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Stress-field-anneal-induced anisotropy in metallic glasses
Journal of Magnetism and Magnetic Materials 104-107 (1992) 119-120 North-Holland
Stress-field-anneal-induced anisotropy in metallic glasses Anna Slawska-Waniewska, Andrzej Siemko and Henryk K. Lachowicz Institute of Physics of the Polish Academy of Sciences, al. Lotnik6w 32/46, 02-668 Warsaw, Poland The experimental results for the stress and/or field induced anisotropies in the Co-Fe-Ni-Si-B metallic glass are presented. The results were analyzed in order to find possible contributions to the observed anisotropies as well as to explain the so-called anisotropy "enhancement" effect. Numerous papers have recently reported on the anisotropy induced in metallic glasses by their annealing under simultaneous action of the externally applied tensile stress and magnetic field (e.g. refs. [1,2]). It has been shown that the anisotropy induced in such way (stress-field-induced anisotropy, K,(o-+ H ) is usually larger than a sum of the anisotropies induced separately by stress, K,(~), and field, Ku(H), under the same annealing conditions. This so called "enhancement" (or "reinforcement") effect defined as k = Ku(~r + H ) - [Ku(o-)+ K~(H)] has many times been shown for a variety of Co- as well as Fe-based metallic glasses (e.g. refs. [3,4]). One can find in the literature a number of attempts to interpret the origin of this effect but none of them seems to be quite satisfactorily (see e.g. refs. [5,6]). In the present paper experimental results of the stress a n d / o r field anisotropies induced in C o - F e N i - S i - B metallic glass samples are presented. These anisotropies have been created under various, well controlled, conditions of the annealing process. An analysis of these results is performed in order to achieve a better understanding of the "enhancement" effect. The samples used in the experiment were cut from a metallic glass ribbon of composition Co57.4Fes.6Ni10 SiI1BI6 and were typically 130 mm long, and of 0.095 mm 2 cross-section. The stress a n d / o r field annealing were performed in air on the as-cast samples at various temperatures (up to 400 ° C), using a conventional electrical furnace. Since the kinetics of induced anisotropy was not considered here, a constant annealing time t a = 1 h was arbitrary chosen. Annealings were carried out in a transverse ( H ± ) or longitudinal (Hit) magnetic field of 6.5 and 10 k A / m , respectively, a n d / o r under various tensile stress applied along the ribbon axis. The induced magnetic anisotropy was measured at room temperature by means of two methods, namely: the FMR technique using in this case small discs cut from the annealed samples and the hysteresis loop method which enables to measure only the transverse anisotropy in the ribbon sample. Fig. 1 shows the induced anisotropy as a function of the tensile stress applied during annealing at three different temperatures (the commonly accepted sign convention is used: K u > 0 for the transverse anisotropy
and K u < 0 for the longitudinal one). The anisotropy was induced by either stress or simultaneous stress and transverse field annealing. The important feature of the results presented in fig. 1 is the observed collinearity of the Ku(~)- and Ku(~ +H±)-dependencies (for the same temperature of annealing) when plotted against the tensile stress. This fact implies that both the stress-induced anisotropy process and the field-induced one are not interdependent since in the opposite case a different slope of both dependencies should be expected. The observed deviation from linearity in the high stress range for the characteristics obtained at an annealing temperature of 260°C has already been detected in Fe-based metallic glasses being explained by an increase of the relaxation frequencies with the increasing applied stress and also by the sinh-dependence of the saturation value of the inelastic strain (for details see ref. [7]).
Ku
t
[Jim 3
• - 260 °C x - 236 °C
25[
o-
220°C
# z / z /
,~/_/// ,,8,,,
200
;I
/
150
100
50
,"D:>,<_ -'~ l " ,_,_,-=
..-/-L-o_.L_ -V
.....
,~'o
"
o
'
e['OPal
- 51
Fig. 1. Stress-induced and stress-field-induced anisotropy constants as a function of stress o- applied during annealing at different temperatures.
0312-8853/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
A. Slawska-Waniewska et al. / Anisotropy in metallic glasses
120 Ku [j/m 3]
( Kuz )
25C 3~
K u (~* Hj)
200
bl
\
150 10
~oo
,/
,
20
30 dTldt [°C/mini
I,L~]
I // " Ku(5+~/
50
0
250
300 Tonn[eC ]
350
400 Tonn [°C]
Z T
(H.)
501-(K% ) Fig. 2. Annealing temperature dependence of anisotropy constants obtained under various conditions of their inducement. Inset (a) presents the anisotropy "enhancement" effect k and I Ku(Hll)l as a function of T~,,. Inset (b) shows the cooling rate dependence of the K,((r + H~)-anisotropy induced at T.,,~ > Tc (arrows indicate the same experimental point). Fig. 2 shows the dependencies of the induced anisotropies K.(HII), Ku(H±), Ku(Cr), Ku(cr + Hii) and K ~ ( o - + H ± ) as a function of the annealing temperature T.... One can easily notice that the field-induced anisotropies Ku(HII) and K ~ ( H z ) behave in a typical way - their magnitudes, directions and annealing temperature dependencies are in good agreement with those given in the literature [8]. Analyzing the curves for K , ( ~ + H ± ) , Ku(~) and K , ( H ± ) presented in fig. 2 one can easily find the " e n h a n c e m e n t " effect, defined as k = K~(~r + H ) - [K,(~r) + K , ( H ) ] (see inset (b) in fig. 2). It seems that the essential feature of the curves presented in fig. 2 is the fact, that the K,(~r + Hii)-dependence follows practically (within the measurement accuracy) the one of KoOr). This concurrence can be explained considering the fact that the metallic glass applied for the experiment exhibits in the range of annealing temperatures an extremely small but positive saturation magnetostriction (for details see rcf. [2]). Therefore, during the stress-annealing below the Curie temperature the shape anisotropy effect (as a main reason) and magnetoelastic coupling forces the magnetization of the sample to align with the ribbon (sample) axis. Considering this fact, an important conclusion can be drawn, that the stress-anisotropy induced in the metallic glass at the annealing temperatures below the Curie point cannot be treated as a single process. Assuming that the strain of magnetoelastic origin is negligibly small as compared with the one produced by the stress externally applied, than at least two contributions should be taken into account.
The first is related to the strain-induced structural (topological) anisotropy and the second is of the type of field-induced anisotropy arising from magnetic moments arrangement. In the metallic glass investigated the strain-induced structural anisotropy is transverse with respect to the ribbon axis (which is in agreement with the results obtained in other Co-based amorphous alloys [8]), whereas the field-induced contribution is directed longitudinally. In the case of Ku(~r + H l ) the magnetic moments are forced by the external magnetic field, saturating the sample in the transverse direction, giving the transverse field-induced anisotropy term. Considering the above and also the results shown in figs. 1 and 2, it seems to be obvious that the so-called " e n h a n c e m e n t " effect results from the anisotropy contribution of the field-induced type. This conclusion seems to be well confirmed in the inset (a) in fig. 2 which shows that both dependencies, k and hK,(Hjj)I versus Z,~n, follow each other. The small inconsistency observed particularly for the lower range of T~.... may arise from the internal stress quenched in during the ribbon producing process a n d / o r surface irregularities. Finally it is worth noticing that even if the stress-annealing is performed at a temperature above the Curie point the " e n h a n c e m e n t " effect is also observed, however its magnitude depends dramatically on the cooling rate. This fact can easily be deduced considering the dependence of the stress-field-induced anisotropy versus cooling rate (the sample annealed at "E,,,, = 340 ° C above the Curie temperature, T c, = 280 ° C) shown in the inset (b) in fig. 2. The conclusion which can be drawn taking into account the considerations presented above is that the origin of the so-called " e n h a n c e m e n t " effect of the stress-field-induced anisotropy in the material investigated does not underline complex mechanisms and can be interpreted in a rather unsophisticated way. References [1] J. Gonz~lez, M. V~zquez, J.M. Barandiar~n, M.A. Illaramendi, A. Salazar and A. Hernando, Phys. Stat, Sol. (a) 113 (1989) 187. [2] A. Slawska-Waniewska, A. Siemko, J. Fink-Finowicki, L. Zaluski and H.K. Lachowicz, J. Magn. Magn. Mater. 101 (1991) 40. [3] J. Gonz~lez, M. V~zquez, J.M. Barandiar~n and A. Hernando, J. de Phys. 49 (1988) C8-1335. [4] J. Gonz~lez and J.M. Blanco, J. Non-Cry,st. Solids 126 (1990) 151. [5] J. Gonz~.lez and K. Ku|akowski, J. Magn. Magn. Mater. 86 (1990) 207. [6] K. Kutakowski and J. Gonz~lez, in: Proc. 5th lnt. Conf. on Physics of Magn. Mat., eds. W. Gorzkowski et al. (World Scientific, Singapore, 1991)p. 377. [7] L. Kraus, N. Z~irubov~,, K. Z~vdta and P. Duhaj, J. Magn. Magn. Mater. 72 (1988) 199. [8] H. Fujimori, in: Amorphous Metallic Alloys, ed. F.E. Luborsky (Butterworths, London, 1983) p. 300.
Stress-field-anneal-induced anisotropy in metallic glasses Anna Slawska-Waniewska, Andrzej Siemko and Henryk K. Lachowicz Institute of Physics of the Polish Academy of Sciences, al. Lotnik6w 32/46, 02-668 Warsaw, Poland The experimental results for the stress and/or field induced anisotropies in the Co-Fe-Ni-Si-B metallic glass are presented. The results were analyzed in order to find possible contributions to the observed anisotropies as well as to explain the so-called anisotropy "enhancement" effect. Numerous papers have recently reported on the anisotropy induced in metallic glasses by their annealing under simultaneous action of the externally applied tensile stress and magnetic field (e.g. refs. [1,2]). It has been shown that the anisotropy induced in such way (stress-field-induced anisotropy, K,(o-+ H ) is usually larger than a sum of the anisotropies induced separately by stress, K,(~), and field, Ku(H), under the same annealing conditions. This so called "enhancement" (or "reinforcement") effect defined as k = Ku(~r + H ) - [Ku(o-)+ K~(H)] has many times been shown for a variety of Co- as well as Fe-based metallic glasses (e.g. refs. [3,4]). One can find in the literature a number of attempts to interpret the origin of this effect but none of them seems to be quite satisfactorily (see e.g. refs. [5,6]). In the present paper experimental results of the stress a n d / o r field anisotropies induced in C o - F e N i - S i - B metallic glass samples are presented. These anisotropies have been created under various, well controlled, conditions of the annealing process. An analysis of these results is performed in order to achieve a better understanding of the "enhancement" effect. The samples used in the experiment were cut from a metallic glass ribbon of composition Co57.4Fes.6Ni10 SiI1BI6 and were typically 130 mm long, and of 0.095 mm 2 cross-section. The stress a n d / o r field annealing were performed in air on the as-cast samples at various temperatures (up to 400 ° C), using a conventional electrical furnace. Since the kinetics of induced anisotropy was not considered here, a constant annealing time t a = 1 h was arbitrary chosen. Annealings were carried out in a transverse ( H ± ) or longitudinal (Hit) magnetic field of 6.5 and 10 k A / m , respectively, a n d / o r under various tensile stress applied along the ribbon axis. The induced magnetic anisotropy was measured at room temperature by means of two methods, namely: the FMR technique using in this case small discs cut from the annealed samples and the hysteresis loop method which enables to measure only the transverse anisotropy in the ribbon sample. Fig. 1 shows the induced anisotropy as a function of the tensile stress applied during annealing at three different temperatures (the commonly accepted sign convention is used: K u > 0 for the transverse anisotropy
and K u < 0 for the longitudinal one). The anisotropy was induced by either stress or simultaneous stress and transverse field annealing. The important feature of the results presented in fig. 1 is the observed collinearity of the Ku(~)- and Ku(~ +H±)-dependencies (for the same temperature of annealing) when plotted against the tensile stress. This fact implies that both the stress-induced anisotropy process and the field-induced one are not interdependent since in the opposite case a different slope of both dependencies should be expected. The observed deviation from linearity in the high stress range for the characteristics obtained at an annealing temperature of 260°C has already been detected in Fe-based metallic glasses being explained by an increase of the relaxation frequencies with the increasing applied stress and also by the sinh-dependence of the saturation value of the inelastic strain (for details see ref. [7]).
Ku
t
[Jim 3
• - 260 °C x - 236 °C
25[
o-
220°C
# z / z /
,~/_/// ,,8,,,
200
;I
/
150
100
50
,"D:>,<_ -'~ l " ,_,_,-=
..-/-L-o_.L_ -V
.....
,~'o
"
o
'
e['OPal
- 51
Fig. 1. Stress-induced and stress-field-induced anisotropy constants as a function of stress o- applied during annealing at different temperatures.
0312-8853/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
A. Slawska-Waniewska et al. / Anisotropy in metallic glasses
120 Ku [j/m 3]
( Kuz )
25C 3~
K u (~* Hj)
200
bl
\
150 10
~oo
,/
,
20
30 dTldt [°C/mini
I,L~]
I // " Ku(5+~/
50
0
250
300 Tonn[eC ]
350
400 Tonn [°C]
Z T
(H.)
501-(K% ) Fig. 2. Annealing temperature dependence of anisotropy constants obtained under various conditions of their inducement. Inset (a) presents the anisotropy "enhancement" effect k and I Ku(Hll)l as a function of T~,,. Inset (b) shows the cooling rate dependence of the K,((r + H~)-anisotropy induced at T.,,~ > Tc (arrows indicate the same experimental point). Fig. 2 shows the dependencies of the induced anisotropies K.(HII), Ku(H±), Ku(Cr), Ku(cr + Hii) and K ~ ( o - + H ± ) as a function of the annealing temperature T.... One can easily notice that the field-induced anisotropies Ku(HII) and K ~ ( H z ) behave in a typical way - their magnitudes, directions and annealing temperature dependencies are in good agreement with those given in the literature [8]. Analyzing the curves for K , ( ~ + H ± ) , Ku(~) and K , ( H ± ) presented in fig. 2 one can easily find the " e n h a n c e m e n t " effect, defined as k = K~(~r + H ) - [K,(~r) + K , ( H ) ] (see inset (b) in fig. 2). It seems that the essential feature of the curves presented in fig. 2 is the fact, that the K,(~r + Hii)-dependence follows practically (within the measurement accuracy) the one of KoOr). This concurrence can be explained considering the fact that the metallic glass applied for the experiment exhibits in the range of annealing temperatures an extremely small but positive saturation magnetostriction (for details see rcf. [2]). Therefore, during the stress-annealing below the Curie temperature the shape anisotropy effect (as a main reason) and magnetoelastic coupling forces the magnetization of the sample to align with the ribbon (sample) axis. Considering this fact, an important conclusion can be drawn, that the stress-anisotropy induced in the metallic glass at the annealing temperatures below the Curie point cannot be treated as a single process. Assuming that the strain of magnetoelastic origin is negligibly small as compared with the one produced by the stress externally applied, than at least two contributions should be taken into account.
The first is related to the strain-induced structural (topological) anisotropy and the second is of the type of field-induced anisotropy arising from magnetic moments arrangement. In the metallic glass investigated the strain-induced structural anisotropy is transverse with respect to the ribbon axis (which is in agreement with the results obtained in other Co-based amorphous alloys [8]), whereas the field-induced contribution is directed longitudinally. In the case of Ku(~r + H l ) the magnetic moments are forced by the external magnetic field, saturating the sample in the transverse direction, giving the transverse field-induced anisotropy term. Considering the above and also the results shown in figs. 1 and 2, it seems to be obvious that the so-called " e n h a n c e m e n t " effect results from the anisotropy contribution of the field-induced type. This conclusion seems to be well confirmed in the inset (a) in fig. 2 which shows that both dependencies, k and hK,(Hjj)I versus Z,~n, follow each other. The small inconsistency observed particularly for the lower range of T~.... may arise from the internal stress quenched in during the ribbon producing process a n d / o r surface irregularities. Finally it is worth noticing that even if the stress-annealing is performed at a temperature above the Curie point the " e n h a n c e m e n t " effect is also observed, however its magnitude depends dramatically on the cooling rate. This fact can easily be deduced considering the dependence of the stress-field-induced anisotropy versus cooling rate (the sample annealed at "E,,,, = 340 ° C above the Curie temperature, T c, = 280 ° C) shown in the inset (b) in fig. 2. The conclusion which can be drawn taking into account the considerations presented above is that the origin of the so-called " e n h a n c e m e n t " effect of the stress-field-induced anisotropy in the material investigated does not underline complex mechanisms and can be interpreted in a rather unsophisticated way. References [1] J. Gonz~lez, M. V~zquez, J.M. Barandiar~n, M.A. Illaramendi, A. Salazar and A. Hernando, Phys. Stat, Sol. (a) 113 (1989) 187. [2] A. Slawska-Waniewska, A. Siemko, J. Fink-Finowicki, L. Zaluski and H.K. Lachowicz, J. Magn. Magn. Mater. 101 (1991) 40. [3] J. Gonz~lez, M. V~zquez, J.M. Barandiar~n and A. Hernando, J. de Phys. 49 (1988) C8-1335. [4] J. Gonz~lez and J.M. Blanco, J. Non-Cry,st. Solids 126 (1990) 151. [5] J. Gonz~.lez and K. Ku|akowski, J. Magn. Magn. Mater. 86 (1990) 207. [6] K. Kutakowski and J. Gonz~lez, in: Proc. 5th lnt. Conf. on Physics of Magn. Mat., eds. W. Gorzkowski et al. (World Scientific, Singapore, 1991)p. 377. [7] L. Kraus, N. Z~irubov~,, K. Z~vdta and P. Duhaj, J. Magn. Magn. Mater. 72 (1988) 199. [8] H. Fujimori, in: Amorphous Metallic Alloys, ed. F.E. Luborsky (Butterworths, London, 1983) p. 300.