  We study classical Heisenberg antiferromagnets with uniaxial exchange
anisotropy and a cubic anisotropy term on simple cubic lattices in an external
magnetic field using ground state considerations and extensive Monte Carlo
simulations. In addition to the antiferromagnetic phase field--induced
spin--flop and non--collinear, biconical phases may occur. Phase diagrams and
critical as well as multicritical phenomena are discussed. Results are compared
to previous findings.
