Physica B 312–313 (2002) 846–847 Magnetic behavior of Yb3Cu4Ge4 and Gd3 Cu4 Ge4 S.K. Dhara,*, Surjeet Singha, P. Bonvilleb, C. Mazumdarb, P. Manfrinettic, A. Palenzonac 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, Universita" di Genova, 16146 Genova, Italy Abstract . Magnetic, speciﬁc heat and 170 Yb Mossbauer spectroscopy measurements were carried out in orthorhombic Yb3 Cu4 Ge4 ; where the rare earth occupies two inequivalent crystallographic sites. We show that this compound has an unusually high magnetic ordering temperature of 7:5 K; close to that of isostructural Gd3 Cu4 Ge4 (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; Mossbauer spectroscopy; Ferromagnet A recent study of Ce3 Cu4 Sn4 and Gd3 Cu4 Sn4 ; crystallizing in the Gd6 Cu8 Ge8 -type orthorhombic structure  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 . 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 Yb3 Cu4 Ge4 and Gd3 Cu4 Ge4 : The existence of R3 Cu4 Ge4 is already known in the literature . The electrical resistivity of Yb3 Cu4 Ge4 (see Fig. 1) is qualitatively similar to that of Ce3 Cu4 Sn4 and shows the occurrence of a magnetic transition near 7:5 K: The magnetic susceptibility shows a Curie–Weiss behavior above 80 K with meff ¼ 4:52mB =Yb and yp C 6 K: These data show that the Yb ions at the two different sites in Yb3 Cu4 Ge4 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 1.7–1:8mB =Yb ion, followed by a slow increase, which does not attain saturation up to 10 T: This behavior points to a ferro- or *Corresponding author. Tel.: +91-22-215-2971; fax: +9122-215-2110. E-mail address: [email protected] (S.K. Dhar). 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 Yb3 Cu4 Ge4 and Gd3 Cu4 Ge4 is conﬁrmed by the heat capacity data shown in Fig. 2, which present lanomalies 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 Yb3 Cu4 Ge4 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 Mossbauer spectra, recorded both below and above TC ; are represented in Fig. 3. In the paramagnetic phase (T ¼ 15 K) one observes two subspectra which can both be described by a quadrupolar 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 aQ C5 mm=s; very close to the maximum value 5:8 mm=s associated with the extremely anisotropic jJ ¼ 72; Jz ¼ 772S Kramers doublet. By contrast, the 0921-4526/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 1 ) 0 1 2 6 9 - 8 S.K. Dhar et al. / Physica B 312–313 (2002) 846–847 847 120 80 H(Tesla) 2 M( µΒ /Yb) ρ (µΩ cm) Yb 3 Cu4 Ge 4 40 0 0 1 T = 1.7 K 0 0 2 100 4 6 8 10 200 300 T (K ) Fig. 1. Resistivity in Yb3 Cu4 Ge4 : The insert shows the powder magnetization curve at 1:7 K: 60 C(J/mol K) Yb 3 Cu 4 Ge 4 Gd 3 Cu4 Ge 4 40 . Fig. 3. 170 Yb Mossbauer spectra in Yb3 Cu4 Ge4 in the paramagnetic phase (15 K) and in the magnetically ordered phase (1.45 and 6:5 K). The solid lines show the two subspectra corresponding to the two inequivalent Yb sites. 20 0 0 5 10 15 20 25 T(K) Fig. 2. Speciﬁc heat in Yb3 Cu4 Ge4 and Gd3 Cu4 Ge4 : quadrupole interaction at the 2d site has a strong non-axial character, with aQ C 3:7 mm=s: The ground state at the 2d-site must therefore be close to the jJ ¼ 72; Jz ¼ 712S or jJ ¼ 72; Jz ¼ 732S state, both of which have large negative aQ values. Below 8 K the spectra show the presence of two magnetic hyperﬁne subspectra with weights nearly in the ratio of 2:1, like in the paramagnetic phase. The 4e site subspectrum shows a large saturated hyperﬁne ﬁeld of about 370 T corresponding to a saturated spontaneous moment of 3:6mB = Yb; close to the moment of 4mB of the jJ ¼ 72; Jz ¼ 772S doublet. The 2d site subspectrum has a smaller saturated hyperﬁne ﬁeld corresponding to a moment of about 1:45mB : The thermal variation of the hyperﬁne ﬁelds follows a mean ﬁeld law for S ¼ 12 and with TC C8 K; appropriate for a doublet ground state. The comparison of the saturated magnetization per . Yb ion at 1:6 K and of the Mossbauer derived saturated spontaneous moments allows to determine whether the magnetic structure is ferro- or ferrimagnetic in Yb3 Cu4 Ge4 : For ferromagnetically coupled ions with large anisotropy and saturated moment m0 ; the T ¼ 0 powder magnetization M0 amounts to 12 m0 per ion. Assuming that this rule applies approximately for the three exchange coupled Yb ions in the formula unit of Yb3 Cu4 Ge4 (two of which are extremely anisotropic), then for a ferromagnetic structure one expects M0 ¼ 1:45mB ; and for a ferrimagnetic one M0 ¼ 0:96mB : The experimental M0 value (1.7–1:8mB ), closer to the ferromagnetic case, shows that in Yb3 Cu4 Ge4 the Yb ions occupying the two inequivalent sites are ferromagnetically coupled. As a conclusion, we have shown that Yb3 Cu4 Ge4 is a ferromagnet with a very high Curie temperature of 7:5 K. This large value can be due to the Kondo effect; if so, Yb3 Cu4 Ge4 would be the second example of a Ybbased Kondo ferromagnet, together with YbNiSn . References     W. Rieger, Monatsh. Chem. 101 (1970) 449. S. Singh, et al., J. Alloys Compd. 298 (2000) 68. G. Hanel, H. Nowotny, Monatsh. Chem. 101 (1970) 463. P. Bonville, et al., Phys. B 182 (1992) 105.
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