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Magnetic properties of hexagonal RNi4Si (R = rare earth) compounds
Journal of Alloys and Compounds 442 (2007) 155–157
Magnetic properties of hexagonal RNi4Si (R = rare earth) compounds M. Falkowski ∗ , B. Andrzejewski, A. Kowalczyk Institute of Molecular Physics, Polish Academy of Sciences, Smoluchowskiego 17, 60-179 Pozna´n, Poland Received 15 May 2006; received in revised form 25 July 2006; accepted 25 September 2006 Available online 30 January 2007
Abstract RNi4 Si (R = rare earth) compounds have been studied by means of the X-ray powder diffraction, magnetization and magnetic susceptibility measurements. These compounds crystallize in the hexagonal CaCu5 -type of structure, space group P6/mmm. Apart from La, Ce, Pr and Yb, these compounds order ferromagnetically between 5.7 K (Tm) and 22 K (Gd). A strong thermomagnetic irreversibility is observed, i.e., the temperature dependence of the magnetic moment is different in the zero-field cooling (ZFC) and field-cooling (FC) mode. The difference between the ZFC and FC magnetization curves could be induced by the magnetocrystalline anisotropy of the R ion. © 2007 Elsevier B.V. All rights reserved. PACS: 71.20.Eh; 71.20.Lp; 75.60.−d Keywords: Rare earth alloys and compounds; Magnetically ordered materials; Magnetic measurements
1. Introduction RNi4 X (X = Al, Cu, Si) compounds crystallize in the hexagonal CaCu5 -type structure, space group P6/mmm. Recently, we have intensively studied the RNi4 Al and RNi4 Cu compounds [1–5], where R denotes a rare earth. In our previous paper [6] we presented results of magnetic measurements for RNi4 Si with R = Ce and Yb. The Ce (3d) XPS spectra have confirmed the mixed-valence state of the Ce ion. The f occupancy nf and the coupling Δ between the f level and the conduction band were derived to be about 0.91 and 36 meV, respectively. The intermediate valence state was also confirmed by the magnetic susceptibility measurements. The resistivity behaviour was found to be typical of a Kondo impurity systems. Magnetic susceptibility and XPS spectra suggests that the valence of ytterbium ion in YbNi4 Si is close to 3+ [6]. The GdNi4 Si compound is ferromagnetically ordered (TC = 22 K) [7]. The calculated total magnetic moment is equal to 7.10 B /f.u., therefore is very close to the experimental value of 7.13 B /f.u. The values of the local magnetic moments for Ni atoms are equal to about 0.03 B /atom.
∗
Corresponding author. E-mail address: [email protected] (M. Falkowski).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.09.151
In this work we have studied the magnetic properties of RNi4 Si (R = rare earth) compounds. 2. Experimental details The RNi4 Si compounds were prepared by induction melting of the stoichiometric amounts of the constituent elements in a water-cooled boat, under an argon atmosphere. The crystal structure was established by a powder Xray diffraction technique, using Cu K␣ radiation. The room temperature X-ray diffraction measurements showed that the RNi4 Si compounds were single-phase material in most cases. RNi4 Si compounds crystallize in the hexagonal CaCu5 type structure, space group P6/mmm. R atoms occupy the 1a site, Ni(1) the 2c site and Ni(2) and Si are statistically distributed on the 3g positions. The unit cell volume decreases with increasing the atomic number of the lanthanide metal. Magnetic measurements were carried out using an extraction sample magnetometer in a magnetic field of up to 9 T.
3. Experimental results and discussion In Fig. 1 the temperature dependences of the magnetic moment measured in the zero-field cooling (ZFC) and field cooling (FC) mode for R = Nd, Sm, Dy, Ho and Er show a magnetic phase transition (paramagnetic–ferromagnetic) at temperatures 9.2, 14.9, 15.6, 8.8 and 7.4 K, respectively. The measured ordering temperatures increase in the case of the light rare earths, whereas a decrease for heavy rare earth metals occurs with the growing R atomic number. TC values scale approximately with
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Fig. 2. Temperature dependence of the dc magnetic susceptibility for PrNi4 Si.
to the phase transition both the FC and ZFC curves converge. Considering that we do not observe such a strong difference between FC and ZFC behaviour of GdNi4 Si [7], it is evident that the observed thermomagnetic irreversibility in RNi4 Si arises from the anisotropy due to the R ion. Gd ion is characterized by the orbital momentum L = 0 implying a negligible magnetic anisotropy. A difference in FC and ZFC magnetization is well known in the spin-glass systems. Above TC the Curie-Weiss law is fulfilled providing the effective magnetic moment consistent with the corresponding values of the free rare earth ions. The magnetic moment of PrNi4 Si was measured in a magnetic field of 0.2 T and the resulting d.c. susceptibility χ(T) (Fig. 2) was fitted with a standard Curie law χ(T) = C/(T − θ) giving θ = −8.7 K and C = 1.61 (K emu/mol). The effective magnetic moment μeff = 3.6 B /f.u. derived from the Curie constant C is similar to the magnetic moment of free Pr3+ ion (≈3.52 B ). Relation between the values of TC and the de Gennes factor TC ∝ G = (gJ − 1)2 J(J + 1) is found for the RNi4 Si compounds (Fig. 3). TC follows the de Gennes factor (normalized to TC of
Fig. 1. Temperature dependence of the magnetization for the studied RNi4 Si compounds.
the de Gennes factor indicating that the coupling of the R–R moments is due to the indirect exchange interactions via the conduction electrons (RKKY interaction). In the ordered state RNi4 Si exhibits a remarkable thermomagnetic irreversibility in measurements of the temperature dependence of magnetization (Fig. 1). In the field-cooled conditions M(T) measured at H = 10 mT is typical of a ferromagnet; i.e., rises rapidly with the lowering of temperature below TC and then saturates at low temperatures. However, the thermomagnetic curve obtained under zero-field-cooled conditions is quite different, namely, at low temperatures the magnetization is smaller than in the fieldcooling mode and shows a maximum. Above a temperature close
Fig. 3. Correlation between the values of TC and the de Gennes factor for the RNi4 Si compounds.
M. Falkowski et al. / Journal of Alloys and Compounds 442 (2007) 155–157
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Sm-based alloys are usually characterized by a large coercive field and are used in a production of permanent magnets. SmNi4 Si shows a huge magnetic hysteresis with coercive force of 1 T at 4.2 K (Fig. 5), whereas RNi4 Si compounds with other rare earths are characterized by HC below 0.05 T at 4.2 K. The coercive field HC is small in contrast to HC = 7 T for SmNi4 B [8,9]. The significant coercive field of SmNi4 B has been ascribed by Mazumdar et al. [8] to a creation of a very narrow domain wall, down to one atomic layer, being a result of a competition between the magnetocrystalline anisotropy and the ferromagnetic coupling. 4. Conclusions A magnetic characterization of the compounds RNi4 Si has been performed. The main results are summarized as follows: Fig. 4. Magnetization curves of the RNi4 Si compounds.
GdNi4 Si) quite well (except Tm) in the region of the heavy rare earths. The values of TC for light rare earths always reveal a deviation from the simple formula. This deviation was taken as a characteristic feature of light rare earth ions of Nd and Sm metals, possibly related to the crystal-field phenomena. The magnetization curves of the RNi4 Si compounds are presented in Fig. 4. The M(H) dependence for RNi4 Si saturates more slightly than for GdNi4 Si indicating a presence of a noncollinear magnetic structure in these compounds and reflecting the negligible magnetocrystalline anisotropy of Gd being in the S state. The magnetic moments in the applied magnetic field of 9 T are reduced in comparison with the free rare earth ion values. This reduction is probably due to the crystal field interactions.
(1) RNi4 Si compounds crystallize in the hexagonal CaCu5 -type of structure. Apart from La, Ce, Pr and Yb, these compounds order ferromagnetically with TC in the range from 5.7 K (Tm) to 22 K (Gd). The transition temperatures are smaller than for their respective parent compounds RNi5 . (2) A strong thermomagnetic irreversibility is observed, i.e., the temperature dependence of the magnetic moment is different in the zero-field cooling and field-cooling mode. The difference between the ZFC and FC magnetization curves could be induced by the magnetocrystalline anisotropy of the R ion. (3) The magnetic moments of rare earth R are reduced compared to the free ion values. This occurs probably due to the crystal field interactions. (4) The coercive field HC = 1 T for SmNi4 Si is smaller in contrast to 7 T for SmNi4 B compound. References
Fig. 5. Hysteresis loop of the SmNi4 Si compound at 4.2 K.
[1] A. Kowalczyk, T. Toli´nski, B. Andrzejewski, A. Szlaferek, J. Alloys Compd. 413 (2006) 1. [2] T. Toli´nski, A. Kowalczyk, G. Chełkowska, M. Pugaczowa-Michalska, B. Andrzejewski, V. Ivanov, A. Szewczyk, M. Gutowska, Rhys. Rev. B 70 (2004) 064413. [3] A. Kowalczyk, M. Pugaczowa-Michalska, T. Toli´nski, Phys. Status Solidi (b) 242 (2005) 433. [4] T. Toli´nski, A. Kowalczyk, M. Pugaczowa-Michalska, G. Chelkowska, J. Phys.: Condens. Matter 15 (2003) 1397. [5] T. Toli´nski, W. Sch¨afer, W. Kockelmann, A. Kowalczyk, A. Hoser, Phys. Rev. B 68 (2003) 144403. [6] A. Kowalczyk, M. Falkowski, T. Toli´nski, G. Chełkowska, Solid State Commun. 139 (2006) 5–8. [7] A. Kowalczyk, A. Szajek, M. Falkowski, G. Chełkowska, J. Magn. Magn. Mater. 305 (2006) 348–351. [8] C. Mazumdar, R. Nagarajan, L.C. Gupta, B.D. Padalia, R. Vijayaraghavan, Appl. Phys. Lett. 77 (2000) 895. [9] T. Toli´nski, G. Chełkowska, B. Andrzejewski, A. Kowalczyk, M. Timko, J. Kovac, Phys. Status Solidi (a) 196 (2003) 294.
Magnetic properties of hexagonal RNi4Si (R = rare earth) compounds M. Falkowski ∗ , B. Andrzejewski, A. Kowalczyk Institute of Molecular Physics, Polish Academy of Sciences, Smoluchowskiego 17, 60-179 Pozna´n, Poland Received 15 May 2006; received in revised form 25 July 2006; accepted 25 September 2006 Available online 30 January 2007
Abstract RNi4 Si (R = rare earth) compounds have been studied by means of the X-ray powder diffraction, magnetization and magnetic susceptibility measurements. These compounds crystallize in the hexagonal CaCu5 -type of structure, space group P6/mmm. Apart from La, Ce, Pr and Yb, these compounds order ferromagnetically between 5.7 K (Tm) and 22 K (Gd). A strong thermomagnetic irreversibility is observed, i.e., the temperature dependence of the magnetic moment is different in the zero-field cooling (ZFC) and field-cooling (FC) mode. The difference between the ZFC and FC magnetization curves could be induced by the magnetocrystalline anisotropy of the R ion. © 2007 Elsevier B.V. All rights reserved. PACS: 71.20.Eh; 71.20.Lp; 75.60.−d Keywords: Rare earth alloys and compounds; Magnetically ordered materials; Magnetic measurements
1. Introduction RNi4 X (X = Al, Cu, Si) compounds crystallize in the hexagonal CaCu5 -type structure, space group P6/mmm. Recently, we have intensively studied the RNi4 Al and RNi4 Cu compounds [1–5], where R denotes a rare earth. In our previous paper [6] we presented results of magnetic measurements for RNi4 Si with R = Ce and Yb. The Ce (3d) XPS spectra have confirmed the mixed-valence state of the Ce ion. The f occupancy nf and the coupling Δ between the f level and the conduction band were derived to be about 0.91 and 36 meV, respectively. The intermediate valence state was also confirmed by the magnetic susceptibility measurements. The resistivity behaviour was found to be typical of a Kondo impurity systems. Magnetic susceptibility and XPS spectra suggests that the valence of ytterbium ion in YbNi4 Si is close to 3+ [6]. The GdNi4 Si compound is ferromagnetically ordered (TC = 22 K) [7]. The calculated total magnetic moment is equal to 7.10 B /f.u., therefore is very close to the experimental value of 7.13 B /f.u. The values of the local magnetic moments for Ni atoms are equal to about 0.03 B /atom.
∗
Corresponding author. E-mail address: [email protected] (M. Falkowski).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.09.151
In this work we have studied the magnetic properties of RNi4 Si (R = rare earth) compounds. 2. Experimental details The RNi4 Si compounds were prepared by induction melting of the stoichiometric amounts of the constituent elements in a water-cooled boat, under an argon atmosphere. The crystal structure was established by a powder Xray diffraction technique, using Cu K␣ radiation. The room temperature X-ray diffraction measurements showed that the RNi4 Si compounds were single-phase material in most cases. RNi4 Si compounds crystallize in the hexagonal CaCu5 type structure, space group P6/mmm. R atoms occupy the 1a site, Ni(1) the 2c site and Ni(2) and Si are statistically distributed on the 3g positions. The unit cell volume decreases with increasing the atomic number of the lanthanide metal. Magnetic measurements were carried out using an extraction sample magnetometer in a magnetic field of up to 9 T.
3. Experimental results and discussion In Fig. 1 the temperature dependences of the magnetic moment measured in the zero-field cooling (ZFC) and field cooling (FC) mode for R = Nd, Sm, Dy, Ho and Er show a magnetic phase transition (paramagnetic–ferromagnetic) at temperatures 9.2, 14.9, 15.6, 8.8 and 7.4 K, respectively. The measured ordering temperatures increase in the case of the light rare earths, whereas a decrease for heavy rare earth metals occurs with the growing R atomic number. TC values scale approximately with
156
M. Falkowski et al. / Journal of Alloys and Compounds 442 (2007) 155–157
Fig. 2. Temperature dependence of the dc magnetic susceptibility for PrNi4 Si.
to the phase transition both the FC and ZFC curves converge. Considering that we do not observe such a strong difference between FC and ZFC behaviour of GdNi4 Si [7], it is evident that the observed thermomagnetic irreversibility in RNi4 Si arises from the anisotropy due to the R ion. Gd ion is characterized by the orbital momentum L = 0 implying a negligible magnetic anisotropy. A difference in FC and ZFC magnetization is well known in the spin-glass systems. Above TC the Curie-Weiss law is fulfilled providing the effective magnetic moment consistent with the corresponding values of the free rare earth ions. The magnetic moment of PrNi4 Si was measured in a magnetic field of 0.2 T and the resulting d.c. susceptibility χ(T) (Fig. 2) was fitted with a standard Curie law χ(T) = C/(T − θ) giving θ = −8.7 K and C = 1.61 (K emu/mol). The effective magnetic moment μeff = 3.6 B /f.u. derived from the Curie constant C is similar to the magnetic moment of free Pr3+ ion (≈3.52 B ). Relation between the values of TC and the de Gennes factor TC ∝ G = (gJ − 1)2 J(J + 1) is found for the RNi4 Si compounds (Fig. 3). TC follows the de Gennes factor (normalized to TC of
Fig. 1. Temperature dependence of the magnetization for the studied RNi4 Si compounds.
the de Gennes factor indicating that the coupling of the R–R moments is due to the indirect exchange interactions via the conduction electrons (RKKY interaction). In the ordered state RNi4 Si exhibits a remarkable thermomagnetic irreversibility in measurements of the temperature dependence of magnetization (Fig. 1). In the field-cooled conditions M(T) measured at H = 10 mT is typical of a ferromagnet; i.e., rises rapidly with the lowering of temperature below TC and then saturates at low temperatures. However, the thermomagnetic curve obtained under zero-field-cooled conditions is quite different, namely, at low temperatures the magnetization is smaller than in the fieldcooling mode and shows a maximum. Above a temperature close
Fig. 3. Correlation between the values of TC and the de Gennes factor for the RNi4 Si compounds.
M. Falkowski et al. / Journal of Alloys and Compounds 442 (2007) 155–157
157
Sm-based alloys are usually characterized by a large coercive field and are used in a production of permanent magnets. SmNi4 Si shows a huge magnetic hysteresis with coercive force of 1 T at 4.2 K (Fig. 5), whereas RNi4 Si compounds with other rare earths are characterized by HC below 0.05 T at 4.2 K. The coercive field HC is small in contrast to HC = 7 T for SmNi4 B [8,9]. The significant coercive field of SmNi4 B has been ascribed by Mazumdar et al. [8] to a creation of a very narrow domain wall, down to one atomic layer, being a result of a competition between the magnetocrystalline anisotropy and the ferromagnetic coupling. 4. Conclusions A magnetic characterization of the compounds RNi4 Si has been performed. The main results are summarized as follows: Fig. 4. Magnetization curves of the RNi4 Si compounds.
GdNi4 Si) quite well (except Tm) in the region of the heavy rare earths. The values of TC for light rare earths always reveal a deviation from the simple formula. This deviation was taken as a characteristic feature of light rare earth ions of Nd and Sm metals, possibly related to the crystal-field phenomena. The magnetization curves of the RNi4 Si compounds are presented in Fig. 4. The M(H) dependence for RNi4 Si saturates more slightly than for GdNi4 Si indicating a presence of a noncollinear magnetic structure in these compounds and reflecting the negligible magnetocrystalline anisotropy of Gd being in the S state. The magnetic moments in the applied magnetic field of 9 T are reduced in comparison with the free rare earth ion values. This reduction is probably due to the crystal field interactions.
(1) RNi4 Si compounds crystallize in the hexagonal CaCu5 -type of structure. Apart from La, Ce, Pr and Yb, these compounds order ferromagnetically with TC in the range from 5.7 K (Tm) to 22 K (Gd). The transition temperatures are smaller than for their respective parent compounds RNi5 . (2) A strong thermomagnetic irreversibility is observed, i.e., the temperature dependence of the magnetic moment is different in the zero-field cooling and field-cooling mode. The difference between the ZFC and FC magnetization curves could be induced by the magnetocrystalline anisotropy of the R ion. (3) The magnetic moments of rare earth R are reduced compared to the free ion values. This occurs probably due to the crystal field interactions. (4) The coercive field HC = 1 T for SmNi4 Si is smaller in contrast to 7 T for SmNi4 B compound. References
Fig. 5. Hysteresis loop of the SmNi4 Si compound at 4.2 K.
[1] A. Kowalczyk, T. Toli´nski, B. Andrzejewski, A. Szlaferek, J. Alloys Compd. 413 (2006) 1. [2] T. Toli´nski, A. Kowalczyk, G. Chełkowska, M. Pugaczowa-Michalska, B. Andrzejewski, V. Ivanov, A. Szewczyk, M. Gutowska, Rhys. Rev. B 70 (2004) 064413. [3] A. Kowalczyk, M. Pugaczowa-Michalska, T. Toli´nski, Phys. Status Solidi (b) 242 (2005) 433. [4] T. Toli´nski, A. Kowalczyk, M. Pugaczowa-Michalska, G. Chelkowska, J. Phys.: Condens. Matter 15 (2003) 1397. [5] T. Toli´nski, W. Sch¨afer, W. Kockelmann, A. Kowalczyk, A. Hoser, Phys. Rev. B 68 (2003) 144403. [6] A. Kowalczyk, M. Falkowski, T. Toli´nski, G. Chełkowska, Solid State Commun. 139 (2006) 5–8. [7] A. Kowalczyk, A. Szajek, M. Falkowski, G. Chełkowska, J. Magn. Magn. Mater. 305 (2006) 348–351. [8] C. Mazumdar, R. Nagarajan, L.C. Gupta, B.D. Padalia, R. Vijayaraghavan, Appl. Phys. Lett. 77 (2000) 895. [9] T. Toli´nski, G. Chełkowska, B. Andrzejewski, A. Kowalczyk, M. Timko, J. Kovac, Phys. Status Solidi (a) 196 (2003) 294.