ON THE ANOMALY OF 0.75Fe - 0.25Co ALLOY NEAR 500 °C

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ON THE ANOMALY OF 0.75Fe - 0.25Co ALLOY

NEAR 500 °C

N. Eissa, M. El-Ockr, A. Bahgat, S. El-Henawii

To cite this version:

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JOURNAL DE PHYSIQUE Colloque C6, suppliment au no 12, Tome 37, -Dicembre 1976, page (26-3189

ON

THE

ANOMALY OF 0.75Fe

-

0.25Co ALLOY NEAR 500

O C N. A. EISSA, M. M. EL-OCKR, A. A. BAHGAT and S. A. EL-HENAWII

Mossbauer effect Laboratory, Physics Department, Faculty of Science Al-Azhar University, Nasr city, Cairo, Egypt

RQum6. - L'effet Mossbauer a 6te utilise pour l'etude de la dependance de la tempQature du champ magnetique hyperfine du Fer dans l'alliage Fe0,75-C00,25. Un pic paramagnktique central superpose aux 6 raies bien resolues a kt6 observe 500 "C environ. Ceci a ett explique par ce qu'on appelle une anomalie au voisinage de 500 "C. La nature de cette anomalie est brikvement discut6e. Abstract.

-

The Mossbauer effect was applied for the study of the temperature dependence of the hyperfine magnetic field at iron in Fe0.75-Co0.25 alloy. A paramagnetic central peak superimposed on well resolved six line pattern was observed at about 500 "C. This was explained in terms of the so called anomaly near to 500 "C. A brief discussion the nature of this anomaly is given.

1. Introduction.

-

The magnetic behaviour of alloys is of great interest specially that of Co-Fe alloys since they are widely used in the up to date technology. The temperature dependence of electrical and magnetic properties of these alloys showed certain anomaly at about 500 OC [I-61, an effect which is known as the anomaly near 500 OC.

Eguchi et al. [I] succeeded in explaining the observed order disorder transition at 730 OC in Fe-Co alloy containing 50

%

Co. The experimental results of magnetization, specific heat and neutron diffraction have been interpreted by a theoretical statistical approach [I]. Their theory also implied that the nonmagnetic states, if it exists, would undergo the order- disorder transitions at about 550 OC. The coexistence of magnetic and nonmagnetic states jn this alloy was used to explain the observed anomaly.

The coexistence of magnetic and nonmagnetic regions in this alloy system was predicted in the framework of the rigid band model by Shimizu [6]. His assumption was based on the fact that of the equilibrium values of free energies in the ferromagnetic and paramagnetic states are the same, there should be a mixing of a 'ferro and paramagnetic phases.

For an equiatiomic composition the changes in the magnetic properties in the course of heat treatment were studied 121. It was assumed that ordering takes place in a two stage process. The first below 550 OC and the second (the superlattice formation) in the range between 500 OC and 7300C.

The above mentioned anomaly between 550 and 730 OC was observed by studying the kinetics of the formation of an ordered structure of a-FeCo, and was attributed to the magnetic character of exchange interaction in the ordered alloys [3].

On the other hand, the electrical resistivity of the ordered Fe-Co structure [4, 51 did not show the 550 OC anomaly. However, the dynamic measurements indi- cated that the degree of ordering of the specime,n j,ust below 500 OC could be changed rapidly to approach the equilibrium degree of ordering at the corresponding temperature [5].

The above mentioned review shows that the existence of 550 OC anomaly and its nature, is still a matter of discussion since the Me technique provides a useful tool for the investigation of this problem. The present work was aimed to use the ME spectrosc6py for observing the behaviour of the ME parameter of'the 0.75

' ~ e -

0.25 Co alloy near 500 OC, and try to understand its nature.

2. Experimental. - The alloys .were prepared (at the Faculty of Chemistry, Leviv State Uqiversity USSR) by the arc method in an inert atmosphere of argon using high purity iron Fe and cobalt metals. The alloy was then subjected to aging at about 850 OC for 150 hours and then cooled to room temperature at a cooling rate of approximately 10 degrees per hour. The X-ray diffraction results showed the homogeneity of the sample.

The ME absorbers were prepared from the annealed alloy powder.

A constant acceleration ME spectrometer coupled to 256 multichannel analyser was used. The temperature of the sample was varied from room temperature to about 750 5 I .T in . a vacuum. furnace.

3. Results. - Representative ME spectra of the 0.75Fe

-

0.25Co alloy at different temperatures are shown in figure 1.

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C6-390 N . A. EISSA, M. M. EL-OCKR, A. A. BAHGAT AND S. A. EL-HENAWII

Velocity ( m m l s )

FIG. 1. - Mossbauer spectra of Feo.75-Coo.zs at various temperatures.

The internal magnetic field decreases by increasing the temperature in the usual manner. At 778 K (505 OC) a central paramagnetic peak superimposed on well resolved six-line pattern was observed.

Raising the temperature the intensity of the:central peak increased at the expense of the six-line pattern up to 1 003 K, where the latter completely disappeared. 4. Discussion. - The alloy under investigation is a substitutional solid solution of cobalt in iron. The phase diagram shows that introducing cobalt does not alter the original structure of iron which means that the Brillouin zolie does not undergo any considerable change. Moreover, the X-ray spectroscopic studies of the emission K,,,;-band of iron in this alloy show that the K,,,;-band ,does not considerably differ from that of pure iron. Tliis indicates that it is possible to analyse

[I] EGUCHI, T., MATSUDA, H., OKI, K., ZEEE Trans. Mag. 4

(1968) 476. ,

[2] D E K H T Y ~ , M. V., ,Fiz. Zh. 15 (1970) 120.

[3] DEKHTYAR, M. V., Fiz. Met. Metalloved. 33 (1972) 746.

[4] YOKOYAMA, T., TAKESAWA, T., HIGASHIDA, Y., Trans. Japan

Znst. Met. 12 (1971) 30.

the experimental results in the frame of the rigid band model.

The fact that no charge in the isomer shift was observed, indicates that the density of s-electrons and consequently the nature of the bonding was not considerably affected by introducing the cobalt atoms. The sudden appearance of the central peak at about (778 K) may be interpreted in terms of the anomaly near to 500 0C.

Taking into consideration the ME parameters (I. S. and line width) of the paramagnetic peak appear- ed at 500 OC, it can be assumed that it is probably due to the formation of paramagnetic centers of iron atoms as a result of the heating process. These iron atoms have different electronic structure which means that they exist in different environment. This causes the change of the crystal field due to the neighbours affecting the iron atoms resulting in the paramagnetic behaviour. This means the formation of paramagnetic regions as a result of the heating process.

The coexistence of the six-line pattern and the paramagnetic peak could be due to the occurence of para- and ferromagnetic states at the same time.

The temperature dependence of the intensities of both the six-line pattern and the paramagnetic peak shows that raising the temperature leads to an increase in the paramagnetic regions at the expense of the ferromagnetic regions.

At temperatures higher than 500 OC the process of transformation from ferromagnetic state to paramagnetic one takes place up to 730 OC, where the intensity of the six-line pattern vanishes which means the formation of completely magnetically disordered solid solution.

Adopting Dektyar's assumptions [2], we may say that in the second stage of the ordering process which takes place in the temperature range 500-730 OC, the paramagnetic state begins to be decetable.

In general the order-disorder transition is a process of a complicated nature. According to the theoretical treatment of the problem [7], it is assumed that the atom movements take place in two different ways, a direct change of atoms and an interchange through vacancies. Both of these mechanisms play an important role in the ordering process. It seems that these mechanisms are responsible for the formation of the paramagnetic atoms, which may act as centers of formation of paramagnetic regions.

rences

[ 5 ] YOKOYAMA, T., TAKEWASA, T., Bull. Fac. Eng. Yokohama

Nut. Univ. Japan 19 (1970) 1.

[6] SHIMISU, M., Inter. Symposium 1972 Electronic structure of transition metals, their alloys and compounds, Kiev 1974. [7] MATSUDA, H., KUROKI, R., EGUCHI, T., J. Japan Inst. Met.