (1978). 18. J.E. Davies, E.A. Giess and Kuptsis, Materials

9. E.B. Moore, B.A. Calhoun and K. Lee, J. Research Bulletin 10, 65 (1975).Applied Physics, 49,1879 (1978).