Hello Alex!
There is no set standard across the Army with what to do/how to send it.  In my last unit, please included some quick background information in the email and a formal letter attached. I have not seen anyone use a memorandum format.  I would recommend using a formal letter format, also found in AR 25-50.  Below is part of a post I wrote earlier this year.  I included the relevant info here.

Letter of introduction:
-For some reason many officers do not send any sort of message to their future unit. This is a missed opportunity. Sending a message is your first opportunity to communicate with your future unit and chain of command. The most common message is a formal letter of introduction. Key topics to cover is your professional background (previous assignments, commissioning source, etc), your projected timeline, family information (spouse/significant other, dependents, marital status), and personal interests.
-Other documents to consider including with your first message is your ORB, career plan, and/or bio. (you may not have everything with you just starting out)

Let me know if you have any additional questions.  I am sure a few others will chime in here as well.

Chad