You will have to use "RedirectResult" which represents a redirection to a new URL.
return RedirectResult(url);
This should solve your problem.
The following listed types are the available derivations of ActionResult:
1 ContentResult —Represents a text result
2 EmptyResult —Represents no result
3 FileResult —Represents a downloadable file (abstract class)
4 FileContentResult —Represents a downloadable file (with the binary content)
5 FilePathResult —Represents a downloadable file (with a path)
6 FileStreamResult —Represents a downloadable file (with a file stream)
7 HttpUnauthorizedResult —Represents the result of an unauthorized HTTP
request
8 JavaScriptResult —Represents a JavaScript script
9 JsonResult —Represents a JavaScript Object Notation (JSON) result that can
be used in an AJAX application
10 RedirectResult —Represents a redirection to a new URL
11 RedirectToRouteResult —Represents a result that performs a redirection
given a route values dictionary
12 PartialViewResult —Base class used to send a partial view to the response
13 ViewResult —Represents HTML and markup
14 ViewResultBase —Base class used to supply the model to the view and then render
the view to the response
15 XmlResult —Action result that serializes the specified object into XML and outputs
it to the response stream (provided by the MvcContrib library)