- Email: [email protected]
Magnetic and electronic properties of NdNi5−xCux compounds
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 290–291 (2005) 371–373 www.elsevier.com/locate/jmmm
Magnetic and electronic properties of NdNi5xCux compounds Emil Burzoa,, Traian Crainica,b, Manfred Neumannb, Liviu Chioncelc, Corina Lazara a
Faculty of Physics, Babes-Bolyai University, Str. M. Kogalniceanu Nr. 1, 400084 Cluj-Napoca RO-3400, Romania b Fachbereich Physik, Universita¨t Osnanbru¨ck, D-49069, Osnanbru¨ck, Germany c Department of Physics, University of Nijmigen 6500 GL, Nijmegen, The Netherlands Available online 7 January 2005
Abstract The NdNi5xCux compounds crystallize in a CaCu5-type structure for xp2: Band structure calculations show that the magnetic moments, at 0 K, for Ni (2c) and (3 g) sites decrease when increasing Ni content and are nil for x ffi 2: XPS measurements, at room temperature, show the presence of unfilled Ni3d band in the entire composition range, in agreement with paramagnetic studies. The magnetic behaviour of nickel is analysed in models which take into account the electron correlation effects in d bands. r 2004 Elsevier B.V. All rights reserved. PACS: 75.30.Cr; 75.50.Gg Keywords: Rare-earth compounds; Magnetic properties; Band structures; XPS studies
The RNi5 compounds, where R is a rare-earth, crystallize in a hexagonal structure of a CaCu5-type, having P6/mmm space group. The R atoms in this structure are located in 1a-type site while Ni occupies 2c and 3g positions. The compounds were intensively studied due to their ability to absorb hydrogen. The analysis of electronic structures and magnetic properties of RNi5xMx where M is a nonmagnetic element is also of interest [1]. Previously, we studied the electronic structures and magnetic properties of LaNi5xCux [2] and DyNi5xAlx [3] systems. In Cu-doped samples, separate Cu bands are formed at ffi3.3 eV below the Fermi level. A strong hybridization of Ni3d and Al3p bands was shown in an aluminium-substituted system. In this paper we analyse the effect of Cu substitution at Corresponding author. Tel.: +40264 403500; +40264 591906. E-mail address: [email protected] (E. Burzo).
fax:
Ni sites in the NdNi5xCux system. Unlike with heavy rare-earth compounds, at low temperatures, if Ni moment exists, it is believed to be parallely aligned to that of neodymium [1]. The NdNi5xCux compounds with xp2 were prepared in an induction furnace in purified argon atmosphere. The samples were thermally treated in vacuum at 900 1C for 1 week. The X-ray analyses show the presence of only one phase. The compounds crystallize in CaCu5type structure. Both a and c lattice parameters increase as copper content is higher. Magnetic measurements were performed in the temperature range 1.7–300 K and fields up to 9 T. The saturation magnetizations, Ms, were determined from magnetization isotherms according to approach to saturation law M ¼ Ms(1–a/H) by extrapolating the measured values at H1-0. The X-ray photoelectron spectroscopy (XPS) measurements were performed using a PHI 5600ci multitechnique system. The spectra were recorded using the monochromatized
0304-8853/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2004.11.263
ARTICLE IN PRESS E. Burzo et al. / Journal of Magnetism and Magnetic Materials 290–291 (2005) 371–373
12
6
0
-6
-12 -6
spin down
DOS (states/eV )
spin up
NdNi5
Nd-f NI(2c) Ni(3g) -4
-2
0 E-EF (eV )
2
d 4
Fig. 1. Projected densities of states of NdNi5.
6
150
2.5
x=0
2.4
x=2
x=0.5
2.3
x=0.5
2.1 2.0
x=1
x=2.0
2.2
5
6
7 8 µ0H (T)
100
9
50
0
100
200
χ−11 (emu/mol)-1
Ka radiation of Al (1486.6 eV). Binding energies are given with reference of Fermi level. Band structure calculations were performed by using the TB–LMTO method within LDA+U approach [4,5]. For nickel, values of average Coulomb interaction U ¼ 2 eV and exchange interaction J ¼ 0.9 eV, widely accepted [6], were used. For Nd values U ¼ 6.25 eV and J ¼ 1.25 eV were chosen to reproduce the estimated moment from experimental data. Relativistic effects, except spin–orbit coupling, were included. Convergence was reached within 106 Ry for a number of 216 k points in the Brillouin zone. The Cu atoms for x ¼ 1 were supposed to occupy the 3g sites while for higher Cu concentrations both 2c and 3g positions. The projected densities of states for compound with x ¼ 0 are plotted in Fig. 1. The magnetic moments of nickel at 2c and 3g sites in NdNi5 are 0.168 and 0.244 mB, respectively, parallely aligned to Nd moment. The moment at Ni(3g) site is higher than that at 2c site and can be correlated with a higher number of Nd as nearest neighbours. The strength of exchange interactions between nickel and neodymium atoms is more important than between those of nickel, the nickel moments being essentially induced by the presence of Nd. The substitution of Ni atoms by Cu has a strong effect on Ni moments. Both Ni(2c) and Ni(3g) moments decrease up to 0.056 mB/atom in NdNi4Cu. The Ni moments, for xX1; unlike NdNi5, are antiparallely oriented to Nd ones. We note that a negative-induced polarization has been already observed on highly delocalized 3d vanadium states in YFe2xVx system [7]. Delocalized Ni3d states are also expected to be present in NdNi4Cu, as suggested by very small Ni polarization. The Ni moments are practically nil in NdNi3Cu2 compound. Some magnetization isotherms obtained at 1.7 K are plotted in Fig. 2. The shape of magnetization isotherm for NdNi5 indicates the presence of high anisotropy. The coefficient of magnetic hardness decreases when Ni is
M (µΒ/f.u.)
372
0 300
T (K)
Fig. 2. Magnetization isotherms at 1.7 K and thermal variations of reciprocal susceptibilities.
replaced by Cu. A small discontinuity in magnetization curves has been seen between 7 and 8 T for samples having x ¼ 0 and 0.5. This may be correlated with a possible itinerant electron metamagnetic transition at some Ni sites. The saturation moments, Ms, per formula unit, at 1.7 K, obtained by extrapolation, are 2.83 mB for NdNi5 and decrease to 2.45 mB for x ¼ 1:5: The errors introduced by extrapolation procedure are of the order of 0.3 mB. The Ms values obtained by band structure calculation are higher by 0.6 and 0.4 mB for compounds with x ¼ 0 and xX1; respectively. The thermal variations of reciprocal susceptibilities, at T420 K, follow linear dependences. According to the addition law of magnetic susceptibilities and supposing that neodymium contribution to the Curie constants is the same as that of Nd3+ ion, we determined the effective nickel moments, Meff(Ni). The Meff(Ni) values decrease from ffi1.25 mB/atom for xp1:0 up to 0.6–0.8 mB/atom at higher copper content. The Ni 2p3/2 and 2p1/2 core levels for some NdNi5xCux compounds, at room temperature, are plotted in Fig. 3. For comparison, the Ni2p spectrum of pure nickel is also given. The positions of the 2p3/2 and 2p1/2 lines are situated at 852.570.1 and 869.770.1 eV, respectively, and are not changed when increasing Cu content. These lines are very close to those reported in pure nickel. In all spectra the presence of 6 eV Ni satellite at 858.570.1 eV can be seen. This suggests that there are unoccupied states in the Ni3d band. The XPS valence bands of some NdNi5xCux samples as well as that of pure nickel are plotted in Fig. 4. These are due to the superposition of Nd, Cu and Ni lines. The Nd lines are difficult to be analysed since of their low intensities and of the superposition with cooper ones. Although the valence band spectrum of NdNi5 is similar to that of pure nickel, the density of states at the Fermi level is diminished since of 5d–3d hybridisation. In addition, the valence band of Ni in NdNi5 is shifted to
ARTICLE IN PRESS E. Burzo et al. / Journal of Magnetism and Magnetic Materials 290–291 (2005) 371–373
Ni2P1/2
Intensity (arb.units)
NdNi3Cu2
Ni2P3/2
NdNi4Cu
NdNi5 Ni
890
880
870 860 Binding energy (eV)
850
840
Fig. 3. Ni 2p XPS spectra for NdNi5xCux system with x ¼ 0; 1 and 2 and of pure nickel.
Intensity (arb.units)
NdNi3cu2 NdNi4cu
NdNi5 Ni
20
15 10 5 Binding energy (eV)
0
373
an exchange field of the order of 30 T for the appearance of nickel moment at 0 K. Effective nickel moments are present in paramagnetic range even in compounds in which nickel moment, at 0 K, is nil. The presence of unfilled Ni3d states at room temperature are also confirmed by XPS measurements. The ratio r between the number of spins determined from Curie constants and those obtained at 0 K, increases from r ¼ 3 (x ¼ 0) to r ¼ 10 in NdNi4Cu compound. These data suggest that the magnetic behaviour of Ni can be analysed in models that take into account the electron correlation effects in d bands. These models reconcile the dual character of electron which, as a particle, requires a real space description and as a wave, a momentum space description. Thus, the spin fluctuation model [8] considers the balance between the frequencies of longitudinal spin fluctuations, which are determined by their life-time and of transverse fluctuations that are of thermal origin. The dynamical mean field theory [9] combined with standard LDA band calculations [10] can also describe the magnetic behaviour of Ni in the present compounds. In a strongly correlated system, leading Curie–Weiss behaviour, at high temperatures, is predicted. For an itinerant electron system, the time dependence of the correlation function results in temperature dependence of local spin, hS2loc i: Fluctuating moments and atomic-like configurations are large at short time scales. The moment is reduced at longer time scales, corresponding to a more less-correlated electronic structure near the Fermi level.
-5
Fig. 4. XPS valence band spectra of some NdNi5xCux samples and that of pure nickel.
higher energy. This may be correlated with an increase of the d-state occupancies. When substituting Ni by Cu, important changes in the valence band spectra can be shown. A rather independent Cu d band is formed around 3.4 eV binding energy. By increasing the Cu content, the relative intensities of Cu d bands increase but their positions and the linewidths are not changed with the composition. The Cu 3d states are filled, as previously reported in LaNi5xCux system [2]. The Ni in NdNi5xCux system, at low temperatures, shows a weak ordered moment for xp1:0 and practically nil for x ¼ 2:0: By analysing the exchange splitting of Ni3d states, obtained from band structure calculations, in correlation with nickel moments, we estimate
References [1] E. Burzo, A. Chelkovski, H.R. Kirckmayr, Landolt Bo¨rnstein Handbook, Vol. III/19d2, Springer, Berlin, 1990. [2] E. Burzo, S. Chiuzbaian, L. Chioncel, M. Neumann, J. Phys.: Cond. Matter 12 (2000) 5897. [3] E. Burzo, et al., J. Appl. Phys. 92 (2002) 7362. [4] A.I. Lichtenstein, J. Zaanen, V.I. Anisimov, Phys. Rev. B 52 (1995) R5467. [5] V.I. Anisimov, F. Aryasetiavan, A.I. Lichtenstein, J. Phys.: Condens. Matter 9 (1997) 767. [6] I. Yang, et al., Phys. Rev. Lett. 87 (2001) 216405. [7] R. Tetean, E. Burzo, L. Chioncel, O. Garlea, J. Magn. Magn. Mater. 242–243 (2002) 836. [8] T. Moriya, J. Magn. Magn. Mater. 100 (1991) 261. [9] A. Georges, G. Kothar, W. Krauth, M.J. Rosenberg, Rev. Mod. Phys. 68 (1996) 13. [10] A.I. Lichtenstein, M.I. Katsnelson, G. Kothar, Phys. Rev. Lett. 87 (2001) 067205.
Journal of Magnetism and Magnetic Materials 290–291 (2005) 371–373 www.elsevier.com/locate/jmmm
Magnetic and electronic properties of NdNi5xCux compounds Emil Burzoa,, Traian Crainica,b, Manfred Neumannb, Liviu Chioncelc, Corina Lazara a
Faculty of Physics, Babes-Bolyai University, Str. M. Kogalniceanu Nr. 1, 400084 Cluj-Napoca RO-3400, Romania b Fachbereich Physik, Universita¨t Osnanbru¨ck, D-49069, Osnanbru¨ck, Germany c Department of Physics, University of Nijmigen 6500 GL, Nijmegen, The Netherlands Available online 7 January 2005
Abstract The NdNi5xCux compounds crystallize in a CaCu5-type structure for xp2: Band structure calculations show that the magnetic moments, at 0 K, for Ni (2c) and (3 g) sites decrease when increasing Ni content and are nil for x ffi 2: XPS measurements, at room temperature, show the presence of unfilled Ni3d band in the entire composition range, in agreement with paramagnetic studies. The magnetic behaviour of nickel is analysed in models which take into account the electron correlation effects in d bands. r 2004 Elsevier B.V. All rights reserved. PACS: 75.30.Cr; 75.50.Gg Keywords: Rare-earth compounds; Magnetic properties; Band structures; XPS studies
The RNi5 compounds, where R is a rare-earth, crystallize in a hexagonal structure of a CaCu5-type, having P6/mmm space group. The R atoms in this structure are located in 1a-type site while Ni occupies 2c and 3g positions. The compounds were intensively studied due to their ability to absorb hydrogen. The analysis of electronic structures and magnetic properties of RNi5xMx where M is a nonmagnetic element is also of interest [1]. Previously, we studied the electronic structures and magnetic properties of LaNi5xCux [2] and DyNi5xAlx [3] systems. In Cu-doped samples, separate Cu bands are formed at ffi3.3 eV below the Fermi level. A strong hybridization of Ni3d and Al3p bands was shown in an aluminium-substituted system. In this paper we analyse the effect of Cu substitution at Corresponding author. Tel.: +40264 403500; +40264 591906. E-mail address: [email protected] (E. Burzo).
fax:
Ni sites in the NdNi5xCux system. Unlike with heavy rare-earth compounds, at low temperatures, if Ni moment exists, it is believed to be parallely aligned to that of neodymium [1]. The NdNi5xCux compounds with xp2 were prepared in an induction furnace in purified argon atmosphere. The samples were thermally treated in vacuum at 900 1C for 1 week. The X-ray analyses show the presence of only one phase. The compounds crystallize in CaCu5type structure. Both a and c lattice parameters increase as copper content is higher. Magnetic measurements were performed in the temperature range 1.7–300 K and fields up to 9 T. The saturation magnetizations, Ms, were determined from magnetization isotherms according to approach to saturation law M ¼ Ms(1–a/H) by extrapolating the measured values at H1-0. The X-ray photoelectron spectroscopy (XPS) measurements were performed using a PHI 5600ci multitechnique system. The spectra were recorded using the monochromatized
0304-8853/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2004.11.263
ARTICLE IN PRESS E. Burzo et al. / Journal of Magnetism and Magnetic Materials 290–291 (2005) 371–373
12
6
0
-6
-12 -6
spin down
DOS (states/eV )
spin up
NdNi5
Nd-f NI(2c) Ni(3g) -4
-2
0 E-EF (eV )
2
d 4
Fig. 1. Projected densities of states of NdNi5.
6
150
2.5
x=0
2.4
x=2
x=0.5
2.3
x=0.5
2.1 2.0
x=1
x=2.0
2.2
5
6
7 8 µ0H (T)
100
9
50
0
100
200
χ−11 (emu/mol)-1
Ka radiation of Al (1486.6 eV). Binding energies are given with reference of Fermi level. Band structure calculations were performed by using the TB–LMTO method within LDA+U approach [4,5]. For nickel, values of average Coulomb interaction U ¼ 2 eV and exchange interaction J ¼ 0.9 eV, widely accepted [6], were used. For Nd values U ¼ 6.25 eV and J ¼ 1.25 eV were chosen to reproduce the estimated moment from experimental data. Relativistic effects, except spin–orbit coupling, were included. Convergence was reached within 106 Ry for a number of 216 k points in the Brillouin zone. The Cu atoms for x ¼ 1 were supposed to occupy the 3g sites while for higher Cu concentrations both 2c and 3g positions. The projected densities of states for compound with x ¼ 0 are plotted in Fig. 1. The magnetic moments of nickel at 2c and 3g sites in NdNi5 are 0.168 and 0.244 mB, respectively, parallely aligned to Nd moment. The moment at Ni(3g) site is higher than that at 2c site and can be correlated with a higher number of Nd as nearest neighbours. The strength of exchange interactions between nickel and neodymium atoms is more important than between those of nickel, the nickel moments being essentially induced by the presence of Nd. The substitution of Ni atoms by Cu has a strong effect on Ni moments. Both Ni(2c) and Ni(3g) moments decrease up to 0.056 mB/atom in NdNi4Cu. The Ni moments, for xX1; unlike NdNi5, are antiparallely oriented to Nd ones. We note that a negative-induced polarization has been already observed on highly delocalized 3d vanadium states in YFe2xVx system [7]. Delocalized Ni3d states are also expected to be present in NdNi4Cu, as suggested by very small Ni polarization. The Ni moments are practically nil in NdNi3Cu2 compound. Some magnetization isotherms obtained at 1.7 K are plotted in Fig. 2. The shape of magnetization isotherm for NdNi5 indicates the presence of high anisotropy. The coefficient of magnetic hardness decreases when Ni is
M (µΒ/f.u.)
372
0 300
T (K)
Fig. 2. Magnetization isotherms at 1.7 K and thermal variations of reciprocal susceptibilities.
replaced by Cu. A small discontinuity in magnetization curves has been seen between 7 and 8 T for samples having x ¼ 0 and 0.5. This may be correlated with a possible itinerant electron metamagnetic transition at some Ni sites. The saturation moments, Ms, per formula unit, at 1.7 K, obtained by extrapolation, are 2.83 mB for NdNi5 and decrease to 2.45 mB for x ¼ 1:5: The errors introduced by extrapolation procedure are of the order of 0.3 mB. The Ms values obtained by band structure calculation are higher by 0.6 and 0.4 mB for compounds with x ¼ 0 and xX1; respectively. The thermal variations of reciprocal susceptibilities, at T420 K, follow linear dependences. According to the addition law of magnetic susceptibilities and supposing that neodymium contribution to the Curie constants is the same as that of Nd3+ ion, we determined the effective nickel moments, Meff(Ni). The Meff(Ni) values decrease from ffi1.25 mB/atom for xp1:0 up to 0.6–0.8 mB/atom at higher copper content. The Ni 2p3/2 and 2p1/2 core levels for some NdNi5xCux compounds, at room temperature, are plotted in Fig. 3. For comparison, the Ni2p spectrum of pure nickel is also given. The positions of the 2p3/2 and 2p1/2 lines are situated at 852.570.1 and 869.770.1 eV, respectively, and are not changed when increasing Cu content. These lines are very close to those reported in pure nickel. In all spectra the presence of 6 eV Ni satellite at 858.570.1 eV can be seen. This suggests that there are unoccupied states in the Ni3d band. The XPS valence bands of some NdNi5xCux samples as well as that of pure nickel are plotted in Fig. 4. These are due to the superposition of Nd, Cu and Ni lines. The Nd lines are difficult to be analysed since of their low intensities and of the superposition with cooper ones. Although the valence band spectrum of NdNi5 is similar to that of pure nickel, the density of states at the Fermi level is diminished since of 5d–3d hybridisation. In addition, the valence band of Ni in NdNi5 is shifted to
ARTICLE IN PRESS E. Burzo et al. / Journal of Magnetism and Magnetic Materials 290–291 (2005) 371–373
Ni2P1/2
Intensity (arb.units)
NdNi3Cu2
Ni2P3/2
NdNi4Cu
NdNi5 Ni
890
880
870 860 Binding energy (eV)
850
840
Fig. 3. Ni 2p XPS spectra for NdNi5xCux system with x ¼ 0; 1 and 2 and of pure nickel.
Intensity (arb.units)
NdNi3cu2 NdNi4cu
NdNi5 Ni
20
15 10 5 Binding energy (eV)
0
373
an exchange field of the order of 30 T for the appearance of nickel moment at 0 K. Effective nickel moments are present in paramagnetic range even in compounds in which nickel moment, at 0 K, is nil. The presence of unfilled Ni3d states at room temperature are also confirmed by XPS measurements. The ratio r between the number of spins determined from Curie constants and those obtained at 0 K, increases from r ¼ 3 (x ¼ 0) to r ¼ 10 in NdNi4Cu compound. These data suggest that the magnetic behaviour of Ni can be analysed in models that take into account the electron correlation effects in d bands. These models reconcile the dual character of electron which, as a particle, requires a real space description and as a wave, a momentum space description. Thus, the spin fluctuation model [8] considers the balance between the frequencies of longitudinal spin fluctuations, which are determined by their life-time and of transverse fluctuations that are of thermal origin. The dynamical mean field theory [9] combined with standard LDA band calculations [10] can also describe the magnetic behaviour of Ni in the present compounds. In a strongly correlated system, leading Curie–Weiss behaviour, at high temperatures, is predicted. For an itinerant electron system, the time dependence of the correlation function results in temperature dependence of local spin, hS2loc i: Fluctuating moments and atomic-like configurations are large at short time scales. The moment is reduced at longer time scales, corresponding to a more less-correlated electronic structure near the Fermi level.
-5
Fig. 4. XPS valence band spectra of some NdNi5xCux samples and that of pure nickel.
higher energy. This may be correlated with an increase of the d-state occupancies. When substituting Ni by Cu, important changes in the valence band spectra can be shown. A rather independent Cu d band is formed around 3.4 eV binding energy. By increasing the Cu content, the relative intensities of Cu d bands increase but their positions and the linewidths are not changed with the composition. The Cu 3d states are filled, as previously reported in LaNi5xCux system [2]. The Ni in NdNi5xCux system, at low temperatures, shows a weak ordered moment for xp1:0 and practically nil for x ¼ 2:0: By analysing the exchange splitting of Ni3d states, obtained from band structure calculations, in correlation with nickel moments, we estimate
References [1] E. Burzo, A. Chelkovski, H.R. Kirckmayr, Landolt Bo¨rnstein Handbook, Vol. III/19d2, Springer, Berlin, 1990. [2] E. Burzo, S. Chiuzbaian, L. Chioncel, M. Neumann, J. Phys.: Cond. Matter 12 (2000) 5897. [3] E. Burzo, et al., J. Appl. Phys. 92 (2002) 7362. [4] A.I. Lichtenstein, J. Zaanen, V.I. Anisimov, Phys. Rev. B 52 (1995) R5467. [5] V.I. Anisimov, F. Aryasetiavan, A.I. Lichtenstein, J. Phys.: Condens. Matter 9 (1997) 767. [6] I. Yang, et al., Phys. Rev. Lett. 87 (2001) 216405. [7] R. Tetean, E. Burzo, L. Chioncel, O. Garlea, J. Magn. Magn. Mater. 242–243 (2002) 836. [8] T. Moriya, J. Magn. Magn. Mater. 100 (1991) 261. [9] A. Georges, G. Kothar, W. Krauth, M.J. Rosenberg, Rev. Mod. Phys. 68 (1996) 13. [10] A.I. Lichtenstein, M.I. Katsnelson, G. Kothar, Phys. Rev. Lett. 87 (2001) 067205.