Dear Pooja,
Memo is a short form of memorandum. The meaning of the word memorandum is
a written statement, record, or communication such as within an office. However, in the office context and especially in Indian context, the word memo has been used with negative context.
Considering your requirement, you can give either "Warning Letter" or "Show Cause Notice". However, to issue either, you must get a formal complaint from the HOD. "Behavioural Issues" is a vague concept. You must find out whether there was defiance to the authority or insubordination.
Even before issue of either, you need to verify the facts. Once the facts are established, you may call the employee to your office and understand his viewpoint also. If the employee does not have any defence or has poor defence, then you may issue either.
If the show cause notice is issued then you need to specify the time limit by which the recipient has to give a reply. Generally, it is 48 hours. If no reply is received or no merit is found in the reply then you may initiate a disciplinary action. It could be warning letter also.
Earlier, I have given draft of the warning letters. To refer these, click the following links:
https://www.citehr.com/327002-warnin...ml#post1493525
https://www.citehr.com/552639-warnin...ml#post2285828
https://www.citehr.com/473418-warnin...ml#post2096672
Thanks,
Dinesh Divekar