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Magnetic properties and specific heat of Dy1−xLaxNi2 compounds
ARTICLE IN PRESS Journal of Magnetism and Magnetic Materials 321 (2009) 2821–2826
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Magnetic properties and specific heat of Dy1xLaxNi2 compounds J. C`wik a,b,, T. Palewski a, K. Nenkov a,b, J. Lyubina b, J. Warchulska a, J. Klamut a, O. Gutfleisch b a b
International Laboratory of High Magnetic Fields and Low Temperatures, Gajowicka 95, 53-421 Wroclaw, Poland IFW Dresden, Postfach 270016, 01171 Dresden, Germany
a r t i c l e in fo
abstract
Article history: Received 12 December 2008 Received in revised form 17 March 2009 Available online 16 April 2009
Magnetic and specific heat measurements have been carried out on polycrystalline series of singlephase Dy1xLaxNi2 (0pxp1) solid solutions. The compounds have a Laves-phase superstructure (space group F4¯3m) with the lattice parameter gradually increasing with decreasing Dy content. The samples with xp0.8 are ferromagnetic with the Curie temperature below 22 K. At high temperatures, all solid solutions are Curie–Weiss paramagnets. The Debye temperature, phonon and conduction electron contributions as well as a magnetic contribution to the heat capacity have been determined from specific heat measurements. The magnetocaloric effect was estimated from specific heat measurements performed in a magnetic field of 0.42 and 4.2 T. & 2009 Elsevier B.V. All rights reserved.
PACS: 61.66.Dk 75.60.Ej 75.40.s 75.80.+q Keywords: Rare earth intermetallics Magnetization Specific heat Magnetocaloric effect
1. Introduction Due to their unique physical properties, some of the RM2 compounds have found many modern applications such as permanent magnets, magneto-optical recording, magneto-acoustic materials, etc. The unique properties of the RM2 compounds are partly due to the rare-earth sublattice, which is characterized by the high magnetocrystalline anisotropy, magnetostriction and high magnetic moments, and partly due to the 3D-metal sublattice, which is responsible for high magnetic-ordering temperatures [1]. It has been reported that the RNi2 (R-rare-earth element) compounds crystallize in cubic C15 structure [1]. The majority of the RNi2 compounds crystallize in a cubic structure characterized by regular arrangement of vacancies at rare-earth sites, which stabilize these compounds in a structure derived from the ideal C15 cubic structure [2,3]. The ordering of the R vacancies on special lattice sites leads either to a tetragonal [4] or cubic superstructure [3,5,6]. At room temperature, the number of ordered R vacancies has been shown to decrease with decreasing atomic radius of the rare-earth element [6]. Order–disorder transitions of these R vacancies corresponding to a transition
Corresponding author at: International Laboratory of High Magnetic Fields and Low Temperatures, Gajowicka 95, 53-421 Wroclaw, Poland. E-mail address: [email protected] (J. C`wik).
0304-8853/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2009.04.014
from the cubic superstructure to the C15 cubic structure were observed at high temperature as well as at high pressures [7,8]. Both the transition pressure and temperature depend on the nature of the R element: e.g. the higher the atomic number of the rare earth the lower the transition pressure and temperature. The majority of the RNi2 compounds are ferromagnetic at low temperatures [1,9]. A distinctive feature of this series of compounds is the absence of the magnetic moment at nickel atoms. The magnetic interactions involve the R sublattice only. However, the existence of the residual magnetic moment on nickel resulting in an increase of the total magnetic moment has been reported [10,11]. Our recent study confirmed this assumption: the magnetic moments of poorly diluted Ho0.9La0.1Ni2, Ho0.85La0.15Ni2 [12], Ho0.8Lu0.2Ni2 [13] and Ho0.8Sc0.2Ni2 [14] solid solutions have been found to be higher than those due to holmium atoms only. In the R-Ni alloys, the main sources of the magnetic moment are electrons located in the unfilled f-level of lanthanide atoms. Moreover, the exchange interaction takes place between the localized 4f magnetic moments through the conduction electrons and produce the magnetic order at low temperatures. The crystalline electric field (CEF) and exchange interaction increase the degeneracy of the rare-earth ground multiplet. Depending on their relative magnitude, one or more CEF levels contribute to the magnetic ordering. The ferromagnetic order and relatively low Curie temperature are characteristic for DyNi2 [1]. Wallace et al. [9] reported the Curie temperature of DyNi2 of 32 K, whereas Markosyan observed the Curie
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temperature of 21 K [15]. Our recent magnetic susceptibility measurements on LaNi2 have shown that this compound exhibits the Pauli paramagnetism in the temperature range 2–298 K [16]. The purpose of the present work is to study the Dy1xLaxNi2 system over a wide composition range and to analyze the influence of the nonmagnetic La ions on physical properties of magnetically ordered DyNi2. The results are compared to other systems, in which the nonmagnetic La atoms substitute for holmium [12] and terbium [16].
2. Experimental details Polycrystalline samples of Dy1xLaxNi2 (x ¼ 0.0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8 and 1.0) solid solutions were prepared by repeated arc-melting of appropriate amounts of DyNi2 and LaNi2 in a purified argon atmosphere at a pressure of 1.5 atm. The purity of nickel and rare-earth metals was 99.99% and 99.9%, respectively. The alloys were remelted four times to ensure good homogeneity. The mass losses after the melting were less than 1 wt%. The buttons obtained were wrapped in a tantalum foil, sealed in an evacuated quartz ampoule and annealed at 450 1C for one month. The structure of the samples was characterized using the X-ray powder diffraction with Co Ka radiation. Magnetic measurements were carried out using a superconducting quantum interference device (SQUID) magnetometer (Quantum Design) in fields up to 5 T. The heat capacity was measured using Quantum Design PPMS 14 heat capacity system in a temperature range of 2–295 K without the magnetic field and in a magnetic field of 0.42 T. For samples with x ¼ 0.2 and0.5, the magnetic fields up to 4.2 T has been used.
3. Results and discussion 3.1. Structural investigation The X-ray diffraction patterns were collected at room temperature for all the Dy1xLaxNi2 solid solutions. As an example, the X–ray diffraction patterns for Dy0.2La0.8Ni2 are shown in Fig. 1. The fundamental reflections in the X-ray diffraction patterns of Dy1xLaxNi2 correspond to those observed in the C15-type structure (space group Fd3m). However, in the experimental
Fig. 1. The X-ray diffraction patterns for (a) theoretical C15-type superstructure (space group F4¯3m) and (b) Dy0.2La0.8Ni2. Asterisks indicate the angular positions of reflections corresponding to the C 15-type structure.
Fig. 2. Lattice parameter dependence for the Dy1xLaxNi2 solid solutions on the chemical composition.
patterns, several relatively weak reflections not belonging to C15 have been observed. Earlier, it was shown for the Ho1xAxNi2 (A ¼ Sc, Y, La and Lu) and Tb1xLaxNi2 systems that the adequate description of the atomic position parameters can be obtained within space group F4¯3m. The lattice parameter of the structure is twice as large as that reported for the RNi2 Laves phase. The lattice parameter refinement was performed using a least-squares method that yields the lattice parameter a ¼ 1.432 and 1.472 nm for DyNi2 and LaNi2, respectively. The substitution of lanthanum for dysprosium in the Dy1xLaxNi2 solid solutions leads to an increase in the lattice parameter (Fig. 2). This behavior is similar to that in the Tb1xLaxNi2 [16] and Ho1xLaxNi2 [12] systems, where the lattice parameter increases as the La content increases. For all the systems, the compositional dependence exhibits the expected linear behavior and obeys the Vegard’s law. 3.2. Magnetic measurements Table 1 presents the magnetic data of the Dy1xLaxNi2 (0pxp1) compounds. In the samples with xp0.8, the ferromagnetic order with a relatively low Curie temperature has been observed. The magnetization curves measured at 5 K for the samples with various lanthanum contents are shown in Fig. 3. For all the studied samples, similarly to Tb1xLaxNi2 [16] and Ho1xLaxNi2 [12], no saturation is reached even in a maximum applied magnetic field of 5 T. The values of saturation magnetic moment (mS) per dysprosium atom were determined by extrapolation to m0H ¼ 0, using the magnetization value observed in a magnetic field of 5 T. The mS values (Table 1) are well below gJ ¼ 10 mB, which is expected for an assembly of free Dy3+ ions. The difference between the expected and the experimental mS values is the lowest in DyNi2 (1.6 mB) and the highest in the case of Dy0.2La0.8Ni2 (3.5 mB). The large difference can be caused by the quenching effect of the crystal field. Apparently, higher magnetic fields are required to saturate the samples. In our previous experiments on Tb1xLaxNi2 [16], the values of saturation magnetic moment per terbium atom, with exception of TbNi2, were considerably lower. Only in the case of the Ho1xLaxNi2 [12] system for x ¼ 0.0, 0.1 and 0.15, the mS values were slightly higher than expected for the pure Ho metal. The relatively low saturation values are apparently connected to the presence of antiferromagnetic components, which naturally would account for low saturation moments [17]. Neutron diffraction is necessary to confirm our assumption.
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Table 1 Magnetic characteristics of Dy1xLaxNi2 solid solutions. Ordered state Compound
DyNi2 Dy0.9La0.1Ni2 Dy0.8La0.2Ni2 Dy0.7La0.3Ni2 Dy0.6La0.4Ni2 Dy0.5La0.5Ni2 Dy0.4La0.6Ni2 Dy0.3La0.7Ni2 Dy0.2La0.8Ni2 LaNi2
Paramagnetic state
mS (mB) T ¼ 5 K; moH ¼ 5 T
TC
(mB/f.u.)
(mB/Dy)
(K)
8.4 6.8 6.0 5.5 4.5 3.6 3.1 2.2 1.3 Pauli paramagnetic
8.4 7.6 7.5 7.9 7.5 7.2 7.8 7.3 6.5
21.3 20.2 19.1 16.5 14.3 12.2 10.1 7.2 6.2
Fig. 3. Magnetization curves measured at 5 K for the Dy1xLaxNi2 (x ¼ 0–1.0) solid solutions.
Type
C–W C–W C–W C–W C–W C–W C–W C–W C–W
YP
meff (mB) m0H ¼ 0.42 T
(K)
(mB/Dy)
26.7 25.3 20.4 17.8 15.7 11.2 8.3 4.5 1.9
10.6 10.4 10.5 10.5 10.4 10.3 10.6 10.6 10.5
the ferromagnetic temperature region is slightly higher in comparison with x ¼ 0.0. Similar behavior is observed for x ¼ 0.2 and x ¼ 0.3. The inverse magnetic susceptibility in a temperature range of 50–290 K is a linear function of the temperature that obeys the Curie–Weiss law. The paramagnetic Curie temperature yP of the DyNi2 compound is equal to 26.7 K and yP of solid solutions decreases monotonously with increasing lanthanum content down to 1.9 K for x ¼ 0.8. The very weak temperature dependence of the magnetic susceptibility of the nonmagnetic LaNi2 compound is close to the temperatureindependent paramagnetism. At room temperature, the magnetic susceptibility wg of LaNi2 is 2.9 106 cm3 g1, which is close to that found for LuNi2 2.8 106 cm3 g1 [13]. The values of the effective paramagnetic moment meff were calculated from the Curie–Weiss constant. For all the solid values are very close to the value solutions under study, the mDy eff mDy ¼ 10:60 mB found for the dysprosium atom in pure Dy metal eff (Table 1). The ferromagnetic Curie temperatures TC (Table 1) were determined using the temperature position of the maximum in the qw(T)/qT plots. The substitution of La for Dy in the Dy1xLaxNi2 solid solutions leads to a decrease in the magnetic-ordering temperature. The Curie temperature of DyNi2 is equal to 21.3 K; in the case of Dy0.2La0.8Ni2 the magnetic ordering is observed below 6.2 K. It should be noted that the increased amount of the lanthanum in the Dy1xLaxNi2 solid solutions weakens the ferromagnetic interaction between the dysprosium atoms not so drastically as lanthanum in the Ho1xLaxNi2 system [12]. In the case of the Ho1xLaxNi2 solid solutions, the ferromagnetic order of the holmium atoms disappears after the substitution of 80 at% of holmium by lanthanum, whereas in the Dy1xLaxNi2 and Tb1xLaxNi2 systems, the magnetic order is present in the solid solutions containing up to 80 at% of the nonmagnetic lanthanum.
3.3. Heat capacity
Fig. 4. Temperature dependence of the magnetic susceptibility of the Dy1xLaxNi2 (x ¼ 0–1.0) solid solutions measured in an applied magnetic field of 0.42 T.
The temperature dependence of the magnetic susceptibility wg measured in a static magnetic field of 0.42 T is shown in Fig. 4. All the solid solutions under study and DyNi2 are magnetically ordered below 22 K. For the sample with x ¼ 0.1, the value of wg in
The results of heat capacity measurements for the solid solutions Dy1xLaxNi2 with xp0.5 are presented in Table 2. Sharp l-like maxima at the heat capacity curves (Figs. 5 and 6) correspond to the magnetic ordering at TC. The values of TC are summarized in Table 2 and are close to those evaluated from the magnetic measurements (Table 1). The relatively low Curie temperature does not allow accurate determination of the electronic heat capacity coefficient g and Debye temperature YD in the low temperature range. Using the heat capacity data of LaNi2 as the lattice contribution, similarly as
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Table 2 Some physical data for Dy1xLaxNi2 solid solutions from heat capacity measurements. Compound
g (mJ/molK2)
TC (K)
yD (K)
DTad m0H ¼ 0.42 T (K)
DTad m0H ¼ 4.2 T (K)
DyNi2 Dy0.9La0.1Ni2 Dy0.8La0.2Ni2 Dy0.7La0.3Ni2 Dy0.6La0.4Ni2 Dy0.5La0.5Ni2
17.0 17.0 17.0 17.0 17.0 17.0
21.2 19.7 18.4 16.3 14.2 12.2
250 251 252 253 255 257
0.55 – 0.5 – 0.45 0.35
– – 4 – – 2
Fig. 5. Total heat capacity Ctot(T) of Dy0.5La0.5Ni2 measured in zero magnetic field. The insets show a low-temperature portion of the Ctot(T) curve measured in zero, 0.42 and 4.2 T magnetic field.
Fig. 6. Total heat capacity Ctot(T) of Dy0.8La0.2Ni2 measured in zero magnetic field. The insets show a low-temperature portion of the Ctot(T) curve measured in zero, 0.42 and 4.2 T magnetic field.
in the Ho1xLaxNi2 solid solutions [12], yields a negative magnetic heat capacity. In order to estimate the magnetic contribution, a theoretical calculation of the Debye function was made [16]. The Debye temperatures and g values were calculated using the Debye function in the temperature range of 2–300 K and are shown in Table 2. The best fit for the wide temperature range could be obtained by fixing the parameters g ¼ 17 mJ/mol K2 for all the measured samples, while the Debye temperature was
Fig. 7. The low-temperature (below 100 K) Cmag versus T dependence of the Dy1xLaxNi2 solid solutions measured in zero magnetic field.
Fig. 8. The low-temperature (below 100 K) Cmag versus T dependence of the Dy1xLaxNi2 solid solutions measured in 0.42 T magnetic field.
increased as the La content increases, similarly to the Ho1xLaxNi2 system [12]. As an example, the temperature dependence of the total heat capacity Ctot(T), the sum of electron and phonon Cel+ph(T) and magnetic contributions Cmag(T) for the solid solutions Dy0.5La0.5 Ni2 and Dy0.8La0.2Ni2 measured in zero magnetic field are shown in Figs. 5 and 6, respectively. The insets of Figs. 5 and 6 show the low-temperature dependence Ctot(T) measured in zero, 0.42 T and 4.2 T magnetic field. In the case of Dy0.8La0.2Ni2 (Fig. 6), a welldefined l-like anomaly observed at 18.4 K corresponds to the
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magnetic ordering. For Dy0.5La0.5Ni2 a similar anomaly is observed at 12.2 K. Heat capacity measurements in a magnetic field (the insets of Figs. 5 and 6) show that the magnetic field causes the broadening and displacement of the maximum. The amplitude of the maximum decrease with increasing the magnetic field and it eventually disappears at 4.2 T. Figs. 7 and 8 show Cmag(T) versus T of the Dy1xLaxNi2 solid solutions (x ¼ 0.0, 0.1, 0.2, 0.3, 0.4 and 0.5) measured in zero and 0.42 T magnetic field, accordingly. These dependences are characterized by maxima observed in the temperature range near the Curie temperature. For the solid solutions with x ¼ 0.0, 0.1, 0.2, 0.3, 0.4 and 0.5, the maxima are observed at 18, 19, 17, 15, 13 and 11 K, respectively. The temperature dependence of the heat capacity measured in a magnetic field of 0.42 T shows that the field leads to a decreasing in the amplitude of the maximum. For DyNi2, Dy0.4La0.6Ni2 and Dy0.5La0.5Ni2 in the paramagnetic region, a broad maximum has been observed, which apparently corresponds to a Schottky anomaly that is caused by a crystal field splitting of the Dy3+ ground state level. The Dy ions have a large orbital moment L ¼ 5 and, therefore, not only the lowsymmetry structure, but also the local anisotropy of Dy ions gives rise to the crystal field effect.
Fig. 9. Magnetic part of the entropy Smag of Dy1xLaxNi2 (x ¼ 0–0.5) measured in zero magnetic field. The dotted lines indicate the (1x) Rln (16) limits.
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Fig. 11. Temperature dependence of DTad of Dy1xLaxNi2 (x ¼ 0.2 and 0.5) measured in an applied magnetic field of 4.2 T.
The magnetic part of the entropy Smag(T) shown in Fig. 9 was calculated by integrating Cmag(T)/T. The entropy does not reach the theoretical maximum Smax ¼ Rln(16) at the Curie temperature. Moreover, a noticeable magnetic contribution to the heat capacity persists up to at least 200 K, which is apparently due to the existence of the spin fluctuations far above the magnetic-ordering temperature. The magnetocaloric effect was calculated by a method suggested by von Ranke et al. [18]. Fig. 10 shows the temperature dependence of the adiabatic temperature change DTad for Dy1xLaxNi2 (x ¼ 0.5, 0.4, 0.2 and 0) measured at a magnetic field of 0.42 T. The magnetocaloric effect in 0.42 T is moderate. The maximum DTad reaches about 0.55 K near 22 K for DyNi2, DTadE0.5 K near 18 K for Dy0.8La0.2Ni2, DTadE0.45 K near 14 K for Dy0.6La0.4Ni2 and DTadE0.35 K near 12 K for Dy0.5La0.5Ni2. Fig. 11 shows the temperature dependence of DTad for Dy1xLaxNi2 (x ¼ 0.5 and 0.2) measured in an applied magnetic field of 4.2 T. In this case the maximum magnetocaloric effect reaches about 4 K for Dy0.8La0.2Ni2 near 18 and 2 K for Dy0.5La0.5Ni2 near 12 K. The sharp peaks in the curves are related to the paramagnetic– ferromagnetic phase transitions observed in the vicinity of the Curie temperature, since in the vicinity of TC the maximum magnetization difference is observed. For solid solutions with xX0.6, the ordering temperatures are too low to allow for accurate estimation of the magnetocaloric effect.
4. Conclusions
Fig. 10. Temperature dependence of DTad of Dy1xLaxNi2 (x ¼ 0, 0.2, 0.4 and 0.5) measured in an applied magnetic field of 0.42 T.
The effect of the substitution of Dy for La on the structure and physical properties of the Dy1xLaxNi2 solid solutions has been studied. It was confirmed that the crystal lattice of the compounds, similarly to Tb1xLaxNi2 [16] and Ho1xLaxNi2 [12], can be adequately described by the superstructure derived from C15-type (space group F4¯3m) Laves-phase structure with the doubled lattice parameter a. The latter increases with increasing La content. The observed linearity of the compositional dependence of the lattice parameter confirms the formation of the substitutional solid solutions. The substitution of magnetic Dy for nonmagnetic La results in the common magnetic dilution accompanied by marked modification of the magnetic behavior of the Dy1xLaxNi2 solid solutions. On the substitution, the exchange interactions are weakened leading to a decrease in the ordering temperature, as
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observed both by the magnetization and heat capacity measurements. The Curie temperature decreases from 21.3 K for DyNi2 to 6.2 K for Dy0.2La0.8Ni2. The ferromagnetic interaction is reduced to a lesser extent, as compared to that in the La-substituted Ho1xLaxNi2 system [12]. The magnetic part of the heat capacity showed the presence of the Schottky anomaly phenomenon. The moderate magnetocaloric effect in a magnetic field of 0.42 and 4.2 T is not as attractive for magnetic cooling as in the case of the Ho1xLaxNi2 or Tb1xLaxNi2 systems. The maximum magnetocaloric effect DTad reaches about 4 K near 18 K for Dy0.8La0.2Ni2 and 2 K near 12 K for Dy0.5La0.5Ni2 for a field change of 4.2 T. Acknowledgement Financial support by the Deutsche Forschungsgemeinschaft (SFB 463) is gratefully acknowledged. References [1] H.R. Kirchmayer, E. Burzo, in: H.P.J. Wijn (Ed.), Landolt–Bo¨rnstein, New Series III/19d2, Berlin, 1990.
[2] V. Paul-Boncour, A. Percheron-Guegan, M. Diaf, J.C. Achard, J. Less-Common Met. 131 (1987) 201. [3] M. Latroche, V. Paul-Boncour, A. Percheron-Guegan, J.C. Achard, J. LessCommon Met. 161 (1990) L27. [4] A.V. Klimyenko, J. Seuntjens, L.L. Miller, B.J. Beaudry, R.A. Jacobson, K.A. Gschneidner Jr., J. Less-Common Met. 144 (1988) 133. [5] M. Latroche, V. Paul-Boncour, A. Percheron-Guegan, J.C. Achard, Eur. J. Solid State Inorg. Chem. 28 (1991) 597. [6] M. Latroche, V. Paul-Boncour, A. Percheron-Guegan, Z. Phys. Chem. 179 (1993) 261. [7] E. Gratz, A. Kottar, A. Lindbaum, M. Mantler, M. Latroche, V. Paul-Boncour, J. Phys. Condens. Matter 8 (1996) 8351. [8] A. Lindbaum, E. Gratz, S. Heathman, Phys. Rev. B 65 (2002) 134114. [9] E.A. Skrabek, W.E. Wallace, J. Appl. Phys. 34 (1963) 1356. [10] R. Malik, P.L. Paulose, E.V. Sampathkumatan, S. Patil, V. Nagarajan, Phys. Rev. B 55 (1997) 8369. [11] M. Mizumaki, K. Yano, I. Umehara, F. Ishikawa, K. Sato, N. Sakai, T. Muro, Phys. Rev. B 67 (2003) 132404. [12] J. C`wik, T. Palewski, K. Nenkov, G.S. Burkhanov, J. Alloys Compd. 399 (2005) 7. [13] J. C`wik, T. Palewski, K. Nenkov, G.S. Burkhanov, O.D. Chistyakov, N. Kolchugina, H. Ma¨dge, Physica B 358 (2005) 323. [14] J. C`wik, T. Palewski, K. Nenkov, G.S. Burkhanov, O.D. Chistyakov, J. Warchulska, Phys. Status Solidi B 242/10 (2005) 1969. [15] A.S. Markosyan, Fiz. Tverd. Tela 23 (1981) 1153. [16] J. C`wik, T. Palewski, K. Nenkov, G.S. Burkhanov, O.D. Chistyakov, J. Klamut, J. Warchulska, J. Alloys Compd. 460 (2008) 41. [17] H. Oesterreicher, J. Stanley, R. Pitts, Phys. Status Solidi A 12 (1972) K65. [18] P.J. von Ranke, D.F. Grangeia, A. Caldas, N.A. de Oliveira, J. Appl. Phys. 93 (2003) 4055.
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Magnetic properties and specific heat of Dy1xLaxNi2 compounds J. C`wik a,b,, T. Palewski a, K. Nenkov a,b, J. Lyubina b, J. Warchulska a, J. Klamut a, O. Gutfleisch b a b
International Laboratory of High Magnetic Fields and Low Temperatures, Gajowicka 95, 53-421 Wroclaw, Poland IFW Dresden, Postfach 270016, 01171 Dresden, Germany
a r t i c l e in fo
abstract
Article history: Received 12 December 2008 Received in revised form 17 March 2009 Available online 16 April 2009
Magnetic and specific heat measurements have been carried out on polycrystalline series of singlephase Dy1xLaxNi2 (0pxp1) solid solutions. The compounds have a Laves-phase superstructure (space group F4¯3m) with the lattice parameter gradually increasing with decreasing Dy content. The samples with xp0.8 are ferromagnetic with the Curie temperature below 22 K. At high temperatures, all solid solutions are Curie–Weiss paramagnets. The Debye temperature, phonon and conduction electron contributions as well as a magnetic contribution to the heat capacity have been determined from specific heat measurements. The magnetocaloric effect was estimated from specific heat measurements performed in a magnetic field of 0.42 and 4.2 T. & 2009 Elsevier B.V. All rights reserved.
PACS: 61.66.Dk 75.60.Ej 75.40.s 75.80.+q Keywords: Rare earth intermetallics Magnetization Specific heat Magnetocaloric effect
1. Introduction Due to their unique physical properties, some of the RM2 compounds have found many modern applications such as permanent magnets, magneto-optical recording, magneto-acoustic materials, etc. The unique properties of the RM2 compounds are partly due to the rare-earth sublattice, which is characterized by the high magnetocrystalline anisotropy, magnetostriction and high magnetic moments, and partly due to the 3D-metal sublattice, which is responsible for high magnetic-ordering temperatures [1]. It has been reported that the RNi2 (R-rare-earth element) compounds crystallize in cubic C15 structure [1]. The majority of the RNi2 compounds crystallize in a cubic structure characterized by regular arrangement of vacancies at rare-earth sites, which stabilize these compounds in a structure derived from the ideal C15 cubic structure [2,3]. The ordering of the R vacancies on special lattice sites leads either to a tetragonal [4] or cubic superstructure [3,5,6]. At room temperature, the number of ordered R vacancies has been shown to decrease with decreasing atomic radius of the rare-earth element [6]. Order–disorder transitions of these R vacancies corresponding to a transition
Corresponding author at: International Laboratory of High Magnetic Fields and Low Temperatures, Gajowicka 95, 53-421 Wroclaw, Poland. E-mail address: [email protected] (J. C`wik).
0304-8853/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2009.04.014
from the cubic superstructure to the C15 cubic structure were observed at high temperature as well as at high pressures [7,8]. Both the transition pressure and temperature depend on the nature of the R element: e.g. the higher the atomic number of the rare earth the lower the transition pressure and temperature. The majority of the RNi2 compounds are ferromagnetic at low temperatures [1,9]. A distinctive feature of this series of compounds is the absence of the magnetic moment at nickel atoms. The magnetic interactions involve the R sublattice only. However, the existence of the residual magnetic moment on nickel resulting in an increase of the total magnetic moment has been reported [10,11]. Our recent study confirmed this assumption: the magnetic moments of poorly diluted Ho0.9La0.1Ni2, Ho0.85La0.15Ni2 [12], Ho0.8Lu0.2Ni2 [13] and Ho0.8Sc0.2Ni2 [14] solid solutions have been found to be higher than those due to holmium atoms only. In the R-Ni alloys, the main sources of the magnetic moment are electrons located in the unfilled f-level of lanthanide atoms. Moreover, the exchange interaction takes place between the localized 4f magnetic moments through the conduction electrons and produce the magnetic order at low temperatures. The crystalline electric field (CEF) and exchange interaction increase the degeneracy of the rare-earth ground multiplet. Depending on their relative magnitude, one or more CEF levels contribute to the magnetic ordering. The ferromagnetic order and relatively low Curie temperature are characteristic for DyNi2 [1]. Wallace et al. [9] reported the Curie temperature of DyNi2 of 32 K, whereas Markosyan observed the Curie
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temperature of 21 K [15]. Our recent magnetic susceptibility measurements on LaNi2 have shown that this compound exhibits the Pauli paramagnetism in the temperature range 2–298 K [16]. The purpose of the present work is to study the Dy1xLaxNi2 system over a wide composition range and to analyze the influence of the nonmagnetic La ions on physical properties of magnetically ordered DyNi2. The results are compared to other systems, in which the nonmagnetic La atoms substitute for holmium [12] and terbium [16].
2. Experimental details Polycrystalline samples of Dy1xLaxNi2 (x ¼ 0.0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8 and 1.0) solid solutions were prepared by repeated arc-melting of appropriate amounts of DyNi2 and LaNi2 in a purified argon atmosphere at a pressure of 1.5 atm. The purity of nickel and rare-earth metals was 99.99% and 99.9%, respectively. The alloys were remelted four times to ensure good homogeneity. The mass losses after the melting were less than 1 wt%. The buttons obtained were wrapped in a tantalum foil, sealed in an evacuated quartz ampoule and annealed at 450 1C for one month. The structure of the samples was characterized using the X-ray powder diffraction with Co Ka radiation. Magnetic measurements were carried out using a superconducting quantum interference device (SQUID) magnetometer (Quantum Design) in fields up to 5 T. The heat capacity was measured using Quantum Design PPMS 14 heat capacity system in a temperature range of 2–295 K without the magnetic field and in a magnetic field of 0.42 T. For samples with x ¼ 0.2 and0.5, the magnetic fields up to 4.2 T has been used.
3. Results and discussion 3.1. Structural investigation The X-ray diffraction patterns were collected at room temperature for all the Dy1xLaxNi2 solid solutions. As an example, the X–ray diffraction patterns for Dy0.2La0.8Ni2 are shown in Fig. 1. The fundamental reflections in the X-ray diffraction patterns of Dy1xLaxNi2 correspond to those observed in the C15-type structure (space group Fd3m). However, in the experimental
Fig. 1. The X-ray diffraction patterns for (a) theoretical C15-type superstructure (space group F4¯3m) and (b) Dy0.2La0.8Ni2. Asterisks indicate the angular positions of reflections corresponding to the C 15-type structure.
Fig. 2. Lattice parameter dependence for the Dy1xLaxNi2 solid solutions on the chemical composition.
patterns, several relatively weak reflections not belonging to C15 have been observed. Earlier, it was shown for the Ho1xAxNi2 (A ¼ Sc, Y, La and Lu) and Tb1xLaxNi2 systems that the adequate description of the atomic position parameters can be obtained within space group F4¯3m. The lattice parameter of the structure is twice as large as that reported for the RNi2 Laves phase. The lattice parameter refinement was performed using a least-squares method that yields the lattice parameter a ¼ 1.432 and 1.472 nm for DyNi2 and LaNi2, respectively. The substitution of lanthanum for dysprosium in the Dy1xLaxNi2 solid solutions leads to an increase in the lattice parameter (Fig. 2). This behavior is similar to that in the Tb1xLaxNi2 [16] and Ho1xLaxNi2 [12] systems, where the lattice parameter increases as the La content increases. For all the systems, the compositional dependence exhibits the expected linear behavior and obeys the Vegard’s law. 3.2. Magnetic measurements Table 1 presents the magnetic data of the Dy1xLaxNi2 (0pxp1) compounds. In the samples with xp0.8, the ferromagnetic order with a relatively low Curie temperature has been observed. The magnetization curves measured at 5 K for the samples with various lanthanum contents are shown in Fig. 3. For all the studied samples, similarly to Tb1xLaxNi2 [16] and Ho1xLaxNi2 [12], no saturation is reached even in a maximum applied magnetic field of 5 T. The values of saturation magnetic moment (mS) per dysprosium atom were determined by extrapolation to m0H ¼ 0, using the magnetization value observed in a magnetic field of 5 T. The mS values (Table 1) are well below gJ ¼ 10 mB, which is expected for an assembly of free Dy3+ ions. The difference between the expected and the experimental mS values is the lowest in DyNi2 (1.6 mB) and the highest in the case of Dy0.2La0.8Ni2 (3.5 mB). The large difference can be caused by the quenching effect of the crystal field. Apparently, higher magnetic fields are required to saturate the samples. In our previous experiments on Tb1xLaxNi2 [16], the values of saturation magnetic moment per terbium atom, with exception of TbNi2, were considerably lower. Only in the case of the Ho1xLaxNi2 [12] system for x ¼ 0.0, 0.1 and 0.15, the mS values were slightly higher than expected for the pure Ho metal. The relatively low saturation values are apparently connected to the presence of antiferromagnetic components, which naturally would account for low saturation moments [17]. Neutron diffraction is necessary to confirm our assumption.
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Table 1 Magnetic characteristics of Dy1xLaxNi2 solid solutions. Ordered state Compound
DyNi2 Dy0.9La0.1Ni2 Dy0.8La0.2Ni2 Dy0.7La0.3Ni2 Dy0.6La0.4Ni2 Dy0.5La0.5Ni2 Dy0.4La0.6Ni2 Dy0.3La0.7Ni2 Dy0.2La0.8Ni2 LaNi2
Paramagnetic state
mS (mB) T ¼ 5 K; moH ¼ 5 T
TC
(mB/f.u.)
(mB/Dy)
(K)
8.4 6.8 6.0 5.5 4.5 3.6 3.1 2.2 1.3 Pauli paramagnetic
8.4 7.6 7.5 7.9 7.5 7.2 7.8 7.3 6.5
21.3 20.2 19.1 16.5 14.3 12.2 10.1 7.2 6.2
Fig. 3. Magnetization curves measured at 5 K for the Dy1xLaxNi2 (x ¼ 0–1.0) solid solutions.
Type
C–W C–W C–W C–W C–W C–W C–W C–W C–W
YP
meff (mB) m0H ¼ 0.42 T
(K)
(mB/Dy)
26.7 25.3 20.4 17.8 15.7 11.2 8.3 4.5 1.9
10.6 10.4 10.5 10.5 10.4 10.3 10.6 10.6 10.5
the ferromagnetic temperature region is slightly higher in comparison with x ¼ 0.0. Similar behavior is observed for x ¼ 0.2 and x ¼ 0.3. The inverse magnetic susceptibility in a temperature range of 50–290 K is a linear function of the temperature that obeys the Curie–Weiss law. The paramagnetic Curie temperature yP of the DyNi2 compound is equal to 26.7 K and yP of solid solutions decreases monotonously with increasing lanthanum content down to 1.9 K for x ¼ 0.8. The very weak temperature dependence of the magnetic susceptibility of the nonmagnetic LaNi2 compound is close to the temperatureindependent paramagnetism. At room temperature, the magnetic susceptibility wg of LaNi2 is 2.9 106 cm3 g1, which is close to that found for LuNi2 2.8 106 cm3 g1 [13]. The values of the effective paramagnetic moment meff were calculated from the Curie–Weiss constant. For all the solid values are very close to the value solutions under study, the mDy eff mDy ¼ 10:60 mB found for the dysprosium atom in pure Dy metal eff (Table 1). The ferromagnetic Curie temperatures TC (Table 1) were determined using the temperature position of the maximum in the qw(T)/qT plots. The substitution of La for Dy in the Dy1xLaxNi2 solid solutions leads to a decrease in the magnetic-ordering temperature. The Curie temperature of DyNi2 is equal to 21.3 K; in the case of Dy0.2La0.8Ni2 the magnetic ordering is observed below 6.2 K. It should be noted that the increased amount of the lanthanum in the Dy1xLaxNi2 solid solutions weakens the ferromagnetic interaction between the dysprosium atoms not so drastically as lanthanum in the Ho1xLaxNi2 system [12]. In the case of the Ho1xLaxNi2 solid solutions, the ferromagnetic order of the holmium atoms disappears after the substitution of 80 at% of holmium by lanthanum, whereas in the Dy1xLaxNi2 and Tb1xLaxNi2 systems, the magnetic order is present in the solid solutions containing up to 80 at% of the nonmagnetic lanthanum.
3.3. Heat capacity
Fig. 4. Temperature dependence of the magnetic susceptibility of the Dy1xLaxNi2 (x ¼ 0–1.0) solid solutions measured in an applied magnetic field of 0.42 T.
The temperature dependence of the magnetic susceptibility wg measured in a static magnetic field of 0.42 T is shown in Fig. 4. All the solid solutions under study and DyNi2 are magnetically ordered below 22 K. For the sample with x ¼ 0.1, the value of wg in
The results of heat capacity measurements for the solid solutions Dy1xLaxNi2 with xp0.5 are presented in Table 2. Sharp l-like maxima at the heat capacity curves (Figs. 5 and 6) correspond to the magnetic ordering at TC. The values of TC are summarized in Table 2 and are close to those evaluated from the magnetic measurements (Table 1). The relatively low Curie temperature does not allow accurate determination of the electronic heat capacity coefficient g and Debye temperature YD in the low temperature range. Using the heat capacity data of LaNi2 as the lattice contribution, similarly as
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Table 2 Some physical data for Dy1xLaxNi2 solid solutions from heat capacity measurements. Compound
g (mJ/molK2)
TC (K)
yD (K)
DTad m0H ¼ 0.42 T (K)
DTad m0H ¼ 4.2 T (K)
DyNi2 Dy0.9La0.1Ni2 Dy0.8La0.2Ni2 Dy0.7La0.3Ni2 Dy0.6La0.4Ni2 Dy0.5La0.5Ni2
17.0 17.0 17.0 17.0 17.0 17.0
21.2 19.7 18.4 16.3 14.2 12.2
250 251 252 253 255 257
0.55 – 0.5 – 0.45 0.35
– – 4 – – 2
Fig. 5. Total heat capacity Ctot(T) of Dy0.5La0.5Ni2 measured in zero magnetic field. The insets show a low-temperature portion of the Ctot(T) curve measured in zero, 0.42 and 4.2 T magnetic field.
Fig. 6. Total heat capacity Ctot(T) of Dy0.8La0.2Ni2 measured in zero magnetic field. The insets show a low-temperature portion of the Ctot(T) curve measured in zero, 0.42 and 4.2 T magnetic field.
in the Ho1xLaxNi2 solid solutions [12], yields a negative magnetic heat capacity. In order to estimate the magnetic contribution, a theoretical calculation of the Debye function was made [16]. The Debye temperatures and g values were calculated using the Debye function in the temperature range of 2–300 K and are shown in Table 2. The best fit for the wide temperature range could be obtained by fixing the parameters g ¼ 17 mJ/mol K2 for all the measured samples, while the Debye temperature was
Fig. 7. The low-temperature (below 100 K) Cmag versus T dependence of the Dy1xLaxNi2 solid solutions measured in zero magnetic field.
Fig. 8. The low-temperature (below 100 K) Cmag versus T dependence of the Dy1xLaxNi2 solid solutions measured in 0.42 T magnetic field.
increased as the La content increases, similarly to the Ho1xLaxNi2 system [12]. As an example, the temperature dependence of the total heat capacity Ctot(T), the sum of electron and phonon Cel+ph(T) and magnetic contributions Cmag(T) for the solid solutions Dy0.5La0.5 Ni2 and Dy0.8La0.2Ni2 measured in zero magnetic field are shown in Figs. 5 and 6, respectively. The insets of Figs. 5 and 6 show the low-temperature dependence Ctot(T) measured in zero, 0.42 T and 4.2 T magnetic field. In the case of Dy0.8La0.2Ni2 (Fig. 6), a welldefined l-like anomaly observed at 18.4 K corresponds to the
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magnetic ordering. For Dy0.5La0.5Ni2 a similar anomaly is observed at 12.2 K. Heat capacity measurements in a magnetic field (the insets of Figs. 5 and 6) show that the magnetic field causes the broadening and displacement of the maximum. The amplitude of the maximum decrease with increasing the magnetic field and it eventually disappears at 4.2 T. Figs. 7 and 8 show Cmag(T) versus T of the Dy1xLaxNi2 solid solutions (x ¼ 0.0, 0.1, 0.2, 0.3, 0.4 and 0.5) measured in zero and 0.42 T magnetic field, accordingly. These dependences are characterized by maxima observed in the temperature range near the Curie temperature. For the solid solutions with x ¼ 0.0, 0.1, 0.2, 0.3, 0.4 and 0.5, the maxima are observed at 18, 19, 17, 15, 13 and 11 K, respectively. The temperature dependence of the heat capacity measured in a magnetic field of 0.42 T shows that the field leads to a decreasing in the amplitude of the maximum. For DyNi2, Dy0.4La0.6Ni2 and Dy0.5La0.5Ni2 in the paramagnetic region, a broad maximum has been observed, which apparently corresponds to a Schottky anomaly that is caused by a crystal field splitting of the Dy3+ ground state level. The Dy ions have a large orbital moment L ¼ 5 and, therefore, not only the lowsymmetry structure, but also the local anisotropy of Dy ions gives rise to the crystal field effect.
Fig. 9. Magnetic part of the entropy Smag of Dy1xLaxNi2 (x ¼ 0–0.5) measured in zero magnetic field. The dotted lines indicate the (1x) Rln (16) limits.
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Fig. 11. Temperature dependence of DTad of Dy1xLaxNi2 (x ¼ 0.2 and 0.5) measured in an applied magnetic field of 4.2 T.
The magnetic part of the entropy Smag(T) shown in Fig. 9 was calculated by integrating Cmag(T)/T. The entropy does not reach the theoretical maximum Smax ¼ Rln(16) at the Curie temperature. Moreover, a noticeable magnetic contribution to the heat capacity persists up to at least 200 K, which is apparently due to the existence of the spin fluctuations far above the magnetic-ordering temperature. The magnetocaloric effect was calculated by a method suggested by von Ranke et al. [18]. Fig. 10 shows the temperature dependence of the adiabatic temperature change DTad for Dy1xLaxNi2 (x ¼ 0.5, 0.4, 0.2 and 0) measured at a magnetic field of 0.42 T. The magnetocaloric effect in 0.42 T is moderate. The maximum DTad reaches about 0.55 K near 22 K for DyNi2, DTadE0.5 K near 18 K for Dy0.8La0.2Ni2, DTadE0.45 K near 14 K for Dy0.6La0.4Ni2 and DTadE0.35 K near 12 K for Dy0.5La0.5Ni2. Fig. 11 shows the temperature dependence of DTad for Dy1xLaxNi2 (x ¼ 0.5 and 0.2) measured in an applied magnetic field of 4.2 T. In this case the maximum magnetocaloric effect reaches about 4 K for Dy0.8La0.2Ni2 near 18 and 2 K for Dy0.5La0.5Ni2 near 12 K. The sharp peaks in the curves are related to the paramagnetic– ferromagnetic phase transitions observed in the vicinity of the Curie temperature, since in the vicinity of TC the maximum magnetization difference is observed. For solid solutions with xX0.6, the ordering temperatures are too low to allow for accurate estimation of the magnetocaloric effect.
4. Conclusions
Fig. 10. Temperature dependence of DTad of Dy1xLaxNi2 (x ¼ 0, 0.2, 0.4 and 0.5) measured in an applied magnetic field of 0.42 T.
The effect of the substitution of Dy for La on the structure and physical properties of the Dy1xLaxNi2 solid solutions has been studied. It was confirmed that the crystal lattice of the compounds, similarly to Tb1xLaxNi2 [16] and Ho1xLaxNi2 [12], can be adequately described by the superstructure derived from C15-type (space group F4¯3m) Laves-phase structure with the doubled lattice parameter a. The latter increases with increasing La content. The observed linearity of the compositional dependence of the lattice parameter confirms the formation of the substitutional solid solutions. The substitution of magnetic Dy for nonmagnetic La results in the common magnetic dilution accompanied by marked modification of the magnetic behavior of the Dy1xLaxNi2 solid solutions. On the substitution, the exchange interactions are weakened leading to a decrease in the ordering temperature, as
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observed both by the magnetization and heat capacity measurements. The Curie temperature decreases from 21.3 K for DyNi2 to 6.2 K for Dy0.2La0.8Ni2. The ferromagnetic interaction is reduced to a lesser extent, as compared to that in the La-substituted Ho1xLaxNi2 system [12]. The magnetic part of the heat capacity showed the presence of the Schottky anomaly phenomenon. The moderate magnetocaloric effect in a magnetic field of 0.42 and 4.2 T is not as attractive for magnetic cooling as in the case of the Ho1xLaxNi2 or Tb1xLaxNi2 systems. The maximum magnetocaloric effect DTad reaches about 4 K near 18 K for Dy0.8La0.2Ni2 and 2 K near 12 K for Dy0.5La0.5Ni2 for a field change of 4.2 T. Acknowledgement Financial support by the Deutsche Forschungsgemeinschaft (SFB 463) is gratefully acknowledged. References [1] H.R. Kirchmayer, E. Burzo, in: H.P.J. Wijn (Ed.), Landolt–Bo¨rnstein, New Series III/19d2, Berlin, 1990.
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