Competing magnetic interactions in CeNi9-xCoxGe4

TL;DR

This study investigates how substituting Ni with Co in CeNi9Ge4 affects magnetic interactions, revealing unexpected antiferromagnetic order that challenges initial expectations of a transition to a paramagnetic state.

Contribution

It provides new insights into the effects of Co substitution on the magnetic properties and spin degeneracy in CeNi9Ge4, highlighting the role of effective spin degeneracy in magnetic ordering.

Findings

AFM order appears even at low Co concentrations

Co substitution increases Kondo and RKKY interactions

Modification of spin degeneracy influences magnetic ground state

Abstract

CeNi9Ge4 exhibits outstanding heavy fermion features with remarkable non-Fermi- liquid behavior which is mainly driven by single-ion effects. The substitution of Ni by Cu causes a reduction of both, the RKKY coupling and Kondo interaction, coming along with a dramatic change of the crystal field (CF) splitting. Thereby a quasi-quartet ground state observed in CeNi9Ge4 reduces to a two-fold degenerate one in CeNi8CuGe4. This leads to a modiffcation of the effective spin degeneracy of the Kondo lattice ground state and to the appearance of antiferromagnetic (AFM) order. To obtain a better understanding of consequences resulting from a reduction of the effective spin degeneracy, we stepwise replaced Ni by Co. Thereby an increase of the Kondo and RKKY interactions through the reduction of the effective d-electron count is expected. Accordingly, a paramagnetic Fermi liquid ground state should arise. Our experimental studies, however, reveal AFM order already for small Co concentrations, which becomes even more pronounced with increasing Co content x. Thereby the modiffcation of the effective spin degeneracy seems to play a crucial role in this system.