- Email: [email protected]
Magnetic and magnetoelastic properties of GdNi: Single-crystal study
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 310 (2007) 1753–1754 www.elsevier.com/locate/jmmm
Magnetic and magnetoelastic properties of GdNi: Single-crystal study K. Uhlı´ rˇ ova´a,, J. Proklesˇ kaa, J. Poltierova´ Vejpravova´a, V. Sechovsky´a, K. Maezawab a
Charles University, Faculty of Mathematics and Physics, DCMP, Ke Karlovu 5, 121 16 Prague 2, Czech Republic b Toyama Prefectural University, Kosugi, Toyama 939-0398, Japan Available online 13 November 2006
Abstract A GdNi single crystal has been prepared by the Czochralski method and studied by measuring the magnetization, electrical resistivity and thermal expansion. The ferromagnetic ordering below TC ¼ 69 K has been confirmed. The saturated magnetization at 2 K was found to be identical (7.2 mB/f.u.) along all the three principal crystallographic directions. This result is in agreement with theoretical calculations, which yield magnetic moment of 7.0 mB/f.u. for Gd3+ ion and 0.18 mB/f.u. for conduction electrons with magnetic moment of the same direction as the Gd moment. An anomalous spontaneous magnetostriction is dominating the thermal expansion behavior below TC. In particular, we have observed a large contraction in the c-direction and a smaller expansion in the a and b directions (nearly identical). The unit cell volume expands below TC (DV/V ¼ 8 104). The observed anisotropy of the magnetostriction in GdNi is attributed to anisotropic magnetoelastic coupling. r 2006 Elsevier B.V. All rights reserved. PACS: 75.20.En; 75.50.Cc; 75.80.+q Keywords: GdNi; Ferromagnetism; Magnetization; Electrical resistivity; Spontaneous magnetostriction
GdNi crystallizes in the orthorhombic CrB-type structure and exhibits ferromagnetism below TC70 K [1]. A magnetocrystalline anisotropy with the b-axis as the easy magnetization direction was reported in Ref. [2]. A large anomalous thermal expansion below TC (an expansion in the ab-plane and a strong contraction in the c-direction) were revealed by X-ray powder diffraction [3]. To clarify the issue of magnetocrystalline anisotropy in GdNi with magnetism due to Gd3+ ions possessing a spin only magnetic moment, and to study the thermal expansion directly by dilatometry we have grown a GdNi single crystal and studied the magnetization, AC susceptibility, thermal expansion and electrical resistance. The GdNi crystal was grown by Czochralski method from starting melt containing stoichiometric amounts of the constituent elements of the purity 4 and 3 N for nickel and gadolinium, respectively. Crystallinity and crystal orientation of the sample were determined by Laue method. Part of the crystal was powderized and studied Corresponding author. Tel.: +420 2 21911352; fax: +420 2 21911393.
E-mail address: [email protected] (K. Uhlı´ rˇ ova´). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.10.581
by X-ray powder diffraction in order to test phase purity and determine the lattice parameters which were found as a ¼ 3.774, b ¼ 10.327, and c ¼ 4.238 A˚. The magnetization and AC susceptibility were measured in the temperature range from 2 to 300 K and a magnetic field up to 9 T applied along a-, b- and c-axis, respectively, using ACMS option of the PPMS apparatus (quantum design). A sample with dimensions 2.36 1.63 1.04 mm3 (a b c) was used. The a-, b- and c-axis thermal expansion, respectively, was measured on the same sample with a ‘‘Vienna type’’ microdilatometric capacitance cell [4] using three terminal connections to the ultra-precision capacitance bridge (Andeen Hagerling 2500 A, 1 kHz). The cell was implemented in the PPMS. The volume change was calculated as a sum of length changes of principal axes. The temperature dependence of the electrical resistivity r(T) was measured at Toyama Prefectural University in Japan. Fig. 1 shows the magnetization curves measured at 2 K in magnetic fields applied along the a-, b- and c-axis, respectively. All the three curves saturate in fields above 1 T. The anisotropy of M(B) curves below 1 T in our opinion was due to the anisotropic response of the ferromagnetic domains.
ARTICLE IN PRESS 1754
K. Uhlı´rˇova´ et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 1753–1754
Fig. 2. Temperature dependence of the electrical resistivity for IJc, reciprocal DC susceptibility in a field along the c-axis and the temperature derivative of AC susceptibility measured on a GdNi single crystal. Fig. 1. Magnetization curves measured on a GdNi single crystal at 2 K in a magnetic field applied along the a-, b- and c-axis, respectively.
The saturated magnetic moment ms ¼ 7.2 mB/f.u. is, however, isotropic. The observed value is in agreement with theoretical calculations [5] providing the moments of 7.0 mB/f.u. because of the 4f electrons in the Gd3+ ion and 0.18 mB/f.u. due to the conduction electrons with polarized spins along the Gd moments. The parallel orientation of Gd and Ni moments was also reported by Mallik et al. [6,7]. In contrast, Yano et al. [8] reported recently an antiferromagnetic coupling between the Gd and Ni moments. The value of the saturated moment observed in our work corroborates rather the scenario of parallel (ferromagnetic) coupling the of Gd 4f and conduction electron moments. The sharp anomaly in the AC susceptibility and the sudden change of slope the r(T) curve (see Fig. 2) consistently point to the Curie temperature T C ¼ 69 K of GdNi. The paramagnetic susceptibility follows the Curie– Weiss law with yp ¼ 72 K and meff ¼ 8.3 mB/f.u. The latter value is noticeably larger than 7.94 mB expected for the 4f electrons of free Gd3+ ion. The discrepancy, which is frequently observed in Gd intermetallics, is most likely due to the polarized spins of conduction electrons in the vicinity of the Gd3+ ion [6,7]. In Fig. 3, the temperature dependences of the relative length change Dli/li (i ¼ a, b, c, respectively), and the relative volume change SDli/li are shown. At ToTC we observe anomalous behavior mainly due to strong anisotropic spontaneous magnetostriction (a large contraction in c and smaller expansion (almost identical) along a and b. Also the relative volume expansion is anomalous below TC. The effects are attributed to anisotropic magnetoelastic coupling of the Gd moments with the crystal lattice. In conclusion: we have grown a GdNi single crystal and studied magnetization, AC susceptibility, electrical resistivity and thermal expansion. Ferromagnetism below TC ¼ 69 K has been confirmed. The 2-K magnetization saturates above 2 T at the value (isotropic) of ms ¼ 7.2 mB/f.u. that is consistent with the moment due to the 4f electrons of the
Fig. 3. Temperature dependence of the relative linear thermal expansion along the a-, b- and c-axis, respectively, measured on a GdNi single crystal and a relative volume expansion calculated as SDli/li.
Gd3+ ion that are enhanced by the polarized spins of conduction electrons. The observed anomalous thermal expansion in the ferromagnetic range, is mainly due to the spontaneous magnetostriction which may be conceived to be due to the anisotropic magnetoelastic coupling. This work is a part of the research plan MSM 0021620834 financed by the Ministry of Education of the Czech Republic.
References [1] [2] [3] [4] [5] [6] [7] [8]
Y. Isikawa, et al., J. Magn. Magn. Mater 31–34 (1983) 1057. K. Sato, et al., J. Appl. Phys. 52 (1981) 2084. E. Gratz, et al., J. Magn. Magn. Mater. 177–181 (1998) 1077. M. Rotter, et al., Rev. Sci. Instr. 69 (1998) 2742. J. Rusz, et al., Phy. Rev. B 71 (2005) 174408. R. Mallik, et al., Phys. Rev. B 55 (1997) R8650. R. Mallik, et al., Phys. Rev. B 55 (1997) 8369. K. Yano, et al., Solid State Commun. 136 (2005) 67.
Journal of Magnetism and Magnetic Materials 310 (2007) 1753–1754 www.elsevier.com/locate/jmmm
Magnetic and magnetoelastic properties of GdNi: Single-crystal study K. Uhlı´ rˇ ova´a,, J. Proklesˇ kaa, J. Poltierova´ Vejpravova´a, V. Sechovsky´a, K. Maezawab a
Charles University, Faculty of Mathematics and Physics, DCMP, Ke Karlovu 5, 121 16 Prague 2, Czech Republic b Toyama Prefectural University, Kosugi, Toyama 939-0398, Japan Available online 13 November 2006
Abstract A GdNi single crystal has been prepared by the Czochralski method and studied by measuring the magnetization, electrical resistivity and thermal expansion. The ferromagnetic ordering below TC ¼ 69 K has been confirmed. The saturated magnetization at 2 K was found to be identical (7.2 mB/f.u.) along all the three principal crystallographic directions. This result is in agreement with theoretical calculations, which yield magnetic moment of 7.0 mB/f.u. for Gd3+ ion and 0.18 mB/f.u. for conduction electrons with magnetic moment of the same direction as the Gd moment. An anomalous spontaneous magnetostriction is dominating the thermal expansion behavior below TC. In particular, we have observed a large contraction in the c-direction and a smaller expansion in the a and b directions (nearly identical). The unit cell volume expands below TC (DV/V ¼ 8 104). The observed anisotropy of the magnetostriction in GdNi is attributed to anisotropic magnetoelastic coupling. r 2006 Elsevier B.V. All rights reserved. PACS: 75.20.En; 75.50.Cc; 75.80.+q Keywords: GdNi; Ferromagnetism; Magnetization; Electrical resistivity; Spontaneous magnetostriction
GdNi crystallizes in the orthorhombic CrB-type structure and exhibits ferromagnetism below TC70 K [1]. A magnetocrystalline anisotropy with the b-axis as the easy magnetization direction was reported in Ref. [2]. A large anomalous thermal expansion below TC (an expansion in the ab-plane and a strong contraction in the c-direction) were revealed by X-ray powder diffraction [3]. To clarify the issue of magnetocrystalline anisotropy in GdNi with magnetism due to Gd3+ ions possessing a spin only magnetic moment, and to study the thermal expansion directly by dilatometry we have grown a GdNi single crystal and studied the magnetization, AC susceptibility, thermal expansion and electrical resistance. The GdNi crystal was grown by Czochralski method from starting melt containing stoichiometric amounts of the constituent elements of the purity 4 and 3 N for nickel and gadolinium, respectively. Crystallinity and crystal orientation of the sample were determined by Laue method. Part of the crystal was powderized and studied Corresponding author. Tel.: +420 2 21911352; fax: +420 2 21911393.
E-mail address: [email protected] (K. Uhlı´ rˇ ova´). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.10.581
by X-ray powder diffraction in order to test phase purity and determine the lattice parameters which were found as a ¼ 3.774, b ¼ 10.327, and c ¼ 4.238 A˚. The magnetization and AC susceptibility were measured in the temperature range from 2 to 300 K and a magnetic field up to 9 T applied along a-, b- and c-axis, respectively, using ACMS option of the PPMS apparatus (quantum design). A sample with dimensions 2.36 1.63 1.04 mm3 (a b c) was used. The a-, b- and c-axis thermal expansion, respectively, was measured on the same sample with a ‘‘Vienna type’’ microdilatometric capacitance cell [4] using three terminal connections to the ultra-precision capacitance bridge (Andeen Hagerling 2500 A, 1 kHz). The cell was implemented in the PPMS. The volume change was calculated as a sum of length changes of principal axes. The temperature dependence of the electrical resistivity r(T) was measured at Toyama Prefectural University in Japan. Fig. 1 shows the magnetization curves measured at 2 K in magnetic fields applied along the a-, b- and c-axis, respectively. All the three curves saturate in fields above 1 T. The anisotropy of M(B) curves below 1 T in our opinion was due to the anisotropic response of the ferromagnetic domains.
ARTICLE IN PRESS 1754
K. Uhlı´rˇova´ et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 1753–1754
Fig. 2. Temperature dependence of the electrical resistivity for IJc, reciprocal DC susceptibility in a field along the c-axis and the temperature derivative of AC susceptibility measured on a GdNi single crystal. Fig. 1. Magnetization curves measured on a GdNi single crystal at 2 K in a magnetic field applied along the a-, b- and c-axis, respectively.
The saturated magnetic moment ms ¼ 7.2 mB/f.u. is, however, isotropic. The observed value is in agreement with theoretical calculations [5] providing the moments of 7.0 mB/f.u. because of the 4f electrons in the Gd3+ ion and 0.18 mB/f.u. due to the conduction electrons with polarized spins along the Gd moments. The parallel orientation of Gd and Ni moments was also reported by Mallik et al. [6,7]. In contrast, Yano et al. [8] reported recently an antiferromagnetic coupling between the Gd and Ni moments. The value of the saturated moment observed in our work corroborates rather the scenario of parallel (ferromagnetic) coupling the of Gd 4f and conduction electron moments. The sharp anomaly in the AC susceptibility and the sudden change of slope the r(T) curve (see Fig. 2) consistently point to the Curie temperature T C ¼ 69 K of GdNi. The paramagnetic susceptibility follows the Curie– Weiss law with yp ¼ 72 K and meff ¼ 8.3 mB/f.u. The latter value is noticeably larger than 7.94 mB expected for the 4f electrons of free Gd3+ ion. The discrepancy, which is frequently observed in Gd intermetallics, is most likely due to the polarized spins of conduction electrons in the vicinity of the Gd3+ ion [6,7]. In Fig. 3, the temperature dependences of the relative length change Dli/li (i ¼ a, b, c, respectively), and the relative volume change SDli/li are shown. At ToTC we observe anomalous behavior mainly due to strong anisotropic spontaneous magnetostriction (a large contraction in c and smaller expansion (almost identical) along a and b. Also the relative volume expansion is anomalous below TC. The effects are attributed to anisotropic magnetoelastic coupling of the Gd moments with the crystal lattice. In conclusion: we have grown a GdNi single crystal and studied magnetization, AC susceptibility, electrical resistivity and thermal expansion. Ferromagnetism below TC ¼ 69 K has been confirmed. The 2-K magnetization saturates above 2 T at the value (isotropic) of ms ¼ 7.2 mB/f.u. that is consistent with the moment due to the 4f electrons of the
Fig. 3. Temperature dependence of the relative linear thermal expansion along the a-, b- and c-axis, respectively, measured on a GdNi single crystal and a relative volume expansion calculated as SDli/li.
Gd3+ ion that are enhanced by the polarized spins of conduction electrons. The observed anomalous thermal expansion in the ferromagnetic range, is mainly due to the spontaneous magnetostriction which may be conceived to be due to the anisotropic magnetoelastic coupling. This work is a part of the research plan MSM 0021620834 financed by the Ministry of Education of the Czech Republic.
References [1] [2] [3] [4] [5] [6] [7] [8]
Y. Isikawa, et al., J. Magn. Magn. Mater 31–34 (1983) 1057. K. Sato, et al., J. Appl. Phys. 52 (1981) 2084. E. Gratz, et al., J. Magn. Magn. Mater. 177–181 (1998) 1077. M. Rotter, et al., Rev. Sci. Instr. 69 (1998) 2742. J. Rusz, et al., Phy. Rev. B 71 (2005) 174408. R. Mallik, et al., Phys. Rev. B 55 (1997) R8650. R. Mallik, et al., Phys. Rev. B 55 (1997) 8369. K. Yano, et al., Solid State Commun. 136 (2005) 67.