Pergarnon Solid State Cmnmunications, Vol. 102, No. 4

01997 &“c&!-F~~

Printed in Great FSri~l~g,

PII: SOOS-1098(96)00781-8

RESISTMTY AND MAGNETO-RESISTMTY OF ANTIFERROMAGNETIC UzPtzSn

Andre M. Strydom and Paul de V. du Plessis

Department of Physics, University of Witwatersrand, Private Bag 3, Wits 2050, Johannesburg, South Africa

(Received 16 October 1996; accepted 16 December 1996 by M.F. Colhs)

The temperature dependence of the resistivity, p(T) and magnetoresistivity (T z 1.5 K, B I 8 T) of polycrystalline UrPtrSn are reported. p(T) shows maxima at 12 K and 45 .K and a minimum at 19 K. A minimum is observed in d@)/dT at the Neel temperature of 15 K. The magnetoresistance at low temperatures is negative and below TN it displays a step-like decrease in an applied field of about 2 T indicating the presence of a phase transition. 0 1997 Elsevier Science Ltd. All rights reserved

Keywords: A. magnetically ordered materials, D. electronic transport.

1. INTRODUCTION

A rapidly expanding literature exists already on the magnetic and electronic properties of the UzTzX com- pounds (T = 3d, 4d or 5d transition elements and X = Sn or In). Results of magnetic susceptibility and specific heat studies on polycrystalline U2T2X samples have been summarized by Havela et al. [ 11. A weakening of magnetism as a result of increased 5f-d hybridization occurs when moving from right to left in a d-transition metal row. Thus it is observed that the following U ZTrSn compounds order antiferromagnetically below their respective NCel temperatures TN as indicated below:

T atom : Ni Pd Pt Rh

TN(K): 25 41 15.5 24

The compounds UrCorSn and U21rzSn on the other hand are classified as spin fluctuators which are characterized by a large increase in susceptibility, but without under- going magnetic order when cooled down to 1.6 K. The electronic contribution to the specific heat of the anti- ferromagnetic compounds and of the spin fluctuators is considerably enhanced [l]. UzFerSn and UzRuzSn are considered as weak paramagnets.

Neutron diffraction studies to determine the spin structures have only been performed on powder samples of U2Pd2Sn [2], UZPdzIn [2] U2Ni2Sn [3] and U&In [4] which order in noncollinear structures and! for UrRhrSn [4] which orders in a collinear structure. The only study on a single crystal U2TrX compound that we

know of is an investigation of c-axis and basal-plane susceptibilities of UzPdzIn [5]. High field magnetization measurements on free powders indicate metamagnetic transitions in fields varying between 20 and 40 T for UzRhzSn, U2Ni&, UrPd$Sn and UrPtzSn [2, 6, 71.

Electrical resistivity, p(T), has been measured for UrNi& 18-111, UzPdrSn [2, 81, UrCorSn [lo], U2Rh2Sn [8-111 and UrFeZSn [ll]. Studies of magne- toresistance and thermo-electric power on U;?NitSn [12] and U$h$n [13] were performed by our group. The current paper is concerned with the resistivity and magnetoresistivity of UrPtrSn presenting results which to our knowledge are still outstanding in the literature.

2. EXPERIMENTAL DETAILS

Metals of the following purities were used in arc- melting the constituents on a water-cooled copper hearth in an argon atmosphere: U (99.98 wt%), Pt (99.97 wt%) and Sn (99.999 wt%). Weight loss during melting was less than 0.2%. Measurements of resistivity were per- formed on a bar shaped sample (6.5 mm X 0.9 mm X 0.7 mm) using a current reversing four-probe d.c. tech- nique. A superconducting solenoid was used to supply magnetic fields up to 8 T and an Oxford Instruments variable temperature insert used in conjunction with an ITCM3 temperature controller enabled temperatures to be regulated within 10 mK. Calibrated carbon glass, capacitance and germanium temperature sensors were used.

307

308 RESISTMTY OF ANTIFERROMAGNETIC UzPtzSn Vol. 102, No. 4

780

740

720 0 100 200 300

Temperature [K]

Fig. 1. Temperature dependence of the resistivity p of U2Pt2Sn. The solid line in the insert is given by p = (786 + O.O927T*) X lo-* Qm.

3. RESULTS

The temperature dependence of the resistivity p is indicated in Fig. 1. The Neel temperature occurs at TN = 15 K as indicated by the minimum in dp/dT in Fig. 2. The small peak in p(T) below TN is presumably due to new superzone boundary formation upon antifer- romagnetic ordering. The negative slope (dpldT < 0) exhibited by p(7) for most of the paramagnetic region is reminiscent of the behaviour of UzNizSn [ll] and UzPdlIn [2]. The resistivity of UzRhzSn shows only a weak temperature dependence between 100 and 500 K

0 _ 0

0.5 - _

-*

a 0.0 -

s -5 -0

-0.5 -

-1.0 -

U,Pt*Sn i

L* I I I I I I I6 I, I I I,,,,,,,,,

0 10 20 30 40 50

Temperature [K]

Fig. 2. Temperature dependence of dpldT for UzPtzSn indicating a magnetic transition temperature TN = 15 K.

0 2 4 6 8

Applied Field [Tesla]

Fig. 3. Magnetoresistance of UzPtzSn for selected iso- therms. Measurements were performed in increasing field sweeps at a rate of 0.3 T min-‘. Note the steps in magnetoresistance near 2 T.

with dpldT positive below 300 K and negative above 300 K [ll]. UzPdzSn exhibits a small positive slope for p(T) between TN and room temperature [2]. The drop in resistivity of U$t;?Sn associated with magnetic ordering is small compared with the large decreases observed for UzNi2Sn [ll], U2Rh2Sn [ll] and U2Pd2Sn [2] in their antiferromagnetic regions. It is indicated in Fig. 1 that the resistivity of UzPt;?Sn at low temperatures in the ordered region follows a T*-temperature dependence.

Magnetoresistance results for a few selected iso- therms taken during a field-increase sweep are given in Fig. 3. The overall magnetoresistance is small compared with that of the previously investigated antiferromagnets U2NizSn [12] and U$h$n [13]. It is also observed that UzNizSn exhibits positive magnetoresistance at low temperatures while UzRhzSn and U2PtzSn have negative magnetoresistance. A step-like decrease in the resistivity is evident for the 2.5 and 8.5 K isotherms in the vicinity of 2 T. This suggests the existence of some spin- reorientation transition. For a single-crystal specimen with the field directed along a specific crystal axis such a transition is likely to be sharp. However, for our polycrystalline sample one will expect a smeared-out transition if such a transition takes place at different critical fields for different crystallographic orientations. In Fig. 4 we indicate that, associated with this step, a hysteresis in the resistivity results is observed taken in increasing and decreasing field sweeps. Such hysteretic behaviour has been observed for all isotherms measured in the magnetic ordered region.

It is evidence that diverse and interesting behaviours

Vol. 102, No. 4 RESISTIVITY OF’ ANTIFERROMAGNETIC UzPtzSn 309

u,Pt2sn

T = 2.8 K:

1 2 3 4

Applied Field [Tesla]

Fig. 4. A magnetoresistance isotherm of UzPt& taken during increasing followed by decreasing field sweeps (0.3 T min-‘) showing hysteresis in the vicinity of 2 T.

are observed for the resistivity and magnetoresistivity of the magnetically ordered U2T2Sn compounds. lJnder- standing these differences will require more detailed information about their magnetic spin-structums and electronic structures. Experimental [2-41 and theoretical [14, 151 work in this regard have commenced.

Acknowledgement-Financial support from the Foundation for Research Development is acknowledged.

REFERENCES

Havela, L., Sechovslj, V., Svoboda, P., Nakotte, H., Prokel, K., de Boer, F.R., Seret, A., Winand, J.M., Rebizant, J., Spirlet, J.C., Purwanto, A. and Robinson, R.A, J. Magn. Magn. Mater., 140-144, 1995,1367. Purwanto, A., Robinson, R.A., Havela, L., SechovsG, V., Svoboda, P., Nakotte, H., Proke6, K.,

3.

4.

5.

6.

7.

8.

9.

10.

11.

12.

13.

14.

15.

de Boer, F.R., Seret, A., Winand, J.M., Rebizant, J. and Spirlet, J.C., Phys. Rev., B50,1994,6792. Bourke, F., Chevalier, B., Foum&, L., Mirambet, F., Roisnel, T., Tran, V.H. and Zolnierek, Z., .I. Magn. Magn. Mater., 138, 1994,307. Nakotte, H., Purwanto, A., Robinson, R.A., ProkeS, K., Klaassen, J.C.P., de Chitel, P.F., de Boer, F.R., Havela, L., Sechovsk$, V., Pereira, L.C.J., Seret, A., Rebizant, J., Spirlet, J.C. and Trouw, F., Phys. Rev., B53,1996,3263. Prokgs., K., Nakotte, H., Havela, L., Sechovskf, V., Pereira, L.C.J., Rijkeboer, C., Seret, A., Spirlet, J.C., Svoboda, P. and de Boer, F.R., Physica, B223&224,1996,225. Nakotte, H., ProkeS, K., Briick, E., Tang, N., de Boer, F.R., Svoboda, P., Sechovslj, V., Havela, L., Winand, J.M., Seret, A., Rebizant, J. and Spirlet, J.C., Physica, B201,1994,247. Fukushima, T., Matsuyama, S., Kumada, T., Kindo, K., ProkeS, K., Nakotte, H., de Boer, F.R., Havela, L., Sechovksf, V., Winand, J.M., Rebizant, J. and Spirlet, J.C., Physica, B211,1995,142. Mirambet, F., Chevalier, .B., Foum&, L., Gravereau, P. and Etoumeau, J., .I. Alloys Compounds, 203,1994,29. Kindo, K., Fukushima, T., Kumada, T., de Boer, F.R., Nakotte, H., ProkeS, K., Havela, L., Sechovskjr, V., Seret, A., Winand, J.M., Spirlet, J.C. and Rebizant, J., J. Magn. Magn. Mater., 140-l&$1995,1369. Pinto, R.P., Amado, M.M., Salgueiro, M.A., Braga, M.E., Sousa, J.B., Chevalier, B., Mirambet, F. and Etoumeau, J., J. Magn. Magn. Mater., 140-144, 1995,1371. du Plessis, P. de V., Strydom, A.M. and Baran, A., Physica, B206-207, 1995, 495. Strydom, A.M., du Plessis, P. de V. and Gridin, V.V., Solid State Commun., 95, 1995, 867. Strydom, A.M., du Plessis, P. de V. and Gridin, V.V., Physica, B225, 1996, 89. DiviS, M., OlSovec, M., Richter, M. and Es&rig, H., J. Magn. Magn. Mater., 140-144,1995,1365. Sandratskii, L.M. and Kiibler, J., Physica, B217, 1996, 167.