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Mictomagnetism and shifted magnetic hysteresis cycle in SmFeAl
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 284 (2004) 13–16 www.elsevier.com/locate/jmmm
Mictomagnetism and shifted magnetic hysteresis cycle in SmFeAl Yuntao Wanga, Huaiyu Shaoa, Xingguo Lia,, Lefu Zhangb, Seiki Takahashib a
College of Chemistry and Molecular Engineering, The State Key Laboratory of Rare Earth Materials Chemistry and Applications, Peking University, Beijing, 100871, China b Department of Materials Science and Engineering, Faculty of Engineering, Iwate University, 4-3-5 Ueda, Morioka, 020-8551, Japan Received 14 January 2004; received in revised form 21 May 2004 Available online 26 June 2004
Abstract Magnetic properties of SmFeAl with hexagonal structure were investigated by a superconducting quantum interference device from 4.2 to 300 K in different magnetic fields and different cooled processes. SmFeAl is a mictomagnetic phase showing different magnetic behaviors in the field-cooled (FC) process and zero-field-cooled process. The mictomagnetic transition temperature is about 80 K. In an FC sample, a shifted magnetic hysteresis cycle towards magnetization was observed below 80 K. r 2004 Elsevier B.V. All rights reserved. Keywords: SmFeAl compound; Mictomagnetism; Shifted hysteresis cycle; Field-induced magnetic transition
1. Introduction Magnetic properties of rare earth metals and alloys have been widely investigated and as a result, many interesting phenomena have been reported. The research focuses specifically on the compounds consisting of rare earth metals and 3d transition metals because of the applicable magnetic properties [1–5]. The binary intermetallic R–T compounds (R=rare earth metal, T=transition metal) have been widely investigated so far Corresponding author. Tel./fax: +861062765930.
E-mail address: [email protected] (X. Li).
and the research attention is now gradually extended to the ternary intermetallic R–T–M compounds (M=p-electron element). Among the several series of compounds studied, some notable properties have been discovered [6–12]. One series among them is RNiAl compounds with the Fe2P structure. Ehlers et al. firstly reported the differences of magnetic transition between the zerofield-cooled (ZFC) and field-cooled (FC) samples and found that the crystalline electric field around the rare earth ion often induced strong anisotropy and Ni had an important influence on the magnetic behavior, although no magnetic moment was found at the Ni sites [13].
0304-8853/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2004.06.011
ARTICLE IN PRESS 14
Y. Wang et al. / Journal of Magnetism and Magnetic Materials 284 (2004) 13–16
Since RFeAl has a composition and structure similar to RNiAl, we are interested in whether RFeAl compounds have similar or new magnetic properties. The compounds consisting of light rare earth elements usually have more multiformity in magnetism than those consisting of heavy rare earth elements [14–16]. Thus, we selected SmFeAl as our target compound in this study.
2. Experiment The intermetallic compound SmFeAl was prepared from 99.9% pure Sm, 99.5% pure Fe and 99.99% pure Al by arc melting in an argon gas atmosphere. Arc-melted ingot was flipped over and remelted thrice. To obtain a single phase, the ingot was homogenized at 1073 K, for 24 h in vacuum. The ingot was pulverized into powders as experimental sample. The structure of the sample was identified by powder X-ray diffraction (XRD) using monochromated Cu Ka radiation. About 10 mg sample powders were packaged into Teflon for magnetic measurements. The magnetic properties were measured at temperatures from 4.2 to 300 K by a superconducting quantum interference device (SQUID) magnetometry with an applied field of up to 20 and 50 kOe.
Fig. 1. The powder XRD pattern of SmFeAl.
3. Results and discussion Fig. 1 shows the XRD result of the sample. The SmFeAl compound crystallizes in a homogeneous hexagonal structure with the lattice constants of a ¼ 0:542 and c ¼ 0:878 nm. It agrees with the report of Dwight that the lattice constants are a ¼ 0:544 and c ¼ 0:884 nm [17]. Some weak peaks of FeAl, SmAl2 and Sm2(Fe0.44Al0.56)17 are also observed in the XRD pattern. These impure phases were formed during the arc-melting and annealing processes and caused by the violent evaporation of Sm at high temperature. However, the content of the impure phases is so small that they cannot dominate the magnetic properties of SmFeAl, especially the shifted hysteresis cycle of FC SmFeAl discussed later.
Fig. 2. Temperature dependence of the magnetization in 10 kOe for FC and ZFC SmFeAl. The cooling field of FC SmFeAl is 10 kOe and that of ZFC SmFeAl is 0 kOe.
Fig. 2 shows the temperature dependence of magnetization in 10 kOe for FC and ZFC SmFeAl. Below 150 K, ZFC curve departures from FC curve. The magnetization of ZFC SmFeAl process is lower than that in the FC one and decreases with decreasing temperature. The rapid reduction of magnetization with decreasing temperature in ZFC process implies that a mictomagnetic transition takes place. The transition temperature estimated from the intersection of the two tangent lines as shown in Fig. 2 is 80 K, below which the magnetic moments are blocked.
ARTICLE IN PRESS Y. Wang et al. / Journal of Magnetism and Magnetic Materials 284 (2004) 13–16
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In Fig. 3(a), the hysteresis cycle is almost the same in FC and ZFC SmFeAl and an inflection point in the cycle around 6 kOe is noticeable in both FC and ZFC SmFeAl. Spontaneous magnetization approaches a saturation value by two steps that are divided at 6 kOe. The staircase behavior expresses the occurrence of a fieldinduced magnetic transition. This is probably due to the fact that the sample has several axes of easy magnetization that demand different driving fields. A similar phenomenon is usually observed in the rare earth compounds. In Fig. 3(b), a shifted hysteresis cycle towards positive magnetization can be clearly observed below 80 K in FC sample. Although the displace-
ment of the hysteresis cycle towards negative magnetic field is reported, the displacement along with the direction of the magnetization has not been reported. In order to confirm this phenomenon, we measured the hysteresis cycle under different conditions and the results illustrate that the shift disappears when the temperature is elevated up to 80 K or above, but reappears when the temperature is lowered below 80 K as shown in Fig. 4. Moreover, the shift cannot be observed in ZFC sample as shown in Fig. 3(b). These results convince us that the shift is true and takes place below 80 K. This shift means that part of the magnetic moment does not change with the magnetic field in the FC process. The difference between positive and negative saturated magnetic moment is about 14 emu/g. The shift takes place in the FC sample, thus the shift is independent of the mictomagnetic transition. Fig. 5 shows a comparison between a 30 kOe FC SmFeAl (a) and a 10 kOe FC SmFeAl (b). For the 30 kOe FC sample, the hysteresis cycle is taken with the applied field up to 50 kOe. As can be seen from Fig. 5, the shift is more distinct in the 30 kOe FC SmFeAl. The difference between positive and negative saturated magnetic moment in this case is about 19 emu/g. This suggests that more magnetic moments in the sample are fixed when it is cooled in a higher magnetic field.
Fig. 3. Hysteresis cycles of ZFC SmFeAl and FC SmFeAl in 10 kOe at (a) 80 K and (b) 4.2 K. All the field ranges of hysteresis loops are from 20 to 20 kOe.
Fig. 4. Hysteresis loops at different temperatures of 10 kOe FC SmFeAl. All the field ranges of hysteresis loops are from 20 to 20 kOe.
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Y. Wang et al. / Journal of Magnetism and Magnetic Materials 284 (2004) 13–16
process and ZFC process, and the mictomagnetic transition temperature is approximately 80 K. In the FC sample, the magnetic hysteresis cycle presents a displacement towards positive magnetization below 80 K and the displacement increases with increasing magnetic field in the cooled process. Magnetism and magnetic structure of SmFeAl shows a strong dependence on the magnetic field.
Acknowledgements
Fig. 5. Influence of different cooled fields on hysteresis loops at 4.2 K; (a): the 30 kOe FC SmFeAl, (b): the 10 kOe FC SmFeAl.
The behavior of SmFeAl is sensitive to magnetic field and magnetization process, indicating that magnetic structure changes with these factors. Although the reason for the shift is not clear at present, the possible one is that a special magnetic structure forms accompanied by the magnetic transition such as cooperative Jahn–Teller effect, charge ordering and so on [5]. Since anisotropy usually plays an important role in rare earth intermetallic compounds, more information can be obtained if a single crystal SmFeAl is studied in the higher magnetic field. In addition, the remarkable behaviors in other properties such as magnetostriction, resistivity and specific heat can be expected to occur accompanied by the fieldinduced magnetic transition. A detailed study on this field-induced magnetic transition is in progress.
4. Conclusions SmFeAl compound has several axes of easy magnetization and approaches the saturation state through two or more steps with increasing magnetic field. It is a mictomagnetic phase exhibiting different magnetic behaviors in the FC
This work was supported by the National Natural Science Foundation of China (Grants No. 20025103, 50274002, 20221101 and 10335040) and MOST of China (No. 2001CB610503). References [1] L. Paolasini, G.H. Lander, J. Alloys Compounds 303–304 (2000) 232. [2] S.J. Lee, R.J. Lange, P.C. Canfield, B.N. Harmon, D.W. Lynch, Phys. Rev. B 61 (2000) 9669. [3] V.S. Pokatilov, J. Magn. Magn. Mater. 189 (1998) 189. [4] X.G. Li, J. Magn. Magn. Mater. 205 (1999) 307. [5] H. Onodera, Y. Koshikawa, M. Kosaka, M. Ohashi, H. Yamauchi, Y. Yamaguchi, J. Magn. Magn. Mater. 182 (1998) 161. [6] H. Oesterreicher, J. Less-Common Met. 30 (1973) 225. [7] A. Kolomiets, L. Havela, A.V. Andreev, V. Sechovsky, V.A. Yartys, J. Alloys Compounds 262–263 (1997) 206. [8] A. Kolomiets, L. Havela, V.A. Yartys, A.V. Andreev, J. Alloys Compounds 253–254 (1997) 343. [9] F. Merlo, S. Cirafici, F. Canepa, J. Alloys Compounds 266 (1998) 22. [10] G. Ehlers, C. Geibel, F. Steglich, H. Maletta, Z. Phys. B 104 (1997) 393. [11] P. Javorsky, P. Bulet, V. Sechovsky, R.R. Arons, E. Ressouche, G. lapertot, Physica B 234–236 (1997) 665. [12] G. Ehlers, H. Maletta, Physica B 234–236 (1997) 667. [13] G. Ehlers, H. Maletta, Z. Phys. B 101 (1996) 317. [14] H. Oesterreicher, J. Less-Common Met. 25 (1971) 341. [15] A.E. Dwight, C.W. Kimball, R.S. Preston, S.P. Taneja, L. Weber, J. Less-Common Met. 40 (1975) 285. [16] H. Drulis, W. Petrynski, B. Stalinski, J. Less-Common Met. 101 (1984) 229. [17] A.E. Dwight, Proceedings of the Seventh Rare Earth Research Conference, 1968, p. 273.
Journal of Magnetism and Magnetic Materials 284 (2004) 13–16 www.elsevier.com/locate/jmmm
Mictomagnetism and shifted magnetic hysteresis cycle in SmFeAl Yuntao Wanga, Huaiyu Shaoa, Xingguo Lia,, Lefu Zhangb, Seiki Takahashib a
College of Chemistry and Molecular Engineering, The State Key Laboratory of Rare Earth Materials Chemistry and Applications, Peking University, Beijing, 100871, China b Department of Materials Science and Engineering, Faculty of Engineering, Iwate University, 4-3-5 Ueda, Morioka, 020-8551, Japan Received 14 January 2004; received in revised form 21 May 2004 Available online 26 June 2004
Abstract Magnetic properties of SmFeAl with hexagonal structure were investigated by a superconducting quantum interference device from 4.2 to 300 K in different magnetic fields and different cooled processes. SmFeAl is a mictomagnetic phase showing different magnetic behaviors in the field-cooled (FC) process and zero-field-cooled process. The mictomagnetic transition temperature is about 80 K. In an FC sample, a shifted magnetic hysteresis cycle towards magnetization was observed below 80 K. r 2004 Elsevier B.V. All rights reserved. Keywords: SmFeAl compound; Mictomagnetism; Shifted hysteresis cycle; Field-induced magnetic transition
1. Introduction Magnetic properties of rare earth metals and alloys have been widely investigated and as a result, many interesting phenomena have been reported. The research focuses specifically on the compounds consisting of rare earth metals and 3d transition metals because of the applicable magnetic properties [1–5]. The binary intermetallic R–T compounds (R=rare earth metal, T=transition metal) have been widely investigated so far Corresponding author. Tel./fax: +861062765930.
E-mail address: [email protected] (X. Li).
and the research attention is now gradually extended to the ternary intermetallic R–T–M compounds (M=p-electron element). Among the several series of compounds studied, some notable properties have been discovered [6–12]. One series among them is RNiAl compounds with the Fe2P structure. Ehlers et al. firstly reported the differences of magnetic transition between the zerofield-cooled (ZFC) and field-cooled (FC) samples and found that the crystalline electric field around the rare earth ion often induced strong anisotropy and Ni had an important influence on the magnetic behavior, although no magnetic moment was found at the Ni sites [13].
0304-8853/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2004.06.011
ARTICLE IN PRESS 14
Y. Wang et al. / Journal of Magnetism and Magnetic Materials 284 (2004) 13–16
Since RFeAl has a composition and structure similar to RNiAl, we are interested in whether RFeAl compounds have similar or new magnetic properties. The compounds consisting of light rare earth elements usually have more multiformity in magnetism than those consisting of heavy rare earth elements [14–16]. Thus, we selected SmFeAl as our target compound in this study.
2. Experiment The intermetallic compound SmFeAl was prepared from 99.9% pure Sm, 99.5% pure Fe and 99.99% pure Al by arc melting in an argon gas atmosphere. Arc-melted ingot was flipped over and remelted thrice. To obtain a single phase, the ingot was homogenized at 1073 K, for 24 h in vacuum. The ingot was pulverized into powders as experimental sample. The structure of the sample was identified by powder X-ray diffraction (XRD) using monochromated Cu Ka radiation. About 10 mg sample powders were packaged into Teflon for magnetic measurements. The magnetic properties were measured at temperatures from 4.2 to 300 K by a superconducting quantum interference device (SQUID) magnetometry with an applied field of up to 20 and 50 kOe.
Fig. 1. The powder XRD pattern of SmFeAl.
3. Results and discussion Fig. 1 shows the XRD result of the sample. The SmFeAl compound crystallizes in a homogeneous hexagonal structure with the lattice constants of a ¼ 0:542 and c ¼ 0:878 nm. It agrees with the report of Dwight that the lattice constants are a ¼ 0:544 and c ¼ 0:884 nm [17]. Some weak peaks of FeAl, SmAl2 and Sm2(Fe0.44Al0.56)17 are also observed in the XRD pattern. These impure phases were formed during the arc-melting and annealing processes and caused by the violent evaporation of Sm at high temperature. However, the content of the impure phases is so small that they cannot dominate the magnetic properties of SmFeAl, especially the shifted hysteresis cycle of FC SmFeAl discussed later.
Fig. 2. Temperature dependence of the magnetization in 10 kOe for FC and ZFC SmFeAl. The cooling field of FC SmFeAl is 10 kOe and that of ZFC SmFeAl is 0 kOe.
Fig. 2 shows the temperature dependence of magnetization in 10 kOe for FC and ZFC SmFeAl. Below 150 K, ZFC curve departures from FC curve. The magnetization of ZFC SmFeAl process is lower than that in the FC one and decreases with decreasing temperature. The rapid reduction of magnetization with decreasing temperature in ZFC process implies that a mictomagnetic transition takes place. The transition temperature estimated from the intersection of the two tangent lines as shown in Fig. 2 is 80 K, below which the magnetic moments are blocked.
ARTICLE IN PRESS Y. Wang et al. / Journal of Magnetism and Magnetic Materials 284 (2004) 13–16
15
In Fig. 3(a), the hysteresis cycle is almost the same in FC and ZFC SmFeAl and an inflection point in the cycle around 6 kOe is noticeable in both FC and ZFC SmFeAl. Spontaneous magnetization approaches a saturation value by two steps that are divided at 6 kOe. The staircase behavior expresses the occurrence of a fieldinduced magnetic transition. This is probably due to the fact that the sample has several axes of easy magnetization that demand different driving fields. A similar phenomenon is usually observed in the rare earth compounds. In Fig. 3(b), a shifted hysteresis cycle towards positive magnetization can be clearly observed below 80 K in FC sample. Although the displace-
ment of the hysteresis cycle towards negative magnetic field is reported, the displacement along with the direction of the magnetization has not been reported. In order to confirm this phenomenon, we measured the hysteresis cycle under different conditions and the results illustrate that the shift disappears when the temperature is elevated up to 80 K or above, but reappears when the temperature is lowered below 80 K as shown in Fig. 4. Moreover, the shift cannot be observed in ZFC sample as shown in Fig. 3(b). These results convince us that the shift is true and takes place below 80 K. This shift means that part of the magnetic moment does not change with the magnetic field in the FC process. The difference between positive and negative saturated magnetic moment is about 14 emu/g. The shift takes place in the FC sample, thus the shift is independent of the mictomagnetic transition. Fig. 5 shows a comparison between a 30 kOe FC SmFeAl (a) and a 10 kOe FC SmFeAl (b). For the 30 kOe FC sample, the hysteresis cycle is taken with the applied field up to 50 kOe. As can be seen from Fig. 5, the shift is more distinct in the 30 kOe FC SmFeAl. The difference between positive and negative saturated magnetic moment in this case is about 19 emu/g. This suggests that more magnetic moments in the sample are fixed when it is cooled in a higher magnetic field.
Fig. 3. Hysteresis cycles of ZFC SmFeAl and FC SmFeAl in 10 kOe at (a) 80 K and (b) 4.2 K. All the field ranges of hysteresis loops are from 20 to 20 kOe.
Fig. 4. Hysteresis loops at different temperatures of 10 kOe FC SmFeAl. All the field ranges of hysteresis loops are from 20 to 20 kOe.
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process and ZFC process, and the mictomagnetic transition temperature is approximately 80 K. In the FC sample, the magnetic hysteresis cycle presents a displacement towards positive magnetization below 80 K and the displacement increases with increasing magnetic field in the cooled process. Magnetism and magnetic structure of SmFeAl shows a strong dependence on the magnetic field.
Acknowledgements
Fig. 5. Influence of different cooled fields on hysteresis loops at 4.2 K; (a): the 30 kOe FC SmFeAl, (b): the 10 kOe FC SmFeAl.
The behavior of SmFeAl is sensitive to magnetic field and magnetization process, indicating that magnetic structure changes with these factors. Although the reason for the shift is not clear at present, the possible one is that a special magnetic structure forms accompanied by the magnetic transition such as cooperative Jahn–Teller effect, charge ordering and so on [5]. Since anisotropy usually plays an important role in rare earth intermetallic compounds, more information can be obtained if a single crystal SmFeAl is studied in the higher magnetic field. In addition, the remarkable behaviors in other properties such as magnetostriction, resistivity and specific heat can be expected to occur accompanied by the fieldinduced magnetic transition. A detailed study on this field-induced magnetic transition is in progress.
4. Conclusions SmFeAl compound has several axes of easy magnetization and approaches the saturation state through two or more steps with increasing magnetic field. It is a mictomagnetic phase exhibiting different magnetic behaviors in the FC
This work was supported by the National Natural Science Foundation of China (Grants No. 20025103, 50274002, 20221101 and 10335040) and MOST of China (No. 2001CB610503). References [1] L. Paolasini, G.H. Lander, J. Alloys Compounds 303–304 (2000) 232. [2] S.J. Lee, R.J. Lange, P.C. Canfield, B.N. Harmon, D.W. Lynch, Phys. Rev. B 61 (2000) 9669. [3] V.S. Pokatilov, J. Magn. Magn. Mater. 189 (1998) 189. [4] X.G. Li, J. Magn. Magn. Mater. 205 (1999) 307. [5] H. Onodera, Y. Koshikawa, M. Kosaka, M. Ohashi, H. Yamauchi, Y. Yamaguchi, J. Magn. Magn. Mater. 182 (1998) 161. [6] H. Oesterreicher, J. Less-Common Met. 30 (1973) 225. [7] A. Kolomiets, L. Havela, A.V. Andreev, V. Sechovsky, V.A. Yartys, J. Alloys Compounds 262–263 (1997) 206. [8] A. Kolomiets, L. Havela, V.A. Yartys, A.V. Andreev, J. Alloys Compounds 253–254 (1997) 343. [9] F. Merlo, S. Cirafici, F. Canepa, J. Alloys Compounds 266 (1998) 22. [10] G. Ehlers, C. Geibel, F. Steglich, H. Maletta, Z. Phys. B 104 (1997) 393. [11] P. Javorsky, P. Bulet, V. Sechovsky, R.R. Arons, E. Ressouche, G. lapertot, Physica B 234–236 (1997) 665. [12] G. Ehlers, H. Maletta, Physica B 234–236 (1997) 667. [13] G. Ehlers, H. Maletta, Z. Phys. B 101 (1996) 317. [14] H. Oesterreicher, J. Less-Common Met. 25 (1971) 341. [15] A.E. Dwight, C.W. Kimball, R.S. Preston, S.P. Taneja, L. Weber, J. Less-Common Met. 40 (1975) 285. [16] H. Drulis, W. Petrynski, B. Stalinski, J. Less-Common Met. 101 (1984) 229. [17] A.E. Dwight, Proceedings of the Seventh Rare Earth Research Conference, 1968, p. 273.