- Email: [email protected]
Structural and magnetic properties of Nd3(Fe1−xCox)27.7Ti1.3 (0<x≤0.4) alloys
Journal of Alloys and Compounds 325 (2001) 59–66
L
www.elsevier.com / locate / jallcom
Structural and magnetic properties of Nd 3 (Fe 12x Co x ) 27.7 Ti 1.3 (0,x#0.4) alloys a, a b b a O. Kalogirou *, C. Sarafidis , M. Gjoka , T. Bakas , M. Giannouri a
Third laboratory of Physics, Department of Physics, Aristotle University of Thessaloniki, 54006 Thessaloniki, Greece b Department of Physics, University of Ioannina, 45110 Ioannina, Greece Received 15 March 2001; accepted 3 April 2001
Abstract Structural and magnetic properties of a novel series of intermetallic compounds, with nominal stoichiometry Nd 3 (Fe 12x Co x ) 27.7 Ti 1.3 (0,x#0.4) are presented. The samples crystallise in the Nd 3 (Fe,Ti) 29 -type structure with monoclinic symmetry (space group A2 /m). The unit cell volume is decreasing as the Co content increases; the cell parameters show anisotropic decrease with the Co content. The Curie temperature increases monotonically with x from 437 to 878 K and the room temperature saturation magnetisation increases from 143.3 for x50 to 172.5 Am 2 / kg for x50.3 and remains practically the same for x50.4. For x50 and 0.1 a tilted magnetic structure is observed. For x$0.2 the compounds present an easy-magnetisation direction along the [4 0 22] direction. Ac susceptibility curves in the whole range of the Co content (x50–0.4) reveal a broad transition at about 160 K, whereas for x50–0.2 a sharp one with the corresponding transition temperature decreasing with increasing Co content. The observed changes of the critical temperatures observed in the ac susceptibility curves and the obtained anisotropy field values are related to the change of the magnetic anisotropy at x50.2. The average hyperfine field values depend on the Co content in a way similar to the dependence of the saturation magnetisation. 2001 Elsevier Science B.V. All rights reserved. ¨ spectroscopy Keywords: Rare earth compounds; Permanent magnet materials; Crystal structure; Magnetic measurements; Mossbauer
1. Introduction The new class of intermetallic compounds, R 3 (Fe,Ti) 29 (3:29) which have monoclinic symmetry has been extensively studied due to their potential for permanent magnet applications [1–15]. The Nd 3 (Fe,Ti) 29 -type compounds crystallise in the A2 /m space group [3] and are structurally related to both the 2:17 and 1:12 phase. Up to now, it is not known whether R 3 (Co,Ti) 29 compounds exist. However, recent attempts to partially substitute Co for Fe up to about 40% have been successful. Shah et al. [16] have been able to synthesise Pr 3 (Fe 12x Co x ) 27.5 Ti 1.5 (0# x#0.3) and studied their structural and magnetic properties [17,18]. Huo et al. have reported the synthesis of Cosubstituted 3:29 compounds for R5Gd [19] and they have found that the solid solution limit of Co in
*Corresponding author. Tel.: 130-31-99-8148; fax: 130-31-99-8003. E-mail address: [email protected] (O. Kalogirou).
Gd 3 (Fe 12x 2y Co y Ti x ) 29 increases with the Ti content [20]. It is interesting that Sun et al. [21] trying to prepare Pr 2 Co 172x Mn x compounds at high Mn concentration (x5 13) have obtained a 3:29 phase (Pr 3 Co 6 Mn 23 ), which is paramagnetic above 5 K. In a recent work, we have reported the synthesis of Nd 3 (Fe 12x Cox ) 27.5 Ti 1.5 (0# x# 0.4) [22], extending the occurrence of the 3:29 phase in the R–Fe–Co–T system for R5Nd and x50.4. We have shown that at room temperature the compounds with x$ 0.2 present an easy-magnetisation direction along the [4 0 22] direction. However, the samples studied in Ref. [22] were not single phase, but contained a relatively large amount of Nd(Fe,Co,Ti) 12 (about 20–25 wt%) and a small amount of a-Fe(Co), which limited a detailed study of their magnetic properties. In this work we report the synthesis and the study of the structural and magnetic properties of practically single-phase 3:29-type Nd 3 (Fe 12x Co x ) 27.7 Ti 1.3 (0,x#0.4) compounds. It is shown that the Curie temperature, the saturation magnetisation, the anisotropy field and the hyperfine parameters of the compounds are strongly affected by the Co content.
0925-8388 / 01 / $ – see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 01 )01379-2
O. Kalogirou et al. / Journal of Alloys and Compounds 325 (2001) 59 – 66
60
aligned powder samples with the alignment direction normal to the sample holder were taken in order to study ¨ the magnetic anisotropy of the compounds. 57 Fe Mossbauer spectra were obtained at 85 and 293 K on a conventional constant acceleration spectrometer with a 57 Co(Rh) source moving at room temperature while the absorber was at the desired temperature. Ac susceptibility measurements were done in the temperature range between 77 and 300 K using a home made ac susceptometer with a primary coil and two secondary coils wound in opposition, and a lock-in amplifier of Standford Applied Research. In the following data taken from Ref. [3] for Nd 3 Fe 27.5 Ti 1.5 (x50) are also given for comparison.
3. Results and discussion Fig. 1. Observed (crosses) and calculated (solid line) X-ray powder diffraction patterns (Cu Ka) of Nd 3 (Fe 12x Co x ) 27.7 Ti 1.3 .
2. Experimental The Nd 3 (Fe 12x Co x ) 27.7 Ti 1.3 (0,x#0.4) samples were prepared by arc melting in a high purity argon atmosphere. The raw materials of Nd, Fe, Co, Ti were at least 99.9% pure. The ingots were wrapped in tantalum foil, encapsulated in evacuated quartz tubes and annealed for 72 h at 1423 K, then quenched in water. X-ray diffraction data were obtained using Cu Ka radiation. The unit cell parameters and the phase compositions were determined by Rietveld refinement. The values of the Curie temperature were derived from dM / dH versus T curves based on thermomagnetic curves obtained by means of a Vibrating Sample Magnetometer (VSM) in a field of 0.1 T. Magnetisation curves of magnetically aligned powder samples were recorded by using a SQUID magnetometer at 5 and 300 K and a field up to 5 T. XRD patterns of magnetically
Fig. 1 shows the observed and calculated XRD patterns of all compounds. Detailed structural analyses by Rietveld refinement using the A2 /m space group showed that the alloys are practically single-phase 3:29 compounds with small traces of the rhombohedral Th 2 Ni 17 -type structure, the tetragonal ThMn 12 -type structure and a-Fe or Co. ¨ From the 57 Fe Mossbauer spectra, lower values for the a-Fe fraction than those form Rietveld analysis were found. It was concluded that this impurity was neither a-Fe nor Co but rather a Fe–Co solid solution. The lattice parameters, the unit cell volume and the phase composition of each sample in wt.% are summarised in Table 1. The lattice parameters and the unit cell volume of the 3:29 phases decrease as the Co content (parameter x) in the sample increases (Fig. 2). This behaviour has been observed in the Gd 3 (Fe 12x 2y Co y Ti x ) 29 series [20] and the related R 2 (Fe 12x Cox ) 17 compounds [23] and has been attributed to the fact that the Fe atoms are replaced by the smaller Co atoms. The lattice contraction is anisotropic, i.e., the lattice parameters a and b decrease faster than c with the Co content (Da /a50.58%, Db /b50.49%, Dc /c5
Table 1 Phase composition, lattice parameters and unit cell volume of Nd 3 (Fe 12x Co x ) 27.7 Ti 1.3 compounds x
Phases (wt.%)
a ˚ (A)
b ˚ (A)
c ˚ (A)
aˆ (8)
V ˚ 3) (A
R p (%)
0
3:29
10.638(1)
8.589(1)
9.746(1)
96.93(1)
883.98
4.10
0.1
3:29 96.8 1:12 0.7 Fe–Co 2.5
10.622(1)
8.587(1)
9.753(1)
96.76
883.32
5.02
0.2
3:29 91.6 2:17R 3.9 Fe–Co 4.6
10.609(1)
8.578(1)
9.747(1)
96.69(1)
881.04
4.36
0.3
3:29 93.1 2:17R 1.9 Fe–Co 5.0
10.594(2)
8.564(1)
9.737(1)
96.67(1)
877.34
5.74
0.4
3:29 94.2 1:12 0.8 2:17R 3.1 Fe–Co 1.9
10.576(1)
8.547(1)
9.721(1)
96.70
872.71
5.76
O. Kalogirou et al. / Journal of Alloys and Compounds 325 (2001) 59 – 66
61
Fig. 3. M(T ) curves of Nd 3 (Fe 12x Co x ) 27.7 Ti 1.3 .
Fig. 2. The dependence of the cell parameters of Nd 3 (Fe 12x Co x ) 27.7 Ti 1.3 on the Co content.
0.26%). Such a behaviour has been observed in Gd 3 (Fe 12x 2y Co y Ti x ) 29 and has been attributed to a possible preferential substitution of Co for Fe [20]. As a matter of fact, Harris et al. [18] have shown by means of neutron diffraction studies on Pr 3 (F 12x Co x ) 27.5 Ti 1.5 (x5 0.1 and 0.3) that the Co atoms occupy those Fe sites not shared with the Ti atoms. Magnetisation versus temperature curves are shown in Fig. 3. The Curie temperature increases monotonically with x, from 437 to 876 K (Fig. 4). Magnetisation measurements at 5 and 300 K were carried out on powder samples magnetically oriented in epoxy resin, at room temperature and at 2 T, parallel and perpendicular to the alignment direction at a field up to 5 T (Fig. 5). The saturation magnetisation for each compound was deduced from the law of approach to saturation (LAS) (Fig. 6a). The room temperature saturation magnetisation increases from 143.3 for x50–172.5 Am 2 / kg for x50.3 and remains practically the same for x50.4. The low temperature values reach a maximum for x50.2 and then decrease with Co content. By extrapolating the (Mi 2M' ) versus H plots the anisotropy fields were found (Fig. 6b). The anisotropy field values are lower for x50 and x50.1 than those for x$0.2. As it is discussed below this is may be related to
the change of the magnetic anisotropy which occurs at x50.2. In Table 2 the saturation magnetisation and the anisotropy field values of Nd 3 (Fe 12x Co x ) 27.7 Ti 1.3 are summarised. In order to study the effect of Co on the magnetic anisotropy in the Nd 3 (Fe 12x Co x ) 27.7 Ti 1.3 compounds XRD patterns of magnetically aligned powder samples with the alignment direction normal to the sample holder were carried out. In Fig. 7 the obtained spectra are compared to a randomly oriented powder sample of Nd(Fe,Ti) 29 (x50). It has been established that for the magnetic anisotropy of the 3:29 compounds the planes related to the (4 0 22), (0 4 0) and (2 0 4) reflections play an important role [3]. The [4 0 22] direction is related to the c-axis of the 1:12 phase, the [2 0 4] direction to the c-axis of the 2:17 phase and the [0 4 0], [4 0 22] directions to the basal plane of the 2:17 phase [3]. In the case of x50 the (4 0 22) reflection increases significantly, the (2 3 21) reflection increases, the (2 0 4) reflection vanishes and the (0 4 0) remains constant. The peak corresponding to the overlap of the strongest general reflections of the 3:29-type structure,
Fig. 4. Curie temperature values of the Nd 3 (Fe 12x Co x ) 27.7 Ti 1.3 compounds versus Co content.
62
O. Kalogirou et al. / Journal of Alloys and Compounds 325 (2001) 59 – 66
Fig. 5. Magnetisation curves at 5 and 300 K, parallel and perpendicular to the applied magnetic field, of Nd 3 (Fe 12x Co x ) 27.7 Ti 1.3 compounds.
i.e., (3 3 1), (4 2 22) remains almost unaffected. The exact nature of the magnetic anisotropy of the Nd 3 (Fe,Ti) 29 compound has not been clarified, so far. It has been interpreted as an easy-cone anisotropy [3,13], as planar anisotropy [24] and with an easy magnetisation direction along the [22 0 1] direction [25] or along the a-axis [26]. We believe that the XRD pattern of the aligned powder sample x50 (Fig. 7) supports the conclusion for the presence of a tilted magnetic structure. In the case of x50.1, an increase of the reflection corresponding to (4 0 22) and to the general reflection (3 1 1) is observed,
whereas the intensities of the reflections corresponding to the characteristic (0 4 0) reflection and the general reflections (2 3 21), (3 3 1) and (4 2 22), are lower compared to those of the x50 compound, implying a reorientation of the easy magnetisation direction. In other words, the x50.1 compound should present a tilted magnetic structure of a different kind than that of the x50 compound. It seems that the monoclinic 3:29 structure should present a complex magnetocrystalline anisotropy. Recently, Tang et al. have shown through symmetry analysis that if the anisotropy constants K1 , K2 , K3 are
O. Kalogirou et al. / Journal of Alloys and Compounds 325 (2001) 59 – 66
63
Fig. 7. X-ray diffraction patterns (Cu Ka) of random and magnetically aligned powder samples of Nd 3 (Fe 12x Co x ) 27.5 Ti 1.5 compounds.
Fig. 6. Saturation magnetisation (a) and anisotropy field (b) of Nd 3 (Fe 12x Co x ) 27.7 Ti 1.3 compounds versus Co content.
taken into account, there are eight preferential directions with a tilt angle of p / 4 with respect to the basal plane [27]. For x$0.2, the situation is rather different. The reflection corresponding to (4 0 22) is very strong and that corresponding to (8 0 24) is enhanced, the rest being very weak, suggesting that the compounds became uniaxial with the easy magnetisation direction along the [4 0 22] direction. Among the Nd 3 (Fe,Ti) 29 -type compounds only Sm(Fe,Ti) 29 presents this kind of anisotropy [28]. In Fig. 8 the ac magnetic susceptibility against temperature for all samples is plotted. It has been reported that in the ac magnetic susceptibility curves of Nd 3 (Fe,Ti) 29 (i.e., without Co, x50) one sharp transition is observed, at 230 K, and a broad shoulder at around 150 K [13,24]. As is shown in Fig. 3, for x50.1 and 0.2 a sharp peak is observed at 210 and 150 K, respectively. A broad shoulder at around 160 K can be detected for x50.1 and maybe for x50.2. In the latter the sharp one overlaps the existence of the broad peak. On the other hand, for the samples with
Fig. 8. AC-magnetic susceptibility Nd 3 (Fe 12x Co x ) 27.7 Ti 1.3 compounds.
against
temperature
of
x50.3 and 0.4 only a broad shoulder is observed at around 160 K. To summarise, it seems that in the whole range of the Co content (x50–0.4) a broad transition appears at a constant temperature, 150–160 K, whereas for x50–0.2 there is a sharp one, with the corresponding transition temperature decreasing with increasing Co content. The presence of the broad transition in Nd 3 (Fe,Ti) 29 has been interpreted as a first-order magnetization process (FOMP)
Table 2 Curie temperature, saturation magnetisation, anisotropy field and average hyperfine field values of Nd 3 (Fe 12x Co x ) 27.7 Ti 1.3 compounds x
0.0 0.1 0.2 0.3 0.4
TC (K)
MS (Am 2 / kg) T55 K
T5300 K
T55 K
T5300 K
T585 K
T5293 K
437 583 694 798 876
159.4 183.9 189.9 189.3 178.1
143.3 167.3 170.6 172.6 172.2
11.7 11.2 12.3 12.8 12.6
3.7 3.6 4.1 3.8 3.8
27.7 30.5 31.2 31.0 30.6
21.1 26.2 28.3 29.3 29.3
HA (T)
Heff (T)
64
O. Kalogirou et al. / Journal of Alloys and Compounds 325 (2001) 59 – 66
[13,24]. The sharp transition at 230 K has been attributed by Morellon et al. [24] to spin reorientation from planar to easy-cone anisotropy. Kalogirou et al. have shown by means of ac susceptibility measurements and 57 Fe ¨ Mossbauer spectroscopy on magnetically aligned powder samples of single-phase Nd 3 (Fe,Ti) 29 that this should also be attributed to first-order magnetisation process [13]. Considering the change of the magnetic anisotropy from a tilted magnetic structure for x50 and 0.1 to the presence of an easy-magnetisation direction along the [4 0 22] direction for x$0.2 the sharp peak observed in the x50.1 and 0.2 samples could be attributed to a spin reorientation
Fig. 9.
57
which is absent for x50.3 and 0.4. The broad peak observed in all samples could be attributed to domain wall motion. 57 ¨ Fe Mossbauer spectra were collected at 85 and 293 K for the compounds under study (Fig. 9). The fitting procedure was made in a way similar to that described in Ref. [10] for Pr 3 (Fe,Ti) 29 . In that model the 11 crystallographically non-equivalent iron sites of Nd 3 (Fe,Ti) 29 have been divided into four groups according to the number of nearest neighbours and the bond length values. Thus, for the compounds under study the fit was done with four components; a fifth component was introduced for
¨ Fe Mossbauer spectra at RT and 85 K of Nd 3 (Fe 12x Co x ) 27.7 Ti 1.3 compounds.
O. Kalogirou et al. / Journal of Alloys and Compounds 325 (2001) 59 – 66
65
the x50.1 and 0.2 compounds may be indicative of spin reorientation phenomena. The dependence of the corresponding critical temperatures and of the anisotropy field values on Co content should be related to the change of the nature of the magnetic anisotropy at x50.2.
Acknowledgements This work was partially supported by the European Social Fund and the General Secretariat of Research and Technology, Greece (project 99ED 535).
Fig. 10. Average hyperfine field values of Nd 3 (Fe 12x Co x ) 27.7 Ti 1.3 compounds versus Co content.
a-Fe. The area ratio was constrained in order to correspond to the populations of the selected iron site groups taking into account the Ti and Co occupancy of the different sites. In the case of Ti atoms this approach is described in Ref. [10]. In the case of Co atoms the neutron diffraction results on Pr 3 (F 12x Co x ) 27.5 Ti 1.5 (x50.1 and 0.3) of Harris et al. [18] that the Co atoms occupy those Fe sites not shared with the Ti atoms were used. A small amount of line broadening was allowed for each component to simulate the distribution of the environments within each com¨ ponent. A detailed study of the Mossbauer spectra in order to assign the subspectra according to the selected iron atom groups taking into account the Co and Ti occupancy and in order to determine preferential sites for the Co atoms is in progress and will be published soon [29]. As it is shown in Fig. 10, the values of the average 57 Fe hyperfine field yielded by the fit increase with the Co content up to x50.3 and then decreases for x50.4 in accordance to the behaviour of the corresponding saturation magnetisation values (Fig. 6a). The values for x50 were taken from Ref. [13].
4. Conclusions The synthesis of Nd–(Fe,Co)–Ti intermetallic compounds with stoichiometry of Nd 3 (Fe 12x Co x ) 27.7 Ti 1.3 (x5 0.1, 0.2, 0.3, 0.4) and the Nd 3 (Fe,Ti) 29 -type structure with monoclinic symmetry (A2 /m) is reported. The unit cell parameters decrease anisotropically with the Co content present in the samples. The Curie temperature increases monotonically with x. For x$0.2 and at room temperature the compounds present an easy-magnetisation direction along the [4 0 22] direction. The maximum values of saturation magnetisation, anisotropy field and average hyperfine are obtained for x50.2 and x50.3. The low temperature magnetic transitions observed in the case of
References [1] Ye.V. Schrebakova, G.V. Ivanova, A.S. Yermolenko, Ye.V. Belozerov, V.S. Gaviko, J. Alloys Comp. 182 (1992) 199. [2] H.S. Li, J.M. Cadogan, R.L. Davis, A. Margarian, J.B. Dunlop, Solid State Commun. 90 (1994) 487. [3] O. Kalogirou, V. Psycharis, L. Saettas, D. Niarchos, J. Magn. Magn. Mater. 146 (1995) 335. [4] H.S. Li, D. Courtois, J.M. Cadogan, J.M. Xu, S.X. Dou, J. Phys.: Condens. Matter 6 (1994) L771. [5] C.D. Fuerst, F.E. Pinkerton, J.F. Herbst, J. Appl. Phys. 76 (1994) 6144. [6] C.D. Fuerst, F.E. Pinkerton, J.F. Herbst, J. Magn. Magn. Mater. 129 (1994) L115. [7] F.M. Yang, B. Nasunjilegal, H.Y. Pan, J.L. Wang, R.W. Zhao, B.P. Hu, Y.Z. Wang, H.S. Li, J.M. Cadogan, J. Magn. Magn. Mater. 135 (1994) 298. [8] O. Kalogirou, V. Psycharis, M. Gjoka, D. Niarchos, J. Magn. Magn. Mater. 147 (1995) L7. [9] D. Courtois, H.S. Li, J.M. Cadogan, IEEE Trans. Magn. 31 (1995) 3677. [10] V. Psycharis, O. Kalogirou, E. Devlin, M. Gjoka, A. Simopoulos, D. Niarchos, J. Magn. Magn. Mater. 153 (1996) 75. [11] Z. Hu, W.B. Yelon, O. Kalogirou, V. Psycharis, J. Appl. Phys. 80 (1996) 2955. [12] A. Margarian, H.S. Li, J.B. Dunlop, J.M. Cadogan, J. Alloys Comp. 239 (1996) 27. [13] O. Kalogirou, V. Psycharis, D. Niarchos, Solid State Commun. 97 (1996) 471. ` Yang, C.P. Chen, N. Tang, X.F. Han, J.L. Wang, J.F. [14] H.G. Pan, F.I. Hu, K.W. Zhou, R.W. Zhao, Q. D Wang, Solid State Commun. 98 (1996) 259. [15] X.F. Han, F.M. Yang, J.J. Zhu, H.G. Pan, Y.G. Wang, J.L. Wang, N. Tang, R.W. Zhao, J. Appl. Phys. 81 (1997) 3248. [16] V.R. Shah, G. Markandeyulu, K.V.S. Rama Rao, M.Q. Huang, K. Sirisha, M.E. McHenry, J. Magn. Magn. Mater. 190 (1998) 233. [17] V.R. Shah, G. Markandeyulu, K.V.S. Rama Rao, M.Q. Huang, K. Sirisha, M.E. McHenry, J. Appl. Phys. 85 (1999) 4678. [18] V.G. Harris, V.R. Shah, G. Markandeyulu, K.V.S. Rama Rao, M.Q. Huang, K. Sirisha, M.E. McHenry, IEEE Trans. Magn. 35 (1999) 3286. [19] G. Huo, Z. Qiao, G. Rao, J. Liang, B. Shen, J. Alloys Comp. 285 (1999) 216. [20] G.Y. Huo, G.H. Rao, Z.Y. Qiao, Q.L. Liu, X.L. Chen, J.K. Liang, B.G. Shen, J. Phys.: Condens. Matter 12 (2000) 1161. [21] Z.-G. Sun, H.-W. Zhang, J.-Y. Wang, B.-G. Shen, Appl. Phys. Lett. 75 (1999) 3850. [22] D.K. Gioureli, O. Kalogirou, A.C. Stergiou, G. Makaronides, G. Litsardakis, S. Katemliades, M. Gjoka, in: Proceedings of the 11th
66
O. Kalogirou et al. / Journal of Alloys and Compounds 325 (2001) 59 – 66
Intern. Symp. on Magnetic Anisotropy and Coercivity in Rare-Earth Transition Metal Alloys, 2000, p. S15. [23] M. Katter, J. Wecker, C. Kuhrt, L. Schultz, J. Magn. Magn. Mater. 114 (1992) 35. [24] L. Morellon, L. Pareti, P.A. Algarabel, F. Albertini, M.R. Ibarra, J. Phys.: Condens. Matter 6 (1994) L379. [25] D. Courtois, H.-S. Li, J.M. Cadogan, Solid State Commun. 98 (1996) 565.
[26] Z. Hu, W.B. Yelon, Solid State Commun. 91 (1994) 223. [27] N. Tang, X.C. Kou, F.R. de Boer, K.H.J. Buschow, J.L. Wang, F. Yang, J. Phys.: Condens. Matter 11 (1999) 5313. [28] F.M. Yang, B. Nasunjilegal, W. Gong, G.C. Hadjipanayis, IEEE Trans. Magn. 30 (1994) 4957. [29] O. Kalogirou, C. Sarafidis, T. Bakas, M. Gjoka, to be presented at ¨ the International Conference on the Applications of the Mossbauer Effect, Oxford, 2–7 September 2001.
L
www.elsevier.com / locate / jallcom
Structural and magnetic properties of Nd 3 (Fe 12x Co x ) 27.7 Ti 1.3 (0,x#0.4) alloys a, a b b a O. Kalogirou *, C. Sarafidis , M. Gjoka , T. Bakas , M. Giannouri a
Third laboratory of Physics, Department of Physics, Aristotle University of Thessaloniki, 54006 Thessaloniki, Greece b Department of Physics, University of Ioannina, 45110 Ioannina, Greece Received 15 March 2001; accepted 3 April 2001
Abstract Structural and magnetic properties of a novel series of intermetallic compounds, with nominal stoichiometry Nd 3 (Fe 12x Co x ) 27.7 Ti 1.3 (0,x#0.4) are presented. The samples crystallise in the Nd 3 (Fe,Ti) 29 -type structure with monoclinic symmetry (space group A2 /m). The unit cell volume is decreasing as the Co content increases; the cell parameters show anisotropic decrease with the Co content. The Curie temperature increases monotonically with x from 437 to 878 K and the room temperature saturation magnetisation increases from 143.3 for x50 to 172.5 Am 2 / kg for x50.3 and remains practically the same for x50.4. For x50 and 0.1 a tilted magnetic structure is observed. For x$0.2 the compounds present an easy-magnetisation direction along the [4 0 22] direction. Ac susceptibility curves in the whole range of the Co content (x50–0.4) reveal a broad transition at about 160 K, whereas for x50–0.2 a sharp one with the corresponding transition temperature decreasing with increasing Co content. The observed changes of the critical temperatures observed in the ac susceptibility curves and the obtained anisotropy field values are related to the change of the magnetic anisotropy at x50.2. The average hyperfine field values depend on the Co content in a way similar to the dependence of the saturation magnetisation. 2001 Elsevier Science B.V. All rights reserved. ¨ spectroscopy Keywords: Rare earth compounds; Permanent magnet materials; Crystal structure; Magnetic measurements; Mossbauer
1. Introduction The new class of intermetallic compounds, R 3 (Fe,Ti) 29 (3:29) which have monoclinic symmetry has been extensively studied due to their potential for permanent magnet applications [1–15]. The Nd 3 (Fe,Ti) 29 -type compounds crystallise in the A2 /m space group [3] and are structurally related to both the 2:17 and 1:12 phase. Up to now, it is not known whether R 3 (Co,Ti) 29 compounds exist. However, recent attempts to partially substitute Co for Fe up to about 40% have been successful. Shah et al. [16] have been able to synthesise Pr 3 (Fe 12x Co x ) 27.5 Ti 1.5 (0# x#0.3) and studied their structural and magnetic properties [17,18]. Huo et al. have reported the synthesis of Cosubstituted 3:29 compounds for R5Gd [19] and they have found that the solid solution limit of Co in
*Corresponding author. Tel.: 130-31-99-8148; fax: 130-31-99-8003. E-mail address: [email protected] (O. Kalogirou).
Gd 3 (Fe 12x 2y Co y Ti x ) 29 increases with the Ti content [20]. It is interesting that Sun et al. [21] trying to prepare Pr 2 Co 172x Mn x compounds at high Mn concentration (x5 13) have obtained a 3:29 phase (Pr 3 Co 6 Mn 23 ), which is paramagnetic above 5 K. In a recent work, we have reported the synthesis of Nd 3 (Fe 12x Cox ) 27.5 Ti 1.5 (0# x# 0.4) [22], extending the occurrence of the 3:29 phase in the R–Fe–Co–T system for R5Nd and x50.4. We have shown that at room temperature the compounds with x$ 0.2 present an easy-magnetisation direction along the [4 0 22] direction. However, the samples studied in Ref. [22] were not single phase, but contained a relatively large amount of Nd(Fe,Co,Ti) 12 (about 20–25 wt%) and a small amount of a-Fe(Co), which limited a detailed study of their magnetic properties. In this work we report the synthesis and the study of the structural and magnetic properties of practically single-phase 3:29-type Nd 3 (Fe 12x Co x ) 27.7 Ti 1.3 (0,x#0.4) compounds. It is shown that the Curie temperature, the saturation magnetisation, the anisotropy field and the hyperfine parameters of the compounds are strongly affected by the Co content.
0925-8388 / 01 / $ – see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 01 )01379-2
O. Kalogirou et al. / Journal of Alloys and Compounds 325 (2001) 59 – 66
60
aligned powder samples with the alignment direction normal to the sample holder were taken in order to study ¨ the magnetic anisotropy of the compounds. 57 Fe Mossbauer spectra were obtained at 85 and 293 K on a conventional constant acceleration spectrometer with a 57 Co(Rh) source moving at room temperature while the absorber was at the desired temperature. Ac susceptibility measurements were done in the temperature range between 77 and 300 K using a home made ac susceptometer with a primary coil and two secondary coils wound in opposition, and a lock-in amplifier of Standford Applied Research. In the following data taken from Ref. [3] for Nd 3 Fe 27.5 Ti 1.5 (x50) are also given for comparison.
3. Results and discussion Fig. 1. Observed (crosses) and calculated (solid line) X-ray powder diffraction patterns (Cu Ka) of Nd 3 (Fe 12x Co x ) 27.7 Ti 1.3 .
2. Experimental The Nd 3 (Fe 12x Co x ) 27.7 Ti 1.3 (0,x#0.4) samples were prepared by arc melting in a high purity argon atmosphere. The raw materials of Nd, Fe, Co, Ti were at least 99.9% pure. The ingots were wrapped in tantalum foil, encapsulated in evacuated quartz tubes and annealed for 72 h at 1423 K, then quenched in water. X-ray diffraction data were obtained using Cu Ka radiation. The unit cell parameters and the phase compositions were determined by Rietveld refinement. The values of the Curie temperature were derived from dM / dH versus T curves based on thermomagnetic curves obtained by means of a Vibrating Sample Magnetometer (VSM) in a field of 0.1 T. Magnetisation curves of magnetically aligned powder samples were recorded by using a SQUID magnetometer at 5 and 300 K and a field up to 5 T. XRD patterns of magnetically
Fig. 1 shows the observed and calculated XRD patterns of all compounds. Detailed structural analyses by Rietveld refinement using the A2 /m space group showed that the alloys are practically single-phase 3:29 compounds with small traces of the rhombohedral Th 2 Ni 17 -type structure, the tetragonal ThMn 12 -type structure and a-Fe or Co. ¨ From the 57 Fe Mossbauer spectra, lower values for the a-Fe fraction than those form Rietveld analysis were found. It was concluded that this impurity was neither a-Fe nor Co but rather a Fe–Co solid solution. The lattice parameters, the unit cell volume and the phase composition of each sample in wt.% are summarised in Table 1. The lattice parameters and the unit cell volume of the 3:29 phases decrease as the Co content (parameter x) in the sample increases (Fig. 2). This behaviour has been observed in the Gd 3 (Fe 12x 2y Co y Ti x ) 29 series [20] and the related R 2 (Fe 12x Cox ) 17 compounds [23] and has been attributed to the fact that the Fe atoms are replaced by the smaller Co atoms. The lattice contraction is anisotropic, i.e., the lattice parameters a and b decrease faster than c with the Co content (Da /a50.58%, Db /b50.49%, Dc /c5
Table 1 Phase composition, lattice parameters and unit cell volume of Nd 3 (Fe 12x Co x ) 27.7 Ti 1.3 compounds x
Phases (wt.%)
a ˚ (A)
b ˚ (A)
c ˚ (A)
aˆ (8)
V ˚ 3) (A
R p (%)
0
3:29
10.638(1)
8.589(1)
9.746(1)
96.93(1)
883.98
4.10
0.1
3:29 96.8 1:12 0.7 Fe–Co 2.5
10.622(1)
8.587(1)
9.753(1)
96.76
883.32
5.02
0.2
3:29 91.6 2:17R 3.9 Fe–Co 4.6
10.609(1)
8.578(1)
9.747(1)
96.69(1)
881.04
4.36
0.3
3:29 93.1 2:17R 1.9 Fe–Co 5.0
10.594(2)
8.564(1)
9.737(1)
96.67(1)
877.34
5.74
0.4
3:29 94.2 1:12 0.8 2:17R 3.1 Fe–Co 1.9
10.576(1)
8.547(1)
9.721(1)
96.70
872.71
5.76
O. Kalogirou et al. / Journal of Alloys and Compounds 325 (2001) 59 – 66
61
Fig. 3. M(T ) curves of Nd 3 (Fe 12x Co x ) 27.7 Ti 1.3 .
Fig. 2. The dependence of the cell parameters of Nd 3 (Fe 12x Co x ) 27.7 Ti 1.3 on the Co content.
0.26%). Such a behaviour has been observed in Gd 3 (Fe 12x 2y Co y Ti x ) 29 and has been attributed to a possible preferential substitution of Co for Fe [20]. As a matter of fact, Harris et al. [18] have shown by means of neutron diffraction studies on Pr 3 (F 12x Co x ) 27.5 Ti 1.5 (x5 0.1 and 0.3) that the Co atoms occupy those Fe sites not shared with the Ti atoms. Magnetisation versus temperature curves are shown in Fig. 3. The Curie temperature increases monotonically with x, from 437 to 876 K (Fig. 4). Magnetisation measurements at 5 and 300 K were carried out on powder samples magnetically oriented in epoxy resin, at room temperature and at 2 T, parallel and perpendicular to the alignment direction at a field up to 5 T (Fig. 5). The saturation magnetisation for each compound was deduced from the law of approach to saturation (LAS) (Fig. 6a). The room temperature saturation magnetisation increases from 143.3 for x50–172.5 Am 2 / kg for x50.3 and remains practically the same for x50.4. The low temperature values reach a maximum for x50.2 and then decrease with Co content. By extrapolating the (Mi 2M' ) versus H plots the anisotropy fields were found (Fig. 6b). The anisotropy field values are lower for x50 and x50.1 than those for x$0.2. As it is discussed below this is may be related to
the change of the magnetic anisotropy which occurs at x50.2. In Table 2 the saturation magnetisation and the anisotropy field values of Nd 3 (Fe 12x Co x ) 27.7 Ti 1.3 are summarised. In order to study the effect of Co on the magnetic anisotropy in the Nd 3 (Fe 12x Co x ) 27.7 Ti 1.3 compounds XRD patterns of magnetically aligned powder samples with the alignment direction normal to the sample holder were carried out. In Fig. 7 the obtained spectra are compared to a randomly oriented powder sample of Nd(Fe,Ti) 29 (x50). It has been established that for the magnetic anisotropy of the 3:29 compounds the planes related to the (4 0 22), (0 4 0) and (2 0 4) reflections play an important role [3]. The [4 0 22] direction is related to the c-axis of the 1:12 phase, the [2 0 4] direction to the c-axis of the 2:17 phase and the [0 4 0], [4 0 22] directions to the basal plane of the 2:17 phase [3]. In the case of x50 the (4 0 22) reflection increases significantly, the (2 3 21) reflection increases, the (2 0 4) reflection vanishes and the (0 4 0) remains constant. The peak corresponding to the overlap of the strongest general reflections of the 3:29-type structure,
Fig. 4. Curie temperature values of the Nd 3 (Fe 12x Co x ) 27.7 Ti 1.3 compounds versus Co content.
62
O. Kalogirou et al. / Journal of Alloys and Compounds 325 (2001) 59 – 66
Fig. 5. Magnetisation curves at 5 and 300 K, parallel and perpendicular to the applied magnetic field, of Nd 3 (Fe 12x Co x ) 27.7 Ti 1.3 compounds.
i.e., (3 3 1), (4 2 22) remains almost unaffected. The exact nature of the magnetic anisotropy of the Nd 3 (Fe,Ti) 29 compound has not been clarified, so far. It has been interpreted as an easy-cone anisotropy [3,13], as planar anisotropy [24] and with an easy magnetisation direction along the [22 0 1] direction [25] or along the a-axis [26]. We believe that the XRD pattern of the aligned powder sample x50 (Fig. 7) supports the conclusion for the presence of a tilted magnetic structure. In the case of x50.1, an increase of the reflection corresponding to (4 0 22) and to the general reflection (3 1 1) is observed,
whereas the intensities of the reflections corresponding to the characteristic (0 4 0) reflection and the general reflections (2 3 21), (3 3 1) and (4 2 22), are lower compared to those of the x50 compound, implying a reorientation of the easy magnetisation direction. In other words, the x50.1 compound should present a tilted magnetic structure of a different kind than that of the x50 compound. It seems that the monoclinic 3:29 structure should present a complex magnetocrystalline anisotropy. Recently, Tang et al. have shown through symmetry analysis that if the anisotropy constants K1 , K2 , K3 are
O. Kalogirou et al. / Journal of Alloys and Compounds 325 (2001) 59 – 66
63
Fig. 7. X-ray diffraction patterns (Cu Ka) of random and magnetically aligned powder samples of Nd 3 (Fe 12x Co x ) 27.5 Ti 1.5 compounds.
Fig. 6. Saturation magnetisation (a) and anisotropy field (b) of Nd 3 (Fe 12x Co x ) 27.7 Ti 1.3 compounds versus Co content.
taken into account, there are eight preferential directions with a tilt angle of p / 4 with respect to the basal plane [27]. For x$0.2, the situation is rather different. The reflection corresponding to (4 0 22) is very strong and that corresponding to (8 0 24) is enhanced, the rest being very weak, suggesting that the compounds became uniaxial with the easy magnetisation direction along the [4 0 22] direction. Among the Nd 3 (Fe,Ti) 29 -type compounds only Sm(Fe,Ti) 29 presents this kind of anisotropy [28]. In Fig. 8 the ac magnetic susceptibility against temperature for all samples is plotted. It has been reported that in the ac magnetic susceptibility curves of Nd 3 (Fe,Ti) 29 (i.e., without Co, x50) one sharp transition is observed, at 230 K, and a broad shoulder at around 150 K [13,24]. As is shown in Fig. 3, for x50.1 and 0.2 a sharp peak is observed at 210 and 150 K, respectively. A broad shoulder at around 160 K can be detected for x50.1 and maybe for x50.2. In the latter the sharp one overlaps the existence of the broad peak. On the other hand, for the samples with
Fig. 8. AC-magnetic susceptibility Nd 3 (Fe 12x Co x ) 27.7 Ti 1.3 compounds.
against
temperature
of
x50.3 and 0.4 only a broad shoulder is observed at around 160 K. To summarise, it seems that in the whole range of the Co content (x50–0.4) a broad transition appears at a constant temperature, 150–160 K, whereas for x50–0.2 there is a sharp one, with the corresponding transition temperature decreasing with increasing Co content. The presence of the broad transition in Nd 3 (Fe,Ti) 29 has been interpreted as a first-order magnetization process (FOMP)
Table 2 Curie temperature, saturation magnetisation, anisotropy field and average hyperfine field values of Nd 3 (Fe 12x Co x ) 27.7 Ti 1.3 compounds x
0.0 0.1 0.2 0.3 0.4
TC (K)
MS (Am 2 / kg) T55 K
T5300 K
T55 K
T5300 K
T585 K
T5293 K
437 583 694 798 876
159.4 183.9 189.9 189.3 178.1
143.3 167.3 170.6 172.6 172.2
11.7 11.2 12.3 12.8 12.6
3.7 3.6 4.1 3.8 3.8
27.7 30.5 31.2 31.0 30.6
21.1 26.2 28.3 29.3 29.3
HA (T)
Heff (T)
64
O. Kalogirou et al. / Journal of Alloys and Compounds 325 (2001) 59 – 66
[13,24]. The sharp transition at 230 K has been attributed by Morellon et al. [24] to spin reorientation from planar to easy-cone anisotropy. Kalogirou et al. have shown by means of ac susceptibility measurements and 57 Fe ¨ Mossbauer spectroscopy on magnetically aligned powder samples of single-phase Nd 3 (Fe,Ti) 29 that this should also be attributed to first-order magnetisation process [13]. Considering the change of the magnetic anisotropy from a tilted magnetic structure for x50 and 0.1 to the presence of an easy-magnetisation direction along the [4 0 22] direction for x$0.2 the sharp peak observed in the x50.1 and 0.2 samples could be attributed to a spin reorientation
Fig. 9.
57
which is absent for x50.3 and 0.4. The broad peak observed in all samples could be attributed to domain wall motion. 57 ¨ Fe Mossbauer spectra were collected at 85 and 293 K for the compounds under study (Fig. 9). The fitting procedure was made in a way similar to that described in Ref. [10] for Pr 3 (Fe,Ti) 29 . In that model the 11 crystallographically non-equivalent iron sites of Nd 3 (Fe,Ti) 29 have been divided into four groups according to the number of nearest neighbours and the bond length values. Thus, for the compounds under study the fit was done with four components; a fifth component was introduced for
¨ Fe Mossbauer spectra at RT and 85 K of Nd 3 (Fe 12x Co x ) 27.7 Ti 1.3 compounds.
O. Kalogirou et al. / Journal of Alloys and Compounds 325 (2001) 59 – 66
65
the x50.1 and 0.2 compounds may be indicative of spin reorientation phenomena. The dependence of the corresponding critical temperatures and of the anisotropy field values on Co content should be related to the change of the nature of the magnetic anisotropy at x50.2.
Acknowledgements This work was partially supported by the European Social Fund and the General Secretariat of Research and Technology, Greece (project 99ED 535).
Fig. 10. Average hyperfine field values of Nd 3 (Fe 12x Co x ) 27.7 Ti 1.3 compounds versus Co content.
a-Fe. The area ratio was constrained in order to correspond to the populations of the selected iron site groups taking into account the Ti and Co occupancy of the different sites. In the case of Ti atoms this approach is described in Ref. [10]. In the case of Co atoms the neutron diffraction results on Pr 3 (F 12x Co x ) 27.5 Ti 1.5 (x50.1 and 0.3) of Harris et al. [18] that the Co atoms occupy those Fe sites not shared with the Ti atoms were used. A small amount of line broadening was allowed for each component to simulate the distribution of the environments within each com¨ ponent. A detailed study of the Mossbauer spectra in order to assign the subspectra according to the selected iron atom groups taking into account the Co and Ti occupancy and in order to determine preferential sites for the Co atoms is in progress and will be published soon [29]. As it is shown in Fig. 10, the values of the average 57 Fe hyperfine field yielded by the fit increase with the Co content up to x50.3 and then decreases for x50.4 in accordance to the behaviour of the corresponding saturation magnetisation values (Fig. 6a). The values for x50 were taken from Ref. [13].
4. Conclusions The synthesis of Nd–(Fe,Co)–Ti intermetallic compounds with stoichiometry of Nd 3 (Fe 12x Co x ) 27.7 Ti 1.3 (x5 0.1, 0.2, 0.3, 0.4) and the Nd 3 (Fe,Ti) 29 -type structure with monoclinic symmetry (A2 /m) is reported. The unit cell parameters decrease anisotropically with the Co content present in the samples. The Curie temperature increases monotonically with x. For x$0.2 and at room temperature the compounds present an easy-magnetisation direction along the [4 0 22] direction. The maximum values of saturation magnetisation, anisotropy field and average hyperfine are obtained for x50.2 and x50.3. The low temperature magnetic transitions observed in the case of
References [1] Ye.V. Schrebakova, G.V. Ivanova, A.S. Yermolenko, Ye.V. Belozerov, V.S. Gaviko, J. Alloys Comp. 182 (1992) 199. [2] H.S. Li, J.M. Cadogan, R.L. Davis, A. Margarian, J.B. Dunlop, Solid State Commun. 90 (1994) 487. [3] O. Kalogirou, V. Psycharis, L. Saettas, D. Niarchos, J. Magn. Magn. Mater. 146 (1995) 335. [4] H.S. Li, D. Courtois, J.M. Cadogan, J.M. Xu, S.X. Dou, J. Phys.: Condens. Matter 6 (1994) L771. [5] C.D. Fuerst, F.E. Pinkerton, J.F. Herbst, J. Appl. Phys. 76 (1994) 6144. [6] C.D. Fuerst, F.E. Pinkerton, J.F. Herbst, J. Magn. Magn. Mater. 129 (1994) L115. [7] F.M. Yang, B. Nasunjilegal, H.Y. Pan, J.L. Wang, R.W. Zhao, B.P. Hu, Y.Z. Wang, H.S. Li, J.M. Cadogan, J. Magn. Magn. Mater. 135 (1994) 298. [8] O. Kalogirou, V. Psycharis, M. Gjoka, D. Niarchos, J. Magn. Magn. Mater. 147 (1995) L7. [9] D. Courtois, H.S. Li, J.M. Cadogan, IEEE Trans. Magn. 31 (1995) 3677. [10] V. Psycharis, O. Kalogirou, E. Devlin, M. Gjoka, A. Simopoulos, D. Niarchos, J. Magn. Magn. Mater. 153 (1996) 75. [11] Z. Hu, W.B. Yelon, O. Kalogirou, V. Psycharis, J. Appl. Phys. 80 (1996) 2955. [12] A. Margarian, H.S. Li, J.B. Dunlop, J.M. Cadogan, J. Alloys Comp. 239 (1996) 27. [13] O. Kalogirou, V. Psycharis, D. Niarchos, Solid State Commun. 97 (1996) 471. ` Yang, C.P. Chen, N. Tang, X.F. Han, J.L. Wang, J.F. [14] H.G. Pan, F.I. Hu, K.W. Zhou, R.W. Zhao, Q. D Wang, Solid State Commun. 98 (1996) 259. [15] X.F. Han, F.M. Yang, J.J. Zhu, H.G. Pan, Y.G. Wang, J.L. Wang, N. Tang, R.W. Zhao, J. Appl. Phys. 81 (1997) 3248. [16] V.R. Shah, G. Markandeyulu, K.V.S. Rama Rao, M.Q. Huang, K. Sirisha, M.E. McHenry, J. Magn. Magn. Mater. 190 (1998) 233. [17] V.R. Shah, G. Markandeyulu, K.V.S. Rama Rao, M.Q. Huang, K. Sirisha, M.E. McHenry, J. Appl. Phys. 85 (1999) 4678. [18] V.G. Harris, V.R. Shah, G. Markandeyulu, K.V.S. Rama Rao, M.Q. Huang, K. Sirisha, M.E. McHenry, IEEE Trans. Magn. 35 (1999) 3286. [19] G. Huo, Z. Qiao, G. Rao, J. Liang, B. Shen, J. Alloys Comp. 285 (1999) 216. [20] G.Y. Huo, G.H. Rao, Z.Y. Qiao, Q.L. Liu, X.L. Chen, J.K. Liang, B.G. Shen, J. Phys.: Condens. Matter 12 (2000) 1161. [21] Z.-G. Sun, H.-W. Zhang, J.-Y. Wang, B.-G. Shen, Appl. Phys. Lett. 75 (1999) 3850. [22] D.K. Gioureli, O. Kalogirou, A.C. Stergiou, G. Makaronides, G. Litsardakis, S. Katemliades, M. Gjoka, in: Proceedings of the 11th
66
O. Kalogirou et al. / Journal of Alloys and Compounds 325 (2001) 59 – 66
Intern. Symp. on Magnetic Anisotropy and Coercivity in Rare-Earth Transition Metal Alloys, 2000, p. S15. [23] M. Katter, J. Wecker, C. Kuhrt, L. Schultz, J. Magn. Magn. Mater. 114 (1992) 35. [24] L. Morellon, L. Pareti, P.A. Algarabel, F. Albertini, M.R. Ibarra, J. Phys.: Condens. Matter 6 (1994) L379. [25] D. Courtois, H.-S. Li, J.M. Cadogan, Solid State Commun. 98 (1996) 565.
[26] Z. Hu, W.B. Yelon, Solid State Commun. 91 (1994) 223. [27] N. Tang, X.C. Kou, F.R. de Boer, K.H.J. Buschow, J.L. Wang, F. Yang, J. Phys.: Condens. Matter 11 (1999) 5313. [28] F.M. Yang, B. Nasunjilegal, W. Gong, G.C. Hadjipanayis, IEEE Trans. Magn. 30 (1994) 4957. [29] O. Kalogirou, C. Sarafidis, T. Bakas, M. Gjoka, to be presented at ¨ the International Conference on the Applications of the Mossbauer Effect, Oxford, 2–7 September 2001.