if and only if (shortened iff) is a biconditional logical connective between statements. Is it proper to use iff to denote if and only if in academic writing (papers)?
The answer will vary from journal to journal. I suggest you browse those journals you respect most and imitate them. Broadly speaking, yes, as an abbreviation many journals will allow abbreviations if they are used over and over again; in such cases, the purpose is to ease the burden of readers.
If you're using it within a logical expression, then yes. If you're using it as an abbreviation in standard prose, then no.