31P NMR in the ferromagnetic state of amorphous Fe75P15C10

Solid State Communications, Vol. 24, pp. 189—192, 1977.

Pergamon Press

Printed in Great Britain

31P NMR IN THE FERROMAGNETIC STATE OF AMORPHOUS FeisPisCio* K. Rajt Yale University, New Haven, CT 06520 U.S.A. and J. Durand California Institute of Technology, Pasadena, CA 91125 U.S.A. and J.I. Budnick The University of Connecticut, Storrs, CT 06268 U.S.A. and C.C. Tsuei IBM Thomas J. Watson Research Center, Yorktown Heights, NY 10598, U.S.A. and S. Skaiski Fordham University, Bronx, NY 10458 U.S.A. (Received l6May 1977 byA.G. Chynoweth) The first observation of a metalloid site hyperfine field (hi) distribution in a ferromagnetic amorphous alloy is reported using a spin-echo NMR technique. The 31P nuclei in amorphous Fe 75P15 C10 show a hf distribution with a maximum at about 56Fe.27AkOe. comparison The sample of the wasNMR prepared spectra with on Fe samples enriched 99.93%Feinand 56Fe also provides the 57Fe hf distribution containingtonatural whose peak value agrees with Mössbauer results. THE ferromagnetic amorphous alloy Fe 57P15C10 has been relatively well studied [1, 2]. According to the most recent measurements, this ferromagnet has a saturation moment of 1 .8.t B/Fe atom at 0°Kand a Curie temperature of 596.5 K [2]. The Fe atoms are surrounded by 12—13 nearest neighbors consisting of iron and metalloid atoms [1]. The Fe hyperfine field (hi) distribution at room tepiperature was first reported by Tsuei eta!. [2] using the MOssbauer technique. The distribution was shown to be asymmetric, spread from 0 to 330 kOe with a maximum occurring at about 265 kOe and its full width at halfintensity (~H)was about 130 kOe. The Fe field at 4.2°Kwas measured by Frankel [3] and showed an increase to 278 kOe. To this date hf distributions for the metalloid atoms have not been observed. This information is of special significance since not much is known about the detailed environments of “glass formers” in amorphous metallic systems. X-ray scattering, which has traditionally provided knowledge of co-ordination numbers and near neighbor distances, is limited in its usefulness to the ____________

Supported in part by the University of Connecticut Research Foundation, t Supported by the National Science Foundation. *

transition metals because of their large scattering amplitudes as compared to the metalloid atoms. Neutron scattering by itself, on the other hand, has not been successful due to complications arising from the d~fficulty in separatmg contnbutions from individual pair correlation functions. A combined approach utilizing both X-ray and neutron diffraction has, however, been used to deduce the phosphorus near neighbor environments in amorphous Co81P19 alloy [4]. The results show that contrary to the transition metal atoms, the P atoms on the average are surrounded by 9 Co and have no Pin the first near neighbor shell. An NMR experiment can provide information on local environments around a given atom provided the hf distributions for such atoms can be obtained. The NMR spectrum of amorphous Fe75P15C10 would consist, in principle, of resonances arising from all of the nuclei Fe, P and C. Due to broad nature of the hf distributions in amorphous alloys it is very likely that these resonances might overlap making their assignment to individual nuclei difficult. It will be shown later that this was found to be the case in the present work. The strength of an NMR signal directly depends on the isotopic abundance and the rf enhancement factor for the nucleus under investigation [6]. Thus simply based on the isotopic abundance of the resonating 189

31P NMR IN AMORPHOUS Fe

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Vol. 24, No. 2

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Fig. 1. Plots of echo amplitude in arbitrary units versus frequency (raw data) for the amorphous Fe75P15C10 and 56Fe 75P15C10 samples at 1 .3°Kunder similar excitation conditions. The scale for echo amplitude starts from zero. The error bars spectrum as a function frequencybetween are shown. The dotted is theofdifference

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( M Hz) 31P nuclei and Fig.57Fe 2. Hyperfine distributions of (a) (b) nuclei in field amorphous Fe 75P15C10. isotopically enriched in 56Fe which has no nuclear spin FR E 0 ~ EN C Y

the two spectra in the frequency 57Ferange nuclei. where they do not overlap and is due to the and thus does not produce a resonance. One is then sure nuclei, 57Fe (2.2%), 31P (100%) and 13C (1.1%), the ‘~ of measuring only the P resonance. This approach was signal intensity for the composition Fe 75P15C10 would used in the present study. be much stronger than the other two nuclei. The rf 56Fe The amorphous samples, Fe75P15C10 and enhancement has its origin in the coupling of the nuclear 75P15C10, used here were prepared rapid quench56Fe isotopebywas acquired spin with the atomic magnetization and among ing meltNational [7]. TheLaboratory and was 99.93% factors is proportional to, for a particular atom,other the fromfrom Oakthe Ridge hyperfme field at the nucleus of that atom. As will be pure. X-ray data on both of these samples showed that shown later, the Fe hyperfine field in amorphous they were amorphous. The compositions were checked Fe 75P15C10 is about 10 times larger than that of P. It by measuring T,, values on several foils using the inducwill not be unreasonable to assume that the Fe field is tance bridge method. The average values of T~for three 56Fe samples were even greater than 10 times the field at the C nucleus. All foils of each of the natural Fe and these considerations lead us to conclude that the NMR 59l°Kand 588°Krespectively, in good agreement with signal strengths are about the same for the Fe and ‘~ the previous work [1, 21. The magnitization measurenuclei and both are far greater than for the C nucleus. ~ ments performed at 4.2°Kand in an external magnetic fact, the NMR intensity for the C is expected to be so field of 30 kOe yielded average values of 1 .88 pB and small and its hf so low that it would not be detectable in 1.95 pB/Fe atom for the samples Fe 75P15C10 and 56Fe the present experiments. 75P15C10 respectively. These results are in agreement In order to obtain on!y the P resonance in with the published [1, 2]. From Mossbauer 57Fevalues was detected in the a56Fe sample [81. Fe75P15C10, one could use two approaches. From a experiment no Mossbauer experiment the Fe field distribution can be In Fig. 1, we have shown the spin-echo spectra (raw obtained and from a NMR experiment both Fe and P data) of amorphous Fe 56Fe 75P15C10 and 75P15C10 resonances are observed (assuming the C resonance to be too small to detect). By comparing the results of these two experiments, it should be possible to determine the

P field distribution. This approach will not yield accurate results if both the Fe and P NMR signals are mixed and appear approximately in the same range of frequencies because of a lack of knowledge about the precise value of enhancement factors, A second method would be to prepare a sample

samples at 1 .3°K.The same low turn angle excitation

conditions were used for both the samples. The absolute echo amplitudes have been corrected for a very small difference (~-~ 5%) in the weights of the two samples. Outside the frequency range shown in Fig. l,the echoes were too weak to be measured. It should be noted that the two sets of data points on the high frequency side overlap within the experimental error indicating that the

Vol. 24, No. 2

31P NMR IN AMORPHOUS Fe 75P15C10

same nuclei are producing the echo intensity in the two samples. Clearly the high frequency sides of the spectra are associated with only the P atoms. On the low frequency side, the data points for the 56Fesample Fe,5P15C10 are evidently higher than for the 75P15C10 sample. This is expected because of the contribution from the 57Fe nuclei in this frequency range. The dotted spectrum is obtained by subtraction of the two sets of data points 57Fe where they do not overlap and is attributed to the nuclei. The hyperfine field distribution for the 31P nuclei is shown in Fig. 2(a). This distribution is obtained from the raw NMR data by dividing the echo amplitude 56Fe observed in 75P15C10 at each frequency by the frequency [6]. A maximum in the spectrum appears at about 46 MHz or 27 kOe with a line-width of about 25 MHz or 14 kOe. Figure 2(b) shows the distribution for the “Fe nuclei in the Fe75P15C10 sample with a peak at about 39MHz or 282 kOe. This value is in good agreement with that obtained by Frankel [3] (278 kOe) using the Mössbauer technique. It should be noted that the “Fe distribution is asymmetric. The S/N ratio prohibits a determination of the low cut off.decreases On the 57Fefrequency echo amplitude high frequency side the sharply to zero in the neighborhood of 47 MHz or 340kOe consistent with the Mässbauer experiments [2, 3]. The ratio of Fe hyperfine field (H) to its line-width (~H)from the Mossbauer experiments [2] is (H/L’~H)Fe 2. Approximately the same ratio is observed for the P resonance (H/i~H)p 2. One possible explanation of the observed constancy of the H/AH ratio is that there is a distribution of Fe moments around a mean value in this amorphous alloy and that the spread in the fields arises from the distribution of Fe moments. Even thoughhave the different nuclei contributing to the spectrum different Fe near -~

neighbor environments and the P nuclei have the same environments, if the Fe and P hyperfine fields are both proportional to the average Fe moment, the hyperfme field at any site as well as the distribution of these hyperfme fields will be determined by the distribution of Fe moments. Proportionality between the Fe hyperfme field and its moment, at sites with different local environments, has been found in a wide variety of interstitial compounds [9]. Bemas eta!. [10] have suggested that the reason for the observed scaling is that in interstitial compounds there is very little conduction electron polanzation. We can obtain an estimate of the average configur. ation of near neighbors in amorphous Fe75P15C10 by comparing it to its closest crystalline analogue Fe3P [2, 11]. Fe3P is an interstitial compound in which

191

scaling between the Fe fields and Fe moments is observed. In this compound there are three Fe sites which differ in their near neighbor configurations in the following way: the Fe1 site has 12 Fe and 2 P nearest neighbors, the Fe11 site has 10 Fe and 3 P and the Fe111 site has 10 Fe and 4 P nearest neighbors. Theshow neutron diffraction experiments of Usher eta!. [12] that the magnetic moments on the sites Fe 1, Fe11 and Fe111 are 2.12, 1.84 and 1.25 pB respectively. The average hyperfine field values, as obtained by Koster eta!. [13] using NMR method, on these sites are: Fe 1: 304 kOe, Fe11: 262 kOe and Fe111: 190 kOe. It is obvious that for the different environments the moments and hyperfine fields scale such that H = Ap where A is the hyperfine coupling constant 140 kOe/pB. The value of the Fe moment and hyperfine field in amorphous Fe75P15C10 lie close to the results for the Fe11 site in crystalline Fe3P. From this observation it may be inferred that in the amorphous alloy short range atomic ordering, similar to that in Fe3P, exists and that an average configuration for Fe in Fe75P15C10 is 10 Fe + 3 (P and C) consistent with that one would expect from a statistical The 31Pmodel. resonance in Fe 3P has not been reported [13]. It is thus not possible at this time to compare the P resonance in the amorphous alloy with its crystalline counterpart. The mechanism responsible for producing a transferred hyperfine field at the P site seems to be the polarization of local s electrons by the neighboring Fe spins. Such a polarization and thus the resulting hyperfine field would be proportional to the total magnetic moment distribution surrounding a P atom. The broadening of the P line then arises from the distribution Fe moments in the 31P fieldofvalue, line-width, andamorphous line shape sample. are veryThe similar in the two amorphous systems Fe~P 16B5[14] and Fe75P15C10 in which the transition metal contents are different. l’his suggests that the magnetic inhomogeneities play a more dominant role in determining the 31 field distribution than the structural disorder. The proportionality between the P fields and the Fe moment distribution in Fe75P15C10 is further supported by the observed hyperfine fields at the two P sites in Fe2P [15]. In this crystalline system both P sites have only Fe nearest neighbors, as expected to be the case in amorphous Fe75P15C10, and the P fields do scale with the average Fe moments surrounding these sites. In amorphous transition metal—metalloid systems, radial distribution function measurements indicate the presence of atomic correlations extending further than nearest neighbors [161. We have considered here only the first near neighbor environments since the Fe and P fields are most sensitive to the moment distribution in ‘~

192

31P NMR IN AMORPHOUS Fe 75P15C10

the first neighbor shell. The effect of the distribution of more distant neighbours is expected to be much smaller [5] and could not account for the large magnitude of the spread in hyperfine fields observed in amorphous systems. Thewithout distant further neighbor contributions could not of be resolved much more detailed study the field distributions. These experiments point out the value of 56Fe isotopic substitution in providing direct information about metalloid site hyperfine fields in Fe based amorphous alloys. It is thus possible to probe, by comparison with analogous crystalline compounds, the first near neighbor environments for both the transition

Vol. 24, No. 2

metal and the metalloid sites. Further NMR experments on the crystalline Fe3P and the b.c.c. Fe80P13C7 compounds would be most helpful in describing the metalbid atomic environments in amorphous We 31P hf determination willFe75P15C10. provide a basis expect that the with theoretical calculations of metalloid for comparison hyperfine fields, especially with respect to the relative role of long range conduction electron polarization contribution and local spin transfer effects. Acknowledgement The authors K. Raj and ii. Budnick wish to thank R. Alben for his interest in this work. —

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LIN S.C.H. & DUWEZ P.,Phys. Status So!idi 34,469 (1969); WAGNER C.N.J., J. Vac. Sci. Tech. 6, 650 (1969); CHEN H.S., Phys. Status So!idi Al7, 561 (1973); AXE J.D., PASSEL L. & TSUEI CC., AlP Conf Proc. 24, 119 (1974); LIN S.C.H.,J. App!. Phys. 40, 2173 and 2175 (1969); SINHA A.K.,J. App!. Phys. 42, 338 (1971).

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TSEUI C.C. & LILIENTHAL H., Phys. Rev. B13, 4899 (1976).

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FRANKEL R.B. (Private communication).

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SADOC J.F. & DIXIMIER J.,Mat. Sci. Engr. 23, 187 (1976).

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NICULESCU V., RAJ K., BUDNICK J.I., BURCH T.J., HINES W.A. & MENOTTI A.H., Phys. Rev. B14, 4160 (1976). BUDNICK J.I. & SKALSKI S., in Hyperfine Interactions, p. 724. (Eds. FREEMAN A.J. & FRANKEL R.), Academic Press, New York, (1967). DUWEZ p. & WILLENS R.H., Trans. Am. Inst. Mm. Engr. 227, 362 (1962); PIETROKOWSKY P., J. Sci. Instrum. 34, 445 (1962). We are grateful to AMAMOU A. for performing the Mossbauer experiment on the 56Fe sample.

6. 7. 8. 9. 10.

BERNAS H., CAMPBELL L.A. & FRUCHART R.,J. Phys. Chem. Solids 28, 17 (1967); SHINOHARA T. & WATANABE H., J. Phys. Soc. Japan 20, 2020 (1965). BERNAS H. & CAMPBELL I.A.,Phys. Lett. 24A, 74 (1967).

11.

The recently reported b.c.c. crystalline alloy Fe

12.

80P13C7 with T~= 593°Kand a saturation moment value of 2.01 pB per Fe atom may also be regarded as an alternative crystalline counterpart to amorphous Fe75P15C10. The reference for the b.c.c. alloy: KAZAMA N., KAMEDA M. & MASUMOTO T., AlP Conf Proc. 34, 307 (1976). LISHER E., WILKINSON C., ERICSSON T., HAGGSTROM L., LUNDGREN L. & WAPPLING R., Proc. mt. Conf. Magnetism (Moscow) IV, 581 (1973).

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KOSTER E. & TURRELL B.G., Phys. Lett. 39A, 211(1972). RAJ K., DURAND J., BUDNICK J.I. & SKALSKI S. (to be published). KOSTER E. & TURREL B.G., Can. J. Phys. 51,830(1973).

16.

See for example: CARGILL III G.S., in Solid State Phys. 30, p. 227. (Eds. EHRENRELCH H., SEITZ F. & TURNBULL D.), Academic Press, New York, (1975).