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Magnetism in Ho5SixGe1−x compounds
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 316 (2007) 313–316 www.elsevier.com/locate/jmmm
Magnetism in Ho5SixGe1x compounds Jan Proklesˇ ka, Jana Vejpravova´, Vladimı´ r Sechovsky´ Charles University, Faculty of Mathematics and Physics, Department of Condensed Matter Physics, Ke Karlovu 5, Praha 2, 121 16, Czech Republic Available online 2 March 2007
Abstract We have prepared polycrystalline samples of the Ho5SixGe4x compounds for 0pxp4 and studied the magnetization, AC magnetic susceptibility, specific heat and electrical resistivity as functions of temperature and applied magnetic filed. The literature data [F. Holtzberg, R.J. Gambino, T.R. McGuire, J. Phys. Chem. Solids 28 (1967) 2283] on the magnetic ordering transitions of the parent compounds Ho5Si4 (ferromagnetic ordering below TC70 K) and Ho5Ge4 (antiferromagnetic ordering below TN23 K) have been confirmed by our results. Besides that, however, we have observed additional anomalies in the temperature dependences of the electrical resistivity, and specific heat at 23 K for Ho5Si4 and at 16 K for both the parent compounds, which we attribute to order–order magnetic phase transitions. The ferromagnetism of Ho5Si4 is suppressed by the Ge concentration in the vicinity of x ¼ 1. The evolution of magnetic phases with varying composition of the Si–Ge sublattice is discussed. r 2007 Elsevier B.V. All rights reserved. PACS: 72.15.Eb; 75.50.Ee; 75.40.Cx Keywords: RE5X4; Resistivity; Specific heat; Magnetic phase transition
1. Introduction The extensive investigation of RE5(Si,Ge)4 systems was motivated by the discovery of a large magnetocaloric effect in the Gd-based compound [1]. Notable attention was paid to the compounds based on the heavy rare-earth (RE ¼ Tb, Dy, Er) owing to their potential application in magnetic refrigerators. The lack of information on the Hobased compounds motivated us to prepare these materials and investigate them by measuring magnetic, thermodynamic and transport properties. The polycrystalline samples of selected Ho5SixGe4x compounds of the typical mass of 3 g were prepared by arc melting the high-purity constituents (Ho—3N, Si—6N, Ge—5N) under Ar atmosphere. The nominal compositions of x ¼ 0, 0.5, 1.5, 2.0, 2.5, 3.5, 4.0 were prepared and studied. The magnetization, the AC susceptibility (w), the electrical resistivity (R) and the specific heat (Cp) were measured in magnetic fields up to 14 T and at temperatures
Corresponding author. Tel.: +420 221 911 653; fax: +420 221 911 617.
E-mail address: [email protected] (J. Proklesˇ ka). 0304-8853/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2007.02.170
down to 2 K using a PPMS 14 T equipment (Quantum Design). Our main interest was to investigate the influence of the composition of the Si–Ge sublattice and the applied magnetic field on magnetic phase transitions and the corresponding critical temperatures. 2. Results Ho5Si4 undergoes a magnetic phase transition at 70 K, which is expressed as a sharp peak in the temperature dependence of the AC magnetic susceptibility, which is seen in Fig. 1. The transition shows up also as a change of slope of the R vs. T curve (see Fig. 1). These observations are in good agreement with results presented in Ref. [2] and interpreted in terms of a paramagnetic to ferromagnetic phase transition. Contrary to Ref. [2], however, we have observed symptoms of additional magnetic phase transitions, which occur in Ho5Si4 at lower temperatures. In particular, pronounced anomalies can be seen in Figs. 1 and 2 in the Cp vs. T, R vs. T dependences at 23 and 16 K. The AC susceptibility, on the other hand, shows a clear maximum at 28 K. The 23 and 16 K features are moving to
ARTICLE IN PRESS J. Proklesˇka et al. / Journal of Magnetism and Magnetic Materials 316 (2007) 313–316
R (a.u.)
314
T (K)
Fig. 1. Electrical resistivity (top curve) and real part of AC susceptibility (bottom curve) for the Ho5Si4 compound.
order-to-order magnetic phase transitions. With small substitution of Ge (x ¼ 3.5) there is no significant change in the transition temperatures and behavior in magnetic field. As can be seen in Fig. 3 the specific heat of Ho5Ge4 shows two round maxims, one at 23 K and the other at 16 K but no indication of any magnet phase transition has been traced at higher temperatures up to room temperature, contrary to the 70-K transition observed in Ho5Si4. Also the temperature dependence of the electrical resistivity (also displayed in Fig. 3) exhibits anomalies in the vicinity of these temperatures. When applying a magnetic field the 23-K anomaly is first shifted to lower temperatures and smeared out. In a field of 3 T it is suppressed whereas the 16-K feature is shifted below 15 K and becomes considerably smeared. In the light of these results we conclude that Ho5Ge4 orders antiferromagnetically at 23 K and undergoes an order-to-order magnetic phase transition at 16 K. The materials with the composition close the 1:1 ratio for the Si and Ge (x ¼ 1.5, 2.0, 2.5) show three subsequent transitions (Table 1) contrary to the previous report [3] of varying character and sensitivity to the applied magnetic field. The resistivity data show the same evolution of transition temperatures with composition. The transition temperatures are shifted to lower values with applying magnetic field. In case of Ho5Si1.5Ge2.5 the
Fig. 2. Low-temperature part of electrical resistivity (top) and specific heat (bottom) for the Ho5Si4 compound.
lower temperatures with increasing the applied magnetic field, which points to a dominant role of antiferromagnetic interactions in the magnetic phases in Ho5Si4 at temperatures below 23 K and the features at 23 and 16 K reflect two
Fig. 3. Electrical resistivity (top) and specific heat (bottom) for the Ho5Ge4 compound.
ARTICLE IN PRESS J. Proklesˇka et al. / Journal of Magnetism and Magnetic Materials 316 (2007) 313–316
application of 1 T almost suppresses the anomaly related to the transition both in the specific heat and the resistivity data. The compositions with x ¼ 2.0 and 2.5 are less sensitive to the applied magnetic field; the transitions are clearly visible in fields up to 3 T. The evolution of magnetization curves (M vs. B dependence) with temperature displayed in Figs. 4 and 5 for Ho5Si4 and Ho5Ge4, respectively, allows making tentative conclusions about Ho magnetic moment arrangements in the two compounds at various temperatures. Comparing the 80 and 60 K magnetization curves observed for Ho5Si4 confirms that at 70 K a long-range order of Ho magnetic moments appears yielding a spontaneous magnetization. The size of magnetization, however, is more than order of magnitude smaller than the expected full moment of five Ho3+ magnetic moments per formula unit. Therefore we propose that the magnetic phase existing between 23 and 70 K is not simple ferromagnetic. The spontaneous magnetization increases with decreasing temperature but does not exceed 10 mB/f.u. even at the lowest temperature of measurement (2 K). The magnetization curve poorly saturates in magnetic fields up to 14 T, however, it show no feature in the entire range of applied fields. The poor saturation may be reflecting gradual
Table 1 Characteristic temperatures of magnetic phase transitions for Ho5SixGe4x compounds in the vicinity of the 1:1 Si:Ge ratio and the magnitude of magnetoresistance at 2 K and 5 T with respect to zero field Ho5SixGe4x
TN (K)
T1 (K)
T2 (K)
MR (%)
x ¼ 1.5 x ¼ 2.0 x ¼ 2.5
37 35 34
20 22 21
10 15 14
14 5.5 15
The critical temperatures are determined from specific-heat data.
315
B (T)
Fig. 5. Magnetization curves for the Ho5Ge4 compound. Curves are shifted by 5 mB for clarity. Dashed lines are data measured with decreasing field.
aligning of Ho magnetic moments from the originally noncollinear magnetic structure. The magnetic phase transition at 23 K reflects in a considerable change of shape of the M vs. B dependences at lower temperatures. In particular, a field-induced magnetic phase transition around 3 T gradually emerges with decreasing temperature indicating increasing role of antiferromagnetic interactions. Below 16 K, the S-shape of the transition becomes considerably more pronounced and at temperatures below 10 K also a considerable hysteresis develops. A field-induced magnetic phase transition develops analogously also on the magnetization curves measured on Ho5Ge4 (in this case around 15 K) at temperatures below 23 K and simultaneously, also a small spontaneous magnetization seems to emerge in this compound with decreasing temperature. These results suggest that the lowtemperature (To23 K) magnetic structures in Ho5Ge4 may be analogous to these in existing the Ho5Si4. 3. Conclusions
B (T)
Fig. 4. Magnetization curves for the Ho5Si4 compound. Curves are shifted by 5 mB for clarity. Dashed lines are data measured with decreasing field.
In conclusion, we have investigated the Ho5SixGe4x system for x ¼ 0, 0.5, 1.5, 2.0, 2.5, 3.5, 4.0 by means of measurements of transport, magnetic and thermodynamic properties. Our results are in partial agreement with previous studies and extend knowledge of this system. In Ho5Si4, magnetic ordering is observed below 70 K characterized a small spontaneous magnetization indicating a complex ordering of Ho magnetic moments, which is changed by magnetic phase transitions at 23 and 16 K to another complex spin structure. Ho5Ge4, on the other hand, seems to exhibit antiferromagnetism below 23 K ( ¼ TN). The striking similarity of behavior of Ho5Si4 and Ho5Ge4 (similar features in temperature dependences of the specific heat and resistivity at 23 and 16 K and a
ARTICLE IN PRESS 316
J. Proklesˇka et al. / Journal of Magnetism and Magnetic Materials 316 (2007) 313–316
metamagnetic transition seen on magnetization curves at temperatures below 23 K) allows speculations about common microscopic mechanism. Based on our results we expect the suppression of the high-temperature (423 K) spontaneous magnetization in Si rich part of phase diagram for the composition x ¼ 1. The specific heat and resistivity data of Ho5Si2Ge2 show an additional anomaly indicating the onset of the magnetic order at higher temperatures than previously reported. Acknowledgment This work is a part of the research plan MSM 0021620834 that is financed by the Ministry of Education of the Czech Republic.
References [1] V.K. Pecharsky, K.A. Gschneidner Jr., Phys. Rev. Lett. 78 (1997) 14494. [2] F. Holtzberg, R.J. Gambino, T.R. McGuire, J. Phys. Chem. Solids 28 (1967) 2283. [3] N. Thuy, et al., J. Magn. Magn. Mater. 262 (2003) 432.
Journal of Magnetism and Magnetic Materials 316 (2007) 313–316 www.elsevier.com/locate/jmmm
Magnetism in Ho5SixGe1x compounds Jan Proklesˇ ka, Jana Vejpravova´, Vladimı´ r Sechovsky´ Charles University, Faculty of Mathematics and Physics, Department of Condensed Matter Physics, Ke Karlovu 5, Praha 2, 121 16, Czech Republic Available online 2 March 2007
Abstract We have prepared polycrystalline samples of the Ho5SixGe4x compounds for 0pxp4 and studied the magnetization, AC magnetic susceptibility, specific heat and electrical resistivity as functions of temperature and applied magnetic filed. The literature data [F. Holtzberg, R.J. Gambino, T.R. McGuire, J. Phys. Chem. Solids 28 (1967) 2283] on the magnetic ordering transitions of the parent compounds Ho5Si4 (ferromagnetic ordering below TC70 K) and Ho5Ge4 (antiferromagnetic ordering below TN23 K) have been confirmed by our results. Besides that, however, we have observed additional anomalies in the temperature dependences of the electrical resistivity, and specific heat at 23 K for Ho5Si4 and at 16 K for both the parent compounds, which we attribute to order–order magnetic phase transitions. The ferromagnetism of Ho5Si4 is suppressed by the Ge concentration in the vicinity of x ¼ 1. The evolution of magnetic phases with varying composition of the Si–Ge sublattice is discussed. r 2007 Elsevier B.V. All rights reserved. PACS: 72.15.Eb; 75.50.Ee; 75.40.Cx Keywords: RE5X4; Resistivity; Specific heat; Magnetic phase transition
1. Introduction The extensive investigation of RE5(Si,Ge)4 systems was motivated by the discovery of a large magnetocaloric effect in the Gd-based compound [1]. Notable attention was paid to the compounds based on the heavy rare-earth (RE ¼ Tb, Dy, Er) owing to their potential application in magnetic refrigerators. The lack of information on the Hobased compounds motivated us to prepare these materials and investigate them by measuring magnetic, thermodynamic and transport properties. The polycrystalline samples of selected Ho5SixGe4x compounds of the typical mass of 3 g were prepared by arc melting the high-purity constituents (Ho—3N, Si—6N, Ge—5N) under Ar atmosphere. The nominal compositions of x ¼ 0, 0.5, 1.5, 2.0, 2.5, 3.5, 4.0 were prepared and studied. The magnetization, the AC susceptibility (w), the electrical resistivity (R) and the specific heat (Cp) were measured in magnetic fields up to 14 T and at temperatures
Corresponding author. Tel.: +420 221 911 653; fax: +420 221 911 617.
E-mail address: [email protected] (J. Proklesˇ ka). 0304-8853/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2007.02.170
down to 2 K using a PPMS 14 T equipment (Quantum Design). Our main interest was to investigate the influence of the composition of the Si–Ge sublattice and the applied magnetic field on magnetic phase transitions and the corresponding critical temperatures. 2. Results Ho5Si4 undergoes a magnetic phase transition at 70 K, which is expressed as a sharp peak in the temperature dependence of the AC magnetic susceptibility, which is seen in Fig. 1. The transition shows up also as a change of slope of the R vs. T curve (see Fig. 1). These observations are in good agreement with results presented in Ref. [2] and interpreted in terms of a paramagnetic to ferromagnetic phase transition. Contrary to Ref. [2], however, we have observed symptoms of additional magnetic phase transitions, which occur in Ho5Si4 at lower temperatures. In particular, pronounced anomalies can be seen in Figs. 1 and 2 in the Cp vs. T, R vs. T dependences at 23 and 16 K. The AC susceptibility, on the other hand, shows a clear maximum at 28 K. The 23 and 16 K features are moving to
ARTICLE IN PRESS J. Proklesˇka et al. / Journal of Magnetism and Magnetic Materials 316 (2007) 313–316
R (a.u.)
314
T (K)
Fig. 1. Electrical resistivity (top curve) and real part of AC susceptibility (bottom curve) for the Ho5Si4 compound.
order-to-order magnetic phase transitions. With small substitution of Ge (x ¼ 3.5) there is no significant change in the transition temperatures and behavior in magnetic field. As can be seen in Fig. 3 the specific heat of Ho5Ge4 shows two round maxims, one at 23 K and the other at 16 K but no indication of any magnet phase transition has been traced at higher temperatures up to room temperature, contrary to the 70-K transition observed in Ho5Si4. Also the temperature dependence of the electrical resistivity (also displayed in Fig. 3) exhibits anomalies in the vicinity of these temperatures. When applying a magnetic field the 23-K anomaly is first shifted to lower temperatures and smeared out. In a field of 3 T it is suppressed whereas the 16-K feature is shifted below 15 K and becomes considerably smeared. In the light of these results we conclude that Ho5Ge4 orders antiferromagnetically at 23 K and undergoes an order-to-order magnetic phase transition at 16 K. The materials with the composition close the 1:1 ratio for the Si and Ge (x ¼ 1.5, 2.0, 2.5) show three subsequent transitions (Table 1) contrary to the previous report [3] of varying character and sensitivity to the applied magnetic field. The resistivity data show the same evolution of transition temperatures with composition. The transition temperatures are shifted to lower values with applying magnetic field. In case of Ho5Si1.5Ge2.5 the
Fig. 2. Low-temperature part of electrical resistivity (top) and specific heat (bottom) for the Ho5Si4 compound.
lower temperatures with increasing the applied magnetic field, which points to a dominant role of antiferromagnetic interactions in the magnetic phases in Ho5Si4 at temperatures below 23 K and the features at 23 and 16 K reflect two
Fig. 3. Electrical resistivity (top) and specific heat (bottom) for the Ho5Ge4 compound.
ARTICLE IN PRESS J. Proklesˇka et al. / Journal of Magnetism and Magnetic Materials 316 (2007) 313–316
application of 1 T almost suppresses the anomaly related to the transition both in the specific heat and the resistivity data. The compositions with x ¼ 2.0 and 2.5 are less sensitive to the applied magnetic field; the transitions are clearly visible in fields up to 3 T. The evolution of magnetization curves (M vs. B dependence) with temperature displayed in Figs. 4 and 5 for Ho5Si4 and Ho5Ge4, respectively, allows making tentative conclusions about Ho magnetic moment arrangements in the two compounds at various temperatures. Comparing the 80 and 60 K magnetization curves observed for Ho5Si4 confirms that at 70 K a long-range order of Ho magnetic moments appears yielding a spontaneous magnetization. The size of magnetization, however, is more than order of magnitude smaller than the expected full moment of five Ho3+ magnetic moments per formula unit. Therefore we propose that the magnetic phase existing between 23 and 70 K is not simple ferromagnetic. The spontaneous magnetization increases with decreasing temperature but does not exceed 10 mB/f.u. even at the lowest temperature of measurement (2 K). The magnetization curve poorly saturates in magnetic fields up to 14 T, however, it show no feature in the entire range of applied fields. The poor saturation may be reflecting gradual
Table 1 Characteristic temperatures of magnetic phase transitions for Ho5SixGe4x compounds in the vicinity of the 1:1 Si:Ge ratio and the magnitude of magnetoresistance at 2 K and 5 T with respect to zero field Ho5SixGe4x
TN (K)
T1 (K)
T2 (K)
MR (%)
x ¼ 1.5 x ¼ 2.0 x ¼ 2.5
37 35 34
20 22 21
10 15 14
14 5.5 15
The critical temperatures are determined from specific-heat data.
315
B (T)
Fig. 5. Magnetization curves for the Ho5Ge4 compound. Curves are shifted by 5 mB for clarity. Dashed lines are data measured with decreasing field.
aligning of Ho magnetic moments from the originally noncollinear magnetic structure. The magnetic phase transition at 23 K reflects in a considerable change of shape of the M vs. B dependences at lower temperatures. In particular, a field-induced magnetic phase transition around 3 T gradually emerges with decreasing temperature indicating increasing role of antiferromagnetic interactions. Below 16 K, the S-shape of the transition becomes considerably more pronounced and at temperatures below 10 K also a considerable hysteresis develops. A field-induced magnetic phase transition develops analogously also on the magnetization curves measured on Ho5Ge4 (in this case around 15 K) at temperatures below 23 K and simultaneously, also a small spontaneous magnetization seems to emerge in this compound with decreasing temperature. These results suggest that the lowtemperature (To23 K) magnetic structures in Ho5Ge4 may be analogous to these in existing the Ho5Si4. 3. Conclusions
B (T)
Fig. 4. Magnetization curves for the Ho5Si4 compound. Curves are shifted by 5 mB for clarity. Dashed lines are data measured with decreasing field.
In conclusion, we have investigated the Ho5SixGe4x system for x ¼ 0, 0.5, 1.5, 2.0, 2.5, 3.5, 4.0 by means of measurements of transport, magnetic and thermodynamic properties. Our results are in partial agreement with previous studies and extend knowledge of this system. In Ho5Si4, magnetic ordering is observed below 70 K characterized a small spontaneous magnetization indicating a complex ordering of Ho magnetic moments, which is changed by magnetic phase transitions at 23 and 16 K to another complex spin structure. Ho5Ge4, on the other hand, seems to exhibit antiferromagnetism below 23 K ( ¼ TN). The striking similarity of behavior of Ho5Si4 and Ho5Ge4 (similar features in temperature dependences of the specific heat and resistivity at 23 and 16 K and a
ARTICLE IN PRESS 316
J. Proklesˇka et al. / Journal of Magnetism and Magnetic Materials 316 (2007) 313–316
metamagnetic transition seen on magnetization curves at temperatures below 23 K) allows speculations about common microscopic mechanism. Based on our results we expect the suppression of the high-temperature (423 K) spontaneous magnetization in Si rich part of phase diagram for the composition x ¼ 1. The specific heat and resistivity data of Ho5Si2Ge2 show an additional anomaly indicating the onset of the magnetic order at higher temperatures than previously reported. Acknowledgment This work is a part of the research plan MSM 0021620834 that is financed by the Ministry of Education of the Czech Republic.
References [1] V.K. Pecharsky, K.A. Gschneidner Jr., Phys. Rev. Lett. 78 (1997) 14494. [2] F. Holtzberg, R.J. Gambino, T.R. McGuire, J. Phys. Chem. Solids 28 (1967) 2283. [3] N. Thuy, et al., J. Magn. Magn. Mater. 262 (2003) 432.