The synthesis, crystal structures and magnetic properties of two families of heterospin complexes containing lanthanide ions and a bis(imino nitroxide) biradical (IPhIN = 1-iodo-3,5-bis(4′,4′,5′,5′-tetramethyl-4′,5′-dihydro-1H-imidazole-1′-oxyl)benzene) are reported: in [Ln₂(hfac)₆(IPhIN)(H₂O)₂] compounds, two lanthanide ions [Ln = Gd^III (1) and Dy^III (2)] are coordinated to the biradical, and in [Ln(hfac)₃(IPhIN)(H₂O)] compounds, one lanthanide ion (Ln = Tb^III (3), Gd^III (4) or Dy^III (5)) is coordinated to the biradical. Ferromagnetic intramolecular magnetic interactions between Gd^III and the biradical were found for 1 and 4, while intramolecular magnetic interactions between the radicals were ferro- and antiferromagnetic, respectively. Compound 2 shows a field induced slow relaxation of magnetization, which (under an external applied field of 2 kOe) exhibits an activation energy barrier of ΔE/k_B = 27 K and a pre-exponential factor of 1.4 × 10⁻⁸ s. To support the magnetic characterization of compound 3ab initio calculations were also performed.