Access to model functions throught custom library

Hi everybody!
I am not strong in English so please excuse me in advance if it is not very clear.
I'm creating my own library for authentication of my users but the problem is that apparently, it is impossible to access the to models functions from the library.
So it's impossible for me to do something like

PHP Code:

$this->CI->zauth_model->get_by('email', $email) 


Is it me who does it badly or there is  another way to do it?
Thank you

That will work if CI has the right value. This is the usual way to do that.

PHP Code:

class Example
{
     protected $CI;

     public function __construct()
    {
          $this->CI = & get_instance();
          $this->CI->load->model('zauth_model');
    }

    public function use_model()
    {
          $this->CI->zauth_model->get_by('email', $email);
    }
}