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Submitted on 1 Jan 1979
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Electrical resistivity, thermopower and X-ray structure investigations on RECo2 (RE = Nd, Tb, Dy, Ho, Er) as
a function of temperature
E. Gratz, H. Sassik, H. Nowotny, W. Steiner, G. Mair
To cite this version:
E. Gratz, H. Sassik, H. Nowotny, W. Steiner, G. Mair. Electrical resistivity, thermopower and X-ray
structure investigations on RECo2 (RE = Nd, Tb, Dy, Ho, Er) as a function of temperature. Journal
de Physique Colloques, 1979, 40 (C5), pp.C5-186-C5-187. �10.1051/jphyscol:1979568�. �jpa-00218985�
Electrical resistivity, thermopower and X-ray structure investigations on RECo 2 (RE = Nd, Tb, Dy, Ho, Er) as a function of temperature
E. Gratz, H. Sassik
Institut fiir Experimental Physik, TU Wien (*) H. Nowotny
Institut fiir Theoretische Physik, TU Wien (*)
W. Steiner
Institut fiir Angewandte Physik, TU Wien (*)
and G. Mair
Institut fiir Festkorperforschung, KFA Jiilich, BRD
Abstract. — Electrical resistivity and thermopower measurements on some RECo
2compounds (RE = Er, Ho, Dy) show marked discontinuities at the magnetic ordering points. This behaviour, typical for compounds with first order phase transitions, is explained by a discontinuous variation of the Fermi of surface at the first order transition point. This variation could be caused by a distortion of the MgCu
2unit cell as detected by X-ray investigations.
In our previous investigations [1,2], we have shown that the P-T behaviour of RECo
2(RE = Er, Ho, Dy) compounds is characterized by marked discontinuities at T
c. According to Bloch et al. [3] it is known that in the above mentioned RECo
2compounds the magnetic transition is of first order type. In figure 1 we collected our resistivity measurements from a number of different RECo
2compounds to show the influence of a first order and a second order transition. In the present work we discuss the possible causes of the drastic change in transport phenomena for the case of first order transitions. Therefore we extended our transport investigations to thermopower and X-ray structure measurements. In figure 2 the absolute thermopower is shown together with the resistivity in the vicinity of the transition point for the case of ErCo
2. The same behaviour is found for HoCo
2and DyCo
2. In contrast to this behaviour an example of a second order transition (TbCo
2) is included in this figure. In the theory of transport phenomena [4] one can derive the following expression for thermopower
A, mean free path of conduction electrons;
S(s), area of the energy surface with energy e, This formula is valid for cubic crystals assuming the relaxation time approximation. As discussed in [4], the first term can be taken to be positive and slowly varying. That means that a sudden drop of the ther- mopower (as shown for ErCo
2in figure 2) is mainly dictated by the second factor (the derivative of the area of the Fermi surface with respect to the energy) in the above equation. Therefore one can deduce from these measurements that for a first order transi- tion an alteration of the shape of the Fermi surface must take place, which causes a change in the sign of the second factor in the above equation. This alteration of the shape of the Fermi surface can be caused on the one hand by the jump of the internal magnetic field (which is characteristic of a magnetic first order transition), on the other hand by an alteration of the Brillouin zone (in this case a distortion of the unit cell must be detected). The aim of our previous X-ray investigations was to find out whether the discontinuity of the lattice parameter at the transition point [5, 6]
is associated with a distortion of the cubic MgCu
2structure. For the case of ErCo
2we found a trigonal distortion of the MgCu
2structure (see also [7]) which is connected with the known easy axis [111] of this
(*) Techn. Univ. Vienna, Karlsplatz 13, A-1040 Austria.
JOURNAL DE PHYSIQUE Colloque C5, supplément au n° 5, Tome 40, Mai 1979, page C5-186
Résumé. — Des mesures de résistivité électrique et de pouvoir thermoélectrique de quelques RECo
2(RE = Er, Ho, Dy) montrent des discontinuités remarquables aux températures d'ordre magnétique. Ce comportement, typique pour des composés avec une transition de premier ordre, est à expliquer par une variation discontinue de la surface de Fermi au point de transition du premier ordre. Cette variation pourrait être provoquée par une déformation de la cellule unitaire de MgCu
2détectée par diffraction des rayons X.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979568
ELECTRICAL RESISTIVITY, THERMOPOWER AND X-RAY STRUCTURE INVESTIGATIONS ON RECO, C5-187
60
Fig. 2. -Electrical resistivity and thermopower of ErCo, and TbCo, in the vicinity of T,.
40 -
compound. The detailed results with respect to X-ray investigations will be published elsewhere.
In conclusion the sudden large change of resistivity and thermopower (some hundred percent) in a small temperature range (figure 2) may be explained by the above mentioned alteration of the Fermi surface which influences in a dramatic way the conduction electrons at these first order transition points. Such
TLKI
- critical behaviour should particularly be expected when the 3d-moment is strongly influenced by the RE-moment as is known to be the case in this family
Fig. 1. - The p vs T curves of RECo,. of RECO, compounds.
References
[I]
GRATZ, E., Proceedings of
EPS Study Conference, Cavtat,[4] ZIMAN, J. M., Electrons and Phonons (Oxford University Press,
Yugoslavia (1977). London) 1960.
[2]
