- Email: [email protected]
Specific heat study of the magnetic properties of ErFe4Al8
~
Journalof
-,~_" magnetism and magnetic materials ELSEVIER
Journal of Magnetism and Magnetic Materials 196-197 (1999) 625-626
Specific heat study of the magnetic properties of ErFe4A18 I.H. Hagmusa, E. BrOck, F.R. de Boer, K.H.J. Buschow* Van der Waals-Zeeman Institute, University of Amsterdam, Valckenierstraat 65, 1018 XE Amsterdam, The Netherlands
Abstract
Specific heat measurements have been performed on the tetragonal ThMnl 2-type compounds ErFe4A18 and YFe4A18 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. A sharp peak in the specific heat, signalling the magnetic ordering of the Er moments at 5.5 K is observed in ErFe4A18. In order to separate out the effect of the Er sublattice we have subtracted the data obtained for YFe4A18 in the same temperature range. From the temperature dependence of the magnetic contribution of the Er moments to the specific heat of ErFe4AI8 we obtained the temperature dependence of the magnetic entropy. We discussed our results in terms of the crystal-field level scheme proposed earlier on the basis of inelastic neutron scattering. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Rare-earth compounds; Erbium iron aluminide; Magnetic ordering; Specific heat anomaly
Rare-earth compounds of the type RFe4A18 crystallise in the relatively simple ThMn12 structure. Neutron diffraction has shown that the Fe atoms occupy almost exclusively the 8f position in the latter structure type [1,2] while there is only a single rare-earth site. From results of magnetic measurements, 57Fe-M6ssbauer spectroscopy and neutron diffraction, it can be derived that the magnetic properties are dominated by antiferromagnetic F e - F e interactions leading to magnetic ordering temperatures in the range 135-200K [-2,3]. The R moments order at much lower temperature but the nature of the magnetic ordering of the R moments is still unclear. In order to contribute to the understanding of the magnetic interactions in this interesting class of magnetic materials we have performed specific-heat measurements on ErFe4AI8 in the temperature range 1.5-200 K. For comparative purposes we have investigated also the YFegA18 compound. Details of sample preparation and specific heat measurements have been given in Ref. [4]. Specific heat measurements made on ErFe4A18 and YFegA18 and several other RFe4A18 compounds [-4] have
*Corresponding author. Tel.: + 31-20-5255714; fax: + 3120-5255788; e-mail: [email protected].
shown that there is hardly any anomaly in the specific heat curve of these compounds at the Fe-ordering temperatures in the 135-200 K range. By contrast, we found that there is a very pronounced low-temperature anomaly in the temperature dependence of the specific heat of ErFegA18. This anomaly occurs in the form of a sharp peak at 5.5 K. Very likely the latter anomaly has to be associated with a magnetic phase transition in which also the Er moments participate. In view of the fact that the Fe sublattice orders already at about 180 K it seems surprising that the Er sublattice becomes magnetically ordered at much lower temperatures. In the crystal structure of the ErFe4AI8 compound, the Er atoms reside in the centre of tetragonal prims formed by the Fe atoms. Details regarding the magnetic ordering of the iron sublattice in RFegA18 compounds can be found in a recent report of a neutron diffraction study of several RFe4A18 compounds [-2] in which R is nonmagnetic (R = La, Ce, Y and Lu). This study has confirmed that the Fe sublattice orders antiferromagnetically at fairly high temperatures [-3], but it has also shown that the magnetic ordering of the Fe moments is far from simple. The prevailing antiferromagnetism of the Fe sublattice is the reason why the molecular field due the Fe moments at the R sites
0304-8853/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 9 8 ) 0 0 8 74-9
626
L H. Hagmusa et al. / Journal of MagnetLsm and Magnetic: Materials 196-197 ( 1999) 625- 626
vanishes in this crystal structure or, at best, is only fairly weak. In fact, the sharpness of the transition observed for ErFe4A18 suggests that the ordering of the Er moment at 5.5 K proceeds independently from the Fe sublattice. Therefore, it seems reasonable to determine the magnetic contribution of the Er sublattice by subtracting from the ErFe4AI8 data the specific-heat contributions of YFe4AIs, the latter representing the phonon contributions and the magnetic contribution of the Fe sublattice to the specific heat. For this purpose the data of YFe4AIs have first been corrected for the mass difference with the Er compound. More details of the analysis of the specific heat of YFe4Als are presented in Ref. [4]. The temperature dependence of the Er sublattice contribution to the specific heat obtained in this way is shown in Fig. 1. These data have been used, in turn, to obtain the temperature dependence of SIn~R, where Sm is the magnetic entropy of the Er sublattice. The temperature dependence of Sm/R is also shown in Fig. 1. The crystal-field splitting in the ErFe4Als compound has been studied earlier by inelastic neutron-scattering experiments [5]. These experiments have been interpreted by means of a level scheme in which the ground state is a doublet. The first excited level, lying at about 10 K above the ground state, is also a doublet dominated by the lJ : = -+ 9 > wave function. The overall crystalfield splitting is about 130 K. The magnetic ordering at about 5.5 K is therefore expected to involve mainly the ground state doublet, and the entropy reached at the magnetic transition is expected to be equal to R In 2 = 0.7. This is in satisfactory agreement with the data shown in Fig. 1. The shoulder on the high-temperature side of the peak has probably to be associated with the population of the first excited doublet state at about 10 K mentioned above. Finally, at high temperatures, the entropy is expected to reach its maximum value R ln(2J + 1). For Er with J = 15/2 this maximum value corresponds to Sm/R = ln(16) = 2.77. This value is seen in Fig. 1 to be almost reached at 100 K. Concluding, the present specific heat data are in concord with the crystal-field level scheme derived previously on the basis of inelastic neutron spectroscopy [5]. The original aim of the latter investigation was to derive reliable values of crystal-field parameters by means of which a realistic description could be given of the crystal-field-induced anisotropy in permanent magnet materials of the type RFe12 ~M~. The A2° parameter is c o m m o n l y considered as the leading quantity in descriptions of the crystal-field-induced anisotropy. Experimental values for several of the RFex2 xM~ compounds are compared in Table 1. Unlike the RzFe14B and R2Co~4B compounds, where A2° changes only slightly when going from Fe to Co, one finds that this quantity depends very strongly on the transition metal component in the RFe~2 ~M~-type materials. This strongly hampers a priory predictions of the crystal-field-induced anisot-
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.
ErFe4AI 8
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0.6
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,
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T(K) Fig. 1. Temperature dependence of the specific heat iinset) and temperature dependence of the magnetic contribution to the specific heat (Cm'T) of ErFe4Als (left scalel. The solid curve represents the temperature dependence of the magnetic entropy (right scale).
Table 1 Comparision of the second-order crystal-field parameter ,4~ in several ThMnx2-type compounds. A value averaged over several R components has been listed for RFe4AI8 and RMn4AI,; R represents Dy and Gd for the first and second compound, respectively Compound
A° (K/ao)
References
RFel 1Ti RCol 1Ti RFe~Als RMn4AIs
32 + 80 + 255 175
[6] [7] [5] [5]
ropy when substituting other elements for Fe in these materials.
References [1] O. Moze, R.M, lbberson, K.H.J. Buschow, J. Plays.: Condens. Matter 2 (1990) 1677. [2] P. Schobinger-Papamantellos, KH.J. Buschow, C. Ritter, J. Magn. Magn. Mater. 186 (1998)21. [3] K.H.J. Buschow, A.M. van der Kraan, J. Phys. F 8 (1978) 921. [4] I.H. Hagmusa, E. Briick, F.R. de Boer, K.HJ. Buschow, J. Alloys Compounds 80 (1998) 278. [5] R. Caciuffo, G. Amoretti, K.H.J. Buschow, O. Moze. A.P. Murani, B. Paci, J. Phys.: Condends. Matter 7 (1995) 798l. [6] B.-P. Hu, P.-S. Li, J.P. Gavigan, J.M.D. Co W, Phys. Rcv. B 41 (1990) 2221. [7] F.M. Mulder, R.C. Thiel, L.D. Tung, J.J.M. Franse, K.H.J. Buschow, J. Alloys Compounds 264 (1998) 43.
Journalof
-,~_" magnetism and magnetic materials ELSEVIER
Journal of Magnetism and Magnetic Materials 196-197 (1999) 625-626
Specific heat study of the magnetic properties of ErFe4A18 I.H. Hagmusa, E. BrOck, F.R. de Boer, K.H.J. Buschow* Van der Waals-Zeeman Institute, University of Amsterdam, Valckenierstraat 65, 1018 XE Amsterdam, The Netherlands
Abstract
Specific heat measurements have been performed on the tetragonal ThMnl 2-type compounds ErFe4A18 and YFe4A18 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. A sharp peak in the specific heat, signalling the magnetic ordering of the Er moments at 5.5 K is observed in ErFe4A18. In order to separate out the effect of the Er sublattice we have subtracted the data obtained for YFe4A18 in the same temperature range. From the temperature dependence of the magnetic contribution of the Er moments to the specific heat of ErFe4AI8 we obtained the temperature dependence of the magnetic entropy. We discussed our results in terms of the crystal-field level scheme proposed earlier on the basis of inelastic neutron scattering. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Rare-earth compounds; Erbium iron aluminide; Magnetic ordering; Specific heat anomaly
Rare-earth compounds of the type RFe4A18 crystallise in the relatively simple ThMn12 structure. Neutron diffraction has shown that the Fe atoms occupy almost exclusively the 8f position in the latter structure type [1,2] while there is only a single rare-earth site. From results of magnetic measurements, 57Fe-M6ssbauer spectroscopy and neutron diffraction, it can be derived that the magnetic properties are dominated by antiferromagnetic F e - F e interactions leading to magnetic ordering temperatures in the range 135-200K [-2,3]. The R moments order at much lower temperature but the nature of the magnetic ordering of the R moments is still unclear. In order to contribute to the understanding of the magnetic interactions in this interesting class of magnetic materials we have performed specific-heat measurements on ErFe4AI8 in the temperature range 1.5-200 K. For comparative purposes we have investigated also the YFegA18 compound. Details of sample preparation and specific heat measurements have been given in Ref. [4]. Specific heat measurements made on ErFe4A18 and YFegA18 and several other RFe4A18 compounds [-4] have
*Corresponding author. Tel.: + 31-20-5255714; fax: + 3120-5255788; e-mail: [email protected].
shown that there is hardly any anomaly in the specific heat curve of these compounds at the Fe-ordering temperatures in the 135-200 K range. By contrast, we found that there is a very pronounced low-temperature anomaly in the temperature dependence of the specific heat of ErFegA18. This anomaly occurs in the form of a sharp peak at 5.5 K. Very likely the latter anomaly has to be associated with a magnetic phase transition in which also the Er moments participate. In view of the fact that the Fe sublattice orders already at about 180 K it seems surprising that the Er sublattice becomes magnetically ordered at much lower temperatures. In the crystal structure of the ErFe4AI8 compound, the Er atoms reside in the centre of tetragonal prims formed by the Fe atoms. Details regarding the magnetic ordering of the iron sublattice in RFegA18 compounds can be found in a recent report of a neutron diffraction study of several RFe4A18 compounds [-2] in which R is nonmagnetic (R = La, Ce, Y and Lu). This study has confirmed that the Fe sublattice orders antiferromagnetically at fairly high temperatures [-3], but it has also shown that the magnetic ordering of the Fe moments is far from simple. The prevailing antiferromagnetism of the Fe sublattice is the reason why the molecular field due the Fe moments at the R sites
0304-8853/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 9 8 ) 0 0 8 74-9
626
L H. Hagmusa et al. / Journal of MagnetLsm and Magnetic: Materials 196-197 ( 1999) 625- 626
vanishes in this crystal structure or, at best, is only fairly weak. In fact, the sharpness of the transition observed for ErFe4A18 suggests that the ordering of the Er moment at 5.5 K proceeds independently from the Fe sublattice. Therefore, it seems reasonable to determine the magnetic contribution of the Er sublattice by subtracting from the ErFe4AI8 data the specific-heat contributions of YFe4AIs, the latter representing the phonon contributions and the magnetic contribution of the Fe sublattice to the specific heat. For this purpose the data of YFe4AIs have first been corrected for the mass difference with the Er compound. More details of the analysis of the specific heat of YFe4Als are presented in Ref. [4]. The temperature dependence of the Er sublattice contribution to the specific heat obtained in this way is shown in Fig. 1. These data have been used, in turn, to obtain the temperature dependence of SIn~R, where Sm is the magnetic entropy of the Er sublattice. The temperature dependence of Sm/R is also shown in Fig. 1. The crystal-field splitting in the ErFe4Als compound has been studied earlier by inelastic neutron-scattering experiments [5]. These experiments have been interpreted by means of a level scheme in which the ground state is a doublet. The first excited level, lying at about 10 K above the ground state, is also a doublet dominated by the lJ : = -+ 9 > wave function. The overall crystalfield splitting is about 130 K. The magnetic ordering at about 5.5 K is therefore expected to involve mainly the ground state doublet, and the entropy reached at the magnetic transition is expected to be equal to R In 2 = 0.7. This is in satisfactory agreement with the data shown in Fig. 1. The shoulder on the high-temperature side of the peak has probably to be associated with the population of the first excited doublet state at about 10 K mentioned above. Finally, at high temperatures, the entropy is expected to reach its maximum value R ln(2J + 1). For Er with J = 15/2 this maximum value corresponds to Sm/R = ln(16) = 2.77. This value is seen in Fig. 1 to be almost reached at 100 K. Concluding, the present specific heat data are in concord with the crystal-field level scheme derived previously on the basis of inelastic neutron spectroscopy [5]. The original aim of the latter investigation was to derive reliable values of crystal-field parameters by means of which a realistic description could be given of the crystal-field-induced anisotropy in permanent magnet materials of the type RFe12 ~M~. The A2° parameter is c o m m o n l y considered as the leading quantity in descriptions of the crystal-field-induced anisotropy. Experimental values for several of the RFex2 xM~ compounds are compared in Table 1. Unlike the RzFe14B and R2Co~4B compounds, where A2° changes only slightly when going from Fe to Co, one finds that this quantity depends very strongly on the transition metal component in the RFe~2 ~M~-type materials. This strongly hampers a priory predictions of the crystal-field-induced anisot-
1.6 f ! 1.4 1.2
~, E oa
1.0 ,
*
0.8
I-~l:
0
20
p*
.
ErFe4AI 8
-- 15 /
~1' "
0.6
3.0 2.5
,
/
1"5 09E
E "~
/
~
1.0
0.4 0,2 0.0
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0 25
50
50 75
100
0.5 0.0
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T(K) Fig. 1. Temperature dependence of the specific heat iinset) and temperature dependence of the magnetic contribution to the specific heat (Cm'T) of ErFe4Als (left scalel. The solid curve represents the temperature dependence of the magnetic entropy (right scale).
Table 1 Comparision of the second-order crystal-field parameter ,4~ in several ThMnx2-type compounds. A value averaged over several R components has been listed for RFe4AI8 and RMn4AI,; R represents Dy and Gd for the first and second compound, respectively Compound
A° (K/ao)
References
RFel 1Ti RCol 1Ti RFe~Als RMn4AIs
32 + 80 + 255 175
[6] [7] [5] [5]
ropy when substituting other elements for Fe in these materials.
References [1] O. Moze, R.M, lbberson, K.H.J. Buschow, J. Plays.: Condens. Matter 2 (1990) 1677. [2] P. Schobinger-Papamantellos, KH.J. Buschow, C. Ritter, J. Magn. Magn. Mater. 186 (1998)21. [3] K.H.J. Buschow, A.M. van der Kraan, J. Phys. F 8 (1978) 921. [4] I.H. Hagmusa, E. Briick, F.R. de Boer, K.HJ. Buschow, J. Alloys Compounds 80 (1998) 278. [5] R. Caciuffo, G. Amoretti, K.H.J. Buschow, O. Moze. A.P. Murani, B. Paci, J. Phys.: Condends. Matter 7 (1995) 798l. [6] B.-P. Hu, P.-S. Li, J.P. Gavigan, J.M.D. Co W, Phys. Rcv. B 41 (1990) 2221. [7] F.M. Mulder, R.C. Thiel, L.D. Tung, J.J.M. Franse, K.H.J. Buschow, J. Alloys Compounds 264 (1998) 43.