Hidden magnetic transitions in thermoelectric layered cobaltite, [Ca$_2$CoO$_3$]$_{0.62}$[CoO$_2$]

TL;DR

This study reveals two magnetic transitions in the thermoelectric cobaltite [Ca2CoO3]0.62[CoO2], with detailed insights into their nature, temperature dependence, and correlation with transport properties, using muon spin rotation and relaxation experiments.

Contribution

The paper provides new evidence of incommensurate spin density wave transitions in layered cobaltite and clarifies their magnetic propagation and temperature evolution.

Findings

Incommensurate SDW exists in the CoO2 subsystem.

Long-range SDW order below ~30 K.

Transitions correlate with transport properties.

Abstract

A positive muon spin rotation and relaxation ($\mu^+$SR) experiment on [Ca$_2$CoO$_3$]$_{0.62}$[CoO$_2$], ({\sl i.e.}, Ca$_3$Co$_4$O$_9$, a layered thermoelectric cobaltite) indicates the existence of two magnetic transitions at $\sim$ 100 K and 400 - 600 K; the former is a transition from a paramagnetic state to an incommensurate ({\sf IC}) spin density wave ({\sf SDW}) state. The anisotropic behavior of zero-field $\mu^+$SR spectra at 5 K suggests that the {\sf IC-SDW} propagates in the $a$-$b$ plane, with oscillating moments directed along the c-axis; also the {\sf IC-SDW} is found to exist not in the [Ca$_2$CoO$_3$] subsystem but in the [CoO$_2$] subsystem. In addition, it is found that the long-range {\sf IC-SDW} order completes below $\sim$ 30 K, whereas the short-range order appears below 100 K. The latter transition is interpreted as a gradual change in the spin state of Co ions %% at temperatures above 400 K. These two magnetic transitions detected by $\mu^+$SR are found to correlate closely with the transport properties of [Ca$_2$CoO$_3$]$_{0.62}$[CoO$_2$].