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Remarks on stress-anneal-induced anisotropy in metallic glasses
Journal of Magnetism and Magnetic Materials 112 (1992) 281-283 North-Holland
Remarks on stress-anneal-induced anisotropy in metallic glasses ,a,. Slawska-Waniewska, A. Siemko and H.K. Lachowicz Institute of Physics of the Polish Academy o] Sciences, AI. Lotnik6w 32/46, 02-668 Warsaw, Poland
The experimental results of the magnetic anisotropy induced during annealing under tensile stress and/or longitudinal magnetic field in F e - C r - S i - B metallic glasses are presented. The possible origins of the induced anisotropies are connected with the type of 3d metal atoms arranging into pairs during annealing.
I. Introduction The phenomenon of stress-anneal-induced anisotropy is known for a relatively long time. Experiments showing the occurrence of this effect have been performed on polycrystalline samples of Ni-Co, Ni-Fe and A I - F e alloys [1-3]. The magnitude of the uniaxial anisotropy induced in these materials was relatively small because of the low elastic limit exhibited by polycrystalline magnets. It is, however, interesting to notice that the experiments cited above, have shown that the easy axis of magnetization of the induced anisotropy can be parallel (for N i - C o alloys) as well as perpendicular (for N i - F e ) to the direction of the applied tensile stress. Since the discovery of metallic glasses, which show an extremally high elastic limit (two order of magnitude higher than those for conventional crystalline magnets) as well as large anelasticity, the stress-anneal-induced ar, isotropy has intensively been investigated, being motivated chiefly s ~ i i O i l l t y giV e f l '--" Lly "-: U l l b. . .p .l V J ~.... ,v~ by a p o :---"-"". .V C u u"~l .~ .. ,. . . t.v. . a ~ H~': an anisotropy much larger than those obtainable using other methods (e.g. field-annealing). As it has been shown in a large number of experiments Correspondence to: Dr. A. Slawska-Waniewska, Institute of Physics of the Polish Academy of Sciences, AI. Lotnik6w 32/46, PL-02-668 Wars'x,, Foland.
(see e.g. refs. [4,5]) the stress-anneal-induced anisotropy appears even if annealing is performed at temperatures above the Curie-point of metallic glasses under investigation. Therefore, it is obvious that the origin of the anisotropy induced by stress-annealing must be related to the structural changes occurred in the material. Magnetic interactions which give finally the observed effect of the anisotropy switch on only then as the temperature of the cooled ,down sample reaches a range below the Curie-point. Though, the origin of the anisotropy considered has been related to a number of phenomeaa, nevertheless, the socalled bond orientational anisotropy seems to be the one which is now most commonly accepted [6]. Within this model, as a result of the stress-annealing process, more atomic bonds become oriented in the plane perpendicular to the stress axis (due to anelasticity) than along this axis. Since the stress-anneal-induced anisotropy in the Co-based metallic glasses has been investigated very intensively whereas there are in the !;~-,~,..~-,,,-,~ ,~nh, f~,v ro~l,it~ rolate~d to this anisotropy in the Fe-based metallic glasses [5.7], the results obtained for the F e - C r - S i - B :,amples will be presented here. These results, toge.ther with those obtained earlier for Co-based alloys subjected to stress annealing [8], allow to speculate about the possible origin of the phenomenon considered.
030.~.-8853/92/$05.00 © 1992 - Elsevier Science Publisher~ B.V. All rights reserved
,~o~,~
A. Slawska-Waniewska et aL
/ Stress-anneai-induced anisotropy
2. Experimental procedure
tkJl
Ku
2.0i-
The metallic glass Fes, Cr2Bt4Si4 ribbon (kindly submitted to us by Dr. K. Zzv6ta from the Institute of Physics, Czechoslovak Acad. Sci.) is characterized by the room temperature saturation magnetization /z0Ms = 1.4 T, the Curie and crystallization temperature of 347 and 470°C, respectively, and positive magnetostriction constant [5]. The samples used in the present work were 120 mm long with their cross-section of about 0.09 mm 2. The stress a n d / o r field annealing runs were performed in air at various temperatures up to 400°C using conventional electric furnace. Each annealing run was composed of two steps, namely: pre-annealing at 300°C for 1 h followed by stress a n d / o r field annealing at the temperature T, for 1 h. Before the stress a n d / o r field was removed, the samples were rapidly cooled down to nearly room temperature. Annealing runs were carried out subjecting samples to the longitudinal magnetic field HII = 10 k A / m a n d / o r tensile stress o r - 600 MPa acting also in the axial direction. The anisotropy field was measured at room temperature by means of the FMR technique on 3 mm diameter discs cut from the annealed sampies. Since a thinning of the samples makes no influence on the angular dependence of the resonance line, thus it was assumed that the measured anisotropy field equals that of the bulk material.
3. Results and discussion The obtained dependencies of stress K t'~l ' longitudinal field --uKHtl, and stress-field K'~+t~'~, induced anisotropies on the annealing temperature T,d are shown in fig. 1. The dependence of Kun, vs. T,, behaves similarly as those reported for other metallic glasses [7-9] reflecting the pair directional ordering model [10]. The stress-induced anisotropy increases with an increase of the annealing temperature T,~ reaching its maximum for the temperature above the Curie point To, being in excellent agreement with a similar dependence obtained for the same material by Kraus et al. [5]. For temperatures T~
1.5-
O"
~ 0.5 _
~-~,~
~ ~ ,,~
'\\ 0"'~ I"111
H,
\ l Ta i.. "~." ~20 3O0 3z,O 380 to& Fig. 1. Annealing temperature dependencies of anisotropy constants induced by different annealing treatments (sign convention: K u > 0 for the easy ayis parallel to the ribbon axis). _
~
lower than Tc there exists a contribution to the stress anisotropy which is of field-induced origin since the magnetization of the sample is aligned with the ribbon axis due to magnetoelastic interaction (positive magnetostriction) and the shape anisotropy effect (this contribution is expected to be similar to the one obtained for the field annealing procedure Ku&~ vs. Ta). However, the most interesting feature of the stress-induced anisotropy in the alloy investigated is that its easy axis is parallel to the ribbon axis as opposed to that observed in the Co-based materials, for which the hard axis lies along the ribbon length [4,6-9]. Considering the results mentioned above one could speculate about the mechanisms underlying magnetic anisotropy created by stress-annealing. It seems to be reasonable to assume that the
A. S!awska-Waniewska et aL i Stress-anneal-induced ~,nisotropy
anisotropy exchange (pseudodipolar interactions) between two like-atoms could be responsible for this phenomenon. The most probable atomic pairs are in these two cases the F e - F c or Co-Co pairs, depending which of the metallic glass we are dealing with. To estimate the anisotropy of such atomic pairs the structure and anisotropy of pure Fe and Co monocrystals can be considered (similar approach has successfully been done in other works [11,12]). ct-Fe monocrystal exhibits a spatially centered cubic structure with nearestneighbor iron atoms laying along the [111] direction which is the hard axis of magnetization. In the hexagonal structure of the Co monocrystal the situation is more complex but one can find that the furthest cobalt atoms are placed along the [1010] direction which is the hard axis and moreover the [1120] hard axis is perpendicular to the nearest neighboring C o - C o atoms. Thus, it can be expected that in Fe-rich metallic glasses rearrangement of the atoms caused by stress annealing should lead to the magnetic anisotropy the hard axis of which is perpendicular to the applied stress, as it was observed here. On the contrary, in Co-rich amorphous alloys the hard axis is expected to follow the stress direction, in agreement with experimental results. The more detailed quantitative analysis of the anisotropy of cobalt or iron atom pairs placed in an amorphous matrix requires, as it was suggested in ref. [12], to consider the modification of the valence 3d shell of Fe (or Co) atom by neighboring metalloids atoms (and other transition metal atoms) leading in turn to the changes in spin-orbit coupling and exchange interaction.
'
283
The longitudinal magnetic field applied during stress annealing does not change the observed stress-induced anisotropy and the dependence u , on Ta fohows the K'utTa) curve (see fig. 1). As mentioned above, during stress annealing at Ta < Tc, the magnetization is aligned along the ribbon axis leading to the longitudinal anisotropy term of the field induced type. Additional external longitudinal magnetic field does not change the spatial distribution of magnetization and its value thus not aff:cting the stress-induced anisotropy.
Reterenccs [1] R. Vergne, Compt. Rend. 252 (1961) 82. [2] R. Vergne, J. Phys. Radium 20 (1959) 254. [3] H.J. Birkenbeii and R.W. Cahn, Proc. Phys. Soc. (London) 79 (1962) 831; J. Appl. Phys. 16 (1961) 2625. [4] H.R. Hilzinger, in: Proc. 4th Intern. Conf. on Rapidly Quenched Metals. eds. T. Masumoto and K. Suzuki (Sendal, 1982)p. 791. [5] L. Kraus, N. Z~irubov:i, K. Z~iv6ta and P. Duhaj, J. Magn. Magn. Mater. 72 (1988) 199. [6] J. Haimowich, T. Jagielifiski and T. Egami, J. Appl. Phys. 57 (1985) 3581. [7] J. Gonzfilez and J.M. Blanco, J. Non-C.'3'st. Solids 126 (1990) 151. [8] A. Slawska-Waniewska, A. Siemko and H.K. Lachowicz, J. Magn. Magn. Mater. 104-11)7 (1992) 199. [9] J. Gonzfilez and K. Ku|akowski, J. Magn. Magn. Mater. 82 (1989) 94. [10] H. Fuijmori, in: Amorphous Metal Alloys, ed. F.E. Luborsky (Butterworths, London, 1983) p. 300. [11] R.J. Gambino, P. Chaudhari and J.J. Cuomo, AlP Conf. Proc. 18 (1974) 578. [12] L. Kraus, P. Duhaj and S. Bode, J. Magn. Magn. Mater. 101 (1991) 1.
Remarks on stress-anneal-induced anisotropy in metallic glasses ,a,. Slawska-Waniewska, A. Siemko and H.K. Lachowicz Institute of Physics of the Polish Academy o] Sciences, AI. Lotnik6w 32/46, 02-668 Warsaw, Poland
The experimental results of the magnetic anisotropy induced during annealing under tensile stress and/or longitudinal magnetic field in F e - C r - S i - B metallic glasses are presented. The possible origins of the induced anisotropies are connected with the type of 3d metal atoms arranging into pairs during annealing.
I. Introduction The phenomenon of stress-anneal-induced anisotropy is known for a relatively long time. Experiments showing the occurrence of this effect have been performed on polycrystalline samples of Ni-Co, Ni-Fe and A I - F e alloys [1-3]. The magnitude of the uniaxial anisotropy induced in these materials was relatively small because of the low elastic limit exhibited by polycrystalline magnets. It is, however, interesting to notice that the experiments cited above, have shown that the easy axis of magnetization of the induced anisotropy can be parallel (for N i - C o alloys) as well as perpendicular (for N i - F e ) to the direction of the applied tensile stress. Since the discovery of metallic glasses, which show an extremally high elastic limit (two order of magnitude higher than those for conventional crystalline magnets) as well as large anelasticity, the stress-anneal-induced ar, isotropy has intensively been investigated, being motivated chiefly s ~ i i O i l l t y giV e f l '--" Lly "-: U l l b. . .p .l V J ~.... ,v~ by a p o :---"-"". .V C u u"~l .~ .. ,. . . t.v. . a ~ H~': an anisotropy much larger than those obtainable using other methods (e.g. field-annealing). As it has been shown in a large number of experiments Correspondence to: Dr. A. Slawska-Waniewska, Institute of Physics of the Polish Academy of Sciences, AI. Lotnik6w 32/46, PL-02-668 Wars'x,, Foland.
(see e.g. refs. [4,5]) the stress-anneal-induced anisotropy appears even if annealing is performed at temperatures above the Curie-point of metallic glasses under investigation. Therefore, it is obvious that the origin of the anisotropy induced by stress-annealing must be related to the structural changes occurred in the material. Magnetic interactions which give finally the observed effect of the anisotropy switch on only then as the temperature of the cooled ,down sample reaches a range below the Curie-point. Though, the origin of the anisotropy considered has been related to a number of phenomeaa, nevertheless, the socalled bond orientational anisotropy seems to be the one which is now most commonly accepted [6]. Within this model, as a result of the stress-annealing process, more atomic bonds become oriented in the plane perpendicular to the stress axis (due to anelasticity) than along this axis. Since the stress-anneal-induced anisotropy in the Co-based metallic glasses has been investigated very intensively whereas there are in the !;~-,~,..~-,,,-,~ ,~nh, f~,v ro~l,it~ rolate~d to this anisotropy in the Fe-based metallic glasses [5.7], the results obtained for the F e - C r - S i - B :,amples will be presented here. These results, toge.ther with those obtained earlier for Co-based alloys subjected to stress annealing [8], allow to speculate about the possible origin of the phenomenon considered.
030.~.-8853/92/$05.00 © 1992 - Elsevier Science Publisher~ B.V. All rights reserved
,~o~,~
A. Slawska-Waniewska et aL
/ Stress-anneai-induced anisotropy
2. Experimental procedure
tkJl
Ku
2.0i-
The metallic glass Fes, Cr2Bt4Si4 ribbon (kindly submitted to us by Dr. K. Zzv6ta from the Institute of Physics, Czechoslovak Acad. Sci.) is characterized by the room temperature saturation magnetization /z0Ms = 1.4 T, the Curie and crystallization temperature of 347 and 470°C, respectively, and positive magnetostriction constant [5]. The samples used in the present work were 120 mm long with their cross-section of about 0.09 mm 2. The stress a n d / o r field annealing runs were performed in air at various temperatures up to 400°C using conventional electric furnace. Each annealing run was composed of two steps, namely: pre-annealing at 300°C for 1 h followed by stress a n d / o r field annealing at the temperature T, for 1 h. Before the stress a n d / o r field was removed, the samples were rapidly cooled down to nearly room temperature. Annealing runs were carried out subjecting samples to the longitudinal magnetic field HII = 10 k A / m a n d / o r tensile stress o r - 600 MPa acting also in the axial direction. The anisotropy field was measured at room temperature by means of the FMR technique on 3 mm diameter discs cut from the annealed sampies. Since a thinning of the samples makes no influence on the angular dependence of the resonance line, thus it was assumed that the measured anisotropy field equals that of the bulk material.
3. Results and discussion The obtained dependencies of stress K t'~l ' longitudinal field --uKHtl, and stress-field K'~+t~'~, induced anisotropies on the annealing temperature T,d are shown in fig. 1. The dependence of Kun, vs. T,, behaves similarly as those reported for other metallic glasses [7-9] reflecting the pair directional ordering model [10]. The stress-induced anisotropy increases with an increase of the annealing temperature T,~ reaching its maximum for the temperature above the Curie point To, being in excellent agreement with a similar dependence obtained for the same material by Kraus et al. [5]. For temperatures T~
1.5-
O"
~ 0.5 _
~-~,~
~ ~ ,,~
'\\ 0"'~ I"111
H,
\ l Ta i.. "~." ~20 3O0 3z,O 380 to& Fig. 1. Annealing temperature dependencies of anisotropy constants induced by different annealing treatments (sign convention: K u > 0 for the easy ayis parallel to the ribbon axis). _
~
lower than Tc there exists a contribution to the stress anisotropy which is of field-induced origin since the magnetization of the sample is aligned with the ribbon axis due to magnetoelastic interaction (positive magnetostriction) and the shape anisotropy effect (this contribution is expected to be similar to the one obtained for the field annealing procedure Ku&~ vs. Ta). However, the most interesting feature of the stress-induced anisotropy in the alloy investigated is that its easy axis is parallel to the ribbon axis as opposed to that observed in the Co-based materials, for which the hard axis lies along the ribbon length [4,6-9]. Considering the results mentioned above one could speculate about the mechanisms underlying magnetic anisotropy created by stress-annealing. It seems to be reasonable to assume that the
A. S!awska-Waniewska et aL i Stress-anneal-induced ~,nisotropy
anisotropy exchange (pseudodipolar interactions) between two like-atoms could be responsible for this phenomenon. The most probable atomic pairs are in these two cases the F e - F c or Co-Co pairs, depending which of the metallic glass we are dealing with. To estimate the anisotropy of such atomic pairs the structure and anisotropy of pure Fe and Co monocrystals can be considered (similar approach has successfully been done in other works [11,12]). ct-Fe monocrystal exhibits a spatially centered cubic structure with nearestneighbor iron atoms laying along the [111] direction which is the hard axis of magnetization. In the hexagonal structure of the Co monocrystal the situation is more complex but one can find that the furthest cobalt atoms are placed along the [1010] direction which is the hard axis and moreover the [1120] hard axis is perpendicular to the nearest neighboring C o - C o atoms. Thus, it can be expected that in Fe-rich metallic glasses rearrangement of the atoms caused by stress annealing should lead to the magnetic anisotropy the hard axis of which is perpendicular to the applied stress, as it was observed here. On the contrary, in Co-rich amorphous alloys the hard axis is expected to follow the stress direction, in agreement with experimental results. The more detailed quantitative analysis of the anisotropy of cobalt or iron atom pairs placed in an amorphous matrix requires, as it was suggested in ref. [12], to consider the modification of the valence 3d shell of Fe (or Co) atom by neighboring metalloids atoms (and other transition metal atoms) leading in turn to the changes in spin-orbit coupling and exchange interaction.
'
283
The longitudinal magnetic field applied during stress annealing does not change the observed stress-induced anisotropy and the dependence u , on Ta fohows the K'utTa) curve (see fig. 1). As mentioned above, during stress annealing at Ta < Tc, the magnetization is aligned along the ribbon axis leading to the longitudinal anisotropy term of the field induced type. Additional external longitudinal magnetic field does not change the spatial distribution of magnetization and its value thus not aff:cting the stress-induced anisotropy.
Reterenccs [1] R. Vergne, Compt. Rend. 252 (1961) 82. [2] R. Vergne, J. Phys. Radium 20 (1959) 254. [3] H.J. Birkenbeii and R.W. Cahn, Proc. Phys. Soc. (London) 79 (1962) 831; J. Appl. Phys. 16 (1961) 2625. [4] H.R. Hilzinger, in: Proc. 4th Intern. Conf. on Rapidly Quenched Metals. eds. T. Masumoto and K. Suzuki (Sendal, 1982)p. 791. [5] L. Kraus, N. Z~irubov:i, K. Z~iv6ta and P. Duhaj, J. Magn. Magn. Mater. 72 (1988) 199. [6] J. Haimowich, T. Jagielifiski and T. Egami, J. Appl. Phys. 57 (1985) 3581. [7] J. Gonzfilez and J.M. Blanco, J. Non-C.'3'st. Solids 126 (1990) 151. [8] A. Slawska-Waniewska, A. Siemko and H.K. Lachowicz, J. Magn. Magn. Mater. 104-11)7 (1992) 199. [9] J. Gonzfilez and K. Ku|akowski, J. Magn. Magn. Mater. 82 (1989) 94. [10] H. Fuijmori, in: Amorphous Metal Alloys, ed. F.E. Luborsky (Butterworths, London, 1983) p. 300. [11] R.J. Gambino, P. Chaudhari and J.J. Cuomo, AlP Conf. Proc. 18 (1974) 578. [12] L. Kraus, P. Duhaj and S. Bode, J. Magn. Magn. Mater. 101 (1991) 1.