How does Donne's love poetry display the metaphysical aspect?
Donne's love poetry is not "seprated" or separated from his metaphysical poetry. His love poetry is metaphysical poetry. There's no separation.
Donne uses stretched metaphors or conceits in his love poetry. He compares things that aren't usually associated with one another, and compares them in odd ways. This is the definition of metaphysical poetry, as the term is applied to Donne.
For instance, Donne compares the two legs of a compass to two lovers, in his love peom, "A Valediction: Forbidding Mourning." The compass is a mechanical instrument and is not something that one would think of as being compared to lovers. This is one characteristic of metaphysical poetry, as we use the term today.