THE DISAPPEARENCE OF THE RESISTIVITY MINIMUM ON ANNEALING AMORPHOUS Fe40Ni40B20 ALLOY

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THE DISAPPEARENCE OF THE RESISTIVITY MINIMUM ON ANNEALING AMORPHOUS

Fe40Ni40B20 ALLOY

E. Babić, Ž. Marohnić

To cite this version:

E. Babić, Ž. Marohnić. THE DISAPPEARENCE OF THE RESISTIVITY MINIMUM ON AN-

NEALING AMORPHOUS Fe40Ni40B20 ALLOY. Journal de Physique Colloques, 1978, 39 (C6),

pp.C6-948-C6-950. �10.1051/jphyscol:19786420�. �jpa-00217892�

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JOURNAL DE PHYSIQUE

Colloque C6, supplément au n° 8, Tome 39, août 1978, page C6-948

THE DISAPPEARENCE OF THE RESISTIVITY MINIMUM ON ANNEALING AMORPHOUS Fe

^{A 0}

Ni

^{4 0}

B

^{2 0}

ALLOY

E. Babic and Z. Marohnic

Institute of Physios of the University, P.O. Box 304, Zagreb, Yugoslavia

Résumé.- Nous avons étudié la résistivité des alliages Fei»oNii,oB2o entre 1,5 et 30 K. Nous avons uti- lisé des échantillons amorphes et des échantillons recuits. Le minimum dans la résistivité disparaît complètement pour les échantillons recuits au-dessus de 750°C. Nous avons comparé la variation de la résistivité (à basse température) d'un échantillon complètement recristallisé avec la variation dans un alliage cristallin FesoNiso.

Abstract.- The resistivity of Fe^oNiitoBao has been measured in the temperature range 1.5-30 K, for both amorphous and annealed samples. After annealing above 750°C the resistance minimum disappears.

The low temperature resistivity of a completely recrystallized sample is compared with that of crystalline FesoNiso alloy.

One of the most striking features of amorphous ferromagnets is a Kondo like resistance minimum. As a resistivity minimum also appears in the majority of non-magnetic amorphous alloys it seems rather ap- pealing to associate it with structural disorder.

The simplest test of this hypothesis is made by annealing the initially amorphous alloys and monito- ring the subsequent changes in the resistance minimum. An earlier attempt/1/ to do so with and Fe32Ni3eCrlllP12B6 alloy (Metglass 2826A) was only partially successful since the resistance minimum although much shallower and shifted to lower temperatures, persisted even after annealing for six hours at 800°C. We note that after annealing at

these temperatures the sample should be completely crystalline. Further complications in the interpre- tation of these results lie in the rather complex composition and above all in the crystallographical- ly unknown final state of the annealed alloy.

Here we present a systematic study of the changes in the low temperature resistivity which occur during the annealing of an amorphous FeitoNiito B2 0 alloy. This system is a soft ferromagnet whose

thermal stability /2-3/, magnetic/4/, electrical and galvanomagnetic properties /5/ are rather well known. Recent X-ray diffraction studies/3/ indicate

that a fairly simple phase transformation occurs during the annealing of the initially amorphous alloy.

The samples were ribbons/6/ 0.3 mm wide and about 20 \im thick. The resistivity measurements were performed in the temperature range 1.5-30 K by using a potentiometric set up with 2 x 10 6 reso- lution. Usually the same samples were successively annealed and then remeasured in the same temperatu-

re interval. The annealing was performed under va- cuum at several fixed temperatures both above and below the crystallization temperature (T =393°C).

At each annealing temperature (T ) the samples were ' a

kept for 30 minutes in order to obtain results which can be compared with our phase transformation inves- tigations/3/.

The changes in the resistivity of the amorphous and annealed samples normalized to the resistivity at the temperature of the resistivity minimum t'p . ) are shown in figure 1 . The normalization

m m °

was made in order to account for the decrease in the residual resistivity which occurs on annealing (Ta- ble I ) . The resistivity variation of the unannealed samples is discussed elsewhere/5-7/ and will not be mentioned here. We also note that the samples annealed at temperatures below T showed only a minor change in the residual resistivity. However the temperature dependence of the resistivity remained the same as in unannealed samples and so these samples are not included in figure 1. Such a resistivity behaviour is strongly indicative of the structural origin of the resistance anomaly. Thus we expect a rather sharp disappearence of the resistance minima on crystallization. In order to study this disap- pearance more closely we annealed several samples at temperatures closely above T . Two of these samples (T = 398 and 400 K respectively) are shown in figure 1. We note that the resistivity minima are progressively shifted to lower temperatures and become more shallow while the increase of the resistivity after minima becomes steeper. However

— n — = r values (see also table I) at the lo- p . dlogx

nun

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19786420

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Fig. 1 : The normalized resistivity

(A)

^{of amor-}

Pmin

phous and annealed F e ~ o N i ~ o B z o alloys vs. lo@. The numbers denote the annealing temperature. Note that the resistivity scale for the three highest annealed samples is enlarged.

Table I : Data relevant to our alloys. Ta is the annealing temperature, pt, 2 is the resistivity at 4.2 K. A is the logarithm:c slop of the resistivity

( ) at lowest temperatures, D = A/pmin, Tmin is dlogl'

the temperature of the resistivity minimum and

-

dP

dT / ^R.T.denotes the room temperature coefficient of the resistivity.

west temperatures are almost unaffected by annealing at these temperatures which possibly indicates that this behaviour is caused by the decrease in the size occupied by the amorphous phase and not so much by the change in the processes causing the resistance anomaly.

After annealing our samples well above Tx the resistance minima became extremely shallow (note the large scale in figure 1 ) and are shifted to very low temperatures so that the resistivity behaviour at the lowest temperatures is difficult to asses. However the minimum persists even after annealing at 600'~ at which point the sample is largely crystallized/3/. After annealing at 750°C the resistivity minimum disappears. As X-ray diffractions

~atterns/3/ indicated only an (FeNi) austenite phase remaining at these temperatures, we have compared in figure 2 the resistivity of this alloy with that of a crystalline Fe50Ni5~alloy.

Fig. 2 : The posi our alloys vs. T~

ling temperature.

alloy.

tive parts of the resistivity of

.

The numbers denote the annea- + represent a crystalline FesoNis0 Although the resistivities of these two alloys are fairly similar there are however some differences.

While the resistivity of Fe Ni obeys rather well 50 50

a T~ law, recrystallized Fe40Ni40B20 alloy gives rise to a stronger temperature dependence. Also the residual resistivity and the room temperature coefficient of the resistivity of this alloy are higher than those of the crystalline FeSONigO alloy (Ta- ble I) probably indicating that the phase transfor- mations are still not completed. In figure 2 we also show the positive temperature dependent parts of the resistivity of the alloys shown in figure 1.

These data are obtained by subtracting the low temperature logarithmic term from the measured resistivity. They show a strong increase in the positive

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resistivity term of the annealed alloys in compari- son to that the amorphous Fe40Ni+0B20 alloy.

Finally we make a rough estimate as to whether the same processes are causing the resistance minimum in both amorphous and largely crystallized alloys. Roughly speaking, if the structural disorder is causing the resistance minima one might expect

1 dp

that

- -

d"LngT values (at the lowest temperatu-

"min

res) would be approximately equal in all these alloys as they were for the samples annealed just above T The data in table I show that these va-

x'

lues decrease very fast after annealing above 450'~.

This seems to indicate that a different mechanism causes the resistivity minimum in largely crystallized alloys. Further measurements of the resistivity and magnetoresistivity of annealed samples at even lower temperatures may clarify this point.

References

/I/ Cochrane,R.W. and Strom Olsen,J.O., Physica 86-88B (1977) 779

--

/ 2 / Luborsky,F.E., Mater. Sci. Eng.

2

(1977) 139 /3/ ~tubigar,~.

,

Babid,E

. ^,

~ubagik,~.

,

Pavuna,D. and

~arohnii,g., Phys. Status Solidi, (a)

44

⁽¹⁹⁷⁷⁾

339

/4/ Becker,J.J., Luborsky,F.E. and Walter,J.L., IEEE Trans. Magn.

13

(1977) 988

/ 5 / ~arohnic',i., Babi6,E. and Pavuna,D., Phys. Lett.

63A (1977) 348

-

/6/ Liebermann,H.H. and Graham,L.D.Jr., IEEE Trans.

Magn.

12

(1976) 921

/7/ ~abie,~., ~arohnic,?., 1vezi6,T. and Ivkov,J., to be published

We thank Dr. H.H. Liebermann for giving us the amorphous ribbon.