While both documents may contain similar elements, it's the angle from which each is written that makes the difference.
Traditionally a pitch deck is created by the startup entrepreneurs. Its goal is for the founders to communicate their business idea, achievements, and where they're heading. The pitch deck usually takes the form of a presentation. A pitch deck may be biased as it is written by the same people who are seeking an investment.
In comparison, a deal memo is created by an investor to make the case for an investment and to make an investment decision on whether or not to proceed to the next phase of due diligence. The deal memo usually takes the form of a document. And although it's written by someone who would like to proceed with making an investment, a deal memo can be more objective since it's written by a third-party and not the startup's founders themselves. For example, a deal memo may also include reasons for not investing, foreseen challenges, and weaknesses of the startup.
Both a pitch deck and a deal memo contain similar elements like the business opportunity, market size, team, traction and valuation.