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Short-range order in α-Fe–Si single crystals
Journal of Magnetism and Magnetic Materials 254–255 (2003) 346–348
Short-range order in a-Fe–Si single crystals Yu.P. Chernenkova, V.I. Fedorova, V.A. Lukshinab, B.K. Sokolovb, N.V. Ershovb,* a
b
Petersburg Nuclear Physics Institute, Russian Academy of Science, Gatchina, St. Petersburg 188350, Russia Institute of Metal Physics, Urals Branch of Russian Academy of Science, S. Kovalevskaya Street, 18, GSP-18, Yekaterinburg 620219, Russia
Abstract Short-range order in a-Fe–Si single crystals with 4.9 and 5.8 at% of silicon subjected to thermomagnetic treatment inducing magnetic anisotropy was studied by X-ray diffraction. Direct experimental evidence for Neel’s hypothesis concerning the oriented ordering of Si atoms in soft magnetic alloys Fe–Si is provided for the first time. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Thermomagnetic treatment; Anisotropy—induced; X-ray diffraction; Short-range order
The 50 years old Neel’s idea [1,2] of oriented ordering of Si atoms is fruitfully used to explain the magnetic properties of the iron-rich Fe–Si alloys having heavyduty applications as magnetic core materials. Neel’s theory was most successfully applied for interpretation of the magnetic anisotropy induced by thermomagnetic treatment (TMT). TMT consists of the successive annealing and cooling of a-Fe–Si alloy in magnetic field parallel to an easy magnetization axis of [0 0 1] type lying in the steel sheet plane. TMT leads to the magnetic permeability increase in [0 0 1] direction and to its decrease in transverse direction. Nevertheless, there was no direct evidence for the structural origin of the magnetic anisotropy. Numerous studies of ordering in Fe–Si (with 0– 20 at% of Si) have revealed the existence of disordered solid solution A2, and two phases DO3 and B2, whose combination and quota depend on composition, the measurement temperature and method of alloy treatment [3]. Two series of samples were used in our investigations. Thin disks about 10 mm in diameter having cubic texture (1 0 0)—Ca and Ci with 4.9 at% Si, and Goss’s texture (1 1 0)—Ga and Gi with 5.8 at% Si were spark*Corresponding author. Tel.: +7-3432-745244; fax: +73432-757476. E-mail address: [email protected] (N.V. Ershov).
cut from a bulk single crystal and a grain of industrial steel sheet, respectively. TMT in saturating magnetic field at 4501C was applied to Ca and Ga before their thinning (to 40–50 mm) with the aim to align all the domain magnetic moments along [0 0 1] and, as a consequence, to induce magnetic anisotropy. Two other samples Ci and Gi were prepared in the same way but Ci in rotating magnetic field and Gi in alternating magnetic field perpendicular to [0 0 1] to produce as low magnetic anisotropy as possible. The X-ray diffused scattering measurements were made at room temperature on a four-circle diffract( monochroometer using Mo Ka radiation (l ¼ 0:71 A) mated by a pyrolytic graphite crystal. To increase the intensity of weak diffused reflections, the X-ray beam with cross-section f ¼ 2 mm and divergence about 0.51 was used. The samples were set in transmission geometry and scattering vector lied in the sample plane. This geometry guarantees the identical measurement conditions for disk-like samples under investigation and any observed difference in scattering has to be attributed to the sample properties. There was no indication of the DO3 type reflections with half-integer indices whereas y22y-scans across the diffused superlattice reflections with odd sum of Miller indices h þ k þ l; (1 0 0), (0 0 1), (1 1 1), etc. reveal their slight shift to lower scattering angles from calculated positions for BCC structure. More pronounced shift of
0304-8853/03/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 2 ) 0 0 8 4 5 - 4
Y.P. Chernenkov et al. / Journal of Magnetism and Magnetic Materials 254–255 (2003) 346–348
347
Table 1 ( Average size of regions with B2 short-range order (in A) Direction
001 100
Sample Ca
Ci
Ga
Gi
8(1) 6(1)
8(1) 9(1)
10(1) 8(1)
11(1) 10(1)
(1 1 1) is shown on the intensity contour map. Reflections (1 0 0) and (0 0 1) originate from regions with B2 type ordering and their scattering vectors are parallel and perpendicular to the TMT magnetic field, respectively. The reflections have different widths in the samples Ca and Ga with magnetic anisotropy and equal ones in Ci and Gi : From the full-width at half-maximum of a reflection, one can estimate the average size of ordered domains or the extension of B2 regions [4]. The results of such evaluation for all samples in [1 0 0] and [0 0 1] directions are given in Table 1. The measurement accuracy is rather poor in view of the low intensity of (1 0 0) and (0 0 1) which is 5–6 orders of magnitude less than that of the main structure reflection (1 0 1). Nevertheless, we could roughly estimate the average ( in a-Fe–(4.9 at%)Si, B8– size of the B2 regions B6–9 A ( ( in 11 A in a-Fe–(5.8 at%)Si, and their anisotropy B2 A Ca and Ga : Thus after TMT in permanent magnetic field the regions with B2 short-range order have different sizes along and across the magnetic field direction; thereby Neel’s hypothesis concerning the oriented ordering of Si atoms in soft magnetic alloys Fe–Si is proved directly for the first time. The cause of the superstructural reflection shift to lower scattering angles can be explained in the frames of a simple qualitative model. If an atom in pure iron ( is substituted by an lattice with parameter aFe ¼ 2:866 A ( less than the Fe atom of Si having radius rSi ¼ 0:98 A ( then eight nearest Fe neighbours move one rFe ¼ 1:22 A, closer to the Si atom forming a B2 type cell (CsCl structure). Its parameter a oaFe ; therefore at least 26 nearest BCC cells are deformed (stretched) and their averaged lattice parameter is larger than aFe : It is obvious that in case of B2 core consisting of two, three or more neighbouring B2 cells (Neel’s hypothesis) the relaxation of distorted lattice takes place at distances longer than aFe and thickness of a coat being made of stretched BCC cells surrounding the B2 core increases. In diffraction such lattice distortions appear as superstructural reflections with odd sum of Miller indices h þ k þ l: Rough estimation shows that contribution of the coat consisting of deformed BCC cells to their intensity is at least not less than the contribution of the B2 core. The averaged lattice parameter in a BCC stretched coat can be evaluated from the shift of a diffused peak maximum.
Fig. 1.
( is more than in pure iron. The Its value ac ¼ 2:98 A ( is enough to compensate the difference ac aFe E0:1 A misfit between the iron lattice and the bulk B2 structure of FeSi compound which was experimentally observed ( [5]. in a stable epitaxial FeSi film having aFeSi ¼ 2:77 A Thus the appearance of the B2 cores in the iron lattice results in a stretching of the nearest cells and in a shift of the superstructural diffused reflections to the lower scattering angle. According to theoretical calculations [6] and experimental observations in epitaxial films [5] the magnetic moment of an atom in FeSi compound with B2 type ordering is either very small or is absent at all. However, enlargement of the lattice parameter that probably takes place in distorted BCC cells of coat leads to increase of atomic magnetic moment [7]. Assuming the occurrence of the same tendency in Fe–Si alloys under investigation, one should expect the decrease of magnetic moment in the B2 core and its increase in the BCC coat. Hence, magnetic anisotropy is probably induced by spatial anisotropy of regions with B2 type ordering in Fe–Si (5– 6 at% Si) alloys subjected to TMT. In other words, the annealing temperature 4501C being below TC B7001C for the alloys is high enough to activate the diffusion of Si atoms and, consequently, for reordering of the nearest neighbourhood during TMT process whose duration is about 1 h. Thus, presence of permanent saturating magnetic field during TMT leads to energetically more preferable spatial distribution of Si atoms, which is most likely anisotropic (Fig. 1).
References [1] L. Neel, J. Phys. Radiat. 12 (1951) 339. [2] L. Neel, J. Phys. Radiat. 13 (1952) 249. [3] K. Nilfrich, W. Kolker, W. Petry, O. Scharpf, Acta Metall. Mater. 42 (1994) 743.
348
Y.P. Chernenkov et al. / Journal of Magnetism and Magnetic Materials 254–255 (2003) 346–348
[4] W. Kolker, R. Wagner, E. Nembach, J. Phys. F: Met. Phys. 18 (1988) 2513. [5] K.A. Mader, Hans von Kanel, A. Balderschi, Phys. Rev. B 48 (1993) 4364.
[6] J. Deniszczyk, Acta Phys. Polon. 98 (2000) 543. [7] A.K. Arzhnikov, L.V. Dobysheva, Phys. Rev. B 62 (2000) 5324.
Short-range order in a-Fe–Si single crystals Yu.P. Chernenkova, V.I. Fedorova, V.A. Lukshinab, B.K. Sokolovb, N.V. Ershovb,* a
b
Petersburg Nuclear Physics Institute, Russian Academy of Science, Gatchina, St. Petersburg 188350, Russia Institute of Metal Physics, Urals Branch of Russian Academy of Science, S. Kovalevskaya Street, 18, GSP-18, Yekaterinburg 620219, Russia
Abstract Short-range order in a-Fe–Si single crystals with 4.9 and 5.8 at% of silicon subjected to thermomagnetic treatment inducing magnetic anisotropy was studied by X-ray diffraction. Direct experimental evidence for Neel’s hypothesis concerning the oriented ordering of Si atoms in soft magnetic alloys Fe–Si is provided for the first time. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Thermomagnetic treatment; Anisotropy—induced; X-ray diffraction; Short-range order
The 50 years old Neel’s idea [1,2] of oriented ordering of Si atoms is fruitfully used to explain the magnetic properties of the iron-rich Fe–Si alloys having heavyduty applications as magnetic core materials. Neel’s theory was most successfully applied for interpretation of the magnetic anisotropy induced by thermomagnetic treatment (TMT). TMT consists of the successive annealing and cooling of a-Fe–Si alloy in magnetic field parallel to an easy magnetization axis of [0 0 1] type lying in the steel sheet plane. TMT leads to the magnetic permeability increase in [0 0 1] direction and to its decrease in transverse direction. Nevertheless, there was no direct evidence for the structural origin of the magnetic anisotropy. Numerous studies of ordering in Fe–Si (with 0– 20 at% of Si) have revealed the existence of disordered solid solution A2, and two phases DO3 and B2, whose combination and quota depend on composition, the measurement temperature and method of alloy treatment [3]. Two series of samples were used in our investigations. Thin disks about 10 mm in diameter having cubic texture (1 0 0)—Ca and Ci with 4.9 at% Si, and Goss’s texture (1 1 0)—Ga and Gi with 5.8 at% Si were spark*Corresponding author. Tel.: +7-3432-745244; fax: +73432-757476. E-mail address: [email protected] (N.V. Ershov).
cut from a bulk single crystal and a grain of industrial steel sheet, respectively. TMT in saturating magnetic field at 4501C was applied to Ca and Ga before their thinning (to 40–50 mm) with the aim to align all the domain magnetic moments along [0 0 1] and, as a consequence, to induce magnetic anisotropy. Two other samples Ci and Gi were prepared in the same way but Ci in rotating magnetic field and Gi in alternating magnetic field perpendicular to [0 0 1] to produce as low magnetic anisotropy as possible. The X-ray diffused scattering measurements were made at room temperature on a four-circle diffract( monochroometer using Mo Ka radiation (l ¼ 0:71 A) mated by a pyrolytic graphite crystal. To increase the intensity of weak diffused reflections, the X-ray beam with cross-section f ¼ 2 mm and divergence about 0.51 was used. The samples were set in transmission geometry and scattering vector lied in the sample plane. This geometry guarantees the identical measurement conditions for disk-like samples under investigation and any observed difference in scattering has to be attributed to the sample properties. There was no indication of the DO3 type reflections with half-integer indices whereas y22y-scans across the diffused superlattice reflections with odd sum of Miller indices h þ k þ l; (1 0 0), (0 0 1), (1 1 1), etc. reveal their slight shift to lower scattering angles from calculated positions for BCC structure. More pronounced shift of
0304-8853/03/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 2 ) 0 0 8 4 5 - 4
Y.P. Chernenkov et al. / Journal of Magnetism and Magnetic Materials 254–255 (2003) 346–348
347
Table 1 ( Average size of regions with B2 short-range order (in A) Direction
001 100
Sample Ca
Ci
Ga
Gi
8(1) 6(1)
8(1) 9(1)
10(1) 8(1)
11(1) 10(1)
(1 1 1) is shown on the intensity contour map. Reflections (1 0 0) and (0 0 1) originate from regions with B2 type ordering and their scattering vectors are parallel and perpendicular to the TMT magnetic field, respectively. The reflections have different widths in the samples Ca and Ga with magnetic anisotropy and equal ones in Ci and Gi : From the full-width at half-maximum of a reflection, one can estimate the average size of ordered domains or the extension of B2 regions [4]. The results of such evaluation for all samples in [1 0 0] and [0 0 1] directions are given in Table 1. The measurement accuracy is rather poor in view of the low intensity of (1 0 0) and (0 0 1) which is 5–6 orders of magnitude less than that of the main structure reflection (1 0 1). Nevertheless, we could roughly estimate the average ( in a-Fe–(4.9 at%)Si, B8– size of the B2 regions B6–9 A ( ( in 11 A in a-Fe–(5.8 at%)Si, and their anisotropy B2 A Ca and Ga : Thus after TMT in permanent magnetic field the regions with B2 short-range order have different sizes along and across the magnetic field direction; thereby Neel’s hypothesis concerning the oriented ordering of Si atoms in soft magnetic alloys Fe–Si is proved directly for the first time. The cause of the superstructural reflection shift to lower scattering angles can be explained in the frames of a simple qualitative model. If an atom in pure iron ( is substituted by an lattice with parameter aFe ¼ 2:866 A ( less than the Fe atom of Si having radius rSi ¼ 0:98 A ( then eight nearest Fe neighbours move one rFe ¼ 1:22 A, closer to the Si atom forming a B2 type cell (CsCl structure). Its parameter a oaFe ; therefore at least 26 nearest BCC cells are deformed (stretched) and their averaged lattice parameter is larger than aFe : It is obvious that in case of B2 core consisting of two, three or more neighbouring B2 cells (Neel’s hypothesis) the relaxation of distorted lattice takes place at distances longer than aFe and thickness of a coat being made of stretched BCC cells surrounding the B2 core increases. In diffraction such lattice distortions appear as superstructural reflections with odd sum of Miller indices h þ k þ l: Rough estimation shows that contribution of the coat consisting of deformed BCC cells to their intensity is at least not less than the contribution of the B2 core. The averaged lattice parameter in a BCC stretched coat can be evaluated from the shift of a diffused peak maximum.
Fig. 1.
( is more than in pure iron. The Its value ac ¼ 2:98 A ( is enough to compensate the difference ac aFe E0:1 A misfit between the iron lattice and the bulk B2 structure of FeSi compound which was experimentally observed ( [5]. in a stable epitaxial FeSi film having aFeSi ¼ 2:77 A Thus the appearance of the B2 cores in the iron lattice results in a stretching of the nearest cells and in a shift of the superstructural diffused reflections to the lower scattering angle. According to theoretical calculations [6] and experimental observations in epitaxial films [5] the magnetic moment of an atom in FeSi compound with B2 type ordering is either very small or is absent at all. However, enlargement of the lattice parameter that probably takes place in distorted BCC cells of coat leads to increase of atomic magnetic moment [7]. Assuming the occurrence of the same tendency in Fe–Si alloys under investigation, one should expect the decrease of magnetic moment in the B2 core and its increase in the BCC coat. Hence, magnetic anisotropy is probably induced by spatial anisotropy of regions with B2 type ordering in Fe–Si (5– 6 at% Si) alloys subjected to TMT. In other words, the annealing temperature 4501C being below TC B7001C for the alloys is high enough to activate the diffusion of Si atoms and, consequently, for reordering of the nearest neighbourhood during TMT process whose duration is about 1 h. Thus, presence of permanent saturating magnetic field during TMT leads to energetically more preferable spatial distribution of Si atoms, which is most likely anisotropic (Fig. 1).
References [1] L. Neel, J. Phys. Radiat. 12 (1951) 339. [2] L. Neel, J. Phys. Radiat. 13 (1952) 249. [3] K. Nilfrich, W. Kolker, W. Petry, O. Scharpf, Acta Metall. Mater. 42 (1994) 743.
348
Y.P. Chernenkov et al. / Journal of Magnetism and Magnetic Materials 254–255 (2003) 346–348
[4] W. Kolker, R. Wagner, E. Nembach, J. Phys. F: Met. Phys. 18 (1988) 2513. [5] K.A. Mader, Hans von Kanel, A. Balderschi, Phys. Rev. B 48 (1993) 4364.
[6] J. Deniszczyk, Acta Phys. Polon. 98 (2000) 543. [7] A.K. Arzhnikov, L.V. Dobysheva, Phys. Rev. B 62 (2000) 5324.