- Email: [email protected]
Correlation between domain structure and permeability under stress in metallic glasses with induced anisotropy
Materials Science and Engineering, A133 ( 1991 ) 136-139
136
Correlation between domain structure and permeability under stress in metallic glasses with induced anisotropy A. Siemko, J. Fink-Finowicki, A. ~lawska-Waniewska and M. Ku~mifiski Institute of Physics, Polish Academy of Sciences, AI. Lotnikrw 32/46, 02-668 Warsaw (Poland)
Abstract Experiments were performed on C067Fe4Cr7SisB14 metallic glass in which the transverse magnetic anisotropy was induced by a proper annealing process. Measurements of the magnetic permeability/~ at high frequency of 20 and 200 kHz were performed versus tensile stress applied to the sample. It was found that for the low-stress region the value of the initial permeability increases with the stress but at higher stress, depending on the induced anisotropy, the behavior of # can be significantly modified. The results obtained are interpreted as an influence of the stress-dependent modification of the domain pattern, as shown by the observation of the Kerr effect domain structure.
1. Introduction Many of the magnetic properties of metallic glasses are influenced by the external stress applied to the ribbon [1-3]. In particular, it is interesting to observe how the domain structure is modified by the applied stress [4-7]. Such an observation can be very useful for interpretation of the stress dependence of the other magnetic parameters. In metallic glasses, because of the lack of crystalline anisotropy, the domain pattern depends on [8]: (1) the shape anisotropy due to the stray field of the specimen, which prefers longitudinal alignment of the stripe domains, (2) the magnetoelastic energy due to (a) internal or (b) external stress, (3) the structural anisotropy related to the atomic pair ordering. Domain pattern observed in as-cast amorphous ribbons is usually very irregular due to the distribution of local magnetic anisotropies. After annealing, the domain structure becomes more regular because of the stress relief [reduction of the term (2a)]. Depending on the direction of the magnetic anisotropy induced by annealing [term (3)], the stripe domains can be oriented longitudinally or transversely with respect to the ribbon axis. This domain orientation may be further modified by applying to the sample an external 0921-5093/91/$3.50
tensile stress [term (2b)]. The magnetoelastic energy can be written as follows: E
-
3
(1) 1,2
where 0"1, 0"2 are the in-plane principal stress components and ~1, ~2 denotes the direction cosines with respect to the ribbon's axis. In the case of a uniaxial applied stress, minimization of Eme with respect to 7i leads to the inplane easy direction of magnetization which is parallel to the tensile or to the compressive stress directions for positive and negative magnetostriction, respectively. In an alternating magnetic field the response of the sample (proportional to the effective magnetic permeability p) depends on the existing domain structure. The aim of this work is to find the correlation between the domain structure and the high-frequency initial permeability of a metallic glass under tensile stress.
2. Experimental and results In the present paper a metallic glass of the nominal composition of Co67Fe4Cr7Si8B14 has been investigated. The sample was annealed under a tensile stress of 130 MPa for 1 h at a temperature of 620 K (above the Curie temperature © Elsevier Sequoia/Printed in The Netherlands
137
Tc=460 K). Such a treatment creates in this material a magnetic anisotropy with the easy direction transverse to the ribbon's axis. The magnetostriction constant of the investigated sample is positive and after annealing is equal to ;L~= 4.6 x 10 -7. The influence of an external uniaxial tensile stress on the domain structure has been investigated by means of the magnetooptical Kerr effect applying a computer-controlled set-up. Figure 1 shows the effect of increasing the external stress on the domain pattern. In the unstressed state (see Fig. la) a regular stripe-like domain pattern is observed. Such a type of structure appears for stresses within the range 0
(a)
Fig. 1. Domain patterns of stress-annealed Co67Fe4Cr75i8 B14 alloy under stress (a) 0, (b) 170 MPa, (c) 190 MPa, (d) 210 MPa (both, the stress and sample axes are vertical).
138
The domain pattern in the unstressed state (Fig. la) is in the form of almost pure transverse stripes. Domain walls in such structures are more stable in a.c. magnetic field and the magnetizing process occurs mainly by rotation of the magnetic moments inside the domains. Therefore, the total losses are much smaller than for the longitudinal structure and the value of/z is higher. If one compares the two curves in Fig. 2 obtained for two different frequencies it is easy to see that the difference between them is significant only for the high-stress region, confirming additionally that the magnetizing process in this case is dominated by the domain wall movement in contrast to its rotational character in the unstressed and lowstress regions. The proposed explanation is also consistent with experiment on the stress-dependent permeability aftereffect. For this purpose the timedependent induction B(t) was measured and the disaccommodation, defined as: D = [B( tl )-B( t2)]/
'4 ~.,,,..,,. 2~kHz
Fig. 2. Stress dependence of the initial magnetic permeability.
0
B(t2) = A/~//z was calculated. The results obtained are shown in Fig. 3, where the disaccommodation D versus the amplitude Hex of the exciting field for various tensile stresses is plotted. As one can see, for the low-stress region the magnetic aftereffect is negligibly small, because the permeability aftereffect is strictly connected with the time-dependent influence of structural defects on the domain wall motion during the magnetizing process. Therefore, this effect should not appear for transverse domain structure, due to the rotational character of the magnetizing process. The magnetic aftereffect starts to be observed for stresses higher than 170 MPa. This critical value of stress, as it was already shown for permeability measurements, corresponds to the domain structure rearrangement caused by magnetoelastic interactions. 3. Conclusions
In the present paper it was shown that the stress dependence of the initial magnetic permeability as well as the disaccommodation and the dominant type of domain structure are directly correlated. This correlation also allows one to distinguish the stress values for which the domain pattern changes qualitatively. The sensitivity of the magnetic permeability to external stress in the positive-magnetostrictive metallic glasses with induced transverse anisotropy can be utilized in construction of two types of transducers, namely: (1) the stress transducer--applying a slow increase of permeability in the low-stress region. (2) the threshold detector--applying a sharp permeability decrease caused by a change of domain configuration. It is worth noticing that the value of the induced anisotropy as well as the magnetostriction constant in metallic glasses can be changed over a wide range, so the parameters of the transducers proposed can be well controlled.
References
1
3
s
7
He~ [mOe]
Fig. 3. Disaccommodation vs. amplitude of exciting magnetic field for various tensile stresses.
1 M. Vazquez, W. Fernengel and H. Kronmiiller, Phys. Stat. Sol. (a), 80(1983) 195. 2 J. Fink-Finowicki, G. Konczos, J. Krzywiriski, A. Siemko, T. Tarnoczi and Z. Vertesy, Acta Phys. Pol. A, 76 (1989) 163. 3 H. K. Lachowicz and A. Siemko, IEEE Trans. Magn., MAG-25 (1989) 3605.
139 4 X. Z. Dong and H. Kronm/iller, Phys. Stat. Sol. (a), 70 (1982) 451. 5 M. August, Phys. Stat. Sol. (a), 103(1987) 231, 6 A. Siemko, H. K. Lachowicz, N. Moser, A. Forkl and H. Kronm/iller, J. Magn. Magn. Mater., 83 (1990) 171.
7 A. Veider, G. Badurek, R. Gr6ssinger and H. Kronm/iiler, J. Magn. Magn. Mater., 60(1986) 182. 8 J. Fink-Finowickiand B. Lisowski, in H. Matyja and E G. Zielifiski (eds.), Amorphous Metals, World Scientific Publishing Co. Pte. Ltd., 1986, p. 263.
136
Correlation between domain structure and permeability under stress in metallic glasses with induced anisotropy A. Siemko, J. Fink-Finowicki, A. ~lawska-Waniewska and M. Ku~mifiski Institute of Physics, Polish Academy of Sciences, AI. Lotnikrw 32/46, 02-668 Warsaw (Poland)
Abstract Experiments were performed on C067Fe4Cr7SisB14 metallic glass in which the transverse magnetic anisotropy was induced by a proper annealing process. Measurements of the magnetic permeability/~ at high frequency of 20 and 200 kHz were performed versus tensile stress applied to the sample. It was found that for the low-stress region the value of the initial permeability increases with the stress but at higher stress, depending on the induced anisotropy, the behavior of # can be significantly modified. The results obtained are interpreted as an influence of the stress-dependent modification of the domain pattern, as shown by the observation of the Kerr effect domain structure.
1. Introduction Many of the magnetic properties of metallic glasses are influenced by the external stress applied to the ribbon [1-3]. In particular, it is interesting to observe how the domain structure is modified by the applied stress [4-7]. Such an observation can be very useful for interpretation of the stress dependence of the other magnetic parameters. In metallic glasses, because of the lack of crystalline anisotropy, the domain pattern depends on [8]: (1) the shape anisotropy due to the stray field of the specimen, which prefers longitudinal alignment of the stripe domains, (2) the magnetoelastic energy due to (a) internal or (b) external stress, (3) the structural anisotropy related to the atomic pair ordering. Domain pattern observed in as-cast amorphous ribbons is usually very irregular due to the distribution of local magnetic anisotropies. After annealing, the domain structure becomes more regular because of the stress relief [reduction of the term (2a)]. Depending on the direction of the magnetic anisotropy induced by annealing [term (3)], the stripe domains can be oriented longitudinally or transversely with respect to the ribbon axis. This domain orientation may be further modified by applying to the sample an external 0921-5093/91/$3.50
tensile stress [term (2b)]. The magnetoelastic energy can be written as follows: E
-
3
(1) 1,2
where 0"1, 0"2 are the in-plane principal stress components and ~1, ~2 denotes the direction cosines with respect to the ribbon's axis. In the case of a uniaxial applied stress, minimization of Eme with respect to 7i leads to the inplane easy direction of magnetization which is parallel to the tensile or to the compressive stress directions for positive and negative magnetostriction, respectively. In an alternating magnetic field the response of the sample (proportional to the effective magnetic permeability p) depends on the existing domain structure. The aim of this work is to find the correlation between the domain structure and the high-frequency initial permeability of a metallic glass under tensile stress.
2. Experimental and results In the present paper a metallic glass of the nominal composition of Co67Fe4Cr7Si8B14 has been investigated. The sample was annealed under a tensile stress of 130 MPa for 1 h at a temperature of 620 K (above the Curie temperature © Elsevier Sequoia/Printed in The Netherlands
137
Tc=460 K). Such a treatment creates in this material a magnetic anisotropy with the easy direction transverse to the ribbon's axis. The magnetostriction constant of the investigated sample is positive and after annealing is equal to ;L~= 4.6 x 10 -7. The influence of an external uniaxial tensile stress on the domain structure has been investigated by means of the magnetooptical Kerr effect applying a computer-controlled set-up. Figure 1 shows the effect of increasing the external stress on the domain pattern. In the unstressed state (see Fig. la) a regular stripe-like domain pattern is observed. Such a type of structure appears for stresses within the range 0
(a)
Fig. 1. Domain patterns of stress-annealed Co67Fe4Cr75i8 B14 alloy under stress (a) 0, (b) 170 MPa, (c) 190 MPa, (d) 210 MPa (both, the stress and sample axes are vertical).
138
The domain pattern in the unstressed state (Fig. la) is in the form of almost pure transverse stripes. Domain walls in such structures are more stable in a.c. magnetic field and the magnetizing process occurs mainly by rotation of the magnetic moments inside the domains. Therefore, the total losses are much smaller than for the longitudinal structure and the value of/z is higher. If one compares the two curves in Fig. 2 obtained for two different frequencies it is easy to see that the difference between them is significant only for the high-stress region, confirming additionally that the magnetizing process in this case is dominated by the domain wall movement in contrast to its rotational character in the unstressed and lowstress regions. The proposed explanation is also consistent with experiment on the stress-dependent permeability aftereffect. For this purpose the timedependent induction B(t) was measured and the disaccommodation, defined as: D = [B( tl )-B( t2)]/
'4 ~.,,,..,,. 2~kHz
Fig. 2. Stress dependence of the initial magnetic permeability.
0
B(t2) = A/~//z was calculated. The results obtained are shown in Fig. 3, where the disaccommodation D versus the amplitude Hex of the exciting field for various tensile stresses is plotted. As one can see, for the low-stress region the magnetic aftereffect is negligibly small, because the permeability aftereffect is strictly connected with the time-dependent influence of structural defects on the domain wall motion during the magnetizing process. Therefore, this effect should not appear for transverse domain structure, due to the rotational character of the magnetizing process. The magnetic aftereffect starts to be observed for stresses higher than 170 MPa. This critical value of stress, as it was already shown for permeability measurements, corresponds to the domain structure rearrangement caused by magnetoelastic interactions. 3. Conclusions
In the present paper it was shown that the stress dependence of the initial magnetic permeability as well as the disaccommodation and the dominant type of domain structure are directly correlated. This correlation also allows one to distinguish the stress values for which the domain pattern changes qualitatively. The sensitivity of the magnetic permeability to external stress in the positive-magnetostrictive metallic glasses with induced transverse anisotropy can be utilized in construction of two types of transducers, namely: (1) the stress transducer--applying a slow increase of permeability in the low-stress region. (2) the threshold detector--applying a sharp permeability decrease caused by a change of domain configuration. It is worth noticing that the value of the induced anisotropy as well as the magnetostriction constant in metallic glasses can be changed over a wide range, so the parameters of the transducers proposed can be well controlled.
References
1
3
s
7
He~ [mOe]
Fig. 3. Disaccommodation vs. amplitude of exciting magnetic field for various tensile stresses.
1 M. Vazquez, W. Fernengel and H. Kronmiiller, Phys. Stat. Sol. (a), 80(1983) 195. 2 J. Fink-Finowicki, G. Konczos, J. Krzywiriski, A. Siemko, T. Tarnoczi and Z. Vertesy, Acta Phys. Pol. A, 76 (1989) 163. 3 H. K. Lachowicz and A. Siemko, IEEE Trans. Magn., MAG-25 (1989) 3605.
139 4 X. Z. Dong and H. Kronm/iller, Phys. Stat. Sol. (a), 70 (1982) 451. 5 M. August, Phys. Stat. Sol. (a), 103(1987) 231, 6 A. Siemko, H. K. Lachowicz, N. Moser, A. Forkl and H. Kronm/iller, J. Magn. Magn. Mater., 83 (1990) 171.
7 A. Veider, G. Badurek, R. Gr6ssinger and H. Kronm/iiler, J. Magn. Magn. Mater., 60(1986) 182. 8 J. Fink-Finowickiand B. Lisowski, in H. Matyja and E G. Zielifiski (eds.), Amorphous Metals, World Scientific Publishing Co. Pte. Ltd., 1986, p. 263.