Calculations were carried out using the planewave density-functional theory code CASTEP version 23.1. The PBE general gradient approximation exchange-correlation functional was used 82 with norm-conserving pseudopotentials from the built-in NCP19 library. Calculated exchange interactions were robust to changes in plane-wave cut-off energy for the basis set. Van der Waals forces between each layer were described using the many-body semiemprical dispersion correction MBD*. An effective on-site interaction parameter, U eff = U -J, was necessary to impose a strong localisation on the Fe and Ni d-states, where U is the onsite Coulomb term and J is the site exchange term. U eff is applied as a correction to the total energy of the system,