Minimum required docs for private (family) loan for tax purposes

[Juan F.]

I looked around and didn't see this exact question answered.

For tax purposes, what is the minimum required amount of documentation between family members for a loan?

When I say tax purposes, I mean that (1) the lender isn't hit with some fee or unexpected tax, and that (2) the borrower is able to consider loan payback as a deductible expense every month.

From my research it seems like the only requirement from the IRS is that you meet the Applicable Federal Rate. I assume a promissory note between parties would be enough to substantiate the relationship?

Any direction or advise would be great - thanks.

(there are no concerns about collateral or default 'best practices', merely asking what the minimum amount of paperwork required by law is)