~ Solid State Communications, Voi.39, pp.1157-1|61. Perga=on Press Ltd. 198l. Printed in Great Britain. 0038-1098/81/35! 157-05502.00/0 SUPERCONOUCTIVITY OF Nd 3+ AND Od 3÷ IN ZrIr2* ESR ANO R.M. Stein, G.E. Barberis end C. Rettori Instltuto de F~sica, UNICAMP, 13100 Campinas (S.P.), Brazil (Received April 01 th 1981 by R.C.C. Leita) We r e p o r t low temperatures ESR and Superconductivity experiments on Nd 3+ and 3+ Gd in the superconducting cubic intermetallic compound ZrIr 2. The ratio of the squares of the effective 4f-conduction electron exchange parameters cortes- + 2_Nd/J2 ponding to Nd 3+ and Gd 3 [Js s_Od ) were extracted both from the thermal broadening of the ESR linewidths and from the initial depression of the super- conducting critical temperature, and were found to be comparable. Nd 3+ was found to have a larger exchange parameter than of Gd 3+. INTRODUCTION The relatively large values of the exchange parameters between the rare- earth (RE) localized magnetic moments and the conduction electrons for the first half of the RE series as compared with that of the second half has been Known for some time. Evidence for this fact is obtained from the initial de- pression of the superconducting tempe- rature, Knight shift, and resistivity (1) measurements in (RE La)A12 . But only recently has this "anomalous" behaviour received increasing attention due to the unusual low temperature properties (2) of (NdLa)Sn 3_ . In a recent worK, Bar- (3) beris et el., have inferred from the ESR Korringa rates, that in a large number of compounds the exchange para- meters for Nd 3+ were systematically larger than that of Gd 3÷ impurities. This was attributed to the larger ionic radius of Nd 3+ as compared with that of Gd 3+. On the other hand, experiments similar to ref. 3 with dopings from the second half of the RE series in Th [4j" " [5) [6) AZ and LaOs 2 show that comparable values of the exchange parameters are obtained from the Korringa rates and the initial depression of the supercon- ducting critic~l temperature T c. These are the features that we study in RExZr1_xIr 2 [RE:Nd,Gd), comparing the ESR and superconductivity properties. EXPERIMENTS The compounds ZrIr 2 and REIr 2 were prepared by arc-melting stoichiometric amounts of the corresponding elements in a pure argon atmosphere. The dilute alloys R~xZrl_xIr 2 were obtained from sucessive dilutions of a master alloy RE0.2Zr0.eIr2 . X-ray analyses indicate (7) the presence of a single phase (MgCu 2 structure, C15] for all our samples. X- ray difraction analiees also show latti- ce parameters that interpolate linearly between the pure REIr 2 and ZrIr 2 values. The ESR experiments were performed in e conventional X-band Varian E-15 spectrometer. The samples were grained to fine powder, and placed in e quartz tail, liquid helium dewar, which fits into a ro'om temperature microwave cavity modulated at 100 KHz. The dewar contains facilities to work with liquid He-3 and a heating system which can change the temperature of the samples from 0.5 K up to 300 K. The superconducting critical tempe- Partially supported by FAPESP and CNPq, Brazil. 1157