Advances in methods to obtain and characterise room temperature magnetic ZnO

Abstract

We report the existence of magnetic order at room temperature in Li-doped ZnO microwires after low energy H+ implantation. The microwires with diameters between 0.3 and 10 μm were prepared by a carbothermal process. We combine spectroscopy techniques to elucidate the influence of the electronic structure and local environment of Zn, O, and Li and their vacancies on the magnetic response. Ferromagnetism at room temperature is obtained only after implanting H+ in Li-doped ZnO. The overall results indicate that low-energy proton implantation is an effective method to produce the necessary amount of stable Zn vacancies near the Li ions to trigger the magnetic order.

Conductivity fluctuations in proton-implanted ZnO microwires

“…1 in [18]). The defects concentration (Zn-vacancies) is important for the magnetism [10,14]. In fact, it was experimentally verified that most of the magnetic signal comes from the near surface region [18].…”

“…Due to the very nature of DIM and the difficulties in creating an adequate density of defects that would be homogeneously distributed all over the entire volume of an oxide sample, different experimental strategies were used to obtain large ferromagnetic magnetization values at saturation. These include, for example, ion irradiation [5,11], co-doping, [12,13,10] and sample size reduction [10,14]. By implantation of protons at energy 300 eV one can obtain a magnetically ordered 10 nm thick surface regions in ZnO:Li microwires.…”

“…By implantation of protons at energy 300 eV one can obtain a magnetically ordered 10 nm thick surface regions in ZnO:Li microwires. These regions show clear ferromagnetic characteristics, as revealed by SQUID magnetometry and x-ray magnetic circular dichroism (XMCD) [10] as well as a negative magnetoresistance [14]. Therefore, increase of the noise associated with decrease of the effective size of the magnetically ordered sample volume may become a limiting factor for the applications of magnetic oxide nanowires in spintronics.…”

“…The first ZnO wire is doped with Li and shows magnetic order at room temperatures after proton implantation [10]. For comparison, a second ZnO wire without Li doping but with a similar proton implantation dose as the first one, has also been measured.…”

“…However, the phenomenon of defect-induced magnetism (DIM), i.e., magnetic order produced by defects and/or added ions (not necessarily magnetic) in the atomic lattice of oxides and other materials, indirectly requires a pronounced sample size reduction [2,3,4,5,6,7,8]. Due to the O-2p hole-spin polarization from oxygen atoms around Zn-vacancies in ZnO [9,10], the DIM phenomenon opens up new possibilities for spintronics applications of the ZnO compound. Due to the very nature of DIM and the difficulties in creating an adequate density of defects that would be homogeneously distributed all over the entire volume of an oxide sample, different experimental strategies were used to obtain large ferromagnetic magnetization values at saturation.…”