Temporary wellbore barrier using ferromagnetic fluid

A ferromagnetic fluid can be used to provide a temporary barrier in a wellbore. A magnetic sub can generate an electromagnetic field within the wellbore. The ferromagnetic fluid is retained at a desired location within the wellbore by the electromagnetic field. Ferromagnetic particles in the fluid can clump together or settle to form the temporary barrier. A zonal isolation fluid such as a metallic fluid, a cement composition, or a curable resin composition can be introduced on top of the temporary barrier. The zonal isolation fluid can solidify to form a permanent or semi-permanent barrier to provide zonal isolation of different zones of the subterranean formation.