Fine structure in the precursor behaviour of the thermoelectric power of Cr near the Néel transition

Solid State Communications, Vol. 61, No. 10, pp. 641-643, 1987. Printed in Great Britain.

0038-1098/87 $3.00 + .00 © 1987 Pergamon Journals Ltd.

FINE STRUCTURE IN THE PRECURSOR BEHAVIOUR OF THE THERMOELECTRIC POWER OF Cr NEAR THE NI~EL TRANSITION S.K. Patapis Department of Physics, Solid State Section, University of Athens, 104 Solonos Str., Athens, Greece

(Received 21 October 1986 by S. Amelinckx) The detailed behaviour of the thermoelectric power of high purity single crystal of chromium just before the Nrel transition temperature is presented. The change of the thermoelectric power in this temperature region shows a set of small scale anomalies, observed for the first time, which accompany the well-known large dip at the Nrel temperature. Similar precursor anomalies one can distinguish in other researcher's measurements (Fote et al, [8]). Also a comparison of the thermoelectric power with electrical resistivity of chromium in the same temperature region shows similar fine structure behaviour as it is expected. INTRODUCTION RECENTLY MEASUREMENTS of the transport properties near magnetic critical points have been undertaken; however studies on high purity single crystals of chromium are relatively scarce. In this paper the thermoelectric power behaviour of a high purity single crystal of chromium is presented when approaching the Nrel temperature from below. In spite of the fact that many studies of the transport properties behaviour near magnetic critical points present inconsistencies and non-reproducible effects [1] the detailed behaviour of the thermoelectric power as it comes out from the present measurements is similar to the one investigated before by other researchers and it can be compared to the behaviour of the resistivity in the same temperature region. Chromium is a VI B transition metal, antiferromagnetic up to the temperature of about 311 K which is the Nrel temperature T~¢. It also exhibits a reversal of spin direction near l12K. Its itinerant antiferromagnetism [2] arises from some of the conduction electrons which condense through an exchange interaction to form a static spin density wave (SDW) [3]. Between 112 K and the Nrel temperature the SDW has a polarization transverse to the spin while below the spin-flip temperature (112 K) it has a longitudinal one. The SDW is incommensurate with the reciprocal lattice [4]. The phase transition at Tt¢ from antiferromagnetic to paramagnetic is contradictory concerning its order. Although, concerning the pure material, this transition is believed to be of first order [4-7] there are contradictions supporting a second order transition [8, 9].

There have been many measurements of various physical and mechanical properties near TN and anomalous change has been observed. This anomalous behaviour ranges from small changes in the lattice parameter [6] to anomalies in the transport properties. Among the latter the thermoelectric power is included but its behaviour is not extensively investigated. Most of the meaurements are concerned with the influence of alloying on the behaviour of this property and few with its detailed behaviour in high purity material [8, 10]. To the best of my knowledge this is the first detailed investigation of the thermoelectric power in a high purity single crystal. EXPERIMENTAL CONSIDERATIONS AND RESULTS The Seebeck coefficient was measured on a single crystal chromium specimen of purity as high as 99.995% obtained from Metals Research Limited (England). The thermoelectric power was measured for a thermal gradient along the [0 0 1] direction of the crystal which had a length of 8 mm. The Nrel temperature T~¢of this sample was at about 316 K as it was determined from the change of lattice constant with temperature through X-ray studies [11]. The sample was mounted on a worked flat end of a ceramic rod which was mounted in a thermostat with a bath of water continuously furnished, through a closed circuit, from a tank the temperature of which was electrically controlled. The Seebeck coefficient was measured with the differential method. The temperature difference AT, between the two ends of the sample was about 0.5 K, sufficient to measure the thermoelectric potential

641

T H E R M O E L E C T R I C P O W E R O F Cr

642

22

Y 21 > 20

I I% 290

500

310

T (K)

Fig. 1. The thermoelectric power of chromium as a function of temperature during heating. across the sample with a good accuracy but small enough to monitor details o f the transition. This potential difference (Seebeck e.m.f.) was directly measured on a digital voltmeter of D A N A Laboratories with 6 significant figures. The mean temperature of the sample could be stabilized as well as to change in a rate of 3 K per hour, conditions quite good to be considered steady. The time needed for steady state conditions to be established for each data point was about 15 min after the bath of the thermostat had been raised to a new temperature. At each temperature, the thermoelectric potential was measured in polarity - - reversed pairs and the mean value was considered. The consecutive measurements were taken under steady-state conditions in the order of increasing temperature (from about 280 K); their accuracy was about 0.5% and the magnitude of their reproducibility was approaching that of 1%. The resulting thermoelectric power change with temperature below TN is shown in Fig. 1. In the above figure one clearly sees the characteristic rapid decrease with increasing temperature as the Nrel temperature is approached. Such a dip is reported by all authors. The interesting point o f the present measurements is the behaviour of the thermoelectric power just before this dip. A careful consideration of this part shows a set of small precursor anomalies accompanying the large scale dip. This fine structure from a first point o f view might not be considered as a real physical event and ascribed to accidental events in the experimental technique, but it seems that the latter is not the reason because the same type of results was found by Fote et al. [8]. Their measurements, with an accuracy 0.3%, were made on a high purity sample composed of three large crystals [8]. Figure 2 is a drawing based on the measurements as listed in the table o f [8]. (The original figure of this paper cannot be used since it is drawn in a smaller

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scale and the detailed structure o f the thermoelectric power cannot be observed; its fine structure is intimately related to that presented in this paper.) Small anomalies with the same features and with similarity in their sequence can be observed between the two figures (Figs. 1 and 2) as the temperature of the sample approaches TN from below. The main difference lies in the magnitude of the observed anomalies. They are smaller (about 3 times) than those of Fig. 1. These differences may be attribed to a difference in the purity of the two samples. That this fine structure of the thermoelectric power is a real effect can be also argued by the fact that a similar behaviour in the same temperature region can be observed in electrical resistivity measurements. This can be seen in the measurements of Williams and Street [1] with a high purity (99.999%) polycrystalline sample both when heating and cooling. The same effect related in a more intimate way to the thermoelectric power behaviour was also found in the resistivity measurements by Meaden and Sze [12]. The similarity in the behaviour of the above two transport properties is consistent with the theoretical prediction of a universal critical behaviour of all transport properties [13, 14]. This similarity has been noticed before in various materials [14, 15] and it is known in Cr from the above mentioned large scale dip present in both transport properties [16]; but this similarity seems to extend to the fine structure of the above properties as well. This fine structure of the thermoelectric power with temperature observed in the present measurements may be ascribed to "precursor" effects which occur at the antiferromagnetic-paramagnetic transition o f Cr or may be related to the "anomaly" of the first order transition which masks the second order occurring at a higher temperature if one approaches Ts from below with effects of fluctuations still noticeable [8].

22 21.5 *'~q~~ 21 I

I

290

300 T(K)

Fig. 2. The thermoelectric power of chromium drawn according to the data by Fote et al. [8].

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