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Magnetic properties of the RCo3Sn (R=Gd to Tm) compounds
Journal of Alloys and Compounds 312 (2000) 9–11
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Magnetic properties of the RCo 3 Sn (R5Gd to Tm) compounds a b c, a a J. Mudryk , D. Fruchart , D. Gignoux *, L.P. Romaka , R.V. Skolozdra a
Department of Chemistry, Ivan Franko National University, 790005 Lviv, Ukraine Laboratoire de Cristallographie, CNRS, BP 166, 38042 Grenoble Cedex 9, France c ´ ´ , CNRS, BP 166, 38042 Grenoble Cedex 9, France Laboratoire de Magnetisme Louis Neel b
Received 1 June 2000; accepted 5 June 2000
Abstract The RCo 3 Sn compounds where R5Gd to Tm, as well as the new SmCo 3 Sn, have been synthesised and their magnetic properties investigated from 4 to 300 K and under 7–9 T. These alloys order ferromagnetically, the Curie temperatures ranging from 10 K for R5Tm to 130 K for R5Gd. Their magnitude together with their linear variation versus the ‘de Gennes’ factor tend to show that Co is not magnetic in this series. Besides, a smooth superimposed metamagnetic type behaviour occurs in GdCo 3 Sn below 22 K around 1.2 T. 2000 Elsevier Science B.V. All rights reserved. Keywords: Rare earth compounds; Transition metal compounds; Magnetically ordered materials; Magnetization measurements
1. Introduction The RCo 3 Sn (R5Y, Gd–Yb) stannides [1] crystallise in the BaLi 4 structure type [2] (space group P63 /mmc, a5 ˚ for YCo 3 Sn). Note that the 8.844, c57.446 A GdCo 32x Sn 11x compounds have an appreciable homogeneity range (0#x#0.25). This structure is built from the alternative stacking along the c-axis of two different types of layers. In z51 / 4 and 3 / 4, R atoms in a 6h site form a first type of layer together with one-third of Co atoms also in a 6h site. Note that the point symmetry of this site is mm2. The second type of layer, at z around 0 and 1 / 2, is formed by the Co atoms of the 2a and the 4f sites and a statistical distribution (in principle) of Co and Sn both occupying the 12k site. Up to now, magnetisation of the RCo 3 Sn compounds has been measured above 80 K and only for weak applied magnetic fields. These measurements have revealed a weak magnetic component up to 1000 K, that has been explained in terms of Co clusters formation [1]. However, the presence of some ferromagnetic impurities was not excluded. Recently, the magnetic behaviour of Lu 3 Co 7.77 Sn 4 , a compound also having a crystal structure related to the BaLi 4 type, was investi-
*Corresponding author. Fax: 133-4-7688-1191. E-mail address: [email protected] (D. Gignoux).
gated. It exhibits a weak itinerant ferromagnet behaviour with T c 563 K and a mean value of the Co moments of 0.19 m B at 4 K [3]. Then, the study of the low temperature magnetic properties of RCo 3 Sn (R5Sm to Tm) was undertaken in order to have a better insight of the magnetic contributions and couplings in the series.
2. Experimental The RCo 3 Sn compounds were prepared by melting the constituents (purity better than 99.9%) by using an arc furnace in pure argon gas and then annealing at 8008C for 30 days. Besides the earliest known RCo 3 Sn (R5Gd to Tm), the new SmCo 3 Sn stannide was prepared. The X-ray patterns, carried out on a DRON-2.0 diffractometer (Fe Ka radiation), were indexed on the basis of a hexagonal unit cell (for the new compound SmCo 3 Sn the cell parameters ˚ Moreover, a broadening are a58.952(4), c57.567(5) A). of some diffraction peaks of RCo 3 Sn indicates the presence of a small amount of a R 2 Co 17 type impurity. The YCo 3 Sn and SmCo 3 Sn were found to contain a larger amount of R 2 Co 17 than the other compounds. In order to eliminate this extra phase many attempts were done by varying the starting composition, without a complete success. The magnetic measurements were carried out up to 9 T and between 4 and 300 K using an automated extraction magnetometer.
0925-8388 / 00 / $ – see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 00 )01095-1
10
J. Mudryk et al. / Journal of Alloys and Compounds 312 (2000) 9 – 11 Table 1 Curie temperatures of the RCo 3 Sn compounds for R5Gd to Tm
Fig. 1. Magnetisation curves of HoCo 3 Sn at different temperatures.
3. Results and discussion Field dependences of magnetisation performed on HoCo 3 Sn at different temperatures are shown in Fig. 1. All the measured variations reveal a ferromagnetic behaviour. Note however the difference of behaviour between the curve at 300 K and those recorded at lower temperature, the latter showing a superimposed susceptibility above 4 T much larger than the former. From Arrott plots (M 2 versus H /M) the spontaneous magnetisation has been deduced at each temperature. Its thermal variation is reported in Fig. 2 as full and open circles. It first decreases rapidly when temperature increases (full circles) and then slightly increases up to 300 K (open circles). This is particularly
Fig. 2. Temperature dependence of the spontaneous magnetisation Ms of HoCo 3 Sn. The full and the open circles correspond to the HoCo 3 Sn and Ho 2 Co 17 contributions, respectively. The dashed line is a guide for the Ho 2 Co 17 spontaneous magnetisation in accordance with Ref. [4] normalised with the value observed at 300 K.
R
Gd
Tb
Dy
Ho
Er
Tm
T c (K) (DT c 562)
130
86
57
30
24
10
clear from Fig. 1 when comparing the M versus H variations at 80 and 300 K. The slight increase observed above 80 K can be ascribed to the large magnetic contribution of Ho 2 Co 17 . Indeed, the thermal variation of Ms of this ferrimagnetic compound, as for the other compounds of the series with heavy rare earths, is minimum at low temperature and exhibits a broad maximum between 400 and 700 K [4]. This thermal variation normalised to the value observed at 300 K is reported in dashed line which follows rather well the open circles distribution. At low temperature its contribution is much smaller than that of HoCo 3 Sn (full circles in Fig. 1). Then a Curie temperature of T c 53262 K was deduced. A similar behaviour was observed for all the other compounds from R5Gd to Tm, where an amount of R 2 Co 17 impurity was estimated to range between 8 and 12%. The same analysis as that made for the Ho-based compound led to the determination of the Curie temperatures, the values of which are reported in Table 1. Their variation as a function of the ( gJ 21)2 J(J11) de Gennes factor is shown in Fig. 3. The almost perfect linear dependence as well as the order magnitude of the Curie temperatures tend to prove that the 3d-Co contribution to magnetism is likely negligible in these compounds. Magnetism then mainly arises from rare earth atoms which are coupled via the usual long range RKKY exchange interaction. Except for R5Gd, the spontaneous magnetisation of the RCo 3 Sn compounds is 1.8–3 times smaller than the magnetic moments of the corresponding free 3 1 ions. This feature together with the large superimposed susceptibility
Fig. 3. Curie temperatures of RCo 3 Sn compounds (R5Gd to Tm) as a function of the de Gennes factor ( gJ 21)2 J(J11).
J. Mudryk et al. / Journal of Alloys and Compounds 312 (2000) 9 – 11
in high field and low temperature come from the large magnetocrystalline anisotropy of compounds with rare earth elements having L±0. Unfortunately, in YCo 3 Sn and SmCo 3 Sn the contribution of the R 2 Co 17 impurities was too large (about 20% in weight) to deduce a reliable estimation of T c . As shown in Fig. 4, GdCo 3 Sn exhibits a unusual and puzzling behaviour below 22 K. Indeed the magnetisation curves at low temperature, in particular at 4 K, reveal the
11
existence of a weak superimposed metamagnetic type transition around 1.2 T. At 20 K a small kink on the M versus H curve is still observed whereas it has completely disappeared at 22 K. An estimate of the spontaneous magnetisation (3.8 m B / f.u.) suggests that at low temperature GdCo 3 Sn exhibits a rather complicated magnetic structure which transforms into a collinear structure under high fields since the magnetisation saturates close to 7 m B / Gd, i.e. the free Gd 31 ion value. From these first considerations it appears that the 3d-Co contribution to magnetism is negligible in this series. However, this question would be definitely solved thanks to the determination of the magnetic structures by means of neutron diffraction experiments.
References [1] R.V. Skolozdra, Stannides of rare earth and transition metals, in: K.A. Gschneidner Jr., L. Eyring (Eds.), Handbook on the Physics and Chemistry of Rare Earths, Vol. 24, Elsevier, Amsterdam, 1997, p. 399, Ch. 164. [2] F.E. Wang, F.A. Konda, C.F. Miskell, A.J. King, Acta Crystallogr. 18 (1965) 24. [3] R.V. Skolozdra, B. Garcia-Landa, D. Fruchart, D. Gignoux, J.L. Soubeyroux, L.G. Akselrud, J. Alloys Comp. 235 (1996) 10. [4] J. Laforest, R. Lemaire, R. Pauthenet, J. Schweizer, CR Acad. Sci. Paris, Ser. B 262 (1966) 1260. Fig. 4. Magnetisation curves of GdCo 3 Sn at different temperatures.
L
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Magnetic properties of the RCo 3 Sn (R5Gd to Tm) compounds a b c, a a J. Mudryk , D. Fruchart , D. Gignoux *, L.P. Romaka , R.V. Skolozdra a
Department of Chemistry, Ivan Franko National University, 790005 Lviv, Ukraine Laboratoire de Cristallographie, CNRS, BP 166, 38042 Grenoble Cedex 9, France c ´ ´ , CNRS, BP 166, 38042 Grenoble Cedex 9, France Laboratoire de Magnetisme Louis Neel b
Received 1 June 2000; accepted 5 June 2000
Abstract The RCo 3 Sn compounds where R5Gd to Tm, as well as the new SmCo 3 Sn, have been synthesised and their magnetic properties investigated from 4 to 300 K and under 7–9 T. These alloys order ferromagnetically, the Curie temperatures ranging from 10 K for R5Tm to 130 K for R5Gd. Their magnitude together with their linear variation versus the ‘de Gennes’ factor tend to show that Co is not magnetic in this series. Besides, a smooth superimposed metamagnetic type behaviour occurs in GdCo 3 Sn below 22 K around 1.2 T. 2000 Elsevier Science B.V. All rights reserved. Keywords: Rare earth compounds; Transition metal compounds; Magnetically ordered materials; Magnetization measurements
1. Introduction The RCo 3 Sn (R5Y, Gd–Yb) stannides [1] crystallise in the BaLi 4 structure type [2] (space group P63 /mmc, a5 ˚ for YCo 3 Sn). Note that the 8.844, c57.446 A GdCo 32x Sn 11x compounds have an appreciable homogeneity range (0#x#0.25). This structure is built from the alternative stacking along the c-axis of two different types of layers. In z51 / 4 and 3 / 4, R atoms in a 6h site form a first type of layer together with one-third of Co atoms also in a 6h site. Note that the point symmetry of this site is mm2. The second type of layer, at z around 0 and 1 / 2, is formed by the Co atoms of the 2a and the 4f sites and a statistical distribution (in principle) of Co and Sn both occupying the 12k site. Up to now, magnetisation of the RCo 3 Sn compounds has been measured above 80 K and only for weak applied magnetic fields. These measurements have revealed a weak magnetic component up to 1000 K, that has been explained in terms of Co clusters formation [1]. However, the presence of some ferromagnetic impurities was not excluded. Recently, the magnetic behaviour of Lu 3 Co 7.77 Sn 4 , a compound also having a crystal structure related to the BaLi 4 type, was investi-
*Corresponding author. Fax: 133-4-7688-1191. E-mail address: [email protected] (D. Gignoux).
gated. It exhibits a weak itinerant ferromagnet behaviour with T c 563 K and a mean value of the Co moments of 0.19 m B at 4 K [3]. Then, the study of the low temperature magnetic properties of RCo 3 Sn (R5Sm to Tm) was undertaken in order to have a better insight of the magnetic contributions and couplings in the series.
2. Experimental The RCo 3 Sn compounds were prepared by melting the constituents (purity better than 99.9%) by using an arc furnace in pure argon gas and then annealing at 8008C for 30 days. Besides the earliest known RCo 3 Sn (R5Gd to Tm), the new SmCo 3 Sn stannide was prepared. The X-ray patterns, carried out on a DRON-2.0 diffractometer (Fe Ka radiation), were indexed on the basis of a hexagonal unit cell (for the new compound SmCo 3 Sn the cell parameters ˚ Moreover, a broadening are a58.952(4), c57.567(5) A). of some diffraction peaks of RCo 3 Sn indicates the presence of a small amount of a R 2 Co 17 type impurity. The YCo 3 Sn and SmCo 3 Sn were found to contain a larger amount of R 2 Co 17 than the other compounds. In order to eliminate this extra phase many attempts were done by varying the starting composition, without a complete success. The magnetic measurements were carried out up to 9 T and between 4 and 300 K using an automated extraction magnetometer.
0925-8388 / 00 / $ – see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 00 )01095-1
10
J. Mudryk et al. / Journal of Alloys and Compounds 312 (2000) 9 – 11 Table 1 Curie temperatures of the RCo 3 Sn compounds for R5Gd to Tm
Fig. 1. Magnetisation curves of HoCo 3 Sn at different temperatures.
3. Results and discussion Field dependences of magnetisation performed on HoCo 3 Sn at different temperatures are shown in Fig. 1. All the measured variations reveal a ferromagnetic behaviour. Note however the difference of behaviour between the curve at 300 K and those recorded at lower temperature, the latter showing a superimposed susceptibility above 4 T much larger than the former. From Arrott plots (M 2 versus H /M) the spontaneous magnetisation has been deduced at each temperature. Its thermal variation is reported in Fig. 2 as full and open circles. It first decreases rapidly when temperature increases (full circles) and then slightly increases up to 300 K (open circles). This is particularly
Fig. 2. Temperature dependence of the spontaneous magnetisation Ms of HoCo 3 Sn. The full and the open circles correspond to the HoCo 3 Sn and Ho 2 Co 17 contributions, respectively. The dashed line is a guide for the Ho 2 Co 17 spontaneous magnetisation in accordance with Ref. [4] normalised with the value observed at 300 K.
R
Gd
Tb
Dy
Ho
Er
Tm
T c (K) (DT c 562)
130
86
57
30
24
10
clear from Fig. 1 when comparing the M versus H variations at 80 and 300 K. The slight increase observed above 80 K can be ascribed to the large magnetic contribution of Ho 2 Co 17 . Indeed, the thermal variation of Ms of this ferrimagnetic compound, as for the other compounds of the series with heavy rare earths, is minimum at low temperature and exhibits a broad maximum between 400 and 700 K [4]. This thermal variation normalised to the value observed at 300 K is reported in dashed line which follows rather well the open circles distribution. At low temperature its contribution is much smaller than that of HoCo 3 Sn (full circles in Fig. 1). Then a Curie temperature of T c 53262 K was deduced. A similar behaviour was observed for all the other compounds from R5Gd to Tm, where an amount of R 2 Co 17 impurity was estimated to range between 8 and 12%. The same analysis as that made for the Ho-based compound led to the determination of the Curie temperatures, the values of which are reported in Table 1. Their variation as a function of the ( gJ 21)2 J(J11) de Gennes factor is shown in Fig. 3. The almost perfect linear dependence as well as the order magnitude of the Curie temperatures tend to prove that the 3d-Co contribution to magnetism is likely negligible in these compounds. Magnetism then mainly arises from rare earth atoms which are coupled via the usual long range RKKY exchange interaction. Except for R5Gd, the spontaneous magnetisation of the RCo 3 Sn compounds is 1.8–3 times smaller than the magnetic moments of the corresponding free 3 1 ions. This feature together with the large superimposed susceptibility
Fig. 3. Curie temperatures of RCo 3 Sn compounds (R5Gd to Tm) as a function of the de Gennes factor ( gJ 21)2 J(J11).
J. Mudryk et al. / Journal of Alloys and Compounds 312 (2000) 9 – 11
in high field and low temperature come from the large magnetocrystalline anisotropy of compounds with rare earth elements having L±0. Unfortunately, in YCo 3 Sn and SmCo 3 Sn the contribution of the R 2 Co 17 impurities was too large (about 20% in weight) to deduce a reliable estimation of T c . As shown in Fig. 4, GdCo 3 Sn exhibits a unusual and puzzling behaviour below 22 K. Indeed the magnetisation curves at low temperature, in particular at 4 K, reveal the
11
existence of a weak superimposed metamagnetic type transition around 1.2 T. At 20 K a small kink on the M versus H curve is still observed whereas it has completely disappeared at 22 K. An estimate of the spontaneous magnetisation (3.8 m B / f.u.) suggests that at low temperature GdCo 3 Sn exhibits a rather complicated magnetic structure which transforms into a collinear structure under high fields since the magnetisation saturates close to 7 m B / Gd, i.e. the free Gd 31 ion value. From these first considerations it appears that the 3d-Co contribution to magnetism is negligible in this series. However, this question would be definitely solved thanks to the determination of the magnetic structures by means of neutron diffraction experiments.
References [1] R.V. Skolozdra, Stannides of rare earth and transition metals, in: K.A. Gschneidner Jr., L. Eyring (Eds.), Handbook on the Physics and Chemistry of Rare Earths, Vol. 24, Elsevier, Amsterdam, 1997, p. 399, Ch. 164. [2] F.E. Wang, F.A. Konda, C.F. Miskell, A.J. King, Acta Crystallogr. 18 (1965) 24. [3] R.V. Skolozdra, B. Garcia-Landa, D. Fruchart, D. Gignoux, J.L. Soubeyroux, L.G. Akselrud, J. Alloys Comp. 235 (1996) 10. [4] J. Laforest, R. Lemaire, R. Pauthenet, J. Schweizer, CR Acad. Sci. Paris, Ser. B 262 (1966) 1260. Fig. 4. Magnetisation curves of GdCo 3 Sn at different temperatures.