Real-space investigation of short-range magnetic correlations in fluoride pyrochlores NaCaCo$_2$F$_7$ and NaSrCo$_2$F$_7$ with magnetic pair distribution function analysis

TL;DR

This study uses neutron scattering and magnetic pair distribution function analysis to reveal persistent short-range magnetic correlations in fluoride pyrochlores NaCaCo$_2$F$_7$ and NaSrCo$_2$F$_7$, confirming a specific magnetic ground state and its stability up to high temperatures.

Contribution

It demonstrates the effectiveness of mPDF analysis in characterizing short-range magnetic order in pyrochlore compounds and confirms the magnetic ground state in these frustrated materials.

Findings

Short-range XY-like correlations with 10-15 Å scale are confirmed.

Correlations persist up to approximately 200 K.

The magnetic ground state is accurately modeled and verified.

Abstract

We present time-of-flight neutron total scattering and polarized neutron scattering measurements of the magnetically frustrated compounds NaCaCo$_2$F$_7$ and NaSrCo$_2$F$_7$, which belong to a class of recently discovered pyrochlore compounds based on transition metals and fluorine. The magnetic pair distribution function (mPDF) technique is used to analyze and model the total scattering data in real space. We find that a previously-proposed model of short-range XY-like correlations with a length scale of 10-15 \AA, combined with nearest-neighbor collinear antiferromagnetic correlations, accurately describes the mPDF data at low temperature, confirming the magnetic ground state in these materials. This model is further verified by the polarized neutron scattering data. From an analysis of the temperature dependence of the mPDF and polarized neutron scattering data, we find that short-range correlations persist on the nearest-neighbor length scale up to 200 K, approximately two orders of magnitude higher than the spin freezing temperatures of these compounds. These results highlight the opportunity presented by these new pyrochlore compounds to study the effects of geometric frustration at relatively high temperatures, while also advancing the mPDF technique and providing a novel opportunity to investigate a genuinely short-range-ordered magnetic ground state directly in real space.