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Magnetic properties of TbNi4Al and DyNi4Al compounds: investigation via neutron diffraction and magnetometry
Journal of Alloys and Compounds 385 (2004) 28–32
Magnetic properties of TbNi4 Al and DyNi4 Al compounds: investigation via neutron diffraction and magnetometry T. Toli´nski a,∗ , W. Schäfer b , A. Kowalczyk a , B. Andrzejewski a , A. Hoser c , A. Szlaferek a a
Institute of Molecular Physics, Polish Academy of Sciences, Smoluchowskiego 17, 60-179 Pozna´n, Poland b Mineralogisches Institut, Univ. Bonn, in Forschungszentrum Jülich, 52425 Jülich, Germany c Institut für Kristallographie, RWTH-Aachen, Germany Received 12 February 2004; received in revised form 20 April 2004; accepted 20 April 2004
Abstract The hexagonal heavy rare-earth based TbNi4 Al and DyNi4 Al compounds have been investigated using ac and dc magnetometry and neutron diffraction measurements. Both techniques provide consistent values of magnetic moments and ordering temperatures. Long-range magnetic order is established in the hexagonal plane and in the case of TbNi4 Al the possibility of an additional contribution of short range order is revealed. The Curie temperature TC and the magnetic moments are 11 K and 6.4 µB /fu for DyNi4 Al and 23 K and 5.5 µB /fu for TbNi4 Al. © 2004 Elsevier B.V. All rights reserved. PACS: 71.20.Eh; 71.20.Lp; 61.12.Ld Keywords: Rare earth compounds; Neutron diffraction; Magnetic properties
1. Introduction Apart from a basic scientific interest the RNi4 Al and RNi4 B compounds (R = Y or rare earth) have been widely investigated due to the possible and partly realized commercial applications. RNi4 B compounds crystallize in the structure of CeCo4 B, space group P6/mmm. The Ni atoms occupy the crystallographic sites (2c) and (6c), the rare earth atoms are also located in two sites (1a), (1b) and boron atoms are located in the (2d) positions. These compounds exhibit interesting anisotropic properties, especially for R = Sm [1,2]. The YNi4 B compound reveals the presence of a superconducting phase [1] and RNi4 B for R = Ce, Nd and Pr exhibits mixed valence properties [3]. The RNi4 Al compounds crystallize in the hexagonal CaCu5 -type structure, space group P6/mmm. The R atoms occupy the (1a) site, Ni(1) the (2c) site and Ni(2) and Al are statistically distributed over the (3g) positions. These materials have attracted attention after successful commercial use of the LaNi5 alloys in batteries and for hydrogen storage due to the large hydrogen absorption capacity [4,5]. Amid
other the TbNi4 Al and DyNi4 Al compounds were also studied from the point of view of their thermodynamic and structural properties [6,7]. It was found that TbNi5−x Alx was single phase for x ≤ 1.5 and TC did not depend on composition for x ≤ 1 [8]. The crystallographic structure of TbNi5−x Alx and DyNi5−x Alx is hexagonal of the CaCu5 type for x ≤ 2 [5,6]. It was observed that the crystallographic structure of DyNi5−x Alx was not changed after hydrogen exposure but the unit-cell volume increased up to 13% [6], which means a significant hydrogen absorption. In this paper, we concentrate on the magnetic properties of TbNi4 Al and DyNi4 Al. The studies have been performed employing neutron diffraction yielding both structural and magnetic information. The magnetic characterization has been supported by the measurements of the temperature dependence of the ac and dc magnetic susceptibility and field dependence of the magnetization. The obtained results are compared with the magnetometric studies of the B-based counterparts, i.e., TbNi4 B and DyNi4 B compounds [1,9].
2. Experiment ∗
Corresponding author. Tel.: +48-61-8695-282; fax: +48-61-8684-524 E-mail address: [email protected] (T. Toli´nski).
0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2004.04.130
The rare earth, nickel and aluminum (boron) were placed in a water-cooled boat in stoichiometric amounts. The
T. Toli´nski et al. / Journal of Alloys and Compounds 385 (2004) 28–32
polycrystalline compounds were obtained by induction melting in an argon atmosphere. The dc magnetization versus temperature was measured in a magnetic field of 1 mT, 2 mT and 0.2 T. The real, χ , and the imaginary, χ , part of the ac susceptibility was measured in an ac magnetic field with an amplitude of 0.1 mT and a frequency of 1 kHz. The magnetic field dependence of the magnetization was measured at T = 4.2 K in magnetic fields up to 9 T. All the magnetic experiments were carried out on a MagLab2000 instrument. Diffraction patterns have been collected on the neutron powder diffractometer SV7-a at the FRJ-2 reactor in the Forschungszentrum Jülich, Germany [10]. The neutron wavelength was λ = 1.0957 Å. The sample consisted of a dense piece of polycrystalline material, therefore any rotation of the grains’ c-axis could be excluded. The sample was contained in a cylindrical vanadium can mounted in a refrigerator cryostat and in a cryomagnet with external magnetic fields up to 5 T perpendicular to the horizontal diffraction plane. Neutron diffraction results were analyzed by full-pattern Rietveld refinements using FULLPROF [11]. The standard deviation of the refinement was about 2%.
3. Results and discussion The temperature TC of the transition from the paramagnetic state to the ferromagnetic order is equal to 11 K for DyNi4 Al. The corresponding ac susceptibility separated into the real, χ , and the imaginary, χ , part is shown for the DyNi4 Al compound in Fig. 1a. The dc susceptibility case (Fig. 1b) reveals typical irreversibility between ZFC (zero field cooling) and FC (field cooling) curves, which was previously explained by pinning of domain walls [9]. One can see that using a large magnetic field (Fig. 1b, inset) removes the irreversibility. Fig. 2 shows the dc susceptibility for the TbNi4 Al compound yielding a transition at 22 K. The dc magnetization curve of TbNi4 Al and DyNi4 Al in an external magnetic field swept up to 9 T is displayed in the inset of Fig. 2. A characteristic feature is the visible lack of saturation even in the large field range, which seems to be an inherent property connected with anisotropy. This conclusion stems from the fact that in the case of the RNi4 B series [1,9] the only compound, for which we have observed a quick and full saturation was GdNi4 B. It is known that Gd has zero orbital momentum, L = 0, leading to an only small magnetic anisotropy. In the following discussion we compare the above results for RNi4 Al with our neutron diffraction studies as well as with the previous magnetometric studies of RNi4 B counterparts. Fig. 3a presents the RT neutron diffraction pattern of DyNi4 Al fitted with the Rietveld model. The (hkl) reflections with the most significant magnetic scattering contributions at 4.2 K are shown in Fig. 3b. The low-angle part is analyzed by including an appropriate magnetic model (solid line), which
29
(a)
(b)
Fig. 1. (a) The real χ (full symbols) and the imaginary χ (opened symbols) part of the ac magnetic susceptibility for DyNi4 Al with magnetic field amplitude of 0.1 mT and frequency of 1 kHz. (b) dc magnetic susceptibility measured at H = 2 mT and 0.2 T (inset).
assumes hexagonal structure, space group P6/mmm. The refined lattice constants are a = 4.945 (2) Å, c = 4.045 (1) Å at 293 K and a = 4.935 (1) Å, c = 4.039 (1) Å at 4.2 K. The Dy atoms are located on the (1a) (0 0 0) site, Ni(1) on the 2c (1/3 2/3 0) site and Ni(2) and Al are statistically distributed over the 3 g (1/2 0 1/2) positions. The extracted magnetic moment of the ferromagnetically ordered DyNi4 Al is µ(Dy)
Fig. 2. The dc magnetic susceptibility measured for TbNi4 Al compound. Inset: magnetization curve of the DyNi4 Al and TbNi4 Al compounds.
30
T. Toli´nski et al. / Journal of Alloys and Compounds 385 (2004) 28–32
(a)
(b) Fig. 3. (a) Neutron diffraction pattern of DyNi4 Al at 293 K: experimental data (dots) and refinement (line). The bottom curve represents the difference between measurement and calculation. (b) Reflections with the most significant magnetic scattering contributions at 4.2 K.
= 6.4 µB at 4.2 K. It is in a good agreement with the conventional magnetization measurements (Fig. 2, inset) giving only the resultant magnetization of the sample (6.5 µB /fu in 5 T). This suggests that the magnetic contribution of Ni can be neglected. The ferromagnetic order of DyNi4 Al sample is established by analyzing the influence of the external magnetic field at 4.2 K on the behavior of the magnetic reflections. The low-angle part of the diffraction patterns collected in various magnetic fields is shown in Fig. 4. The magnetization process is especially well visible for the (0 0 1) reflection. Apart from the increase of the intensity with magnetic field one can also see that after switching the magnetic field off at 4.2 K the remaining peaks are larger than after warming up above TC into the virgin state (top pattern), i.e. a remanence magnetization is visible (Hoff , 4.2 K). The integrated magnetic intensity of the (0 0 1) peak, which is proportional
to the square of magnetization, is displayed as a function of the external magnetic field in Fig. 5. From the increase of the (0 0 1) and the decrease of the (h 0 0) peaks it is evident that the moments are ordered in the hexagonal basis plane. The crystallographic structure is illustrated in the inset of Fig. 5. Due to the domain effects it is not possible to determine the magnetic structure within the hexagonal plane. We have also carried out neutron diffraction in a wide range of temperature, which has revealed a transition from the magnetically ordered to the paramagnetic state at TC = 11 (1) K. The sum of all integrated magnetic intensities is shown as a function of temperature in Fig. 6. A similar set of neutron diffraction experiments as in the case of DyNi4 Al has been carried out for the TbNi4 Al compound. In Fig. 7 the 4.2 K and RT low-angle patterns are displayed together with the resulting difference pattern. An important observation is that even at 4.2 K the total volume
T. Toli´nski et al. / Journal of Alloys and Compounds 385 (2004) 28–32
DyNi4Al
001 |
101 |
31
110 |
Intensity (arb. units)
H off, warming up
5
H off,
4.2K
H=5T,
4.2K
H=3T,
4.2K
H=2T,
4.2K
H=1T,
4.2K
10
15
20
Fig. 5. (a) Field dependence of the (0 0 1) integrated magnetic intensity of DyNi4 Al proportional to the square of magnetization. Solid line is a guide to the eye. Inset: the crystallographic structure of RNi4 Al compounds.
25
2θ (deg)
200
integr.magn.intensity(arb.units)
Fig. 4. Low-angle part of the diffraction patterns of DyNi4 Al measured in the cryomagnet and showing the influence of the external magnetic field mainly on the (0 0 1) reflection; chronological sequence of the field (H)–temperature (T) conditions is shown from bottom to top. ‘H off warming up’ means the increase of the temperature above TC after switching off the magnetic field H = 5 T.
is not yet completely long-range ordered. This becomes evident from broad diffuse magnetic peaks indicating magnetic short-range order. The diffuse scattering is beyond the Bragg peaks (1 0 0), (0 0 1) and (1 0 1), which represent magnetic long-range order. The Rietveld analysis leads to the magnetic moment µ(Tb) = 5.5 (3) µB at 4.2 K. The TC value obtained from the temperature dependence of the sum of integrated magnetic intensities is equal to 23 (1) K (Fig. 8).
4
4
T=4.2K
DyNi4Al sum of all hkl
150
100
50
0 0
5
10
15
20
25
temperature (K) Fig. 6. The sum of all integrated magnetic intensities as a function of temperature for DyNi4 Al compound revealing TC = 11 K. Solid line is a guide for the eye.
4.2K - 293K
T=293K
Intensity (arb. units)
3 3
3 2
2
2
1
1
0
0
1
0 10
15
20
25
10
15
20
25
10
15
20
25
2θ (deg) Fig. 7. Comparison of the low-angle parts of the diffraction patterns at 4.2 K (left) and 293 K (center) and of the temperature difference pattern (4.2–293 K) containing only magnetic scattering contributions (right) for TbNi4 Al. Broad background modulations of diffuse magnetic scattering beyond the Bragg peaks (1 0 0/0 0 1) and (1 0 1) are visible at 4.2 K and in the temperature difference pattern indicating magnetic short-range order at 4.2 K in addition to the long-range magnetic order represented by the Bragg peaks.
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T. Toli´nski et al. / Journal of Alloys and Compounds 385 (2004) 28–32
Table 1 Magnetic and structural parameters of DyNi4 Al and TbNi4 Al in comparison with DyNi4 B and TbNi4 B [1,9] compounds. The magnetic moment obtained from dc magnetization measurements (dc) is given at H = 5 T, i.e., the maximum field used in neutron studies and T = 4.2 K. The data for NdNi4 Al and NdNi4 B from [9,12] are also included Compound
Lattice constants (Å) RT
TbNi4 Al DyNi4 Al NdNi4 Al TbNi4 B DyNi4 B NdNi4 B a
Magnetic moment 4.2 K
Neutrons
dc
Ordering temperature Neutrons
dc
a
c
a
c
(µB )
(µB /fu)
(K)
(K)
4.9536 (11) 4.945 (2) 5.0103 (13) 5.020 5.000 5.053
4.0454 (6) 4.045 (1) 4.0601 (7) 6.970 6.966 6.954
4.9417 (5) 4.935 (1) 4.9955 (14) – – –
4.0382 (6) 4.039 (1) 4.0551 (7) – – –
5.5 (3) 6.4 (3) 1.85 (5)a – – –
5.8 6.5 1.52 6.3 8.2 1.68
23 (1) 11 (1) 6.0 (5) – – –
22 11 6 21 15 11.7
At 1.8 K.
mainly in the case of light and heavy rare earths (Nd, Dy). For Tb, which is in the transition region between the light and the heavy rare earths the obtained values of TC are similar both for TbNi4 Al and TbNi4 B. This behavior resembles somehow the dependence of the Curie temperature on the de Gennes factor. 3. DyNi4 Al shows long-range order in the hexagonal basis plane but for TbNi4 Al also the presence of short-range order is revealed.
Acknowledgements
Fig. 8. The sum of all integrated magnetic intensities as a function of temperature for TbNi4 Al compound. TC = 28 K is obtained.
4. Summary In this section we show in a compact way the main results for DyNi4 Al and TbNi4 Al in comparison with DyNi4 B and TbNi4 B compounds. The lattice constants, magnetic moments and ordering temperatures are compiled in Table 1. For B-based compounds only magnetometric investigations have been performed due to the large absorption of natural boron. The results for NdNi4 Al and NdNi4 B from Ref. [9,12] are also included. The resulting conclusions are as given further. 1. The magnetic moments and ordering temperatures extracted from the neutron diffraction experiment and magnetometry are in good accordance. This consistency of the magnetic moments is indicative of a negligible contribution of the Ni atoms to the magnetic properties of RNi4 Al. 2. The magnetic moments and ordering temperatures of RNi4 Al compounds are reduced with respect to the RNi4 B counterparts. However, TC seems to be affected
The work was supported by the Centre of Excellence for Magnetic and Molecular Materials for Future Electronics within the European Commission contract No. G5MA-CT-2002-04049. References [1] T. Toli´nski, A. Kowalczyk, A. Szlaferek, B. Andrzejewski, J. Kováˇc, M. Timko, J. Alloys Compd. 347 (2002) 31. [2] C. Mazumdar, R. Nagarajan, L.C. Gupta, B.D. Padalia, R. Vijayaraghavan, Appl. Phys. Lett. 77 (2000) 895. [3] T. Toli´nski, A. Kowalczyk, G. Chełkowska, Phys. Lett. A 308 (2003) 75. [4] E.L. Houston, G.D. Sandrock, J. Less-Common Met. 74 (1980) 435. [5] M. Wada, Sci. Technol. Japan 51 (1994) 54. [6] B. Šorgi´c, A. Drašner, Ž. Blažina, J. Phys. Condens. Matter 7 (1995) 7209. [7] Ž. Blažina, B. Šorgi´c, A. Drašner, J. Phys. Condens. Matter 9 (1997) 3099. [8] A.G. Kuchin, A.S. Ermolenko, V.I. Khrabrov, N.I. Kourov, G.M. Makarova, Ye.V. Belozerov, T.P. Lapina, Yu.A. Kulikov, J. Magn. Magn. Mater. 238 (2002) 29. [9] T. Toli´nski, A. Kowalczyk, A. Szlaferek, M. Timko, J. Kováˇc, Solid State Commun. 122 (2002) 363. [10] W. Schäfer, E. Jansen, R. Skowronek, A. Kirfel, Physica B 234–236 (1997) 1146. [11] J. Rodriguez-Carvajal, FullProf-version 1999, Physica B 192 (1999) 55. [12] T. Toli´nski, W. Schäfer, W. Kockelmann, A. Kowalczyk, A. Hoser, Phys. Rev. B 68 (2003) 144403.
Magnetic properties of TbNi4 Al and DyNi4 Al compounds: investigation via neutron diffraction and magnetometry T. Toli´nski a,∗ , W. Schäfer b , A. Kowalczyk a , B. Andrzejewski a , A. Hoser c , A. Szlaferek a a
Institute of Molecular Physics, Polish Academy of Sciences, Smoluchowskiego 17, 60-179 Pozna´n, Poland b Mineralogisches Institut, Univ. Bonn, in Forschungszentrum Jülich, 52425 Jülich, Germany c Institut für Kristallographie, RWTH-Aachen, Germany Received 12 February 2004; received in revised form 20 April 2004; accepted 20 April 2004
Abstract The hexagonal heavy rare-earth based TbNi4 Al and DyNi4 Al compounds have been investigated using ac and dc magnetometry and neutron diffraction measurements. Both techniques provide consistent values of magnetic moments and ordering temperatures. Long-range magnetic order is established in the hexagonal plane and in the case of TbNi4 Al the possibility of an additional contribution of short range order is revealed. The Curie temperature TC and the magnetic moments are 11 K and 6.4 µB /fu for DyNi4 Al and 23 K and 5.5 µB /fu for TbNi4 Al. © 2004 Elsevier B.V. All rights reserved. PACS: 71.20.Eh; 71.20.Lp; 61.12.Ld Keywords: Rare earth compounds; Neutron diffraction; Magnetic properties
1. Introduction Apart from a basic scientific interest the RNi4 Al and RNi4 B compounds (R = Y or rare earth) have been widely investigated due to the possible and partly realized commercial applications. RNi4 B compounds crystallize in the structure of CeCo4 B, space group P6/mmm. The Ni atoms occupy the crystallographic sites (2c) and (6c), the rare earth atoms are also located in two sites (1a), (1b) and boron atoms are located in the (2d) positions. These compounds exhibit interesting anisotropic properties, especially for R = Sm [1,2]. The YNi4 B compound reveals the presence of a superconducting phase [1] and RNi4 B for R = Ce, Nd and Pr exhibits mixed valence properties [3]. The RNi4 Al compounds crystallize in the hexagonal CaCu5 -type structure, space group P6/mmm. The R atoms occupy the (1a) site, Ni(1) the (2c) site and Ni(2) and Al are statistically distributed over the (3g) positions. These materials have attracted attention after successful commercial use of the LaNi5 alloys in batteries and for hydrogen storage due to the large hydrogen absorption capacity [4,5]. Amid
other the TbNi4 Al and DyNi4 Al compounds were also studied from the point of view of their thermodynamic and structural properties [6,7]. It was found that TbNi5−x Alx was single phase for x ≤ 1.5 and TC did not depend on composition for x ≤ 1 [8]. The crystallographic structure of TbNi5−x Alx and DyNi5−x Alx is hexagonal of the CaCu5 type for x ≤ 2 [5,6]. It was observed that the crystallographic structure of DyNi5−x Alx was not changed after hydrogen exposure but the unit-cell volume increased up to 13% [6], which means a significant hydrogen absorption. In this paper, we concentrate on the magnetic properties of TbNi4 Al and DyNi4 Al. The studies have been performed employing neutron diffraction yielding both structural and magnetic information. The magnetic characterization has been supported by the measurements of the temperature dependence of the ac and dc magnetic susceptibility and field dependence of the magnetization. The obtained results are compared with the magnetometric studies of the B-based counterparts, i.e., TbNi4 B and DyNi4 B compounds [1,9].
2. Experiment ∗
Corresponding author. Tel.: +48-61-8695-282; fax: +48-61-8684-524 E-mail address: [email protected] (T. Toli´nski).
0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2004.04.130
The rare earth, nickel and aluminum (boron) were placed in a water-cooled boat in stoichiometric amounts. The
T. Toli´nski et al. / Journal of Alloys and Compounds 385 (2004) 28–32
polycrystalline compounds were obtained by induction melting in an argon atmosphere. The dc magnetization versus temperature was measured in a magnetic field of 1 mT, 2 mT and 0.2 T. The real, χ , and the imaginary, χ , part of the ac susceptibility was measured in an ac magnetic field with an amplitude of 0.1 mT and a frequency of 1 kHz. The magnetic field dependence of the magnetization was measured at T = 4.2 K in magnetic fields up to 9 T. All the magnetic experiments were carried out on a MagLab2000 instrument. Diffraction patterns have been collected on the neutron powder diffractometer SV7-a at the FRJ-2 reactor in the Forschungszentrum Jülich, Germany [10]. The neutron wavelength was λ = 1.0957 Å. The sample consisted of a dense piece of polycrystalline material, therefore any rotation of the grains’ c-axis could be excluded. The sample was contained in a cylindrical vanadium can mounted in a refrigerator cryostat and in a cryomagnet with external magnetic fields up to 5 T perpendicular to the horizontal diffraction plane. Neutron diffraction results were analyzed by full-pattern Rietveld refinements using FULLPROF [11]. The standard deviation of the refinement was about 2%.
3. Results and discussion The temperature TC of the transition from the paramagnetic state to the ferromagnetic order is equal to 11 K for DyNi4 Al. The corresponding ac susceptibility separated into the real, χ , and the imaginary, χ , part is shown for the DyNi4 Al compound in Fig. 1a. The dc susceptibility case (Fig. 1b) reveals typical irreversibility between ZFC (zero field cooling) and FC (field cooling) curves, which was previously explained by pinning of domain walls [9]. One can see that using a large magnetic field (Fig. 1b, inset) removes the irreversibility. Fig. 2 shows the dc susceptibility for the TbNi4 Al compound yielding a transition at 22 K. The dc magnetization curve of TbNi4 Al and DyNi4 Al in an external magnetic field swept up to 9 T is displayed in the inset of Fig. 2. A characteristic feature is the visible lack of saturation even in the large field range, which seems to be an inherent property connected with anisotropy. This conclusion stems from the fact that in the case of the RNi4 B series [1,9] the only compound, for which we have observed a quick and full saturation was GdNi4 B. It is known that Gd has zero orbital momentum, L = 0, leading to an only small magnetic anisotropy. In the following discussion we compare the above results for RNi4 Al with our neutron diffraction studies as well as with the previous magnetometric studies of RNi4 B counterparts. Fig. 3a presents the RT neutron diffraction pattern of DyNi4 Al fitted with the Rietveld model. The (hkl) reflections with the most significant magnetic scattering contributions at 4.2 K are shown in Fig. 3b. The low-angle part is analyzed by including an appropriate magnetic model (solid line), which
29
(a)
(b)
Fig. 1. (a) The real χ (full symbols) and the imaginary χ (opened symbols) part of the ac magnetic susceptibility for DyNi4 Al with magnetic field amplitude of 0.1 mT and frequency of 1 kHz. (b) dc magnetic susceptibility measured at H = 2 mT and 0.2 T (inset).
assumes hexagonal structure, space group P6/mmm. The refined lattice constants are a = 4.945 (2) Å, c = 4.045 (1) Å at 293 K and a = 4.935 (1) Å, c = 4.039 (1) Å at 4.2 K. The Dy atoms are located on the (1a) (0 0 0) site, Ni(1) on the 2c (1/3 2/3 0) site and Ni(2) and Al are statistically distributed over the 3 g (1/2 0 1/2) positions. The extracted magnetic moment of the ferromagnetically ordered DyNi4 Al is µ(Dy)
Fig. 2. The dc magnetic susceptibility measured for TbNi4 Al compound. Inset: magnetization curve of the DyNi4 Al and TbNi4 Al compounds.
30
T. Toli´nski et al. / Journal of Alloys and Compounds 385 (2004) 28–32
(a)
(b) Fig. 3. (a) Neutron diffraction pattern of DyNi4 Al at 293 K: experimental data (dots) and refinement (line). The bottom curve represents the difference between measurement and calculation. (b) Reflections with the most significant magnetic scattering contributions at 4.2 K.
= 6.4 µB at 4.2 K. It is in a good agreement with the conventional magnetization measurements (Fig. 2, inset) giving only the resultant magnetization of the sample (6.5 µB /fu in 5 T). This suggests that the magnetic contribution of Ni can be neglected. The ferromagnetic order of DyNi4 Al sample is established by analyzing the influence of the external magnetic field at 4.2 K on the behavior of the magnetic reflections. The low-angle part of the diffraction patterns collected in various magnetic fields is shown in Fig. 4. The magnetization process is especially well visible for the (0 0 1) reflection. Apart from the increase of the intensity with magnetic field one can also see that after switching the magnetic field off at 4.2 K the remaining peaks are larger than after warming up above TC into the virgin state (top pattern), i.e. a remanence magnetization is visible (Hoff , 4.2 K). The integrated magnetic intensity of the (0 0 1) peak, which is proportional
to the square of magnetization, is displayed as a function of the external magnetic field in Fig. 5. From the increase of the (0 0 1) and the decrease of the (h 0 0) peaks it is evident that the moments are ordered in the hexagonal basis plane. The crystallographic structure is illustrated in the inset of Fig. 5. Due to the domain effects it is not possible to determine the magnetic structure within the hexagonal plane. We have also carried out neutron diffraction in a wide range of temperature, which has revealed a transition from the magnetically ordered to the paramagnetic state at TC = 11 (1) K. The sum of all integrated magnetic intensities is shown as a function of temperature in Fig. 6. A similar set of neutron diffraction experiments as in the case of DyNi4 Al has been carried out for the TbNi4 Al compound. In Fig. 7 the 4.2 K and RT low-angle patterns are displayed together with the resulting difference pattern. An important observation is that even at 4.2 K the total volume
T. Toli´nski et al. / Journal of Alloys and Compounds 385 (2004) 28–32
DyNi4Al
001 |
101 |
31
110 |
Intensity (arb. units)
H off, warming up
5
H off,
4.2K
H=5T,
4.2K
H=3T,
4.2K
H=2T,
4.2K
H=1T,
4.2K
10
15
20
Fig. 5. (a) Field dependence of the (0 0 1) integrated magnetic intensity of DyNi4 Al proportional to the square of magnetization. Solid line is a guide to the eye. Inset: the crystallographic structure of RNi4 Al compounds.
25
2θ (deg)
200
integr.magn.intensity(arb.units)
Fig. 4. Low-angle part of the diffraction patterns of DyNi4 Al measured in the cryomagnet and showing the influence of the external magnetic field mainly on the (0 0 1) reflection; chronological sequence of the field (H)–temperature (T) conditions is shown from bottom to top. ‘H off warming up’ means the increase of the temperature above TC after switching off the magnetic field H = 5 T.
is not yet completely long-range ordered. This becomes evident from broad diffuse magnetic peaks indicating magnetic short-range order. The diffuse scattering is beyond the Bragg peaks (1 0 0), (0 0 1) and (1 0 1), which represent magnetic long-range order. The Rietveld analysis leads to the magnetic moment µ(Tb) = 5.5 (3) µB at 4.2 K. The TC value obtained from the temperature dependence of the sum of integrated magnetic intensities is equal to 23 (1) K (Fig. 8).
4
4
T=4.2K
DyNi4Al sum of all hkl
150
100
50
0 0
5
10
15
20
25
temperature (K) Fig. 6. The sum of all integrated magnetic intensities as a function of temperature for DyNi4 Al compound revealing TC = 11 K. Solid line is a guide for the eye.
4.2K - 293K
T=293K
Intensity (arb. units)
3 3
3 2
2
2
1
1
0
0
1
0 10
15
20
25
10
15
20
25
10
15
20
25
2θ (deg) Fig. 7. Comparison of the low-angle parts of the diffraction patterns at 4.2 K (left) and 293 K (center) and of the temperature difference pattern (4.2–293 K) containing only magnetic scattering contributions (right) for TbNi4 Al. Broad background modulations of diffuse magnetic scattering beyond the Bragg peaks (1 0 0/0 0 1) and (1 0 1) are visible at 4.2 K and in the temperature difference pattern indicating magnetic short-range order at 4.2 K in addition to the long-range magnetic order represented by the Bragg peaks.
32
T. Toli´nski et al. / Journal of Alloys and Compounds 385 (2004) 28–32
Table 1 Magnetic and structural parameters of DyNi4 Al and TbNi4 Al in comparison with DyNi4 B and TbNi4 B [1,9] compounds. The magnetic moment obtained from dc magnetization measurements (dc) is given at H = 5 T, i.e., the maximum field used in neutron studies and T = 4.2 K. The data for NdNi4 Al and NdNi4 B from [9,12] are also included Compound
Lattice constants (Å) RT
TbNi4 Al DyNi4 Al NdNi4 Al TbNi4 B DyNi4 B NdNi4 B a
Magnetic moment 4.2 K
Neutrons
dc
Ordering temperature Neutrons
dc
a
c
a
c
(µB )
(µB /fu)
(K)
(K)
4.9536 (11) 4.945 (2) 5.0103 (13) 5.020 5.000 5.053
4.0454 (6) 4.045 (1) 4.0601 (7) 6.970 6.966 6.954
4.9417 (5) 4.935 (1) 4.9955 (14) – – –
4.0382 (6) 4.039 (1) 4.0551 (7) – – –
5.5 (3) 6.4 (3) 1.85 (5)a – – –
5.8 6.5 1.52 6.3 8.2 1.68
23 (1) 11 (1) 6.0 (5) – – –
22 11 6 21 15 11.7
At 1.8 K.
mainly in the case of light and heavy rare earths (Nd, Dy). For Tb, which is in the transition region between the light and the heavy rare earths the obtained values of TC are similar both for TbNi4 Al and TbNi4 B. This behavior resembles somehow the dependence of the Curie temperature on the de Gennes factor. 3. DyNi4 Al shows long-range order in the hexagonal basis plane but for TbNi4 Al also the presence of short-range order is revealed.
Acknowledgements
Fig. 8. The sum of all integrated magnetic intensities as a function of temperature for TbNi4 Al compound. TC = 28 K is obtained.
4. Summary In this section we show in a compact way the main results for DyNi4 Al and TbNi4 Al in comparison with DyNi4 B and TbNi4 B compounds. The lattice constants, magnetic moments and ordering temperatures are compiled in Table 1. For B-based compounds only magnetometric investigations have been performed due to the large absorption of natural boron. The results for NdNi4 Al and NdNi4 B from Ref. [9,12] are also included. The resulting conclusions are as given further. 1. The magnetic moments and ordering temperatures extracted from the neutron diffraction experiment and magnetometry are in good accordance. This consistency of the magnetic moments is indicative of a negligible contribution of the Ni atoms to the magnetic properties of RNi4 Al. 2. The magnetic moments and ordering temperatures of RNi4 Al compounds are reduced with respect to the RNi4 B counterparts. However, TC seems to be affected
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