MEASUREMENT OF A MAGNETIC AFTER-EFFECT IN THE INTERMETALLIC COMPOUND CoGa

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MEASUREMENT OF A MAGNETIC

AFTER-EFFECT IN THE INTERMETALLIC

COMPOUND CoGa

A. van Ommen, A. Reckman, M. van Feggelen, H. Bakker

To cite this version:

JOURNAL DE PHYSIQUE Colloque C7, supplkment au no 12, Tome 38, dkcembre 1977, page C7-337

MEASUREMENT

OF

A MAGNETIC AFTER-EFFECT

IN

THE INTERMETALLIC

COMPOUND

CoGa

A. H. VAN OMMEN, A. P. F. M. RECKMAN, M. VAN FEGGELEN and H. BAKKER

Natuurkundig Laboratorium Universiteit van Amsterdam, Valckenierstraat 65, Amsterdam, The Netherlands

RCsumC. - Un effet magnktique secondaire a CtC CtudiC dans le compol intermktallique CoGa

apres avoir CtC trempk A la tempkrature du laboratoire et recuit ensuite. Les enthalpies de migration des lacunes ont Cte dkterminkes pour cinq compositions de 48

A

56 at % Co de la dkpendance de la temperature de la mobilitk des dkfauts.

Abstract. - A magnetic after-effect, after quenching to room temperature and subsequent anneal-

ing, was measured in the intermetallic compound CoGa. From the temperature dependence of the defect mobility, vacancy migration enthalpies were deduced for five compositions in the range of

48-56 at O/, Co.

Introduction. - The ordered compound CoGa is a From the temperature dependence of the defect

typical example of a VIII-IIIA compound. I t crys- mobility, we were able to deduce vacancy migration

tallizes in the BZstructure and exists over a wide enthalpies in various compositions of the compound.

homogeneity range (46-68 at

%

Co) around the

stoichiometric composition. One of the interesting features of these compounds is the difference of the microscopic structure on both sides of the stoichio- metry. E.g. in the case of CoGa, on the cobalt rich side of the stoichiometry, cobalt atoms are substituted on the gallium sublattice (so called anti-structure atoms), whereas on the gallium rich side structural vacancies are formed on the cobalt sublattice.

Furthermore the compound exhibits a special type

of disorder : at higher temperatures thermal vacancies

combined with thermal anti-structure atoms are easily formed. The creation of a thermal anti-structure atom is always accompanied by the formation of a new elementary cell. This means that the formation of one thermal anti-structure atom generates two thermal vacancies on the cobalt sublattice. In this way the thermal vacancy and anti-structure atom content may

even amount to some percents [I].

Since a cobalt anti-structure atom is surrounded by eight cobalt nearest neighbours, these atoms are mainly responsible for the magnetic behaviour of the compound [ I , 2, 3, 4, 51. Therefore its magnetic susceptibility will depend on the thermal history. At higher temperatures the magnetic susceptibility becomes even time dependent. This is related to the formation or annihilation of thermal anti-structure atoms and so of thermal vacancies.

The aim of this investigation was to study the magne- tic after-effect that occurs in CoGa after quenching

from high temperature (e.g. I 000 OC) to room

temperature and subsequently annealing at inter- mediate temperatures (e.g. 600 0C) where defects become mobile.

Experimental procedure. - SAMPLE PREPARATION.

-- Weighed charges of cobalt (99.998

%

pure) and of

gallium (99.999 9

%

pure) were heated together to

1 200 OC in an alumina crucible, placed in an eva- cuated quartz ampulla. In this way the cobalt was dissolved in the liquid gallium. T o ensure homogeneity the compound was arc melted several times under argon atmosphere. Subsequently it was arc melted again and cast in a water-cooled copper mold. From this ingot a number of samples were cut by spark erosion. Then the samples were quenched from high temperature. This quenching was performed in the following way. A number of samples (e.g. 10) of a certain composition were placed in an evacuated quartz ampulla with a very thin bottom. The ampulla was heated in a resistance furnace of which the tem- perature was controlled within 0.1 OC and was mea-

sured by means of a Pt-Pt 10

%

Rh thermocouple.

Then this special ampulla was dropped into water, where it broke on an aluminium disk. To prevent oxidation an argon flow was led over the water. This resulted in specimens that were supersaturated with thermal vacancies and thermal anti-structure atoms. Specimens of the following compositions were pre- pared in this way : 48,50,52,54 and 56 at

%

Co.

MAGNETIC MEASUREMENTS.

-

TO measure the

after-effect of the magnetic susceptibility a quenched sample was heated rapidly in a Faraday magneto- meter [3] to a temperature where defects became mobile. For each composition a number of isothermal measurements were carried out in a temperature range of about 500 OC to 600 OC. The two most cobalt rich compounds were measured using a magnetic field

C7-338 A. H. VAN OMMEN, A. RECKMAN, M. VAN FEGGELEN AND H. BAKKER

of 7.6 kOe. Because of their lower susceptibility the remaining compositions were measured in a field of 9.8 kOe. As the magnetometer was controlled by a micro-processor we were able to perform 400 to 600 susceptibility measureinents per specimen. The mea- surements took place over periods of time between 16 and 60 hours. T o prevent oxidation during the measu- rements, the sample was placed in an evacuated quartz ampulla. All measured samples showed a para- magnetic behaviour of the susceptibility.

Results. -- As was shown by Berner et al. [l] the high temperature susceptibility of CoGa can be described by a Curie-Weiss term, proportional to the number of antistructure atoms plus a term that only

depends on temperature :

When a sample, quenched from high temperature and so containing high thermal defect concentrations, is annealed at a fixed intermediate temperature, thermodynamic equilibrium will be established by the elimination of both types of defects. This results in a decrease of the magnetic susceptibility, proportional to the decrease of the number of thermal anti-structure atoms and so of the number of thermal vacancies.

If we assume this to be a first order process, it is

easily seen that the time dependence of

x

should have

the form :

r being the relaxation time of the process.

Figure 1 shows a typical plot of a susceptibility measurement as a function of time. As can be seen from this figure and even better from figure 2, repre-

FIG. 1 . - Typical plot of a susceptibility measurement as a func- tion of time.

FIG. 2. -Typical semi-logarithmic plot of ( ~ ( t ) - ~ ( c o ) ) as a function of time.

senting a semi-logarithmic plot of ( ~ ( t ) - ~ ( m ) ) , the major part of the kinetics is very well described by this first order process.

Measurements always took place over at least ten relaxation times, so that n o variation in the suscep- tibility was measured anymore. The constant value obtained in this way was assigned to ~ ( c o ) .

From the slope of the straight line in the semi-

logarithmic plot the time constant z was derived

graphically. Computer fits of the measured curves led to similar results.

It turned out that the annealing temperature dependence of the time constant could be well des-

cribed by the Arrhenius relation :

This can be seen from figure 3 that shows a semi- logarithmic plot of r(T) versus 1 000/T for the compo-

sitions Co,,Ga,,, Co,,Ga,, and Co,,Ga,,. The

straight line is a least square fit of the data from which

the activation energy Q and the constant z, were

calculated. -

For Co,,Ga,, a possible influence of the quenching

temperature on the activation energy was also investi- gated. For the same set of samples successive quenches and susceptibility measurements were performed. Within the experimental error no variation of the

activation energy was found as can be seen from

table I in which a schematic representation of all measurements is given.

In figure 4 the results for the compounds contain-

MEASUREMENT OF MAGNETIC IN INTERMETALLIC COMPOUND C7-339

FIG. 3. — Semi-logarithmic plot of the time constants versus the inverse annealing temperature, for the cobalt-rich compounds.

FIG. 4. — Semi-logarithmic plot of the time constants versus the inverse annealing temperature, for a gallium-rich and for the

stoichiometric compound.

irreproducible character, may be attributed to dis-solved hydrogen. (It turned out that hydrogen is taken up easily by CoGa, even at extreme low pressures. Moreover, etching of the quenched samples in concentrated acids, gave an enormous peak shaped variation of the magnetic susceptibility as a function of time. A similar behaviour was found for an electro-lytically hydrogen-charged specimen. So contact of the samples with hydrogen was always avoided.) Though corrections could be made we decided to perform additional measurements for these two compositions, in which, due to an even more thorough sample treatment, disturbing effects were absent.

Finally the composition dependence of the acti-vation energy is displayed in figure 5.

Discussion. — The time constant that is derived from the after-effect of the magnetic susceptibility corresponds to the migration process of vacancies to annihilation-centra combined with the migration of anti-structure atoms to their own sublattice. From this we may conclude that the activation energy obtained from the Arrhenius relation (3) can be interpreted as the migration enthalpy for vacancies. When we assume the vacancies to migrate to pre-existing stationary dislocations, one can derive from

TABLE I

Schematic representation of the results of this work on CoGa and the results of [7] and [8] on FeAl

A . H. VAN OMMEN, A. RECKMAN, M. V A N FEGGELEN A N D H. BAKKER

FIG. 5. -The composition dependence of the migration enthalpy for vacancies.

the solution for the vacancy concentration as a func- tion of time, given by Koehler et al. [6], that :

in which a is the lattice parameter, v the vibrational frequency of the atoms and 1/R2 the dislocation

density. For v = 1013 s-', a = 2 x cm and

.ro = lo-' we can calculate a dislocation density of about 1010/cm2, which appears to be a reasonable value. Further it can be seen from (4), that the large shift in 2, that has been observed in Co,,Ga,, after the second quench may be ascribed to a large difference in dislocation density of the specimens.

The variation of the migration enthalpy for vacan- cies as a function of composition shows, as can be seen from figure 5, a decrease with increasing cobalt content. A comparison to migration enthalpies in another VIII-IIIA compound shows that in FeAl from dilatometric studies [7] and from electrical resistivity measurements [8] a similar behaviour of the vacancy

migration enthalpy can be derived. The results of

these measurements are also shown in table I. Addi- tional sources of information, to which a comparison of our results can be made, are tracer diffusion and helium desorption measurements.

Recently Stolwijk et al. [9] have measured diffusion

coefficients of Co-60 in CoGa for five compositions.

Their results for Co,,Ga,, lead to an activation

[I] BERNER, D., GEIBEL, G . , GEROLD, V. and WACHTEL, E., J . Phys. Chem. Sol. 36 (1 975) 22 1.

[2] WACHTEL, E., LINSE, V. and GEROLD, V., J. Phys. Chem. Sol. 34 (1973) 1461.

[3] TAMMINGA, Y., Thesis University of Amsterdam (1973). [4] AMAMOU, A. and GAUTIER, F., J. Phys. F. : Metal Phys. 4

(1974) 563.

[5] BOOTH, J. G . and MARSHALL, J . D., P h y s Lett. 32A (1970) 149. [6] KOEHLW, J. S., SEITZ, F. and BAUERLE, E., Phys. Rev. 107

(1957) 1499.

energy for diffusion of 67 kcal/mole. A comparison to our result for the vacancy migration enthalpy of 35 kcal/mole and an estimate from the measurements of Berner et al. [l] of 10 kcal/mole for the vacancy formation enthalpy, for this composition, shows that there exists a discrepancy between the sum of vacancy migration and formation enthalpy on,one hand.and the activation energy for cobalt diffusion on the other

hand. A reason for this discrepancy may be found

from the fact that the diffusion measurements were carried out in a higher temperature range than our after effect measurements. In this respect the curvature in the In D versus l/Tplots found by Stolwijk et al. [9]

could give rise to the removal of this discrepancy. Another type of experiments a comparison of our results can be made to, are desorption measurements of injected helium [lo]. From the shape of the desorption curve it is concluded that helium diffuses as a substitu- tional in CoGa. If the helium vacancy exchange frequency is high compared to the other vacancy exchange frequencies. it can be shown that the activation energy for the desorption process can be interpreted as a self-diffusion activation energy. An investigation of the desorption of helium in Co,,Ga,,, in the same temperature range as the after effect measurements, yields an activation energy of 46 f 5 kcal/mole, which is in good agreement with

the sum of 45 kcal/mole of vacancy formation and

migration enthalpy

.

T o be able to make a good comparison of our results to other sources of information, reliable vacancy concentration measurements are necessary. We recently started dilatometric and density measure- ments from which the vacancy concentration in ther- modynamic equilibrium can be obtained. Furthermore the magnetic after effect measurements will be extended to other VIIT-IIIA compounds. In this way we hope to get more insight in the kinetics and defect properties of this group of interesting intermetallic compounds.

Acknowledgments. - We thank Prof. Dr. G . de Vries for helpful discussions and criticism. We

are indebted to Dr. Y. Tamminga for his useful

suggestions. We are obliged to Drs. S. Arlman for advices concerning the computer programming and Mr. A. Monster and Mr. G. Janssen for elabo- rating some of the results. Financial support by the

(( Stichting voor Fundamenteel Onderzoek der Mate-

rie )) is gratefully acknowledged.

rences

[7] RIEU, J . and Goux, C.. Mem. Sci. Rev. MdtaN. LXVl (1969) 869.

[8] RIVI~RE, J. P. and GRILHL?, J., Phys. Status Solidi (a) 25 (1974)

429.

[9] STOLWUK, N. A , , SPRUUT, T., HOETJES-EUKEL, M. A. and BAKKER, H., Phys. Status Solidi (a) 42 (1977) 537.