Localized versus itinerant magnetic moments in Na0.72CoO2

TL;DR

This study uses 59Co-NMR data to investigate the magnetic nature of Co 3d-electrons in Na0.72CoO2, revealing itinerant electrons with antiferromagnetic interactions and temperature-dependent orbital contributions, suggesting complex magnetic behavior.

Contribution

It provides a detailed NMR analysis showing that Co 3d-electrons are itinerant and exhibit temperature-dependent orbital effects, advancing understanding of magnetism in Na0.72CoO2.

Findings

Co 3d-electrons are itinerant below room temperature

Antiferromagnetic coupling is favored among Co electrons

Temperature-dependent orbital contributions influence magnetic properties

Abstract

Based on experimental 59Co-NMR data in the temperature range between 0.1 and 300 K, we address the problem of the character of the Co 3d-electron based magnetism in Na0.7CoO2. Temperature dependent 59Co-NMR spectra reveal different Co environments below 300 K and their differentiation increases with decreasing temperature. We show that the 23Na- and 59Co-NMR data may consistently be interpreted by assuming that below room temperature the Co 3d-electrons are itinerant. Their magnetic interaction appears to favor an antiferromagnetic coupling, and we identify a substantial orbital contribution corb to the d-electron susceptibility. At low temperatures corb seems to acquire some temperature dependence, suggesting an increasing influence of spin-orbit coupling. The temperature dependence of the spin-lattice relaxation rate T1-1(T) confirms significant variations in the dynamics of this electronic subsystem between 200 and 300K, as previously suggested. Below 200 K, Na0.7CoO2 may be viewed as a weak antiferromagnet with TN below 1 K but this scenario still leaves a number of open questions.