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Abstract:
Unusual phases and phase transitions are seen at the
magnetic-nonmagnetic boundary in Ce-, Eu- and Yb-based compounds.
EuNi2P2 is a very unusual valence fluctuating Eu system, because at low
temperatures the Eu valence stays close to 2.5 instead of approaching an
integer value. The Eu valence, and thus the magnetic property in this
system, can be tuned by Ge substitution in the P site as EuNi2Ge2 is
known to exhibit the antiferromagnetc (AFM) ordering of divalent Eu
moments with T-N = 30 K. We have grown EuNi2(P1-xGex)(2) (0.0 <= x <=
0.5) single crystals and studied their magnetic, thermodynamic and
transport properties. Increasing Ge doping to x > 0.4 results in a
well-defined AFM ordered state with T-N = 12 K for x = 0.5. Moreover,
the reduced value of magnetic entropy for x = 0.5 at T-N suggests the
presence of valance fluctuation/the Kondo effect in this compound.
Interestingly, the specific heat exhibits an enhanced Sommerfeld
coefficient upon Ge doping. Subsequently, electronic structure
calculations lead to a non-integral valence in EuNi2P2 but a stable
divalent Eu state in EuNi2Ge2, which is in good agreement with the
experimental results.
