I'm using drupal 7.28 and node references 7.x-2.1

I need to know the nid of anode selected using cck node reference autocomplete, please note that I'm not looking for #default_value of the field

I've added ajax (at hook_form_alter) to perform further tasks using the nid of the selected node before submiting the form

I've searched at $form_state['imput'] but there is just the text I entered to filter the node reference field selection

also looked at $form['values'] but there the nid of the referenced field is null ($form_state['values']['my_field']['und'][0]['nid'] = null)

how / where should I look for the nid of a selected node through cck nodereference autocomplete field ?

please , any guide is appreciated, thanks in advance