Vacancies and magnetic properties of FeAl-alloys

Vacancies and magnetic properties of FeAl-alloys

Journal of Magnetism and Magnetic Materials 45 (1984) 305-308 North-Holland, A m s t e r d a m 305 VACANCIES AND MAGNETIC PROPERTIES OF FeAI-ALLOYS ...

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Journal of Magnetism and Magnetic Materials 45 (1984) 305-308 North-Holland, A m s t e r d a m

305

VACANCIES AND MAGNETIC PROPERTIES OF FeAI-ALLOYS H. D O M K E and L.K. T H O M A S Institut fur Metallforschung der Technischen Universit(2t Berlin, Joachimstaler StraBe 31/32, 1000 Berlin 15, West Germany

The temperature dependence of the magnetic susceptibility of FexA11 - x alloys with 0,5 < x _< 0.59 was measured between 2 and 300 K. By quenching from 800 and annealing at 450 o C the vacancy concentration was changed in the range 0 < c v < 0.012. There is a transition from the paramagnetic to the spin glass state at a temperature Tr between 3 and 15 K. Tf does not depend on Cv. The values X - Xn with XB ~- const, follow the Curie-Weiss law between 3Tf and 160 K. The effective magnetic m o m e n t is about 5/~B, it depends slightly on the vacancy and Fe-concentration.

1. Introduction Equiatomic ordered FeAl-alloys are able to contain unusually high concentrations of vacancies [1]. According to a generally accepted model in these alloys [2] the number of Fe-atoms NFeAS on Al-sites depends on the concentration of the thermal vacancies. The variation of the Fe-content also changes NFeAS. The magnetic properties of FeA1 are mainly determined by these so-called Fe-antistructure atoms (FeAS). At sufficiently high temperatures FeAl-alloys are paramagnetic and they exhibit typical spin glass behaviour at low temperatures [3]. In this paper we report measurements of the magnetic susceptibility on FeAl-samples with Feconcentrations 0.5 < CFe _~<0.59 in the temperature region 2 K _< T < 300 K. Various vacancy concentrations were produced by quenching and annealing. This work was started to investigate the influence of vacancies on the magnetic properties of these alloys. A well known model [1,2] relates the number of vacancies to the number of Featoms. FeA1 alloys of the concentration range mentioned crystallize in the B 2 structure. According to theory and experiment the thermal vacancies - concentration c v - at about 800 ° C at first are thought to exist in both the A1- and Fe-sublattices. But all vacancies of the Al-lattice move into the Fe-lattice. Therefore the concentration of the Fe-antistructure atoms CFeAS is CFoAS = (CFo -- 0.5) + 0.5Cv.

With no vacancies in the lattice each Fe-antistructure atom has 8 Fe nearest neighbours and causes a localized magnetic moment. There is no agreement in the literature on the magnitude of the localized moments [4]. Quenched alloys contain more CF~ASbut less Fe-atoms in the Fe-sublattice. Therefore the effective magnetic moment is expected to change by quenching and annealing.

2. Experiments The samples were prepared by induction melting in an Ar-atmosphere. Then they were annealed for 5 days at 1000°C, furnace cooled and ground to their final shape. After additional annealing at 450 o C for elimination of vacancies and mechanical strain the magnetic measurements were performed in fields between 10 and 500 G with a Faraday balance and a vibrating sample magnetometer. The temperature range was 2 K < T < 300 K. In order to obtain samples with various vacancy concentrations the samples were heated to about 8 0 0 ° C and quenched with a speed of 100 K / s . This low cooling speed is sufficient to retain all vacancies because of the ordered structure of the alloys. Magnetic measurements of the quenched samples followed. The concentration c o was calculated from the change of the length AI/I measured with a dilatometer at 450 o C [1].

(1)

0304-8853/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

H. Domke, L.K. Thomas / Vacancies of FeAl-alloys

306

3. Results

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-20

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It was n o t p o s s i b l e to fit the susceptibility values with a C u r i e - W e i s s law only. W e used the relation

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s X

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-5

X = Z, + c/(r-

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• vacancies annealed ~

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. quencnea

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for t e m p e r a t u r e s T < 160 K. XB is the t e m p e r a t u r e i n d e p e n d e n t b a n d c o n t r i b u t i o n . The low t e m p e r a ture limit of this a p p r o x i m a t i o n is given b y the spin glass region. T h e values o f XB are b e t w e e n 9 a n d 12 x 10 - 6 e m u / g . T h e y are slightly conc e n t r a t i o n d e p e n d e n t . Fig. 1 shows ( X - XB) - 1 = f ( T ) . F r o m the straight lines we f o u n d the Curie Constant C = NF~AS / ~ f f / 3 k B a n d the t e m p e r a t u r e /9. I n o r d e r to calculate the effective m a g n e t i c m o m e n t / ~ f f the n u m b e r NF~AS of the F e - a n t i s t r u c ture a t o m s has to b e known. It was c a l c u l a t e d f r o m eq. (1) with CF~ given b y the alloy F e - c o n t e n t a n d c v f r o m the A l / l - m e a s u r e m e n t s . Fig. 2 shows 8 and/x~e f as a f u n c t i o n of CZeAS. Fig. 3 gives the values of Xa, 8 and/%eel for the alloy Fes0.rA149.4; different values o f CFeAS were o b t a i n e d o n l y b y q u e n c h i n g a n d annealing. Figs. 4 a n d 5 refer to spin glass properties. Fig. 4 shows the m a x i m u m of the susceptibility at the t e m p e r a t u r e T~ for two different s a m p l e s with different h e a t t r e a t m e n t . T h e curves l a a n d 2 have

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I

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I

I

I

I

I

1

l

I

6 /A ~

o



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I

i

I

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vocancies

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annealed I

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0,08

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Fig. 2. Effective magnetic moment Fcff of each Fe-antistructure atom and Curie-Weiss-temperature O after different heat treatment as a function of the Fe-antistructure concentration, calculated by eq. (1).

Feso.6 A I ~ Cv

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b: Cv: 0.0034 c: Cv= 0,0074

d: C v = 0,0115

. . . .

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~,

,

"-~ 5.5

. . . . . .

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150

Fig. 1. Reciprocal susceptibility (X XB)-1 versus temperature for Fes0.6A149.4. The variation of the vacancy concentration was achieved by the following heat treatments: (a) annealed at 450 °C for 3 days; (b) quenched from 810 °C to room-temperature; with an additional heat treatment at 400°C for 35 h (controlled by dilatometry); (c) quenched from 785 °C; (d) quenched from 820 o C.

5,2

./ f o

..

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i

° ,

0.005 0.01

-

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Fig. 3. Effective magnetic moment/-"¢rf, Curie-Weiss-temperature 0 and the temperature independent band contribution Xa for Feso.rA149.4 versus concentration of vacancies cv and of antistructure atoms CFcAS.Values obtained by a least squares fit to eq. (1) of the data of fig. 1.

H. Domke,

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LK.

Thomas / Vacancies of FeAl-alloys

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~X

,5",

///

I

-

2

-

la

2a

T[K]

Fig. 4. Temperature dependence of the magnetic susceptibility for 2 FeAl-alloys with different heat treatments: (1) Fe52.5A147.5 vacancies annealed, CFeAS= 0.025; (la) Fe52.sA147.5quenched, CFeAS=0.028; (2) Fe52.sA147.2 vacancies annealed, CFeAS= 0.028; (2a) Fe52.sAl,17.2quenched, CVcAS= 0.031.

different alloy composition and different vacancy concentration b u t b y means of heat treatment the same n u m b e r of Fe-antistructure atoms. We see the same Tf, but different X-values. Fig. 5 shows the temperatures Tf as a function of CF~AS.

4. Discussion We use the model [5,6], that each Fe-antistructure atom with m o r e than 6 nearest neighbour-Fe-

oquenched • vacancies a n n e a l e d / tO

/

t

/

/

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307

atoms carries the normal moment/~ -- 2.2/z a k n o w n f r o m the unit cell of a-Fe. The value of ~eff is higher and depends slightly on the value of CF,AS. The value /~,ff = 5/~a is thought to be caused by clustering of various unit cells with FeAS in the alloy. This value is of the same order of magnitude as found in the similar alloys C o G a [7], CoAl [8] and spin glasses C u M n [9], A u M n and A g M n [10]. The comparison of 2 samples with the same value of CF,AS = 0.006 but different vacancy concentrations c v = 0 and c v -- 0.012 - see fig. 2 - results in different effective m o m e n t s ]Lef t ~ 5.4/~ B and/£eff ~ 4/~a. The higher vacancy concentration causes a dilution of the n u m b e r s of Fe-atoms on Fe-sites. Therefore the possibility of clustering decreases, the resulting m o m e n t is smaller. The increase of CF, by alloying supersedes this effect. F o r CF~ >_ 0.506 the quenched samples show higher values of /~,ff than the samples fully annealed. Since the b a n d - c o n t r i b u t i o n Xa rises in a similar way - see fig. 3 - this effect seems to refer to an increasing n u m b e r of d-electrons b e c o m m i n g delocalized. Therefore the u n c o m p e n s a t e d localized m o m e n t s of the remaining d-states are enhanced. According to fig. 5 there is no influence of the vacancy concentration on the value of spin glasstransition temperature Tf. This seems to be reasonable within the range of the model. The rand o m l y distributed exchange interaction between nearest n e i g h b o u r s - f e r r o m a g n e t i c and next nearest n e i g h b o u r s - a n t i f e r r o m a g n e t i c - does not change b y r a n d o m l y distributed vacancies in the Fe-sublattice. A n o t h e r possibility is that the vacancy concentration was too small to detect a relation between c v and Tf.

Acknowledgement

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,o/ I

I

0.02

I

W e thank the Deutsche Forschungsgemeinschaft for the support of this work. I

0,04

I

I

0,06

i

I

O,08 CFeAS

Fig. 5. The temperatures of the susceptibility maximum Tf in fields of 10 G versus the concentration of the antistructure Fe-atoms CFeAS.CV,AS was calculated from the alloy composition and the vacancy concentration according to eq. (1).

References [1] D. Paris and P. Lesbat, J. Nucl. Mater. 69, 70 (1978) 628. [2] H. Jacobi and H.J. Engell, Acta Met. 19 (1971) 701. [3] R.D. Shull, H. Okamoto and P.A. Beck, Solid State Commun. 20 (1976) 863.

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H. Domke, L.K. Thomas/ Vacancies of FeAl-alloys

[4] A. Parthasarathi and P.A. Beck, Solid State Commun. 18 (1976) 211. [5] H. Okamoto and P.A. Beck, Monatshefte Chemie 103 (1971) 907. [6] P. Shukla and M. Wortis, Phys. Rev. B21 (1979) 159. [7] D. Berner, G. Geibel, V. Gerold and E. Wachtel, J. Phys. Chem. Solids 36 (1974) 221.

[8] R. Meyer, E. Wachtel and V. Gerold, Z. Metallkde. 67 (1976) 97. [9] A.F. Morgownik and J.A. Mydosh, Phys. Rev. B24 (1981) 5277. [10] A.K. Majumdar, V. Oestreich and D. Weschenfelder, Solid State Commun. 45 (1983) 907.