1249 T H E R M A L C O N D U C T I V I T Y OF N E O D Y M I U M AND E U R O P I U M AT L O W
TEMPERATURES V. H A J K O , S. J A N O ~ , A. F E H E R and P. P E T R O V I ( ~ Department of Experimental Physics, Faculty of Sciences, Safarik University, 041 54 Kogice, Czechoslovakia The thermal conductivity of antiferromagnetic neodymium and europium has a linear temperature dependence below 5 K; no anomalous behaviour was observed in neodymium below 6 K. The electrical and thermal magnetoresistance of neodymium below 4 K in the magnetic fields higher than 5 kG is negative, while in the case of europium is positive.
1. Introduction The investigation of the thermal conductivity of pure h e a v y rare-earth metals below 4 K  shows that the thermal conductivity can be separated into a t e r m linear and a quadratic in the temperature. The linear term represents an electronic contribution of impurity scattering. The quadratic term is mostly due to a p h o n o n contribution. H o w e v e r , there is also evidence of m a g n o n heat conduction in thulium and in erb i u m - y t t r i u m alloys . A detailed investigation of the thermal conductivity of the light rareearth metals below 5 K was not reported up to now. The results of m e a s u r e m e n t s  showed an additional contribution to the thermal conductivity of n e o d y m i u m below its N6el temperature TN = 7.5 K, which can be of magnetic origin. The thermal conductivity of europium at low t e m p e r a t u r e s , as far as we know, has not b e e n reported yet.
2. Experimental procedure The thermal conductivity was m e a s u r e d b y a conventional steady-state heat flow method. The specimen was m o u n t e d with one end thermally anchored to a liquid 4He or 3He bath. The electrical heater attached to the other end provided a heat current through the specimen. The resulting t e m p e r a t u r e gradient along the specimen was m e a s u r e d with the aid of two AllenBradley carbon resistance t h e r m o m e t e r s calibrated against 4He and 3He v a p o u r pressure m e a s u r e d b y means of a M K S Baratron capacitance m a n o m e t e r type 170 with a sensor head type 145 BHS. The magnetic field was parallel to the heat current and was provided b y a superconducting solenoid. The electrical resistivity was m e a s u r e d b y the four-terminal m e t h o d using p o t e n t i o m e t e r with 10 - a V sensitivity. Polycrystalline n e o d y m i u m and europium were Physica 86--88B (1977) 1249-1250 @ North-HoUand
obtained f r o m T e c h s n a b e x p o r t M o s k v a , USSR. The analysis certificate stated that the neodymium contained 0.01% Fe, 0.02% Si, 0.005% Ca, 0.05% Pr, 0.05% Ce, 0.02% La, 0.05% Sm, 0.001% H and 0.007% N. Polycrystalline europium contained 0.01% Fe, 0.01% Nd, 0.01% Gd, 0.01% Sm, 0.04% Si, 0.005% N, 0.02% C and 0.002% H. Our m e a s u r e m e n t s were carried out on rod-shaped samples which had a diameter of 5 mm. The resistivity ratios p3oo/p4,2 of neodymium and europium were found to be 6.2 and 11.5, respectively.
3. Results The results of the longitudinal magnetoresistance Ap/po of n e o d y m i u m are plotted in fig. 1. We o b s e r v e d small positive m a g n e t o r e s i s t a n c e in the magnetic fields up to 5 k G and a b o v e 5 kG up to 36 kG a negative magnetoresistance. The negative longitudinal m a g n e t o r e s i s t a n c e a b o v e 1 0 k G was also o b s e r v e d in . We o b s e r v e d some hysteresis of the magnetoresistance, which is also presented in fig. 1. The negative m a g n e t o r e s i s t a n c e of n e o d y m i u m is thus probably related to the change of domain structure. The thermal conductivity results of n e o d y m i u m 10-
15 B (kG)20
Fig. 1. The electrical magnetoresistance of neodymium at 4.2 K. The arrows indicate change of the magnetic field.
T: L,2 K
-o,3~ ~ , B T(K) - -
Fig. 2. The thermal c o n d u c t i v i t y and thermal magnetoresis-
lance of neodymium.
Fig. 4. The thermal conductivity and thermal magnetoresistance of europium.
are presented in fig. 2. The magnetic field dependence of the thermal magnetoresistance AW[Wo at temperature 4. 2K is shown in the inset inserted in fig. 2. The thermal conductivity K in zero magnetic field could be fitted by the expression K = 3.3 T (mW/cmK]. For the magnetic field 30kG we obtained the expression K = 4 . 4 T (mW/cmK]. Our results did not confirm the anomalous behaviour of the thermal conductivity which has been reported below 6 K in . It seems to be probable that different behaviour is related to the relative fraction of the d.h.c.p, and f.c.c, phases of a given sample. The results for the longitudinal electrical mag-
B (kO) - - - -
netoresistance of europium are plotted in fig. 3. In magnetic fields up to 36 kG we observed a positive magnetoresistance and a larger hysteresis than in the case of neodymium. The magnetic field dependence of the thermal resistance at temperature 4.2 K is shown in the inset inserted in fig. 4. The thermal conductivity of europium in zero magnetic field could be fitted by the expression K = 6.48 T [mW/cmK] and for a magnetic field 30 kG we obtained the expression K = 6 . 1 5 T (mW/cmK]. The dependence of the thermal conductivity of europium and neodymium upon the magnetic field is similar to the dependence of the electrical magnetoresistance. However, the increase of the thermal conductivity of neodymium in a magnetic field is larger than the change calculated from the Wiedemann-Franz law.
Fig. 3. The electrical magnetoresistance of europium at 4.2K.
 R. Ratnalingam and J.B. Sousa, J. Low. Temp. Phys. 4 (1971) 401.  V. Hajko and S. Jfi.no~, in: Proceedings 14th Int. Conf. Low. Temp. Phys., M. Krusius and M. Vuorio, eds. (North-Holland, Amsterdam, 1975) p. 306.  K.T. Tee, K.V. Rao and G.T. Meaden, J. Less Comm. Metals 31 (1973) 181.  H. Nagasawa, Physics Letters 41A (1972) 39.