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Magnetic properties of RFe4Al8 compounds studied by specific heat measurements
Journal of Alloys and Compounds 278 (1998) 80–82
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Magnetic properties of RFe 4 Al 8 compounds studied by specific heat measurements ¨ I.H. Hagmusa*, E. Bruck, F.R. de Boer, K.H.J. Buschow Van der Waals-Zeeman Institute, University of Amsterdam, Valckenierstraat 65, 1018 XE Amsterdam, The Netherlands Received 6 May 1998
Abstract Specific-heat measurements have been performed on the tetragonal ThMn 12 -type compounds NdFe 4 Al 8 , HoFe 4 Al 8 , ErFe 4 Al 8 and YFe 4 Al 8 in the temperature range 1.5–200 K. There are only very small anomalies associated with the magnetic ordering of the Fe sublattice in the high-temperature part of the specific-heat curves and with the magnetic ordering of Nd and Ho in the low-temperature part. A sharp peak in the specific heat, indicative of magnetic ordering of the Er moments at 5.5 K, is observed in ErFe 4 Al 8 . 1998 Elsevier Science S.A. All rights reserved. Keywords: Ternary rare-earth compounds; Rare-earth iron aluminides; Magnetic properties; Specific heat
1. Introduction The rare-earth compounds of the type RFe 4 Al 8 form in the relatively simple ThMn 12 structure. Neutron diffraction investigations have shown that the Fe atoms occupy almost exclusively the 8f position in the latter structure type [1,2]. There is only a single rare-earth site. From results of ¨ magnetic measurements, 57 Fe-Mossbauer spectroscopy and neutron diffraction, it can be derived that the magnetic properties are dominated by antiferromagnetic Fe–Fe interactions leading to magnetic ordering temperatures in the range 135–200 K [2,3]. The R moments order at much lower temperature, but much experimental information on the magnetic ordering of the R moments is still lacking. In order to obtain a better understanding of the magnetic interactions in this interesting class of magnetic materials we have performed specific-heat measurements on several of these RFe 4 Al 8 compounds (R5Nd, Ho, Er and Y) in the temperature range 1.5–200 K.
2. Experimental The RFe 4 Al 8 compounds with R5Nd, Ho, Er and Y were prepared in polycrystalline form by melting stoichiometric amounts of the elements (of at least 99.9% purity) *Corresponding author. 0925-8388 / 98 / $19.00 1998 Elsevier Science S.A. All rights reserved. PII: S0925-8388( 98 )00643-4
in an arc furnace under a reduced argon atmosphere. The samples were subsequently vacuum annealed at 8008C for several weeks. All samples were characterized by X-ray diffraction and shown to be approximately single phase with Bragg peaks consistent with the ThMn 12 structure. We have measured the specific heat in zero field in the temperature range from 1.5 to 200 K. About 200–400 mg of each compound were mounted on a sapphire plate in a thermal frame by using apiezone. This setup comprises the possibilities for measurements using the standard adiabatic method.
3. Results and discussion The specific heat of the compounds NdFe 4 Al 8 , HoFe 4 Al 8 , ErFe 4 Al 8 and YFe 4 Al 8 has been measured at zero magnetic field as a function of temperature. Plots of c /T versus T of the first three compounds can be compared with the corresponding plot of YFe 4 Al 8 in Fig. 1. Because Y is non-magnetic, the data of YFe 4 Al 8 can be taken to be representative of the phonon contributions and the magnetic contribution of the Fe sublattice to the specific heat. For the comparison made in Fig. 1, the data of YFe 4 Al 8 have been corrected for the mass difference with the corresponding other RFe 4 Al 8 compounds shown. The results for YFe 4 Al 8 are shown in more detail in Fig. 2,
I.H. Hagmusa et al. / Journal of Alloys and Compounds 278 (1998) 80 – 82
81
Fig. 2. Temperature dependence of the specific heat (c) of YFe 4 Al 8 . The solid curve represents a fit to the data points made on the basis of Eq. (1).
Fig. 1. Temperature dependence of the specific heat (c /T ) of the compounds NdFe 4 Al 8 , HoFe 4 Al 8 , ErFe 4 Al 8 and YFe 4 Al 8 .
where they can be compared with a curve calculated by using the formula uD ] T
T 3 c 5 g T 1 9nk B s ] uD d
x e dx E ]]] (e 2 1) 4 x
x
2
(1)
0
where g is the electronic specific-heat coefficient, QD the
Debye temperature and x5hn /KB T. From a plot of c /T versus T 2 of the low-temperature data, we have determined g to be equal to 0.06 J K 22 mol 21 . From fitting the specific-heat curve over the whole temperature range considered, we derived QD 5429 K. Previous studies of YFe 4 Al 8 made by means of neutron¨ diffraction [2] and Fe-Mossbauer spectroscopy [3] have shown that the Fe atoms are magnetic and give rise to complex antiferromagnetic ordering at about 185 K. In view of these results, it is surprising that the magnetic phase transition does not show up as an anomaly in the specific-heat curve. Previous studies of the magnetic properties of RFe 4 Al 8 compounds have shown that the Fe sublatice becomes magnetically ordered at 142 K in NdFe 4 Al 8 [3], at 185 K in HoFe 4 Al 8 [4] and at 183 K in ErFe 4 Al 8 [3]. Inspection of the results displayed in Fig. 1 shows that there is a small anomaly in the specific-heat curve of ErFe 4 Al 8 at the Fe-ordering temperatures reported for this compound, but there is no indication of the Fe moment ordering in NdFe 4 Al 8 and HoFe 4 Al 8 . In the low-temperature regime, below 50 K, all three compounds show some type of anomaly. In NdFe 4 Al 8 there is a small anomaly at about 20 K. In HoFe 4 Al 8 there is a small anomaly at about 10 K, which corresponds to the sharp peak observed at this temperature in the temperature dependence of the ACsusceptibility by Talik et al. [4]. The strong rise of the specific heat shown for this compound below 2 K in Fig. 1 is attributed to a nuclear contribution of Ho and is left out of consideration here. The low-temperature anomaly is
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I.H. Hagmusa et al. / Journal of Alloys and Compounds 278 (1998) 80 – 82
most pronounced in ErFe 4 Al 8 where it is seen to lead to a sharp peak in the temperature dependence of the specific heat at 5.5 K. It is very likely that all these anomalies are associated with magnetic phase transition in which the R moments also participate. In the crystal structure of the RFe 4 Al 8 compounds, the R atoms reside in the centre of tetragonal prisms formed by the Fe atoms. In view of the prevailing antiferromagnetism of the Fe sublattice, this means that the molecular field of the Fe moments experienced by the R moments is fairly weak. An estimate of the degree of R moment ordering as a function of temperature can be obtained from the difference in specific-heat curves shown in Fig. 1 for RFe 4 Al 8 and the curve for YFe 4 Al 8 . It can be seen that the contribution of the R moment ordering extends to much higher temperature in NdFe 4 Al 8 than in HoFe 4 Al 8 and ErFe 4 Al 8 . In fact, the sharpness of the transition observed for ErFe 4 Al 8 suggests that the ordering of the Er moments proceeds independently from the Fe sublattice at 5.5 K. This opens the possibility of determining the magnetic contribution of the Er sublattice by subtracting the specific-heat contribution of YFe 4 Al 8 from the ErFe 4 Al 8 data. A more detailed analysis of the temperature dependence of the magnetic Er sublattice contribution to the specific heat will be given elsewhere [5] and compared with the crystal field level scheme derived from inelastic neutron scattering data [6]. The interpretation of the low-temperature peak in the temperature dependence of ErFe 4 Al 8 as being due to magnetic ordering of the Er sublattice is in agreement with ¨ results obtained by rare earth Mossbauer spectroscopy on several of the RFe 4 Al 8 compounds [7]. From the temperature dependence of the quadrupole splitting of the 169 Tm spectra of TmFe 4 Al 8 and the temperature dependence of the Zeeman splitting of the 161 Dy spectra of DyFe 4 Al 8 , experimental information was obtained on the molecular field experienced by the corresponding rare earth moments. By using a molecular field approach these values have
subsequently been used to obtain an estimate of the magnetic ordering temperatures of the corresponding rare earth sublattices, leading to values of 2 K for TmFe 4 Al 8 and 43 K for DyFe 4 Al 8 . A much lower magnetic ordering temperature for the Dy sublattice (25 K) was reported in a recent neutron diffraction study of DyFe 4 Al 8 [8]. The value of the Er ordering temperature of 5.5 K obtained in the course of the present investigation for ErFe 4 Al 8 nicely fits into this sequence. However, the three ordering temperatures do not scale to the DeGennes factor, even if accepting the latter value for DyFe 4 Al 8 . This shows that the assumption that the R sublattice ordering is exclusively caused by the R–R interaction may be an oversimplification. It is interesting to note, though, that the ordering temperature of 5.5 K for ErFe 4 Al 8 is very close to the value of 5.2 K found for ErCu 4 Al 8 [9], a compound in which any other type of magnetic coupling between the R moments is absent.
References [1] O. Moze, R.M. Ibberson, K.H.J. Buschow, J. Phys. Condens. Matter 2 (1990) 1677. [2] P. Schobinger-Papamantellos, K.H.J. Buschow, C. Ritter, J. Magn. Magn. Mater. 186 (1998) 21. [3] K.H.J. Buschow, A.M. van der Kraan, J. Phys. F: Met. Phys. 8 (1978) 921. [4] E. Talik, J. Heimann, J. Szade, T. Mydlarz, Physica B 205 (1995) 127. ¨ [5] I.H. Hagmusa, E. Bruck, F.R. de Boer, K.H.J. Buschow, J. Magn. Magn. Mater., submitted. [6] R. Caciuffo, G. Amoretti, K.H.J. Buschow, O. Moze, A.P. Murani, B. Paci, J. Phys. Condens. Matter 7 (1995) 7981. [7] P.C. Gubbens, A.M. van der Kraan, K.H.J. Buschow, J. Magn. Magn. Mater. 27 (1982) 61. [8] J.A. Paixao, S. Langridge, S.Aa. Sørensen, B. Lebech, A.P. Gonc¸alves, G.H. Lander, P.J. Brown, P. Burlet, E. Talik, Physica B 234–236 (1997) 614. ¨ [9] I.H. Hagmusa, F.E. Kayzel, E. Bruck, K.H.J. Buschow, J. Appl. Phys. (1998), in press.
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Magnetic properties of RFe 4 Al 8 compounds studied by specific heat measurements ¨ I.H. Hagmusa*, E. Bruck, F.R. de Boer, K.H.J. Buschow Van der Waals-Zeeman Institute, University of Amsterdam, Valckenierstraat 65, 1018 XE Amsterdam, The Netherlands Received 6 May 1998
Abstract Specific-heat measurements have been performed on the tetragonal ThMn 12 -type compounds NdFe 4 Al 8 , HoFe 4 Al 8 , ErFe 4 Al 8 and YFe 4 Al 8 in the temperature range 1.5–200 K. There are only very small anomalies associated with the magnetic ordering of the Fe sublattice in the high-temperature part of the specific-heat curves and with the magnetic ordering of Nd and Ho in the low-temperature part. A sharp peak in the specific heat, indicative of magnetic ordering of the Er moments at 5.5 K, is observed in ErFe 4 Al 8 . 1998 Elsevier Science S.A. All rights reserved. Keywords: Ternary rare-earth compounds; Rare-earth iron aluminides; Magnetic properties; Specific heat
1. Introduction The rare-earth compounds of the type RFe 4 Al 8 form in the relatively simple ThMn 12 structure. Neutron diffraction investigations have shown that the Fe atoms occupy almost exclusively the 8f position in the latter structure type [1,2]. There is only a single rare-earth site. From results of ¨ magnetic measurements, 57 Fe-Mossbauer spectroscopy and neutron diffraction, it can be derived that the magnetic properties are dominated by antiferromagnetic Fe–Fe interactions leading to magnetic ordering temperatures in the range 135–200 K [2,3]. The R moments order at much lower temperature, but much experimental information on the magnetic ordering of the R moments is still lacking. In order to obtain a better understanding of the magnetic interactions in this interesting class of magnetic materials we have performed specific-heat measurements on several of these RFe 4 Al 8 compounds (R5Nd, Ho, Er and Y) in the temperature range 1.5–200 K.
2. Experimental The RFe 4 Al 8 compounds with R5Nd, Ho, Er and Y were prepared in polycrystalline form by melting stoichiometric amounts of the elements (of at least 99.9% purity) *Corresponding author. 0925-8388 / 98 / $19.00 1998 Elsevier Science S.A. All rights reserved. PII: S0925-8388( 98 )00643-4
in an arc furnace under a reduced argon atmosphere. The samples were subsequently vacuum annealed at 8008C for several weeks. All samples were characterized by X-ray diffraction and shown to be approximately single phase with Bragg peaks consistent with the ThMn 12 structure. We have measured the specific heat in zero field in the temperature range from 1.5 to 200 K. About 200–400 mg of each compound were mounted on a sapphire plate in a thermal frame by using apiezone. This setup comprises the possibilities for measurements using the standard adiabatic method.
3. Results and discussion The specific heat of the compounds NdFe 4 Al 8 , HoFe 4 Al 8 , ErFe 4 Al 8 and YFe 4 Al 8 has been measured at zero magnetic field as a function of temperature. Plots of c /T versus T of the first three compounds can be compared with the corresponding plot of YFe 4 Al 8 in Fig. 1. Because Y is non-magnetic, the data of YFe 4 Al 8 can be taken to be representative of the phonon contributions and the magnetic contribution of the Fe sublattice to the specific heat. For the comparison made in Fig. 1, the data of YFe 4 Al 8 have been corrected for the mass difference with the corresponding other RFe 4 Al 8 compounds shown. The results for YFe 4 Al 8 are shown in more detail in Fig. 2,
I.H. Hagmusa et al. / Journal of Alloys and Compounds 278 (1998) 80 – 82
81
Fig. 2. Temperature dependence of the specific heat (c) of YFe 4 Al 8 . The solid curve represents a fit to the data points made on the basis of Eq. (1).
Fig. 1. Temperature dependence of the specific heat (c /T ) of the compounds NdFe 4 Al 8 , HoFe 4 Al 8 , ErFe 4 Al 8 and YFe 4 Al 8 .
where they can be compared with a curve calculated by using the formula uD ] T
T 3 c 5 g T 1 9nk B s ] uD d
x e dx E ]]] (e 2 1) 4 x
x
2
(1)
0
where g is the electronic specific-heat coefficient, QD the
Debye temperature and x5hn /KB T. From a plot of c /T versus T 2 of the low-temperature data, we have determined g to be equal to 0.06 J K 22 mol 21 . From fitting the specific-heat curve over the whole temperature range considered, we derived QD 5429 K. Previous studies of YFe 4 Al 8 made by means of neutron¨ diffraction [2] and Fe-Mossbauer spectroscopy [3] have shown that the Fe atoms are magnetic and give rise to complex antiferromagnetic ordering at about 185 K. In view of these results, it is surprising that the magnetic phase transition does not show up as an anomaly in the specific-heat curve. Previous studies of the magnetic properties of RFe 4 Al 8 compounds have shown that the Fe sublatice becomes magnetically ordered at 142 K in NdFe 4 Al 8 [3], at 185 K in HoFe 4 Al 8 [4] and at 183 K in ErFe 4 Al 8 [3]. Inspection of the results displayed in Fig. 1 shows that there is a small anomaly in the specific-heat curve of ErFe 4 Al 8 at the Fe-ordering temperatures reported for this compound, but there is no indication of the Fe moment ordering in NdFe 4 Al 8 and HoFe 4 Al 8 . In the low-temperature regime, below 50 K, all three compounds show some type of anomaly. In NdFe 4 Al 8 there is a small anomaly at about 20 K. In HoFe 4 Al 8 there is a small anomaly at about 10 K, which corresponds to the sharp peak observed at this temperature in the temperature dependence of the ACsusceptibility by Talik et al. [4]. The strong rise of the specific heat shown for this compound below 2 K in Fig. 1 is attributed to a nuclear contribution of Ho and is left out of consideration here. The low-temperature anomaly is
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most pronounced in ErFe 4 Al 8 where it is seen to lead to a sharp peak in the temperature dependence of the specific heat at 5.5 K. It is very likely that all these anomalies are associated with magnetic phase transition in which the R moments also participate. In the crystal structure of the RFe 4 Al 8 compounds, the R atoms reside in the centre of tetragonal prisms formed by the Fe atoms. In view of the prevailing antiferromagnetism of the Fe sublattice, this means that the molecular field of the Fe moments experienced by the R moments is fairly weak. An estimate of the degree of R moment ordering as a function of temperature can be obtained from the difference in specific-heat curves shown in Fig. 1 for RFe 4 Al 8 and the curve for YFe 4 Al 8 . It can be seen that the contribution of the R moment ordering extends to much higher temperature in NdFe 4 Al 8 than in HoFe 4 Al 8 and ErFe 4 Al 8 . In fact, the sharpness of the transition observed for ErFe 4 Al 8 suggests that the ordering of the Er moments proceeds independently from the Fe sublattice at 5.5 K. This opens the possibility of determining the magnetic contribution of the Er sublattice by subtracting the specific-heat contribution of YFe 4 Al 8 from the ErFe 4 Al 8 data. A more detailed analysis of the temperature dependence of the magnetic Er sublattice contribution to the specific heat will be given elsewhere [5] and compared with the crystal field level scheme derived from inelastic neutron scattering data [6]. The interpretation of the low-temperature peak in the temperature dependence of ErFe 4 Al 8 as being due to magnetic ordering of the Er sublattice is in agreement with ¨ results obtained by rare earth Mossbauer spectroscopy on several of the RFe 4 Al 8 compounds [7]. From the temperature dependence of the quadrupole splitting of the 169 Tm spectra of TmFe 4 Al 8 and the temperature dependence of the Zeeman splitting of the 161 Dy spectra of DyFe 4 Al 8 , experimental information was obtained on the molecular field experienced by the corresponding rare earth moments. By using a molecular field approach these values have
subsequently been used to obtain an estimate of the magnetic ordering temperatures of the corresponding rare earth sublattices, leading to values of 2 K for TmFe 4 Al 8 and 43 K for DyFe 4 Al 8 . A much lower magnetic ordering temperature for the Dy sublattice (25 K) was reported in a recent neutron diffraction study of DyFe 4 Al 8 [8]. The value of the Er ordering temperature of 5.5 K obtained in the course of the present investigation for ErFe 4 Al 8 nicely fits into this sequence. However, the three ordering temperatures do not scale to the DeGennes factor, even if accepting the latter value for DyFe 4 Al 8 . This shows that the assumption that the R sublattice ordering is exclusively caused by the R–R interaction may be an oversimplification. It is interesting to note, though, that the ordering temperature of 5.5 K for ErFe 4 Al 8 is very close to the value of 5.2 K found for ErCu 4 Al 8 [9], a compound in which any other type of magnetic coupling between the R moments is absent.
References [1] O. Moze, R.M. Ibberson, K.H.J. Buschow, J. Phys. Condens. Matter 2 (1990) 1677. [2] P. Schobinger-Papamantellos, K.H.J. Buschow, C. Ritter, J. Magn. Magn. Mater. 186 (1998) 21. [3] K.H.J. Buschow, A.M. van der Kraan, J. Phys. F: Met. Phys. 8 (1978) 921. [4] E. Talik, J. Heimann, J. Szade, T. Mydlarz, Physica B 205 (1995) 127. ¨ [5] I.H. Hagmusa, E. Bruck, F.R. de Boer, K.H.J. Buschow, J. Magn. Magn. Mater., submitted. [6] R. Caciuffo, G. Amoretti, K.H.J. Buschow, O. Moze, A.P. Murani, B. Paci, J. Phys. Condens. Matter 7 (1995) 7981. [7] P.C. Gubbens, A.M. van der Kraan, K.H.J. Buschow, J. Magn. Magn. Mater. 27 (1982) 61. [8] J.A. Paixao, S. Langridge, S.Aa. Sørensen, B. Lebech, A.P. Gonc¸alves, G.H. Lander, P.J. Brown, P. Burlet, E. Talik, Physica B 234–236 (1997) 614. ¨ [9] I.H. Hagmusa, F.E. Kayzel, E. Bruck, K.H.J. Buschow, J. Appl. Phys. (1998), in press.