German citation not shown properly

I have a german article to cite, and in my bib file I typed something like the following

@article{müjaaa2005,
  title={aaaa},
  author={Müjaaa},
  journal={aaaa},
  pages={111},
  year={2005},
  publisher={aaa}
}

however cite{müjaaa2005} only shows a question mark.

The beginning part of my tex file looks like the following:

 begin{filecontents}{preliminary.sty}
ProvidesPackage{preliminary}
DeclareOption{draft}{%
  AtBeginDocument{%
    renewcommandmaketitlehookc{%
      begin{center}(Preliminary and Incomplete)end{center}}}}
ProcessOptions
RequirePackage{titling}
endinput
end{filecontents}
documentclass[12pt]{article}
usepackage{preliminary}
usepackage[ngerman]{babel}
usepackage[utf8x]{inputenc}
usepackage[T1]{fontenc}
usepackage[round,authoryear]{natbib}

Anyone knows how to fix this problem, and let it show müjaaa(2005) ?

Thanks!

@article{mujaaa2005,
  title={aaaa},
  author={M{"{u}}jaaa},
  journal={aaaa},
  pages={111},
  year={2005},
  publisher={aaa}
}