Where should I place the custom user library file in CodeIgniter's directories?

I have a custom user library file in the systems libraries folder of my CodeIgniter installation. However, I'm facing issues when trying to call the user model in the admin directory using fcpath. I've tried using the following code:

$this->CI->load->model(dirname(FCPATH) . '/admin/application/modules/backend/models/user/user_model.php');

Do you have any suggestions to resolve this problem?