Mar 13, 2016 - V Niculescut, K Rajtz, T Burch# and J I Budnickt t The University of Connecticut, Storrs. Connecticut 06268 USA. 0 Department of Physics ...
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Hyperfine interactions and structural disorder of Fe3Si1-xAlx alloys
This content has been downloaded from IOPscience. Please scroll down to see the full text. 1977 J. Phys. F: Met. Phys. 7 L73 (http://iopscience.iop.org/0305-4608/7/3/004) View the table of contents for this issue, or go to the journal homepage for more
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J. Phys. F: Metal Phys.. Vol. 7, No. 3, 1977. Printed in Great Britain. @ 1977
LETTER TO THE EDITOR
Hyperfine interactions and structural disorder of Fe,Si ,Al, alloys -
V Niculescut, K Rajtz, T Burch# and J I Budnickt t The University of Connecticut, Storrs. Connecticut 06268 USA Yale University, New Haven, Connecticut 06520 USA
0 Department of Physics Marquette University, Milwaukee, Wisconsin 53233 USA Received 25 November 1976
Abstract. Spin echo, x ray and magnetization measurements are performed on Fe3Si, -xAl, alloys for 0 < x < 0.25. The results are analysed by considering near neighbour environments for various sites.
1. Introduction Fe3Si is an ideal system in which to study chemical, magnetic and hyperfine interactions because it is highly ordered crystallographically and because impurities enter only one of the available sites. The effect of a large number of transition metal impurities on these properties has already been studied (Burch et a1 1974). In this paper we report a study of the substitution of the sp element, Al, into this matrix using spin echo, x ray and magnetization. Both Fe3Si and Fe3Al have the DO3 crystal structure and a continuous range of solid solutions is formed between them. All of these compounds are ferromagnetic with comparable moments on the two Fe sites. Figure 1 shows a unit cell of DO, structure and the near neighbour (NN) configurations for each site are listed in table 1.
Figure 1. A unit cell of the D O 3 crystal structure. The sites are: 0, A ; 0, B ; 0, C and A, D where A, C : Fe; B : Fe and D: Si.
$ Supported by the National Science Foundation.
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Letter to the Editor Table 1. The number and kind of near neighbours for each site. Neighbour configurations Fe,Si Site
1"
2NN
3NN
A,C B D
4B,4D 8A,C 8A,C
6A,C 6D 6B
12A,C 12B 12D
2. Alloy preparation
Samples of Fe3Si, -xAlx (x = 0, 0.04, 0.08, 0.15 and 0.25) were prepared by arc melting in an argon atmosphere. Ingots with x < 0.08 were annealed at 800°C for 40 hours, slow cooled to 600°C for one hour and furnace cooled. This treatment is suitable for Fe,Si alloys. Ingots with x > 0.15 were given a heat treatment suited for Fe3Al alloys, i.e. annealing at 950°C for 3 days, water quenching, powdering with a tungsten carbide file to -200 mesh, then annealing at 550°C for several hours and slowly cooling to room temperature.
3. Experimental The spin echo NMR spectra of these alloys taken at 1-3K are shown in figure 2. In the spectrum of Fe,%, sharp NMR lines characteristic of a high degree of crystallographic order are observed. The resonances from nuclei at Fe(B), Fe(A,C) and Si(D) sites are observed at 46.6, 30.0 and 31.5 MHz respectively (Stearns 1963, Budnick et al 1967). The addition of A1 broadens these lines and also produces satellites. In addition, a new resonance is seen at about 15.8 MHz with a satellite at about 19.2 MHz, and the net intensity of these lines increases with the A1 concentration. We assign these new lines to A1 nuclei. The effect of further substitution of A1 is that the frequencies of all of the line shift and the relative intensity of the satellites increase. The Fe(B), Fe(A,C) and Si(D) lines have linear shifts of about the same size to lower frequency while the A1 line shifts to higher frequency at about double this rate (see figure 3). The satellites associated with these main lines show similar concentration dependences. The x ray measurements on these samples showed the presence of only one phase, DO3. The degrees of disorder as well as the lattice constants were calculated from x ray measurements using the procedure outlined in Niculescu et al (1976). The values of lattice constants increase linearly with the A1 composition from 5.653 for x = 0 to 5.691 A for x = 0.25 and for x = 1.0 the line extrapolates to 5.793 A, the measured value for Fe3A1. The A1 substitution introduces a B D disorder which increases with the concentration. Small amounts of A,C c1 D disorder was found in all of these alloys. The room temperature saturation magnetization for our Fe3Si, -xAlx alloys showed increases above the value for Fe3Si, 4.4p,/formula unit (the magnetization reported for Fe3Al is 5.2pe/formula unit). This increase implies a small increase of one or both of the Fe sites moments with x. Neutron diffraction results which measure 2.2 p, Fe(B) moments in both Fe3% and Fe3Al and 1.35 and 1.5 p, Fe(A,C)
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Letter to the Editor
I
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1
1 Fe1A.C)
I ...... 22
,, ,,.-?.
10
A.7i,...
x=l
.... .................!, .... ',..
3L
58
Frequency IMHzI Figure 2. Plots of normalized spin echo amplitude against frequency for Fe,Si, -xAIx alloys at 1.3 with .Y = 0. 0.08. 0.15. 0.25 and 1.0. The x = 1 spectrum is from Burch er a/ ( I 970).
33 7
3 31 219
0
0.1
0.2
3
Concentration ( X I Figure 3. Concentration dependence of the internal fields at Fe(B), Fe(A.C), Si and A I nuclei in Fe&, -xAlx alloys at 1.3 K. The labels (0) and (1) correspond to main line and satellite respectively.
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Letter to the Editor
moments in Fe,Si and Fe3Al respectively (Hines et a1 1976, Nathans et a1 1958) suggest that the principal increase is in the Fe(A,C) moment. 4. Discussion
It is evident that A1 enters the FeJSi matrix with one less sp electron. One therefore expects a monotonic change in the conduction electron polarization term of the hyperfine field contribution on both the transition metal and sp elements as the A1 content is increased. Our results (see figure 3) are in agreement with this expectation. The A1 enters in a site which is surrounded by 8 Fe(A,C) 1”. For Fe3Si, ~~ in Fe,Al it is about 1 . 5 ~ ~ . the average moment on 1” Fe(A,C) is 1 . 3 5 whereas The observation of smaller values of the A1 internal field in Fe,(AlSi) alloys and a larger value in Fe,Al(26 k 3 kOe) (Burch et al 1970) suggests a possible relationship between the A1 field and the INN Fe(C,A) moments. The satellite which we observe on both the A1 and Si resonances could be explained by the changes in this Fe(A,C) moment. This idea is supported by the fact that the A1 satellites intensity increases with respect to the main line increases with x and that extrapolation of the satellite’s field to x = 1 (Fe,Al) yields a value of 28 & 2 kOe in very good agreement with the reported value. This work was supported in part by the University of Connecticut Research Foundation. References Budnick J I, Skalski S, Burch T J and Wernick J H 1967 J . AppL Phys. 38 1137 Burch T J. Li’trenta T and Budnick J I 1974 Phys. Rev. Lett. 33 421 Burch T J. Murphy J J. Budnick J I and Skalski S 1970 J . Appl. Phys 41 1327 Hines W .A. Menotti A H. Budnick J I, Burch T J, Litrenta T, Niculescu V and Raj K 1976 Phys. Reu. B 13 4060 Nathans R, Pigott M T and Shull C G 1958 J . Phys. Chem. Solids 6 38 Niculescu V, Raj K. Burch T J and Budnick J I 1976 Phys. Rrc. B 13 3176 Stearns M B 1963 Phys. Rev. 129 1136
