Remnant magnetization above room temperature in semiconducting Y_0.5Ca_0.5BaCo_4O_7

TL;DR

This study reveals that the semiconducting Y_0.5Ca_0.5BaCo_4O_7 exhibits remnant magnetization above room temperature and displays multiple magnetic anomalies linked to spin freezing and complex magnetic interactions.

Contribution

It reports the observation of remnant magnetization above room temperature in a semiconducting cobalt oxide with detailed analysis of its magnetic anomalies and underlying mechanisms.

Findings

Multiple magnetic anomalies at 387, 281, 52, and 14 K.

Presence of remnant magnetization above room temperature.

Complex interplay of double-exchange and super-exchange interactions.

Abstract

The Y_0.5Ca_0.5BaCo_4O_7 compound exhibits four magnetic anomalies at the temperatures 387, 281, 52, and 14 K; all anomalies show characteristics typical for spin freezing into disordered states: frequency dependent transition temperature in the AC magnetic susceptibility together with relaxation of thermoremnant magnetization. Y_0.5Ca_0.5BaCo_4O_7 is a semiconductor with a small band-gap of 0.17 eV concluded from four-point conductivity measurements and the conductivity is proportional to T^(-3/4), suggesting an electron hopping mechanism involving Co^(2+) and Co^(3+) ions; at higher temperatures, double-exchange is the proposed reason for the strong magnetic interaction. At lower temperatures, super-exchange interactions start to compete with the double-exchange for domination and this competition together with geometrical frustrations in the structure are responsible for magnetic disorder down to 2 K. Covalence between Co and O is also discussed as additional reason for the strong magnetic interactions.