  The electronic and magnetic properties of neutral substitutional
transition-metal dopants in dia- mond are calculated within density functional
theory using the generalized gradient approximation to the exchange-correlation
potential. Ti and Fe are nonmagnetic, whereas the ground state of V, Cr and Mn
are magnetic with a spin entirely localized on the magnetic ion. For Co, Ni,
and Cu, the ground state is magnetic with the spin distributed over the
transition-metal ion and the nearest-neighbor carbon atoms; furthermore a bound
state is found in the gap that originates from the hybridization of the
3d-derived level of the dopant and the 2p-derived dangling bonds of the
nearest-neighbor carbons. A p{d hybridization model is developed in order to
describe the origin of the magnetic interaction. This model predicts high-spin
to low-spin transitions for Ni and Cu under compressive strain.
