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Induced anisotropy and magnetostriction behaviour of an annealed Co–Fe (Co-rich) amorphous wire1
Journal of Magnetism and Magnetic Materials 186 (1998) 135—138
Induced anisotropy and magnetostriction behaviour of an annealed Co—Fe (Co-rich) amorphous wire1 J.M. Blanco!,*, L. Domı´ nguez!, P. Aragone´ses!, J. Gonza´lez" ! Dpto de Fı& sica Aplicada I, Escuela Universitaria de Ingenierı& a Te& cnica e Industrial, Universidad del Paı& s Vasco, 20011 San Sebastia& n, Spain " Dpto de Fı& sica de Materiales, Facultad de Quı& micas, Universidad del Paı& s Vasco, P.O. Box 1072, 20080 San Sebastia& n, Spain Received 28 October 1997
Abstract We present results on the induced anisotropy and magnetostriction constant of a very low magnetostrictive amorphous wire after annealing with an applied stress (stress annealing) which can develop a transverse magnetic anisotropy followed by a subsequent annealing without the applied stress (stress relief) which can eliminate the transverse induced anisotropy. The evolution of the magnetostriction constant with these treatments has also been determined. It changes from a negative value (as-cast) to positive values after stress annealing, while the effect of the stress relief treatment changes the magnetostriction constant towards a saturation value independent of the thermal history of the sample. ( 1998 Elsevier Science B.V. All rights reserved. Keywords: Amorphous systems — wires; Anisotropy — induced; Magnetostriction constant
1. Introduction Some amorphous alloys are known to suitable systems for studies in view of their peculiar magnetostrictive behaviour. In particular, Co-rich amorphous alloys exhibit the following magnetic behaviour: (i) the macroscopic anisotropy is nearly zero as a result of random orientation of local easy axes, (ii) the saturation magnetostriction is
* Corresponding author. Fax: #34 43 47 1098; e-mail: [email protected]. 1 This paper was presented in part at ICM’97 in Cairns, Australia.
verysensitive to the chemical composition, heat treatment and applied mechanical stress and (iii) uniaxial magnetic anisotropy can be induced by stress and/or field annealing. Noticeable changes in the physical properties take place at an annealing temperature around 340°C indicating that some kind of structural transformation, similar to the martensitic first-order transformations observed in crystalline materials, takes place in Co-rich amorphous alloys [1]. However, O’Handley [2] and Furthmu¨ller et al. [3] recently emphasized the similarity found between the magnetostrictive behaviour of crystalline and amorphous ferromagnets which suggests that large local magnetic anisotropies are to be expected. In
0304-8853/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 9 7 ) 0 1 1 1 4 - 1
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J.M. Blanco et al. / Journal of Magnetism and Magnetic Materials 186 (1998) 135—138
particular, Co-rich ‘non-magnetostrictive’ alloys present a peculiar magnetostrictive behaviour with j decreasing toward negative values as the applied stress increases, according to the expression j(p)"j(0)#Ap,
tention will be paid to the linear dependence of the saturation magnetostriction found recently in some Co-rich ribbons and wire shaped amorphous alloys [4,5].
(1)
where j(p) and j(0) are the saturation magnetostriction for an applied tensile stress p, and without applied stress, respectively, and A is a negative coefficient which ranges from !6 to !1]10~10 MPa~1 [4,5]. As a consequence, for those compositions with small and positive j(+10~7) at zerostress, a change in the sign of j takes place as the stress is increased. Different microscopic mechanisms account for the stress dependence of j. The microscopic model of Fa¨hnle et al. [6] correlates A with the second strain derivative of the local anisotropy coefficients which are determined by the local symmetry and chemistry. Therefore, only those thermal treatments which produce irreversible phase transformations in the local symmetry of the amorphous structure are expected to affect the subsequent value of A. It is to be noted that various hints of such type of transformations have been observed and reported for Co-rich metallic glasses [7]. The model developed by Szymczak [8] describes the stress dependence of j as a consequence of the bond orientational anisotropy induced by the stress. Therefore, the thermally activated processes are invoked to be responsible for the stress dependence of j. Moreover, it is expected that the action of the tensile stress at room temperature should be drastically affected by the strength and orientation of any bond anisotropy induced previously at higher temperature. Hernando et al. [9] have analyzed the behaviour of j(p) in amorphous ribbons taking into account the fluctuations of the local anisotropy and, therefore, of the local magnetostriction. In this model, the influence of the thermal treatments on the local magnetostriction fluctuations should be reflected in a similar influence on the coefficient A. In this work we present some results on the induced magnetic anisotropy and changes of the magnetostriction constant in a nearly non-magnetostriction amorphous wire subjected to stress annealing and stress-relief treatments. Special at-
2. Experimental technique Amorphous wire of nominal composition (Co Fe ) Si B was kindly supplied by 0.95 0.05 72.5 12.5 15 UNITIKA Ltd. Co. (Japan). Its diameter was 0.125 mm and it was cut to a length of 10 cm for magnetic measurements. Axial hysteresis loops of the samples were obtained by a conventional induction method at 50 Hz. Details can be found in Ref. [10]. Stress annealing and subsequent annealing without stress (stress relief) were performed by the so-called current annealing technique. The stress annealing can induce in the sample a macroscopic transverse magnetic anisotropy. This transverse induced magnetic anisotropy has been evaluated as the difference of the magnetization work of the stress annealed with respect to that of the as-quenched sample. The stress annealing was performed with an applied tensile stress of p "0, 400, 550, !// 825 and 1100 MPa at different current densities, while the stress relief treatment was carried out without tensile stress in samples which were previously stress annealed. Both kinds of thermal treatments were performed by the so-called current annealing technique [10]. The measurements of the magnetostriction constant, j, were performed using the small angle magnetization rotation (SAMR) method. As detailed elsewhere, this method makes use of the variations in the signal, » , picked up by 2u a narrow coil when applying simultaneously to the sample a saturating DC axial field, H , a small AC z transversal field, H , and a tensile stress p. The y experimental set-up described above was used to apply the DC field and the tensile stress, and to detect the induced signal. When a tensile stress, p, is applied, the induced signal » , measured by a lock-in amplifier, in2u creases or decreases as a consequence of the increment of magnetoelastic anisotropy having a transverse or an axial easy axis, respectively. This change in » can be compensated by an 2u
J.M. Blanco et al. / Journal of Magnetism and Magnetic Materials 186 (1998) 135—138
adequate modification of H so that » takes the z 2u same value as that existing before the stress was applied. In this way, the magnetostriction, j , is S obtained as j "!(k M /3)(*H /*p)» "cte, (2) S 0 S z 2u where M is the saturation magnetization (k M " S 0 S 0.8 T) of the wire.
3. Experimental results and their analysis The transverse-induced anisotropy as a function of the annealing time with the current density as a dependent parameter has previously been determined. These curves showed different behaviour depending on the electric current annealing range, as has been studied previously [10,11]. For low current annealing, the induced anisotropy increases monotonically with annealing time, with a faster rate as the current density increases, until it reaches a maximum value, at high current densities. After an initial increase, the anisotropy decreases with increasing annealing time. In both cases a maximum induced anisotropy, K.!9, can be obtained. The current density dependence of K.!9 is plotted in Fig. 1 with the stress applied during the treatment as a parameter showing a maximum around j "30.0 A/mm2. Similar behaviour for this kind !// of anisotropy was found in amorphous ribbons [10].
Fig. 1. Maximum anisotropy induced by annealing under various stresses in (Co Fe ) Si B amorphous wire. 0.95 0.05 72.5 12.5 15
137
The magnetostrictive behaviour of this amorphous wire is described by a linear stress dependence of the saturation magnetostriction where the stress derivative of j is always negative and quite S insensitive to both kinds of annealing treatments. For the as-cast state, the magnetostriction at zero applied stress, j (0), takes the value of !0.40]10~7. Upon S heat treatment with stress, this parameter is modified. Fig. 2 shows a typical evolution of j (0) as S a function of the annealing time for several isothermal treatments. This kind of treatment shifts j (0) S toward positive values, and a change of sign from negative to positive is then observed. When annealing around the Curie point ( j "34.6 A/mm2), the # maximum variation of j (0) is obtained. It should S be noted that the variation of j (0) does not depend S on the magnitude of the stress applied during the stress annealing treatment. Samples treated with j "0 and 550 MPa were !// subsequently subjected to two stress-relief treatments carried out by current annealing at 38.7 A/ mm2 for 45 min and 40.7 A/mm2 for 2 min respectively. The results of j (0) are shown in Fig. 3 and S they indicate that the evolution of j (0) with the S stress-relief treatment seems to be independent of the thermal history of the sample. Concluding, the results obtained on the induced anisotropy and magnetostriction j (0) clearly show S that the variation of j (0) is not related to the S
Fig. 2. Evolution of the magnetostriction at zero applied stress j(0) as a function of the annealing parameters of the stress annealed amorphous wire (p "825 MPa). !//
138
J.M. Blanco et al. / Journal of Magnetism and Magnetic Materials 186 (1998) 135—138
Acknowledgements This work has been supported by the Basque Country Government (PI95/78) and UPV (057.263EA075/96).
References
Fig. 3. T(1) stress annealing at 37.4 A/mm2 S.R. (1) stress relief at 38.7 A/mm2 for 45 min and S.R. (2) stress relief at 40.7 A/mm2 for 2 min.
magnitude of the stress applied during the annealing. In fact, this polarization effect should give rise to directional atomic rearrangements responsible for the induced magnetic anisotropy as was reported in Ref. [12]. The changes of the magnetostriction are assumed to be a consequence of the atomic rearrangements of topological character associated with structural relaxation processes. Consequently, the evolution of j (0) with the annealing S treatments should be mainly determined by the evolution of the microstructure to make the amorphous configuration more stable.
[1] J.M. Riveiro, V. Madurga, A. Hernando, Phys. Rev. B 39 (1989) 11950. [2] R.C. O’Handley, in: R.A. Levy, R. Hasegawa (Eds.), Amorphous Magnetism II, Plenum, New York, 1977, p. 379. [3] J. Furthmu¨ller, M. Fa¨hnle, G. Herzer, J. Phys. F 16 (1986) L255. [4] G. Herzer, Ann. Fis. B 86 (1990) 157. [5] A. Hernando, J. Gonza´lez, J.M. Blanco, M. Va´zquez, G. Rivero, J.M. Barandiaran, E. Ascasibar, Phys. Rev. B 46 (1992) 3401. [6] M. Fa¨hnle, J. Fu¨rthmu¨ller, R. Pawellek, T. Beuerle, Appl. Phys. Lett. 59 (1991) 2049. [7] B.W. Corb, R.C. O’Handley, J. Megusar, M.J. Grant, Phys. Rev. Lett. 51 (1983) 1386. [8] H. Szymczak, J. Magn. Magn. Mater. (1987) 227. [9] A. Hernando, C. Go´mez-Polo, E. Pulido, G. Rivero, M. Va´zquez, A. Garcı´ a-Escorial, J.M. Barandiara´n, Phys. Rev. B 42 (1990) 6471. [10] M. Va´zquez, J. Gonza´lez, A. Hernando, J. Magn. Magn. Mater. 53 (1986) 323. [11] J. Gonza´lez, K. Kulakowski, J. Magn. Magn. Mater. 82 (1989) 94. [12] J.M. Blanco, P.G. Barbo´n, J. Gonza´lez, C. Go´mez-Polo, M. Va´zquez, J. Magn. Magn. Mater. 104—107 (1992) 137.
Induced anisotropy and magnetostriction behaviour of an annealed Co—Fe (Co-rich) amorphous wire1 J.M. Blanco!,*, L. Domı´ nguez!, P. Aragone´ses!, J. Gonza´lez" ! Dpto de Fı& sica Aplicada I, Escuela Universitaria de Ingenierı& a Te& cnica e Industrial, Universidad del Paı& s Vasco, 20011 San Sebastia& n, Spain " Dpto de Fı& sica de Materiales, Facultad de Quı& micas, Universidad del Paı& s Vasco, P.O. Box 1072, 20080 San Sebastia& n, Spain Received 28 October 1997
Abstract We present results on the induced anisotropy and magnetostriction constant of a very low magnetostrictive amorphous wire after annealing with an applied stress (stress annealing) which can develop a transverse magnetic anisotropy followed by a subsequent annealing without the applied stress (stress relief) which can eliminate the transverse induced anisotropy. The evolution of the magnetostriction constant with these treatments has also been determined. It changes from a negative value (as-cast) to positive values after stress annealing, while the effect of the stress relief treatment changes the magnetostriction constant towards a saturation value independent of the thermal history of the sample. ( 1998 Elsevier Science B.V. All rights reserved. Keywords: Amorphous systems — wires; Anisotropy — induced; Magnetostriction constant
1. Introduction Some amorphous alloys are known to suitable systems for studies in view of their peculiar magnetostrictive behaviour. In particular, Co-rich amorphous alloys exhibit the following magnetic behaviour: (i) the macroscopic anisotropy is nearly zero as a result of random orientation of local easy axes, (ii) the saturation magnetostriction is
* Corresponding author. Fax: #34 43 47 1098; e-mail: [email protected]. 1 This paper was presented in part at ICM’97 in Cairns, Australia.
verysensitive to the chemical composition, heat treatment and applied mechanical stress and (iii) uniaxial magnetic anisotropy can be induced by stress and/or field annealing. Noticeable changes in the physical properties take place at an annealing temperature around 340°C indicating that some kind of structural transformation, similar to the martensitic first-order transformations observed in crystalline materials, takes place in Co-rich amorphous alloys [1]. However, O’Handley [2] and Furthmu¨ller et al. [3] recently emphasized the similarity found between the magnetostrictive behaviour of crystalline and amorphous ferromagnets which suggests that large local magnetic anisotropies are to be expected. In
0304-8853/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 9 7 ) 0 1 1 1 4 - 1
136
J.M. Blanco et al. / Journal of Magnetism and Magnetic Materials 186 (1998) 135—138
particular, Co-rich ‘non-magnetostrictive’ alloys present a peculiar magnetostrictive behaviour with j decreasing toward negative values as the applied stress increases, according to the expression j(p)"j(0)#Ap,
tention will be paid to the linear dependence of the saturation magnetostriction found recently in some Co-rich ribbons and wire shaped amorphous alloys [4,5].
(1)
where j(p) and j(0) are the saturation magnetostriction for an applied tensile stress p, and without applied stress, respectively, and A is a negative coefficient which ranges from !6 to !1]10~10 MPa~1 [4,5]. As a consequence, for those compositions with small and positive j(+10~7) at zerostress, a change in the sign of j takes place as the stress is increased. Different microscopic mechanisms account for the stress dependence of j. The microscopic model of Fa¨hnle et al. [6] correlates A with the second strain derivative of the local anisotropy coefficients which are determined by the local symmetry and chemistry. Therefore, only those thermal treatments which produce irreversible phase transformations in the local symmetry of the amorphous structure are expected to affect the subsequent value of A. It is to be noted that various hints of such type of transformations have been observed and reported for Co-rich metallic glasses [7]. The model developed by Szymczak [8] describes the stress dependence of j as a consequence of the bond orientational anisotropy induced by the stress. Therefore, the thermally activated processes are invoked to be responsible for the stress dependence of j. Moreover, it is expected that the action of the tensile stress at room temperature should be drastically affected by the strength and orientation of any bond anisotropy induced previously at higher temperature. Hernando et al. [9] have analyzed the behaviour of j(p) in amorphous ribbons taking into account the fluctuations of the local anisotropy and, therefore, of the local magnetostriction. In this model, the influence of the thermal treatments on the local magnetostriction fluctuations should be reflected in a similar influence on the coefficient A. In this work we present some results on the induced magnetic anisotropy and changes of the magnetostriction constant in a nearly non-magnetostriction amorphous wire subjected to stress annealing and stress-relief treatments. Special at-
2. Experimental technique Amorphous wire of nominal composition (Co Fe ) Si B was kindly supplied by 0.95 0.05 72.5 12.5 15 UNITIKA Ltd. Co. (Japan). Its diameter was 0.125 mm and it was cut to a length of 10 cm for magnetic measurements. Axial hysteresis loops of the samples were obtained by a conventional induction method at 50 Hz. Details can be found in Ref. [10]. Stress annealing and subsequent annealing without stress (stress relief) were performed by the so-called current annealing technique. The stress annealing can induce in the sample a macroscopic transverse magnetic anisotropy. This transverse induced magnetic anisotropy has been evaluated as the difference of the magnetization work of the stress annealed with respect to that of the as-quenched sample. The stress annealing was performed with an applied tensile stress of p "0, 400, 550, !// 825 and 1100 MPa at different current densities, while the stress relief treatment was carried out without tensile stress in samples which were previously stress annealed. Both kinds of thermal treatments were performed by the so-called current annealing technique [10]. The measurements of the magnetostriction constant, j, were performed using the small angle magnetization rotation (SAMR) method. As detailed elsewhere, this method makes use of the variations in the signal, » , picked up by 2u a narrow coil when applying simultaneously to the sample a saturating DC axial field, H , a small AC z transversal field, H , and a tensile stress p. The y experimental set-up described above was used to apply the DC field and the tensile stress, and to detect the induced signal. When a tensile stress, p, is applied, the induced signal » , measured by a lock-in amplifier, in2u creases or decreases as a consequence of the increment of magnetoelastic anisotropy having a transverse or an axial easy axis, respectively. This change in » can be compensated by an 2u
J.M. Blanco et al. / Journal of Magnetism and Magnetic Materials 186 (1998) 135—138
adequate modification of H so that » takes the z 2u same value as that existing before the stress was applied. In this way, the magnetostriction, j , is S obtained as j "!(k M /3)(*H /*p)» "cte, (2) S 0 S z 2u where M is the saturation magnetization (k M " S 0 S 0.8 T) of the wire.
3. Experimental results and their analysis The transverse-induced anisotropy as a function of the annealing time with the current density as a dependent parameter has previously been determined. These curves showed different behaviour depending on the electric current annealing range, as has been studied previously [10,11]. For low current annealing, the induced anisotropy increases monotonically with annealing time, with a faster rate as the current density increases, until it reaches a maximum value, at high current densities. After an initial increase, the anisotropy decreases with increasing annealing time. In both cases a maximum induced anisotropy, K.!9, can be obtained. The current density dependence of K.!9 is plotted in Fig. 1 with the stress applied during the treatment as a parameter showing a maximum around j "30.0 A/mm2. Similar behaviour for this kind !// of anisotropy was found in amorphous ribbons [10].
Fig. 1. Maximum anisotropy induced by annealing under various stresses in (Co Fe ) Si B amorphous wire. 0.95 0.05 72.5 12.5 15
137
The magnetostrictive behaviour of this amorphous wire is described by a linear stress dependence of the saturation magnetostriction where the stress derivative of j is always negative and quite S insensitive to both kinds of annealing treatments. For the as-cast state, the magnetostriction at zero applied stress, j (0), takes the value of !0.40]10~7. Upon S heat treatment with stress, this parameter is modified. Fig. 2 shows a typical evolution of j (0) as S a function of the annealing time for several isothermal treatments. This kind of treatment shifts j (0) S toward positive values, and a change of sign from negative to positive is then observed. When annealing around the Curie point ( j "34.6 A/mm2), the # maximum variation of j (0) is obtained. It should S be noted that the variation of j (0) does not depend S on the magnitude of the stress applied during the stress annealing treatment. Samples treated with j "0 and 550 MPa were !// subsequently subjected to two stress-relief treatments carried out by current annealing at 38.7 A/ mm2 for 45 min and 40.7 A/mm2 for 2 min respectively. The results of j (0) are shown in Fig. 3 and S they indicate that the evolution of j (0) with the S stress-relief treatment seems to be independent of the thermal history of the sample. Concluding, the results obtained on the induced anisotropy and magnetostriction j (0) clearly show S that the variation of j (0) is not related to the S
Fig. 2. Evolution of the magnetostriction at zero applied stress j(0) as a function of the annealing parameters of the stress annealed amorphous wire (p "825 MPa). !//
138
J.M. Blanco et al. / Journal of Magnetism and Magnetic Materials 186 (1998) 135—138
Acknowledgements This work has been supported by the Basque Country Government (PI95/78) and UPV (057.263EA075/96).
References
Fig. 3. T(1) stress annealing at 37.4 A/mm2 S.R. (1) stress relief at 38.7 A/mm2 for 45 min and S.R. (2) stress relief at 40.7 A/mm2 for 2 min.
magnitude of the stress applied during the annealing. In fact, this polarization effect should give rise to directional atomic rearrangements responsible for the induced magnetic anisotropy as was reported in Ref. [12]. The changes of the magnetostriction are assumed to be a consequence of the atomic rearrangements of topological character associated with structural relaxation processes. Consequently, the evolution of j (0) with the annealing S treatments should be mainly determined by the evolution of the microstructure to make the amorphous configuration more stable.
[1] J.M. Riveiro, V. Madurga, A. Hernando, Phys. Rev. B 39 (1989) 11950. [2] R.C. O’Handley, in: R.A. Levy, R. Hasegawa (Eds.), Amorphous Magnetism II, Plenum, New York, 1977, p. 379. [3] J. Furthmu¨ller, M. Fa¨hnle, G. Herzer, J. Phys. F 16 (1986) L255. [4] G. Herzer, Ann. Fis. B 86 (1990) 157. [5] A. Hernando, J. Gonza´lez, J.M. Blanco, M. Va´zquez, G. Rivero, J.M. Barandiaran, E. Ascasibar, Phys. Rev. B 46 (1992) 3401. [6] M. Fa¨hnle, J. Fu¨rthmu¨ller, R. Pawellek, T. Beuerle, Appl. Phys. Lett. 59 (1991) 2049. [7] B.W. Corb, R.C. O’Handley, J. Megusar, M.J. Grant, Phys. Rev. Lett. 51 (1983) 1386. [8] H. Szymczak, J. Magn. Magn. Mater. (1987) 227. [9] A. Hernando, C. Go´mez-Polo, E. Pulido, G. Rivero, M. Va´zquez, A. Garcı´ a-Escorial, J.M. Barandiara´n, Phys. Rev. B 42 (1990) 6471. [10] M. Va´zquez, J. Gonza´lez, A. Hernando, J. Magn. Magn. Mater. 53 (1986) 323. [11] J. Gonza´lez, K. Kulakowski, J. Magn. Magn. Mater. 82 (1989) 94. [12] J.M. Blanco, P.G. Barbo´n, J. Gonza´lez, C. Go´mez-Polo, M. Va´zquez, J. Magn. Magn. Mater. 104—107 (1992) 137.