Stress dependence of magnetostriction in amorphous ferromagnets: its variation with temperature and induced anisotropy

Journal of Magnetism and Magnetic Materials 114 (1992) 7.5-81 North-Holland

Stress dependence of magnetostriction in amorphous ferromagnets: its variation with temperature and induced anisotropy J. GonzCilez a, J.M. Blanc0 a, A. Hernando d, J.M. Barandiarin and G. River0 d a Dpto. Fisica de Materiales, Univ. Pais Vasco, San Sibasticin, Spain

b, M. VSlzquez ’

b Dpto. Electricidad y Electrhica, Univ. Pak Vasco, Lejona, Spain ’ Institute de Ciencia de Materiales, CSIC, Madrid, Spain d Institute de Magnetism0 Aplicado, Lab. “Salvador Velayos”, Univ. Complutense, Las Rotas ‘155, Madrid 28230, Spain

Received 25 October 1991; in revised form 24 December 1991

In this work we report magnetostriction measurements obtained in a Co-rich amorphous alloy in which macroscopic anisotropies of different easy axis directions and strengths were induced. Particular interest was focused on the determination of the thermal evolution of the stress dependence of the magnetostriction as well as the influence of the anisotropy on such evolution. Although the results are within the experimental accuracy limit some conclusions have been drawn. In particular, the influence of the easy magnetization direction on the thermal variation of the stress dependence at low temperature seems clear from the results we show here. Also we report for the first time the appearance of a decrease in the absolute value of the stress derivative of the magnetostriction after thermal treatment.

1. Introduction

Magnetic anisotropy can be induced in amorphous ferromagnets alloys by field [l], stress [2] and more recently by stress and field annealing [3,4]. The microscopic mechanisms responsible of these anisotropies are not well known, especially those concerning the anisotropy induced by stress and/or stress and field. Readers are referred to a recent review on these subjects [5]. On the other hand, Herzer [6], Barandiaran et al. [7] and Lachowicz and Siemko [8] reported the stress dependence of the magnetostriction constant. Furthermore, it was reported that in the compositions with small magnetostriction (I lo-‘), which is positive at zero applied stress, its

Correspondence to: Prof. A. Hernando, Instituto de Magnetismo, Lab. Salvador Velayos, Universidad Complutense de Madrid, Apdo. 155, 28230 Las Rozas (Madrid), Spain.

sign evolved from positive to negative as the magnitude of stress increased. Different authors (see ref. [5] and references therein) have performed similar studies in order to get a better understanding of this phenomenon. Most of the data reported up to now have been obtained in Co-rich compounds for which the saturation magnetostriction is low enough to allow the experimental observation of its stress dependence through magnetoelastic methods [7]. The experimental values seem likely to follow the law A(a) =h(O) -Aa,

(1)

where h(O) is the saturation magnetostriction constant when the applied stress (+ is zero and A is a positive coefficient experimentahy determined and ranges from 1 to 6 X lo-la MPa-‘. Different interpretations have been suggested for explaining the origin of the parameter A. Herzer [91 has recently shown that previous an-

0304~8853/92/$05.00 0 1992 - Elsevier Science Publishers B.V. All rights reserved

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J. Gonzdez et al. / Magnetostriction in amorphous ferromagnets

nealing treatments do not remarkably affect the strength of A. Measurements carried out in low magnetostrictive amorphous wires lead some of the authors to conclude also that there is not a noticeable influence of the annealing on A [lo]. However, some of the models invoked to account for the stress dependence of A lead one to expect quantitative changes of the phenomenon after annealing [ill. In order to increase our understanding of the stress dependence of A we have measured the thermal evolution of A after subjecting the samples to thermal annealing under different conditions.

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-

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-

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-

\ \ I \ \CRIST&LIZATION I I I

I

TPC) 0

100

200

300

200

\ 500

Fig. 1. Saturation magnetization as a function of the temperature. The onset of crystallization has been shown.

2. Experimental procedure Glassy (Co,,,Fe,,),,Si,,B,, ribbons were prepared by melt spinning onto a copper wheel about 40 m s-l surface speed. The sample were 0.5 mm wide, 25 pm thick and 8 cm long. Five different thermal treatments have been applied in order to induce magnetic anisotropy, namely: transverse field annealing (TFA), longitudinal field annealing (LFA), stress annealing (SA), stress plus transverse field annealing (STFA) and stress plus longitudinal field annealing (SLFA). The magnetic field applied during the thermal treatment was 60 Ge when applied longitudinally and 4599 Qe ybep applied transversely. These values are both high enough to saturate the sample along the magnetic field direction. The tensile stress applied longitudinally during the SA, STFA and SLFA processes was 400 MPa. In all cases, the annealing temperature was 340°C and the annealing time 5 h. These annealing conditions were chosen to reach the maximum anisotropy according to previous results [4,12]. The heating was achieved by means of the current-annealing technique. Basically, it consists of flowing an electric current along the sample which produces .an increase of temperature by the Joule effect. Details on this heating method can be found in ref. [13]. The induced anisotropy strength, K, reached after annealing had the following values: K(TFA) = 380 J rnm3, K(LFA) = -290 J rne3, K(SA) =

400 J me3, KWFA) = 535 J me3 and K(SLFA1 = -390 J rnm3, where the positive values indicate that the easy axis of magnetization is transverse to the ribbon axis, while the negative values correspond to the easy magnetization direction along the ribbon axis. The spontaneous magnetization of the sample at room temperature, p&f&T = ZO’C), was found to be 1.21 T. The thermal dependence of the magnetization was studied in a furnace (fig. I), and these results were used to construct the correspondence between the intensity of the heating electrical current and the temperature T, after deteqining the dependence of the spontaneous magnetization on the curewt. We crptallization temperature for this composition, 4tWC, is lower than the Curie temperature of the amorphous state. The experimental study we have carried out basically consists in determining the thermal variation of the stress dependence of the magnetostriction through the measurement of the parameter A as a function of T. Therefore, the method we have used can be summarized as follows: (i> the temperature is fixed by keeping constant the intensity of the ac current flowing along the sample; (ii> by applying a longitudinal field we reach the saturation, which allows us to determine the temperature, from results shown in fig. 1, and also to fix a magnetization value, which generally was 1 T for temperatures below 320°C

et al. / Magnetostriction in amorphous ferromagnets

.I. Godlez

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J. Gondez

0.2

et al. / Magnetostriction

in amorphous ferromagnets

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Fig. 3. Thermal evolution of the magnetostriction

constant at zero stress, MO), for as cast, LFA, SA and TFA samples.

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Fig. 4. The coefficient A is plotted as a function of the temperature for as-cast, SA, TFA, LFA, STFA and SLFA samples.

J. Gondez

et al. / Magnetostriction in amorphous ferromagnets

and 0.5 T for higher temperatures, both values corresponding to the region above the knee of the magnetization curve; (iii) once the initial longitudinal field HZ and the magnetization values have been selected and fixed, different tensile stress strengths (+ are applied and then the longitudinal field is changed to keep constant the magnetization; (iv) the experimental HZ-a curves corresponding to M and T constants are fitted to the parabolic expression H,(a)

=H,(O)

--A’VB’&

(2)

where A’ and B’ are constants. The magnetostriction A can then be obtained as A(@>T) = - (/-GW)/3)(dWd& = h(0, T) -A(T

3. Experimental

(3)

results

To illustrate the procedure used for all samples we have plotted in fig. 2 the HZ-a curves, for M = constant, corresponding to the sample subjected to LFA for temperatures T = 20, 30, 80,220,240,3OOC (M = 1 T), and 330°C (M = 0.5 T). Every curve has been fitted to the expression given by eq. (2) and from this fitting we obtain for each T, through the expression (3), A as well as its stress dependence, and therefore the thermal dependence of the parameter A. The evolution of A(O) with temperature, after the thermal treatments, is shown in fig. 3 for samples as quenched and after SA, TFA and ’ LFA. Fig. 4 summarizes the experimental values of A as a function of T for samples subjected to the five different annealing treatments mentioned above. The error bar, due to the measurement accuracy, can be considered to be 0.3 X lo-” MPa-‘.

4. Discussion The results shown in fig. 4 indicate that after the annealing process the value of A, at room

79

temperature, has decreased from its absolute value with respect to the A value of the as quenched sample. In particular, A(20”C), in units of lo-” MPa-‘, is -3.5 for the sample as cast, and after the annealing treatments which induce a transverse easy axis evolves to - 1.3 for SA, - 0.8 for TFA and - 1.1 for STFA. After performing those annealing processes which lead to a longitudinal easy axis a similar decrease is observed; A(20”C), expressed in the same units, becomes - 2.5 for SLFA and - 1.1 for LFA. Therefore, we have observed a correlation between neither the direction nor the strength of the anisotry and the decrease of A. However, the general trend shown by A to decrease after the induction of the anisotropy seems to be clear in spite of the measurement accuracy. This result might seem to contradict what was reported by Herzer [9] and also some recent results in amorphous wires reported by some of us [lo]. In order to clarify such apparent contradiction we have repeated the measurements at room temperature in another batch of the same composition and have obtained the following values: A(O)= - 4 X lo-’ and A = - 1.6 x lo-” for the as-cast sample; after SA at 340°C for 3 h these values change, using the same units, to A(O)= - 2.8 and A = - 1.1; and finally after 6 h of SA at the same temperature the values become A(O)= - 2.1 and A = -0.6. Nevertheless, it must be emphasized that in the experiments published here we have used a higher annealing temperature and a different composition and, furthermore, we have induced anisotropy, i.e. macroscopic polarization of local axes, while Herzer’s report only dealt with the influence of annealing on A. In the case of amorphous wires, the thermal treatments also produced induction of anisotropy; however, no changes in A were observed. It is noticeable that the decrease in the absolute value of A(20”C) observed after annealing roughly corresponds to the decrease observed in the absolute value of A(0, 2O”C), which according to the results plotted in fig. 3 evolves from - 6 x lo-’ in the sample as cast to approximately -2 X lo-’ in the annealed samples. Therefore, the change of A and A produced by the induction of

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J. Gonzrilez et al. / Magnetosfriction in amorphous ferromagnets

the anisotropies results for both properties in a decrease in an absolute value by a factor of l/3 approximately. The same behavior is observed for the results obtained in the other batch of the same composition as indicated above, although in this case a factor closer to l/2 was obtained. As the measurement of A requires the application of high tensile stresses the interpretation of the results corresponding to the thermal dependence of A must be carried out with caution. The previous studies performed on the kinetics and characteristics of the stress induced anisotropy have clearly shown that two components of the anisotropy, closely related to the anelastic and plastic strain, coexist. The anelastic anisotropy is reversible or recoverable as being due to processes thermally activated at equilibrium. On the other hand, the plastic anisotropy is permanent, and is originated by irreversible processes activated during the structural relaxation [14]. The kinetic studies thoroughly performed by Nielsen et al. in similar compositions to that used in this report [15] have shown an onset of the induction of anelastic anisotropy at temperatures of about 250°C approximately, for usual annealing times (fan = 1 h). Finally, it must be considered that as the annealing temperature was 340°C any subsequent annealing at temperatures close to below or above 340°C should produce structural relaxation and irreversible changes that, when biased by the stress, would induce plastic anisotropy. The possible induction of such anisotropies can mask the effect of the temperature on the parameter A. It must also be taken into account that in the high temperature range,i.e. from 320 to 36O”C, the thermal variation of the magnetostriction shown in fig. 3 exhibits compensation temperatures. Therefore, we can distinguish three temperature ranges: (i) between room temperature and 250°C the anisotropy induced by the stress on the sample is similar to that induced at room temperature, which originates the stress dependence of h, (ii) between 250 and 310°C the processes which produce the anelastic anisotropy can be activated during the measurement time, thus producing a new component of the anisotropy which would affect the A determination and (iii) the range of

temperatures between 320 and 380°C (onset of crystallization), where compensation temperatures for A(O)occur and the plastic anisotropy can also be induced during the measurement process. In range (i) we are far from the temperature for which equilibrium or nonequilibrium processes are activated in short times (measurement time). On the other hand, the thermal evolution of MO, T) is smooth for the temperatures well below that of compensation, as shown in fig. 3. By taking into account the error bar it is possible to distinguish two opposite types of behavior. For the as cast sample as well as for the samples with the longitudinal easy axis, the bsolute value of A increases as the temperature ri \ es and it exhibits a maximum at 170°C for the as cast and SLFA samples and at 250°C for the LFA sample. The samples with the transverse easy axis do not exhibit a similar behavior or at least it is not so clear. If we define a variation percentage in the range from 20 to 250°C as: * = (A(20”C) -A( maximum)) /A ( 20°C))

(4)

where A(maximum) is the maximum absolute value of A within this temperature range, we obtain for samples with the macroscopic easy axis parallel to the ribon axis the following # values: 40% as cast; 67% LFA and 25% SLFA and for samples with the transverse easy axis, 8% SA, 12% TFA and 9% STFA. The analysis of the results leads to the conclusion that the variation of A in the range (i> is smaller for samples with the easy axis perpendicular to the ribbon axis. A similar trend is observed in fig. 3, where it is shown that between 20 and 250°C A(0, T) of the LFA sample decreases by 95% in absolute value while the corresponding decrease for the SFA and TFA samples is roughly 40%. Within range (ii) the absolute value of A decreases with a high slope for all samples. Notice that a corresponding enhancement of the rate of decrease can also be observed for A(O) in fig. 3. In the low limit of range (iii) the absolute value of A exhibits a minimum which is only not clear in the SLFA sample. The important question is whether there exists some correlation between this minimum and the compensation tem-

J. Gonzrilez et al. / Magnetostriction in amorphous ferromagnets

perature of A(O). For the as cast sample the minimum occurs at 37O”C, which is roughly its compensation temperature, 365”C, as illustrated in fig. 3. The compensation temperature for the SA and TFA samples is 340°C while the minimum occurs at 330 and 31O”C, respectively. However, for the LFA sample the compensation temperature is 330°C which coincides with the temperature for which the minimum takes place. Within the experimental error it is possible, with caution, to conclude that there exists a correlation between the minimum in A and the compensation temperature of A. Nevertheless, the minimum also may be due to the onset of plastic anisotropy superimposed on the thermal decrease of the absolute value of A. In the upper limit of range (iii), the curves shown in fig. 4 clearly indicate that the thermal dependence of A undergoes anomalous variations for the temperature range between the minimum to 380°C. This reflects the induction of plastic anisotropy and the influence of irreversible structural changes at the onset of crystallization.

5. Conclusions In the present report we have shown the thermal variation of the stress dependence of the magnetostriction for a nearly zero magnetostriction amorphous alloy, in which macroscopic anisotropies were induced by combining different agents. For the low temperature range some quantitative and qualitative differences in the behavior of A between samples with longitudinal and transverse easy axes were observed. Some correlation between the compensation temperature of the magnetostriction at zero stress and the temperature for which a minimum in the absolute value of the parameter A occurs must not be discarded. The decrease in the absolute value of MO, 20°C) produced by the thermal treatments is yghly proportional to the decrease in the absolute value of A at 20°C. This result seems to be in contradiction with previous results reported by Herzer 191 and by some of the authors of this

81

work in a previous study on amorphous wires [lo]. By this reason we have checked this behavior in samples of another batch with the same composition and a similar decrease in the absolute value of A was observed. The difficulties found in performing these measurements lead to considerable experimental error. It should be desirable, however, to perform a similar systematic study in a composition with lower compensation temperatures in order to eliminate the unavoidable activation of processes at high temperature which mask the thermal equilibrium evolution of these properties. Furthermore, the change observed in A after annealing, which has never been observed before, must be carefully studied in other compositions with different annealing conditions.

References 111 F.E. Luborsky and J.L. Walker, IEEE Trans. Magn. MAG-13 (1977) 953. [2] O.V. Nielsen and H.J.V. Nielsen, J. Magn. Magn. Mater. 22 (1980121. 131 M. Vazquez, E. Ascasibar, A. Hernando and O.V. Nielsen, J. Magn. Magn. Mater. 66 (1987) 37. [41 J. Gonzalez and k. Kulakowski, J. Magn. Magn. Mater. 82 (1989) 94. 151 A. Hernando, M. Vazquez, G. River0 and J.M.Barandiarln, J. Magn. Magn. Mater. 101 (1991) 6. lb1 G. Herzer, Proc. SMM 7 Conf., Blackpool. 1985 (Wolfson Centre, Cardiff, 1986) p. 335. 171 J.M. Barandiarin, A. Hernando, V. Madurga, O.V. Nielsen, M. V&zquez and M. V&zquez Lopez, Phys. Rev. B35 (1987) 5066. @I A. Siemko, H.K. Lachowicz and B. Lisowski, Acta Phys. Pol. A72 (1987) 197. [91 G. Herzer, Ann. Fis. B86 (19901 157. HOI J.M. Blanco, P.G. Barb&, J. Gonzalez, C. Gomez-Polo and M. Vazquez, J. Magn. Magn. Mater. 104-107 (1992) 137. Dll H. Szymczak and H.K. Lachowicz, IEEE Trans. Magn. MAG-24 (1988) 1747. WI J. Gonzalez, J.M. Blanco, J.M.BarandiarLn, M. V&zquez and A. Hernando, in: Physics of Magnetic Materials, eds. W. Gorzkowski, M. Gulowski, H.K. Lachowicz and H. Szymczak (World Scientific, Singapore, 1991) p. 354. 1131 M. Vbzquez, J. Gonzalez and A. Hernando, J. Magn. Magn. Mater. 53 (1986) 323. 1141 O.V. Nielsen, L.K. Hansen, A. Hemando and V. Madurga, J. Magn. Magn. Mater. 36 (1983) 73. 1151 O.V. Nielsen, IEEE Trans. Magn. MAG-21(1985) 20008.