Atomic structure of zero-magnetostrictive amorphous Fe4.7Co70.3Si15B10 alloy

Materials Science and Engineering, 60 (1983) 87-93

Atomic S t r u c t u r e

87

o f Z e r o - m a g n e t o s t r i c t i v e A m o r p h o u s Fe4.7Co70.3Si15B10

Alloy

H. MATSUDA* and T. EGAMI Department o f Materials Science and Engineering, and Laboratory for Research on the Structure o f Matter, University o f Pennsylvania, Philadelphia, PA 19104 (U.S.A.)

(Received December 18, 1982)

SUMMARY

The atomic radial distribution function (RDF) o f zero-magnetostrictive Fe4.7Co70.sSilsBlo amorphous alloys was determined by the energy-dispersive X-ray diffraction technique. The nature o f the atomic short-range order in the alloy was investigated by examining the structure o f the crystalline compounds with similar compositions and by comparing the R D F o f this alloy with those o f similar amorphous alloys. It is suggested that the metalloid elements in this alloy retain a high degree of compositional short-range order, just as in many other transition metal-metalloid amorphous alloys.

1. INTRODUCTION

Amorphous alloys obtained by liquid quenching have been extensively studied in the last 10 years, mainly owing to their superior magnetic and mechanical properties. The composition of the alloys most widely studied is the 3d transition metals (TMs) (7585 at.%) plus metalloids (Ms) {25-15 at.%), which are commercially produced and frequently mentioned for possible applications [ 1-4]. Small-scale applications have already been commenced, for electronic devices such as tape recorder heads or phonograph pickups. For such applications, most often zeromagnetostrictive cobalt-base amorphous alloys are used. The purpose of this work is to study the atomic structure of one such zeromagnetostrictive amorphous alloy, Fe~TCOT0.aSilsB10. The atomic structure was studied by the energy-dispersive X-ray diffrac*Present address: Sony Corporation Research Center, 174 Fujitsuka-cho, Hodogaya-ku, Yokohama 240, Japan. 0025-5416/83/$3.00

tion (EDXD) technique and was compared with those of the crystalline and amorphous alloys with similar compositions.

2. EXPERIMENTAL DETAILS AND RESULTS

Amorphous Fe4.TCOT0.aSilsB10 ribbon was prepared by the chill-block-casting, or meltspinning, technique [ 5]. A typical crosssectional dimension is 30 pm × 15 mm. The EDXD technique is a spectroscopic method of structural investigation using a white X-ray source [6-8]. White X-ray radiation was obtained from a tungsten target X-ray tube operated at 46 kV and placed at 45 ° away from the diffraction plane in order to eliminate the polarization effect (Fig. 1). The sample was made of 20 layers of the amorphous alloy ribbons and was placed in a 0-20 diffractometer in reflection geometry. The spectroscopy of the diffracted beam was carried out using an intrinsic germanium detector (Princeton Gamma-Tech IGP-25) and a multichannel pulse height analyzer (Tracor-Northern 1705). The spectra were processed by a PDP-11/10 minicomputer to determine the structure factor i{q) which was Fourier transformed to yield the radial distribution function (RDF). The spectral intensity of diffracted X-rays is given by [8]

I(E) = A(E){Iel(E) + linel(E) + + Ira(E) +/air(E)) + Is(E)

(1)

where E is the energy of the diffracted photon, A(E) is the combined detection efficiency, Iel(E) is the elastic scattering intensity, Iinel(E) is the inelastic scattering intensity, Im(E) is the multiple scattering intensity, /air{E) is the air scattering intensity and Is(E) is the spurious photon counts, mainly the escape peaks. A(E),/air(E) and Is(E ) were © Elsevier Sequoia/Printed in The Netherlands

88

~ ~

preamplifier collimator Ge detector ! /

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detector ~iits

detector

dewar

power

_ //I L\/,4

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]

h pulsep,le-up

I rejector

/

' X-ray tube

e-2etable

I

/

~

lineargate I~1 multichannel pulse shaper analyzer pulse height Jl

~

supply

Fig. 1. Schematic diagram of the EDXD system [8].

10.0 7.5 50

v

2.5

-2.5 -5.0

0

I

6

I

9

ll2

i

15

118 2il

i

24

Fig. 2. Structure factor qi(q) of Fe4.7Co70.3SilsBlo determined by the EDXD technique.

determined experimentally, while Ii,el(E) and Ira(E) were calculated theoretically. The total contribution of Iai~(E) and Im(E) a m o u n t s to about 7% of the total intensity, at 0 = 40 °. In the data analysis it is crucial to determine the primary spectrum lp(E), or in practice Ip(E)/ p(E) where p(E) is the absorption coefficient, with sufficient accuracy. This was done selfconsistently at large angles in a similar way to t h a t described in ref. 7 in which a detailed description is given. The structure factor i(q) was determined from Iel(E) at six diffraction angles using the energy range 18-40 keV. The diffraction angles were chosen such t h a t the

portions of i(q) determined at each diffraction angle have sufficient overlaps, so t h a t the sellconsistency can be checked [7]. The selfconsistency was indeed satisfactory. The values of i(q) were thus determined up to 23.5 A -1 as shown in Fig. 2. As a consequence the termination error is completely negligible. This wide range of q space yields the maxim u m theoretical resolution of the RDF of about 0.13 A which is small enough to resolve the fine details of the RDF. The shape of F(q) = qi(q) shown in Fig. 2 is similar to that for Fe4oNi4oP14B6 [7] except that the second peak is considerably higher. The RDF is given by

G(r) = 4Tcr{p(r) -- Po} 2 7r J 0

i(q) sin(qr) q dq

(2)

where Po is the average number density of the sample. The Fourier transformation was computed w i t h o u t any damping function and with no artificial treatment to improve the low r region. The result shown in Fig. 3 is similar to those for other TM-base alloys in terms of the general shape. An amorphous alloy of composition identical with that of our sample was recently

89 8.0

~6.o o<[ ~ 40

2o

~ 00 8O

-2.0 i

~'60

L

r (i)

~ 4o ~ 20 -~00 ~ -2() -40 00

0

120 r

180

24D

(A)

Fig. 3. R D F o f Fe4.TCOTo.3Si15Blo alloys. T h e i n s e t s h o w s t h e small r region.

studied by Williams et al. [9, 10] using neutron and conventional X-ray diffraction techniques. In their study, however, the maximum value of q was limited to 4 A -1 for neutron and 10 A -1 for X-ray diffraction. Consequently their r space resolution is appreciably lower than ours; their first peak width is wider than ours by a b o u t 50%.

3. D I S C U S S I O N

It is widely accepted that the dense random packing model provides an excellent starting point for describing the atomic structure of amorphous metals [ 11], although the effect of compositional short-range order (CSRO) must be taken into account for polyatomic glasses [12]. It has been further proposed that the local structure of TM-M amorphous alloys is similar to that in the corresponding crystalline c o m p o u n d s [ 1 3 - 1 5 ] . In particular, Gaskell [ 13 ] has suggested that there is a high degree of CSRO between TM and M atoms, so that the structure may be composed of a welldefined unit, a capped trigonal prism of TM atoms with an M atom at the center [13]. Such trigonal prisms are c o m m o n l y found in c o m p o u n d s involving TMs and Ms [16]. It is therefore worthwhile to examine the structure of crystalline c o m p o u n d s with similar compositions.

In order to gain insight into the nature of interaction between cobalt atoms and boron atoms, we shall first consider the c o m p o u n d Co3B. The crystal structure of Co3B is k n o w n to be orthorhombic cementite structure (D011, Pnma), isomorphous to Ni3B [17]. Although the atomic positions in CoaB have not y e t been determined, since the lattice constants are very close to those of Ni3B we could safely assume that the atomic environment of a boron atom in Co3B is very similar to that in Ni3B. In Ni3B a boron atom has no boron atom at the nearest-neighbor sites and is at the center of a trigonal prism of six nickel atoms, with an average B-Ni distance of 2.04 A [16]. Three more nickel atoms are present at the three cap positions of the prism, a b o u t 2.40 A away from boron. Since the average contact Ni-Ni distance is 2.52 •, the apparent boron radius, i.e. rNi_B - - rNi_Ni/2, is 0.78 A. If we consider hard sphere models of clusters of nickel atoms with a radius of 1.26 )~, we find that the inscribed radius of an ideal capped trigonal prism is equal to 0.92 A, while that of an ideal trigonal prism without a cap is 0.66 A. Therefore, the nickel coordination around a boron atom, a crushed capped trigonal prism with the caps pushed away outward, is well understood using the hard sphere model, as has been pointed out by Aronsson and Rundqvist [16]. The apparent boron radius in NiaB, 0.78 A, is appreciably smaller than the atomic radius of boron, 0.86 A, according to Kiessling [18], or 0.98A, according to Pauling [19]. As suggested also by Aronsson and Rundqvist [16], this reduction in the TM-M distance compared with the sum of the t w o atomic radii is n o t likely to be electrostatic in origin. Although the charge transfer model is frequently used to explain the magnetic properties of the TM-M alloys (see for example ref. 20), the results of photoemission studies [21, 22], C o m p t o n scattering [23] and electronic structure calculations [ 2 4 - 2 6 ] all agree in that the charge transfer is negligible in most cases and in that TM and M form a weak covalent bonding, the strength of which depends on the specific combination of the elements. For instance, the Fe-B interaction is weaker than the Ni-B interaction [25], so that the Fe-B distance (2.21 A) is greater than the Ni-B distance (2.04 A) [27]. From these considerations we conclude that in

90

CoaB a boron atom is located at the center of a deformed capped trigonal prism of cobalt atoms, forming covalent bonds with cobalt neighbors, although the strength of the bond is weaker than in Ni3B as discussed later. Another related c o m p o u n d worth studying is Ni6Si2B, which is isotypic to Fe2P [28]. In this c o m p o u n d also, boron and silicon atoms are located at the center of capped prisms of nickel atoms with an Ni-B distance of 2.08 )k and an Ni-Si distance of 2.34 A. Since the Ni-Ni contact distance is 2.55 A, the apparent boron radius is 0.80 )k and the apparent silicon radius is 1.06 A. The ratio rsi/rNi of the radius of the silicon atom to that of the nickel atom equals 0.83 and is larger than the ideal size ratio for a capped trigonal prism, which equals 0.73. Therefore, in the liquid state, silicon atoms may prefer a nickel coordination number of around 10 rather than 9. The only Co-Si-B c o m p o u n d for which atomic positions have been determined and which has a composition close to Co75Si~5B10 is Co4.7Si2B [29]. The apparent boron radius in this c o m p o u n d is 0.85 A, if the cobalt radius is assumed to be 1.30 A as determined from the average of the seven closest Co-Co distances, and the apparent silicon radius is 1.08 A. In this c o m p o u n d , however, M atoms have t w o M neighbors, since the M concentration is simply t o o high to avoid such occurrences. Thus the examination of the crystal structures suggests that there would be a high degree of CSRO between cobalt and silicon and b e t w e e n cobalt and boron. The interatomic distances estimated from the structure of crystalline c o m p o u n d s are as follows: rco-co ~ 2.52 A, rco_si ~ 2.32 A and rco-B ~ 2.11 A. The results in Fig. 3 and Table 1 show

that these estimates are indeed reasonable. The first peak position of the R D F is 2.52 )k as expected. The second peak is at 4.27 )~, further away by 0.25 • than the corresponding value for Ni~0Fe2~B25 [15]. This reflects primarily the difference between the size of silicon atoms and that of boron atoms ( r s i - - r B -= 0.23 A). The subpeak of the second peak is of course the collinear C o - C o Co peak, with the distance close to 2rco-co. Even the small peaks m a y be accounted for, although the possibility that some of them may be due to experimental noise cannot be completely eliminated; the small peak at a b o u t 2.1 A is probably the Co-B peak, and a peak at 3.2 A may correspond to one of the TM-TM distances found in Ni6Si2B (3.26 A). It is also instructive to compare our results with the structure of amorphous alloys of similar compositions. In Fig. 4, our results are compared with those for Fe75Si1~B10 [27]. It can be observed that the peak positions and the shapes of the second peak are similar, although the height of the first peak is significantly different. Such a difference in the first peak height can also be seen in the comparison between Fev5B25 and NisoFe25B25 shown in Fig. 5 [15]. If we estimate the first peak height of amorphous Ni75B25, which cannot be obtained using liquid quenching, by simply linearly extrapolating the peak heights of Fe75B25 and NisoFe25B25 with respect to composition, it is very close to the first peak height of the present cobalt-rich alloy. It is also close to the first peak height of Ni75Si10B15 [30]. Thus, the first peak height

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oq

6.0

TABLE 1 4.5

Experimental results

o cL I

i(q) Position of t h e first peak W i d t h o f t h e first peak Position of the second peak Position of the second peak shoulder

(A -1 )

4/rr{p(ri) -- PO} (A)

3.12

2.52

0.41 5.34

0.42 4.27

30

cL 0.0

-I.5 -5.0

i

210

L

I

I

6'0

8'.0 ',;o

r (A)

--

4.95

Fig. 4. R D F s of Fe4.7Co70.3Si15B10

Fe75SilsBlo ( ...... ) [ 27 ].

) and

91

TABLE 2 75

p ( r 2 ) / p ( r 2 s ) values

60 45 50 G(r) 15

oo

~

-15

-50

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I0

"

:~. . . .

20

50

/"

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40

50

Composition

p ( r 2 ) / p ( r 2 s) a

Fe4.TCOTo.3Si15Blo Fe75B25 [15] Fe75Si15B10 [27] Ni75SiloB15 [30]

1.120 -+0.01 1.164 1.168 1.048

a P(r2), height of the second peak of the RDF;p(r2s), height of the second peak shoulder of the RDF.

I

3

60

r(~,) Fig. 5. RDFs of Fe75B25 ( ...... ) and Ni50Fe25B25

(

2

7

) [15].

for the cobalt-base alloy is similar to that for the nickel-base alloy, while the shape of the second peak for the cobalt-base alloy is intermediate between those for the nickel-base alloy and the iron-base alloy. The fact that the cobalt-base alloy has characteristics of a nature in between those of the nickel- and iron-base alloys is also seen in the TM-B interaction strength and the TM-B distance as we discussed earlier (rNi-B = 2.04 A; rco-S = 2.11 A; r~e-B = 2.21 A) and is obviously a reflection of the fact that cobalt lies between nickel and iron in the periodic table. For a close examination we compared the ratio of the height p(r2) of the second peak to the height p(r2s) of the second peak shoulder for FeT~Sil~Blo, Fe4.TCoT0.3Si15Blo and Ni75SiloB15. The values of P0 were determined from density measurements [31]. The second peak ratio K = p(r2)/P(r2s) for the cobalt-base alloy is clearly about halfway between those for the iron-base alloy and L~ickel-base alloy, as shown in Table 2. For TM-B alloys this trend can be explained in terms of the high degree of CSRO and the different size ratios of boron to TM [27]. If the ratio r B / r W M is close to 0.73 as is the case for Fe75B25, then the capped trigonal prism is nearly ideal, and m a n y of the second-nearestneighbor F e - F e distances such as R15 and R17 in Fig. 6 are similar, resulting in a high second peak. If the CSRO is destroyed as is probably the case for amorphous particles produced by

5

Fig. 6. Capped trigonal prism of TM atoms around an M atom.

spark erosion, the height of the second peak is reduced [27]. In Ni-B alloys the ratio FB/rT M is much smaller than the ideal ratio (rB/rNi ~ 0.62), SO that the capped trigonal prism is distorted as we discussed earlier and the second peak height is smaller. Such a trend persists even when some boron atoms are replaced by silicon atoms [27], although this is somewhat surprising given the size difference between boron and silicon. The fact that the second peak height ratio K for cobalt is between those for nickel and iron indicates t h a t a high degree of CSRO exists also in the cobalt-base alloy. While the second peak ratio K shows a smooth transition from nickel to iron, the peak heights themselves do not show a monotonic trend. As shown in Figs. 4 and 5, this is primarily because the peaks of the iron-base alloys are always lower than those of the cobalt- and nickel-base alloys. It is interesting to note that, in Fe3B, most of the iron atom pairs have c o m m o n boron nearest neighbors, while some of t h e m do not. The iron pairs which do not have a c o m m o n boron nearest neighbor have five or six c o m m o n iron neighbors and form seven or eight iron atom

92

clusters, pentagonal or hexagonal bipyramids. The pentagonal bipyramid, or heptatope [ 32 ], is made up of five tetrahedra and may be viewed as a possible nucleus of the amorphous iron phase. In contrast, the hexagonal bipyramid, or octatope, is found in the b.c.c. lattice in a distorted form. In the b.c.c, lattice the interatomic distance between the neighboring atoms in the hexagonal ring is the nearest-neighbor distance, 2.48 A. In Fe3B the average near-neighbor distance within the hexagonal ring is 2.44 A, and the distance between the pair of atoms sandwiching the hexagon is 2.34 A. These distances are appreciably smaller than the average Fe-Fe distance, 2.61 A, and contribute to the widening of the first peak of the RDF and to a reduction in its height compared with those of the nickeland cobalt-base alloys, since such TM atom clusters are not found in Ni3B or COBB. It is most likely that the absence of the TM atom clusters in Ni3B is the consequence of the strong Ni-B interaction. In the Fe-B system the interaction between iron and boron is not strong enough for this to occur, so that the iron atoms may microscopically "segregate" to form the TM atom clusters. 4. CONCLUSION

The atomic structure of zero-magnetostrictive Fe4.TCoT0.3SilsB10 amorphous alloy was determined using the EDXD method. The structure factor was determined up to 23.5 A -1, thus eliminating the termination error in the Fourier transform used to obtain the RDF. In order to gain insight into the nature of the atomic interaction in this rather complex alloy, we examined the structure of crystalline compounds with similar compositions and determined the effective atomic sizes. We noted that the apparent M radius varies depending on the specific combination (most probably according to the different strengths of the covalent bonding). The structure of both crystalline and amorphous alloys can be understood on the basis of the packing of atoms with different radii, and the difference in the interatomic interactions. By comparing the structure of the cobalt-base alloy that we had determined with those of the iron- and nickel-base alloys, we suggested that the cobalt-base alloy retains a high degree of TM-M CSRO and that the strength of the

interaction between cobalt and boron or between cobalt and silicon is intermediate between the strengths of the Fe-(B, Si) interactions and the Ni-(B, Si) interactions.

ACKNOWLEDGMENTS

The work reported in this paper was supported by the Sony Corporation and by the National Science Foundation through the use of the X-ray facility in the Laboratory for Research on the Structure of Matter. The present authors thank Dr. Y. Makino, Sony Corporation, for encouragement and support for this work, and Dr. K. Aso, Mr. S. Uedaira, Dr. Y. Ochiai, Mr. M. Hayakawa and Mr. K. Hotai for supplying the amorphous sample. They are also grateful to Dr. S. Aur for technical assistance in the EDXD measurement and the data analysis.

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