Structure and magnetic properties of Pr2Co17−xSix compounds

Journal of Alloys and Compounds 302 (2000) 5–11

L

www.elsevier.com / locate / jallcom

Structure and magnetic properties of Pr 2 Co 172x Si x compounds ¨ b , Z.Y. Liu a , F.R. de Boer b , L. Zhang a,b , D.C. Zeng a , Y.N. Liang a,b , J.C.P. Klaasse b , E. Bruck b, K.H.J. Buschow * a Department of Mechano-electronical Engineering, South China University of Technology, Guangzhou, PR China Van der Waals–Zeeman Instituut, Universiteit van Amsterdam, Valckenierstraat 65, 1018 XE Amsterdam, The Netherlands

b

Received 3 November 1999; accepted 5 November 1999

Abstract The structure and magnetic properties of Pr 2 Co 172x Six compounds with Si concentrations up to x 5 2 were studied by means of X-ray diffraction and magnetic measurements. All compounds have the rhombohedral Th 2 Zn 17 -type structure. At room temperature, the easy magnetization direction is perpendicular to the c-axis in all compounds. Above room temperature, a spin-reorientation transition takes place in Pr 2 Co 172x Si x compounds with x 5 1.5 and 2. The Co moment and the Curie temperature decrease strongly with increasing Si concentration. The rotation-alignment method was employed to magnetically align fine powder particles of Pr 2 Co 172x Si x compounds along their c-axis. The field dependence of the magnetization was measured on the rotation-aligned samples with the field applied along the rotation (c) axis, from which the anisotropy constant K1 was derived by means of the Sucksmith–Thompson method. The absolute value of K1 and the anisotropy field HA decrease monotonically with increasing Si concentration.  2000 Elsevier Science S.A. All rights reserved. Keywords: Pr 2 Co 172x Si x compounds; Spin-reorientation; Rotation-alignment method; Sucksmith–Thompson method

1. Introduction In several previous investigations, we studied the structure and magnetic properties of R 2 Co 172x Si x compounds [1–5]. In the Y 2 Co 172x Si x and Gd 2 Co 172x Si x compounds, the room-temperature anisotropy of the Co sublattice changes with increasing Si concentration from easy plane to easy axis. Accordingly, the anisotropy constant K1 changes sign from negative to positive. After reaching a maximum at about x 5 2, it starts to drop again. In all Ho 2 Co 172x Si x compounds (0 # x # 3), competition between the easy-plane Ho-sublattice anisotropy and easyaxis Co-sublattice anisotropy leads to spin-reorientation transitions. The spin-reorientation temperature, T sr , decreases strongly with increasing Si concentration to the extent that Ho 2 Co 14 Si 3 adopts easy-axis anisotropy at room temperature. However, spin-reorientation transitions do not occur in the corresponding Dy 2 Co 172x Si x compounds, due to the dominance of the easy-plane anisotropy of the Dy sublattice in the whole temperature range below

*Corresponding author. Tel.: 131-20-525-5714; fax: 131-20-5255788. E-mail address: [email protected] (K.H.J. Buschow)

the Curie temperature. In the present study, we have extended our investigation to Pr 2 Co 172x Si x compounds.

2. Experimental Pr 2 Co 172x Six samples with Si concentrations x 5 0, 0.5, 1, 1.5, 2 and 3 were prepared by arc melting starting materials of at least 99.9% purity. After arc melting the samples were wrapped in Ta foil, sealed in an evacuated quartz tube and annealed at 9008C for 2 weeks and subsequently quenched in water. The X-ray-diffraction diagrams showed that the annealed samples were approximately single phase for x # 2 and that all compounds crystallized in the rhombohedral Th 2 Zn 17 -type structure. The sample with x 5 3 could not be prepared in singlephase form. X-ray-diffraction diagrams were also recorded on finely ground powder samples in which the particles were magnetically aligned at room temperature in a static magnetic field of about 1 T and fixed in the alignment direction with epoxy. The magnetic measurements were made in a SQUID magnetometer in the temperature range 5–300 K in magnetic fields up to 5 T. For the measurements above 300 K, we used a home-built magnetometer based on the

0925-8388 / 00 / $ – see front matter  2000 Elsevier Science S.A. All rights reserved. PII: S0925-8388( 99 )00694-5

6

L. Zhang et al. / Journal of Alloys and Compounds 302 (2000) 5 – 11

Faraday principle. In the latter experiment, the measurements were made on pieces of polycrystalline bulk material sealed in an evacuated quartz tube in order to avoid as far as possible oxidation at elevated temperatures.

3. Results The X-ray-diffraction diagrams measured on randomly oriented powder particles are displayed in Fig. 1. The lattice constants derived from these data are listed in Table 1, together with the corresponding unit-cell volumes. Fig. 2 shows the X-ray-diffraction diagrams measured on magnetically aligned powder samples. It can be concluded that all Pr 2 Co 172x Si x compounds with x # 2 have their easy magnetization direction perpendicular to the c-axis at room temperature. The temperature dependence of the magnetization above room temperature is presented in Fig. 3. A spin-reorientation transition from easy-axis anisotropy at high temperatures to easy-plane anisotropy at low temperatures occurs in Pr 2 Co 172x Si x compounds with x 5 1.5 and 2, marked by the occurrence of maxima in the M–T curves. The Curie temperatures and spin-reorientation temperatures derived from these measurements are shown in Table 2. It is seen in Fig. 4 that the magnetic-ordering temperatures of the Pr 2 Co 172x Si x compounds decrease strongly and nearly linearly with increasing Si concentration. The field dependence of the magnetization of Pr 2 Co 172x Si x compounds was measured at 5 and 300 K on powder particles that were free to rotate in their equilibrium position in the applied field. The results are presented in Fig. 5. The values of the spontaneous moment, M0 , were

Table 1 Lattice constants, unit-cell volume and easy magnetization direction of Pr 2 Co 172x Si x compounds x

˚ a (A)

˚ c (A)

˚ 3) V (A

Structure

EMD

0 0.5 1 1.5 2

8.460 8.450 8.439 8.427 8.424

12.255 12.243 12.245 12.246 12.240

759.6 757.2 755.1 753.2 752.1

Rhombohedral Rhombohedral Rhombohedral Rhombohedral Rhombohedral

Easy plane Easy plane Easy plane Easy plane Easy plane

derived by extrapolating the linear parts in the high-field region of the magnetic isotherms to zero field. Assuming that the Pr moment has a free-ion value of 3.2 m B at 5 K, the Co moments of Pr 2 Co 172x Six compounds were evaluated and are plotted versus concentration in Fig. 6, together with the corresponding spontaneous moments. It can be seen that Si substitution results in a stronger decrease of the spontaneous moments than expected according to simple magnetic dilution, represented by the dashed line in the bottom part of Fig. 6. Thus, Si not only acts as a magnetic diluent in Pr 2 Co 172x Six compounds, but reduces the average Co moment as well. As we have shown, all the presently studied Pr 2 Co 172x Six compounds exhibit easy-plane anisotropy at room temperature. Therefore, we applied the rotationalignment method to prepare magnetically aligned samples [6]. In this method, fine powder particles are mixed with epoxy and rotated in an applied magnetic field until the epoxy has solidified. This results in an alignment of the powder particles of the easy-plane compounds along the rotation axis, which actually is the hard axis, that is, the c-axis of the particles. In order to examine the degree of alignment, X-ray-diffraction diagrams were recorded with

Fig. 1. X-ray-diffraction patterns of randomly oriented powder of Pr 2 Co 172x Si x compounds.

L. Zhang et al. / Journal of Alloys and Compounds 302 (2000) 5 – 11

7

Fig. 2. X-ray-diffraction patterns of magnetically aligned powder of Pr 2 Co 172x Si x compounds.

Fig. 3. Temperature dependence of the magnetization of Pr 2 Co 172x Si x compounds, measured in a field of 0.05 T.

Table 2 Curie temperature, spin-reorientation temperature, spontaneous moment at 300 and 5 K, average Co moment at 5 K, anisotropy constant K1 and anisotropy field HA at 300 K of Pr 2 Co 172x Si x compounds x

TC (K)

0 0.5 1 1.5 2

1157 1087 1001 909 806

T sr (K)

M0, 300 K (m B / f.u.)

M0, 5 K (m B / f.u.)

mCo (m B )

K1 (MJ / m 3 )

m0 HA (T)

621 540

29.0 27.6 25.5 22.1 20.1

32.1 30.7 27.1 24.8 23.0

1.51 1.47 1.29 1.19 1.11

21.46 21.12 20.96 20.72 20.59

2.73 2.21 2.04 1.78 1.60

the bisector of the incident and diffracted beam parallel to the rotation axis of the samples. The results are displayed in Fig. 7, where it can be seen that the alignment is fairly good. The field dependence of the magnetization, measured on the rotation-aligned samples at 300 K with the field applied along the c-axis, is shown in Fig. 8. These data were used to plot Hi /J versus J 2 , according to the Sucksmith– Thompson expression for an easy-plane system [7]: H 2K1 1 4K2 4K2 2 ]i 5 2 ]]] 1 ]] J J J 2s J 4s

8

L. Zhang et al. / Journal of Alloys and Compounds 302 (2000) 5 – 11

Fig. 4. Dependence of the Curie temperature of Pr 2 Co 172x Si x compounds on the Si concentration.

where Hi is the internal field, Js the saturation polarization, and K1 and K2 the anisotropy constants. Taking the demagnetizing factor N equal to 0.33 as for approximately spherical particles, Hi can be derived from Hi 5 H 2 NM. For obtaining Js , we used the values of the spontaneous polarization listed in Table 2. Fig. 9 presents an example of such a Sucksmith–Thompson plot. The anisotropy constants K1 and K2 were derived from the slope and the vertical intercept of the linear parts of the Sucksmith– Thompson curves. However, the inevitable misalignment of the particles, which contributes to K2 , prevents an accurate determination of the value of K2 . The K1 value for the Pr 2 Co 17 compound obtained in this way is close to that (21.51 MJ / m 3 , at 250 K) obtained in the single-crystal measurements [8]. The anisotropy field HA , which is the smallest applied field able to force the magnetic-moment direction from the basal plane to the c-axis, can be calculated from HA 5 2 2K1 /Js . The concentration dependence of K1 and HA is shown in Fig. 10. It can be seen that the absolute values of the anisotropy constant K1 and the anisotropy field HA decrease monotonically with increasing Si concentration.

4. Discussion

Fig. 5. Field dependence of the magnetization of Pr 2 Co 172x Si x compounds at 5 and 300 K, measured on powder particles that were free to rotate in the sample holder.

Previous investigations of Ce 2 Co 172x Si x , Gd 2 Co 172x Si x and Y 2 Co 172x Si x compounds with x # 3 have shown that all Ce 2 Co 172x Si x compounds, Gd 2 Co 172x Si x compounds with x $ 1, and Y 2 Co 172x Si x compounds with x . 1 have easy-axis anisotropy at room temperature. Since the Gd and the Y sublattice can be assumed to have no anisotropy, it can be concluded that the Co sublattice favours an easy magnetization direction parallel to the c-axis in

L. Zhang et al. / Journal of Alloys and Compounds 302 (2000) 5 – 11

9

Fig. 6. Spontaneous moment and Co moment in Pr 2 Co 172x Si x compounds at 5 K.

Fig. 7. X-ray-diffraction patterns of rotation-aligned samples of Pr 2 Co 172x Si x compounds, measured with the bisector of the incident and diffracted beam parallel to the rotation axis.

10

L. Zhang et al. / Journal of Alloys and Compounds 302 (2000) 5 – 11

Fig. 8. Field dependence of the magnetization of Pr 2 Co 172x Si x compounds at 300 K, measured on rotation-aligned samples with the field applied parallel to the rotation (c) axis.

R 2 Co 172x Si x compounds with x . 1 at room temperature. It has been established [9] that the Pr sublattice has easy-plane anisotropy in 2:17-type compounds (K Pr 1 , 0). This anisotropy decreases strongly with increasing temperature due to the strong temperature dependence of the thermal average of the Stevens operator kO 02 l in the firstorder anisotropy expression: K 1R 5 2 ]23 aJ kr 2 lkO 20 lA 20 . However, the anisotropy of the Co sublattice is much less temperature dependent. From our X-ray-diffraction results for the magnetically aligned samples, we know that, at room temperature, the easy-plane anisotropy of the Pr

sublattice dominates the Co-sublattice anisotropy in all Pr 2 Co 172x Si x compounds. However, in Pr 2 Co 172x Si x compounds with x . 1, one may expect that a spin-reorientation transition might occur at higher temperature when the easy-axis anisotropy of the Co sublattice becomes dominant. As a matter of fact, this is apparently the case in the compounds Pr 2 Co 15.5 Si 1.5 and Pr 2 Co 15 Si 2 , for which the spin-reorientation temperatures are 621 and 540 K, respectively. The anisotropy constant for compounds formed between rare earths (R) and transition metals (T) is the sum of the

Fig. 9. Sucksmith–Thompson plot for the Pr 2 Co 17 compound and the linear fit for the middle part of the plot.

L. Zhang et al. / Journal of Alloys and Compounds 302 (2000) 5 – 11

11

Fig. 10. Si concentration dependence of the anisotropy field HA and the anisotropy constant K1 of Pr 2 Co 172x Si x compounds at 300 K. R T R- and T-sublattice anisotropies, K tot 1 5 K 1 1 K 1 . For the Gd 2 Co 172x Si x and Y 2 Co 172x Si x compounds, the values of the anisotropy constant K1 (i.e. K Co 1 ) at 300 K were obtained by means of the expression K1 5 ]12 m0 HA Ms and were found to change from negative to positive with increasing Si concentration. After reaching a maximum value at about x 5 2, the K1 values decrease again. In Pr 2 Co 172x Si x compounds, because the negative K 1Pr prevails in all compounds at 300 K, combined with the behavior described above for K Co 1 , a monotonic decrease of the absolute values of K1 with increasing x results, as displayed in Fig. 10.

Acknowledgements This work was carried out within the scientific exchange programme between The Netherlands and P.R. China, and was supported financially by the Dutch Technology Foundation (STW). Some of the authors (L. Zhang, D.C. Zeng, Y.N. Liang and Z.Y. Liu) would like to express their thanks for the support of the NNSFC (59771026), NSFGD

(970500), HEDGD (B2-102-448) and the Rare Earth Office of Guangdong (REOGD).

References ¨ [1] X.Z. Wei, S.J. Hu, D.C. Zeng, X.C. Kou, Z.Y. Liu, E. Bruck, J.C.P. Klaasse, F.R. de Boer, K.H.J. Buschow, Physica B 262 (1999) 306. ¨ [2] X.Z. Wei, S.J. Hu, D.C. Zeng, Z.Y. Liu, E. Bruck, J.C.P. Klaasse, F.R. de Boer, K.H.J. Buschow, Physica B 266 (1999) 249. ¨ [3] S.J. Hu, X.Z. Wei, O. Tegus, D.C. Zeng, E. Bruck, J.C.P. Klaasse, F.R. de Boer, K.H.J. Buschow, J. Alloys Comp. 284 (1999) 60. ¨ [4] S.J. Hu, X.Z. Wei, D.C. Zeng, Z.Y. Liu, E. Bruck, J.C.P. Klaasse, F.R. de Boer, K.H.J. Buschow, Physica B 270 (1999) 157. ¨ [5] L. Zhang, D.C. Zeng, Y.N. Liang, J.C.P. Klaasse, E. Bruck, Z.Y. Liu, F.R. de Boer, K.H.J. Buschow, J. Alloys Comp. 292 (1999) 38. [6] W. Qun, Z. Zhi-gang, L. Wei, X.K. Sun, Y.C. Chuang, F.R. de Boer, J. Magn. Magn. Mater. 109 (1992) 59. [7] W. Sucksmith, J.E. Thompson, Proc. R. Soc. London, Ser. A 225 (1954) 362. [8] S. Sinnema, PhD thesis, Universiteit van Amsterdam, 1988. [9] J.J.M. Franse, R.J. Radwanski, in: K.H.J. Buschow (Ed.), Handbook of Magnetic Materials, Vol. 7, North-Holland, Amsterdam, 1993, p. 468.