Vacancy induced room temperature ferromagnetism in Cu-doped ZnO nanofibers

•ZnO nanofibers were doped by thermally diffusing Cu atoms into the pure ZnO.•Cu-doped ZnO nanofibers fabricated by thermal in-diffusion exhibit RTFM.•Cu doping by diffusion leads high level of oxygen vacancies.•Robust O vancancies in the Cu-diffusing ZnO nanofibers allow for long range FM.