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THE ONSET OF FERROMAGNETISM IN HCP CoCr
ALLOYS
S. Kilcoyne, A. Hannon, R. Cywinski
To cite this version:
JOURNAL DE PHYSIQUE
Colloque (28, Suppldment au no 12, Tome 49, ddcembre 1988
THE ONSET
OFFERROMAGNETISM IN HCP
G C rALLOYS
S. H. Kilcoyne (I), A. C. Hannon (2) and R. Cywinski (I)
(I) JJ Thomson Physical Laboratory, University of Reading, Reading, RG6 ZAF, G.B. (2) Neutron Division, Rutherford Appleton Laboratory, Chilton, OX11 OQX, G.B.
Abstract. - Spin correlations close to the critical concentration for ferromagnetism in HCP &Cr have been studied using SANS and a.c. susceptibility. The.critica1 concentration is 69 at.% Co above which the correlation length does not diverge at Tc but exhibits a scaling behaviour similar to that observed for many inhomogeneous systems.
Cr bas a solid solubility of approximately 40 at% in hexagonal close packed Co. The average magnetic moment decreases a t a rate of 6.5 p ~ / C r atom [I], this decrease being attributed to a reduction of the mo- ment of those Co atoms which are nearest neighbours t o a Cr impurity 121. The Cr moment itself is 0.45 PB oriented parallel to the Co moments. Low field dc magnetic measurements [3] show that ferromagnetism disappears between 25 and 29 at% Cr. In this crit- ical region magnetic order proceeds from superpara- magnetism, via ferromagnetism to a spin glass state. At higher Cr concentrations the superparamagnetic t o spin glass transition is direct. In this study we have used small angle neutron scattering (SANS) and ac susceptibility to investigate the natwre of the magnetic correlations in the critical region of HCP C o l ~ o - ~ C r , (24
<
x<
32).Samples were prepared by argon arc melting appro- priate quantities of 4N5 pure Co and Cr. The result- ing ingots were machined to 10 mm diam x 1 mm thick discs and annealed a t 920 OC for 7 days. Neu- tron diffraction patterns obtained using POLARIS at RAL's ISIS pulsed source showed the alloys to be sin- gle phase HCP. SANS measurements were made using Dl7 a t ILL, Grenoble.
The variation of the scattered neutron intensity with temperature for a range of scattering vectors, Q =
477 sin B / X , for the alloy containing 26 at% Cr is shown in figure 1. The illustrated behaviour is typical of that observed for all the alloys (with the exception of x =
32). At high Q values Q
(
>
0.025A-1
)
a well defined but extremely broad critical peak is observed at Tc, while at lower Q there is a massive increase in intensity as T decreases below Tc towards 4.2 K.Above T, the Q-dependence of the scattering is ex- tremely well fitted by a Lorentdan function, I (Q) = l o / ( K ~ -1- Q ~ )
.
However the range parameter, K-l, does not diverge at Tc but tends to a value<
400A.
A fit of the variation of K~ with reduced tempera-ture { ( T
-
Tc) /Tc)" leads to a critical exponent v = 1.20 f 0.03. Remarkably this scaling relation and even the value of v itself is identical to that observed for a number of disordered systems including PdNi, CrFeand amorphous (FeMn)PC [4]. For the x = 26 alloy shown in figure 1 the fitted T, is 171.03~ 0.5 K (although it should also be noted from the inset to Fig. 1 that the temperature of the critical peak decreases with de- creasing Q).
Below Tc a Lorentzian description of the scatter- ing is no longer appropriate, and the &dependence is better represented by a squared Lorentzian. The pronounced increase in intensity towards low T is not fully understood. In other disordered systems this be- haviour has been interpreted [4] as arising from the growth- of magnetic clusters close t o the percolation threshold, ccdt. Percolation theory leads one to expect this scattering to increase as c,,it is approached from below, and decrease above cc,it once the infinite clus- ter has been formed. In Coloo-,Cr,, however, for the alloy with x = 32 (which shows no critical peak and is therefore just below cc,it), there is no such subcritical scattering, while, on the other hand, it is still promi- nent at x = 24, well above cc,it. We must therefore tentatively ascribe this scattering t o massive magnetic
Temperature (K)
Fig. 1. - Small angle scattering intensity at a given Q as
a function of temperature for Co74Crz6. The inset details the scattering close to T , at the higher Qvalues.
C8 - 224 JOURNAL DE PHYSIQUE
Fig. 2. - Temperature dependence of the a.c. susceptibility (330 Hz and 1 Oe rms field) for & C r alloys in the range 24 to 32 at % Cr. /, ... ".'" ... -. - -.. J : 2 -.a5 0 50 100 150 2M) 250 MO Temperature (Kl
Fig. 3. - First differential of the ac susceptibility with respect to temperature, dX/ dT, for the Co74Cr26 alloy. inhomogeneities which exist only in the ferromagnetic region of the system.
The non-divergence of the correlation length, to- gether with the broad critical peak, and evidence for magnetic clustering obtained from the SANS mea- surements suggests that, despite relatively high Curie points, the ferromagnetic order in Q C r alloys is lim- ited in range. This view is confirmed by the a.c. sus- ceptibility measurements (made upon 1 x 1 x 4 mm3 specimens out from the neutron samples) shown in fig- ure 2. Firstly it should be noted that none of the al- loys shows the demagnetisation cut-off characteristic of
"good" ferromagnets. Indeed the maximum observed ac signal shown in figure 2 is only 30 % that of a pure Co sample of an identical shape and size. Secondly the form of the susceptibility curves are closely similar to the low field dc measurements of [3], with cluster glass- like behaviour being clearly observed for 26
<
x<
32. The sharp rise in X (T) for T<
30 K exhibited by theFig. 4. - Magnetic phase diagram of the & C r system close to the critical concentration. See text for the definition of Tc
.
x = 24 alloy may well represent a contribution from a small amount of FCC Q C r contamination. cc,;t for this phase is believed to be 76 at % Co). The temper- atures at which the maxima in X (T) are found agree well with the previously reported Curie points [3]. Cu- riously, however, there are no obvious features in X (T) which can be related to the critical scattering peaks observed by the SANS. Such a situation has also been found for Q F e alloys at concentrations close to cc,;t [5]. For t h e Q C r system we have noted that differentiation of
x
(T) with respect to T leads to a pronounced min- imum at a temperature close t o that of the observed critical scattering: for example (dX/ dT),,;, is found at 167 K for the x = 26 alloy, as shown in figure 3. Choosing this criterion as the definition of the order- ing temperature we are able to construct the magnetic phase diagram shown in figure 4, where it can be seen that the critical concentration for ferromagnetism is approximately 69 at % Co.Acknowledgments
We are indebted t o Dr. C. Ritter for assistance with the SANS measurements. SHK acknowledges financial support from the NBRC.
[I] Crangle, J., Philos. Mag. 2 (1957) 569.
[2] Cable, J. W. and Hicks, T. J., Phys.
Rev.
B 2 (1970) 176.[3] Gavoille, G., Durupt, S. and Hubsch, J., J. Phys.
France 43 0982) 773.
[4] Rainford, B. D. and Burke, S. K., J. Appl. Phys.
53 (1982) 7660.
