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Microstructural and magnetic investigation of partially crystallized amorphous ribbons
Journal of Magnetism and Magnetic Materials 157/158 (1996) 217-219
ELSEVIER
~
journal of
magnetism and magnetic materials
Microstructural and magnetic investigation of partially crystallized amorphous ribbons V. Basso a,*, G. Bertotti
p. Duhaj b, E. Ferrara a V. Haslar c L. Kraus c J. P o k o r n y c, K. Zaveta c
a
a Istituto Elettrotecnico Nazionale Galileo Ferraris and INFM, C.so M. d'Azeglio 42, 1-10125, Torino, Italy b Slovak Academy of Sciences, Institute of Physics, Dubravska Cesta 8, 824 28 Brat&lava, Slovakia ° Academy of Sciences of the Czech Republic, Institute of Physics, Na Slovance 2, 180 40 Prague 8, Czech Republic
Abstract We investigated microstructural and magnetic properties of partially crystallized Fe64Co21B15 amorphous ribbons, prepared by different heat treatments. We found a deep correlation between microstructure (cr-Fe nanograins ranging from 40 to 70 nm) and hysteresis properties. With the help of Preisach modelling we were able to identify, from the presence of two separated local coercive field peaks in the reconstructed Preisach distribution, two separate magnetization mechanisms associated with the amorphous and the crystalline phase. Both local coercive field peaks rescale according to a function of the mean grain size only. This result can be interpreted as a consequence of domain wall pinning by the nanograins. Keywords: Preisach model; Partial crystallization; Amorphous alloys
Amorphous and nanocrystaUine materials exhibit excellent magnetic properties because of their low anisotropy. In amorphous samples magnetocrystalline anisotropy is absent, but shape anisotropy and local magnetoelastic anisotropies due to quenched-in stresses play a fundamental role. In nanostructured magnetic materials (Finemet) the crystalline anisotropy is averaged out because domain wall (DW) width extends over many nanograins [1]. On the other hand the growth of a crystalline phase dispersed in an amorphous matrix is found to increase the coercivity of the material [2]. The investigation of the hysteresis properties could be a useful tool to understand the magnetization mechanism and to get structural information directly from magnetic measurements. We induced partial crystallization in Fe64Co2~B15 amorphous ribbons and studied the ensuing modification in the hysteresis properties by reconstructing the Preisach distribution [3] from measured minor loops. We found a second peak, at higher coercivity, in the Preisach distribution in all the partially crystallized samples, that was completely absent in the amorphous one. This unambiguously marks the presence of two different magnetic phases, which to some extent recalls the results on two phase hard magnetic materials
* Corresponding author. Fax: basso @omega.ien.it.
+39-11-650-7611;
e-mail:
where a magnetically hard phase is dispersed in a soft one [4]. The coercive field H c is found to be very sensitive to the mean grain dimension s and increases as H ccx s 4. This fact suggests that small grains of the order of DW width affect in a complicated way the energy landscape where DW motion takes place. An explanation of the dependence of coercivity on grain dimension is given below. Partially crystallized ribbons were prepared by pre-annealing Fe64Co21B15 samples at 350°C for a variable time up to 80 minutes. A subsequent annealing under longitudinally applied stress (300°C/4 h / 5 0 0 MPa) was performed in order to induce easy-ribbon-axis anisotropy and well defined domain structure. The samples microstructure was studied by X-ray diffraction and transmission electron microscopy. Crystals of a-Fe, with mean size s ranging from 40 to 70 nm, depending on pre-annealing time, were observed. Correspondingly, the crystal fraction p was found to increase from 3% to 16%. From the relation p = ( s / L ) 3 we found in all the samples the same mean grain distance L ~ 130 nm. The magnetic properties were studied by domain observation, anisotropy constant measurement and quasistatic hysteresis measurements (minor loops and first-order return branches). Domain pictures show an increasing complexity with the crystalline fraction. The fully amorphous sample shows a simple longitudinal stripe domain structure, while in partially crystallized samples small sec-
0304-8853/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. SSD1 0304-8853(95)01018-1
V. Basso et al. / Journal of Magnetism and Magnetic Materials 157/158 (1996) 217-219
218
1.5 4
1
Hc*P(hc/Hc)
0.5
G rn
3
o -0.5 -1
-1.5 -80-60-40-20
0
20
40
60
80
time (minutes)
H (A/m) ,~ Fig. 1. Measured minor loops on stress annealed Fe64Co21B15 with 30 minutes pre-annealing (7.6% a-Fe). ondal3" domains appear. The anisotropy constant Ku was measured by means of transverse susceptibility method and in all the samples was found to be K u ~ 1.5 × 103 J / m 3, as a consequence of the stress annealing. The dependence of hysteresis properties on the amount of partial crystallization was investigated by Preisach modelling. The Preisach distribution of each sample was reconstructed, by the method discussed in [5], from a set of 20 minor loops, measured on single ribbon strips (length 30 cm, width 1 cm, thickness 25 tzm). A typical example of corresponding minor loops and Preisach distribution p(ce,/3) is given in Fig. 1 and Fig. 2. The presence of a two-peak structure is evident. The peak at higher coercivities unambiguously marks the presence of the induced crystal phase. Its evolution versus pre-annealing time can be illustrated by rewriting p(a,/3) as a function of the elementary loop coercive field h c = ( a - / 3 ) / 2 and interaction field hu = ( a + / 3 ) / 2 , and by considering the integrated coercive field distribution P(h c) = f p(hc,hu)dh u. From P ( h c) we get the two local coercive fields hol
0
Fig. 3. Local coercive field distributions P(h c / H c) normalized to the measured coercive field He: no pre-annealing (0 minutes) Ho = 8.5 A / m ; 10 minutes, Ho = 11.9 A / m ; 30 minutes, Hc = 30.3 A / m ; 45 minutes, H e = 57.6 A/m; 60 minutes, H c = 75.9 A / m ; 80 minutes, Ho = 129.2 A/re. (lower one) and ho2 (higher one). Fig. 3 shows P(ho) for various annealing conditions, rescaled according to the coercive field H e of each material. H c ranged from 8 A / m (no pre-annealing) to 130 A / m (80 minutes pre-annealing). The presence of a-Fe nanograins can affect the moving D W energy landscape because of their high anisotropy. W h e n the grain dimension is sufficiently small and the crystal fraction approaches unity the effective anisotropy is some orders of magnitude lower than that of c~-Fe and has a s 6 dependence [1]. In our case the average anisotropy and the D W width are mainly controlled by the amorphous matrix (97% • -. 84%), and nanocrystals contribute to local D W energy fluctuations. Following this approach, the D W width turns out to be of the order of ~ = ( A / K u ) t/2 100 nm, where A is the exchange constant (10 -~1 J / m )
Preisach distribution
0.0006 0.0005 0.0004 0.0003 0.0002 0.0001
.
0
100 50
100 (A/)
-
100
-I00
Fig. 2. Reconstructed Preisach distribution p(a,/3) from minor loops of Fig. 1.
V. Basso et al. / Journal of Magnetism and Magnetic Materials 1 5 7 / 1 5 8 (1996) 217-219 and the D W energy E w ~ 6 K u. W e can estimate the coercive field behaviour as H c ~ A E w / A x ~ 6ZX K u / A x ~ K F e ( s / L ) 3, if we assume that the fluctuation A K u is proportional to the anisotropy energy of the a-Fe crystalline fraction and the fluctuation length h x of the order of the D W width ~ itself. The local coercivity he1 and hc2 and the measured coercive field H c follow a higher power of the mean grain dimension ( ~ s 4 instead of s3). This could be due to the fact that 6 is close to the mean grain distance L. In this frame the second local coercivity seems to be related to the secondary domain structure, which possibly selects the magnetically harder zones. The complexity of the magnetization process did not permit us to directly relate in a quantitative way the magnetic properties to the microstructural one. The correlation between the second peak contribution to magnetization and the amount of the crystalline fraction will be investigated in a m o r e
219
detailed way under various conditions in a subsequent article. Acknowledgements': This work was partially supported by the Commission of the European Communities, P E C O / C O P E R N I C U S Program, contract CIPA-CT930239 and by NATO Project HTECH.LG 93157.
References [1] G. Herzer, IEEE Trans. Magn. 26 (1990) 1397. [2] J.C. Swartz, J.J. Hangh, R.F. Krause and R. Kossowsky, J. Appl. Phys. 52 (1981) 1908. [3] I.D. Mayergoyz, Matematical Models of Hysteresis (Springer, New York, 1991). [4] E.F. Kneller and R. Hawig, IEEE Trans. Magn. 27 (1991) 3588. [5] V. Basso, G. Bertotti, A. Infortuna and M. Pasquale, IEEE Trans. Magn. 31 (1995).
ELSEVIER
~
journal of
magnetism and magnetic materials
Microstructural and magnetic investigation of partially crystallized amorphous ribbons V. Basso a,*, G. Bertotti
p. Duhaj b, E. Ferrara a V. Haslar c L. Kraus c J. P o k o r n y c, K. Zaveta c
a
a Istituto Elettrotecnico Nazionale Galileo Ferraris and INFM, C.so M. d'Azeglio 42, 1-10125, Torino, Italy b Slovak Academy of Sciences, Institute of Physics, Dubravska Cesta 8, 824 28 Brat&lava, Slovakia ° Academy of Sciences of the Czech Republic, Institute of Physics, Na Slovance 2, 180 40 Prague 8, Czech Republic
Abstract We investigated microstructural and magnetic properties of partially crystallized Fe64Co21B15 amorphous ribbons, prepared by different heat treatments. We found a deep correlation between microstructure (cr-Fe nanograins ranging from 40 to 70 nm) and hysteresis properties. With the help of Preisach modelling we were able to identify, from the presence of two separated local coercive field peaks in the reconstructed Preisach distribution, two separate magnetization mechanisms associated with the amorphous and the crystalline phase. Both local coercive field peaks rescale according to a function of the mean grain size only. This result can be interpreted as a consequence of domain wall pinning by the nanograins. Keywords: Preisach model; Partial crystallization; Amorphous alloys
Amorphous and nanocrystaUine materials exhibit excellent magnetic properties because of their low anisotropy. In amorphous samples magnetocrystalline anisotropy is absent, but shape anisotropy and local magnetoelastic anisotropies due to quenched-in stresses play a fundamental role. In nanostructured magnetic materials (Finemet) the crystalline anisotropy is averaged out because domain wall (DW) width extends over many nanograins [1]. On the other hand the growth of a crystalline phase dispersed in an amorphous matrix is found to increase the coercivity of the material [2]. The investigation of the hysteresis properties could be a useful tool to understand the magnetization mechanism and to get structural information directly from magnetic measurements. We induced partial crystallization in Fe64Co2~B15 amorphous ribbons and studied the ensuing modification in the hysteresis properties by reconstructing the Preisach distribution [3] from measured minor loops. We found a second peak, at higher coercivity, in the Preisach distribution in all the partially crystallized samples, that was completely absent in the amorphous one. This unambiguously marks the presence of two different magnetic phases, which to some extent recalls the results on two phase hard magnetic materials
* Corresponding author. Fax: basso @omega.ien.it.
+39-11-650-7611;
e-mail:
where a magnetically hard phase is dispersed in a soft one [4]. The coercive field H c is found to be very sensitive to the mean grain dimension s and increases as H ccx s 4. This fact suggests that small grains of the order of DW width affect in a complicated way the energy landscape where DW motion takes place. An explanation of the dependence of coercivity on grain dimension is given below. Partially crystallized ribbons were prepared by pre-annealing Fe64Co21B15 samples at 350°C for a variable time up to 80 minutes. A subsequent annealing under longitudinally applied stress (300°C/4 h / 5 0 0 MPa) was performed in order to induce easy-ribbon-axis anisotropy and well defined domain structure. The samples microstructure was studied by X-ray diffraction and transmission electron microscopy. Crystals of a-Fe, with mean size s ranging from 40 to 70 nm, depending on pre-annealing time, were observed. Correspondingly, the crystal fraction p was found to increase from 3% to 16%. From the relation p = ( s / L ) 3 we found in all the samples the same mean grain distance L ~ 130 nm. The magnetic properties were studied by domain observation, anisotropy constant measurement and quasistatic hysteresis measurements (minor loops and first-order return branches). Domain pictures show an increasing complexity with the crystalline fraction. The fully amorphous sample shows a simple longitudinal stripe domain structure, while in partially crystallized samples small sec-
0304-8853/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. SSD1 0304-8853(95)01018-1
V. Basso et al. / Journal of Magnetism and Magnetic Materials 157/158 (1996) 217-219
218
1.5 4
1
Hc*P(hc/Hc)
0.5
G rn
3
o -0.5 -1
-1.5 -80-60-40-20
0
20
40
60
80
time (minutes)
H (A/m) ,~ Fig. 1. Measured minor loops on stress annealed Fe64Co21B15 with 30 minutes pre-annealing (7.6% a-Fe). ondal3" domains appear. The anisotropy constant Ku was measured by means of transverse susceptibility method and in all the samples was found to be K u ~ 1.5 × 103 J / m 3, as a consequence of the stress annealing. The dependence of hysteresis properties on the amount of partial crystallization was investigated by Preisach modelling. The Preisach distribution of each sample was reconstructed, by the method discussed in [5], from a set of 20 minor loops, measured on single ribbon strips (length 30 cm, width 1 cm, thickness 25 tzm). A typical example of corresponding minor loops and Preisach distribution p(ce,/3) is given in Fig. 1 and Fig. 2. The presence of a two-peak structure is evident. The peak at higher coercivities unambiguously marks the presence of the induced crystal phase. Its evolution versus pre-annealing time can be illustrated by rewriting p(a,/3) as a function of the elementary loop coercive field h c = ( a - / 3 ) / 2 and interaction field hu = ( a + / 3 ) / 2 , and by considering the integrated coercive field distribution P(h c) = f p(hc,hu)dh u. From P ( h c) we get the two local coercive fields hol
0
Fig. 3. Local coercive field distributions P(h c / H c) normalized to the measured coercive field He: no pre-annealing (0 minutes) Ho = 8.5 A / m ; 10 minutes, Ho = 11.9 A / m ; 30 minutes, Hc = 30.3 A / m ; 45 minutes, H e = 57.6 A/m; 60 minutes, H c = 75.9 A / m ; 80 minutes, Ho = 129.2 A/re. (lower one) and ho2 (higher one). Fig. 3 shows P(ho) for various annealing conditions, rescaled according to the coercive field H e of each material. H c ranged from 8 A / m (no pre-annealing) to 130 A / m (80 minutes pre-annealing). The presence of a-Fe nanograins can affect the moving D W energy landscape because of their high anisotropy. W h e n the grain dimension is sufficiently small and the crystal fraction approaches unity the effective anisotropy is some orders of magnitude lower than that of c~-Fe and has a s 6 dependence [1]. In our case the average anisotropy and the D W width are mainly controlled by the amorphous matrix (97% • -. 84%), and nanocrystals contribute to local D W energy fluctuations. Following this approach, the D W width turns out to be of the order of ~ = ( A / K u ) t/2 100 nm, where A is the exchange constant (10 -~1 J / m )
Preisach distribution
0.0006 0.0005 0.0004 0.0003 0.0002 0.0001
.
0
100 50
100 (A/)
-
100
-I00
Fig. 2. Reconstructed Preisach distribution p(a,/3) from minor loops of Fig. 1.
V. Basso et al. / Journal of Magnetism and Magnetic Materials 1 5 7 / 1 5 8 (1996) 217-219 and the D W energy E w ~ 6 K u. W e can estimate the coercive field behaviour as H c ~ A E w / A x ~ 6ZX K u / A x ~ K F e ( s / L ) 3, if we assume that the fluctuation A K u is proportional to the anisotropy energy of the a-Fe crystalline fraction and the fluctuation length h x of the order of the D W width ~ itself. The local coercivity he1 and hc2 and the measured coercive field H c follow a higher power of the mean grain dimension ( ~ s 4 instead of s3). This could be due to the fact that 6 is close to the mean grain distance L. In this frame the second local coercivity seems to be related to the secondary domain structure, which possibly selects the magnetically harder zones. The complexity of the magnetization process did not permit us to directly relate in a quantitative way the magnetic properties to the microstructural one. The correlation between the second peak contribution to magnetization and the amount of the crystalline fraction will be investigated in a m o r e
219
detailed way under various conditions in a subsequent article. Acknowledgements': This work was partially supported by the Commission of the European Communities, P E C O / C O P E R N I C U S Program, contract CIPA-CT930239 and by NATO Project HTECH.LG 93157.
References [1] G. Herzer, IEEE Trans. Magn. 26 (1990) 1397. [2] J.C. Swartz, J.J. Hangh, R.F. Krause and R. Kossowsky, J. Appl. Phys. 52 (1981) 1908. [3] I.D. Mayergoyz, Matematical Models of Hysteresis (Springer, New York, 1991). [4] E.F. Kneller and R. Hawig, IEEE Trans. Magn. 27 (1991) 3588. [5] V. Basso, G. Bertotti, A. Infortuna and M. Pasquale, IEEE Trans. Magn. 31 (1995).