  Rapidly spinning magnetic grains can acquire large magnetic dipole moments
due to the Barnett effect. Here we study the new effect of Barnett magnetic
dipole-dipole interaction on grain-grain collisions and grain growth, assuming
grains spun up by radiative torques. We find that the collision rate between
grains having embedded iron inclusions can be significantly enhanced due to
Barnett magnetic dipole-dipole interaction when grains rotate suprathermally by
radiative torques. We discuss the implications of enhanced collision rate for
grain growth and destruction in the circumstellar envelope of evolved stars,
photodissociation regions, and protostellar environments. Our results first
reveal the importance of the dust magnetic properties and the local radiation
field on grain growth and destruction.
