Local atomic and magnetic structure of microcrystalline Fe-Al alloys

ELSEVIER

Journal of Magnetism and Magnetic Materials 133 (1994) 86-89

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journal of magnetism and magnetic materials

Local atomic and magnetic structure of microcrystalline Fe-A1 alloys V. Pokatilov Central Research Institute of Ferrous Metallurgy, 2 Baumanskaj 9 / 2 3 107005, Moscow, Russian Federation

Abstract

A detailed magnetization and NMR study of the alloy Fe76AI24 prepared by the rapid quenching technique is reported. The local atomic and magnetic properties of specimens as-quenched and annealed at 150-510°C were measured. The heat treatments change little the saturation magnetization M and mean magnetic moments of specimens m, but the mean hyperfine fields at the 27-A1 nuclei H(A1) and their distributions were greatly changed. Using the so-called local environment model, M, m and the dependence of the local moments of iron atoms on their local environments have been considered.

1. Introduction

Fe-A1 alloys are the basis of a large number of technically important magnetic materials. Some studies of ribbon F e - A I alloys prepared by the rapid quenching technique have been reported. Some new problems in examination of their magnetic properties and structure have resulted from the development and application of the method of rapid cooling of the melt. It has been demonstrated by electron microscopy and X-ray techniques that, depending on the AI concentration, the Fe-A1 ribbons consist of bcc Fe and DO3 [1] or B2 phases [2]. The present paper discusses the mean magnetic moments and 27-A1 hyperfine field (HFF) distributions and their dependence on heat treatment in rapidly quenched specimens of Fe-A1 alloys.

2. Experiment

Electrolytic iron (99.9% purity) and electrolytic aluminium (99.999%) were used as raw materials. The molten alloy was rapidly quenched using the two-disk technique. The thickness of the ribbons was 30-40 ~xm. The composition was analyzed by chemical method and the ribbons contained 24 at% A1. They were annealed at temperatures of 150, 200, 250, 300, 350, 400, 450 or

510°C for 1 h in an argon gas atmosphere. Some ribbons were annealed at 900 or 630°C for 1 h and then quenched into water at room temperature. The structure of the Fe76AI24specimens was controlled with the aid of X-ray diffraction and transmission electron microscopy, (TEM). The saturation magnetizations were measured using a vibrating-sample magnetometer at 293 and 77 K for magnetic fields up to 13 kOe. The magnetization was measured with an accuracy of up to 0.7%. Spin-echo NMR measurements of the HFF distributions were made at 4.2 K with zero external magnetic field and for frequencies ranging from 20 to 80 MHz.

3. Results and discussion

Fig. 1 shows the dependence of the saturation magnetization M and average magnetic moment m on the heat treatment. It is clear that the heat treatment caused practically no change in the values of M and m. Fig. 2 shows the NMR spectra of the Fe-24AI ribbons. It is obvious that the HFF distributions depend on the temperature and conditions of quenching. The ribbons annealed and quenched from 600°C contain the B2 phase and the maxima of the HFF spectra of these specimens (600°C) are at 40 MHz. The diffraction patterns of alloys rapidly quenched from the melt and

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Fig. l. Dependence of saturation magnetization M and mean magnetic moment per Fe atom m in Fe-24AI alloy obtained by the rapid quenching technique at 293 K (e) and 77 K (©).

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Fig. 2. Normalized spin-echo spectra of 27-A1 nuclei for Fe-24AI alloys: as-quenched state from the melt (e); asquenched at 900°C ( • ) and 600°C (o) after annealing for 1 h. The calculated values of Pi and f (res.) using the model described in the text are shown by the dashed lines.

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Fig. 3. Normalized spin-echo spectra of 27-A1 nuclei of Fe24A1 alloy after annealing for 1 h at: 150°C (a), 250°C (b), 350°C (c), 450°C (d), 510°C (e) and 900°C for 0.5h, 600°C for 1 h, 400°C for 4 h (D.

Fig. 3 shows the influence of heat treatment on the 27-A1 H F F distributions. After annealing at 510°C the specimens contain the DO3 phase. The 27-A1 spectrum of this alloy is very narrow, with a maximum at 29 M H z [5]. The annealings at 250°C and higher temperatures turn the phase state of the ribbons into DO3 according to the 27-A1 H F F distributions. The average H F F at the 27-A1 nuclei H(AI) decreases from H(A1) = 47 kOe for rapidly quenched ribbons, to H ( A 1 ) = 28 kOe for the samples annealed at 510°C (Fig. 4). Since the Al atoms do not carry a magnetic moment, the predominant mechanism for the hyperfine field on an A1 nucleus is the 4s polarization due to neighboring moments of the Fe atoms: H(A1) = A . n ( 1 ) ' m ( l ) ,

(1)

where n(1) is the number of Fe nearest neighbors (nn), A is the interaction constant for the transferred hyperfine field, and m(1) is the average magnetic moment of the Fe atoms surrounding the A1 atoms. To estimate H(AI) we need to know A and m(1). In bcc Fe(A1), B2 and DO3 phases of F e - 2 5 A I alloy the A1 atoms have eight Fe atoms (8Fe) as first nearneighbors (lnn). The iron magnetic moments depend greatly on the number of their Fe lnn; this is a well

If. Pokatilov /Journal

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of Magnetism and Magnetic Materials 133 (1994) 86-89

known local environmental effect in Fe-25AI and F e 25Si alloys [6]. There are two sites of Fe atoms (Fe(1) and Fe(2)) in the DO3 phase, which are chemically and magnetically inequivalent. The Fe(1) sites have four Fe(2) (4Fe) and four A1 (4A1) Inn. The Fe(2) sites have 8Fe(1) and magnetic moment of 2.14/, 8 [7]. In DO3 phase the A1 atoms have 8Fe(1) Inn with magnetic moments of 1.46/* B, and the 27-A1 H F F H ( A I ) = 25 kOe. In bcc Fe(AI) the AI atoms have 8Fe(2) inn with m{Fe(2)} = 2.2/* B and H(A1) = 54 kOe [4]. In the B2 ordered lattice (and also in the 'offstoichiometry' Fe3A1 alloy) the A1 and Fe atoms are mixed randomly at the sites at the center of lattice. This means that the A1 atoms have mixtures of different Fe(1) (with various numbers of AI and Fe atoms as lnn), and the iron mean magnetic moments surrounding the A1 atoms are changed from 2.14 to 1.46/, B. This gives rise to the broad distributions of the 27-A1 HFF. It is clear from Figs. 1-3 that although the average moments per Fe atom m are not practically changed after heat treatment, but the 27-A1 H F F distributions show that the structural states of the Fe-24A1 ribbons are quite different and the local magnetic moments of the Fe atoms are changed over a broad range, depending on the local environments in the microscope ranges. Using neutron [7] and 57-Fe H F F data [6] the dependence of the Fe magnetic moment on the number of Fe lnn for the binary Fe-A1 system has been determined, as shown in Fig. 5. The saturation magnetization for the F e - A I alloy is given by [8]:

M = {223.2/M(0)}{(25 + x)(2.14) + (50)~P~mi(1),

(2)

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Fig. 5. Dependence of local magnetic moments at Fe(1) sites versus the number of Fe(2) Inn for the binary Fe-AI system. where M(0) is the gram formula weight, m is the moment for Fe(1) with i n n of Fe(2) atoms (Fig. 5), Pi is the probability of finding k 'impurities' in any lnn shell of n sites, such as for a dilute random alloy with an 'impurity' concentration x, assuming the formula Fe(2)}50Fe(1)25+xAlzs_, for Fe-24A1 alloy. Pi can be calculated from a binominal distribution. From Eq. (2), the values of M and m have been estimated as M = 165 e m u / g and m = 1.42/* w This consideration gives a good fit to the experimental data. In bcc Fe(AI) an impurity A1 atom contains 8Fe(2) lnn with m{Fe(2)} = 2.2/* B and its H F F H(A1)= 54.3 kOe (60.2 MHz). In this case the constant A is estimated as A ( 1 ) = 3.17 k O e / / * u - F e atom. For ordered Fe3AI alloy, an A1 atom has only 8Fe(1) lnn with m(2) = 1.46/* B and, consequently, A(2) = 2.24 k O e/ / * u Fe atom. For the intermediate states of Fei(1) atoms the constants A have intermediate values A(i) between A(1) and A(2), and the local H F F H i (AI) is given by:

H(AI) =A(i)n(1)mi{Fe(1)},

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where A(i) depends on concrete neighbors. Fig. 2 shows the relative values Pi and the local resonance frequencies that were estimated from Eq. (3) and Fig. 5. It is obvious that these resonances overlap the whole range of experimental H F F distributions. Their positions can clarify the poorly resolved peaks and evaluate the short orders in the microscope ranges.

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300 400 500 600 T, C Fig. 4. Dependence of the average HFF at 27-A1 nuclei H(AI) against annealing temperature.

References

[l] H. Sagane and T. Eguchi, Trans. Jpn. Metals 18 (1977) 488.

V. Pokatilov /Journal of Magnetism and Magnetic Materials 133 (1994) 86-89 [2] F. Gadieu, M. Russak and R.F. Purich, J. Magn. Magn. Mater. 54-57 (1986) 1598. [3] R.M. Bozort, Ferromagnetism (Foreign Literature, Moscow, 1956) p. 174. [4] M.B. Stearns, Phys. Rev. B9 (1972) 3326. [5] V. Niculescu, K. Raj, T. Burch and J.I. Budnick, J. Phys. F: Metal Phys. 7 (1977) 73. [6] K. Yamanaka, H. Ito, R. Oshima and F. Fujita, J. Jpn. Inst. Metal 35 (1971) 566.

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[7] W.A. Hines, A.H. Menotti, J.I. Budnick, T.J. Burch, T. Lirenta, N. Niculescu and K. Raj, Phys. Rev. B13 (1976) 4060. [8] J.G. Booth, J.E. Clark, J.D. Ellis, P.J. Webster and S. Yoon, in: Proc. Int. Conf. on Magnetism (Nauka, Moscow, 1973), IV, p. 577.