Hello Karthik,
As suggested by Jaroslaw Kosik mark the change document in the further characteristics in the domain of the field.
Are you using a standard domain or custom domain?
In case of custom domain you can able to set the flag.
While in case of standard domain you can't change it, to achieve the same either create a custom domain or create a new log object (Refer to the below link) and save the changes of the custom field to the custom log in a suitable function exit.
http://scn.sap.com/docs/DOC-27066
Regards,
TP
