# Regensburg 2002 – wissenschaftliches Programm

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# MA: Magnetismus

## MA 14: Schwere Fermionen, Kondo-Systeme II (gemeinsam mit TT)

### MA 14.6: Vortrag

### Donnerstag, 14. März 2002, 15:45–16:00, H20

**Neutron study of the magnetic order in CeCu**_{2}**(Si**_{1−x}**Ge**_{x}**)**_{2}** with low Ge-content** — •M. Deppe^{1}, R. Borth^{1}, J. Custers^{1}, C. Geibel^{1}, P. Hinze^{1}, D. Hohlwein^{2}, E. Lengyl^{1}, N. Oeschler^{1}, R. Schneider^{2}, O. Stockert^{1}, H. Q. Yuan^{1}, and F. Steglich^{1} — ^{1}MPI-CPfS, Noethnitzer Str. 40, 01187 Dresden — ^{2}HMI, Glienicker Str. 100, 14109 Berlin

Studies on polycrystalline samples of the heavy-fermion alloy
CeCu_{2}(Si_{1−x}Ge_{x})_{2} showed that the system can be
tuned from an antiferromagnetically ordered Ce-compound at x=1
to a heavy-fermion superconductor at x=0. However, no magnetic
Bragg peaks could be observed with neutron
scattering for low Ge concentration x<0.6 . Since the nature
of the magnetic order in pure CeCu_{2}Si_{2} is still a matter of
debate, and since it is suggested to be strongly connected with the
superconducting state, a more precise knowledge of the evolution of
the magnetic order in this alloy is crucial for obtaining a deeper
understanding of the magnetism and superconductivity in pure
CeCu_{2}Si_{2}.
We have therefore grown large single crystals
of CeCu_{2}(Si_{1−x}Ge_{x})_{2}
which allowed us to perform neutron diffraction using the flat-cone
diffractometer E2 at the HMI, Berlin. We observed magnetic reflections and
could determine the propagation vector of the antiferromagnetic order for
Ge concentrations down to x=0.05.
We will compare our results with
thermodynamic measurements and the phase diagram established
for the single-crystals. The experimental evidences for an
antiferromagnetic
ordering with very low moments will be discussed in the context of
theoretical calculations.