Evidence of the anomalous fluctuating magnetic state by pressure driven 4f valence change in EuNiGe3

In rare-earth compounds with valence fluctuation, the proximity of the 4f level to the Fermi energy leads to instabilities of the charge configuration and the magnetic moment. For Eubased valence fluctuation materials, the intra-atomic interactions, may play an important role to affect the magnetic ground state since the multielectron configurations 4f6 (Eu3+, L=3, S=3 and J=0) and 4f7 (Eu2+, L=0, S=7/2 and J=7/2) which are dominantly defining the macroscopic magnetic properties. Here, we provide direct experimental evidence for an induced magnetic polarization of the Eu3+ atomic shell with J=0, due to intra-atomic exchange and spin-orbital coupling interactions with Eu2+ atomic shell. By applying external pressure, a transition from antiferromagnetic to a fluctuating behavior in a EuNiGe3 single crystals is probed by element- and orbital-specific x-ray absorption spectroscopy and x-ray magnetic circular dichroism at Eu L2-edge. The enhanced mixing of Eu 4f and 5d electronic orbitals under pressure leads to a monotonic increase of the mean valence of Eu ions, with the onset of a thermally fluctuating state at 30 GPa. Magnetic polarization is observed for both valence states of Eu2+ and Eu3+ across the entire pressure range, with a clear occurrence of an electronic phase transition revealed by a linewidth change of the Eu3+ resonance at 30 GPa. The anomalous magnetic order is discussed in terms of a homogeneous intermediate valence state where frustrated Dzyaloshinskii-Moriya couplings are enhanced by the onset of spin-orbital interaction and engender a chiral spin-liquid-like precursor.