Electronic self-doping of Mo-states in A2FeMoO6 (A=Ca, Sr and Ba) half-metallic ferromagnets - a Nuclear Magnetic Resonance study

TL;DR

This study uses NMR spectroscopy to reveal how the electronic charge at Mo sites in A2FeMoO6 ferromagnetic oxides varies with ionic size, influencing magnetic properties and Curie temperatures.

Contribution

It demonstrates the self-doping process between Fe and Mo sites and links structural parameters to electronic and magnetic behavior in double perovskite oxides.

Findings

Mo charge varies with ionic size in A2FeMoO6.

Self-doping between Fe and Mo sites is observed.

Curie temperature correlates with Mo electron density.

Abstract

A systematic study of (A,A')2FeMoO6 (A,A'=Ca, Sr, Ba) ferromagnetic oxides with double perovskite structure has been performed using 95,97Mo and 57Fe NMR spectroscopy. These oxides are isoelectronic but have substantially different Curie temperatures. The NMR analysis provides clear evidence that the magnetic moment at Mo sites is not constant but varies sensitively with the ionic size of the alkaline ions. The 95,97Mo frequency, and thus the electronic charge at Mo ions, is found to be smaller in Ba and Ca than in Sr-based oxides. The charge release from Mo sites is accompanied by an uptake at Fe sites, and thus a self-doping Fe-Mo process is observed. This process is controlled by relevant structural parameters: the Fe-O-Mo bond length and bending. A clear relationship between the Curie temperature and the magnetic moment and thus electron density at Mo sites has been disclosed. The relevance of these findings for the understanding of ferromagnetic coupling in double perovskites is discussed.