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Anomalous magnetic behavior in Sm(Fe1−xMnx)2Si2 system
Journal of Alloys and Compounds 383 (2004) 140–143
Anomalous magnetic behavior in Sm(Fe1−x Mnx )2 Si2 system J. Vejpravová∗ , V. Sechovský, P. Svoboda, J. Prokleška Department of Electronic Structures, Charles University, 121 16 Prague 2, Czech Republic
Abstract A series of Sm(Fe1−x Mnx )2 Si2 polycrystalline samples (x = 0, 0.25, 0.5, 0.75 and 1) has been prepared, characterized and studied in detail by means of specific heat (C), magnetization (M) and ac susceptibility measurements in the temperature range T = 2–350 K and in magnetic fields (B) up to 9 T. Here we present selected results focused on the anomalous magnetic behavior of Sm(Fe0.5 Mn0.5 )2 Si2 and Sm(Fe0.75 Mn0.25 )2 Si2 . © 2004 Elsevier B.V. All rights reserved. Keywords: Silicides; ThCr2 Si2 type; Magnetism
1. Introduction The RET2 X2 ternary intermetallics (RE: rare earth, T: d-metal, X: p-metal), which crystallize in the ThCr2 Si2 –type structure (space group I4/mmm), attract permanent interest stimulated by numerous exotic physical phenomena ranging from nonconventional superconductivity to strong ferromagnetism [1]. The large variety of possible RE–T–X combinations allows systematic studies of relations between electronic structure and various material properties. Concerning Sm compounds, much attention has been paid to SmMn2 Ge2 , which undergoes several magnetic phase transitions within a wide temperature range and a giant magnetoresistance (GMR) effect [2,3]. For the Si containing compounds, basic studies have been done on SmMn2 Si2 reporting an antiferromagnetic (AF) ordering at TN = 398 K, two order-order transitions at 230 and 120 K, respectively, between AF phases and exhibits a ferromagnetic (F) ordering below 35 K as deduced from magnetization data [4,5]. Practically no studies have been performed on SmFe2 Si2 , for which paramagnetic behavior down to 4.2 K has been reported in [6]. While an antiferromagnetic order of Mn 3d magnetic moments in SmMn2 Si2 is expected [4], Fe has been reported to remain nonmagnetic within the entire REFe2 Si2 series [7]. Both, the non-trivial magnetic behavior of SmMn2 Si2 and missing any thorough study of SmFe2 Si2 have moti∗ Corresponding author. Tel.: +420-2-21911227; fax: +420-2-21911351. E-mail address: [email protected] (J. Vejpravov´a).
0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2004.04.025
vated us to investigate these compounds in more detail. The completely different magnetic behavior of the two compounds provided the main stimulus to study an influence of the composition of the Mn–Fe sublattice substitution on magnetic properties of pseudoternary compounds Sm(Fe1−x Mnx )2 Si2 .
2. Experimental Polycrystalline samples of Sm(Fe1−x Mnx )2 Si2 with x = 0, 0.25, 0.5, 0.75 and 1 have been synthesized from high purity constituents (Sm: 3N, Fe: 5N, Mn: 5N, Si: 6N) by several times repeated arc melting in an argon protective atmosphere (6N purity). The excess amount of 5 at.% Sm and 0.5 at.% Mn were added to the stoichiometric starting composition to compensate a high volatility of the two elements during the melting procedure. All samples were characterized by a microprobe and a standard powder X-ray diffraction technique (using Cu K␣ with monochromator to eliminate Fe extinction). The microprobe analysis confirmed the stoichiometry ±3% for all compounds, for SmFe2 Si2 an additional Fe–Si phase (ferromagnetic at room temperature) of variable stoichiometry has been detected on grain boundaries. The lattice parameters at 300 K of both border compounds agree well with the literature data [4–6]. The lattice parameters and the unit cell volume of pseudoternary samples increase roughly linearly with Mn content. For the sample with x = 0.75, a small amount of an Mn rich phase (x ∼ 0.8) has been observed in the X-ray diffraction pattern although it could not be recognized by microprobe.
J. Vejpravov´a et al. / Journal of Alloys and Compounds 383 (2004) 140–143
2
0.6 0.4 0.2 0.0 10
The temperature dependencies of studied compounds shown in Fig. 1 yield at least one anomaly within the low temperature region (see Fig. 1). For SmFe2 Si2 , the sharp peak at 3.5 K corresponds to the Curie temperature TC , which is in good agreement with magnetization results. For x = 0.25, 0.5 and 0.75, the round anomalies occur around T ∼ 9, 10 and 13 K, respectively, matching the magnetic phase transition to the low temperature ferromagnetic phase indicated by magnetization data. For SmMn2 Si2 , a small anomaly appears at 9 K, has not yet been reported. The ‘double peak’ located around ∼30 K (see Fig. 2) corresponds to the phase transition between the low-temperature ferromagnetic phase and the antiferromagnetic phase above 30 K, which in a good agreement with [4]. The small but nonnegligible anomalies at T ∼ 120 and 240 K reflect the order-to-order transitions between two AF phases proposed in [4]. An additional pronounced peak of unknown origin was observed at T = 306 K. In order to demonstrate the pronounced magnetic contribution to the total specific heat of SmMn2 Si2 , the C(T) experimental data are compared with the phonon specific heat of paramagnetic LaFe2 Si2 in Fig. 2. We could not use LaMn2 Si2 as a nonmagnetic analogue because of the magnetic ordering existing in this compound [8].
While the C(T) data for Sm(Fe1−x Mnx )2 Si2 solidsolutions show anomalies only at low temperatures the temperature dependencies of magnetization exhibit besides the low-temperature phase transitions at least one magnetic-field dependent anomaly within the 100–300 K range for the all three investigated compositions. Strange behavior of dc magnetization, resembling features reported for the parent compound SmMn2 Si2 [4], has been observed for x = 0.25 and 0.5, respectively. As seen in Fig. 3, the temperature dependence of the zero-field cooled (ZFC) magnetization recorded in 0.01T (resp. magnetic susceptibility χ) for x = 0.5 exhibits a pronounced minimum at around T ∼ 270 K, while the corresponding temperature dependencies measured in 1 T and higher fields mimic rather paramagnetic behavior (nevertheless magnetic-filed dependent below 270 K). Also the ac susceptibility reveals an unusual behavior dependent on magnetic fields applied (see Fig. 4). In 0.003 T a pronounced shoulder is present at ∼170 K whereas in 0.01 T an additional broad shoulder appears at ∼100 K. The anomalies become rapidly smeared with increasing magnetic field and are not seen on the χ(T) curve measured in 0.5 T. We should note also that no anomaly was clearly visible in the
2.0e-7
12
Sm(Fe0.5Mn0.5)2Si2 1T 2T 4T 9T 0.01T
1.5e-7
3
χ (m /mol)
x=0 x=0.5 x=0.75 x=0.25 x=1
8
100 T (K)
Fig. 2. Specific heat of SmMn2 Si2 in comparison with a phonon specific heat of nonmagnetic LaFe2 Si2 (solid line). The C/T representation with logarithmic scale at T was selected to enhance the phase transitions.
3. Results and discussion
C (J/molK)
SmMn2Si2
0.8 C/T (J/molK )
This points to a possible non-statistical occupation of the 4d Wyckoff positions by Fe and Mn in. The specific heat (C) was measured within the temperature range 2–360 K on plate-shaped samples (typical mass ∼10 mg) using the double relaxation technique. The DC magnetization and AC susceptibility (with constant amplitude of magnetic field 0.001 T and frequency 73 Hz) were measured at temperatures between 2 and 350 K and in fields up to 9 T on powdered samples (of typical mass ∼100 mg) with fine grains fixed in random orientations by acetone-based glue in a PE capsule. These experiments were performed using the PPMS-9T facility (Quantum Design).
141
4
1.0e-7
5.0e-8 Sm(Fe1-xMnx)2Si2
0 0
10
20
30
T (K) Fig. 1. Low temperature detail of specific heat C(T) of Sm(Fe1−x Mnx )2 Si2 .
100
150
200 T (K)
250
300
Fig. 3. Temperature dependencies of magnetic susceptibility of Sm(Fe0.5 Mn0.5 )2 Si2 in 0.01 T with pronounced minimum at ∼270 K in comparison with that in higher fields.
142
J. Vejpravov´a et al. / Journal of Alloys and Compounds 383 (2004) 140–143
6e-7
2
0.2
0
5K 12K 15K 16K 20K
2
2e-7
50
2K 10K
0.003T 0.01T 0.05T 0.1T 0.5T
4e-7
Sm(Fe0.5Mn0.5)2Si2
0.3 M (µB/ f.u.)
AC susc. (r.u.)
Sm(Fe0.5Mn0.5)2Si2
0.1
0.0
100 150 200 250 300 350
0.3
T (K)
Sm(Fe0.75Mn0.25)2Si2
2K
2K
0.5
T
µB/f.u.
0.4 0.3 0.2 0.1
Sm(Fe0.5Mn0.5)2Si2
0.0
0.4 µB/f.u.
2K 5K 12K
Sm(Fe0.75Mn0.25)2Si2
0.5
14K 10K
0.3 0.2 0.1 0.0
0
2
4
6
8
10
µ0H (T) Fig. 5. Magnetization curves of Sm(Fe0.5 Mn0.5 )2 Si2 (at 2, 5, 10, 12, 14, 116, 17 and 20 K) in comparison with Sm(Fe0.75 Mn0.25 )2 Si2 .
0.2 12K 15K
2
M (µB/ f.u.)
temperature dependence of specific heat in the range between 50 and 300 K. Although one cannot exclude a possibility of some special magnetic ordering (e.g. canted antiferromagnetism in the Mn sublattice) yielding very weak spontaneous magnetization formed on the temperature range under discussion, we conclude that rather a tiny amount of ferromagnetic impurity, which is ordered below 270 K, is responsible for these strange effects in our samples and similar also in the SmMn2 Si2 case reported in [4]. The magnetization isotherms measured for x = 0.5 and 0.25 in the vicinity of the magnetic phase transition do not exhibit clear trend to saturation up to 9 T (see Fig. 5). One of the main reasons is probably the strong uniaxial magnetocrystalline anisotropy, which causes that on a polycrystalline sample we measure a considerable mixing of a
2
10K
Fig. 4. Development of low-field AC susceptibility of Sm(Fe0.5 Mn0.5 )2 Si2 .
0.1
5K
0.0 0
5
10 µ0H/µ
15
20
Fig. 6. A comparison of Arrott plots in the vicinity of the F phase transition for Sm(Fe0.5 Mn0.5 )2 Si2 and Sm(Fe0.75 Mn0.25 )2 Si2 .
hard-magnetization direction signal, which is usually linear with higher magnetic fields. Under presumption of uniaxial anisotropy and the c-axis as the easy magnetization direction, the value of the spontaneous magnetic moment can be then estimated from Arrott plots (Fig. 6) to be ∼0.7 B /f.u. for x = 0.5 and somewhat lower (0.65 B /f.u.) for x = 0.25. The Arrott plots show a different character of magnetic ordering for x = 0.5 and 0.25. The high-field Arrott plots for x = 0.5 (above 0 H/M ∼ 5) are close to straight lines and parallel, which corresponds to a negligible bond disorder in the compound. On the other hand, for x = 0.25 the plots have a significant positive parabolic curvature at T > TC , indicating a possible contribution of a disordered ferromagnetic term (for details, see [9]). In conclusion, we can say that even at very diluted Mn concentration, the ordered itinerant 3d moment is observed, similarly as in pure SmMn2 Si2 . On the other hand, the itinerant character of magnetic interaction is suppressed with increasing content of Mn due to the strengthening of the localized antiferromagnetic coupling of 4f Sm and 3d transition elements moments. This may indicate that the effects observed in the higher temperature range correspond to the antiferromagnetic order of itinerant 3d electrons, while the low temperature ferromagnetic ordering originates in weaker Sm coupling. An additional 4f–3d interaction cannot be excluded from the scenario although two magnetic ordering temperatures (the higher one for the Mn sublattice and the lower for the rare-earth sublattice) are frequently reported for REMn2 X2 compounds [1]. In our compounds, however, we have only a secure proof of ferromagnetic ordering at low
J. Vejpravov´a et al. / Journal of Alloys and Compounds 383 (2004) 140–143
temperatures. The origin of the high-temperature magnetization and ac susceptibility anomalies naturally need further investigation. Certainly, information on magnetism on microscopic scale is strongly desired to put our speculations on a more solid ground.
Acknowledgements This work is a part of the research program MSM113200002 that is financed by the Ministry of Education of the Czech Republic. Part of this work was also supported by the grants provided by the Grant Agency of the Czech Republic (grant # 106/02/0940) and Grant Agency of the Charles University (grant # 165/01).
143
References [1] A. Szytula, J. Leciejewicz, in: K.A. Gschneider Jr., L. Eyring (Eds.), Handbook on the Physics and Chemistry of Rare Earths, vol. 133, Amsterdam, North-Holland, 1989, references therein. [2] R.B. van Dover, E.M. Gyorgy, R.J. Cava, J.J. Krajewski, R.J. Felder, W.F. Peck, Phys. Rev. B 47 (1993) 6134. [3] H.V.J. Barbers, A.J. Nolten, F. Kayzel, S.K.J. Lenczowski, K.H.J. Buschow, F.R. de Boer, Phys. Rev. B 47 (1993) 6134. [4] M. Zhao, C. Sun, L. Wang, W. Li, Q. Su, J. Appl. Phys. 81 (1997) 5534. [5] A. Szytula, I. Szott, Solid State Commun. 40 (1981) 199. [6] I. Felner, I. Mayer, Solid State Commun. 16 (1975) 1005. [7] P. Svoboda, J. Vejpravová, F. Honda, E. Šantavá, O. Schneeweiss, T. Komatsubara, Phys. B 328 (2003) 139. [8] M. Hofmann, S.J. Campbell, K. Knorr, S. Hull, V. Ksenofontov, J. Appl. Phys. 91 (2002) 8126. [9] I. Yenung, R.M. Roshko, G. Williams, Phys. Rev. B 34 (1986) 3456.
Anomalous magnetic behavior in Sm(Fe1−x Mnx )2 Si2 system J. Vejpravová∗ , V. Sechovský, P. Svoboda, J. Prokleška Department of Electronic Structures, Charles University, 121 16 Prague 2, Czech Republic
Abstract A series of Sm(Fe1−x Mnx )2 Si2 polycrystalline samples (x = 0, 0.25, 0.5, 0.75 and 1) has been prepared, characterized and studied in detail by means of specific heat (C), magnetization (M) and ac susceptibility measurements in the temperature range T = 2–350 K and in magnetic fields (B) up to 9 T. Here we present selected results focused on the anomalous magnetic behavior of Sm(Fe0.5 Mn0.5 )2 Si2 and Sm(Fe0.75 Mn0.25 )2 Si2 . © 2004 Elsevier B.V. All rights reserved. Keywords: Silicides; ThCr2 Si2 type; Magnetism
1. Introduction The RET2 X2 ternary intermetallics (RE: rare earth, T: d-metal, X: p-metal), which crystallize in the ThCr2 Si2 –type structure (space group I4/mmm), attract permanent interest stimulated by numerous exotic physical phenomena ranging from nonconventional superconductivity to strong ferromagnetism [1]. The large variety of possible RE–T–X combinations allows systematic studies of relations between electronic structure and various material properties. Concerning Sm compounds, much attention has been paid to SmMn2 Ge2 , which undergoes several magnetic phase transitions within a wide temperature range and a giant magnetoresistance (GMR) effect [2,3]. For the Si containing compounds, basic studies have been done on SmMn2 Si2 reporting an antiferromagnetic (AF) ordering at TN = 398 K, two order-order transitions at 230 and 120 K, respectively, between AF phases and exhibits a ferromagnetic (F) ordering below 35 K as deduced from magnetization data [4,5]. Practically no studies have been performed on SmFe2 Si2 , for which paramagnetic behavior down to 4.2 K has been reported in [6]. While an antiferromagnetic order of Mn 3d magnetic moments in SmMn2 Si2 is expected [4], Fe has been reported to remain nonmagnetic within the entire REFe2 Si2 series [7]. Both, the non-trivial magnetic behavior of SmMn2 Si2 and missing any thorough study of SmFe2 Si2 have moti∗ Corresponding author. Tel.: +420-2-21911227; fax: +420-2-21911351. E-mail address: [email protected] (J. Vejpravov´a).
0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2004.04.025
vated us to investigate these compounds in more detail. The completely different magnetic behavior of the two compounds provided the main stimulus to study an influence of the composition of the Mn–Fe sublattice substitution on magnetic properties of pseudoternary compounds Sm(Fe1−x Mnx )2 Si2 .
2. Experimental Polycrystalline samples of Sm(Fe1−x Mnx )2 Si2 with x = 0, 0.25, 0.5, 0.75 and 1 have been synthesized from high purity constituents (Sm: 3N, Fe: 5N, Mn: 5N, Si: 6N) by several times repeated arc melting in an argon protective atmosphere (6N purity). The excess amount of 5 at.% Sm and 0.5 at.% Mn were added to the stoichiometric starting composition to compensate a high volatility of the two elements during the melting procedure. All samples were characterized by a microprobe and a standard powder X-ray diffraction technique (using Cu K␣ with monochromator to eliminate Fe extinction). The microprobe analysis confirmed the stoichiometry ±3% for all compounds, for SmFe2 Si2 an additional Fe–Si phase (ferromagnetic at room temperature) of variable stoichiometry has been detected on grain boundaries. The lattice parameters at 300 K of both border compounds agree well with the literature data [4–6]. The lattice parameters and the unit cell volume of pseudoternary samples increase roughly linearly with Mn content. For the sample with x = 0.75, a small amount of an Mn rich phase (x ∼ 0.8) has been observed in the X-ray diffraction pattern although it could not be recognized by microprobe.
J. Vejpravov´a et al. / Journal of Alloys and Compounds 383 (2004) 140–143
2
0.6 0.4 0.2 0.0 10
The temperature dependencies of studied compounds shown in Fig. 1 yield at least one anomaly within the low temperature region (see Fig. 1). For SmFe2 Si2 , the sharp peak at 3.5 K corresponds to the Curie temperature TC , which is in good agreement with magnetization results. For x = 0.25, 0.5 and 0.75, the round anomalies occur around T ∼ 9, 10 and 13 K, respectively, matching the magnetic phase transition to the low temperature ferromagnetic phase indicated by magnetization data. For SmMn2 Si2 , a small anomaly appears at 9 K, has not yet been reported. The ‘double peak’ located around ∼30 K (see Fig. 2) corresponds to the phase transition between the low-temperature ferromagnetic phase and the antiferromagnetic phase above 30 K, which in a good agreement with [4]. The small but nonnegligible anomalies at T ∼ 120 and 240 K reflect the order-to-order transitions between two AF phases proposed in [4]. An additional pronounced peak of unknown origin was observed at T = 306 K. In order to demonstrate the pronounced magnetic contribution to the total specific heat of SmMn2 Si2 , the C(T) experimental data are compared with the phonon specific heat of paramagnetic LaFe2 Si2 in Fig. 2. We could not use LaMn2 Si2 as a nonmagnetic analogue because of the magnetic ordering existing in this compound [8].
While the C(T) data for Sm(Fe1−x Mnx )2 Si2 solidsolutions show anomalies only at low temperatures the temperature dependencies of magnetization exhibit besides the low-temperature phase transitions at least one magnetic-field dependent anomaly within the 100–300 K range for the all three investigated compositions. Strange behavior of dc magnetization, resembling features reported for the parent compound SmMn2 Si2 [4], has been observed for x = 0.25 and 0.5, respectively. As seen in Fig. 3, the temperature dependence of the zero-field cooled (ZFC) magnetization recorded in 0.01T (resp. magnetic susceptibility χ) for x = 0.5 exhibits a pronounced minimum at around T ∼ 270 K, while the corresponding temperature dependencies measured in 1 T and higher fields mimic rather paramagnetic behavior (nevertheless magnetic-filed dependent below 270 K). Also the ac susceptibility reveals an unusual behavior dependent on magnetic fields applied (see Fig. 4). In 0.003 T a pronounced shoulder is present at ∼170 K whereas in 0.01 T an additional broad shoulder appears at ∼100 K. The anomalies become rapidly smeared with increasing magnetic field and are not seen on the χ(T) curve measured in 0.5 T. We should note also that no anomaly was clearly visible in the
2.0e-7
12
Sm(Fe0.5Mn0.5)2Si2 1T 2T 4T 9T 0.01T
1.5e-7
3
χ (m /mol)
x=0 x=0.5 x=0.75 x=0.25 x=1
8
100 T (K)
Fig. 2. Specific heat of SmMn2 Si2 in comparison with a phonon specific heat of nonmagnetic LaFe2 Si2 (solid line). The C/T representation with logarithmic scale at T was selected to enhance the phase transitions.
3. Results and discussion
C (J/molK)
SmMn2Si2
0.8 C/T (J/molK )
This points to a possible non-statistical occupation of the 4d Wyckoff positions by Fe and Mn in. The specific heat (C) was measured within the temperature range 2–360 K on plate-shaped samples (typical mass ∼10 mg) using the double relaxation technique. The DC magnetization and AC susceptibility (with constant amplitude of magnetic field 0.001 T and frequency 73 Hz) were measured at temperatures between 2 and 350 K and in fields up to 9 T on powdered samples (of typical mass ∼100 mg) with fine grains fixed in random orientations by acetone-based glue in a PE capsule. These experiments were performed using the PPMS-9T facility (Quantum Design).
141
4
1.0e-7
5.0e-8 Sm(Fe1-xMnx)2Si2
0 0
10
20
30
T (K) Fig. 1. Low temperature detail of specific heat C(T) of Sm(Fe1−x Mnx )2 Si2 .
100
150
200 T (K)
250
300
Fig. 3. Temperature dependencies of magnetic susceptibility of Sm(Fe0.5 Mn0.5 )2 Si2 in 0.01 T with pronounced minimum at ∼270 K in comparison with that in higher fields.
142
J. Vejpravov´a et al. / Journal of Alloys and Compounds 383 (2004) 140–143
6e-7
2
0.2
0
5K 12K 15K 16K 20K
2
2e-7
50
2K 10K
0.003T 0.01T 0.05T 0.1T 0.5T
4e-7
Sm(Fe0.5Mn0.5)2Si2
0.3 M (µB/ f.u.)
AC susc. (r.u.)
Sm(Fe0.5Mn0.5)2Si2
0.1
0.0
100 150 200 250 300 350
0.3
T (K)
Sm(Fe0.75Mn0.25)2Si2
2K
2K
0.5
T
µB/f.u.
0.4 0.3 0.2 0.1
Sm(Fe0.5Mn0.5)2Si2
0.0
0.4 µB/f.u.
2K 5K 12K
Sm(Fe0.75Mn0.25)2Si2
0.5
14K 10K
0.3 0.2 0.1 0.0
0
2
4
6
8
10
µ0H (T) Fig. 5. Magnetization curves of Sm(Fe0.5 Mn0.5 )2 Si2 (at 2, 5, 10, 12, 14, 116, 17 and 20 K) in comparison with Sm(Fe0.75 Mn0.25 )2 Si2 .
0.2 12K 15K
2
M (µB/ f.u.)
temperature dependence of specific heat in the range between 50 and 300 K. Although one cannot exclude a possibility of some special magnetic ordering (e.g. canted antiferromagnetism in the Mn sublattice) yielding very weak spontaneous magnetization formed on the temperature range under discussion, we conclude that rather a tiny amount of ferromagnetic impurity, which is ordered below 270 K, is responsible for these strange effects in our samples and similar also in the SmMn2 Si2 case reported in [4]. The magnetization isotherms measured for x = 0.5 and 0.25 in the vicinity of the magnetic phase transition do not exhibit clear trend to saturation up to 9 T (see Fig. 5). One of the main reasons is probably the strong uniaxial magnetocrystalline anisotropy, which causes that on a polycrystalline sample we measure a considerable mixing of a
2
10K
Fig. 4. Development of low-field AC susceptibility of Sm(Fe0.5 Mn0.5 )2 Si2 .
0.1
5K
0.0 0
5
10 µ0H/µ
15
20
Fig. 6. A comparison of Arrott plots in the vicinity of the F phase transition for Sm(Fe0.5 Mn0.5 )2 Si2 and Sm(Fe0.75 Mn0.25 )2 Si2 .
hard-magnetization direction signal, which is usually linear with higher magnetic fields. Under presumption of uniaxial anisotropy and the c-axis as the easy magnetization direction, the value of the spontaneous magnetic moment can be then estimated from Arrott plots (Fig. 6) to be ∼0.7 B /f.u. for x = 0.5 and somewhat lower (0.65 B /f.u.) for x = 0.25. The Arrott plots show a different character of magnetic ordering for x = 0.5 and 0.25. The high-field Arrott plots for x = 0.5 (above 0 H/M ∼ 5) are close to straight lines and parallel, which corresponds to a negligible bond disorder in the compound. On the other hand, for x = 0.25 the plots have a significant positive parabolic curvature at T > TC , indicating a possible contribution of a disordered ferromagnetic term (for details, see [9]). In conclusion, we can say that even at very diluted Mn concentration, the ordered itinerant 3d moment is observed, similarly as in pure SmMn2 Si2 . On the other hand, the itinerant character of magnetic interaction is suppressed with increasing content of Mn due to the strengthening of the localized antiferromagnetic coupling of 4f Sm and 3d transition elements moments. This may indicate that the effects observed in the higher temperature range correspond to the antiferromagnetic order of itinerant 3d electrons, while the low temperature ferromagnetic ordering originates in weaker Sm coupling. An additional 4f–3d interaction cannot be excluded from the scenario although two magnetic ordering temperatures (the higher one for the Mn sublattice and the lower for the rare-earth sublattice) are frequently reported for REMn2 X2 compounds [1]. In our compounds, however, we have only a secure proof of ferromagnetic ordering at low
J. Vejpravov´a et al. / Journal of Alloys and Compounds 383 (2004) 140–143
temperatures. The origin of the high-temperature magnetization and ac susceptibility anomalies naturally need further investigation. Certainly, information on magnetism on microscopic scale is strongly desired to put our speculations on a more solid ground.
Acknowledgements This work is a part of the research program MSM113200002 that is financed by the Ministry of Education of the Czech Republic. Part of this work was also supported by the grants provided by the Grant Agency of the Czech Republic (grant # 106/02/0940) and Grant Agency of the Charles University (grant # 165/01).
143
References [1] A. Szytula, J. Leciejewicz, in: K.A. Gschneider Jr., L. Eyring (Eds.), Handbook on the Physics and Chemistry of Rare Earths, vol. 133, Amsterdam, North-Holland, 1989, references therein. [2] R.B. van Dover, E.M. Gyorgy, R.J. Cava, J.J. Krajewski, R.J. Felder, W.F. Peck, Phys. Rev. B 47 (1993) 6134. [3] H.V.J. Barbers, A.J. Nolten, F. Kayzel, S.K.J. Lenczowski, K.H.J. Buschow, F.R. de Boer, Phys. Rev. B 47 (1993) 6134. [4] M. Zhao, C. Sun, L. Wang, W. Li, Q. Su, J. Appl. Phys. 81 (1997) 5534. [5] A. Szytula, I. Szott, Solid State Commun. 40 (1981) 199. [6] I. Felner, I. Mayer, Solid State Commun. 16 (1975) 1005. [7] P. Svoboda, J. Vejpravová, F. Honda, E. Šantavá, O. Schneeweiss, T. Komatsubara, Phys. B 328 (2003) 139. [8] M. Hofmann, S.J. Campbell, K. Knorr, S. Hull, V. Ksenofontov, J. Appl. Phys. 91 (2002) 8126. [9] I. Yenung, R.M. Roshko, G. Williams, Phys. Rev. B 34 (1986) 3456.