Magnetic behavior of Yb3Cu4Ge4 and Gd3Cu4Ge4

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
Tata Institute of Fundamental Research, Condensed Matter Department, Homi Bhabha Road, Mumbai-400005, India
Commissariat a" l’Energie Atomique, CE Saclay, 91191 Gif-sur-Yvette, France
INFM and Dipartimento di Ch!ımica, Universita" di Genova, 16146 Genova, Italy
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 [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 Yb3 Cu4 Ge4 and
Gd3 Cu4 Ge4 : The existence of R3 Cu4 Ge4 is already
known in the literature [3].
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
M( µΒ /Yb)
ρ (µΩ cm)
Yb 3 Cu4 Ge 4
T = 1.7 K
T (K )
Fig. 1. Resistivity in Yb3 Cu4 Ge4 : The insert shows the powder
magnetization curve at 1:7 K:
C(J/mol K)
Yb 3 Cu 4 Ge 4
Gd 3 Cu4 Ge 4
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.
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 [4].
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