php json_encode not returning proper json encoded string

I am using a jquery ajax call which accepts json response :

 var posturl = '/admin/getparamdetails/';
    var data = "adnetworkId="+adnetworkId;
    $.ajax({
        type: "POST",
        url: posturl,
        data : data,
        datatype: "json",
        success: function(msg){
            //$("#displayPramForm").html(msg);
            //alert('hello'+msg.length+' '+msg.hello.length);
            console.log(msg);
            if(msg!='')
            {
                alert(msg.hello);
            }
        },
        failure: function(msg){}
    });

in my php backend function , I am using json_encode on a simple array as shown:

 $json_encoded_string =  json_encode(array("hello"=>'abc'));
 echo $json_encoded_string;
 die;               

but alert(msg.hello) returns undefined for me. What is going wrong here ? Also , in my console.log I am able to get the output as :

{"hello":"abc"}