I cannot recover $_SESSION with ajax and codeigniter

Question

I’m having trouble recovering a $_SESSION using php native Session with ajax in codeigniter, because when I try to verify the data informUndefined. follows the code, since thanks to all.

controller login

if($user->cod_user == 1)
    {


        $_SESSION['admin'] = $user->name;

        echo "<script>location.href='admin';</script>";

    }
    else if($user->cod_user == 2)
    {
        $_SESSION['admin'] = $user->name;

        echo "<script>location.href='vendedor';</script>";
    }

in the view you receive (admin or seller) arrives as Undefined

I suspect the way I’m doing the redirect with javascript, because in this msm controller I can visualize the normal.

ajax requisition

$(function(){

        $('#gestao').slideDown('slow');

        // efetua login no sistema
        $('#bt_login').click(function(){

            $.post('<?php echo site_url()?>/Login', $('#form_login').serialize())
            .done(function(data){
                $('#msg_erro').html(data);


            });

        });
    });