  X-ray absorption spectroscopy (XAS) as well as x-ray magnetic circular
dichroism (XMCD) and magnetization of hybrid pyrochlore Eu2-xFexTi2O7 were
investigated, where the rare earth Eu (4f) was replaced with transition metal
Fe (3d) to introduce competing 4f-3d interactions. It is confirmed that the
valence states of Eu and Fe ions are formally trivalent while that of Ti ions
are tetravalent (3d0). The analysis yielded that the tetravalent Ti ions occupy
octahedral sites with distorted Oh symmetry which is triggered by the presence
of vacant 8a anionic site adjacent to TiO6 octahedra. Further study with Fe
doping revealed that it essentially reduces the octahedral distortion by
introducing anionic disorder (migration of 48f oxygen ions to 8a site).
Analysis of O K edge XAS spectra further confirmed the Fe substitution causing
the systematic change in the ligand (O2-) coordination of the Ti4+ cations. On
the other hand, a new field induced transition (with Fe doping) at low
temperature T* (4 K<T*< 8 K) in ac susceptibility with unusually slow spin
relaxation was observed. The transition shifted towards higher temperatures
both with increasing applied field and Fe concentration. However, the single
ion spin freezing (Tf ~35 K) appears to be suppressed with Fe substitution.
Interestingly, small amount of Fe3+ ion substitution showed significant
enhancement in the dc magnetization at lower temperatures (<100 K). Analysis
further indicated rise of dipolar FM exchange interaction with Fe doping.
