Hyperfine field distribution in amorphous Fe40Ni40B20

Solid State Communications, Vol. 27, PP. 1425—1428. © Pergamon Press Ltd. 1978. Printed in Great Britain.

0038—1098/78/0922—1425 $02.00/U

HYPERFINE FIELD DISTRIBUTION IN AMORPHOUS Fe~Ni40B20 I. Vincze Central Research Institute for Physics, H-i 525 Budapest, P.O.B. 49, Hungary and E. Babic Institute of Physics, University of Zagreb, Yugoslavia (Received 1 July 1978 by A. Zawadowski) From the Mössbauer investigation of amorphous Fe~Ni~B20 and Fe~B20 alloys it was found that the substitution of Fe by Ni only shifts but does not influence the shape of the iron hyperfine field distribution contrary to that of crystalline f.c.c. Ni—Fe alloys suggesting a rather localized type of behaviour. The distributions of the linear combinations of isomer shift and quadrupole splitting are affected by this substitution. RECENTLY the amorphous ferromagnets have been a subject of considerable interest. Especially the (FeNi)~B2oglasses have been extensively studied due to their simple structure (only one kind of metalloid) and possible technological applications. However, relatively little is known about the electron structure of these amorphous alloys. Mössbauer or NMR measurements offer a significant information about the local environments via hyperfme interactions, In this paper we report on the distribution of iron hyperfme fields p(E) determined by MOssbauer technique in an amorphous Fe~Ni~B20 alloy. The hyperfme field distribution will be compared with that of glassy Fe80B20 and with the qualitative behaviour of their crystalline counterparts, Fe50Ni50 and Fe, respectively, These system are suitable for the comparison because of their nearly identical Curie temperature and fairly different average magnetic moments (Table 1). The sample was prepared from the master alloy of predetermined concentration by the use of a meltspinning device [21.It was in a form of long ribbon about 0.3mm wide and 2O~zmthick. The amorphous structure of the specimen was verified by X-ray diffraction using iron-filtered cobalt radiation. The crystallization of this sample has been studied in detail earlier

[31.For the Mossbauer measurements a conventional equipment was used. The Mössbauer spectrum of the Fe40Ni40B20 alloy (Fig. 1) consists of the usual six, strongly broadened and overlapping lines characteristic of amorphous alloys. The evaluation of the distribution of hyperfme fields, isomer shifts (&) and quadrupole splittings (~E)from such spectra is a rather difficult task which is not completely solved yet. Especially the texture present in the sample causes serious systematical errors via the uncertain intensities of the second and fifth lines of the Mossbauer spectrum. However, with a simple procedure [41we can get rid of the problems connected with overlapping lines and unknown intensity ratios. The intensity of the second and fifth lines depends strongly on the angle between the directions of the emitted 7-rays and the magnetic moments in the sample. Figures 1(a) and (b) show the measured spectra at room temperature for two different angles (0 90°and 30°,respectively). The sample was polarized in its plane by using a small permanent magnet and 0 is the angle between the plane and the 7-direction. From two linear combinations of these two spectra two sub-spectra can be calculated which separately contain the 1—3--4—6 [Fig. 1 (c)1 and 2—5 [Fig. 1 (d)1 lines of the original spectrum. (The

Table 1. Curie temperature T~,magnetic moment per transition metal atom p, average iron hyperfine field HFe (at 80K) and isomer shift ISFe (relative to pure iron) ofamorphous Fe~B20and Fe,4,Ni.,0B20 T5Fe (mm/see) Alloy T~(K) P (PB) HFe (kOe) Fe~B 20 647* 1.99* 276 ±4 0.075 ±0.01 Fe~Ni~B30 662* 1.29* 242 ±4 0.14 ±0.01 *T~enf

[11. 1425

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HYPERFINE FIELD DISTRIBUTION IN AMORPHOUS Fe~Ni~B20

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0)

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b.)

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d.)

-6

-4

-2

2

4

6 ve[ocity(mm/sec)

Fig. 1. Room temperature MOssbauer spectra of amorphous Fe~Ni~B20 alloy at 0 900 (a) and 0 300 (b). (0 is the angle between the directions of the 7-rays and the magnetic moment in the sample.) The linear combinations of these spectra give the 1 —3---4—6 lines (c) and the 2—5 lines (d) of the spectrum respectively. coefficients of the linear combinations were determined from the overlap-free parts of the spectra.) From these sub-spectra which are free from overlap we can reliably determine the distributions of p(R) and two linear cornbinations of the isomer shift, S and quadrupole splittings, L~E(namely p(6 + ~~E) and p(5 ~~E), respectively). Following the method of [4] the distributions were approximated by a binomial distribution (here z = 20 was used) the shape of which was least-square fitted to the spectra. The distributions obtained this way are shown in Figs. 2 and 3 together with those determined for amorphous Fe~B20alloy (METGLAS 2605) on the same manner [4]. In Fig. 2 the p(H) distribution of Fe~B20was shifted by 34kOe (which is the difference in the average hyperfine fields) to have a better comparison of the two distributions. Also, the p(ff) distributions determined from the 1—6 and 2—5 lines of the spectra are compared separately to avoid systematical errors due to sample-thickness effects. It is very remarkable that though half of the Fe atoms is substituted by Ni atoms the shape of p(I1) does not show any observable change. (An indication for this was found by Chien eta!. [5] in the amorphous (Fe1 _~Ni~)8oPi4B6 system from the qualitatively similar features of the spectra.) This behaviour is quite the opposite to that found in disordered crystalline f.c.c. Ni—Fe alloys. It has been found [61 that in these alloys —

—

the iron hyperfine field decreases by about 10 kOe for the substitution of a first neighbour Fe by Ni. Thus the statistical fluctuation in the environment of the iron atoms results in a considerable broadening of the Mossbauer line width. For example, in the case of a disordered f.c.c. Fe50Ni50 alloy the measured outer line width (~ 0.6 mm sec~)is more than two times larger than that in pure iron. On the other hand, the substitution of Ni affects the distribution of both linear combinations of isomer shift and quadrupole splitting, S + ~/~E and S respectively as it can be seen in Fig. 3. These changes can be attributed both to the perturbation of p(5) and p(~ff)but at present it is not possible to separate them. The insensitivity of the p(H) shape to substitution of Fe by Ni in these amorphous alloys suggests a small nearest neighbour contribution to the iron hyperfine field (via conduction electron polarization or direct overlap) in comparison with the crystalline transition metal alloys. On the other hand, this behaviour is similar to that of the intermetalhic compounds where the nearest neighbour contribution was also found to be small [71. On the other hand, the substitution of Fe by Ni changes the average values of the hyperfine field and isomer shift which are shown in Table 1. The average values of the quadrupole splitting was found to be —

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HYPERFINE FIELD DISTRIBUTION IN AMORPHOUS Fe~Ni~B20

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a,

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200

300

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Fig. 2. Comparison of the room temperature hyperfme field distribution of amorphous Fe40NL0B20 (Continuous line) and amorphous Fe~B20(broken line) determined from the 1—6 lines (a) and from the 2—5 lines (b) of the spectrum, respectively. Here z = 20 was used and the p(If) curve of Fe~,B20was shifted by 34 kOe. —

p(~+~E)

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Fig. 3. Room temperature distributions of the linear combinations of isomer shift (5) and ciuadrupole shift (~Elof amorphous Fe~Ni.~B20 (continuous line) and amorphous Fe~B20(broken line) determined from the 1—6 lines (a) and 6Fe) is also shown. from the 2—5 lines (b) of the spectrum, respectively. The isomer shift of pure iron ( zero in both cases. The tendencies may be explained by distance would result in a stronger overlap of the wave atomic size effects. The density of Fe~Ni.~B is larger functions, fIeld i.e. in[91. an increased isomer shift and decreased 3 and20 hyperfine than gthat of respectively FeseB2o by ~[1]), 5% (7.72 cmdifference in 7.39 cm3, whilegthe the atomic weights would result only in 2% increase. Thus the atomic radius of Ni is smaller than that of Fe and for the Ni substitution a decrease is expected in the atomic distances similarly to that of mixed interAcknowledgements Stimulating discussions with metallic borides [81. The supposed smaller iron—boron Drs. G. Gruner and T. Kemény are acknowledged. —

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