Vacancy defects and monopole dynamics in oxygen-deficient pyrochlores

TL;DR

This paper investigates how oxygen vacancies in pyrochlores influence magnetic monopole dynamics, revealing that such defects slow down monopole motion at very low temperatures, and are mitigated by oxygen annealing.

Contribution

It identifies oxygen deficiency as the main source of magnetic impurities in pyrochlores and demonstrates how these defects affect monopole dynamics in spin ice materials.

Findings

Oxygen vacancies induce Ti3+ ions with magnetic moments.

Oxygen annealing removes these vacancies and related magnetic impurities.

Vacancies significantly slow monopole dynamics at sub-Kelvin temperatures.

Abstract

The idea of magnetic monopoles in spin ice has enjoyed much success at intermediate temperatures, but at low temperatures a description in terms of monopole dynamics alone is insufficient. Recently, numerical simulations were used to argue that magnetic impurities account for this discrepancy by introducing a magnetic equivalent of residual resistance in the system. Here we propose that oxygen deficiency is the leading cause of magnetic impurities in as-grown samples, and we determine the defect structure and magnetism in Y2Ti2O(7-\delta) using diffuse neutron scattering and magnetization measurements. These defects are eliminated by oxygen annealing. The introduction of oxygen vacancies causes Ti4+ to transform to magnetic Ti3+ with quenched orbital magnetism, but the concentration is anomalously low. In the spin-ice material Dy2Ti2O7 we find that the same oxygen-vacancy defects suppress moments on neighbouring rare-earth sites, and that these magnetic distortions dramatically slow down the long-time monopole dynamics at sub-Kelvin temperatures.