A cement plug is for plugging an annular region between a first casing and a second casing in a well. A method includes: perforating the first casing to open a flow path to the annular region; providing a magnetorheological cementitious slurry; inducing a magnetic field for affecting the physical properties of the magnetorheological cementitious slurry and for defining a lower boundary for the cement plug; feeding the magnetorheological cementitious slurry into the annular region through a perforation in the first casing; and impeding the mobility of the magnetorheological cementitious slurry in the annular region by exposing it to the magnetic field such that the magnetorheological cementitious slurry may consolidate substantially at the lower boundary without the need for a base device in the annular region.