Ferromagnetic transition metal implanted ZnO: a diluted magnetic semiconductor?

TL;DR

This study investigates the ferromagnetic properties of transition metal-implanted ZnO, revealing secondary phase formation as the source of ferromagnetism and highlighting challenges in detecting these phases with standard XRD techniques.

Contribution

It demonstrates that ferromagnetism in transition metal-implanted ZnO is due to secondary phases, not uniform doping, and discusses detection challenges and magnetic behavior differences.

Findings

Secondary phases like Fe, Ni, Co nanocrystals cause ferromagnetism.

XRD may fail to detect secondary phases due to orientation issues.

Co-doped ZnO films show paramagnetism, but magnetoresistance suggests s-d exchange interactions.

Abstract

Recently theoretical works predict that some semiconductors (e.g. ZnO) doped with magnetic ions are diluted magnetic semiconductors (DMS). In DMS magnetic ions substitute cation sites of the host semiconductor and are coupled by free carriers resulting in ferromagnetism. One of the main obstacles in creating DMS materials is the formation of secondary phases because of the solid-solubility limit of magnetic ions in semiconductor host. In our study transition metal ions were implanted into ZnO single crystals with the peak concentrations of 0.5-10 at.%. We established a correlation between structural and magnetic properties. By synchrotron radiation X-ray diffraction (XRD) secondary phases (Fe, Ni, Co and ferrite nanocrystals) were observed and have been identified as the source for ferromagnetism. Due to their different crystallographic orientation with respect to the host crystal these nanocrystals in some cases are very difficult to be detected by a simple Bragg-Brentano scan. This results in the pitfall of using XRD to exclude secondary phase formation in DMS materials. For comparison, the solubility of Co diluted in ZnO films ranges between 10 and 40 at.% using different growth conditions pulsed laser deposition. Such diluted, Co-doped ZnO films show paramagnetic behaviour. However, only the magnetoresistance of Co-doped ZnO films reveals possible s-d exchange interaction as compared to Co-implanted ZnO single crystals.