MAGNETIC AFTER - EFFECT IN Co95-xSi5Bx METALLIC GLASSES

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MAGNETIC AFTER - EFFECT IN Co95-xSi5Bx METALLIC GLASSES

H. Wencek, W . Ciurzy_ska, J. Moro_, B. Wys[math]ocki, S. Szymura, A.

Ca[math]ka

To cite this version:

H. Wencek, W . Ciurzy_ska, J. Moro_, B. Wys[math]ocki, S. Szymura, et al.. MAGNETIC AFTER - EFFECT IN Co95-xSi5Bx METALLIC GLASSES. Journal de Physique Colloques, 1983, 44 (C9), pp.C9-145-C9-150. �10.1051/jphyscol:1983917�. �jpa-00223363�

JOURNAL DE PHYSIQUE

Colloque C9, supplbment au n012, Tome 44, dkcembre 1983 page C9-145

MAGNETIC AFTER - ^EFFECT I N C O ~ ~ - METALLIC GLASSES ~ S ~ ~ B ~

H . Wencek, W . ~ i u r z ~ i i s k a * , J.W. Morod, B . wys20ckir, S . Szymurar and A. ca.l.karr

I n s t i t u t e o f Physics and Chemistry o f Metals, S i l e s i a n U n i v e r s i t y , Bankom 22, 40-007 Katowice, Poland

* ~ n s t i t u t e of Physics, Technical University o f Czqstochowa, Zawadzkiego 29, 42-200 Czqstochowa, Poland

" I n s t i t u t e of Materials Science and Engineering, Warsaw Technical U n i v e r s i t y , Narbutta 85, 02-524 Warszawa, Poland

~ e s u m e

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abilite magnetique dans les alliages amorphes C O ~ ~ - /x=15, ~ S ~ ~ B ~ 19,22/ existant dans l'intervalle de temperatures de 200-420K.

Elle se compose de deux parties se recouvrant qui se comwortent diffkremment pendant le recuit dans 1' intervalle 420-470K.

Abstract

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I

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In ferromagnetic alloys there occur magnetic instabilities usually associated with migrational relaxation processes and known as magnetic after-effects.One of the instabilities is the magnetic ~ermeabilitv disaccommodation ~ ( t ) appearing in weak magnetic fields [l].In numerous

papers devoted to this subject / F ~ C ~ / ~ ~ and Fe80T20 T ~ ~ ~ F ~ ~ ~ N ~ ~ ~ T ~ ~ alloys have been in~estigated~where T denotes one or more metalloids.

Up to now only few studies on cobalt-based alloys have been reported.

In this paper investigations on disaccommodation occurring in three C095,xSi5Bx alloys are reported,Results obtained are discussed in de- tail in [2].

I1

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Measurements were conducted on samples of composition C O ~ ~ S C~ O~~B~~ ~ , Si5B19 and Cog0Si5Bl5 made using the standard melt-spinning method.

Ribbons of width 8 mm and thickness 0.035 mm were coiled on quartz glass rings of diameter 30 mm.Magnetic permeability was determined using a Maxwell-Wien type a.c. bridge.

Preliminary measurements made to determine disaccommodation intensity Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983917

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DZp(tl) -p(t0) / p (to) as a function of amplitude of magnetising field /Hm/ and of maximum amplitude of demagnetising field /Hr/ at a cons- tant frequency f=lkHz and for t =25s andtl=295s from finish of demaqne-

0

tisation, showed that [2] : 1. starting from Hr=20 A/m D does not vary, 2. the function D versus Hm exhibits characteristic maxima at 0.68, 0.86 and 1.00 A/m for alloys C O ~ ~ S C~ O~ B~ ~~ and Co80Si5B15, ~S,~ ~ B ~ ~ respectively-Hence for further measurements the samples were demagne- tised in a 50Hz a.c. field of maximum amplitude H =500 A/m,and magneti- r sed in a lkHz a.c.field of amplitude Hm=0.16 A/m.

Isotherms l/p(t) at several temperatures and isochronous curves D(T) for t0=25s and t1=115,175,235 and 295s were determined-Making use of the Arrhenius law

Q

e=eoexp(Eid (1)

where 8 is relaxation tirne,e0 is t h ~ pre-exponential factor,Q is acti- vation energy and k the Boltzmann constantland also of the known rela- tion applied in the case of a single time constant [l] i.e.

@ (T~) Stl-tO)/ln- tl (2)

where @(T ) is the relaxation time at peak temperature,the time cons- P

tant 8 was calculated for several temperatures.

I11

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Fig.1 shows the isothermal disaccommodation curves measured in the temperature range 200-420K for a C O ~ ~ sample.In effect straight S ~ ~ B ~ ~ lines were obtained at 247 and 298K.For the remaining alloys curves of

292 7 K Fig.1 - Isothermal disaccommoda-

tion curves;alloy C O ~ ~ S ~ ~ B ~ ~

4 5 6 7 8

The isochronous disaccommodation curves D/Dmax=f(~) /t1=295s/ determi-

Fig. 3 - ln @ ^{versus T -1.}

Fig.2

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curves obtained in the as-quenched alloy Cog0Si5Bl5 P state for:1-Co80Si5B15r2-Co76Si5Blgr

3-Co73Si5B22

ned for a sample in the as-quenched state are aiven in Fig.2.Comparing the forms of these curves it may be observed that: 1. maxima occur at temperatures T =350-380Kr while T shifts towards higher temperatures

P P

with increasing boron content; 2. the relaxation spectrum consists of at least two parts, i.e.10~ temperature DisI /from 200 to about 320K/

and high temperature DisII /about 320 to 420K/.

For the Cog0Si5Bl5 sample it was ascertained that the isochronous cur- ves maxima,situated in the DisII part,shift towards lower temperatures with increase in tl.Fig.3 shows the function ln@ /formula ( 2 ) / versus reciprocal of peak temperature. A straight line was found, in agree- ment with formula (1). For other alloys no such effect was observed.

On Fig.4 are presented the isochronous disaccommodation curves obtai- ned for the Cog0Si5Bl5 sample; the curves were determined for a sample in the as-quenched state /two successive measurements, I and II/ and

nextrafter successive heatings in the temperature interval ~ ~ = 4 2 0 - 4 7 0 ~ .

1 a

The sample was heated at T =420K for one hour,cooled slowly in the

a 2

furnace and the disaccommodation curve was plotted to temperature Ta=

Ta+lOK. At this temperature the heating process was repeated, the sam- 1 ple was allowed to cool slowly and an isochronous curve was plotted to

3 2

T =T +10K etc.From Fig.4 it may be seen that the repeated measurement a a

/II/ causes a drop in spectrum intensity and shifting of peak tempera- ture towards higher temperatures.A marked drop in peakheight is obser- ved after ageing at 430K. Successive heating at higher temperatures /up to 470K/ produces no significant change in the DisII maximum /Fig.

5/ but it does cause lowering of the low temperature part of the spec- trum. Similar results were obtained for the remaining samples of other compositions.

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Fig.4

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first measurement, xse- cond measurement and af- heating for 1 hour at:

o m , m ₄₈₀

Ttz

I

Fig. 5 - ^Height. of DisII versus heating temperatu- re for Cog0Si5Bl5

400 420 440 460 480

Fig.6 gives the results obtained for a double measurement of curve D=f(T),first with rising and then with decreasing temperat~re~for the Co76Si5B19 sample in the range 200-600K.Prior to measurements the sam- ple was heated at 430K.After such high measurement temperaturestthe curve exhibits distinct temperature hysteresis.Above the peak tempera- ture the effects associated with heating of the sample were superimpo- sed on the processes reversible by demagnetisation.The peak height do- es not change,while a decrease in the low temperature part and a cer- tain rise in the high temperature part of the spectrum is observed.

Fig.6

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alloy C O ~ ~ S ~ ~ B ~ ~ IV

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Before discussing the results of investigations described here it sho- uld be stressed that the appearance of disaccommodation in the tested samples is evidence of the fact that in the measurement temperature

range migrational relaxation processes take placerwell known in the case of crystalline ferro-and ferrimagnetics [l) h he form of the D=f(H) curves,showing marked maximalindicates that in the alloys studied the disaccommodation process is primarily associated with migrational re- laxations in the 180° walls of the domain structure. Similar results have been reported for ferromagnetic metallic glasses of other composi- tions,e.g. FeO. 05C00 95Si8B14 [3J and Fe5C070B15 141

.

Studies conducted on disaccommodation in glassy alloys have shown that the description of this phenomenon requires the introduction of a con- tinuous spectrum of relaxation times.The form of the isothermal curves l/p(t) versus In t obtained for all the tested alloys /~ig.l/ shows, that for 25s C t < 295s l/p(t) changes linearly with In t at tempera- tures frome.g.247to 298K. This resultappearstomeanthattime-constants spectrum is indeed continuous and complies with the logarithmic distri- bution of 8 /p. 44 in [I] /. The logarithmic distribution of relaxation times has also been reported for Fe-B glassy alloys [5].

Results of isochronal measurements /Fig.2,4,6/ indicate that in amor- phous ferromagnetic samples C O ~ ~ - /x=15,19,22/ there occurs a wi- ~ S ~ ~ B ~ de band of magnetic wermeability disaccommodation /200-460K/,with a ma- ximum at T =350-380K,markedly dependent on composition /Fig.2/. This band isof compoundnaturefinit P maybedistinguishedtwo superimposing areas /DisI and DisII/ which exhibit different behaviour after the he- ating applied in these tests.

Heatingattemperatures up to 350Khas virtually no influenceon the ma- gnitude of disaccommodation in interval DisI.Heating at 430K causes a marked fall in D throughout the whole temwerature rangelhowever he- ating at Ta

>

The stabilised height of the isochronous curve /Fig.5/ after heating at 430K indicates the stabilisation of the glassy structure. This stabi- lity is evidence that the low temperature processes of relaxators di- sappearance,which are not of relaxation origin in the sense of relaxa- tions reversible by demagnetisation, i.e. causing magnetic after- effects,are now finished.What remains is only disaccomodation asso-

C9-150 JOURNAL DE PHYSIQUE

ciated with migrating but stable /in the measurements temperature range/

relaxators, most probably tending to directional ordering.

This last proposition may be justified as follows. From the displace- ment of the peak from tl, observed for the Cog0Si5Bl5 alloy, may be calculated the Arrhenius' law parameters /formula (1)/ Q and Go of the migration processes responsible for the DisII disaccommodation. For @

calculated from ( 2 ) values Q=l. 3eV. Q0=1. 5x10-17s were obtained. The value of the pre-exponential factor indicates that the observed disac- commodation originates from a directional ordering type process.

In paper [6] for alloy FegoB20 values of Q=1.66eV and @ =2. ~ x ~ o - ~ ~ s 0

were obtained. It may be concluded that the pre-exponential factors of migrational relaxations are of the order of 10-17s and hence at least one order of magnitude less than obtained for crystals /e.g. for Snoek relaxation in dFe-C, @,,=3.2~10-~~s [7] /.

At present the lack of peak displacement in the case of other alloys is not clear. Recently, Allia et al. [8] suggested that in glassy alloys isochronous disaccommodation peaks are not originated from reversible migrational relaxation processes, but correspond to the beqinning of irreversible structural relaxation effects.

REFERENCES

1. KRONM~~LLER H. ,Nachwirkung in Ferromagnetica, Springer-Verlag ,

Berlin, Heidelberg, New York 1968

2. WENCEK H.,CIURZY~SKA W.,MORO~ J.W.,WYSEOCKI B.,SZYMURA S.,CAEKA A., Archiwum Nauki o Materialach, in press

3. JAGIELI~SKI T. ,WALECKI T., Phys. Status Solidi /a/ 2 (1980) K31 4. MATSUYAMA T.,OHTA K.,KAJIURA M.,TERANISHI T.,Jap.J.Appl.Phys.

19

(1980) 55

5. VOJTANIK P.,KEKALO J.B.,Phys. Status Solidi /a/ 3 (1980) K45 6. KRONM~LLER H., MOSER N., Magnetic after-effects and the hysteresis

loop, in: Amorphous Metallic Alloys, Ed. F.E. Luborsky, Butterworth, London 1983, in press

7. KOZEOWSKI L. , M O R O ~ J.W. ,PRZYBYEA J. ,RASEK J., Acta Phys. Pol.

A40

(1971) 445

8. ALLIA P., LUBORSKY F.E.,SATO TURTELLI R.,SOARDO G.P., VINAI F., IEEE Trans. Magn.

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