Style for cites

[Tim Schröder]

Hi there,

i am using bibtex to manage the literature for my thesis. I dont know 
much about LaTeX except for the basic principles.
I still have a problem with the style of the inserted bib-references. 
They all look like (Fel84) in the Text. But i want them to look like 
[Fel84].

Could somebody tell me how that works ??  The literaturelist at the end 
looks right.

I am using amsalpha.bst style, ams-math,nat-bib and style: author-year.