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THERMOPOWER MEASUREMENTS ON Cr-Al
SINGLE CRYSTALS IN THE MAGNETIC TRIPLE
POINT REGION
J. Yakhmi, R. Walia, S. Bhatia, R. Iyer
To cite this version:
JOURNAL DE PHYSIQUE
Colloque C8, Suppl6ment au no 12, Tome 49, decembre 1988
THERMOPOWER MEASUREMENTS ON Cr-A1 SINGLE CRYSTALS IN THE
MAGNETIC TRIPLE POINT REGION
J. V. Yakhmi (Iv2), R. Walia (3), S. N. Bhatia (3) and R. M. Iyer (2)
( I ) Laboratoire de Physique des Solides, Universitd Paris-Sud, F-91405 Orsay, France (2) Chemistry Division, Bhabha Atomic Research Centre, Bombay-400 085, India
(3) Department of Physics, Indian Institute of Technology, Bombay-400 076, India
Abstract. - Thermopower measurements are reported on samples containing 0.52, 1.02 and 1.41 at % Al. The commensurate-incommensurate SDW transition is resolved for the first time for Cr
+
1.41 at % Al. Cr+
1.02 at % A1 shows a sharp N6el transition at 188 K, and a rather broad transition between 170 K and 135 K, which may arise from inhomogeneity of the sample in the sensitive region of the magnetic triple point.Below its Nkel temperature (TN), chromium is an itinerant antiferromagnet characterized by a trans- versely polarized spin-density wave (SDW) whose wave-vector is incommensurate with the reciprocal lat- tice vector [I, 21. Very unusual and diverse features have been reported about the magnetic state of Cr-A1 alloys in the concentration (c) region c
<
5 at % Al, such as, a single minimum in the TN vs. Al-content curve at about 1.2 at % A1 [3]; the existence of two minima in the TN VS. Al-content plot [4]; and thesuppression of antiferromagnetism at about 3 at %
A1 as concluded from the disappearance of typical electrical resistivity anomalies at TN, and reappear- ance of antiferromagnetism for c
2
4 at % A1 [5,61. Sharp anomalies in the magnetovolume and mag-
netoelastic measurements by Alberts and Laurens [7],
however, confirmed the presence of antiferromagnetism for c
5
4 at % Al. Yakhmi et al. [8] have recentlyreported the existence of measurable panomalies in single-crystalline samples and established the exis- tence of antiferromagnetic order for all concentrations
c
5
4.53 at % Al. No clear panomalies were obtained, however, corresponding to the C-I phase boundary re- gion was identified [9] in the magnetic phase diagram by utilizing the infrared reflectivity data obtained ear- lier [lo].In this paper, we report thermopower data on three single-crystal samples with Al-content in the vicinity
Fig. 1. - Derivative of thermopower dS/dT as a function of temperature for the three Cr-A1 samples. +U.10 +0.05 0 - 0.05 -0.10 -0.15-
-
Yx
-
<
%
- 0.20 \ m -0 +0.10- 0 -0.05 -0.10 -0.15- - 0.20 Temperature ( K ) , I t t I , I - - Cr - A a 0.52at %A1 --
- - - --I:
1.02 at % Al 1.41 at % Al--
--
I I 1 8 I I 1 I I I I I I 0 40 80 120 160 200 240 280 +0.10 +O.u5 - d.05 0 -0.10 --0.15 - 0.20C8
-
218 JOURNAL DE PHYSIQUEof the magnetic triple point. The only other thermo- electric power data available on Cr-A1 system are on polycrystalline samples [3, 111.
The single crystal samples used were the same as used earlier for infra-red reflectivity [lo] and electri- cal resistivity studies [8], and contained 0.52, 1.02
and 1.41 at % A1 as analysed by flame-emission spectroscopy. Accurate data on absolute thermoelec- tric power (S) was obtained below room-temperature. High-purity lead (99.999 % ) wires were used as ref- erence. The error in measurement of temperature did not exceed
f
0.5 K. Derivative plots of thermopower, dS/dT vs. Tare presented in figure 1 for a better per- spective of the nature of different phase transitions, which are then included in the magnetic phase dia- gram shown in figure 2.Fig. 2. - Magnetic phase diagram for Cr-A1 system. Sym- bols P, C and I denote paramagnetic, commensurate and incommensurate SDW phases. The continuous curve repre- sents the phase boundary determined earlier from electrical resistivity measurements on the same samples [9]. Note the large error-bar for the broad transition for Cr +/1.02 at %
Al. Dotted line indicates the newly resolved C-I phase boundary.
The alloy Cr
+
0.52 at % A1 shows a sharp dS/dT- minimum at 270 K in agreement with a first-order P-I transition as concluded earlier [9]. The alloy Cr+
1.02 a t % Al, however, shows interesting fe- tures, possibly due t o its close proximity t o the mag- netic triple-point composition. As it is cooled it showsa very narrow and sharp dS/dT-minimum at 188 K
which can be assigned to a P-I Nee1 transition, pre- dictably, of first order. Upon further cooling, however, it goes through another rather broad minimum which is spread out between 170 K and 135 K. We believe
that the presence of minor concentration fluctuations, which could be present even in a single-crystal, gives rise to this behaviour, particularly due to close prox- imity of three phase boundaries P-I, P-C and C-I. It is likely that some parts of the sample go through a P-I transition at 188 K upon cooling, whereas the remain- ing portions with slightly varying Al-content get con- verted into the I-phase gradually between 170 K and
135 K, giving rise to a smeared transition. Prelimi- nary calorimetric data on this sample too suggests the existence of three different transitions between 180 K and 140 K [12].
The alloy Cr
+
1.41 at % Al shows a broad dS/dT- minimum at 216 K in agreement with a P-C transition which ought to be of second order. Upon cooling fur- ther, however, it shows a first-order transition i.e. a dS/dT-minimum a t 118 K which can only be due to a C-I transition. This is the first time that this transi- tion has been resolved for any Cr-A1 dilute alloy using transport measurements.Acknowledgments
One of the authors (J.V.Y.) would like t o thank
Prof. E. Fawcett and Prof. H. U. Astrom for useful discussions regardin'g the magnetic phase diagram.
[I] Lomer, W. M., Proc. Phys. Soc. 80 (1962) 489. [2] Fedders, P. A. and Martin, P. C., Phys. Rev. 143
(1966) 245.
[3] Arajs, S., Reeves, N. L. and Anderson, E. E., J. Appl. Phys. 42 (1971) 1691.
[4] Pop, I., Dadarlat, P., Petrisov, T. and Giurgiu, A., J. Phys. Chem. Solids 42 (1981) 927. [5] Alberts, H. L. and Burger, S. J., Solid State Com-
mun. 28 (1978) 771.
[6] Sousa, J. B., Amado, M. M., Pinto, R. P., Pin- heiro, M. F., Braga, M. E:, Moreira, J. M., Hed- man, L. E., Astrom, H. U., Khlaif, L., Walker, P., Garton, G. and Hukin, D., J. Phys. F. 10 (1980) 2535.
[7] Alberts, H. L. and Laurens, J. A. J., Phys. Rev.
B 29 (1984) 5279.
[8] Yakhmi, J. V., Gopalakrishnan, I. K., Iyer, R. M. and Stanford, J. L., J. Phys. F 17 (1987) L65. [9] Yakhmi, J. V., Gopalakrishnan, I. K., Iyer, R. M.
and Stanford, J. L., J. Appl. Phys. 61 (1987) 3994.
[lo] Lind, M. A. and Stanford, J. L., J. Phys. Soc. Jpn 53 (1984) 4029.
1111 Sousa, J. B., Amado, M. M., Pinto, R. P., Mor- eira, J. M., Braga, M. E. and Ausloos. M.. J.
