error when visiting pages behind middleware while not logging in

[khaledapps]

first I have made a new auth (admin)
and I made HomeController which has the following methods

public function __construct() {
     $this->middleware('auth:admin');
}

public function index() {
     return view('admin.dashboard.index');
}

when someone try to go to admin/dashboard an error comes:
"Route [login] not defined"

and when I change the middleware in __construct mehtod to 'admin'
everything goes fine except that Auth::user() returns null

I can't understand exactly what is the difference between 'admin' and 'auth:admin'

why 'admin' redirects to the login page but with Auth::user() is null

and 'auth:admin' returns Auth::user() correctly but doesn't redirect to login page

[khaledapps]

solved by using

$this->middleware('admin');

and

for getting the admin
Auth::guard('admin')->user();