Electronic structure by X-ray absorption spectroscopy and observation offield induced unusually slowspin relaxation from magnetic properties in pyrochlore Eu$_{2-x}$Fe$_x$Ti$_2$O$_7$

TL;DR

This study investigates the electronic and magnetic properties of Eu2-xFexTi2O7 using X-ray spectroscopy and magnetization measurements, revealing slow spin relaxation, field-induced transitions, and enhanced magnetization due to Fe doping.

Contribution

It provides new insights into how Fe doping affects the electronic structure, local distortions, and magnetic interactions in Eu2-xFexTi2O7 pyrochlore materials.

Findings

Fe doping reduces octahedral distortion and causes ligand coordination changes.

A new low-temperature field-induced transition with slow spin relaxation is observed.

Fe substitution enhances dc magnetization and dipolar ferromagnetic interactions.

Abstract

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.