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Structural and magnetic properties of R2(Fe,Si)17 compounds with R=Tb and Er
Journal of Alloys and Compounds 284 (1999) 289–294
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Structural and magnetic properties of R 2 (Fe,Si) 17 compounds with R5Tb and Er ¨ b , F.M. Yang a J.L. Wang a ,b , F.R. de Boer b , *, X.F. Han a , N. Tang a , C. Zhang c , D. Zhang c , E. Bruck a
State Key Laboratory of Magnetism, Institute of Physics, Chinese Academy of Sciences, P.O. Box 603, Beijing, People’ s Republic of China b Van der Waals–Zeeman Institute, University of Amsterdam, Valckenierstraat 65, 1018 XE Amsterdam, The Netherlands c Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110015, People’ s Republic of China Received 27 August 1998; received in revised form 25 November 1998
Abstract The structural and magnetic properties of R 2 Fe 172x Si x compounds have been investigated by means of X-ray diffraction and magnetization measurements. Substitution of Si for Fe leads to a clear reduction of the unit-cell volume. With increasing Si content, a phase transition from the Th 2 Ni 17 - to the Th 2 Zn 17 -type of structure has been observed for R5Tb, while all compounds crystallize in the Th 2 Ni 17 -type of structure for R5Er. A clear decrease of the saturation moment and an increase of the Curie temperature have been found upon Si substitution. The exchange–interaction constant JTbFe between the Tb and the Fe moments has been obtained from a mean-field analysis of the Curie temperature T c . The X-ray diffraction patterns of magnetically-aligned powder samples show that the easy magnetization direction is in the plane for all compounds investigated. 1999 Elsevier Science S.A. All rights reserved. Keywords: Tb 2 (Fe,Si) 17 ; Er 2 (Fe,Si) 17 ; Exchange interaction
1. Introduction Recently, much effort has been paid to the determination of the intersublattice magnetic interaction in the rare-earth (R)–transition metal (T) intermetallics. Quantitative information about the strength of this interaction can be obtained by various experimental techniques [1], such as ¨ neutron scattering, Mossbauer experiments and the highfield free-powder (HFFP) technique. A mean-field analysis of the Curie temperature T c is also an important method to obtain information on the R–T interaction. In the search for new permanent-magnet materials and in the study of the physical mechanisms of magnetic properties in these compounds, often other magnetic or non-magnetic elements are substituted for the R or T elements in R–T compounds [2–5]. Because substitution of Al, Ga and Si for Fe in R 2 Fe 17 compounds leads to a pronounced increase of T c and a change of the anisotropy, recently much attention has been paid to the investigation of the effects of substitution of these elements in R 2 Fe 17 compounds [6–8]. A study of the influence of the impurities Al, Ga and Si on the T c of Sm 2 Fe 17 , using the spin*Corresponding [email protected]
author.
Fax:
131-20-5255788;
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fluctuation approach of Mohn and Wohlfarth [9], shows that the calculated T c enhancement weighted with the impurity-site occupancy is in good agreement with experimental data [10]. A neutron-diffraction study on Tb 2 Fe 172x Ga x compounds has shown that, for high Ga concentration, the magnetic moments are oriented along the [001] direction in the whole temperature region below T c [11]. Room-temperature easy-axis anisotropy has been obtained in R 2 Fe 17 compounds by substituting a high Ga concentration for Fe [12]. A study of Dy 2 Fe 172x Si x compounds has been performed by the HFFP method [13]. Shen et al. [14] have reported the structural and magnetic data for the Tm 2 Fe 172x Six system. In the present paper, the effects of Si substitution for Fe in Tb 2 Fe 17 and Er 2 Fe 17 on the magnetic and structural properties are discussed. A mean-field analysis of T c has been performed with the purpose to derive the strength of rare-earth–transition-metal exchange interaction.
2. Experimental procedure Tb 2 Fe 172x Si x and Er 2 Fe 172x Si x (x50.0–3.5) intermetallic compounds were prepared by means of arc melting in high-purity Ar-gas atmosphere. The starting materials with
0925-8388 / 99 / $ – see front matter 1999 Elsevier Science S.A. All rights reserved. PII: S0925-8388( 98 )00946-3
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at least 99.9% purity were weighed in the stoichiometric 2:17 ratio. In order to compensate the loss during melting, a few percents (about 1–2%) of excess Tb and Er were added. The melting procedure was repeated at least three times in order to ensure the homogeneity. After this, the ingots were sealed in quartz tubes in Ar atmosphere, and annealed at 1173 K for 24 h, followed by quenching to room temperature. X-ray diffraction (XRD) was employed to examine the crystal structure of the compounds and to determine the easy magnetization direction at room temperature of the compounds investigated. The lattice parameters were derived from the XRD patterns. The field dependence of the magnetization at 5 K of samples consisting of powder particles with a size small enough to be considered as single crystals, that were free to orient themselves in the applied field, was measured in a SQUID magnetometer. The temperature dependence of the magnetization M(T ) was measured from room temperature up to 800 K by means of a vibrating sample magnetometer (VSM) in a field of 0.1 T. The Curie temperatures were determined by extrapolating the M 2 –T curves to M 2 50.
3. Results and discussion The XRD and thermomagnetic measurements indicate that all samples investigated are single phase except for a small amount of a-Fe which is present as an impurity phase. All Er 2 Fe 172x Si x compounds with x#3.5 crystallize in the hexagonal Th 2 Ni 17 -type of structure. For the Tb 2 Fe 172x Si x compounds, the situation is different. As an example, Fig. 1 shows the XRD patterns of Tb 2 Fe 172x Si x compounds with x50.0, 0.88, 1.47 and 2.5. It can be seen that Tb 2 Fe 17 consists of a mixture of the Th 2 Ni 17 - and the Th 2 Zn 17 -type of structure. The Tb 2 Fe 172x Si x compounds with 0,x,2.0 crystallize in Th 2 Ni 17 -type structure, while for 2.0#x#3.0 the compounds have the Th 2 Zn 17 -type of structure. So, Si substitution in Tb 2 Fe 17 leads to a change of structure from hexagonal Th 2 Ni 17 -type to rhombohedral Th 2 Zn 17 -type. This is similar to the behavior of Ga in Ho 2 Fe 172x Ga x [15]. In contrast, for Er 2 Fe 172x Si x with x50–3, the crystal structure is not influenced by Si substitution. Shen et al. [14] report that the replacement of Si for Fe in both Tm 2 Fe 17 and Y 2 Fe 17 does not change the crystal structure. Tm 2 Fe 172x Si x has the Th 2 Ni 17 -type of structure and, for x#3, Y 2 Fe 172x Si x consists of a mixture of the Th 2 Ni 17 - and the Th 2 Zn 17 -type of structure. It also has been reported [13] that Si substitution for Fe changes the structure of both Dy 2 Fe 17 and Y 2 Fe 17 . In the Dy 2 Fe 172x Si x system, a structural phase transition from the Th 2 Ni 17 - to the Th 2 Zn 17 -type of structure occurs at x53.4 and, in the Y 2 Fe 172x Si x system, at the same x value, the structure changes from Th 2 Ni 17 to CaCu 5 type. The contradictory reports on the Y 2 Fe 172x Si x system may result from the different melting and annealing conditions since the formation of a certain phase is sensitive to these
Fig. 1. X-ray-diffraction patterns of randomly oriented powder samples of Tb 2 Fe 172x Si x compounds.
conditions. In Gd 2 Fe 172x Si x compounds, at x52, a phase transition from the Th 2 Ni 17 -type to the CaCu 5 -type of structure has been reported by Yan et al. [16]. These findings indicate that for the late heavy-rare-earth elements, like Er and Tm, their Th 2 Ni 17 -type of structure is more stable and more difficult to be changed by Si substitution. The lattice parameters of the Er 2 Fe 172x Six and Tb 2 Fe 172x Si x compounds are shown in Tables 1 and 2, respectively. It can be seen that substitution of Si for Fe results in a clear decrease of the unit-cell volume. This is due to the smaller radius of Si compared with Fe and is similar to the observation in other systems [13–15]. The field dependence of the magnetization of the Er 2 Fe 172x Si x and Tb 2 Fe 172x Si x compounds is shown in Fig. 2a and b, respectively. The magnetization was measured at 5 K in decreasing field on powder particles which were free to orient themselves in the applied magnetic field. The spontaneous moments were derived by extrapolating the saturated part of the magnetization curves to B50. Fig. 3 shows the composition dependence of the spontaneous moment M(0) at 5 K for the R 2 Fe 172x Si x compounds. For both R5Tb and Er, M(0) decreases monotonously and approximately linearly with increasing Si content, with a rate of about 22.8 m B / Si-atom. This is similar to the results reported by Shen et al. [14], who report for R5Tm a rate of 22.9 m B / Si-atom. These rates of decrease are clearly faster than those expected on the basis of a simple magnetic dilution, which may result from
J.L. Wang et al. / Journal of Alloys and Compounds 284 (1999) 289 – 294
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Table 1 Structural and magnetic parameters of Er 2 Fe 172x Si x compounds 3
x
Structure type
˚ a (A)
˚ c (A)
˚ ) V (A
T c (K)
M(0) (m B / f.u.)
MT (m B / f.u.)
M(0) (Y 2 Fe 172x Si x ) (m B / f.u.) [14]
0 1 2 3 3.5
Th 2 Ni 17 Th 2 Ni 17 Th 2 Ni 17 Th 2 Ni 17 Th 2 Ni 17
8.451 8.438 8.424 8.403 8.411
8.303 8.429 8.290 8.269 8.260
513.3 511.9 509.9 505.7 506.1
301 410 461 472 473
17.2 12.8 11.4 8.6 7.2
35.2 30.8 29.4 26.6 25.2
34.0 30.1 27.6 24.2
the filling of the Fe 3d-subband by the valence electrons of Si like in the case of Al in R 2 Fe 172x Al x [17]. The magnetic moments of the heavy-R and Fe atoms in R–T intermetallics couple antiparallel, which yields ferrimagnetism for R 2 Fe 172x Si x with R5Tb and Er. By assuming the R moments in Er 2 Fe 172x Si x and Tb 2 Fe 172x Si x to be equal to the free-ion value of 9 m B , we obtain the moments of MT of the (Fe,Si) sublattice in these compounds as listed in Tables 1 and 2, respectively. In Tables 1 and 2, the MT values are compared with the MT values in the Y 2 Fe 172x Si x system reported by Shen et al. [14]. It can be seen that the results are similar. The slight differences may be explained in terms of additional d-band polarization by the R moments. The composition dependence of T c of R 2 Fe 172x Si x compounds with R5Tb and Er is shown in Fig. 4. In order to make a good comparison, the T c values for compounds containing Y are also drawn in the figure. It is seen that in all cases T c increases fast with increasing x until about x52, goes through a broad maximum and then decreases with further increasing Si content. There are two kinds of Fe–Fe exchange interactions in R 2 Fe 17 compounds [18]. For the nearest-neighbour Fe–Fe pairs in which the Fe–Fe distance is larger than a certain critical distance this interaction is positive, while it is negative for Fe–Fe pairs in which the distance is shorter. The behaviour of T c with increasing Si content can be considered to be due to the competition between the positive and the negative interactions and may be understood in the terms of preferential occupation of Si for some crystallographic sites at which ¨ the Fe–Fe interaction is negative. A Mossbauer study on Sm 2 Fe 172x Si x [18] and a neutron-diffraction study on Nd 2 Fe 13 Si 4 [19] have shown that the Si atoms indeed preferentially occupy a specific Fe site. Fig. 4 also indicates that in these systems, the values of T c of the
Er 2 Fe 172x Si x compounds are smaller than those of corresponding Tb-based compounds, which can be explained in terms of the contribution of the R–T exchange interaction. In R–T compounds, three types of exchange interactions exist, the R–T, the R–R and the T–T interaction. Although T c is primarily determined by the Fe–Fe exchange, the R–Fe exchange interaction may not be insignificant. A study of the 4f–3d exchange interaction in intermetallic compounds using the HFFP method [5] has shown that uJRT u decreases with increasing atomic number Z of the R ion. This can be explained to be the result of the distance increase between the 4f and 5d shells with increasing Z [20]. If we neglect the relatively weak interaction between the 4f moments, in a molecular-field description and use the experimental values 2SFe 5m Fe and the T c values of the Y compounds of corresponding composition [13], the exchange–interaction constants JTbFe have been evaluated like in Ref. [13]. The composition dependence of 2JTbFe /k is shown in Fig. 5. It can be seen that 2JTbFe /k does not change clearly with increasing Si content. Similar behavior has been observed when Fe was replaced by Ga in Ho 2 Fe 17 [14]. It has been reported [21] that in Tb 2 Fe 17 the exchange constant JTbFe /k between Tb and Fe is 27.0 K. This result was obtained by fitting the hard-axis magnetization curve measured on a single crystal. This value is only slightly lower than the value of 27.5 K that we obtain in the present mean-field analysis of T c . Because the difference between T c of Er 2 Fe 172x Si x and Y 2 Fe 172x Si x compounds is very small and even has the wrong sign for x#2, the exchange constant 2JErFe /k cannot be obtained in this way. The easy magnetization direction at room temperature has been derived from the XRD patterns of magnetically aligned powder samples. As an example, Fig. 6a, and c
Table 2 Structural and magnetic parameters of Tb 2 Fe 172x Si x compounds x
Structure type
˚ a (A)
˚ c (A)
˚ 3) V (A
T c (K)
M(0) (m B / f.u.)
MT (m B / f.u)
0.00
Th 2 Ni 17 (Th 2 Zn 17 ) Th 2 Ni 17 Th 2 Ni 17 Th 2 Ni 17 Th 2 Zn 17 Th 2 Zn 17 Th 2 Zn 17
8.486 (8.489) 8.426 8.425 8.404 (8.450) (8.446) (8.417)
8.272 (12.376) 8.298 8.288 8.264 (12.358) (12.344) (12.302)
516.0 (772.3) 510.2 509.5 505.5 (764.1) (762.6) (754.9)
403
17.4
35.4
450 485 493 512 520 523
16.1 14.4 12.7 11.4 9.9 8.9
34.1 32.4 30.7 29.1 27.9 26.9
0.59 1.17 1.47 2.0 2.5 3.0
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J.L. Wang et al. / Journal of Alloys and Compounds 284 (1999) 289 – 294
Fig. 3. Composition dependence of the spontaneous moment of R 2 Fe 172x Si x compounds with R5Er and Tb. The dashed lines are guides to the eye.
with the sample surface perpendicular to the rotation axis for the compounds Tb 2 Fe 17 and the Tb 2 Fe 14.5 Si 2.5 , respectively. It can be seen that the (004) peak has become dominant, which means that the cylinder axis is the hard magnetization direction. Since the Tb 2 Fe 17 compound is a mixture of the Th 2 Ni 17 - and Th 2 Zn 17 -type structures, the (006) peak of the Th 2 Zn 17 -type structure is also clearly observed in Fig. 6b. Fig. 7 shows the curves of M versus B and dM 2 / dt 2 versus B measured at room temperature on rotation-aligned Tb 2 Fe 17 and Er 2 Fe 16 Si samples with the external field applied parallel to the hard magnetization direction. It can be seen that the anisotropy fields Ba are clearly detectable.
Fig. 2. Field dependence of the magnetization at 5 K of free-powder samples of (a) Tb 2 Fe 172x Si x , and (b) Er 2 Fe 172x Si x compounds.
show the XRD patterns of magnetically-aligned powder samples for Tb 2 Fe 17 and Tb 2 Fe 14.5 Si 2.5 compounds, respectively. It can be seen that the (300) and (220) peaks become dominant and other peaks are absent after normally magnetic alignment, which indicates that these compounds have easy-plane anisotropy. In order to investigate the anisotropy fields of these compounds the rotationalignment technique has been used to get the aligned hard magnetization direction of these compounds. Fig. 6b and d show the XRD patterns of the rotation-aligned samples
Fig. 4. Composition dependence of the Curie temperature of R 2 Fe 172x Si x compounds with R5Y (data from [13]), Tb and Er.
J.L. Wang et al. / Journal of Alloys and Compounds 284 (1999) 289 – 294
Fig. 5. Concentration dependence of the exchange–interaction constant 2JTbFe /k in Tb 2 Fe 172x Si x compounds.
For Tb 2 Fe 17 and Er 2 Fe 16 Si, the values of Ba are 4.4 T and 1.0 T, respectively. From Fig. 7 it can be seen that in Tb 2 Fe 17 another peak is detected at 2.1 T. One explanation for this peak may be that it corresponds to another anisotropy field since Tb 2 Fe 17 is a mixture of the hexagonal and the rhombohedral structures which may have different anisotropy fields due to the slightly different crystallographic symmetry. This is observed in the case of uniaxial Gd 2 (Co 12x Fe x ) 17 with x,0.5 [22], whereas in
293
Fig. 7. Field dependence of magnetization and dM 2 / dt 2 of the compounds Tb 2 Fe 17 (top) and Er 2 Fe 16 Si (bottom) with the external field applied parallel to the hard magnetization direction.
other cases only one anisotropy field is detected if the two structures have nearly the same anisotropy fields [23,24]. Another possible explanation for the peak at 2.1 T may be that it corresponds to the critical field of a first-ordermagnetization process (FOMP). At 4.2 K, a transition has been reported at 3.9 T in the hard direction of Tb 2 Fe 17 [21]. This latter explanation seems more reasonable than the first one because the difference between 4.4 and 2.1 T seems rather large for two structures with only slightly different crystallographic symmetries.
4. Conclusions The effects of Si substitution for Fe in Tb 2 Fe 17 and Er 2 Fe 17 have been investigated. Si substitution for Fe leads to a decrease of the spontaneous moment and to an increase of T c . A clear reduction of the unit-cell volume and a structural transition in Tb 2 Fe 172x Si x from Th 2 Ni 17 to Th 2 Zn 17 -type at about x52 have been observed. A mean-field analysis of T c shows that the exchange–interaction constant JTbFe /k amounts to 27.5 K in Tb 2 Fe 17 . The anisotropy types at room temperature of all compounds investigated are all easy-plane.
Acknowledgements
Fig. 6. X-ray-diffraction patterns of samples aligned in the normal way (a) and (c) and rotation aligned samples (b) and (d) of the compounds Tb 2 Fe 17 and the Tb 2 Fe 14.5 Si 2.5 , respectively.
The present investigation has been carried out within the scientific exchange program between China and The Netherlands, and was supported by the National Natural Science Foundation of China.
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References [1] F.R. de Boer, Z.G. Zhao, K.H.J. Buschow, J. Magn. Magn. Mater. 157 / 158 (1996) 504. [2] V.K. Sinha, S.F. Cheng, W.E. Wallace, S.G. Sankar, J. Magn. Magn. Mater. 81 (1989) 227. [3] R. Verhoef, F.R. de Boer, T.H. Jacobs, K.H.J. Buschow, J. Appl. Phys. 67 (1990) 4774. [4] N. Plugaru, M. Valeanu, D.P. Lazar, D. Matulescu, J. Wang, N. Tang, F.M. Yang, J. Magn. Magn. Mater. 157 / 158 (1996) 647. ˆ [5] J.P. Liu, F.R. de Boer, P.F. de Chatel, R. Coehoorn, K.H.J. Buschow, J. Magn. Magn. Mater. 132 (1994) 159. [6] A.G. Kuchin, N.I. Kourov, Yu.V. Knyazev, N.M. Kleinerman, V.V. Serikov, G.V. Ivanova, A.S. Ermolenko, Phys. Stat. Sol. (a) 155 (1996) 479. [7] P.C.M. Gubbens, A.M. van der Kraan, T.H. Jacobs, K.H.J. Buschow, J. Less-Common Met. 159 (1990) 173. [8] S.R. Mishra, G.J. Long, Q.A. Pringle, D.P. Middleton, Z. Hu, W.B. Yelon, P. Grandjean, K.H.J. Buschow, J. Appl. Phys. 79 (1996) 3145. [9] P. Mohn, E.P. Wohlfarth, J. Phys. F, Met. Phys. 17 (1987) 2421. [10] R.F. Sabirianov, S.S. Jaswal, J. Appl. Phys. 79 (1996) 5942. [11] Z. Hu, W.B. Yelon, S. Mishra, J.G. Long, Q.A. Pringle, D.P. Middleton, K.H.J. Buschow, J. Appl. Phys. 76 (1994) 443.
[12] B.G. Shen, Z.H. Cheng, B. Liang, J.X. Zhang, F.W. Wang, S.Y. Zhang, J.G. Zhao, W.S. Zhan, Appl. Phys. Lett. 67 (1995) 1621. [13] D.M. Zhang, Y.H. Gao, B.M. Yu, C.Q. Tang, N. Tang, X.P. Zhong, W.G. Lin, F.M. Yang, F.R. de Boer, J. Appl. Phys. 76 (1994) 7452. [14] B.G. Shen, H.Y. Gong, B. Liang, Z.H. Cheng, J.X. Zhang, J. Alloys Compd. 229 (1995) 257. [15] F.M. Yang, J.L. Wang, Y.H. Gao, N. Tang, X.F. Han, H.G. Pan, Q.A. Li, J.F. Hu, J.P. Liu, F.R. de Boer, J. Appl. Phys. 79 (1996) 7883. [16] X.H. Yan, J.K. Liang, S.S. Xie, Phys. Stat. Sol. (a) 134 (1992) 77. [17] T.H. Jacobs, K.H.J. Buschow, G.F. Zhou, X. Li, F.R. de Boer, J. Magn. Magn. Mater. 116 (1992) 220. [18] Z.W. Li, X.Z. Zhou, A.H. Morrish, Phys. Rev. B. 51 (1995) 2891. [19] G.J. Long, G.K. Marasinghe, S. Mishra, Q.A. Pringle, F. Grandjean, K.H.J. Buschow, W.B. Yelon, F. Pourarian, O. Isnard, Solid State Commun. 88 (1993) 761. [20] E. Belorizky, M.F. Fremy, J.P. Gavigan, D. Givord, H.S. Li, J. Appl. Phys. 61 (1987) 3971. [21] R. Verhoef, PhD thesis, University of Amsterdam, 1990. ¨ [22] R. Grossinger, J. Phys. D, Appl. Phys. 15 (1983) 1545. [23] Y. Khan, Acta Crystallogr. B 29 (1973) 2502. ¨ [24] X.C. Kou, E.H.C.P. Sinnecker, R. Grossinger, J. Phys. Condens. Matter 8 (1996) 1557.
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Structural and magnetic properties of R 2 (Fe,Si) 17 compounds with R5Tb and Er ¨ b , F.M. Yang a J.L. Wang a ,b , F.R. de Boer b , *, X.F. Han a , N. Tang a , C. Zhang c , D. Zhang c , E. Bruck a
State Key Laboratory of Magnetism, Institute of Physics, Chinese Academy of Sciences, P.O. Box 603, Beijing, People’ s Republic of China b Van der Waals–Zeeman Institute, University of Amsterdam, Valckenierstraat 65, 1018 XE Amsterdam, The Netherlands c Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110015, People’ s Republic of China Received 27 August 1998; received in revised form 25 November 1998
Abstract The structural and magnetic properties of R 2 Fe 172x Si x compounds have been investigated by means of X-ray diffraction and magnetization measurements. Substitution of Si for Fe leads to a clear reduction of the unit-cell volume. With increasing Si content, a phase transition from the Th 2 Ni 17 - to the Th 2 Zn 17 -type of structure has been observed for R5Tb, while all compounds crystallize in the Th 2 Ni 17 -type of structure for R5Er. A clear decrease of the saturation moment and an increase of the Curie temperature have been found upon Si substitution. The exchange–interaction constant JTbFe between the Tb and the Fe moments has been obtained from a mean-field analysis of the Curie temperature T c . The X-ray diffraction patterns of magnetically-aligned powder samples show that the easy magnetization direction is in the plane for all compounds investigated. 1999 Elsevier Science S.A. All rights reserved. Keywords: Tb 2 (Fe,Si) 17 ; Er 2 (Fe,Si) 17 ; Exchange interaction
1. Introduction Recently, much effort has been paid to the determination of the intersublattice magnetic interaction in the rare-earth (R)–transition metal (T) intermetallics. Quantitative information about the strength of this interaction can be obtained by various experimental techniques [1], such as ¨ neutron scattering, Mossbauer experiments and the highfield free-powder (HFFP) technique. A mean-field analysis of the Curie temperature T c is also an important method to obtain information on the R–T interaction. In the search for new permanent-magnet materials and in the study of the physical mechanisms of magnetic properties in these compounds, often other magnetic or non-magnetic elements are substituted for the R or T elements in R–T compounds [2–5]. Because substitution of Al, Ga and Si for Fe in R 2 Fe 17 compounds leads to a pronounced increase of T c and a change of the anisotropy, recently much attention has been paid to the investigation of the effects of substitution of these elements in R 2 Fe 17 compounds [6–8]. A study of the influence of the impurities Al, Ga and Si on the T c of Sm 2 Fe 17 , using the spin*Corresponding [email protected]
author.
Fax:
131-20-5255788;
e-mail:
fluctuation approach of Mohn and Wohlfarth [9], shows that the calculated T c enhancement weighted with the impurity-site occupancy is in good agreement with experimental data [10]. A neutron-diffraction study on Tb 2 Fe 172x Ga x compounds has shown that, for high Ga concentration, the magnetic moments are oriented along the [001] direction in the whole temperature region below T c [11]. Room-temperature easy-axis anisotropy has been obtained in R 2 Fe 17 compounds by substituting a high Ga concentration for Fe [12]. A study of Dy 2 Fe 172x Si x compounds has been performed by the HFFP method [13]. Shen et al. [14] have reported the structural and magnetic data for the Tm 2 Fe 172x Six system. In the present paper, the effects of Si substitution for Fe in Tb 2 Fe 17 and Er 2 Fe 17 on the magnetic and structural properties are discussed. A mean-field analysis of T c has been performed with the purpose to derive the strength of rare-earth–transition-metal exchange interaction.
2. Experimental procedure Tb 2 Fe 172x Si x and Er 2 Fe 172x Si x (x50.0–3.5) intermetallic compounds were prepared by means of arc melting in high-purity Ar-gas atmosphere. The starting materials with
0925-8388 / 99 / $ – see front matter 1999 Elsevier Science S.A. All rights reserved. PII: S0925-8388( 98 )00946-3
290
J.L. Wang et al. / Journal of Alloys and Compounds 284 (1999) 289 – 294
at least 99.9% purity were weighed in the stoichiometric 2:17 ratio. In order to compensate the loss during melting, a few percents (about 1–2%) of excess Tb and Er were added. The melting procedure was repeated at least three times in order to ensure the homogeneity. After this, the ingots were sealed in quartz tubes in Ar atmosphere, and annealed at 1173 K for 24 h, followed by quenching to room temperature. X-ray diffraction (XRD) was employed to examine the crystal structure of the compounds and to determine the easy magnetization direction at room temperature of the compounds investigated. The lattice parameters were derived from the XRD patterns. The field dependence of the magnetization at 5 K of samples consisting of powder particles with a size small enough to be considered as single crystals, that were free to orient themselves in the applied field, was measured in a SQUID magnetometer. The temperature dependence of the magnetization M(T ) was measured from room temperature up to 800 K by means of a vibrating sample magnetometer (VSM) in a field of 0.1 T. The Curie temperatures were determined by extrapolating the M 2 –T curves to M 2 50.
3. Results and discussion The XRD and thermomagnetic measurements indicate that all samples investigated are single phase except for a small amount of a-Fe which is present as an impurity phase. All Er 2 Fe 172x Si x compounds with x#3.5 crystallize in the hexagonal Th 2 Ni 17 -type of structure. For the Tb 2 Fe 172x Si x compounds, the situation is different. As an example, Fig. 1 shows the XRD patterns of Tb 2 Fe 172x Si x compounds with x50.0, 0.88, 1.47 and 2.5. It can be seen that Tb 2 Fe 17 consists of a mixture of the Th 2 Ni 17 - and the Th 2 Zn 17 -type of structure. The Tb 2 Fe 172x Si x compounds with 0,x,2.0 crystallize in Th 2 Ni 17 -type structure, while for 2.0#x#3.0 the compounds have the Th 2 Zn 17 -type of structure. So, Si substitution in Tb 2 Fe 17 leads to a change of structure from hexagonal Th 2 Ni 17 -type to rhombohedral Th 2 Zn 17 -type. This is similar to the behavior of Ga in Ho 2 Fe 172x Ga x [15]. In contrast, for Er 2 Fe 172x Si x with x50–3, the crystal structure is not influenced by Si substitution. Shen et al. [14] report that the replacement of Si for Fe in both Tm 2 Fe 17 and Y 2 Fe 17 does not change the crystal structure. Tm 2 Fe 172x Si x has the Th 2 Ni 17 -type of structure and, for x#3, Y 2 Fe 172x Si x consists of a mixture of the Th 2 Ni 17 - and the Th 2 Zn 17 -type of structure. It also has been reported [13] that Si substitution for Fe changes the structure of both Dy 2 Fe 17 and Y 2 Fe 17 . In the Dy 2 Fe 172x Si x system, a structural phase transition from the Th 2 Ni 17 - to the Th 2 Zn 17 -type of structure occurs at x53.4 and, in the Y 2 Fe 172x Si x system, at the same x value, the structure changes from Th 2 Ni 17 to CaCu 5 type. The contradictory reports on the Y 2 Fe 172x Si x system may result from the different melting and annealing conditions since the formation of a certain phase is sensitive to these
Fig. 1. X-ray-diffraction patterns of randomly oriented powder samples of Tb 2 Fe 172x Si x compounds.
conditions. In Gd 2 Fe 172x Si x compounds, at x52, a phase transition from the Th 2 Ni 17 -type to the CaCu 5 -type of structure has been reported by Yan et al. [16]. These findings indicate that for the late heavy-rare-earth elements, like Er and Tm, their Th 2 Ni 17 -type of structure is more stable and more difficult to be changed by Si substitution. The lattice parameters of the Er 2 Fe 172x Six and Tb 2 Fe 172x Si x compounds are shown in Tables 1 and 2, respectively. It can be seen that substitution of Si for Fe results in a clear decrease of the unit-cell volume. This is due to the smaller radius of Si compared with Fe and is similar to the observation in other systems [13–15]. The field dependence of the magnetization of the Er 2 Fe 172x Si x and Tb 2 Fe 172x Si x compounds is shown in Fig. 2a and b, respectively. The magnetization was measured at 5 K in decreasing field on powder particles which were free to orient themselves in the applied magnetic field. The spontaneous moments were derived by extrapolating the saturated part of the magnetization curves to B50. Fig. 3 shows the composition dependence of the spontaneous moment M(0) at 5 K for the R 2 Fe 172x Si x compounds. For both R5Tb and Er, M(0) decreases monotonously and approximately linearly with increasing Si content, with a rate of about 22.8 m B / Si-atom. This is similar to the results reported by Shen et al. [14], who report for R5Tm a rate of 22.9 m B / Si-atom. These rates of decrease are clearly faster than those expected on the basis of a simple magnetic dilution, which may result from
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291
Table 1 Structural and magnetic parameters of Er 2 Fe 172x Si x compounds 3
x
Structure type
˚ a (A)
˚ c (A)
˚ ) V (A
T c (K)
M(0) (m B / f.u.)
MT (m B / f.u.)
M(0) (Y 2 Fe 172x Si x ) (m B / f.u.) [14]
0 1 2 3 3.5
Th 2 Ni 17 Th 2 Ni 17 Th 2 Ni 17 Th 2 Ni 17 Th 2 Ni 17
8.451 8.438 8.424 8.403 8.411
8.303 8.429 8.290 8.269 8.260
513.3 511.9 509.9 505.7 506.1
301 410 461 472 473
17.2 12.8 11.4 8.6 7.2
35.2 30.8 29.4 26.6 25.2
34.0 30.1 27.6 24.2
the filling of the Fe 3d-subband by the valence electrons of Si like in the case of Al in R 2 Fe 172x Al x [17]. The magnetic moments of the heavy-R and Fe atoms in R–T intermetallics couple antiparallel, which yields ferrimagnetism for R 2 Fe 172x Si x with R5Tb and Er. By assuming the R moments in Er 2 Fe 172x Si x and Tb 2 Fe 172x Si x to be equal to the free-ion value of 9 m B , we obtain the moments of MT of the (Fe,Si) sublattice in these compounds as listed in Tables 1 and 2, respectively. In Tables 1 and 2, the MT values are compared with the MT values in the Y 2 Fe 172x Si x system reported by Shen et al. [14]. It can be seen that the results are similar. The slight differences may be explained in terms of additional d-band polarization by the R moments. The composition dependence of T c of R 2 Fe 172x Si x compounds with R5Tb and Er is shown in Fig. 4. In order to make a good comparison, the T c values for compounds containing Y are also drawn in the figure. It is seen that in all cases T c increases fast with increasing x until about x52, goes through a broad maximum and then decreases with further increasing Si content. There are two kinds of Fe–Fe exchange interactions in R 2 Fe 17 compounds [18]. For the nearest-neighbour Fe–Fe pairs in which the Fe–Fe distance is larger than a certain critical distance this interaction is positive, while it is negative for Fe–Fe pairs in which the distance is shorter. The behaviour of T c with increasing Si content can be considered to be due to the competition between the positive and the negative interactions and may be understood in the terms of preferential occupation of Si for some crystallographic sites at which ¨ the Fe–Fe interaction is negative. A Mossbauer study on Sm 2 Fe 172x Si x [18] and a neutron-diffraction study on Nd 2 Fe 13 Si 4 [19] have shown that the Si atoms indeed preferentially occupy a specific Fe site. Fig. 4 also indicates that in these systems, the values of T c of the
Er 2 Fe 172x Si x compounds are smaller than those of corresponding Tb-based compounds, which can be explained in terms of the contribution of the R–T exchange interaction. In R–T compounds, three types of exchange interactions exist, the R–T, the R–R and the T–T interaction. Although T c is primarily determined by the Fe–Fe exchange, the R–Fe exchange interaction may not be insignificant. A study of the 4f–3d exchange interaction in intermetallic compounds using the HFFP method [5] has shown that uJRT u decreases with increasing atomic number Z of the R ion. This can be explained to be the result of the distance increase between the 4f and 5d shells with increasing Z [20]. If we neglect the relatively weak interaction between the 4f moments, in a molecular-field description and use the experimental values 2SFe 5m Fe and the T c values of the Y compounds of corresponding composition [13], the exchange–interaction constants JTbFe have been evaluated like in Ref. [13]. The composition dependence of 2JTbFe /k is shown in Fig. 5. It can be seen that 2JTbFe /k does not change clearly with increasing Si content. Similar behavior has been observed when Fe was replaced by Ga in Ho 2 Fe 17 [14]. It has been reported [21] that in Tb 2 Fe 17 the exchange constant JTbFe /k between Tb and Fe is 27.0 K. This result was obtained by fitting the hard-axis magnetization curve measured on a single crystal. This value is only slightly lower than the value of 27.5 K that we obtain in the present mean-field analysis of T c . Because the difference between T c of Er 2 Fe 172x Si x and Y 2 Fe 172x Si x compounds is very small and even has the wrong sign for x#2, the exchange constant 2JErFe /k cannot be obtained in this way. The easy magnetization direction at room temperature has been derived from the XRD patterns of magnetically aligned powder samples. As an example, Fig. 6a, and c
Table 2 Structural and magnetic parameters of Tb 2 Fe 172x Si x compounds x
Structure type
˚ a (A)
˚ c (A)
˚ 3) V (A
T c (K)
M(0) (m B / f.u.)
MT (m B / f.u)
0.00
Th 2 Ni 17 (Th 2 Zn 17 ) Th 2 Ni 17 Th 2 Ni 17 Th 2 Ni 17 Th 2 Zn 17 Th 2 Zn 17 Th 2 Zn 17
8.486 (8.489) 8.426 8.425 8.404 (8.450) (8.446) (8.417)
8.272 (12.376) 8.298 8.288 8.264 (12.358) (12.344) (12.302)
516.0 (772.3) 510.2 509.5 505.5 (764.1) (762.6) (754.9)
403
17.4
35.4
450 485 493 512 520 523
16.1 14.4 12.7 11.4 9.9 8.9
34.1 32.4 30.7 29.1 27.9 26.9
0.59 1.17 1.47 2.0 2.5 3.0
292
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Fig. 3. Composition dependence of the spontaneous moment of R 2 Fe 172x Si x compounds with R5Er and Tb. The dashed lines are guides to the eye.
with the sample surface perpendicular to the rotation axis for the compounds Tb 2 Fe 17 and the Tb 2 Fe 14.5 Si 2.5 , respectively. It can be seen that the (004) peak has become dominant, which means that the cylinder axis is the hard magnetization direction. Since the Tb 2 Fe 17 compound is a mixture of the Th 2 Ni 17 - and Th 2 Zn 17 -type structures, the (006) peak of the Th 2 Zn 17 -type structure is also clearly observed in Fig. 6b. Fig. 7 shows the curves of M versus B and dM 2 / dt 2 versus B measured at room temperature on rotation-aligned Tb 2 Fe 17 and Er 2 Fe 16 Si samples with the external field applied parallel to the hard magnetization direction. It can be seen that the anisotropy fields Ba are clearly detectable.
Fig. 2. Field dependence of the magnetization at 5 K of free-powder samples of (a) Tb 2 Fe 172x Si x , and (b) Er 2 Fe 172x Si x compounds.
show the XRD patterns of magnetically-aligned powder samples for Tb 2 Fe 17 and Tb 2 Fe 14.5 Si 2.5 compounds, respectively. It can be seen that the (300) and (220) peaks become dominant and other peaks are absent after normally magnetic alignment, which indicates that these compounds have easy-plane anisotropy. In order to investigate the anisotropy fields of these compounds the rotationalignment technique has been used to get the aligned hard magnetization direction of these compounds. Fig. 6b and d show the XRD patterns of the rotation-aligned samples
Fig. 4. Composition dependence of the Curie temperature of R 2 Fe 172x Si x compounds with R5Y (data from [13]), Tb and Er.
J.L. Wang et al. / Journal of Alloys and Compounds 284 (1999) 289 – 294
Fig. 5. Concentration dependence of the exchange–interaction constant 2JTbFe /k in Tb 2 Fe 172x Si x compounds.
For Tb 2 Fe 17 and Er 2 Fe 16 Si, the values of Ba are 4.4 T and 1.0 T, respectively. From Fig. 7 it can be seen that in Tb 2 Fe 17 another peak is detected at 2.1 T. One explanation for this peak may be that it corresponds to another anisotropy field since Tb 2 Fe 17 is a mixture of the hexagonal and the rhombohedral structures which may have different anisotropy fields due to the slightly different crystallographic symmetry. This is observed in the case of uniaxial Gd 2 (Co 12x Fe x ) 17 with x,0.5 [22], whereas in
293
Fig. 7. Field dependence of magnetization and dM 2 / dt 2 of the compounds Tb 2 Fe 17 (top) and Er 2 Fe 16 Si (bottom) with the external field applied parallel to the hard magnetization direction.
other cases only one anisotropy field is detected if the two structures have nearly the same anisotropy fields [23,24]. Another possible explanation for the peak at 2.1 T may be that it corresponds to the critical field of a first-ordermagnetization process (FOMP). At 4.2 K, a transition has been reported at 3.9 T in the hard direction of Tb 2 Fe 17 [21]. This latter explanation seems more reasonable than the first one because the difference between 4.4 and 2.1 T seems rather large for two structures with only slightly different crystallographic symmetries.
4. Conclusions The effects of Si substitution for Fe in Tb 2 Fe 17 and Er 2 Fe 17 have been investigated. Si substitution for Fe leads to a decrease of the spontaneous moment and to an increase of T c . A clear reduction of the unit-cell volume and a structural transition in Tb 2 Fe 172x Si x from Th 2 Ni 17 to Th 2 Zn 17 -type at about x52 have been observed. A mean-field analysis of T c shows that the exchange–interaction constant JTbFe /k amounts to 27.5 K in Tb 2 Fe 17 . The anisotropy types at room temperature of all compounds investigated are all easy-plane.
Acknowledgements
Fig. 6. X-ray-diffraction patterns of samples aligned in the normal way (a) and (c) and rotation aligned samples (b) and (d) of the compounds Tb 2 Fe 17 and the Tb 2 Fe 14.5 Si 2.5 , respectively.
The present investigation has been carried out within the scientific exchange program between China and The Netherlands, and was supported by the National Natural Science Foundation of China.
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