Critical behaviour of transport coefficients at a structural-ferromagnetic transition

Accurate measurements of the electrical resistivity, the thermoelectric power and the thermal conductivity of TbZn are reported at the ferromagnetic-structural transition. The temperature derivative of the coefficients is examined. The critical behaviours are understood in terms of conduction electron scattering on spin correlated fluctuations and the presence of structural and magnetic domains. A detailed characterization of the static properties of the equiatomic Cs-C1 compounds of the rare earth metals with Zn is available [I]. Here we extend our knowledge of such systems to the case of transport properties, using a monocrystalline sample of TbZn. It has a ferro-paramagnetic transition at Tc = 200 K, with an apparently simultaneous cubic to tetragonal transition ; the easy magnetic direction occurs along a quaternary axis. At TR = 63 K, the lattice distortion practically disappears and the easy magnetic direction rotates to a binary axis [I]. Figure 1 shows the general behaviour of the electrical resistivity @) and its temperature derivative dp/dT along a quaternary axis. The data have been normalized by a factor pc = p(Tc). The phase transitions in TbZn are clearly revealed in the dp/dTcurve, which is remarkably similar to that of the specific heat (C) (inset of figure 1 ; from [I]). The ferroparamagnetic transition induces a peak in dp/dT, followed by an extended tail in the paramagnetic phase due to short range order. The transition at TR is accompanied by a latent heat and marked by a very high and narrow peak, in a manner characteristic of 1st-order transitions [2]. The resistivity is practically disconfinuous at T,(Ap/p 2. 5 %), whereas dp/dT changes by 20 %. A qualitatively similar mismatch occurs also. in C at TR (see inset). (*) Work supported by INIC, Portugal e), ESIS, Belgium (*), NATO Research Grant 1481 e), (b). Fig. 1. Normalized c-axis resistivity and temperature derivative of single crystal TbZn ; inset : specific heat from [I]. The thermoelectric power (S) has been measured from 90 to 300 K. The temperature gradient was along a quaternary axis. Figure 2 shows that S is negative. The critical temperature T,, as derived from dpld T measurements is indicated. The pronounced increase in S below Tc is indicative of a H,, conducArticle published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979516 CRITICAL BEHAVIOUR OF TRANSPORT COEFFICIENTS AT A STRUCTURAL TRANSITION C5-43 Fig. 2. c-axis thermoelectric power of single crystal TbZn. Fig. 3. c-axis temperature derivative of thermoelectric power and normalized thermal conductivity of TbZn. tion electron-localized spin interaction [3]. This behaviour is similar to that observed in pure Tb along a basal axis [4]. The data for S above T, practically extrapolate through the origin, whereas its slope is rather small and negative. This indicates a holedominated thermopower, in contrast to pure Tb, where the slope above T, is positive. Figure 3 shows the derivative dS/dT for TbZn. It is remarkable that dS/dT presents a characteristic ),-shape anomaly, with a sharp peak at T,, reminiscent of that found in dp/dT near T,. Important differences are apparent at lower temperatures. Whereas dS/dT decreases smoothly and monotonically with T, dp/dT exhibits anomalously high values and a complex behaviour. Any reasonable theory must explain why such effects are so exhuberantly present in dp/dT and not in dS/dT [5]. We have also measured the ihermal conductivity (K), using a novel experimental approach described elsewhere [6]. This enables the small changes of K near T, to be accurately measured (figure 3). For convenience, a linear background has been previously extracted, so as to reveal more clearly the changes in K near T,. The main features are a localized depression in K near T, and a pronounced increase in the slope dK/dT when the sample enters the paramagnetic state. The depression in K is consistent with the expected reduction in the mean free path of the long wavelength phonons caused by the critical fluctuations [7]. The increase in dK/dTcan be qualitatively understood in terms of the electronic contribution to K [a]. Finally the existence of time-dependent effects below T, is most probably due to magnetic and structural domains. The influence of such domains on transport properties is being worked out theoretically [5]. Technical assistance of J. Bessa is gratefully acknowledged.