ANOMALOUS CRITICAL BEHAVIOUR OF TRANSPORT PROPERTIES IN ANTIFERROMAGNETIC Cr-Al ALLOYS

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ANOMALOUS CRITICAL BEHAVIOUR OF

TRANSPORT PROPERTIES IN

ANTIFERROMAGNETIC Cr-Al ALLOYS

J. Sousa, M. Amado, R. Pinto, J. Moreira, M. Braga, M. Ausloos

To cite this version:

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JOURNAL DE PHYSlQUE CoNoque C6, supplPment au no 8, Tome 39, aotit 1978, page ~ 6 - 7 9 2

ANOMALOUS CRITICAL BEHAVIOUR O F TRANSPORT PROPERTIES

I N

ANTIFERROMAGNETIC Cr-AI

ALLOYS(*)

J.B. Sousa, M.M. Amado, R.P. Pinto, J.M. Moreira, M.E. Braga and M.

usl loos'

Centro de Fisica da Universidade do Porto, Porto, Portugal.

+ ~ n t e m a t i o n a ~ Centre for Theor. Solid S t . Phys., Univ. Lidge, Be Zgiwn

Rdsum6.- Le comportement antiferromagndtique des alliages Cr-A1 a Qtd dtudi6 par mesure directe et prCcise de plusieurs coefficients des propridtls de transport (p,S,dp/dT,dS/dT). Pour les basses concentrations (c

6

1,5 % at. Al) dp/dT et dS1dT prdsentent le mSme comportement critique avec le minimum usuel pour TN. On trouve aussi des minima pour c > 4 ; pour les concentrations intermddiai- res nos mesures donnent un nouveau type d'anomalie dans dpTd~. On discute la variation (assez bizar- re) de TN avec la concentration.

Abstract.- The complex antiferromagnetic behaviour of Cr-A1 alloys is studied using accurate measurements of transport coefficients (p,S,dp/dT,dS/dT). For c; 1.5 (at.% Al), dp/dT and dS/dT exhibit the usual minima at TN, with-the

same

critical behaviour in %-cases. Sharp minima reappear for c 4. At intermediate compositions, a new type of anomaly appears in dp/dT. The complex concentration dependence of TN in Cr-A1 is discussed.

Transport property measurements are used here to investigate the complex antiferromagnetic behaviour of dilute Cr-A1 alloys 11-31. Our investiga- tion covers the cases of the electrical resistivity (p), thermoelectric power (S) and the temperature derivatives dp/dT,dS/dT on a set of twelve Cr-A1 samples (0

5

c

2

6 at % Al) over a wide range of temperatures (80 S340 K). Only the main conclusions will be presented here. A full account of our work will be published elswhere.

(i) For low A1 content (c

&

1 at % ; incommensurate magnetic structure) both P and S exhibit the usual bumps just below TN, and sharp minima appear in dp/dT the Ndel temperature /4,5/.

4s shown in figure 1 for a sample with 0.06 at % Al, the minima in dp/dT and dS/dT near TN are remarkably similar. Indeed, if we plot d p / d ~ as a

function of dS/dT, with temperature as an implicit variable, a linear relation is found between dp/dT and dS/dT. This indicates the

same

critical behaviour.for both derivatives in the range of reduced

temperatures investigated (6 X 10-4

~1

E]$, 6 10-~). (ii) For c

2

1.5 at % A1 the magnetic anomalies in p and S are extremely small, and the bumps be- come practically undetectable. TN values are then very difficult to obtain experimentally. However, very accurate dp/dT measurements still reveal faint

(+) Work financially supported by INIC (Portugal), ESIS (Belgium) and a NATO Research Grant

(no. 1481).

minima at the magnetic transition.

Fig. 1 : Temperature derivatives dp/dT and dS/dT for Cr99.94A10. 0 6 '

This has been used to obtain new information on the complex concentration dependence of TN in dilute Cr-A1 alloys (see iv).

(iii) At higher concentrations the magnetic bumps reappear in p and S. This i~~articularlyclear from figure 2, which shows the thermoelectric power for a set of five Cr-A1 samples. As previously no- ticed, there is virtual absence of bumps for c

2

1.5 at % Al. This particular composition agrees with previous indications for the disappearance of the single incommensurate phase /1,6/.

This has been further investigated with ca-

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reful dp/dT measurements. c 4 at % the Ngel temperature increases sharply with A1 content, as in normal commensurate antifer-

romagnetic alloys (e.g. Cr-Mn).

Fig. 2 : Temperature dependence of S in dilute Cr- A1 alloys.

Indeed, for 2

2

c $ 4 at % A1 a new type of anomaly has been found in dp/dT, as shown in figure 3. Whereas above TN the usual "critical" behaviour is

readily seen in dp/dT, for T < TM this derivative Fig. 3 : dp/dT : anomalous critical behaviour in shows a striking linear temperature dependence upto CrS8.2A1

,.,.

inset : concentration dependence of

TN 1n the Cr-A1 system : (e) our data ; (0) from the immediate vicinity of TN. The onset of this li- ref. 161 ; (X) from ref. /l/.

near region is clearly marked by a kink in dp/dT at

a temperature considerably below TN. Since neutron The authors whish to thank Dr.H.Stone for the diffraction studies give evidence for the existence sample preparation, Pr. Araiijo for valuable comments of mixed magnetic phases in Cr-A1 1 3 1 , it is not un- and help, and J. Bessa for technical aid.

reasonable to associate this with the above mention- ned anomalies in dp/dT. Further work is on progress

to clarify this interesting point. References

For c 4 at %, when the antiferromagnetic lat-

/ l / Chakrabarti, D. J. and Beck, P.A., J. Phys. Chem.

tice is definitel-y commensurate 131, dp/dT exhibits Solids

32

(1971) 1609.

the usual critical behaviour on

both

sides of TN /2/ Arajs, S., Rao, K.V., Astrom, H.U. and Young,

(as in Fig. 1). T.F., Phys. Scripta

8

(1973) 109.

(iv)-our TN results are in the inset Of 131 Kallel, A. and DE ~ergevin, F., solid State Commun

5

(1 967) 955.

figure 3, together with the data available in the

141 Akiba, C. and Mitsui, T., J. Phys. Soc. Jn

32

literature /1,6/. For low A1 content- TN decreases (1972) 644.

with increasing A1 content and reaches a sharp mi- / 5 / Fote, A., Axler, R., Schurmann, K. and Mihalisin

'l,

nimum for c Q 1.5 at X. This is in fair agreement T., Phys. Rev. (1973) 2099.

with previous experimental indiCaiions /6/, but di- 161 Arajs, S., Reeves, N.L. and Anderson, LE., J. Appl. Phys.

62

(1971) 1691.

sagrees with standard theories 171, which predict

171 Fedders, P.A. and Martin, P.C., Phys. Rev.

143

an increase of TN when extra electrons are added to (1966) 245.

Cr.