Physica B 312–313 (2002) 846–847 Magnetic behavior of Yb 3 Cu 4 Ge 4 and Gd 3 Cu 4 Ge 4 S.K. Dhar a, *, Surjeet Singh a , P. Bonville b , C. Mazumdar b , P. Manfrinetti c , A. Palenzona c a Tata Institute of Fundamental Research, Condensed Matter Department, Homi Bhabha Road, Mumbai-400005, India b Commissariat " a l’Energie Atomique, CE Saclay, 91191 Gif-sur-Yvette, France c INFM and Dipartimento di Ch ! ımica, Universit " a di Genova, 16146 Genova, Italy Abstract Magnetic, specific heat and 170 Yb M . ossbauer spectroscopy measurements were carried out in orthorhombic Yb 3 Cu 4 Ge 4 ; where the rare earth occupies two inequivalent crystallographic sites. We show that this compound has an unusually high magnetic ordering temperature of 7:5K; close to that of isostructural Gd 3 Cu 4 Ge 4 (8:6 K), and that it is a ferromagnet. The breakdown of the de Gennes scaling indicates a strong exchange interaction between Yb ions, presumably due to 4f-conduction band hybridization. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Yb and Gd intermetallics; M. ossbauer spectroscopy; Ferromagnet A recent study of Ce 3 Cu 4 Sn 4 and Gd 3 Cu 4 Sn 4 ; crystallizing in the Gd 6 Cu 8 Ge 8 -type orthorhombic structure [1] with two symmetry inequivalent rare earth sites at 2d (mmm) and 4e (mm) in the unit cell, revealed multiple magnetic transitions with comparable highest transition temperature 10.4 and 13 K; respectively [2]. This is incompatible with the de Gennes scaling and points to a strong exchange interaction between the Ce ions. In the present work we have investigated the magnetic behavior of iso-structural Yb 3 Cu 4 Ge 4 and Gd 3 Cu 4 Ge 4 : The existence of R 3 Cu 4 Ge 4 is already known in the literature [3]. The electrical resistivity of Yb 3 Cu 4 Ge 4 (see Fig. 1) is qualitatively similar to that of Ce 3 Cu 4 Sn 4 and shows the occurrence of a magnetic transition near 7:5K: The magnetic susceptibility shows a Curie–Weiss behavior above 80 K with m eff ¼ 4:52m B =Yb and y p C  6K: These data show that the Yb ions at the two different sites in Yb 3 Cu 4 Ge 4 are both in the trivalent state. The magnetisation curve at 1:7 K (see inset of Fig. 1) shows a rapid build-up in relatively low field up to 1.7–1:8m B =Yb ion, followed by a slow increase, which does not attain saturation up to 10 T: This behavior points to a ferro- or ferrimagnetic ordering of the Yb ions, the absence of saturation being presumably due to the excited crystal electric field levels. The occurrence of a magnetic transition in Yb 3 Cu 4 Ge 4 and Gd 3 Cu 4 Ge 4 is confirmed by the heat capacity data shown in Fig. 2, which present l- anomalies at almost the same temperatures (respectively 7.5 and 8:6 K). This implies a breakdown of the de Gennes scaling, which predicts a ratio of about 50 between the transition temperatures of isostructural Gd and Yb compounds. The high transition temperature of Yb 3 Cu 4 Ge 4 is thus surprising and points to an anomalously large exchange interaction which may have its origin in the hybridization between the localized Yb-4f and the itinerant conduction band orbitals. Selected 170 Yb M . ossbauer spectra, recorded both below and above T C ; are represented in Fig. 3. In the paramagnetic phase (T ¼ 15 K) one observes two subspectra which can both be described by a quad- rupolar hyperfine interaction. The relative weights of the two subspectra are approximately in the ratio 2:1, corresponding to the two crystallographic 4e and 2d sites. The quadrupolar interaction at the 4e site has an axial character, with a quadrupole coupling parameter a Q C5 mm=s; very close to the maximum value 5:8 mm=s associated with the extremely anisotropic jJ ¼ 7 2 ; J z ¼ 7 7 2 S Kramers doublet. By contrast, the *Corresponding author. Tel.: +91-22-215-2971; fax: +91- 22-215-2110. E-mail address: sudesh@tifr.res.in (S.K. Dhar). 0921-4526/02/$-see front matter r 2002 Elsevier Science B.V. All rights reserved. PII:S0921-4526(01)01269-8