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Magnetic softening of nanocrystalline Fe74Cu1Nb3Si12B10 alloy obtained by two-step heat treatment
Journal of Magnetism and Magnetic Materials 215}216 (2000) 422}424
Magnetic softening of nanocrystalline Fe Cu Nb Si B alloy obtained by two-step heat treatment A. B"achowicz *, J. Zbroszczyk , J. Olszewski , W.H. CiurzynH ska , H. Fukunaga, K. Narita, B. Wys"ocki , M. Hasiak Institute of Physics, Technical University of Cze9 stochowa, Al Armii Krajowej 19, 42-200 Cze9 stochowa, Poland Faculty of Engineering, Nagasaki University, Nagasaki, Japan Faculty of Engineering, Kyushu University, Fukuoka 812, Japan
Abstract The magnetic properties of the nanocrystalline Fe Cu Nb Si B alloy annealed at 573, 623, 673 or 700 K for 1 h and then at 823 K (crystallization temperature) for 0.5 h are investigated. It is found that the preannealing of the samples narrows grain size distribution which involves a higher initial magnetic susceptibility. However, the distribution of the grain size does not in#uence the magnetic susceptibility disaccommodation. 2000 Elsevier Science B.V. All rights reserved. Keywords: Nanocrystalline materials; Grain size distribution; MoK ssbauer e!ect
The excellent magnetic properties of nanocrystalline materials are attributed to "ne grains embedded in an amorphous matrix [1]. These grains grow in the regions rich in iron due to the di!usion of Cu, Nb and B atoms outside these regions. It has been found [2,3] that the magnetic properties of the nanocrystalline alloys depend on preparation conditions of the amorphous ribbons. Moreover, it has been shown that the annealing of amorphous ribbons below the crystallization temperature in#uences the microstructure of the nanocrystalline alloys [4]. The aim of this paper is to study the microstructure and magnetic properties, namely magnetic susceptibility and its disaccommodation, of the nanocrystalline Fe Cu Nb Si B alloy obtained by the heat treat ment of the amorphous ribbons subjected earlier to a preannealing below the crystallization temperature. The microstructure of the nanocrystalline samples was investigated using MoK ssbauer spectrometry, X-ray dif-
* Corresponding author. Tel./fax: #48-34-3250-795. E-mail address: [email protected] (A. B"achowicz).
fractometry and transmission electron microscopy. MoK ssbauer spectrometry was used to determine the distribution of the magnetization in the samples, their phase composition and the iron content in the amorphous and crystalline phases. X-ray di!ractometry was applied in order to check the amorphousness of the samples and for the identi"cation of the crystalline phase. From the transmission electron microscopy measurements grain size distribution has been determined. Magnetic susceptibility (s) and its disaccommodation *(1/s)"1/s !1/s (where s and s are the values of the magnetic susceptibility at t "2 s and t "120 s after the demagnetization, respectively) were investigated for toroidal samples of 2 cm inner diameter in the temperature range from 160 up to 600 K. The amplitude and frequency of the magnetizing "eld were equal to 0.16 A/m and 2 kHz, respectively. All investigations were carried out for the samples preannealed at 573, 623, 673 or 700 K for 1 h and then heat treated at 823 K (the crystallization temperature) for 0.5 h. To compare microstructure and magnetic properties of the annealed samples, one sample was subjected to one-step annealing at 823 K for 0.5 h. All samples in the as-quenched state exhibited almost the same initial magnetic susceptibility, equal to about 700.
0304-8853/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 0 ) 0 0 1 7 7 - 3
A. B!achowicz et al. / Journal of Magnetism and Magnetic Materials 215}216 (2000) 422}424
The magnetic susceptibility versus temperature for the Fe Cu Nb Si B alloy subjected to two-step anneal ing (the "rst step below the crystallization temperature at 673 K for 1 h and the second step * at 823 K for 0.5 h) is shown in Fig. 1. It is seen that the nanocrystalline sample obtained by the heat treatment of the amorphous sample, which was preannealed at 673 K for 1 h, exhibits an initial magnetic susceptibility equal to about 9000 (at room temperature). However, the susceptibility of the nanocrystalline sample, obtained by one-step heat treatment (at 823 K for 0.5 h), reaches a lower susceptibility equal to about 7000. A similar behavior was observed for the samples preannealed at 573, 623 and 700 K and then heat treated at 823 K for 0.5 h. The initial magnetic susceptibility for these samples was equal to 7200, 7700 and 8500, respectively. The disaccommodation of the magnetic susceptibility versus temperature is presented in Fig. 2. It is seen that the shape of the isochronal disaccommodation curves does not depend on the preannealing conditions of the samples. One can see in these curves only temperatureindependent background (from about 175 up to 475 K). Near the Curie temperature of the amorphous matrix (550 K) the intensity of the disaccommodation rapidly increases due to the phase transition (Hopkinson maximum) from the ferromagnetic to paramagnetic state [5]. In order to explain the magnetic properties of the nanocrystalline samples obtained by two-step annealing,
423
the investigations of their microstructure have been carried out. From the analysis of the MoK ssbauer spectra for the as-quenched and preannealed samples it has been found that this treatment leads to the increase of the packing density of atoms. The hyper"ne "eld distribution parameters, evaluated from MoK ssbauer spectra analysis, for the nanocrystalline samples are shown in Table 1. It is seen from this table that after the second step of annealing, the phase composition of all investigated samples is very similar. Moreover, the composition of the amorphous matrix is almost the same. Furthermore, the iron content in the crystalline a-FeSi phase is equal to 85.4 at% in all investigated samples. At the same time, from the electron microscope observation, it is found that the distribution of grain sizes in the nanocrystalline samples depends on the heat treatment. The narrowest grain size distribution is observed for that sample which exhibits the highest susceptibility (annealed at 673 K for 1 h and then at 823 K for 0.5 h). Concerning disaccommodation intensity in the nanocrystalline samples, it does not depend on the grain size distribution.
Fig. 2. Isochronal disaccommodation curves for the nanocrystalline Fe Cu Nb Si B alloy obtained by di!erent heat treatments of the amorphous ribbons at: (*) 673 K for 1 h and then 823 K for 0.5 h, (;) 823 K for 0.5 h.
Fig. 1. Initial magnetic susceptibility of the nanocrystalline Fe Cu Nb Si B alloy obtained by di!erent annealing at: (*) 673 K for 1 h and then 823 K for 0.5 h, (;) 823 K for 0.5 h.
Table 1 The average hyper"ne "eld (B ), second line intensity in Zeeman sextets (A ), volume fraction of the crystalline phase (< ), content of iron in the crystalline and amorphous phases (Fe , Fe , respectively), for the nanocrystalline Fe Cu Nb Si B alloy, after di!erent
heat treatments Treatment
B (T)
A
<
573 K/1 h#823 K/0.5 h 623 K/1 h#823 K/0.5 h 673 K/1 h#823 K/0.5 h 700 K/1 h#823 K/0.5 h 823 K/0.5 h
23.64 23.57 23.61 23.67 23.65
3.45 3.63 3.51 3.61 3.65
0.48 0.48 0.48 0.48 0.50
Fe
61.0 61.0 61.0 61.0 62.0
(%)
<
Fe (%)
0.52 0.52 0.52 0.52 0.50
85.4 85.4 85.4 85.4 85.6
424
A. B!achowicz et al. / Journal of Magnetism and Magnetic Materials 215}216 (2000) 422}424
This work was "nancially supported by the Polish Committee for Scienti"c Research (Grant No. 7 T082 022 17). References [1] Y. Yoshizawa, S. Oguma, K. Yamauchi, J. Appl. Phys. 64 (1988) 6044.
[2] M. Knobel, J.P. Sinnecker, J.F. Saenger, R. Sato Turtelli, J. Magn. Magn. Mater. 133 (1994) 255. [3] M. Knobel, J.P. Sinnecker, J.F. Saenger, R. Sato Turtelli, Philos. Mag. B 68 (1993) 861. [4] A. Makino, T. Bitoh, A. Inoue, T. Masumoto, J. Appl. Phys. 81 (1997) 2736. [5] H. KronmuK ller, J. Magn. Magn. Mater. 41 (1984) 366.
Magnetic softening of nanocrystalline Fe Cu Nb Si B alloy obtained by two-step heat treatment A. B"achowicz *, J. Zbroszczyk , J. Olszewski , W.H. CiurzynH ska , H. Fukunaga, K. Narita, B. Wys"ocki , M. Hasiak Institute of Physics, Technical University of Cze9 stochowa, Al Armii Krajowej 19, 42-200 Cze9 stochowa, Poland Faculty of Engineering, Nagasaki University, Nagasaki, Japan Faculty of Engineering, Kyushu University, Fukuoka 812, Japan
Abstract The magnetic properties of the nanocrystalline Fe Cu Nb Si B alloy annealed at 573, 623, 673 or 700 K for 1 h and then at 823 K (crystallization temperature) for 0.5 h are investigated. It is found that the preannealing of the samples narrows grain size distribution which involves a higher initial magnetic susceptibility. However, the distribution of the grain size does not in#uence the magnetic susceptibility disaccommodation. 2000 Elsevier Science B.V. All rights reserved. Keywords: Nanocrystalline materials; Grain size distribution; MoK ssbauer e!ect
The excellent magnetic properties of nanocrystalline materials are attributed to "ne grains embedded in an amorphous matrix [1]. These grains grow in the regions rich in iron due to the di!usion of Cu, Nb and B atoms outside these regions. It has been found [2,3] that the magnetic properties of the nanocrystalline alloys depend on preparation conditions of the amorphous ribbons. Moreover, it has been shown that the annealing of amorphous ribbons below the crystallization temperature in#uences the microstructure of the nanocrystalline alloys [4]. The aim of this paper is to study the microstructure and magnetic properties, namely magnetic susceptibility and its disaccommodation, of the nanocrystalline Fe Cu Nb Si B alloy obtained by the heat treat ment of the amorphous ribbons subjected earlier to a preannealing below the crystallization temperature. The microstructure of the nanocrystalline samples was investigated using MoK ssbauer spectrometry, X-ray dif-
* Corresponding author. Tel./fax: #48-34-3250-795. E-mail address: [email protected] (A. B"achowicz).
fractometry and transmission electron microscopy. MoK ssbauer spectrometry was used to determine the distribution of the magnetization in the samples, their phase composition and the iron content in the amorphous and crystalline phases. X-ray di!ractometry was applied in order to check the amorphousness of the samples and for the identi"cation of the crystalline phase. From the transmission electron microscopy measurements grain size distribution has been determined. Magnetic susceptibility (s) and its disaccommodation *(1/s)"1/s !1/s (where s and s are the values of the magnetic susceptibility at t "2 s and t "120 s after the demagnetization, respectively) were investigated for toroidal samples of 2 cm inner diameter in the temperature range from 160 up to 600 K. The amplitude and frequency of the magnetizing "eld were equal to 0.16 A/m and 2 kHz, respectively. All investigations were carried out for the samples preannealed at 573, 623, 673 or 700 K for 1 h and then heat treated at 823 K (the crystallization temperature) for 0.5 h. To compare microstructure and magnetic properties of the annealed samples, one sample was subjected to one-step annealing at 823 K for 0.5 h. All samples in the as-quenched state exhibited almost the same initial magnetic susceptibility, equal to about 700.
0304-8853/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 0 ) 0 0 1 7 7 - 3
A. B!achowicz et al. / Journal of Magnetism and Magnetic Materials 215}216 (2000) 422}424
The magnetic susceptibility versus temperature for the Fe Cu Nb Si B alloy subjected to two-step anneal ing (the "rst step below the crystallization temperature at 673 K for 1 h and the second step * at 823 K for 0.5 h) is shown in Fig. 1. It is seen that the nanocrystalline sample obtained by the heat treatment of the amorphous sample, which was preannealed at 673 K for 1 h, exhibits an initial magnetic susceptibility equal to about 9000 (at room temperature). However, the susceptibility of the nanocrystalline sample, obtained by one-step heat treatment (at 823 K for 0.5 h), reaches a lower susceptibility equal to about 7000. A similar behavior was observed for the samples preannealed at 573, 623 and 700 K and then heat treated at 823 K for 0.5 h. The initial magnetic susceptibility for these samples was equal to 7200, 7700 and 8500, respectively. The disaccommodation of the magnetic susceptibility versus temperature is presented in Fig. 2. It is seen that the shape of the isochronal disaccommodation curves does not depend on the preannealing conditions of the samples. One can see in these curves only temperatureindependent background (from about 175 up to 475 K). Near the Curie temperature of the amorphous matrix (550 K) the intensity of the disaccommodation rapidly increases due to the phase transition (Hopkinson maximum) from the ferromagnetic to paramagnetic state [5]. In order to explain the magnetic properties of the nanocrystalline samples obtained by two-step annealing,
423
the investigations of their microstructure have been carried out. From the analysis of the MoK ssbauer spectra for the as-quenched and preannealed samples it has been found that this treatment leads to the increase of the packing density of atoms. The hyper"ne "eld distribution parameters, evaluated from MoK ssbauer spectra analysis, for the nanocrystalline samples are shown in Table 1. It is seen from this table that after the second step of annealing, the phase composition of all investigated samples is very similar. Moreover, the composition of the amorphous matrix is almost the same. Furthermore, the iron content in the crystalline a-FeSi phase is equal to 85.4 at% in all investigated samples. At the same time, from the electron microscope observation, it is found that the distribution of grain sizes in the nanocrystalline samples depends on the heat treatment. The narrowest grain size distribution is observed for that sample which exhibits the highest susceptibility (annealed at 673 K for 1 h and then at 823 K for 0.5 h). Concerning disaccommodation intensity in the nanocrystalline samples, it does not depend on the grain size distribution.
Fig. 2. Isochronal disaccommodation curves for the nanocrystalline Fe Cu Nb Si B alloy obtained by di!erent heat treatments of the amorphous ribbons at: (*) 673 K for 1 h and then 823 K for 0.5 h, (;) 823 K for 0.5 h.
Fig. 1. Initial magnetic susceptibility of the nanocrystalline Fe Cu Nb Si B alloy obtained by di!erent annealing at: (*) 673 K for 1 h and then 823 K for 0.5 h, (;) 823 K for 0.5 h.
Table 1 The average hyper"ne "eld (B ), second line intensity in Zeeman sextets (A ), volume fraction of the crystalline phase (< ), content of iron in the crystalline and amorphous phases (Fe , Fe , respectively), for the nanocrystalline Fe Cu Nb Si B alloy, after di!erent
heat treatments Treatment
B (T)
A
<
573 K/1 h#823 K/0.5 h 623 K/1 h#823 K/0.5 h 673 K/1 h#823 K/0.5 h 700 K/1 h#823 K/0.5 h 823 K/0.5 h
23.64 23.57 23.61 23.67 23.65
3.45 3.63 3.51 3.61 3.65
0.48 0.48 0.48 0.48 0.50
Fe
61.0 61.0 61.0 61.0 62.0
(%)
<
Fe (%)
0.52 0.52 0.52 0.52 0.50
85.4 85.4 85.4 85.4 85.6
424
A. B!achowicz et al. / Journal of Magnetism and Magnetic Materials 215}216 (2000) 422}424
This work was "nancially supported by the Polish Committee for Scienti"c Research (Grant No. 7 T082 022 17). References [1] Y. Yoshizawa, S. Oguma, K. Yamauchi, J. Appl. Phys. 64 (1988) 6044.
[2] M. Knobel, J.P. Sinnecker, J.F. Saenger, R. Sato Turtelli, J. Magn. Magn. Mater. 133 (1994) 255. [3] M. Knobel, J.P. Sinnecker, J.F. Saenger, R. Sato Turtelli, Philos. Mag. B 68 (1993) 861. [4] A. Makino, T. Bitoh, A. Inoue, T. Masumoto, J. Appl. Phys. 81 (1997) 2736. [5] H. KronmuK ller, J. Magn. Magn. Mater. 41 (1984) 366.