Journal of Magnetism and Magnetic Materials 36 (1983) 81-85 81 North-Holland Publishing Company

SEPARATION INTO TWO CONTRIBUTIONS OF STRESS ANNEAL INDUCED MAGNETIC ANISOTROPY IN METALLIC GLASS RIBBONS

O.V. NIELSEN

Department of Electrophysics, The Technical University of Denmark, DK-2800 Lyngby, Denmark

Received 14 October 1982; in revised form 22 November 1982

Stress annealing experiments followed by stress relief experiments on metallic glass ribbons have made it possible quantitatively to separate the stress anneal induced magnetic anisotropy K u into two terms of opposite sign, Kan and Kpl. These terms, labelled anelastic and plastic, respectively, show very different dependences on the annealing conditions. The measurements of Ku of either sign were made possible by use of the small signal inverse Wiedemann effect in twisted ribbons as recently described.

1. Introduction

Whereas the magnetic anisotropy in metallic glasses induced by field annealing has been thor- oughly investigated [1] and seems to be well under- stood [2] after the appearance of continuous rib- bons, less attention has been given to the anisot- ropy induced by stress annealing.

Egami et al. [3] and Mizoguchi et al. [4] have reported improved remanence to saturation ratios and decreased coercivity for Fea0Ni40PlaB6 rib- bons stress annealed above 200°C. Similar im- provements for FesoPl3C 7 alloys have been re- ported by Fujimori and Masumoto [5], and excel- lent low coercivity at high flux levels has been obtained in Fe86B7C 7 by Hatta et al. [6]. The opposite effect was observed by Luborsky and Becker [7] in Feal.sB14.sSi4 samples stress annealed at 331°C. Kisdi-Koszb et al. [8] observed a change of sign near the eutectic composition of the stress anneal induced anisotropy for a series of Fe -B alloys. A large alloying effect on the stress anneal induced anisotropy has been found by Novak et al. [9] in the Fes0T3B17 system, where T is a transition metal. Hilzinger [10] has studied the reversible behavior of stress anneal induced anisot- ropy in a zero magnetostr ic t ive material Co66Fe4Mo2Si16B12. A complicated induced ani-

sotropy dependence on stress annealing time and temperature has been reported [11,12] to occur in

Co73Mo2Si l sB10 and (Co0.89Fe0.11)v2Mo3SilsB10 alloys, and the compositional dependence has been studied [13] in the (Fe, CO)TsSilsB10 system.

In contrast to field induced anisotropy of which the easy axis with a few exceptions [8] is parallel to the annealing field, the easy axis induced by stress annealing may be parallel or perpendicular to the tensile stress annealing axis. The direction depends [11-13] on the annealing temperatures and on the duraction of pre-annealing, stress annealing and subsequent stress relaxation. In previous papers [ 11,12] concerning (Coo.89Feo.l 1)72M°3 Sil5 BI0 rib- bons it was proposed that the stress induced ani- sotropy K u may be composed of at least two contributions of opposite sign

Ku = Kan + gpl, (1)

where Kan > 0 originates from an anelastic and Kpl < 0 from a plastic deformation (strain) of the ribbon. The validity of this assumption has re- cently [13] been demonstrated to cover the whole range (0 _< x ___ 0.9) for a series of (CoxFel_x)75 Si15B10 ribbons.

At the time when the first series of experiments in our labora tory were carried out with (Co0.89Fe0.11)72Mo3SilsBi0 ribbons, only the hard

0304-8853/83/0000-0000/$03.00 © 1983 North-Holland

82 O.V. Nielsen / Stress anneal experiments on metallic glass ribbons

r ibbon axis anisotropy could be measured (by conventional methods). In the meantime we have developed a method of measurements which ena- bles a determination also of the easy ribbon axis anisotropy for positive magnetostrictive materials. This method, originally described in ref. [14] and improved in ref. [13], is based on the small signal inverse Wiedemann effect in a twisted ribbon. By this method the anisotropy energy is measured directly as an equivalent internal stress %. The purpose of the present work is to extent the above mentioned experiments on (Coo.89Fe0.11)72Mo 3 SilsBlo wth special attention to the magnitude of the induced easy ribbon axis anisotropy.

2. Equiva lent anisotropy parameters

Defining the anisotropy constant K u from the expression for the magnetic anisotropy contribu- tion F u to the free energy during a coherent rota- tion of the magnetization in the ribbon plane we may write

F u = K . cos2@, (2)

where @ is the angle between the magnetization and the ribbon axis. If Ku is positive its magnitude can be determined from the ribbon axis magneti- zation curve

K u = f o L H d J , (3)

where H is the magnetic field along the ribbon axis, J is the r ibbon axis polarization and Js is the saturation value of J.

Equivalent anisotropy parameters are

H u = 2 1 g u l / J ~, (4)

where H u is the magnetic field perpendicular to the easy axis necessary to saturate the material along the field, or

°u = - ~ K , / X s , ( 5 )

where % is a uniaxial stress along the ribbon axis which via the magnetoelastic coupling produces an anisotropy of magnitude Ku. h s is the saturation magnetostriction coefficient. In analogy to eq. (1)

we write o u as a sum of two terms

O" u = (7an + Orpl. ( 6 )

Because ou-values are measured directly, the re- ported values of K u and H u are calculated frol~ eqs. (5) and (4), respectively, using X s - 3.1 × 10 -~ and J~=0.75 T [15]. While o u and Ku may be positive as well as negative it is meaningless ot course to distinguish between positive and nega- tive Hu-values.

3. Exper imenta l results

All the annealing experiments were performed at 380°C, which temperature is considered suffi- ciently high to ensure the development of viscoe- lastic strains within reasonable times, and suffi- ciently far below the crystallization temperatur~ ( = 525°C) to avoid crystallization in long tim~ ( = 100 h) annealing experiments.

100 ~ ] 15 • - 0 l .

t r e L = lh ( b )

10 50 • •

-0.2 K p L ~ 5

n ~ e ~: 0 .~ - o \ 5;o Io6o Isb( 2

b= O'on n (MPo) v

[).2 -50 I0

K an + Kpl D .4 -100 - 15

Tclnn = 380"C t p r e = lh ~ O.6 - 20

-150 • to" = lh o ~ 25

08

Fig. 1. (a) Induced anisotropy vs. applied stress during stress, annealing [12]. (b) The ribbons from (a) stress relieved 1 h at 380°C.

O.V. Nielsen / Stress anneal experiments on metallic glass ribbons 83

50

0

==0

-50

-100

-150

i i I

~ ( b )

tre I =lh o - ~

I I I

0.1 1 10 10(] tpre (h)

(a) (a)

Kpl

I I I

"0.2 5

% _

5 0.2

10

0.4 15

0.6 - 20

Fig. 2. (a) Induced anisotropy vs. pre-annealing time [12]. (b) The ribbons from (a) stress relieved 1 h at 380°C.

Keeping the annealing temperature fixed on this value a series of r ibbons were pre-annealed (stabilized) 1 h ( = tpre) after which time annealing stresses oa, n of varying magni tudes were applied during the stress annealing period of length t o =

1 h. The r ibbons were cooled down to 100°C before the stresses were unloaded. Fig. 1 a shows a linear dependence on o~,, of the obtained hard r ibbon axis anisotropy, which we believe consists of compet ing contr ibut ions K~, and Kpv

In order to eliminate Kan these samples were reheated without applied stress to 380°C for a period of tr¢ j = 1 h (stress relief). Fig. lb shows a roughly linear dependence on o~, n of the hereby obta ined easy axis anisotropy which we believe consists solely of Kpv

The effect on K u of the pre-annealing time tpr ~ can be seen in fig. 2 which shows K u obtained for pre-annealed samples exposed to 1 h stress anneal- ing with applied stress o~. n = 630 MPa. Fig. 2a (Kan + Kpl ) shows a hard r ibbon axis anisotropy increasing in magni tude with lpre" Elimination of K~. by reliefing the r ibbons 1 h at 380°C reveals an easy r ibbon axis anisotropy Kpl which d e -

creases in magni tude with increasing tpr e (fig. 2b). In a similar way we studied the effect of stress

annealing time t o on the induced anisotropies. Fig. 3a (K~. + Kp~) shows that the hard r ibbon axis

150:

100

50

~ 0

- 50

-100

i I i I

1 / t tel =lh ° ( b /

/ .

o,1 l lO, IO~ t~ (h]

Ton n = 380 *C f O'on n • 630 MPo , ~ t pre = lh / Kan, Kpl

. / o.........e. , r i o )

I I I I

-0.6-

-0.~

-0.2

E

v

0.2

0.4

20

15

10

5

5

15

Fig. 3. (a) Induced anisotropy vs. stress annealing time [12]. (b) The ribbons from (a) stress relieved 1 h at 380°C.

aniso t ropy decreases in magni tude with increasing t o, whereas the easy axis anisotropy Kpl isolated by stress relief increases with t o .

While the magni tude of Kpl (and Opl ) can be read directly f rom figs. 2b and 3b, values of K , , (and %.) can be calculated as the differences

0 fl_

b

0 , 0

-50

Tan n = 380 °C o tpr e • t o- tre I = lh

0.2

0.4 o o

-100

0.6

-150 • ~ ° ° ° I I

0.1 I 10 100 tpr e or tcr ( h )

Fig. 4. "Anelastic" stress induced anisotropy. Difference be- tween (a) and (b) from figs. 2 and 3.

84 O. V. Nielsen / Stress anneal experiments on metallic glass ribbons

f t . . ~r

150

100

50

Tann= 380 *C tpr e = lh

t(r= 71h

0 I 1 0 10 20

tre I (h)

-I-

"0.6 20

= 15 -0.~; l j ,0 -0.2

5

J 0 0 30

Fig. 5. Thermal stability of "plastic" stress induced anisotropy in ribbon r from fig. 3.

between Ku (and o u)-values measured before and after stress relief. The results are shown in fig. 4 from which we conclude that the anelastic anisot- ropy dependence on tpr e and t o are the same.

Hitherto we have assumed that the stress relief time tre ~ = 1 h has been of sufficient length to ensure a complete anelastic relaxation (cancella- tion of/Can). In order to control this, we exposed r ibbon r from fig. 3 to successive relief treatments. Ku as a function of tre I is shown in fig. 5. A constant value of Kpl (negative) combined with a proceeding relaxation of Kan (positive) would give rise to values of IKul = [Kpl + K ~ , I i n c r e a s i n g with t~e I. As seen from fig. 5 IKul d e c r e a s e s with tre 1, indicating that K,n as assumed has fully relaxed within the first hour of stress relief treatment whereas a constant level of Kpl needs a prolonged treatment. We therefore assume that fig. 5 shows the relaxation behavior of KpI.

Finally, it may be mentioned that the magneti- zation curves for the ribbons show an almost ideal behavior. Ribbons with positive K , have sloped magnetization curves, most of them with a con- stant slope (susceptibility) nearly up to saturation. The magnetization curves for ribbons with nega- tive K , are very square shaped with low coercivity, e.g. ribbon r from fig. 3b ( H ~ = 23 Oe) had a remanence to saturation (H~ppl = 30 Oe) ratio as high as 0.98 and a coercivity of only 20 mOe.

4. Discussion

In this section we discuss the main characteris- tics of the stress induced anisotropy on the basis of the viscoelastic properties of the present material [12] and of related materials [16]. Striking similari- ties have recently [ 13] been described in detail. For details about the viscoelastic behavior of metallic glasses see e.g. refs. [17,18].

From fig. 1 it is evident that K u = gan--I-Kpl is proportional to the applied annealing stress. The proportionality for Kpl is less convincing but still very likely in view of the large shift of K u (com- pared to the magnitude of Kpl) during the relaxa- tion process. Thus, we conclude that Ka, as well as Kpl are proportional to Oan n. A similar behavior has been observed for the anelastic and the plastic strains (,n and ( pl, respectively [ 12,16] (Newtonian flow).

Previously [12], the increase in magnitude of K u with increasing pre-annealing time (fig. 2a) was attributed to a numerical decrease of Kpl due to the increasing stabilization of the amorphous state. This assumption is in fact confirmed by the relaxa- tion experiments leading to fig. 2b.

In an analogous way the decrease in magnitude of Ku with increasing stress annealing time (fig. 3a) was attributed to a numerical increase of Kpl due to the evolution of the plastic strain. This assumption is confirmed as well by the relaxation experiments leading to fig. 3b.

The magnitude of K~n was expected indepen- dent of the pre-annealing time (compare the be- havior of e~,, for FesoPi3C 7 [16]) but fig. 4 reveals an increase of about 75% as the pre-annealing time increases from 0.1 to 100 h. Note, however, that the dependence of K~n on t o is the same, which means that the magnitude of Kan mainly is de- termined by the thermal history. This is in sharp contrast to the behavior of Kpl , compare fig. 4 with figs. 2b and 3b.

As concerns the thermal stability of Kpl, a t the temperature at which the ribbons have been treated, fig. 5 indicates a relaxation of the initial value towards a lower stable value. A logarithmic plot does not fit a simple relaxation process to- wards zero.

At present we have no detailed explanation fo~

O.V. Nielsen / Stress anneal experiments on metallic glass ribbons 85

the observed stress anneal proper t ies . A s imple macroscop ic mode l based on the fo rma t ion of a surface layer giving compress ive or tensile bu lk stresses th rough a di f ference of the the rmal elon- ga t ions can be ru led out because of the large stresses required. A change of the a tomic o rder ing pa r ame te r s as recent ly p r o p o s e d [19] seems more adequate . Fo l lowing [19] the anelas t ic pa r t Kan of K u could be associa ted with shor t - range reversible a tomic d i sp lacements and changes in shor t - range o rde r at low tempera tures , whereas the p las t ic pa r t Kpl could be associa ted with long-range irreversi- b le a tomic mo t ions and changes in shor t - range o rde r at high tempera tures . This exp lana t ion agrees wi th the recoverabi l i ty of Kan and with the pa r t i a l recoverabi l i ty and par t i a l pe rmanence of Kpl. Fu r - ther exper iments , especial ly concern ing the tem- pe ra tu re dependences of the induced anisot ropies , m a y be benefic ia l for a be t te r unders tand ing . Such exper iments have previous ly [20] p roved powerfu l in explan ing the origin of as -quenched an i so t ropy and of field induced an iso t ropy .

5. Conclusion

The present work suppor t s the previous ly pro- posed hypothes is , that the magne t ic an i so t ropy induced in meta l l ic glass r ibbons by stress anneal - ing is closely re la ted to their viscoelast ic p rope r - ties. The an i so t ropy m a y or ig inate pa r t ly f rom reversible changes of the a tomic shor t - range o rde r and par t ly f rom irrevers ible long-range mot ions .

Acknowledgement

The au thor is gra teful to Prof. V. F r a n k for his con t inued interes t and for his careful read ing of the manuscr ip t .

References

[1] F.E. Luborsky, in: Ferromagnetic Materials, Vol. I, Chap. 6, ed. E.P. Wohlfarth (North-Holland, Amsterdam, 1980).

[2] F.E. Luborsky and J.L. Walter, IEEE Trans. Magn. MAG- 13 (1977) 953.

[3] T. Egami, P.J. Flanders and D.C. Graham, Jr., Appl. Phys. Lett. 26 (1975) 128; and AlP Conf. Proc. No. 24 (1975) 697.

[4] T. Mizoguchi, S. Hatta, H. Kato, H. Arai, K. Maeda and N. Akutsu, IEEE Trans. Magn. MAG-16 (1980) 1147.

[5] H. Fujimori and T. Masumoto, Trans. Japan. Inst. Met. 17 (1976) 175.

[6] S. Hatta, T. Egami and C.D. Graham, Jr., Appl. Phys. Lett. 34 (1979) 113.

[7] F.E. Luborsky and J.J. Becker, IEEE Trans. Magn. MAG- 15 (1979) 1939.

[8] I~. Kisdi-Koszb, L. Potock~¢ and L. Novhk, J. Magn. Magn. Mat. 15-18 (1980) 1383.

[9] L. Novak, L. Potock~,, S. Uli~iansky, 1~. Kisdi-Kosz6, A. Lovas, J. Takhcs and R. Ml~'nek, J. Magn. Magn. Mat. 26 (1982) 118.

[10] H.R. Hilzinger, Proc. 4th Intern. Conf. on Rapidly Quenched Metals, Vol. II (The Japan. Inst. Met., 1982) p. 791.

[11] O.V. Nielsen and H.J,V. Nielsen, J. Magn. Magn. Mat. 22 (1980) 21.

[12] O.V. Nielsen, H.J.V. Nielsen, T. Masumoto and H.M. Kimura, J. Magn. Magn. Mat. 24 (1981) 88.

[13] O.V. Nielsen, L.K. Hansen, A. Hernando and V. Madurga, J. Magn. Magn. Mat. 36 (1983) 73.

[14] O.V. Nielsen, J. Magn. Magn. Mat. 24 (1981) 81. [15] O.V. Nielsen and H.J.V. Nielsen, Solid State Commun. 35

(1980) 281. [16] H. Kimura, T. Murata and T. Masnmoto, Sci. Rep. RITU

A26 (1977) 270. [17] A.I. Taub and F.E. Luborsky, Acta Metal1. 29 (1981) 1939. [18] A.L. Greer and F. Spaepen, Ann. New York Acad. Sci.

371 (1981) 953. [19] H. Warlimont and H.R. Hilzinger, Proc. 4th Intern. Conf.

on Rapidly Quenched Metals, Vol. II (The Japan. Inst. Met., 1982) p. 1167.

[20] T. Egami and P.J. Flanders, AIP Conf. Proc. No. 29 (1976) 220.