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Studies on the influence of the order–disorder transition on the magnetic properties of Fe–Al alloys
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 272–276 (2004) 1510–1511
Studies on the influence of the order–disorder transition on the magnetic properties of Fe–Al alloys D. Mart!ın Rodr!ıgueza,*, E. Apin˜aniza, J.S. Garitaonandiab, F. Plazaolaa, D.S. Schmoolc, G. Cuellod a
Elektrika eta Elektronika Saila, Universidad del Pais Vasco-Euskal Herriko Unibertsitatea, P. K. 644, Bilbao 48080, Spain b Fisika Aplikatua II Saila, Euskal Herriko Unibertsitatea, P. K. 644, Bilbao 48080, Spain c Departamento de F!ısica & IFIMUP, Universidade do Porto, Rua do Campo Alegre 687, Porto 4169-007, Portugal d Institute Laue Langevin, Rue Jules Horowitz B. P., Grenoble Cedex 156-38042, France
Abstract The degree of order in Fe–Al intermetallics has an important influence on their magnetic properties. Moreover, the disorder–order transition provokes a dramatical change on the magnetism of these alloys. In order to clarify this . phenomenon, neutron diffraction and Mossbauer spectroscopy measurements have been made. The results show that the reordering process occurs in two stages, the first of which occurs at about 150 C while the second takes place at around 350 C: The change of magnetic properties is observed to be related to the decrease of A2 phase content and decreasing lattice parameter. r 2003 Elsevier B.V. All rights reserved. PACS: 61.12.Ld; 76.80.þy; 75.50.Bb Keywords: Fe–Al alloys; Order–disorder transition; Magnetic enhancement; Defect annealing
Mechanical deformation is known to induce disorder in fully ordered Fe–Al alloys. This effect produces a strong increase in magnetisation in these alloys and may even induce ferromagnetic transition in normally paramagnetic alloys [1,2]. However, mechanically deformed Fe–Al alloys experience a dramatic decrease of magnetisation when heated to 150–200 C; where calorimetric measurements display a large exothermic peak [3]. Over the past several decades different mechanisms have been proposed to explain this phenomenon: such as a change in local Fe environment [2] and volume effects [4]. This phenomenon is still, however, open to question. To clarify the situation, we have performed neutron . diffraction and Mossbauer spectroscopy on the Fe65 Al35 alloy. Fe65 Al35 as-crushed alloy powders have been prepared as discussed in Ref. [3]. In situ neutron diffraction *Corresponding author. Tel.: +34-94-601-5371; fax: +3494-601-3071. E-mail address: [email protected] (D. Mart!ın Rodr!ıguez).
measurements were performed at Institut Laue Langevin (ILL) on the D20 instrument. The as-crushed sample was heated from room temperature to 600 C twice while neutron diffraction spectra were continuously . collected. Mossbauer spectra were also taken on the as-crushed sample (see Ref. [3]). In Fig. 1 we show three neutron diffraction spectra measured at room temperature: the as-crushed sample, the same sample after the first heating cycle and then after the second heating. There are two important featurings to note: the B2 superstructure peaks increase after the first annealing and the spectra do not alter with further annealing, indicating that all the crystallographic changes occur in the first heat treatment. . Fig. 2 shows Mossbauer spectra before and after measuring in ILL sample. We clearly observe that the as-crushed sample has a strong ferromagnetic contribution while the annealed one is essentially paramagnetic. The neutron diffraction spectra were fitted using Rietveld method in order to obtain the quantity of each phase and their lattice parameter (Fig. 3). We observe a
0304-8853/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2003.12.1108
ARTICLE IN PRESS D. Mart!ın Rodr!ıguez et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) 1510–1511
Fig. 1. Neutron diffraction spectra taken at room temperature before and after different heating cycles.
. Fig. 2. Mossbauer spectra taken before and after the annealing treatment performed in ILL.
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phase is progressively eliminated and complete B2 ordering is reached. Once the ordering is complete, the lattice parameters during first and second heating cycles coincide (Fig. 3). No traces of the A2 phase remain after the first annealing, the sample being B2 and the only change is then an increase of lattice parameter due to thermal expansion. Neutron diffraction measurements show that all crystallographic changes occur during the first annealing and, due to the relation between order and magnetism, we conclude that all magnetic changes also occur in the first annealing cycle. In the first reordering stage (100– 150 C), two factors that may be responsible for the reduction in magnetisation: firstly, a decrease of the A2 phase content and secondly, a decrease in lattice parameter of the A2 phase. This can be attributed to the reduction of defects such as vacancies and interstitials [5]. Recent theoretical calculations [6] show that not only crystallographic disorder but also the lattice parameter has an important influence on the magnetic properties of these alloys, therefore experimental results are in good qualitative agreement with these considerations. . In conclusion, Neutron diffraction and Mossbauer spectroscopy measurements on the as-crushed Fe65 Al35 alloy show that reordering is a two stage process. In the first (100–150 C) there is a dramatic reduction of the A2 phase content and lattice parameter. After the second stage (250–350 C), the ordering process is finished. This work was undertaken under projects MAT 20024087-C02-01 and UPV 224.310-14553/2002. Authors are grateful to J. A. Jimenez for helpful discussions. We gratefully acknowledge J. Torregrossa during the experiment. D. M. R. thanks the UPV/EHU for his fellowship.
References Fig. 3. Lattice parameters of A2 and B2 phases as a function of temperature from neutron data fittings. Inset shows the variation of the contents of the two phases during the first annealing process.
dramatic change between 100 C and 180 C in which the quantity and lattice parameter of the A2 phase decrease significantly. The phase percentages then remain approximately constant upto 250 C after which the A2
[1] A. Taylor, R.M. Jones, J. Phys. Chem. Solids 6 (1958) 16. [2] G.P. Huffman, R.M. Fisher, J. Appl. Phys. 38 (1967) 735. [3] E. Apin˜aniz, J.S. Garitaonandia, F. Plazaola, J.J. del Val, J.A. Jimenez, A.R. Pierna, J. Magn. Magn. Mater. 254–255 (2002) 136. [4] A. Hernando, X. Amils, J. Nogu!es, S. Surin˜ach, M. D. ! M.R. Ibarra, Phys. Rev. B 58 (1998) 11864. Baro, ! [5] X. Amils, J. Nogu!es, S. Surin˜ach, J.S. Mun˜oz, M.D. Baro, A. Hernando, J.P. Morniroli, Phys. Rev. B 63 (2001) 52402. [6] E. Apin˜aniz, F. Plazaola, J.S. Garitaonandia, Eur. Phys. J. B 31 (2003) 167.
Journal of Magnetism and Magnetic Materials 272–276 (2004) 1510–1511
Studies on the influence of the order–disorder transition on the magnetic properties of Fe–Al alloys D. Mart!ın Rodr!ıgueza,*, E. Apin˜aniza, J.S. Garitaonandiab, F. Plazaolaa, D.S. Schmoolc, G. Cuellod a
Elektrika eta Elektronika Saila, Universidad del Pais Vasco-Euskal Herriko Unibertsitatea, P. K. 644, Bilbao 48080, Spain b Fisika Aplikatua II Saila, Euskal Herriko Unibertsitatea, P. K. 644, Bilbao 48080, Spain c Departamento de F!ısica & IFIMUP, Universidade do Porto, Rua do Campo Alegre 687, Porto 4169-007, Portugal d Institute Laue Langevin, Rue Jules Horowitz B. P., Grenoble Cedex 156-38042, France
Abstract The degree of order in Fe–Al intermetallics has an important influence on their magnetic properties. Moreover, the disorder–order transition provokes a dramatical change on the magnetism of these alloys. In order to clarify this . phenomenon, neutron diffraction and Mossbauer spectroscopy measurements have been made. The results show that the reordering process occurs in two stages, the first of which occurs at about 150 C while the second takes place at around 350 C: The change of magnetic properties is observed to be related to the decrease of A2 phase content and decreasing lattice parameter. r 2003 Elsevier B.V. All rights reserved. PACS: 61.12.Ld; 76.80.þy; 75.50.Bb Keywords: Fe–Al alloys; Order–disorder transition; Magnetic enhancement; Defect annealing
Mechanical deformation is known to induce disorder in fully ordered Fe–Al alloys. This effect produces a strong increase in magnetisation in these alloys and may even induce ferromagnetic transition in normally paramagnetic alloys [1,2]. However, mechanically deformed Fe–Al alloys experience a dramatic decrease of magnetisation when heated to 150–200 C; where calorimetric measurements display a large exothermic peak [3]. Over the past several decades different mechanisms have been proposed to explain this phenomenon: such as a change in local Fe environment [2] and volume effects [4]. This phenomenon is still, however, open to question. To clarify the situation, we have performed neutron . diffraction and Mossbauer spectroscopy on the Fe65 Al35 alloy. Fe65 Al35 as-crushed alloy powders have been prepared as discussed in Ref. [3]. In situ neutron diffraction *Corresponding author. Tel.: +34-94-601-5371; fax: +3494-601-3071. E-mail address: [email protected] (D. Mart!ın Rodr!ıguez).
measurements were performed at Institut Laue Langevin (ILL) on the D20 instrument. The as-crushed sample was heated from room temperature to 600 C twice while neutron diffraction spectra were continuously . collected. Mossbauer spectra were also taken on the as-crushed sample (see Ref. [3]). In Fig. 1 we show three neutron diffraction spectra measured at room temperature: the as-crushed sample, the same sample after the first heating cycle and then after the second heating. There are two important featurings to note: the B2 superstructure peaks increase after the first annealing and the spectra do not alter with further annealing, indicating that all the crystallographic changes occur in the first heat treatment. . Fig. 2 shows Mossbauer spectra before and after measuring in ILL sample. We clearly observe that the as-crushed sample has a strong ferromagnetic contribution while the annealed one is essentially paramagnetic. The neutron diffraction spectra were fitted using Rietveld method in order to obtain the quantity of each phase and their lattice parameter (Fig. 3). We observe a
0304-8853/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2003.12.1108
ARTICLE IN PRESS D. Mart!ın Rodr!ıguez et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) 1510–1511
Fig. 1. Neutron diffraction spectra taken at room temperature before and after different heating cycles.
. Fig. 2. Mossbauer spectra taken before and after the annealing treatment performed in ILL.
1511
phase is progressively eliminated and complete B2 ordering is reached. Once the ordering is complete, the lattice parameters during first and second heating cycles coincide (Fig. 3). No traces of the A2 phase remain after the first annealing, the sample being B2 and the only change is then an increase of lattice parameter due to thermal expansion. Neutron diffraction measurements show that all crystallographic changes occur during the first annealing and, due to the relation between order and magnetism, we conclude that all magnetic changes also occur in the first annealing cycle. In the first reordering stage (100– 150 C), two factors that may be responsible for the reduction in magnetisation: firstly, a decrease of the A2 phase content and secondly, a decrease in lattice parameter of the A2 phase. This can be attributed to the reduction of defects such as vacancies and interstitials [5]. Recent theoretical calculations [6] show that not only crystallographic disorder but also the lattice parameter has an important influence on the magnetic properties of these alloys, therefore experimental results are in good qualitative agreement with these considerations. . In conclusion, Neutron diffraction and Mossbauer spectroscopy measurements on the as-crushed Fe65 Al35 alloy show that reordering is a two stage process. In the first (100–150 C) there is a dramatic reduction of the A2 phase content and lattice parameter. After the second stage (250–350 C), the ordering process is finished. This work was undertaken under projects MAT 20024087-C02-01 and UPV 224.310-14553/2002. Authors are grateful to J. A. Jimenez for helpful discussions. We gratefully acknowledge J. Torregrossa during the experiment. D. M. R. thanks the UPV/EHU for his fellowship.
References Fig. 3. Lattice parameters of A2 and B2 phases as a function of temperature from neutron data fittings. Inset shows the variation of the contents of the two phases during the first annealing process.
dramatic change between 100 C and 180 C in which the quantity and lattice parameter of the A2 phase decrease significantly. The phase percentages then remain approximately constant upto 250 C after which the A2
[1] A. Taylor, R.M. Jones, J. Phys. Chem. Solids 6 (1958) 16. [2] G.P. Huffman, R.M. Fisher, J. Appl. Phys. 38 (1967) 735. [3] E. Apin˜aniz, J.S. Garitaonandia, F. Plazaola, J.J. del Val, J.A. Jimenez, A.R. Pierna, J. Magn. Magn. Mater. 254–255 (2002) 136. [4] A. Hernando, X. Amils, J. Nogu!es, S. Surin˜ach, M. D. ! M.R. Ibarra, Phys. Rev. B 58 (1998) 11864. Baro, ! [5] X. Amils, J. Nogu!es, S. Surin˜ach, J.S. Mun˜oz, M.D. Baro, A. Hernando, J.P. Morniroli, Phys. Rev. B 63 (2001) 52402. [6] E. Apin˜aniz, F. Plazaola, J.S. Garitaonandia, Eur. Phys. J. B 31 (2003) 167.