- Email: [email protected]
Studies of the relaxed amorphous phase in the Fe80Nb6B14 alloy
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 320 (2008) e770–e773 www.elsevier.com/locate/jmmm
Studies of the relaxed amorphous phase in the Fe80Nb6B14 alloy A. Chrobaka,, G. Haneczokb, D. Chrobakb, Ł. Madejb, G. Che"kowskaa, M. Kulpaa a
b
University of Silesia, Institute of Physics, 40-007 Katowice, 4 Uniwersytecka, Poland University of Silesia, Institute of Materials Science, 40-007 Katowice, 12 Bankowa, Poland Available online 12 April 2008
Abstract The paper presents measurements of magnetic permeability, magnetic after-effects, magnetostriction, DSC and XPS for the Fe80Nb6B14 amorphous alloys preliminary annealed for 1 h at temperatures ranging from 300 to 770 K. It was shown that annealing out of free volume and internal stresses causes a decrease of magnetostriction coefficient and leads to the formation of the energetically stable relaxed amorphous state. The XPS spectra show local fluctuation of boron density. This effect was attributed to the formation of small iron clusters—the characteristic feature for the relaxed amorphous phase. r 2008 Elsevier B.V. All rights reserved. PACS: 75.75.+a; 75.50.y Keywords: Amorphous alloys; Free volume; Magnetic properties
1. Introduction It is well known that soft magnetic properties of amorphous alloys based on iron can be significantly improved by applying a suitable annealing treatment. This effect is usually explained by the formation of iron nanograins embedded into amorphous surroundings. Such a microstructure averages magnetic anisotropy and, according to the random anisotropy model [1,2], leads to the enhancement of soft magnetic properties (ESMP). Refs. [3,4] show that an appropriate chemical composition (e.g., Fe80Nb6B14) allows obtaining the ESMP effect without the formation of iron nanograins, i.e. in the socalled relaxed amorphous phase. This gives a decrease of material brittleness (in comparison with the nanocrystalline magnets) and what follows is a broadening of practical applications. A proper Nb content contributes to the slowing down of diffusion processes and shifts the crystallization into higher temperatures that makes it possible to obtain a thermodynamically stable relaxed amorphous phase [4].
Corresponding author. Tel./fax.: +48 32 3591630.
E-mail address: [email protected] (A. Chrobak). 0304-8853/$ - see front matter r 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2008.04.048
Recently [5], we have formulated a hypothesis that the relaxed amorphous structure can contain small iron clusters (acting as subnanograins) that are magnetically coupled and coherent with the amorphous surrounding. Such a microstructure with iron clusters of dimensions smaller than the ferromagnetic exchange length gives a random distribution of magnetic anisotropy and leads to an increase of magnetic permeability m (e.g. for the Fe80Nb6B14 alloy about 14 times). In a series of papers [6–8], the ESMP effect is usually explained by a decrease of free-volume content and magnetostriction coefficient, which is connected with annealing out of internal stresses. The aim of the present work is to study the structure of the relaxed amorphous phase and the mechanism of the ESMP effect by making use of (i) differential scanning calorimetry (DSC), (ii) magneto-dilatometer (MD), (iii) magnetic after-effects (MAE) and (iv) X-ray photoemission spectroscopy (XPS) measurements in order to characterize the amorphous phase in the context of the ESMP effect. 2. Experimental results The examined alloy—Fe80Nb6B14—was obtained by the melt spinning technique in the form of strips with thickness and width of about 25 mm and 10 mm, respectively.
ARTICLE IN PRESS A. Chrobak et al. / Journal of Magnetism and Magnetic Materials 320 (2008) e770–e773
As-quenched samples in the amorphous state (confirmed by X-ray measurements and high-resolution electron microscopy technique [3]) were annealed for 1 h at different temperatures (denoted as Ta) in the range from 300 to 770 K. The following experimental techniques were used:
Fig. 1 presents the DSC curves for three representative samples: annealed for 1 h at Ta ¼ 470, 700 and 770 K. It is worth noticing that for all samples the crystallization temperature Tx does not change. On the contrary, the heat transition from amorphous to crystalline state Qt (calculated as the area under the DSC peak) changes with the temperature of the preliminary annealing Ta. The relationship between Qt and annealing temperature Ta for all examined samples is presented in Fig. 2 where for comparison the so-called optimization curve m(Ta) is also shown. In the temperature up to Ta ¼ 650 K, Qt is almost stable and strongly decreases up to Ta ¼ 770 K. One can 470 K/1h 700 K/1h 770 K/1h
4
25
30 28
20
26 15 24
μ 10-3
QT [J/g]
DSC, Perkin-Elmer DSC-7, temperature range of 300–850 K, heating rate 5 K/min, MD at room temperature, self-designed, resolution 10 nm, sample length 50 mm, magnetic permeability m and MAE, Dm/m measured at room temperature (where Dm ¼ m(t1)m(t2); t1 ¼ 30 and t2 ¼ 1800 s are times after demagnetization; Maxwell–Wien bridge, frequency of about 1 kHz, magnetic field 0.5 A/m), XPS at room temperature, PHI 5700/660, momochromatized Al Ka radiation (1486.6 eV), resolution about 0.3 eV, vacuum 5 1010 Torr.
32
QT μ
10
22 20
5 18 0
16 500
600
700
800
Ta [K] Fig. 2. The magnetic permeability m and the heat transitions amorphous–crystalline state QT determined at room temperature for samples annealed for 1 h at temperatures Ta.
10
8
6 λ 106
e771
4
2
Heat flow [mW]
3
0
0 2
20
40 60 μ0H [mT]
80
100
Fig. 3. The parallel magnetostriction coefficient l versus magnetic field for the Fe80Nb6B14 amorphous alloy
1
0
750
760
770 T [K]
780
Fig. 1. Experimental DSC maxima measured for Fe80Nb6B14 amorphous alloy (heating rate 5 K/min) preliminary annealed at 470, 700 and 770 K/ 1h
notice that at temperatures close to the optimization annealing temperature Top (corresponding to the highest magnetic permeability), Qt shows a local plateau. Fig. 3 shows the parallel magnetostriction coefficient l versus magnetic field m0H for the samples in the as-quenched state after demagnetization. The presented relationship reflects a typical saturated curve where magnetostriction increases up to about 9 106 for m0H ¼ 40 mT. Magnetostriction coefficient l in saturation and magnetic after-effects Dm/m measured for samples annealed at
ARTICLE IN PRESS A. Chrobak et al. / Journal of Magnetism and Magnetic Materials 320 (2008) e770–e773
e772
12
9 80
50 40
7
30 20 10 0
Nb 3d
6 0
2
4
6 8 Δμ/μ
6
10 12
5 4
4
3
B 1s
Intensity [arb. unit]
Δμ/μ
8
8
λ 106
λ2 [10-6]
10
70 60
770 K/1h
2 2
700 K/1h
Δμ/μ λ
1
0
0 400
300
600 500 Ta [K]
800
700
aq
Fig. 4. The magnetic after effects Dm/m and the magnetostriction coefficient l determined at room temperature for samples annealed for 1 h at temperatures Ta.
210
190 200 Binding energy [eV]
180
Fig. 6. The XPS spectra of Nb 3d and B 1s lines for the Fe80Nb6B14 amorphous alloy (in the as quenched state and annealed at different Ta) determined at room temperature.
Intensity [arb. unit]
Fe80Nb6B14 770 K/1h
Fe80Nb6B14 700 K/1h
Fe80Nb6B14 aq
Fe
14
12
10
8 6 4 Binding energy [eV]
2
0
-2
Fig. 5. The XPS spectra of valence band for the Fe80Nb6B14 amorphous alloy (annealed at different Ta) and crystalline Fe determined at room temperature.
different temperatures Ta are presented in Fig. 4. The inset shows a plot l2(Ta) versus Dm/m. It is worth noting that l decreases with increasing annealing temperature up to Ta ¼ 660 K (60 K below the temperature of the optimization annealing Top) and is almost constant for Ta4660 K. It is generally accepted that the quantity Dm/m is related to the free-volume content frozen into melt-spun ribbons.
Both curves presented in Fig. 4 show similar behavior as l(Ta). Moreover, l2(Ta) correlates with magnetic aftereffects Dm/m (see the inset in Fig. 4), which suggests that both the quantities are closely connected with annealing out of internal stress during structural relaxation. Fig. 5 presents the XPS spectra of valence band obtained for three samples: in the as-quenched state, after annealing at Top ¼ 700 K (the relaxed amorphous state) and after annealing at 770 K (nanocrystalline state) [5]. For comparison the XPS spectrum of pure iron is also shown. All plotted curves are very similar but for samples of the Fe80Nb6B14 alloy, additional electronic states at 9 eV are observed. Apart from that, the Fe 2p, 3p and 3s core lines of all tested samples do not show any changes. Taking into account the latter, we focused on the boron 1s line and the niobium 3d line where some important differences were found. The corresponding XPS spectra for the same samples are shown in Fig. 6. As we can see the B 1s line intensity is sensitive to thermal treatment, i.e. is the highest for the sample annealed at temperature Top ¼ 700 K and is the lowest for the sample in the asquenched state. In contrast to this the Nb 3d band does not show any changes in all the examined cases. 3. Discussion and conclusions Thermodynamic stability of the amorphous and the relaxed amorphous phase is still of great interest from both scientific and practical point of view. This kind of structural disorder affects different physical quantities—in
ARTICLE IN PRESS A. Chrobak et al. / Journal of Magnetism and Magnetic Materials 320 (2008) e770–e773
amorphous alloys based on iron mainly magnetic properties via the ESMP effect. The question about a nature of this coupling is very important because it can allow modeling and optimizing material properties for a number of applications. In general, amorphous alloys are not in thermodynamic equilibrium and permanently tend to states with lower free energy. The difference in energetic levels is the driving force leading to structural changes and can be measured as the heat emitted during full or partial crystallization in the DSC experiment. Typical DSC exothermic peaks (see Fig. 1) are connected with the first stage of crystallization (formation and growth of iron nanograins). The second, well-separated, stage (formation of iron borides) can be observed in higher temperature range than the data presented in Fig. 1 [3]. However, the formation of iron nanostructure can be considered as the main reason for structural changes in the amorphous phase because it is the first energetically metastable state on the way to full crystallization. Therefore, the heat of the first stage of crystallization Qt for the sample preliminary annealed at different temperatures reflects an energetic state in different structural order (from the amorphous phase through the relaxed amorphous phase to the crystalline state). For the Fe80Nb6B14 amorphous alloy it is well documented (see Fig. 2) that just before nanocrystallization (the first nanograins were observed in samples annealed at Ta ¼ 760 K [3,5]), Qt is almost constant. This evident plateau reveals that the relaxed amorphous phase can be considered as a well-defined energetic metastable state. Moreover, this phenomenon is in correlation with the increase of magnetic permeability, which suggests that explanation of the ESMP effect lies in a specific Fe atoms order/disorder. It was already mentioned that the structural relaxation should exert an influence on magnetostriction coefficient l and magnetic after-effects Dm/m. According to Fig. 4 both these quantities decrease up to Ta ¼ 660 K and for higher annealing temperatures they are almost constant. This drop takes place at 40 K before the temperature of the optimization annealing Top and is due to annealing out of free volume and internal stresses frozen during fabrication. Besides, l2 correlates with Dm/m, which is in good agreement with the observations presented in Ref. [9] for different amorphous alloys. Studies of the electronic structure (XPS) of the tested alloy allowed understanding a role of boron and niobium atoms in structural relaxation. As is shown in Fig. 6, intensity of the Nb 3d line does not depend on preliminary annealing, which means that local niobium density does
e773
not change during structural relaxation. The latter can be explained by the fact that Nb atoms are relatively heavy and cause a slowing down of diffusion processes in the Fe–Nb–B group of amorphous alloys [3]. In contrast to niobium, boron atoms show a different behavior. Comparing the XPS spectra measured for the amorphous (asquenched) and the relaxed amorphous phase (Ta ¼ 700 K) of the examined alloy, one can conclude that the change of the B 1s line intensity reflects a fluctuation of local boron density taking place during structural relaxation (see Fig. 6). It can be explained by the formation of iron clusters free of boron atoms. So the observed relaxed amorphous phase consists of some regions enriched and free of boron. For samples annealed at higher temperatures, the boron atoms may be solved in iron crystalline grains and finally during full crystallization borides Fe2B, Fe3B are formed [3]. The main conclusions of the present paper can be formulated as follows:
Annealing out of free volume and internal stresses cause a decrease of magnetostriction coefficient in the examined amorphous alloy (Fe80Nb6B14) and leads to the formation of the energetically stable relaxed amourphous phase. The observed fluctuations of local boron density are caused by the formation of small iron clusters, characteristic of the relaxed amorphous phase. The enhancement of soft magnetic properties effect taking place in the relaxed amorphous phase can be explained by the formation of magnetically coupled iron clusters, which is in agreement with the Herzer model (averaging out of magneto-crystalline anisotropy) [1,2].
References [1] G. Herzer, L.L. Varga, J. Magn. Magn. Mater. 215–216 (2000) 506. [2] G. Herzer, J. Magn. Magn. Mater. 294 (2005) 99. [3] A. Chrobak, D. Chrobak, G. Haneczok, P. Kwapulin´ski, Z. Kwolek, M. Karolus, Mater. Sci. Eng. A 382 (2004) 401. [4] G. Haneczok, A. Chrobak, P. Kwapulin´ski, Z. Stok"osa, J. Rasek, N. Wo´jcik, in: Proceedings of the Soft Magnetic Materials, vol. 16, Du¨sseldorf, Germany, 2003, pp. 603–608. [5] G. Haneczok, J.E. Fra˛ ckowiak, A. Chrobak, P. Kwapulin´ski, J. Rasek, Phys. Stat. Sol. (a) 202 (2005) 2574. [6] T. Naohara, Philos. Mag. Lett. 78 (1998) 229. [7] G. Haneczok, J. Rasek, Def. Diffus. Forum 224–225 (2004) 13. [8] M.E. McHenry, M.A. Willard, D.E. Laughin, Prog. Mater. Sci. 44 (1999) 291. [9] P. Allia, F. Vinai, Phys. Rev. B. 26 (1982) 6141.
Journal of Magnetism and Magnetic Materials 320 (2008) e770–e773 www.elsevier.com/locate/jmmm
Studies of the relaxed amorphous phase in the Fe80Nb6B14 alloy A. Chrobaka,, G. Haneczokb, D. Chrobakb, Ł. Madejb, G. Che"kowskaa, M. Kulpaa a
b
University of Silesia, Institute of Physics, 40-007 Katowice, 4 Uniwersytecka, Poland University of Silesia, Institute of Materials Science, 40-007 Katowice, 12 Bankowa, Poland Available online 12 April 2008
Abstract The paper presents measurements of magnetic permeability, magnetic after-effects, magnetostriction, DSC and XPS for the Fe80Nb6B14 amorphous alloys preliminary annealed for 1 h at temperatures ranging from 300 to 770 K. It was shown that annealing out of free volume and internal stresses causes a decrease of magnetostriction coefficient and leads to the formation of the energetically stable relaxed amorphous state. The XPS spectra show local fluctuation of boron density. This effect was attributed to the formation of small iron clusters—the characteristic feature for the relaxed amorphous phase. r 2008 Elsevier B.V. All rights reserved. PACS: 75.75.+a; 75.50.y Keywords: Amorphous alloys; Free volume; Magnetic properties
1. Introduction It is well known that soft magnetic properties of amorphous alloys based on iron can be significantly improved by applying a suitable annealing treatment. This effect is usually explained by the formation of iron nanograins embedded into amorphous surroundings. Such a microstructure averages magnetic anisotropy and, according to the random anisotropy model [1,2], leads to the enhancement of soft magnetic properties (ESMP). Refs. [3,4] show that an appropriate chemical composition (e.g., Fe80Nb6B14) allows obtaining the ESMP effect without the formation of iron nanograins, i.e. in the socalled relaxed amorphous phase. This gives a decrease of material brittleness (in comparison with the nanocrystalline magnets) and what follows is a broadening of practical applications. A proper Nb content contributes to the slowing down of diffusion processes and shifts the crystallization into higher temperatures that makes it possible to obtain a thermodynamically stable relaxed amorphous phase [4].
Corresponding author. Tel./fax.: +48 32 3591630.
E-mail address: [email protected] (A. Chrobak). 0304-8853/$ - see front matter r 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2008.04.048
Recently [5], we have formulated a hypothesis that the relaxed amorphous structure can contain small iron clusters (acting as subnanograins) that are magnetically coupled and coherent with the amorphous surrounding. Such a microstructure with iron clusters of dimensions smaller than the ferromagnetic exchange length gives a random distribution of magnetic anisotropy and leads to an increase of magnetic permeability m (e.g. for the Fe80Nb6B14 alloy about 14 times). In a series of papers [6–8], the ESMP effect is usually explained by a decrease of free-volume content and magnetostriction coefficient, which is connected with annealing out of internal stresses. The aim of the present work is to study the structure of the relaxed amorphous phase and the mechanism of the ESMP effect by making use of (i) differential scanning calorimetry (DSC), (ii) magneto-dilatometer (MD), (iii) magnetic after-effects (MAE) and (iv) X-ray photoemission spectroscopy (XPS) measurements in order to characterize the amorphous phase in the context of the ESMP effect. 2. Experimental results The examined alloy—Fe80Nb6B14—was obtained by the melt spinning technique in the form of strips with thickness and width of about 25 mm and 10 mm, respectively.
ARTICLE IN PRESS A. Chrobak et al. / Journal of Magnetism and Magnetic Materials 320 (2008) e770–e773
As-quenched samples in the amorphous state (confirmed by X-ray measurements and high-resolution electron microscopy technique [3]) were annealed for 1 h at different temperatures (denoted as Ta) in the range from 300 to 770 K. The following experimental techniques were used:
Fig. 1 presents the DSC curves for three representative samples: annealed for 1 h at Ta ¼ 470, 700 and 770 K. It is worth noticing that for all samples the crystallization temperature Tx does not change. On the contrary, the heat transition from amorphous to crystalline state Qt (calculated as the area under the DSC peak) changes with the temperature of the preliminary annealing Ta. The relationship between Qt and annealing temperature Ta for all examined samples is presented in Fig. 2 where for comparison the so-called optimization curve m(Ta) is also shown. In the temperature up to Ta ¼ 650 K, Qt is almost stable and strongly decreases up to Ta ¼ 770 K. One can 470 K/1h 700 K/1h 770 K/1h
4
25
30 28
20
26 15 24
μ 10-3
QT [J/g]
DSC, Perkin-Elmer DSC-7, temperature range of 300–850 K, heating rate 5 K/min, MD at room temperature, self-designed, resolution 10 nm, sample length 50 mm, magnetic permeability m and MAE, Dm/m measured at room temperature (where Dm ¼ m(t1)m(t2); t1 ¼ 30 and t2 ¼ 1800 s are times after demagnetization; Maxwell–Wien bridge, frequency of about 1 kHz, magnetic field 0.5 A/m), XPS at room temperature, PHI 5700/660, momochromatized Al Ka radiation (1486.6 eV), resolution about 0.3 eV, vacuum 5 1010 Torr.
32
QT μ
10
22 20
5 18 0
16 500
600
700
800
Ta [K] Fig. 2. The magnetic permeability m and the heat transitions amorphous–crystalline state QT determined at room temperature for samples annealed for 1 h at temperatures Ta.
10
8
6 λ 106
e771
4
2
Heat flow [mW]
3
0
0 2
20
40 60 μ0H [mT]
80
100
Fig. 3. The parallel magnetostriction coefficient l versus magnetic field for the Fe80Nb6B14 amorphous alloy
1
0
750
760
770 T [K]
780
Fig. 1. Experimental DSC maxima measured for Fe80Nb6B14 amorphous alloy (heating rate 5 K/min) preliminary annealed at 470, 700 and 770 K/ 1h
notice that at temperatures close to the optimization annealing temperature Top (corresponding to the highest magnetic permeability), Qt shows a local plateau. Fig. 3 shows the parallel magnetostriction coefficient l versus magnetic field m0H for the samples in the as-quenched state after demagnetization. The presented relationship reflects a typical saturated curve where magnetostriction increases up to about 9 106 for m0H ¼ 40 mT. Magnetostriction coefficient l in saturation and magnetic after-effects Dm/m measured for samples annealed at
ARTICLE IN PRESS A. Chrobak et al. / Journal of Magnetism and Magnetic Materials 320 (2008) e770–e773
e772
12
9 80
50 40
7
30 20 10 0
Nb 3d
6 0
2
4
6 8 Δμ/μ
6
10 12
5 4
4
3
B 1s
Intensity [arb. unit]
Δμ/μ
8
8
λ 106
λ2 [10-6]
10
70 60
770 K/1h
2 2
700 K/1h
Δμ/μ λ
1
0
0 400
300
600 500 Ta [K]
800
700
aq
Fig. 4. The magnetic after effects Dm/m and the magnetostriction coefficient l determined at room temperature for samples annealed for 1 h at temperatures Ta.
210
190 200 Binding energy [eV]
180
Fig. 6. The XPS spectra of Nb 3d and B 1s lines for the Fe80Nb6B14 amorphous alloy (in the as quenched state and annealed at different Ta) determined at room temperature.
Intensity [arb. unit]
Fe80Nb6B14 770 K/1h
Fe80Nb6B14 700 K/1h
Fe80Nb6B14 aq
Fe
14
12
10
8 6 4 Binding energy [eV]
2
0
-2
Fig. 5. The XPS spectra of valence band for the Fe80Nb6B14 amorphous alloy (annealed at different Ta) and crystalline Fe determined at room temperature.
different temperatures Ta are presented in Fig. 4. The inset shows a plot l2(Ta) versus Dm/m. It is worth noting that l decreases with increasing annealing temperature up to Ta ¼ 660 K (60 K below the temperature of the optimization annealing Top) and is almost constant for Ta4660 K. It is generally accepted that the quantity Dm/m is related to the free-volume content frozen into melt-spun ribbons.
Both curves presented in Fig. 4 show similar behavior as l(Ta). Moreover, l2(Ta) correlates with magnetic aftereffects Dm/m (see the inset in Fig. 4), which suggests that both the quantities are closely connected with annealing out of internal stress during structural relaxation. Fig. 5 presents the XPS spectra of valence band obtained for three samples: in the as-quenched state, after annealing at Top ¼ 700 K (the relaxed amorphous state) and after annealing at 770 K (nanocrystalline state) [5]. For comparison the XPS spectrum of pure iron is also shown. All plotted curves are very similar but for samples of the Fe80Nb6B14 alloy, additional electronic states at 9 eV are observed. Apart from that, the Fe 2p, 3p and 3s core lines of all tested samples do not show any changes. Taking into account the latter, we focused on the boron 1s line and the niobium 3d line where some important differences were found. The corresponding XPS spectra for the same samples are shown in Fig. 6. As we can see the B 1s line intensity is sensitive to thermal treatment, i.e. is the highest for the sample annealed at temperature Top ¼ 700 K and is the lowest for the sample in the asquenched state. In contrast to this the Nb 3d band does not show any changes in all the examined cases. 3. Discussion and conclusions Thermodynamic stability of the amorphous and the relaxed amorphous phase is still of great interest from both scientific and practical point of view. This kind of structural disorder affects different physical quantities—in
ARTICLE IN PRESS A. Chrobak et al. / Journal of Magnetism and Magnetic Materials 320 (2008) e770–e773
amorphous alloys based on iron mainly magnetic properties via the ESMP effect. The question about a nature of this coupling is very important because it can allow modeling and optimizing material properties for a number of applications. In general, amorphous alloys are not in thermodynamic equilibrium and permanently tend to states with lower free energy. The difference in energetic levels is the driving force leading to structural changes and can be measured as the heat emitted during full or partial crystallization in the DSC experiment. Typical DSC exothermic peaks (see Fig. 1) are connected with the first stage of crystallization (formation and growth of iron nanograins). The second, well-separated, stage (formation of iron borides) can be observed in higher temperature range than the data presented in Fig. 1 [3]. However, the formation of iron nanostructure can be considered as the main reason for structural changes in the amorphous phase because it is the first energetically metastable state on the way to full crystallization. Therefore, the heat of the first stage of crystallization Qt for the sample preliminary annealed at different temperatures reflects an energetic state in different structural order (from the amorphous phase through the relaxed amorphous phase to the crystalline state). For the Fe80Nb6B14 amorphous alloy it is well documented (see Fig. 2) that just before nanocrystallization (the first nanograins were observed in samples annealed at Ta ¼ 760 K [3,5]), Qt is almost constant. This evident plateau reveals that the relaxed amorphous phase can be considered as a well-defined energetic metastable state. Moreover, this phenomenon is in correlation with the increase of magnetic permeability, which suggests that explanation of the ESMP effect lies in a specific Fe atoms order/disorder. It was already mentioned that the structural relaxation should exert an influence on magnetostriction coefficient l and magnetic after-effects Dm/m. According to Fig. 4 both these quantities decrease up to Ta ¼ 660 K and for higher annealing temperatures they are almost constant. This drop takes place at 40 K before the temperature of the optimization annealing Top and is due to annealing out of free volume and internal stresses frozen during fabrication. Besides, l2 correlates with Dm/m, which is in good agreement with the observations presented in Ref. [9] for different amorphous alloys. Studies of the electronic structure (XPS) of the tested alloy allowed understanding a role of boron and niobium atoms in structural relaxation. As is shown in Fig. 6, intensity of the Nb 3d line does not depend on preliminary annealing, which means that local niobium density does
e773
not change during structural relaxation. The latter can be explained by the fact that Nb atoms are relatively heavy and cause a slowing down of diffusion processes in the Fe–Nb–B group of amorphous alloys [3]. In contrast to niobium, boron atoms show a different behavior. Comparing the XPS spectra measured for the amorphous (asquenched) and the relaxed amorphous phase (Ta ¼ 700 K) of the examined alloy, one can conclude that the change of the B 1s line intensity reflects a fluctuation of local boron density taking place during structural relaxation (see Fig. 6). It can be explained by the formation of iron clusters free of boron atoms. So the observed relaxed amorphous phase consists of some regions enriched and free of boron. For samples annealed at higher temperatures, the boron atoms may be solved in iron crystalline grains and finally during full crystallization borides Fe2B, Fe3B are formed [3]. The main conclusions of the present paper can be formulated as follows:
Annealing out of free volume and internal stresses cause a decrease of magnetostriction coefficient in the examined amorphous alloy (Fe80Nb6B14) and leads to the formation of the energetically stable relaxed amourphous phase. The observed fluctuations of local boron density are caused by the formation of small iron clusters, characteristic of the relaxed amorphous phase. The enhancement of soft magnetic properties effect taking place in the relaxed amorphous phase can be explained by the formation of magnetically coupled iron clusters, which is in agreement with the Herzer model (averaging out of magneto-crystalline anisotropy) [1,2].
References [1] G. Herzer, L.L. Varga, J. Magn. Magn. Mater. 215–216 (2000) 506. [2] G. Herzer, J. Magn. Magn. Mater. 294 (2005) 99. [3] A. Chrobak, D. Chrobak, G. Haneczok, P. Kwapulin´ski, Z. Kwolek, M. Karolus, Mater. Sci. Eng. A 382 (2004) 401. [4] G. Haneczok, A. Chrobak, P. Kwapulin´ski, Z. Stok"osa, J. Rasek, N. Wo´jcik, in: Proceedings of the Soft Magnetic Materials, vol. 16, Du¨sseldorf, Germany, 2003, pp. 603–608. [5] G. Haneczok, J.E. Fra˛ ckowiak, A. Chrobak, P. Kwapulin´ski, J. Rasek, Phys. Stat. Sol. (a) 202 (2005) 2574. [6] T. Naohara, Philos. Mag. Lett. 78 (1998) 229. [7] G. Haneczok, J. Rasek, Def. Diffus. Forum 224–225 (2004) 13. [8] M.E. McHenry, M.A. Willard, D.E. Laughin, Prog. Mater. Sci. 44 (1999) 291. [9] P. Allia, F. Vinai, Phys. Rev. B. 26 (1982) 6141.