Structural, optical, magnetic and electrical properties of Zn1-x Co (x) O thin films

TL;DR

This study systematically investigates Zn1-x Co x O:Al thin films, revealing that correlated ferromagnetism only occurs at higher Co concentrations and is linked to secondary phases and Co clustering, with comprehensive structural, optical, magnetic, and electrical analysis.

Contribution

It provides a detailed correlation between Co concentration, ferromagnetism, and secondary phase formation in ZnO-based thin films, clarifying the origin of ferromagnetism.

Findings

Correlated ferromagnetism appears only at x > 0.25.

Samples with x < 0.2 show weak ferromagnetism only in magnetometry.

Anomalous Hall effect is due to secondary phases and Co clusters.

Abstract

Despite a considerable effort aiming at elucidating the nature of ferromagnetism in ZnO-based magnetic semiconductor, its origin still remains debatable. Although the observation of above room temperature ferromagnetism has been reported frequently in the literature by magnetometry measurement, so far there has been no report on correlated ferromagnetism in magnetic, optical and electrical measurements. In this paper, we investigate systematically the structural, optical, magnetic and electrical properties of Zn1-x Co (x) O:Al thin films prepared by sputtering with x ranging from 0 to 0.33. We show that correlated ferromagnetism is present only in samples with x > 0.25. In contrast, samples with x < 0.2 exhibit weak ferromagnetism only in magnetometry measurement which is absent in optical and electrical measurements. We demonstrate, by systematic electrical transport studies that carrier localization indeed occurs below 20-50 K for samples with x < 0.2; however, this does not lead to the formation of ferromagnetic phase in these samples with an electron concentration in the range of 6 x 10(19) cm(-3) 1 x 10(20) cm(-3). Detailed structural and optical transmission spectroscopy analyses revealed that the anomalous Hall effect observed in samples with x > 0.25 is due to the formation of secondary phases and Co clusters.