Ferromagnetism from non-magnetic ions: Ag-doped ZnO

Nasir Ali,Budhi Singh,Subhasis Ghosh,Anuvesh Kumar,Zaheer Ahmed Khan,Vijaya A R,Kartick Tarafder,Manoj K Wadhwa

doi:10.1038/s41598-019-56568-8

Abstract

To develop suitable ferromagnetic oxides with Curie temperature (TC) at or above room temperature for spintronic applications, a great deal of research in doping ZnO with magnetic ions is being carried out over last decade. As the experimental results on magnetic ions doped ZnO are highly confused and controversial, we have investigated ferromagnetism in non-magnetic ion, Ag, doped ZnO. When Ag replaces Zn in ZnO, it adopts 4d9 configuration for Ag2+ which has single unpaired spin and suitable exchange interaction among these spins gives rise to ferromagnetism in ZnO with above room temperature TC. Experimentally, we have observed room temperature ferromagnetism (RTFM) in Ag-doped ZnO with Ag concentration varied from 0.03% to 10.0%. It is shown that zinc vacancy (VZn) enhances the ferromagnetic ordering (FMO) while oxygen vacancy (VO) retards the ferromagnetism in Ag-doped ZnO. Furthermore, the theoretical investigation revealed that VZn along with Ag2+ ions play a pivotal role for RTFM in Ag-doped ZnO. The Ag2+-Ag2+ interaction is ferromagnetic in the same Zn plane whereas anti-ferromagnetic in different Zn planes. The presence of VZn changes the anti-ferromagnetic to ferromagnetic state with a magnetic coupling energy of 37 meV. Finally, it has been established that the overlapping of bound magnetic polarons is responsible for RTFM in low doping concentration. However, anti-ferromagnetic coupling sets in at higher doping concentrations and hence weakens the FMO to a large extent.

Highlights

To develop suitable ferromagnetic oxides with Curie temperature (TC) at or above room temperature for spintronic applications, a great deal of research in doping ZnO with magnetic ions is being carried out over last decade
Whatever be the exchange mechanisms of ferromagnetism, one has to explain the most important experimental facts in ZnO i.e. the observation of ferromagnetic ordering (FMO) at 300 K at a very low doping content (0.03%) which is several order of magnitude smaller than the cation percolation threshold, ~19.8% in wurtzite ZnO22. This indicates that all the short range interaction such as double and super exchange proposed so far[23,24], except the mechanism proposed by Coey et al.[14], to explain FMO can be doubted as the distance (~20 nm) between two dopant atoms is much larger than the lattice constant of ZnO
In view of all these contradictory results regarding the stability and origin of room temperature ferromagnetism (RTFM) in Ag-doped ZnO, we present a detailed experimental and theoretical investigation to understand the origin of RTFM in Ag-doped ZnO films

Summary

Introduction

To develop suitable ferromagnetic oxides with Curie temperature (TC) at or above room temperature for spintronic applications, a great deal of research in doping ZnO with magnetic ions is being carried out over last decade. We have observed room temperature ferromagnetism (RTFM) in Ag-doped ZnO with Ag concentration varied from 0.03% to 10.0%. Whatever be the exchange mechanisms of ferromagnetism, one has to explain the most important experimental facts in ZnO i.e. the observation of ferromagnetic ordering (FMO) at 300 K at a very low doping content (0.03%) which is several order of magnitude smaller than the cation percolation threshold (xp), ~19.8% in wurtzite ZnO22. To rule out the doubt whether ferromagnetism is intrinsic or extrinsic in DMOs, it is being attempted to dope ZnO with non-magnetic ions. Keithley 2182A nanovoltmeter and 6221 AC and DC current source were used to perform room temperature Hall measurements on doped ZnO films

Results

Discussion

Conclusion