Magnetic properties and 57Fe Mössbauer effect in UFe10−xNixSi2 alloys

Journal of Alloys and Compounds, 201 (1993) 61-65 JALCOM 760

61

Magnetic properties and 57Fe M6ssbauer effect in UFelo_xNixSi2 alloys W. Suski, F.G. Vagizov*, K. W o c h o w s k i a n d H . D r u l i s Polish Academy of Sciences, W. Trzebiatowski Institute of Low Temperature and Structure Research, P.O. Box 937, 50-950 Wroctaw 2 (Poland) (Received March 3, 1993)

Abstract The magnetic properties and 57Fe M6ssbauer effect in the UFelo_xNixSi2 system were investigated over a broad temperature range. Magnetic measurements revealed a ferromagnetic character, with two anomalies in the temperature dependence of the susceptibility for x > 6 but only one anomaly for x < 6 . The temperature of the last anomaly increases as x decreases. The high-temperature anomaly seems to be related to the precipitation of free nickel. The field dependence of the magnetization exhibits remanence for x>~4. The 57Fe M6ssbauer effect demonstrates that only iron contributes to the magnetic ordering observed at lower temperatures. It is shown that the substitution of Ni for Fe results in a strong preferential occupation by Ni of the 8(f) site. The decrease in the Curie point and the saturation magnetization is attributed to the decrease in the number of the nearest neighbour Fe atoms.

1. Introduction Preliminary investigation of UFel0_xNixSi2 has shown that it exists in single-phase form for x > 8 only [1]. This system exhibits a tetragonal, ThMn12-type structure (space group I4/mmm) in which the uranium atoms are located in the 2(a) positions, and the Ni atoms and substituted Fe atoms occupy the 8(f), 8(i) and 80) sites. The Si atoms occupy exclusively the 8(j) sites. Occupation of the 8(i) positions by the Fe atoms is very important for the magnetic properties because the Fe atoms, located in these positions, have the largest number of nearest neighbour Fe atoms and the largest Fe-Fe separation which creates the strongest ferromagnetic exchange interactions. As has been shown in ref. 1, the lattice parameters roughly follow Vegard's law, increasing with increasing iron content. The a parameter displays a much stronger increase than the c parameter. For x > 8 , the temperature dependence of the magnetization and susceptibility exhibits two anomalies. Those observed at about 630 K may be related to the Curie point of nickel. However, the anomaly is absent in pure UNiloSi2 [2]. At low temperature, other anomalies are observed at 32 and 45 K for x = 9.5 and 9.0 respectively. These low-temperature anomalies are most probably associated with magnetic *On leave from the Physicotechnical Institute, Russian Academy of Sciences, 420029 Kazan, Sibirski Trakt 10/7, Russian Federation.

0925-8388/93/$6.00

ordering of ferromagnetic character. Because both samples exhibit considerable remanence at low temperature, we have concluded that an admixture of nickel with UFel0Si2 may improve the magnetic parameters of this compound which shows a relatively high Curie point and magnetization but a total lack of remanence [3]. Therefore our preliminary research [1] was extended to a broader composition range (x~<8) despite the presence of a very small admixture (less than 3%) of an unidentified phase ("almost" single-phase samples are currently the subject of extensive examination). Moreover, M6ssbauer effect investigations were carried out to determine whether iron contributes to the magnetic ordering observed in magnetic measurements. In addition, the distribution of Fe atoms in various crystallographic positions has a substantial influence on the magnetic properties and this distribution can be determined from a deconvolution of the M6ssbauer spectra.

2. Experimental details The preparation of the samples and the crystallographic examinations have been described previously [1]. The magnetometric measurements were carried out using a magnetic balance (Faraday method) in a zero magnetic field in the temperature range 4.2-1000 K

© 1993- Elsevier Sequoia. All rights reserved

W. Suski et al. / Magnetic properties and ~ZFe MOssbauer effect in UFem_~N&Si~

62

and an extraction magnetometer at 4.2 K in a magnetic field up to 4 T. The magnetic ordering temperature was determined using the M6ssbauer thermal scan method. The experimental spectra were fitted as in previous investigations [4--6] by three sextets with different widths of the external and internal lines. It is assumed that Hh~(8(i))>Hht(8(j))>Hhf(8(f)); this relation has been determined in other Fe-rich intermetallics with the ThMn12-type structure (see ref. 7) and was confirmed by band structure calculations [8, 9].

3. R e s u l t s

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8

composition, x

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0

Fig. 2. Magnetic (T, x) phase diagram. Tm (squares, right-hand scale) was determined from magnetometric measurements and T~ (circles, left-hand scale) was determined from the M6ssbauer effect temperature scan.

and discussion

3.1. Magnetic measurements In Fig. 1, the temperature dependences of the Tagnetic susceptibility of UFe~o_~Ni~Si2 compounds with 1.0 ~t 8, passes through a minimum for x = 6.0 and subsequently reaches the highest value of Tm = 645 K for x = 0.0. In this figure, the magnetic ordering temperatures, Try, determined from M6ssbauer measurements (see Section 2) are also drawn. The M6ssbauer spectra for x>/8, obtained at room ternperature, do not exhibit any indications of magnetic ordering. The strong difference between the results of the susceptibility and M6ssbauer experiments and the observation that Tm for x >/8 is very close to the Curie

13 ~. ~2 ~: ~ ~ ~o ~ z© ~< 7 ~_ 6 F~z © 4 < ~ 2

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Fig. 3. M a g n e t i z a t i o n M vs. m a g n e t i c field at 4.2 K for diff e r e n t x.

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TEMPERATURE [K] Fig. 1. M a g n e t i c susceptibility XM VS. t e m p e r a t u r e at 300-1000 K for the UFezo_,NixSi2 system ( x = 1, 2, 4, 6, 8, 9.5).

point of Ni indicate that precipitation of free nickel may occur in these samples. In Fig. 3, the field dependence of the magnetization of the samples with 1 ~
W. Sustd et al. / Magnetic properties and arFe MOssbauer effect in UFeto_~NixSi2 -4

T A B L E 1. R e m a n e n c e of the UFet0_xNi, Si2 system

0

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f.u., formula unit.

sition and can be considered as evidence for a magnetic state of uranium in these compounds. 3.2. MOssbauer effect study

Although the X-ray examination of these samples showed them to be "almost" single phase for x~<8.5, the 57Fe M6ssbauer spectrum of UFe2NisSi2 revealed weak Zeeman lines, the parameters of which are close to that of a-Fe (Hhf = 29.3 T). In the M6ssbauer spectra of the compound with x = 8 in Fig. 4, the outermost lines of this impurity are indicated by an arrow (less than 5%). The full lines in Fig. 4 represent a fit to the experimental results, as mentioned in Section 2. It can be seen that an increase in Ni concentration results in a decrease in the Fe hyperfine field which is due to the rearrangement of the individual atoms on the crystallographic sites. In Fig. 5 the distribution of the Fe atoms in different crystallographic positions vs. Ni concentration x is shown. For the 8(j) position it can be seen that the Fe atom occupation number decreases more strongly than for the other positions. This suggests a preferential occupation of this position by the Ni and Si atoms. The nuclear magnetic resonance (NMR) results for UNiloSiz [1] show that the Si atoms are located mainly on the 8(j) sites, which confirms the present observation. Therefore, a pronounced change in slope of the distribution function for the 80) and 8(f) sites apparently corresponds to a redistribution of the Si atoms from the 8(f) to the 8(j) sites for the alloys with high Ni content. In turn, it can be concluded that the Ni atoms enter mainly the 8(f) sites. The average isomer shift IS and the average quadrupole splitting QS vs. the Ni concentrationx are shown in Fig. 6. The linear increase in IS corresponds to an increase in the number of 3d electrons which enhances 4s electron screening and decreases the charge density at the nucleus. The slope of these lines (AIS/Ax=0.15 mm s-1) is slightly smaller than found for the yttrium alloys (for YFell_xNixTi: AIS/ Ax=0.26 mm s -1 [11]). This is apparently related to the specific electronic structure of the uranium compounds. A similar change in IS has also been observed for UFelo_xCoxSiz [4]. In the last case, however, the slope is almost twofold smaller than for the Ni alloys (Fig. 6(a)) which obviously

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Fig. 4. S7Fe M6ssbauer spectra for the UFei0_~Ni~Si2 system at 20 K.

results from the different number of d electrons for Co and Ni compared with Fe. In contrast, the concentration dependence of the average quadrupole splitting (Fig. 6(b)) exhibits a decrease for 6
64

W.. Suski et aL / Magnetic properties and 5rFe M6ssbauer effect in UFem_,Ni~Siz . . . . . . . . .

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Fig. 7. The number of nearest neighbour (NN) Fe atoms (a) and the average Fe-Fe distance on different crystallographic sites (b) vs. the Ni concentration x. observed for the YFen_~Co~Ti system [11]. In this case, however, such behaviour resulted from a change in the easy magnetization direction relative to the c axis from parallel to perpendicular. The decrease in the Fe hyperfine fields with increasing Ni atom substitution is mainly related to the decrease in the number of the nearest neighbour Fe atoms (Fig. 7(a)) because the change in the average Fe-Fe distance is very small (Fig. 7(b)). Consequently, the total magnetic moment of the Fe sublattice decreases almost linearly with an increase in the Ni concentration (Fig. 8(a)). A similar decrease in the magnetic moment of the Fe sublattice has been observed in UF%o_xCoxSi2 [4] with

o ,/?2? 200

. . . . . . . .

400

600

800

temperature, K Fig. 9. Average hyperfine field of UFe6NLSi2 and UFe4Ni6Si2 vs. temperature. an increase in Co atom concentration. In Fig. 8(b), the average magnetic moment of the Fe atoms/Zve is plotted as a function of the Ni concentration. We wish to stress that there is a relatively strong change in /2Fe with x when compared with the isostructural yttrium alloys YFen_xNixTi and YFen_~CoxTi [11-14] for which a satisfactory description was given in terms of the rigid band approximation. Furthermore, the concentration dependence of the saturation magnetization for the uranium system does not follow the simple relation / z = ( 1 - x ) / z v e + X / Z N i . v . T h e r e is a strong effect on the band structure from the 5f-3d interactions. Such interactions have been detected in other uranium compounds [6]. For the determination of the magnetic ordering temperature of the Fe sublattices (Tve), the temperature dependences of the hyperfine field were investigated. In Fig. 9, Hhf Vs. T plots are shown for x = 4 and 6, and the results of such measurements are represented by circles in Fig. 2 the (T, x) magnetic phase diagram. The TF~ values for alloys with x~< 9 depend linearly on /ZFe2 (Fig. 10). This proves that there is a strong ferromagnetic coupling of the Fe atoms and that the magnetic ordering temperatures TF~ are mostly determined by the exchange interactions of the 3d electrons of the Fe atoms. In the light of our M6ssbauer ex-

W. Suski et aL / Magnetic properties and ~TFe MOssbauer effect in UFeto_,NixSi 2 800

.........i........./,,,

600

,.~ 400 t~

~. 200 ~J

~0

15 20 25 3o (/z~,) 2, /za 2

Fig. 10. Ordering temperature of the Fe sublattice (Trc) vs. squared magnetic moment of the Fe atom.

periment, we suspect that the low-temperature anomalies observed in ref. 1 for the alloys with x = 9 and 9.5 are also determined by ferromagnetic ordering of the iron sublattice.

4. Conclusions

The investigation of the magnetic properties and 57Fe M6ssbauer effect proves that the UFel0_xNixSi2 compounds are ferromagnets with Curie points decreasing monotonically with Ni concentration. For x > 6 an additional anomaly at high temperatures (approximately 600-700 K) appears, which seems to be related to the precipitation of free nickel. This hypothesis requires further microscopic confirmation. For the compositions with x/> 4, the field dependence of the magnetization gives rise to remanence, which is the largest observed for the alloys based on UFeloSi2 (e.g. U ( C o , Fe)loSi2, U ( F e , Al)loSi2).

65

The M6ssbauer effect shows that there is preferential occupation of the 8(f) positions by Ni. The concentration dependence of the magnetic moment in UFelo_xNixSi2 suggests that the behaviour of these compounds cannot be described by the rigid band model. This indicates that there is a strong influence of the uranium atom on the magnetic properties in this system. References 1 W. Suski, K. Wochowski, A. Zygmunt, J. Janczak, B. Nowak, K. Nied£'wied~ and O.J. Zogat, J. Alloys Comp., in press. 2 W. Suski, A. Zaleski, D. Badurski, L. Folcik, K. Wochowski, B. Siedel, C. Giebel and F. Steglich, J. Alloys Comp., in press. 3 W. Suski, A. Baran and T. Mydlarz, Phys. Lett. A, 136 (1989) 89. 4 T. Berlureau, B. Chevalier, P. Gravereau, L. Fournes and J. Etourneau, J. Magn. Magn. Mater., 102 (1991) 166. 5 W. Suski, F.G. Vagizov, H. Drulis, J. Janczak and K. Wochowski, J. Magn. Magn. Mater., 117 (1992) 203. 6 A.V. Andreev, F.G. Vagizov, W. Suski and H. Drulis, J. Alloys Comp., 187 (1992) 401. 7 H.S. Li and J.M.D. Coey, in K.H.J. Buschow (ed.), Handbook of Magnetic Materials, Vol. 6, Elsevier Science, Amsterdam, 1991, p. 1. 8 R. Coehoorn, Phys. Rev. B, 41 (1990) 11 790. 9 S.S. Jaswal, Y.G. Ren and D.J. Sellmyer, J. Appl. Phys., 67 (1990) 4564. 10 A.V. Andreev, W. Suski and N.V. Baranov, J. Alloys Comp., 187 (1992) 293. 11 Z.W. Li, X.Z. Zhou and A.H. Morrish, J. Appl. Phys., 69 (1991) 5602. 12 Y.-C. Yang, S. Hong, Z.-Y. Zhang, T. Luo and J.-L. Gao, Solid State Commun., 68 (1988) 175. 13 Z.W. Li, X.Z. Zhou, A.H. Morrish and Y.-C. Yang, J. Phys. Condensed Matter, 2 (1990) 9621. 14 Y.-C. Yang and B.-P. Cheng, J. Less-Common Met., 153 (1989) 9.