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Structural evolution and magnetic properties in Fe70Cr10B20 ribbons
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 294 (2005) e155–e158 www.elsevier.com/locate/jmmm
Structural evolution and magnetic properties in Fe70Cr10B20 ribbons J.D. Santosa,, J. Oliveraa, P. Gorriaa, M.J. Pe´reza, M.L. Sa´ncheza, B. Hernandoa, V.M. Pridaa, A. Ferna´ndez-Martı´ nezb, G. Cuellob a
Departamento de Fı´sica, Universidad de Oviedo, C/Calvo Sotelo s/n, 33007 Oviedo, Spain b Institut Laue-Langevin, BP 156, F-38042 Grenoble Cedex 9, France Available online 21 April 2005
Abstract The evolution of the magnetic properties of Fe70Cr10B20 alloy, initially in amorphous state, is studied during the whole crystallization process, from room temperature to 800 1C. Neutron thermodiffraction has been used in order to follow all the structural changes that take place in the sample, including the appearance of tetragonal phases and polymorphic transformations. The formation of these tetragonal phases, with high magnetic anisotropy, leads to a degradation of the soft magnetic response. The study of the correlation between structure and magnetic response helps to optimize the composition and heat treatments in these materials for their application in sensor devices. r 2005 Elsevier B.V. All rights reserved. PACS: 61.12.Ld; 75.50.Kj; 75.60.Ej; 81.30.Hd Keywords: Amorphous ribbons; Magnetic properties; Crystallization; Neutron diffraction; Coercive force
1. Introduction The increasing interest in the new magnetic materials led to the enhancement of the research activities connected to the processing and characterization of transition metal-metalloid (TM–M) and RE–TM–M (TM ¼ Fe, Ni, Co; RE ¼ Sm, Gd, Nd; M ¼ B, P, Si) amorphous alloys. Corresponding author. Tel.: +34 985 103308;
fax: +34 985 103324. E-mail address: [email protected] (J.D. Santos).
The magnetic and magnetoelastic properties of ferromagnetic TM–M amorphous and nanocrystalline alloys can be varied within large limits in their chemical composition [1]. In fact, the addition of Cr to FeB-based metallic glasses changes the magnetic and mechanical properties of such materials [2,3]. Thermally activated structural relaxation or crystallization processes can be also used to optimize the macroscopic characteristics of the material. In this work, we study the link between the evolution of the magnetic properties and the
0304-8853/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2005.03.074
ARTICLE IN PRESS J.D. Santos et al. / Journal of Magnetism and Magnetic Materials 294 (2005) e155–e158
The Fe70Cr10B20 ribbons, with a cross section of 5 mm 24 mm, were obtained in our laboratory by means of the melt spinning technique in a vacuum chamber. Magnetization vs. temperature curves, M(T), were measured in a Faraday susceptometer under an applied magnetic field of 1 kOe and in the temperature range between 20 and 800 1C. Differential scanning calorymetry (DSC) in the range 20–600 1C has been used in order to follow the crystallization process. Both M(T) and DSC measurements were performed at a heating rate of 10 1C/min. Magnetic polarization, J, and coercive field, Hc, values were estimated from the hysteresis loops, obtained by the conventional induction technique at room temperature. In situ neutron thermodiffraction experiments, in the range 20–800 1C, have been carried out at the D20 high flux diffractometer (ILL, Grenoble, France), between 25 and 125 1 in 2y, and using a lE1.3 A˚. Different thermal treatments, at selected temperatures have been performed in a furnace with argon atmosphere.
3. Results and discussion Two different thermomagnetic curves are shown in Fig. 1. The curve represented with full symbols corresponds to the as-quenched sample. In this curve a clear decrease of the magnetization, M, is observed, with a change of slope at around 120 1C, indicating the Curie temperature, TC, of the amorphous alloy. At higher temperatures, above 450 1C, a rapid increase of M suggests the beginning of the crystallization of some ferromagnetic phase. Further heating leads to an increase in the value of M up to 750 1C, but with a drastic change of slope above 500 1C. For temperatures above 780 1C, M goes to zero, indicating that some amount of bcc–Fe (T C ¼ 770 1C) is formed during the crystallization process. After this heating, the
1st heating 2nd heating
0.7 0.6
Tc (Fe)
0.5 0.4 Tc(Fe30Cr10B20)
0.3 0.2 0.1
Tc(amph)
0 0
100
200
300 400 500 Temperature (˚C)
600
700
800
Fig. 1. Magnetization vs. temperature curves of the asquenched and fully crystallized samples.
Heat Flow (arbitrary units)
2. Experimental
0.8
Magnetization, M (arbitrary units)
structural changes that occur during the crystallization process of Fe70Cr10B20 ribbons, initially in amorphous state.
Heat Flow (arbitrary units)
e156
440
0
450
100
460 470 480 Temperature (C)
490
500
200 300 400 Temperature (C)
500
600
Fig. 2. DSC curve for the studied alloy. A detailed view of the peak is depicted in the inset.
sample was cooled down to 20 1C and a second M(T) curve was measured (open symbols in Fig. 1). In this curve two clear changes of slope are observed, suggesting that during the first heating process two ferromagnetic phases with different values of TC have been formed, as crystallization products of the initially amorphous alloy. In Fig. 2, the DSC curve is presented. A sharp peak above 450 1C confirms the existence of a crystallization process as previously suggested by
ARTICLE IN PRESS J.D. Santos et al. / Journal of Magnetism and Magnetic Materials 294 (2005) e155–e158
the M(T) curve. However, this DSC peak has a small shoulder in the high temperature side (see inset in Fig. 2), indicating a complex crystallization process. Moreover, the heat flow continues increasing above 500 1C (some trend as M in Fig. 1), thus suggesting further transformations in the sample. In order to have a deep insight in the crystallization process of this FeCrB alloy and with the aim to fully understand the observed magnetic changes, an in situ neutron diffraction experiment was performed in the same temperature range. This technique has been previously used to follow the crystallization behavior of other metallic glasses [4] and gives important structural information that helps in understanding magnetic beha-
e157
vior of such materials. In Fig. 3 two diffraction patterns are presented, those corresponding to the sample at 500 1C during the first heating (up) and at room temperature after cooling from 800 1C (down). The upper diffraction pattern corresponds to a temperature just above the change of slope of M (see Fig. 1), and it has been fitted with two different crystalline phases, bcc–Fe (E20%) and a tetragonal phase, 1 4¯ , with composition per unit cell Fe20Cr4B8 (E80%). The other diffraction pattern has also been fitted with two crystalline phases, one of them is again bcc–Fe, but now the percentage is much higher (40%), and the second phase, also with tetragonal structure but different
0.6
as-quenched
0.4 T = 500 °C
0.2
Intensity (arbitrary units)
0 -0.2 -0.4
Intensity (arbitrary units)
T = 20 °C
Magnetic Polarization, Js(T)
-0.6 0.6
heated up to 500°C
0.4 0.2 0 -0.2 -0.4 -0.6 0.6
heated up to 800°C
0.4 0.2 0 -0.2 -0.4 40
60
80 2θ (degrees)
100
120
-0.6 -200
Fig. 3. Neutron diffraction patterns together with their fits of the FeCrB sample. The position of the reflections corresponding to each crystalline phase and the difference between the observed and calculated intensities are also represented.
-100 100 0 Applied Magnetic Field, Happ(Oe)
200
Fig. 4. Room temperature hysteresis loops of the sample in the as-quenched state and after heating up to 500 and 800 1C.
ARTICLE IN PRESS e158
J.D. Santos et al. / Journal of Magnetism and Magnetic Materials 294 (2005) e155–e158
symmetry, I4=mcm, has changed its composition, Fe6Cr2B4 (60%). With these results the evolution of M with temperature can be explained as follows. The crystallization process begins at temperatures above 450 1C and finishes before 500 1C. In this process bcc–Fe and Fe20Cr4B8 are formed, and a sharp increase in the value of M is observed (see Fig. 1) in this temperature range. For temperatures above 500 1C and up to 750 1C, a polymorphic transformation takes place in the sample, the Fe20Cr4B8 phase into Fe6Cr2B4 due to the segregation of some Fe atoms, which is responsible for the continuous increase of the value of M. On the other hand, and if we take a look at the hysteresis loops corresponding to samples submitted to heat treatments (see Fig. 4 for loops obtained at room temperature corresponding to samples heated at 500 and 800 1C compared with the as-quenched one), we observe a great increase in the value of Hc after crystallization, that can be attributed to the formation of ferromagnetic FeCrB crystalline phases but with high values for the magnetic anisotropy due to their tetragonal structure.
4. Conclusions Fe70Cr10B20 metallic glass presents a complex structural and magnetic behavior on heating at
high temperatures. Crystallization of two phases, bcc–Fe and tetragonal Fe20Cr4B8 is followed by a polymorphic transformation of the latter, giving rise to Fe6Cr2B4 and bcc–Fe. The use of neutron thermodiffraction has allowed us to understand the striking thermal evolution of the magnetization and the great increase of coercive field values.
Acknowledgments The authors acknowledge MCYT and FICyT for financial support under projects MCT-03-MAT-06492 and PB02-037, respectively. We also thank ILL for the allocation of neutron beam time. J.O. thanks MCYT for the PhD grant.
References [1] I. Mihalca, A. Ercuta, C. Ionascu, Sensors and Actuators A 106 (2003) 61. [2] J.A. Verduzco, I. Betancourt, F. Saavedra, E. Reynoso, J. Non-Cryst. Solids 329 (2003) 163. [3] U. Gu¨ntzel, K. Westerholt, Phys. Rev. B 41 (1990) 740. [4] L. Ferna´ndez Barquı´ n, J.C. Go´mez Sal, P. Gorria, J.S. Garitaonandia, J.M. Barandiara´n, J. Phys.: Condens. Matter 10 (1998) 5027.
Journal of Magnetism and Magnetic Materials 294 (2005) e155–e158 www.elsevier.com/locate/jmmm
Structural evolution and magnetic properties in Fe70Cr10B20 ribbons J.D. Santosa,, J. Oliveraa, P. Gorriaa, M.J. Pe´reza, M.L. Sa´ncheza, B. Hernandoa, V.M. Pridaa, A. Ferna´ndez-Martı´ nezb, G. Cuellob a
Departamento de Fı´sica, Universidad de Oviedo, C/Calvo Sotelo s/n, 33007 Oviedo, Spain b Institut Laue-Langevin, BP 156, F-38042 Grenoble Cedex 9, France Available online 21 April 2005
Abstract The evolution of the magnetic properties of Fe70Cr10B20 alloy, initially in amorphous state, is studied during the whole crystallization process, from room temperature to 800 1C. Neutron thermodiffraction has been used in order to follow all the structural changes that take place in the sample, including the appearance of tetragonal phases and polymorphic transformations. The formation of these tetragonal phases, with high magnetic anisotropy, leads to a degradation of the soft magnetic response. The study of the correlation between structure and magnetic response helps to optimize the composition and heat treatments in these materials for their application in sensor devices. r 2005 Elsevier B.V. All rights reserved. PACS: 61.12.Ld; 75.50.Kj; 75.60.Ej; 81.30.Hd Keywords: Amorphous ribbons; Magnetic properties; Crystallization; Neutron diffraction; Coercive force
1. Introduction The increasing interest in the new magnetic materials led to the enhancement of the research activities connected to the processing and characterization of transition metal-metalloid (TM–M) and RE–TM–M (TM ¼ Fe, Ni, Co; RE ¼ Sm, Gd, Nd; M ¼ B, P, Si) amorphous alloys. Corresponding author. Tel.: +34 985 103308;
fax: +34 985 103324. E-mail address: [email protected] (J.D. Santos).
The magnetic and magnetoelastic properties of ferromagnetic TM–M amorphous and nanocrystalline alloys can be varied within large limits in their chemical composition [1]. In fact, the addition of Cr to FeB-based metallic glasses changes the magnetic and mechanical properties of such materials [2,3]. Thermally activated structural relaxation or crystallization processes can be also used to optimize the macroscopic characteristics of the material. In this work, we study the link between the evolution of the magnetic properties and the
0304-8853/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2005.03.074
ARTICLE IN PRESS J.D. Santos et al. / Journal of Magnetism and Magnetic Materials 294 (2005) e155–e158
The Fe70Cr10B20 ribbons, with a cross section of 5 mm 24 mm, were obtained in our laboratory by means of the melt spinning technique in a vacuum chamber. Magnetization vs. temperature curves, M(T), were measured in a Faraday susceptometer under an applied magnetic field of 1 kOe and in the temperature range between 20 and 800 1C. Differential scanning calorymetry (DSC) in the range 20–600 1C has been used in order to follow the crystallization process. Both M(T) and DSC measurements were performed at a heating rate of 10 1C/min. Magnetic polarization, J, and coercive field, Hc, values were estimated from the hysteresis loops, obtained by the conventional induction technique at room temperature. In situ neutron thermodiffraction experiments, in the range 20–800 1C, have been carried out at the D20 high flux diffractometer (ILL, Grenoble, France), between 25 and 125 1 in 2y, and using a lE1.3 A˚. Different thermal treatments, at selected temperatures have been performed in a furnace with argon atmosphere.
3. Results and discussion Two different thermomagnetic curves are shown in Fig. 1. The curve represented with full symbols corresponds to the as-quenched sample. In this curve a clear decrease of the magnetization, M, is observed, with a change of slope at around 120 1C, indicating the Curie temperature, TC, of the amorphous alloy. At higher temperatures, above 450 1C, a rapid increase of M suggests the beginning of the crystallization of some ferromagnetic phase. Further heating leads to an increase in the value of M up to 750 1C, but with a drastic change of slope above 500 1C. For temperatures above 780 1C, M goes to zero, indicating that some amount of bcc–Fe (T C ¼ 770 1C) is formed during the crystallization process. After this heating, the
1st heating 2nd heating
0.7 0.6
Tc (Fe)
0.5 0.4 Tc(Fe30Cr10B20)
0.3 0.2 0.1
Tc(amph)
0 0
100
200
300 400 500 Temperature (˚C)
600
700
800
Fig. 1. Magnetization vs. temperature curves of the asquenched and fully crystallized samples.
Heat Flow (arbitrary units)
2. Experimental
0.8
Magnetization, M (arbitrary units)
structural changes that occur during the crystallization process of Fe70Cr10B20 ribbons, initially in amorphous state.
Heat Flow (arbitrary units)
e156
440
0
450
100
460 470 480 Temperature (C)
490
500
200 300 400 Temperature (C)
500
600
Fig. 2. DSC curve for the studied alloy. A detailed view of the peak is depicted in the inset.
sample was cooled down to 20 1C and a second M(T) curve was measured (open symbols in Fig. 1). In this curve two clear changes of slope are observed, suggesting that during the first heating process two ferromagnetic phases with different values of TC have been formed, as crystallization products of the initially amorphous alloy. In Fig. 2, the DSC curve is presented. A sharp peak above 450 1C confirms the existence of a crystallization process as previously suggested by
ARTICLE IN PRESS J.D. Santos et al. / Journal of Magnetism and Magnetic Materials 294 (2005) e155–e158
the M(T) curve. However, this DSC peak has a small shoulder in the high temperature side (see inset in Fig. 2), indicating a complex crystallization process. Moreover, the heat flow continues increasing above 500 1C (some trend as M in Fig. 1), thus suggesting further transformations in the sample. In order to have a deep insight in the crystallization process of this FeCrB alloy and with the aim to fully understand the observed magnetic changes, an in situ neutron diffraction experiment was performed in the same temperature range. This technique has been previously used to follow the crystallization behavior of other metallic glasses [4] and gives important structural information that helps in understanding magnetic beha-
e157
vior of such materials. In Fig. 3 two diffraction patterns are presented, those corresponding to the sample at 500 1C during the first heating (up) and at room temperature after cooling from 800 1C (down). The upper diffraction pattern corresponds to a temperature just above the change of slope of M (see Fig. 1), and it has been fitted with two different crystalline phases, bcc–Fe (E20%) and a tetragonal phase, 1 4¯ , with composition per unit cell Fe20Cr4B8 (E80%). The other diffraction pattern has also been fitted with two crystalline phases, one of them is again bcc–Fe, but now the percentage is much higher (40%), and the second phase, also with tetragonal structure but different
0.6
as-quenched
0.4 T = 500 °C
0.2
Intensity (arbitrary units)
0 -0.2 -0.4
Intensity (arbitrary units)
T = 20 °C
Magnetic Polarization, Js(T)
-0.6 0.6
heated up to 500°C
0.4 0.2 0 -0.2 -0.4 -0.6 0.6
heated up to 800°C
0.4 0.2 0 -0.2 -0.4 40
60
80 2θ (degrees)
100
120
-0.6 -200
Fig. 3. Neutron diffraction patterns together with their fits of the FeCrB sample. The position of the reflections corresponding to each crystalline phase and the difference between the observed and calculated intensities are also represented.
-100 100 0 Applied Magnetic Field, Happ(Oe)
200
Fig. 4. Room temperature hysteresis loops of the sample in the as-quenched state and after heating up to 500 and 800 1C.
ARTICLE IN PRESS e158
J.D. Santos et al. / Journal of Magnetism and Magnetic Materials 294 (2005) e155–e158
symmetry, I4=mcm, has changed its composition, Fe6Cr2B4 (60%). With these results the evolution of M with temperature can be explained as follows. The crystallization process begins at temperatures above 450 1C and finishes before 500 1C. In this process bcc–Fe and Fe20Cr4B8 are formed, and a sharp increase in the value of M is observed (see Fig. 1) in this temperature range. For temperatures above 500 1C and up to 750 1C, a polymorphic transformation takes place in the sample, the Fe20Cr4B8 phase into Fe6Cr2B4 due to the segregation of some Fe atoms, which is responsible for the continuous increase of the value of M. On the other hand, and if we take a look at the hysteresis loops corresponding to samples submitted to heat treatments (see Fig. 4 for loops obtained at room temperature corresponding to samples heated at 500 and 800 1C compared with the as-quenched one), we observe a great increase in the value of Hc after crystallization, that can be attributed to the formation of ferromagnetic FeCrB crystalline phases but with high values for the magnetic anisotropy due to their tetragonal structure.
4. Conclusions Fe70Cr10B20 metallic glass presents a complex structural and magnetic behavior on heating at
high temperatures. Crystallization of two phases, bcc–Fe and tetragonal Fe20Cr4B8 is followed by a polymorphic transformation of the latter, giving rise to Fe6Cr2B4 and bcc–Fe. The use of neutron thermodiffraction has allowed us to understand the striking thermal evolution of the magnetization and the great increase of coercive field values.
Acknowledgments The authors acknowledge MCYT and FICyT for financial support under projects MCT-03-MAT-06492 and PB02-037, respectively. We also thank ILL for the allocation of neutron beam time. J.O. thanks MCYT for the PhD grant.
References [1] I. Mihalca, A. Ercuta, C. Ionascu, Sensors and Actuators A 106 (2003) 61. [2] J.A. Verduzco, I. Betancourt, F. Saavedra, E. Reynoso, J. Non-Cryst. Solids 329 (2003) 163. [3] U. Gu¨ntzel, K. Westerholt, Phys. Rev. B 41 (1990) 740. [4] L. Ferna´ndez Barquı´ n, J.C. Go´mez Sal, P. Gorria, J.S. Garitaonandia, J.M. Barandiara´n, J. Phys.: Condens. Matter 10 (1998) 5027.