Transport and magneto-transport properties of U2Ni2Sn

Journal of Magnetism and Magnetic Materials 157/158 (1996) 698-699

JR

journalof magnetism and magnetic materials

ELSEVIER

Transport

and magneto-transport

properties

o f U 2N i 2 S n

R.P. Pinto a M.M. Amado a M.E. Braga a, M.A. Salgueiro a J.B. Sousa a,* B. Chevalier b, D. Laffargue b, j. Etourneau b a IFIMUP (IMAT) and CFUP, Laborat6rio de Ffsica, Faculdade de Ci~ncias, Universidade do Porto, P-4050 Porto, Portugal b Inst. Chimie Mati~re Condens£e de Bordeaux, Av. Dr. A. Schweitzer, F-33600 Pessac, France Abstract We report the results of measurements of the magnetoresistance ( A p / p ) , thermopower (S) and magneto-thermopower (AS) for antiferromagnetic UzNizSn (T N = 25 K), from 3.7 to 100 K and in magnetic fields up to 1 T. Critical spi n fluctuation effects are observed in d p / d T , d S / d T , A p / p and S. Large negative thermopower values occur below ~ 0.8T N (and also a sharp decrease of p), resembling a spin fluctuation resonance process. However, both A p / p and AS are positive, which is anomalous in this model. Ke3words: Magnetoresistance; Thermopower; Magneto-thermopower; UzNi 2Sn; Intermetallic compounds

The tetragonal stannide U2Ni2Sn orders antiferromagnetically at TN = 25 K, as revealed by neutron diffraction and M~Sssbaner spectroscopy [1]. A sharp cusp occurs in the magnetic susceptibility (X), and characteristic minima are observed in the temperature derivatives of the electrical resistivity ( d p / d T ) and thermopower ( d S / d T ) [2]. A Curie-Weiss fit of x ( T ) above TN gives an effective magnetic moment /xeff = 3.15 / x B / U atom, slightly lower than that for trivalent U 3÷ (3.68/zB). In contrast, U2Co2Sn is paramagnetic down to 4.2 K (/zoff = 2.5 / x B / U atom) and the transport properties behave quite differently [2]. This shows that the 5f magnetic moments, corresponding interactions and electron scattering depend critically on the degree of 5f-electron localization, through the 5f hybridization with the conduction-band electron wavefunctions. UeNiaSn polycrystalline buttons were prepared by rf levitation melting, followed by annealing at 800°C for one week in evacuated quartz tubes. X-ray powder diffraction showed a single-phase tetragonal ordered structure derived from the U3Si2-type structure. The temperature dependence of the thermopower in zero field is shown in Fig. 1 for the range 5 - 6 5 K. At high temperatures S(T) exhibits small and negative values up to room temperature ( S = - 2 /zV K - i ) [2]. The onset of magnetic order is marked by a sharp negative minimum in d S / d T (Fig. lb), suggesting superzone gap effects just below TN associated with the antiferromagnetic periodic-

* Corresponding author. Fax: + 351-2-319267.

ity. This effect (and that of critical fluctuations) gives a positive contribution to the thermopower near TN, making S slightly positive just below T•. However, a competing (negative) thermopower contribution exists below TN, which grows rapidly and completely dominates S(T) for T_< 20 K ( ~ 0.8TN). It produces fairly large thermopower values, towards a pronounced minimum around 8 K, where S -- - 12 /,V K -1. Similar S(T) minima have been reported for heavy-fermion systems such as U2Co3Si 5, CeCu2Si 2 and CeA13 (at

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Fig. 1. Temperature dependence of the thermopower (S) in zero field for UzNi2Sn. Inset (a): temperature dependence of electrical resistivity derivative ( 1 / p ) d p / d T for U2NiaSn. Inset (b): temperature dependence of thermopower derivative (dS/dT) for U2Ni2Sn.

0304-8853/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. SSDI 0 3 0 4 - 8 8 5 3 ( 9 5 ) 0 1 2 1 5 - X

R.P. Pinto et al. / Journal of Magnetism and Magnetic Materials 157/158 (1996) 698-699

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Fig. 2. Field dependence of A p / p for U2Ni2Sn. T = 7, 20 and 4 K, respectively), and have been explained within the spin fluctuation resonance model for such compounds. This model is a modern generalization (to dense crystalline magnetic systems) of the single-site Kondo resonance problem in dilute magnetic alloys [3-5]. The temperature dependence of d p / d T (Fig. la) also exhibits a sharp minimum at TN. Below TN, p(T) decreases abruptly, as indicated by the large positive d p / d T values. This trend is associated with the onset of the low-temperature coherent state [5], since the heavy-fermion crystalline system ceases to be a collection of independent Kondo-like scatterers when the magnetic moments order below T N. The reduction in the resonant electron scattering due to increasing coherence with decreasing T makes other scattering mechanisms gradually more important, such as impurity and electron (Fermi fluid) interactions. This can explain the decrease in IsI towards zero, which starts 12. 8. Q.

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below ~ 8 K, and the approximate T 2 decrease in p(T) over the same temperature range. Further insight into the electron scattering was obtained with measurements of the magnetoresistance (A p / p ) and magneto-thermopower (AS), using magnetic fields /-toH up to 1 T over the temperature range 4 . 2 - 6 0 K. Fig. 2 shows a detailed set of magnetoresistance curves (with A p = p(T,H) - p(T,O)), taken both above and below T N. The magnetoresistance is always positive and reaches saturation at moderate critical fields (Hc). H c is fairly small at low temperatures, reaches a maximum around 0.4T N ( ~ 1 T) and then decreases towards zero at TN. The magnetic field dependence of the thermopower, AS = S(T,H) - S(T,O), is also positive and similar to that observed in A p / p , as it exhibits a linear dependence with A p / p (for values at the same field H). Fig. 3 shows the temperature dependence of A p / p and AS taken at / z 0 H = 0.9 T, over the temperature range 6 - 8 0 K. Both A p / p and IASI exhibit cusps at TN, reflecting the effect of H on the critical spin fluctuations (giving A p > 0 and AS < 0). One expects a rapid decrease in such fluctuations when T decreases below TN, which is consistent with the data in Fig. 3 (both A p / p and A S become nearly zero at T = 0.8TN). At lower temperatures we observe a new effect, producing a very large positive peak in both A p / p and AS around 15 K. This behaviour is anomalous, since all theoretical treatments based on the spin fluctuation resonance model or on the alternative electronic polaron model for dense heavy-fermion systems [4], give negative A p and AS values, with a H 2 field dependence. These predictions have recently been observed in U2Ru3Si 5 [4]. In conclusion, the behaviour of the temperature dependence of the electrical resistivity and the thermopower in UzNi2Sn seems to be consistent with expectations from the spin fluctuation resonance model with a progressively coherent state below T N. However, the behaviour of the magnetoresistance and magneto-thermopower cannot be described by such a model, nor by the alternative electronic polaron model [4]. Acknowledgement: This work has been partially supported by Project S T R D A / C / C E N / 5 2 2 / 9 2 . References

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Fig. 3. Temperature dependence of the magnetoresistance (A p / p ) and magneto-thermopower (AS) for U2NizSn.

[1] F. Mirambert, P. Graverean, B. Chevalier, L. Trut and J. l~toumean, J. Alloys Compounds 191 (1993) L1. [2] R.P. Pinto, M.M. Amado, M.A. Salgueiro, M.E. Braga, J.B. Sousa, B. Chevalier, F. Mirambert and J. ]~tourneau, J. Magn. Magn. Mater. 140-144 (1995) 1371. [3] F. Steglich, C.D. Bredl, W. Lieke, U. Rauchschwalbe and G. Sparn, Physica B 126 (1984) 82. [4] S.H. Liu, in: Handbook on the Physics and Chemistry of Rare Earths, vol. 17, eds. K.A. Gschneidner et al. (Elsevier, Amsterdam, 1993) p. 87. [5] L. Piraux, E. Grivei, B. Chevalier, P. Dordor, E. Masquestant and J. l~tourneau, J. Magn. Magn. Mater. 128 (1993) 313.