The growth of SmCo5 crystals by the bridgman technique

Journal of Crystal Growth 18 (1973) 7-12 © North-H¢,llam~ Publishing Co.

THE GROWTH OF SmCos CRYSTALS BY T H E BRIDGMAN TECHNIQUE* J. F. MILLER and A. E. AUSTIN" Battelle, Columbus Laboratories, Columbus, 0hio43201, U.S.A.

Received 30 July 1972 A method was developed for the growth of SmCos crystals which consistently yields large crystals (utp to about 100 g anti up to 2 cm dimensions). Growth is by directional solidification of a samsrium-rich melt by the Bridgman technique at a rate _<1 mm/hr.

1. Introduction S m C o s and other rare earth-cobalt t:ompounds currently are o f interest and are unuer d e v e l o p m e n t as

permanent magnet materials because of their high coercive forces, high energy products, and high Cur;e temperaturest). The objective of the w,~rk which is to be discussed was the growth of sizeable single crystals of the RCo5 compound; (where R denotes yttrium or a lanthanide rare-earth metal) to be used in studies of properties that are pertinent to the applications technology. Factor~ of pertinence are those such as the intrinsic magnetic properties and the eff~ts of deformation, defects, and impurities on the magnetic domains and domain-waU movement. The growth of macro-crystals of the rare earth--cobalt compounds has proven to be difficult because (!) the majority of the compounds of interest are peritectie phases, and (2) the materials are extremely reactive (especially at elevated temperatures and in the molten state). In the past, techniques such as arc or levitation melting followed by strain annealing have been tried2), but with only limited success because of peritecttc rea_ct.io.ns and inherently poor process control. Although the growth of crystals of the yttrium, neodymium, and dysprosium compounds by the zone-melt technique has been reported3), crystals obtained were small (2-3 mm maximum dimensionsl and no work was done with the samarium compound. In the initial stages of our work on the growth of SmCos several growth methods were explored, in-

duding chemical vapor deposition both from ha~.ide and organometallic precursors, and zone-melt growth both i , refractory and water-cooled copper containers. While vapor deposit;on from organometallic source materials appeared to have potential as a method for the deposition of Sin-Co magnetic films, none of these methods appeared promising for the growth of macrocrystals of SmCos. This paper describes the method which was developed for the growth of SmCos crystals, utilizing the Bridgman technique, that consistently has yielded large crystals (up to approximately 100 g in weig;-t ~.ad 2-cm dimensions). 2. Crystal growth Since SmCo 5 is a peritectLc phase't), growth from the melt must involve solidification from a melt which contains excess samarium. A~ solidification of the equilibrium solid phase occurs, a significant change', of composition must occur, with the excess samarium being rejected from the crystallizing solid into the i nelt in the interface region. To allow the excess samarium to be dissipated by diffusion an~i to avoid constitutional su_percooling, linear crystaUizat,on rates mast be very low. "l~,u3. long periods at elevated temperatures with material in the molten state are required fo~ the growth of sizeable cr-yst,~ls. In attempting to provide the required conditions for * This work was supported by Air Force Materials Laboratory, Air Force Systems t.ommand, Wright-PattersonAir Force Base, Ohio, under Contract F 336! 5-70-C!477.

J. F. M I L L E R A N D A. E. A U S T I N

the growth of SmCo s crystals, two major problems were encounte[,.~l: (1) a container problem, and (2) dit~culties arising because of :he appreciable vapor pressure of Sm at the growth temperature. In the investigation of the container problem, details of which have been published previously s), no container material was found that will completely resist attack by the molten Sin--Co at the growth temperatures. For example, it was found that high-purity recrystallized alumina, which had been reported to be a satisfactory and inert container for the rare earth--cobalt materials up to 1600 °C 3), was attacked rapidly by molten SmCo and Nd--Co materials at 1350 °C. The alumina could not, in fact, be used in the inner growth system assembly at al[l since it was attacked by both samarium melt and vapor, thus effectively removing samarium from the growth system as Sm203 and Sm-AI compounds and producing oxide contamination of the Sin-Co melt. ltowever, it was found that crucibles of p)rolytic boron nitride resisted attack to a much greater degree and were the most satisfactory containers. The rate of attack on the pyrolytic boron nitride was very low, the reaction products (SmN and borides) formed a barrier layer and were soluble to only a limited degree in SmCos. The dit~culties arising from the high vapor pressure of samarium were overcome by utilizing an entirely closed outer tantalum ampoule which served both as a susceptor for rf energy and as a container of the Sm vapor. The system used for the Bridgman growth of SmCo s is shown schematically in fig. 1. As can be seen, the pyrolytic boron nitride cry~tal-growth crucible is held within a closed tantalum susceptor that is suspended within a sealed silica growth ampoule, along with a quantity of samarium metal which is used to getter the growth atmosphere. The boron nitride crucibles used in most of this work were 1-inch diameter, 2 inches high, with a 90-degree conical tip. "rt.~.,~ . . .-=~a,o' . . . . ...~. ' for the ~t ........ ~ U-~ °O U-i-L "O - 5 w~r~ ,. ow tn , distilled samarium metal produced from 99.9% oxide* and vacuum-processed (99.94 + %) cobalt**. * Typical supplier's (Research Chemicals Division, Nuclear Corporation of America) analysis in weight percent: 0.0t ~', 0.02 Ca, 0.005 Ms, 0.001 Si, 0.005 Fe, 0.01 Gd. ** "l ypical supplier's (Materials Research Corpora, ~on) ,~,.mission spe~rographic analysis in ppm by weight: 40 C, 2 M z'~ 3 A I, < 10 Si, <8 S, < l0 Ca, 3 Mn, 400 F¢~ 120 Ni, 10 Cu, < 1 ~b.

Lowering mechanism

. . . . . .

- Silico suspension hook Silica ampoule ,

,, Tantalum bail - Tontalum cover Tubular tantalum rf susceplor

0

0

0

C~

0

0 >,

0

0

J

Pyrolytic BN crucible rf coil Rare earth-cobalt melt

Tantal~im bottom plate

0

0

Tubular tantalum heat sink

Samarium-metal getter Tantalum cup Silica support

Fig. I.

Schematic of crystal-growth system.

The samarium metal was stored in a glove box in dry nitrogen, and the weighing and loading of the materials into the growth crucibles also were done in the glove box. The crucible containing the materials was placed in a tantalum susceptor-container within a silica ampoule as is shown in fig. I. The assembly was vacuum baked at about 600 °C prior to, and after loading. Helium, which was dried by passage through a liquid-nitrogen trap, was used to flush the ampoule during the final bakeout and finally was introduced to serve as the growth atmosphere prior to the sealing of the ampoule. ...... ~l't As a "---' E,,u, step, th=- ~"UWtll . . . . . . . '- ~Lt~Os ph ~*~ WaS tt 5~,,~,~-~ by heating to redoess the samarium metal getter within the ampoule and holding it at temperatu e for 10.-15 rain. In the growth runs, induction heating, employing a 450 kc, 10 kW rf generator, was used to melt the rare earth-cobalt charges. During the "i~ridgman drop, output of the rf generator was held constant by use of a

THE G R O W T H OF

SmCos C R Y S T A L S

feedback, fast-response control sDtem, The maximum temperature employed was held at about 1350 °C to minimize both the vapor transport of samarium and the container-malt ~cti'on.~iAt:i~i:~ Co does not melt, but must ~ d i ~ N ~ b y ~ 6 1 t e n rare-earth metal~ it was f0und that a ~ ~ i o f ] ~ i 2 hr at this temperature was ~ u i r e d to c o m 0 ~ l y ~ o l v e the cm,dimension pieces of cobalt that were used, and to ensure homogenization of the malt. Specifically, the following procedure was used in a growth run: (1) the temperature was raised to 1150 °C and held for about one hour to melt the samarium metat and initiate dissolution of the cobalt. (2) the temperature was raised to 13.50 °C and held for 10-12 hr to dissolve the Co and homogenize the melt, 0 ) the growth was carried out by lowering the ~mlten charge from the hot zone at a selected rate (e.g., 1 mm/hr), (4) the temperature of the hot zone finally was reduced by use of a mechanical drive train to a temperature well below the Curie temperature at a rate of about 12 °C /rain. In the developmental part of the program, initial melt compositions were varied from 62.0 to 67.0 wt % cobalt, and lowering (growth) rates were varied from 0.9 to 15 mm/hr. Data on the growth runs indicate that initial cobalt concentration should be kept at. or just below, 66.0 wt ~. At this concentration, both Sm2Co7 and Sm2Cot~ were found as second phases in the SmCos it~gots (with run conditions apparently determining which phase formed). At cobalt concentrations much below 66 wt% (e.g., 6 5 ~ and below), the Sm-rich Sm2CoTphase was present throughout the ingots. With an initial melt composition of 67.0 wt % cobalt, however, the second phase always was the cobalt-rich Sm2Cot 7 phase. The results obtained also show the necessity for use of low growth rates. At growth rates greater than 2 mm,'hr, polyphase material always has been obtained. At [~ growth rate of 2 mm/hr, single-phase crystals were obtained occasionally, but frequently small second •

,

pHase ili~iUSIOIi~t--WIIIUII wg:l~ ~lg;t~ul~lJlg;

U|tI,T

BY THE B R I D G M A N T E C H N I Q U E

"

9

TABLE 1 Optical emission spectrographic analysis of SmCos Impurity elements detected*, p p m by wt

B Fe Si Mn Cu Ni

IO0'

l0

200

30

10

Io

Io

6o

20 T** 300

20 T** 300

T** 200

. ....

:

-

* Other impurity elements were sought, but were not detected. AcCuracy ~ 5 0 % relative. ** T ~ trace; detected but conceatration not assessable, less than $ ppm. TABLI~ 2 Mass spectrographic analysis o f SmCos Impurity elements detected, p p m by wt Element Sample number 27987-88 279~7-94 B C N O Cr Mn Fe Ni Si Nd

30 30 5 50 20 20 200 200 20 10

30 30 5 50 20 30 70 200 40 l0

crystals is good. The results of analyses of cryst~ds which are given in tables I and 2 show that the principal impurity present is the nickel that was present in relatively high concentration in the cobalt starting material. Other metallic impurities indicated to be present in the tens of parts per million range, Fe, Mn, Cr, Si, C, and Nd, also are present in the starting materials and the development and/or application of effective purification processes will be required I:o reduce their concentrations. Sixty-five other elements were sought by mass spectrographic analys~s and concentrations were found

gtttVi.

prolonged etching with dilute HCI - were found in the crystals. On the other hand, growth rates ~ 1 mm/hr consistently have yielded single-phase SmCos crystals.

3. Analysis of SmCos crystals Detailed chemical analyses which have been made of the single-crystal SmCos, show that purity of the

concentrations above I ppm; the remaining 58 elements were less than I ppm and many were below the detection limit. Although there is some melt-container reaction at the interface, boron and nitrogen contamination of the SmCos from the BN containers is seen to remain low (in the few tens of parts per million rat~ge, as can be

l0

J. F. MILLER AND A. E. A U S T I N

(a)

(b)

Fig. 2. (11.0) face of SmCos crystal 27987-88B. (a) (L22012) As-polished and etched in dilute HCI; magn. 13.5×. (6) (L220|6) After chemical polish l0 seconds in mixed concentrated acids; magn. 13.5 ×. The micrograph (b) shows the appearance of a ~,b,~t.ructure

p..~r~lial

t o *,he C a X l S

*,,,,~.t, gOCS tsutll

tu~;|

right to tipper

seen in tables 3 and 4). Apparently fl,: rea,~tion products prima, ily form and remain in the interface region, and the boron and nitrogen are diss,~i~eC only to a limited degree in tl',e SmCo s.

,_e.

The concentration of oxygen in the crystals is notably low. Both vacuum-fusion and mass-spectrographic analyses show oxygen to be present at concentrations of only 50-75 ppm - in contrast ~vith concentrations

THE G R O W T H OF

SmCo5CRYSTALS

TABLE 3

TABLE 4

Analysis of materials for boron and nitrogen Material

B, ppm by wt*

Distilled samarium (Research Chemicals) VP cobalt, (Materials Research) SmCos from lower center of ingot 27987-28

Analysis for gaseous elements in SmCo5

N, ppm by wt**

< !0

•i 0

< I0

50

30

40

* Determined by semiquantitative optical emission spectrographic analysis. ** Determined by Kjeldahi analysis.

orders of magnitude higher that typically are reported for the RCos compounds.

4. Characterization of SmCos crystals Large single crystal, were obtained from the ingots by successive lapping, etching in dilute HCI and selective cleavage. The single crystals were defined by their bulk magnetic a aisotropy, metallographic examination and back-reflection X-ray Laue patterns taken at intervals across polished faces. A more detailed examination by X-ray topography and rocking crystal breadth

//

I1

BY THE B R I D G M A N T E C H N I Q U E

Sample No.

27987-88 27987-94

Impurity element detected, ppm by wt Kjeldal:~ Vacuum-fusion analysis analysis -N O H N 20 20

<5 <5

70 75

15 30

disclosed the presence of low angle boundaries running parallel to the hexagonal c axis and separating regions of 150 to 300 microns width. The angular misalignment of these subgrains was 0.5 to 1.5 degrees in the basal plane. These low-angle boundaries were also revealed by a chemical polish etch. Fig. 2 illustrates the appearance of these low angle boundaries in a (11.0) face of a SmCo s crystal. The single crystal was magnetically soft and had long magnetic domains parallel to the c-axis with widths of 50 to 200 microns. Fig. 3 shows these domains as revealed by polarized light microscopy in a (11.0) face of a SmCos crystal.

5. Summary The method has been developed ~0r the growth of

I/

Fig. 3. (8F272) Polarized light micro~raph of (I 1.0) surface of SmC( crystal 27987-88B; magn. 100 ×. This shows the long magnetic domains running palrallel to the c axis. The surface was repolished after the treatment in the chemical polishing solution and also shows some residual etch pits.

|2

J.F. MILLER AND A. E, AUSTIN

SmCo.~ cD'stals from the melt, employing the Bridgrnan techrtiq ue, p'.~xolyticBN melt containers.initially Sin-rich melts, and very low(~< I mm/hr) growth rates, which c~axistcntl~ has yield,zd sizeable, pure SmCo., crystals Cup tv about 100 grams in v, ?.ight and having up to 2-era dimensions) 1his ~nethod also has been applied successfully to the growth of sizeable crystals of NdCo5 and mixed cr)~slals of Nd.~Sm~-xCos.

R e f e r e ~ ~s 1) K. Strnat, G. Hoffer, J. Olson, W. Ostertag and J. Becker,

J. Appl. Phys. 38 (1967) 1001. 2) K. Strnat, in: Proc. Third Rare Earth Con~., Clearwater, Florida, April, 1963, Session IV, pp. 89-100. 3) J. F. Nester and J. B. Schroeder, Trans. Met. Soc, AIME 233 (1965) 249. 4) K. H. J. Buschow, Philips Res. l~ept. 26 (1971) 49. 5) J. F. Miller and A. E. Austin, J. Less-Common Metals 25

(1971) 317.