Cannot lock xml write using lock() or Mutex - multithreading

Question

I am writing a simple application which writes to a single xml file for around 1000 iterations from multiple threads .
I have tried using the lock() ,Mutex and the ReaderWriterSlim to lock the file while its being written. It works for some time but gives error about the file is being accessed by another process.
Can somebody please help me to get some solution :
Or is there any other way to achieve this .
public class XmlWrite
{
//private static object _lock = new Object();
static string xmlFilePath = "C:\\Test\\Multi.xml";
public static Mutex m_Mutex = new Mutex(false, #"Global\MyMutex");
public static void LogXML(object threadID)
{
int i = 0;
while (i < 1000)
{
try
{
m_Mutex.WaitOne();
XmlDocument xml = LoadDocument(xmlFilePath);
var xmlElement = XElement.Load(new XmlNodeReader(xml));
var sourceXElement = new XElement("ParentNode");
sourceXElement.Add(new XElement("ChildNode", threadID));
xmlElement.Add(sourceXElement);
SaveDocument(xmlElement, xmlFilePath);
Console.WriteLine("Value " + xml.ToString() + "Index" + i + "Thread" + threadID.ToString());
}
catch (Exception exe)
{
Console.WriteLine(exe.Message);
}
finally
{
m_Mutex.ReleaseMutex();
}
i = i + 1;
}
}
public static XmlDocument LoadDocument(String path)
{
XmlDocument document = new XmlDocument();
using (StreamReader stream = new StreamReader(path, Encoding.GetEncoding("iso-8859-7")))
{
document.Load(stream);
}
return (document);
}
public static XElement SaveDocument(XElement document, String path)
{
using (StreamWriter stream = new StreamWriter(path, false, Encoding.GetEncoding("iso-8859-7")))
{
document.Save(stream);
}
return (document);
}
}
class Program
{
static void Main(string[] args)
{
new Thread(XmlWrite.LogXML).Start("A");
new Thread(XmlWrite.LogXML).Start("B");
new Thread(XmlWrite.LogXML).Start("C");
new Thread(XmlWrite.LogXML).Start("D");
new Thread(XmlWrite.LogXML).Start("E");
new Thread(XmlWrite.LogXML).Start("F");
new Thread(XmlWrite.LogXML).Start("G");
new Thread(XmlWrite.LogXML).Start("H");
new Thread(XmlWrite.LogXML).Start("i");
}
}