Fe-implanted ZnO: Magnetic precipitates versus dilution

TL;DR

This study investigates the origin of ferromagnetism in Fe-implanted ZnO, distinguishing between precipitate-induced and potential carrier-mediated magnetic behavior across different processing conditions.

Contribution

It provides a comprehensive analysis of how Fe implantation and processing parameters influence magnetic phases and ferromagnetism in ZnO, clarifying the role of precipitates versus dilution.

Findings

Ferromagnetism arises from Fe nanocrystals and magnetic precipitates.

Different regimes of Fe incorporation lead to distinct magnetic behaviors.

Ionic Fe alone does not induce ferromagnetism due to lack of carrier coupling.

Abstract

Nowadays ferromagnetism is often found in potential diluted magnetic semiconductor systems. However, many authors argue that the observed ferromagnetism stems from ferromagnetic precipitates or spinodal decomposition rather than from carrier mediated magnetic impurities, as required for a diluted magnetic semiconductor. In the present paper we answer this question for Fe-implanted ZnO single crystals comprehensively. Different implantation fluences and temperatures and post-implantation annealing temperatures have been chosen in order to evaluate the structural and magnetic properties over a wide range of parameters. Three different regimes with respect to the Fe concentration and the process temperature are found: 1) Disperse Fe$^{2+}$ and Fe$^{3+}$ at low Fe concentrations and low processing temperatures, 2) FeZn$_2$O$_4$ at very high processing temperatures and 3) an intermediate regime with a co-existence of metallic Fe (Fe$^0$) and ionic Fe (Fe$^{2+}$ and Fe$^{3+}$). Ferromagnetism is only observed in the latter two cases, where inverted ZnFe$_2$O$_4$ and $\alpha$-Fe nanocrystals are the origin of the observed ferromagnetic behavior, respectively. The ionic Fe in the last case could contribute to a carrier mediated coupling. However, their separation is too large to couple ferromagnetically due to the lack of p-type carrier. For comparison investigations of Fe-implanted epitaxial ZnO thin films are presented.