The word memorandum is singular. Its plural is either memoranda or memorandums. In fact, memorandums now predominates. But you should remember your audience: If your readers get their noses out of joint over memorandums, then capitulate and use memoranda. If you want to press the issue, you can point out that Shakespeare used memorandums in Henry IV (Act 3, Scene II).
Prince Henry to Falstaff:
Charge an honest woman with picking thy pocket! why, thou whoreson, impudent, embossed rascal, if there were anything in thy pocket but tavern-reckonings, memorandums of bawdy-houses, and one poor penny-worth of sugar-candy to make thee long-winded, if thy pocket were enriched with any other injuries but these, I am a villain: and yet you will stand to if; you will not pocket up wrong: art thou not ashamed?